Advancements and challenges of tailored engineering of carbon materials for electrolytic CO₂ reduction to high-value carbon products

Abstract

The extensive release of greenhouse gases, including carbon dioxide (CO₂), poses a significant global challenge linked to fossil fuel use. These emissions contribute to the deterioration of the ecosystem around us, including global warming, rising sea levels, and biodiversity loss. In order to mitigate climate change by limiting greenhouse gas emissions, CO₂ must be electrochemically converted to carbon products. It transforms a waste product into high-value chemicals and fuels that promote resource recovery leading to economic opportunities. Our review provides an extensive examination of current research on carbon-based catalysts used for the reductive electrochemical transformation of CO₂. The focus is on advances and ongoing challenges in converting CO₂ into valuable carbon-based products. The low cost, large surface area, improved reliability, and conductive properties of carbon-based compounds make them excellent catalysts. These materials can be customized through methods such as heteroatom doping and composite formation, allowing for modulation of their catalytic properties to favor specific reaction pathways and increase product selectivity. This review includes a summary table highlighting the reduction products, faradaic efficiency (FE), current density (J), stability, and key features of various carbon-based catalysts. It also discusses major challenges, design principles, strategic recommendations, and future directions for developing effective carbon-based electrocatalysts for CO₂ reduction.

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1. Introduction

The increasing reliance on fossil fuels since the Industrial Revolution has significantly increased CO₂ emissions.^1,2 The scientific community and the general population are deeply concerned about the serious climate change and sustainability problems that arise from this.^3–5 To address these challenges, it is necessary to develop CO₂ mitigation technologies.^6,7

As fossil fuels account for up to 80% of overall energy consumption, converting CO₂ to low-carbon fuels offers significant benefits.⁸ The use of CO₂ as a renewable resource yields commercially valuable compounds such as carbon monoxide gas, methanol, methane, formic acid, and ethylene by means of renewable electricity. CO₂'s high bond energy, however, makes it difficult to convert to valuable goods. CO₂ reduction procedures are kinetically slow due to the numerous steps involved in electron transport.⁹ Various approaches have been employed to transform CO₂ into energy products and materials to address this challenge, including electrochemical,¹⁰ biochemical,¹¹ thermochemical,^12,13 photochemical,¹⁴ and radiochemical.¹⁵ Among these technologies, photochemical CO₂ reduction is precise and selective, but the reduction rate or creation of valuable chemicals is much less efficient. Furthermore, the thermochemical reduction of CO₂ involves high temperature, high energy, high pressure, and hydrogen (H₂) as a reducing agent, which is essentially unattainable for large-scale commercial applications.^16,17 Among these techniques, electrochemical CO₂ reduction offers several benefits due to its easy scale-up applications, moderate chemical usage, controlled process, and typical settings.¹⁸

CO₂ is a highly stable compound due to the intense bond dissociation energy of its carbon–oxygen double bonds, which is approximately 336 kilojoules per mole.¹⁹ Consequently, the reduction of CO₂ is a slow-moving, thermodynamically uphill reaction. Commonly, 2, 4, 6, 8, 12, or even more electron reduction pathways are involved in the CO₂ reduction reaction (CO₂RR). As a result, various products could be produced, such as formate, CO, CH₄, HCOOH, C₂H₄, CH₃OH, and so on.^20,21Table 1 shows the reduction reaction of CO₂ and the corresponding standard potential for creating specific end compounds.²² The table shows that the potential for CO₂ reduction is always more negative. There exists a large thermodynamic constraint to the CO₂ conversion via electrocatalysis into useful products. The principal cause of this can be the initial transfer of a single electron to CO₂, which produces CO₂ and operates under high-voltage conditions (−1.9 V) in comparison to the Standard Hydrogen Electrode (SHE).^23,24 Preparing alcohols or hydrocarbons from CO₂via electrochemical reduction is kinetically more demanding than preparing HCOOH or CO, as a result of the more complicated reaction routes involving numerous electron transfers. As a result, to facilitate the transformation of CO₂ into specific products via electrochemical reduction, a considerable overpotential is generally necessary.

Table 1 At a pressure of 1.0 atm and room temperature, the standard potential of the CO₂ reduction reaction in aqueous solutions (V versus SHE).²²

Half-reaction	Potential E0 (V) at pH = 7
CO2 + e^− → CO2^−	−1.9
2H^+ + 2e^− → H2	−0.42
CO2 + 2H^+ + 2e^− → HCOOH	−0.61
CO2 + 2H^+ + 2e^− → CO + H2O	−0.53
CO2 + 4H^+ + 4e^− → HCHO + H2O	−0.48
CO2 + 6H^+ + 6e^− → CH3OH + H2O	−0.38
2CO2 + 8H^+ + 8e^− → CH3COOH + 2H2O	−0.30
2CO2 + 10H^+ + 10e^− → CH3CHO + 3H2O	−0.35
2CO2 + 12H^+ + 12e^− → C2H5OH + 3H2O	−0.33
2CO2 + 14H^+ + 14e^− → C2H6 + 4H2O	−0.27
3CO2 + 18H^+ + 18e^− → C3H7OH + 5H2O	−0.32

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CO₂ reduction has promoted the use of electrocatalysts incorporating noble metals, including those based on Pd, Au, and Ag, because of their favorable characteristics, which include stability, selectivity, low overpotential, and potent electrocatalytic performance. These substances can facilitate the electrocatalytic transformation of CO₂ into beneficial compounds, such as hydrocarbons and alcohols.²⁵ However, their expensive cost has limited their wide-use applications for CO₂ reduction. Driven by the need for cost-effective alternatives to noble metal catalysts in CO₂ reduction, research has yielded diverse catalytic systems. These include metal oxides, single-site catalysts and single-atom catalysts supported on various substrates, transition metals integrated with or deposited onto graphene, bimetallic and metal–carbon composites, metal-functionalized porphyrin analogues, metal–organic frameworks (MOFs), colloidal nanocrystals, and doped nanostructures.^26–39 Areas needing improvement in electrocatalysts include stability, reduction onset potential, FE, and current density. Therefore, finding an active electrocatalyst that can reduce CO₂ to valuable products while competing with the hydrogen evolution reaction (HER) remains a significant challenge. The elevated reduction overpotential results in substantial energy loss and promotes the HER, a competing process that limits the efficiency of reduction of CO₂.

In contrast to conventional CO₂ electrocatalysts, carbon materials are attracting significant interest in reduction of CO₂ as a result of their advantageous traits such as significant surface area, exceptional conductivity and structural strength. Their cost-effectiveness, environmental benignity, and abundance are also beneficial. However, pristine (nano) carbons show limited efficacy in CO₂ reduction because of the inherent difficulty in activating CO₂ molecules by electroneutral carbon atoms. Carbon has the ability to form diverse structures across different dimensions, and a defining feature is the evolution from 0D carbon nanodots to single dimension carbon nanotubes (CNTs), 2D graphene, and 3D diamonds, as illustrated in Fig. 1.⁴⁰

Fig. 1 Carbon allotropes categorized based on their dimensional characteristics.

Among the foundational carbon materials, pristine carbon structures, those with uniform chemical composition and no intentional modification, are central to the development of advanced carbon-based technologies. CNTs, graphite, graphene, and carbon dots (CDs) are some examples, each with distinct structural and functional properties. The three-dimensional (3D) allotrope graphite is composed of stacked graphene layers, held together by weak van der Waals forces, resulting in high thermal stability and electrical conductivity.⁴¹ It is widely used in electrodes and lubricants and serves as a precursor for graphene production.

Graphene, a single atomic layer of sp²-hybridized carbon atoms arranged in a two-dimensional honeycomb lattice, demonstrates exceptional electrical conductivity, mechanical strength, and thermal conductivity.⁴² Its large surface area and high electron mobility make it highly promising for applications in sensors, flexible electronics, and energy storage devices. CNTs are cylindrical structures formed by rolling graphene sheets. Based on the number of layers, they are classified as single-walled or multi-walled. Their exceptional electrical conductivity, tensile strength, and aspect ratio make CNTs suitable for use in catalytic supports, conductive composites, and field emission devices.⁴³ Their properties are highly dependent on chirality and diameter, which determine whether they behave as metals or semiconductors.

On the other hand, CDs are 0D nanostructures, typically less than 10 nm in size, comprising a core of sp²/sp³-hybridized carbon and a surface rich in functional groups such as –OH, –COOH, and –NH₂. These groups impart excellent solubility and chemical tunability. Due to their bright, excitation-dependent fluorescence, CDs are ideal for biosensing, bioimaging, photocatalysis, and environmental remediation.⁴⁴ The diverse dimensionalities of these pure carbon materials, 3D (graphite), 2D (graphene), 1D (CNTs), and 0D (CDs), underscore the vast range of structural configurations possible within the field of carbon nanoscience.⁴⁵

The development of carbon nanomaterials marks a shift from basic forms of carbon, such as graphite and graphene, toward advanced, functional structures. This transition is driven by the increasing demand for high-efficiency materials capable of delivering superior performance in critical applications like catalysis, energy storage, and environmental remediation. The evolutionary design of carbon materials can be understood through distinct stages, including the development of heteroatom-doped carbons, single-atom catalysts (SACs), carbon-supported metal nanoparticles (NPs), hybrid carbon–metal structures, and metal–organic frameworks. Each step introduces additional chemical complexity, improved control over structural and electronic properties, and greater adaptability for target-specific functions.⁴⁶

Heteroatom-doped carbon materials represent the first advancement beyond pristine carbon. Incorporating non-metal atoms such as nitrogen, sulfur, phosphorus, or boron alters the electronic structure and enhances catalytic performance. Nitrogen doping, in particular, improves electron-donating ability and introduces active sites that facilitate the oxygen reduction reaction (ORR).⁴⁶ Similarly, sulfur doping contributes to charge redistribution, and dual-doping strategies, such as nitrogen and sulfur co-doping, can produce synergistic effects, resulting in improved conductivity and catalytic activity.⁴⁷

The next level of complexity involves single-atom catalysts (SACs) supported on carbon frameworks. These systems consist of isolated metal atoms (e.g., Fe, Co, Ni, and Pt) uniformly dispersed across the carbon matrix, often coordinated with nitrogen atoms in Fe–N₄-like moieties.⁴⁸ SACs address limitations of conventional NP catalysts, such as low atom efficiency and heterogeneous active sites. By ensuring each metal atom serves as a discrete, well-defined catalytic site, SACs achieve near 100% metal utilization and exhibit exceptional selectivity and performance.

Building further, carbon-supported metal NPs involve the anchoring of nanoscale clusters of metals such as Pt, Pd, Ru, or Co onto conductive carbon matrices. Compared to SACs, these feature small atom clusters that remain highly dispersed and stabilized by the carbon substrate. The carbon not only provides physical support but also influences electronic behavior and prevents sintering under harsh reaction conditions. This phenomenon, known as strong metal–support interaction, facilitates electron transfer between the carbon and metal phases, enhancing catalytic activity.⁴⁹ The most advanced structures in this hierarchy are carbon–metal oxide hybrids, which combine the electrical conductivity and flexibility of carbon with the redox activity and structural stability of metal oxides such as MnO₂, TiO₂, Fe₂O₃, or Co₃O₄. These hybrid materials harness synergistic effects: the carbon component enhances charge transport and mechanical flexibility, while the oxide contributes to chemical reactivity and robustness.⁵⁰ This classification outlines the historical and functional evolution of carbon nanomaterials and offers a clear framework for understanding their progressive design strategies and diverse applications.

With a focus on metal–carbon composites, porous carbon structures, heteroatom-doped materials, and metal-free catalysts, this article provides a thorough review of current advancements in carbon materials for CO₂ reduction. It investigates the mechanisms and reaction pathways involved in the conversion of CO₂, and it offers insights into how these constituents interact with CO₂ to promote effective reduction. A further investigation of the difficulties that carbon-based electrocatalysts for the CO₂RR are currently facing, including problems with stability, selectivity, and scalability, is presented in this review. Moreover, it provides design principles and recommendations for enhancing performance and explores future views, describing potential innovations and strategies to improve the actual application of these materials.

2. Fundamentals of electrochemical reduction of CO₂

2.1. Mechanistic overview of CO₂ reduction

CO₂ undergoes electro-reduction, an electron-rich transfer pathway that yields a range of chemical compounds through modification of the number and binding locations of electrons and protons during the reaction. Fig. 2 displays multiple CO₂RR routes that are commonly observed. The pathway to generate C₁ products aims to make a reaction path involving only one carbon atom. The reduction of CO₂ proceeds following surface adsorption of reactant molecules on the catalyst, forming intermediate species. Two types of intermediates were identified: (1) the proton–electron transfer mechanism promotes the formation of a carboxylic acid intermediate. This occurs through interaction of an active site with CO₂, leading to proton transfer to the adsorbed *CO₂^δ− (2) at the reaction site, and the interaction of CO₂ with oxygen results in the formation of a HCOO as an intermediate, following which hydrogenation occurs.⁵¹ The strength of *CO binding to the catalyst influences the reaction pathway. Weak binding favors *CO formation via *COOH dehydration, while the *HCOO intermediate leads to formate. Conversely, the interaction of *CO intermediates with other reactive species leads to formation of C₁, C₂, and polycarbon products. Owing to the capability of both carbon and oxygen in *CO to accept protons, two reaction pathways are possible: (1) oxygen protonation leads to *COH, which can be reduced to CH₄ and water; (2) carbon protonation forms *CHO, which is then converted to methanol via hydrogenation.^52–54

Fig. 2 Mechanism of CO₂ reduction through catalytic processes. Reproduced with permission.⁵⁶ Copyright 2021, Wiley-VCH.

C₂ product formation proceeds through a pathway involving OCCOH and *OCHCH₂ intermediates, ultimately leading to either C₂H₄ or CH₃CH₂OH via deoxygenation or hydrogenation, respectively.^55,56 Further steps involving carbon–carbon bond formation, protonation, and hydrogenation yield CH₃COCH₃. Copper's distinct CO adsorption energy makes it uniquely suited for CO conversion to CHO, COH, or OCCOO precursors to C₂⁺products. Other metals (Ag, Au, Fe, Ni, and Pt) bind CO with insufficient or excessive strength.⁵⁷ By creating metal-based carbon catalysts with certain coordination environments, the electronic framework of the reaction sites, and consequently, catalytic activity can be tailored.⁵⁷

2.2. Evaluating effectiveness of CO₂ reduction

To gain insight into the CO₂RR effectiveness, current density (J), stability, energy efficiency, and FE are the main metrics of performance.⁵⁸ With the substantial carbon loss resulting from carbonate formation in alkaline CO₂RR, researchers have recently proposed evaluating CO₂ utilization efficiency as an additional metric.⁵⁹ The total effectiveness of the catalyst in the CO₂RR relies on several reaction determinants, such as regular temperature and pressure, electrochemical reactors, and electrolyte composition. All these aspects would mainly impact stability, CO₂ utilization efficiency, FE, energy efficiency, and current density. Moreover, to ensure an accurate evaluation, catalytic activity should be assessed across a range of catalysts using comparable reaction parameters. Crucially, the intrinsic activity of any given catalyst must be determined under conditions where the reaction is kinetically controlled.

2.2.1 Cells and electrolyzers. Electrochemical conversion of CO₂via electrochemical methods and experiments is commonly conducted in a tri-electrode system within a divided H-cell. By incorporating an ion-exchange membrane for separating both the electrodes, this cell uses electrolyte fluid at 25 °C and circumambient pressure. The membrane aims to prevent reduction products from re-oxidizing at the anode once they develop at the cathode. Effective CO₂ electroreduction in liquid electrolytes requires the gas to initially dissolve within the electrolyte. While CO₂ exhibits higher solubility in water (34 mM at 25 °C and 1 atm, where 1 atm equals 1.013 × 10⁵ Pa) compared to gases like nitrogen, oxygen, carbon monoxide, and methane, the achievable geometric J is typically limited to below 100 mA cm⁻². To overcome limitations posed by CO₂ solubility, designs incorporating gas diffusion electrodes (GDEs) with extensive gas–liquid–solid interfaces are employed. This approach facilitates the construction of CO₂ electrolyzers capable of achieving industrial-scale current densities (exceeding 200 mA cm⁻²). Flow-cell and electrode–membrane assembly (MEA) configurations are among the most prevalent electrolyzer designs. Research on flow-cell electrolyzers utilizing concentrated alkaline electrolytes has demonstrated CO₂ reduction current densities exceeding 1 A cm⁻². A critical area for future investigation is enhancing the stability of these systems.^60,61 However, MEA electrolyzers with longer-lasting GDE structures and higher energy efficiency appear to be more promising.^62,63

2.2.2 Electrolyte composition. The choice of electrolyte significantly influences the products formed during the CO₂RR. Studies have shown that in bicarbonate electrolytes, larger alkali-metal cations generally enhance both the activity and selectivity toward C₂⁺ products. The observed trend is Cs⁺ > Rb⁺ > K⁺ > Na⁺ > Li⁺. Enhanced performance likely arises from increased specific cation adsorption at the electrode surface or cation accumulation within the outer Helmholtz plane.^64,65 Halide ions, among other anions, can increase C–C coupling by adsorbing strongly on Cu electrodes.⁶⁶ Another significant electrolyte parameter is the pH. Higher pH is typically thought to promote C₂H₄ selectivity over CH₄ while suppressing the competing HER.^67–69 Because CO₂ is acidic, alkaline electrolytes are unlikely to form in H-cells; hence, neutral electrolytes (such as KHCO₃) are commonly utilized. This is achievable in a GDE configuration because CO₂ and electrolyte are kept separate. In contrast, flow-cell electrolyzers operate with highly concentrated alkaline electrolytes and MEA electrolyzers use an alkaline membrane. However, the low CO₂ usage efficiency caused by carbonate production stops the practical deployment of alkaline CO₂RR. Recently, researchers have turned their focus to acidic CO₂RR, which may be an economical solution to prevent carbon loss.

2.2.3 Product analysis. Nuclear Magnetic Resonance (NMR), Gas Chromatography (GC), Ion Chromatography (IC) and High-Performance Liquid Chromatography (HPLC) are approaches to classifying liquid products that are applied in liquid-phase analysis. Gaseous products are often monitored using online GC at frequent intervals. Accurate calibration curves are essential in both cases, requiring multiple standard concentration points spanning the expected product concentration range. FE is then determined by correlating product concentrations with measured currents under steady-state conditions.

3. Carbon-derived materials for the reduction of CO₂

Nanomaterials have experienced unprecedented advancement across multiple fields, driven by enhanced synthesis techniques and deeper understanding of structure–property relationships at the nanoscale.^70–77 In electronics, quantum dots and 2D materials like graphene and transition metal dichalcogenides have enabled ultra-fast processors and flexible displays, while in medicine, targeted drug delivery systems using lipid nanoparticles and gold nanoparticles have revolutionized cancer therapy and vaccine development. Energy storage has been transformed through silicon nanowires and lithium-metal anodes that dramatically increase battery capacity, and photovoltaics have achieved record efficiencies using perovskite nanocrystals and plasmonic nanostructures. Environmental applications have flourished with photocatalytic titanium dioxide nanoparticles for water purification and air cleaning, while advanced manufacturing has embraced nanocomposites that offer superior mechanical properties with reduced weight. These breakthroughs stem from precise control over size, shape, surface chemistry, and assembly of nanomaterials, enabling properties that are impossible to achieve with bulk materials.^77–79

Among these diverse nanomaterial applications, carbon-derived materials have emerged as up-and-coming candidates for CO₂ reduction technologies through the development of highly efficient electrocatalysts and photocatalysts Table 2. Single-atom carbon catalysts, featuring isolated metal atoms anchored on carbon supports, have demonstrated exceptional selectivity and activity for CO₂ electroreduction to valuable chemicals like carbon monoxide, methane, and ethylene. Graphene-based materials and carbon nanotubes functionalized with nitrogen, sulfur, or metal dopants have shown remarkable performance in both electrochemical and photocatalytic CO₂ conversion processes. Additionally, hierarchically structured carbon aerogels and metal–organic framework (MOF)-derived porous carbons provide high surface areas and tunable active sites that enhance CO₂ capture and subsequent reduction efficiency. These carbon-derived materials offer sustainable pathways for atmospheric CO₂ mitigation while simultaneously producing valuable chemical feedstocks, representing a critical advancement toward closing the carbon cycle in industrial processes and bridging the gap between environmental remediation and industrial chemistry through precisely engineered carbon frameworks tailored for optimal catalytic performance.^17,80–84

Table 2 Summary of the performance of carbon-based catalysts converting CO₂ into useful products

Electrocatalyst	Electrolyte	Product	Potential (V vs. RHE)	FE (%)	J (mA cm^−2)	pH	Stability (h)	Key features
Metal-carbon composites
SnO2@u-C^85	0.5 M KHCO3	CO	−1.17 V	89	—	—	10	• The ultrathin carbon shell creates a chemically homogeneous interface, improving electron transfer and stability
								• 3D conductive network facilitating fast electron transport
Cu-PHI/PTI^86	1 M KOH	CO	−0.84	68	+348	—	12	• Cu–N4 coordination centres closely surrounded by N-defects and CN domains
								• The 2D CNx platform ensures excellent conductivity and defect engineering flexibility
Cu2O/CN^87	0.1 M KHCO3	C2H4	−1.1	32.2	−4.3	6.8	4	• BET surface area of Cu2O/CN = 20.6 m^2 g^−1 (vs. 7.5 m^2 g^−1 for Cu2O), aiding mass transport
								• g-C3N4 provides strong CO2 adsorption via N-rich sites and also generates and stabilises the *CO intermediate
In2O3@CNR^88	1 M KOH	CO	−0.8 V	>90	+300	—	12	• Uniform distribution of In2O3 with high BET surface area: 210 m^2 g^−1
								• Combines high activity, selectivity, and scalable flow-cell operation
CdCO3-CNFs^89	0.5 M KHCO3	CO	−0.83	93.4	+10	7.2	24	• CNFs improve dispersion, electron transfer, and active site stability
								• Predominant exposure of CdCO3(104) facets suppresses the HER
Ni@NC-Cl^90	0.5 M KHCO3	CO	−0.76 V	93.3%	−31.3	—	12	• Cl^− adsorbed on the Ni@NC-Cl surface modifies the local electronic structure and improves CO2 adsorption and activation
								• Ni@NC-Cl has the lowest energy barrier for *COOH formation (0.374 eV)
Sn/rGO800 (ref. 91)	0.1 M KHCO3	CO	830	90	+10	—	—	• High electrical conductivity due to the rGO network
								• Sn–rGO interface accelerates electron transfer and facilitates CO2 adsorption
OCSn^92	0.5 M NaHCO3	HCOOH	−0.46	92	+60	—	30	• High surface area with exposed stepped active sites and edge sites
								• The presence of SnOx facilitates formation of the *OCHO intermediate and suppresses *H/*COOH
ZnO/CTO_2 (ref. 93)	0.1 M KHCO3	CO	−1.3	85.8	+75.6	—	—	• Enhanced electrical conductivity via the carbon network from biochar
								• Synergistic interaction between ZnO and carbon boosts activity and selectivity for CO formation over H2
Cu-(Co + Cu)PMOF/CNT-COOH^94	0.1 M KHCO3	CO	−0.9 V	95.98	−3.48	—	12	• CNT-COOH forms coordination bonds (not just π–π interactions) with Cu-MOF, leading to better interface stability and electron transfer
								• DFT analysis identified CoN4 sites as the main active centre due to stronger CO2 adsorption and higher electron density
CuNPs@N–C-2 (ref. 95)	0.1 M KHCO3	CO	−0.7 V	99%	−5.2	—	16	• High surface area and hierarchical porosity (micro- and mesopores)
								• N-doping boosts CO2 adsorption and *CO intermediate stabilization
								• No significant aggregation or oxidation of Cu NPs after the reaction
CoTAP-cov^96	0.5 M KHCO3	CO	−1.20 to −1.40 V	100%	+210	—	24	• Superior selectivity and intrinsic activity
								• Covalent immobilization enables faster electron transfer and improved CO2 diffusion
MWCNT-Por-COF-Co^97	0.5 M KHCO3	CO	−0.6 V	99.3%	+18.77	7.2	—	• The covalent hybrid of porphyrin-COF and MWCNTs improves conductivity
								• Strong interfacial contact promoted electron transfer and product selectivity
Ag75/C^98	1 M KOH	CO	−1.6 V	—	+200	—	30	• The porous support enhanced gas transport and stability vs. pure Ag
								• Lower charge-transfer resistance and binder shielding compared to pure Ag
Ni–N-CNS/CNT-X^99	—	CO	−2.0	91	+28.9	11	26	• The 3D hierarchical structure enhances conductivity and mass transport
								• Encapsulated Ni NPs and Ni–N sites boost CO2RR selectivity and suppress the HER
CQD@MOF^100	1 M KOH	CO	0.637	91	+10	14	17	• Green synthesis using kiwi-derived CQDs
								• Demonstrated long-term stability
								•CQDs enhanced MOF conductivity and introduced abundant oxygen vacancies
Bi/CN^101	0.1 M KHCO3	HCOOH	1.3 to 1.5 V	98%	−0.01	—	20	• The CN matrix improved Bi nanoparticle dispersion and the electronic structure via electron transfer
								• Bi/CN outperformed many other Bi-based CO2RR electrocatalysts
								• Synergistic Bi–CN interaction facilitated CO2 adsorption and intermediate stabilisation
ZnIn2S4/NDCC^102	0.5 M KHCO3	CO	0.7 V	42%	—	—	—	• First report of ZIS-based catalysts producing ethanol via the CO2RR
								• The N-doped carbon substrate enhanced CO2 adsorption and promoted CO–CO coupling
								• Supported by DFT calculations to validate the mechanism
Ni-NZC@NC^103	0.5 M KHCO3	CO	−0.6 V to −0.9 V	97.1%	+300	—	5	• Designed a Ni–carbide interface in N-doped carbon for *COOH stabilization
								• Demonstrates interface engineering for the industrial-scale CO2RR
Ni1/150NCs@NC^104	Zn(NO3)2	CO	−0.88 V	98.7%	−40.4	—	24	• Maintains activity over 100 h in gas-fed electrolysers
								• Demonstrates atomic-level synergy for efficient CO2-to-C2^+ conversion
Ni–N1–C3 (ref. 105)	1 M HCl	CO	−0.9 V	99%	+200	—	45	• Operando spectroscopy and DFT used to decouple active sites from adsorbates
								• Demonstrates how site-intermediate decoupling boosts multi-carbon selectivity
Pd/hNCNCs-9 (ref. 106)	KHCO3	CO	−0.05–0.30 V	>95%	+10.3	9	8	• Small, uniform Pd NPs (2.3 nm) with high dispersion
								• N → Pd electron transfer optimises intermediate adsorption and suppresses the HER
Fe/Cu-NC^107	KHCO3	C2H5OH	−0.8 V	67.4	73	8.3	40	• Synergistic Fe–Cu centres facilitate CO coupling and protonation
								• High selectivity among C2^+ products
Ag/CoO/NCNS^108	KHCO3	CO	−0.88 V	94.7	53	—	20	• Ag provides CO sites; CoO enhances CO desorption and suppresses the HER
								• Carbon nanosheets increase conductivity and dispersion
CNCu2.5 (ref. 109)	KHCO3	HCOOH	−0.9 V	85	—	0	>10	• Hierarchical porosity enhances mass transport and active site exposure
								• New insights into the ‘single-atom’ and ‘single-atom-cluster’ reaction sites
Ni-NC-1000 (ref. 110)	KHCO3	CO	−1.07 V	80%	140	7.2	20	• Ni-NC-1000 with a balanced SAs/NPs ratio showed the best CO2RR activity
								• High CO selectivity and good stability in long-term tests
Sn@NC-700 (ref. 111)	KHCO3	HCOOH	−1.03 V	93.2	22.3	6.8	50	• 3D porous carbon with uniformly distributed Sn and N dopants
								• Uses waste Sn sludge and scalable synthesis
								• A lower energy barrier for *OCHO formation (0.33 eV) and suppressed the HER
Ni@NCNT/Gr-800 Ni@NCNT/Gr-R^112	KHCO3	CO	−0.71–0.91 V	>90	16.9–37.4	—	24	• Graphene enhances conductivity and prevents CNT stacking
								• Pyrrolic N provides lone pair electrons → increases electron density → improves *COOH intermediate adsorption
Fe-CNT/BNNT^113	—	CH4	−0.16 V	—	—	—	—	• Charge transfer from the transition metal to the substrate regulates CO2 activation and *OH binding
								• Moderate charge transfer leads to optimal *OH interaction and better CO2RR
Porous carbon
Ni–N-OMMCs^116	0.5 M KHCO3	CO	−0.7 V	98	—	—	25	• Ni–N-doped 3D porous carbon with macro/mesopores
								• Large surface area (829 m^2 g^−1) and fast mass transport
20%Au/FPC-800 (ref. 117)	0.1 M KHCO3	CO	−0.7 to −1.1 V	92	+10.4	7.2	24	• High current density using a GDE
								• Enhanced CO2 diffusion and bubble release
CNS-NiSA^118	0.5 M KHCO3	CO	−0.8	95	−7.8	—	—	•N and S co-coordination optimises the electronic structure and enhances *COOH intermediate formation
								• Sheet-like morphology offers high conductivity and surface exposure
								• Maintained stable performance with minimal degradation
Ni–N-HCS^119	0.5 M KHCO3	CO	−0.70	95.04	+10.88	7.2	24	• Constructed hollow carbon spheres with maximized single Ni–N4 surface sites
								• Maintained FECO above 95% for 24 hours of continuous operation
								• DFT confirms optimal COOH* intermediate binding energy for enhanced CO2RR
K-defect C-1100 (ref. 120)	KHCO3	CO	−0.45 V	99%	+9.8	—	—	• Used K^+ ions during pyrolysis to generate defect-rich porous carbon
								• DFT suggests enhanced COOH* binding on defect edges
								• The robust carbon matrix demonstrated excellent conductivity and durability
[Fe(C2O4)3]^3− (ref. 122)	0.5 M KHCO3	CO	−0.9 V	84%	−4.9	—	—	• Synthesised hollow Fe single-atom doped carbon nanocages (H-Fe-NCs) using oxalate-assisted ZIF-8 templating
								• The Kirkendall effect enabled formation of a hollow porous structure, enhancing CO2 diffusion and active site exposure
CNT@mC-Ni-T0.5 (ref. 121)	0.5 M KHCO3	CO	−0.81 V	98%	+60	7.8	—	• Uniform Ni NPs (∼5.5 nm) embedded in an N-doped carbon shell enhance stability and electron transfer
								• DFT confirms the synergistic Ni(111)/N-doped carbon interface and improves *COOH intermediate adsorption and CO generation
Ni-NAC^123	KHCO3	CO	−1.4 V	95%	>140	7.2	14	• Exhibited high activity even at large current densities
								• DFT revealed that Ni–N3C1 configuration offers optimal CO2 reduction energetics by balancing *COOH and *CO binding
Ni–CNC-1000 (ref. 124)	1 M KHCO3	CO	−0.8 V	96.6%	−7.9	—	—	• Uses the electrospun-pyrolysis cooperative strategy to create atomically dispersed metal species
								• Modulates the porous carbon support structure for enhanced charge and mass transfer
								• Represents advancement beyond traditional single-atom catalysts
Heteroatom-doped carbon
Cu(N)^125	1 M KOH	CO	−0.55	82.3	+320	—	—	• Develops Cu nanoparticles protected by a quasi-graphitic C shell to prevent catalyst reconstruction
								• The C shell epitaxially grows on Cu via a gas–solid reaction (CO–CO2–C equilibrium)
								• Provides exceptional stability for CO2 reduction over 180 hours of operation
Bi-NRs@NCNTs^126	0.1 M KHCO3	HCOOH	−0.9	90.9	+0.11	—	—	• The nanoconfined structure enhances CO2 adsorption, intermediate retention, and electron transfer
								• NCNT shells prevent Bi aggregation and oxidation, ensuring long-term catalytic stability
								• Demonstrates a promising strategy for low-cost, stable, and selective CO2-to-formate electrocatalysts
Ni0.037-NG-H^127	0.5 M KHCO3	CO	−0.8	97.3	+15	—	64	• Demonstrates that leaching out Ni NPs increases rather than decreases catalytic activity
								• Ends confusion in the field regarding active sites for CO2 electroreduction
2.5Ni@N-CNTs/CP-700 (ref. 128)	0.05 M KHCO3	CO	−0.76	57	+10.6	—	—	• This method eliminates binders and enhances conductivity and active site exposure
								• N species (especially pyridinic and pyrrolic-N) identified as the main active sites for CO2 adsorption and activation
Ni 0.87-NC-1-AE^129	0.1 M KHCO3	CO	−0.9	97.6	+21.81	6.8	16	• DFT calculations confirmed that Ni–N2O2 active centres have lower limiting potentials and more favourable CO2 adsorption compared to Ni–N4 configurations
								• Dual coordination (N and O atoms) lowers the reaction energy barrier for *COOH formation (key intermediate)
Ni–N–C^130	0.5 M KHCO3	CO	−0.7	97.4	+58	—	24	• The catalyst features coexisting Ni–N4 sites and uncoordinated N-doped sites (pyridinic and pyrrolic), leading to synergistic effects that boost CO2 reduction activity
								• The Ni–N4 center's structure remains intact, but uncoordinated N-dopants get activated
Ni@N–C^131	KHCO3	CO	−0.77 V	90%	+19.7	—	—	• XPS and EXAFS confirmed the presence of Ni–Nx moieties as the dominant active sites
								• Utilised MOF-derived architecture with nitrogen-rich ligands, providing a scalable and environmentally friendly route
Fe–N–C^132	0.5 M KHCO3	CO	0.34 V	98%	—	—	24	• Exceptional catalytic performance through atomic-level engineering
								• Fundamental mechanistic insights through phosphorus tuning
CuNi/NC^133	0.1 M KHCO3	CO	−0.6 V	99.7%	−3.8	6.8	48	• The mesoporous structure (∼3.3 nm pores) facilitates mass transport of reactants and intermediates
								• XPS analysis confirmed electron redistribution between Cu and Ni due to d-band overlap, improving CO2 adsorption and intermediate stabilization
Metal-free
NGQDs^134	—	C2H5OH	−0.78 V	26	+21	—	—	• The catalyst outperforms pristine GQDs and N-doped reduced graphene oxides (NRGOs) in activity and selectivity
								• NGQDs show superior performance even at lower overpotentials compared to Cu-based catalysts
BND^135	—	C2H5OH	−1.0 V	93.2	—	—	—	• DFT calculations confirm a thermodynamically favourable multi-step pathway for ethanol formation involving *COCO intermediates
								• The BND shows superior C2 product generation
N3-750 (ref. 136)	—	CO	0.2 to 0.36 V	75.90%	−27.245	—	8	• XPS analysis revealed pyridinic N as the key active site for CO2 adsorption and *COOH intermediate formation
								• Demonstrates the viability of low-cost, sustainable, biomass-derived carbon catalysts for CO2 electroreduction to syngas components
CNP-900 (ref. 137)	0.5 M KHCO3	CO	−0.55 V	80.8%	+1.25	—	—	• Optimized catalyst performance: the N,P-codoped carbon material (CNP-900) achieved a high FE of 80.8% at a low overpotential of −0.55 V
								• Active site identification: balanced ratios of pyridinic N and graphitic N, along with minimal P–N content, were found crucial for enhancing CO2 reduction activity
NPC^138	0.5 M KHCO3	CO	−0.41 V	88%	+1.71	—	27	• Dual-doping design: incorporates both nitrogen and phosphorus into porous carbon nanosheets, enhancing CO2 adsorption and charge transfer
								• Efficient CO2 reduction: achieves high CO selectivity with a FE of ∼95% and excellent stability over long-term electrolysis
NCSs^139	0.5 M KHCO3	CO	−1.25 V	71%	−5	—	—	• DFT results show that pyrrolic N has a low energy barrier (ΔG ∼0.58 eV) for COOH* formation
								• NCS-70 retained morphology, N-content, and performance over extended CO2RR testing
NPCM-1000 (ref. 140)	0.5 M NaHCO3	CO	−0.55	92	—	—	24	• Integrated electrode structure: NiCo NPs embedded in N-doped CNTs are grown directly on carbon paper, forming a binder-free, flexible, and conductive electrode
								• Efficient water splitting: demonstrates low overpotentials for both the HER (89 mV) and OER (276 mV at 10 mA cm^−2), with excellent durability and gas bubble release
NG-1000 (ref. 141)	0.5 M KHCO3	CO	−0.72	95	+9.07	—	10	• Achieved up to 11.0 wt% nitrogen content, with graphitic-N as the dominant active species in NG-1000
								• NG-1000 had a BET surface area of 371 m^2 g^−1, promoting active site exposure and gas diffusion
SeBN-C-1100 (ref. 142)	0.1 M KHCO3	CO	−0.6 V	95.2%	−5	—	—	• Ternary doping introduces charge redistribution, structural defects, and synergistic electronic effects
								• SeBN-C-1100 shows a high surface area (691 m^2 g^−1) and hierarchical porosity beneficial for mass transport

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3.1. Metal–carbon composite materials

Wu and co-workers developed ultrathin carbon-coated SnO₂ quantum dots derived from Sn-MOFs, SnO₂@u-C that were synthesized for optimal CO₂ electroreduction. The SnO₂@u-C electrode demonstrated high performance in formate production, achieving a remarkable FE of up to 95.7% and energy efficiency of 58.2%. This electrode also demonstrated exceptional structural and catalytic stability, sustaining a formate FE of 89% during a 10-hour continuous experiment. A constant potential of 1.17 V vs. the reversible hydrogen electrode (RHE) was applied during the electrolysis experiment. Excellent FE can be understood in terms of the unique framework of SnO₂@u-C, where an ultra-fine layer of carbon atoms forms a 3D network. This configuration offers abundant active sites and promotes facile electron transfer. It mitigates the reductive conversion of SnO₂ QDs (Fig. 3).⁸⁵

Fig. 3 (a) Synthesis of SnO₂@u-C, (b) optical image, (c) TEM image, (d) HRTEM image, (e) particular area electron diffraction pattern, (f) elemental mapping images and (g) TEM and the related SnO₂ QD size statistics of SnO₂@u-C. Reproduced with permission.⁸⁵ Copyright 2024, Elsevier.

Roy and his group synthesized Cu-PHI/PTI, a material featuring single Cu atoms uniformly distributed at high density within the nanoscale N-doped pores of 2D crystalline matrices of carbon nitride, specifically Na-polyheptazine imide (PHI) and Li-polytriazine imide (PTI). This electrocatalyst exhibited a 68% FE and a partial J of 348 mA cm⁻² at an applied potential of −0.84 V against RHE. The exceptional observed performance stems from synergistic catalysis from multiple copper atoms within the triangular 9N pores of PTI. This configuration strengthens the Cu–N interaction, influencing the reaction energetics and promoting catalytic activity (Fig. 4).⁸⁶

Fig. 4 (a) Schematic depiction of the synthetic methods of Li-PTI, Na-PHI, and Cu-PHI/PTI. (b) SEM image of Na-PHI. (c–e) TEM, HRTEM, and the corresponding SAED pattern of Na-PHI. (f) SEM image of Li-PTI. (g–i) TEM, HRTEM, and SAED pattern of Li-PTI. Reproduced with permission.⁸⁶ Copyright 2024, Wiley-VCH.

Yi and collaborators developed a Cu₂O/CN composite of Cu₂O nanocubes supported on graphitic carbon nitride. At −1.1 V vs. RHE, this material achieved 32.2% FE for C₂H₄ production, accompanied by a partial J of −4.3 mA cm⁻². The improved dispersibility of NPs on the N-rich support enhanced both the transfer of mass and the adsorption of CO₂ at the catalyst interface. The study highlighted the synergistic contribution of the support and NPs to the efficient formation of the product due to C–C coupling, ethylene, during the CO₂RR.⁸⁷

Pan et al. developed In₂O₃@CNR, consisting of indium oxide NPs on conductive carbon nanorods, serving as a pre-catalyst for reducing CO₂ to formate (Fig. 5). Using an H-cell configuration with CO₂-saturated 0.5 M KHCO₃, the catalyst exhibited a 90% FE for selective formate generation over a broad potential window. By employing an electrode-based flow cell based on gas diffusion and electrolyte with 1 M KOH, the catalyst obtained a considerable J of 300 mA cm⁻². Selectivity remained above 90%, cathodic energy efficiency was approximately 50%, and stable operation was observed for more than 12 hours.⁸⁸

Fig. 5 (a) Flow cell schematic. (b) Front and top views of the assembled cell. (c) Polarization curves of In@CNR in 1 M KHCO₃ and 1 M KOH. (d) Chronoamperometric stability at −0.65 V (KOH) and −0.92 V (KHCO₃). Reproduced with permission.⁸⁸ Copyright 2021, Wiley-VCH.

Xiao et al. revealed electrochemical conversion of CO₂ explored using CdCO₃-CNFs, a novel electrocatalyst comprising CdCO₃ NPs supported on carbon nanofibers (Fig. 6). The catalyst yielded an FE of 93.4% and a partial J of roughly 10 mA cm⁻² when polarized at −0.83 V. The enhanced electron transfer and secured active sites, resulting from strong catalyst–support interaction, are considered the basis for the high performance of the catalyst.⁸⁹

Fig. 6 (a–e) Optimized COOH and CO adsorption structures and binding energies on CdCO₃ (012)/(104) facets and other catalysts; HER free energy diagrams for CdCO₃ surfaces; a schematic of CdCO₃-decorated carbon nanofibers for enhanced CO₂ electroreduction. Reproduced with permission.⁸⁹ Copyright 2021, Elsevier.

Peng et al. synthesized carbon-coated nickel catalysts by high-temperature pyrolysis and solvothermal methods. For the electrochemical CO₂RR, a series of Ni@NC-X (X = Cl, N, S) catalysts were prepared with hydrochloric acid, nitric acid, and sulfuric acid, respectively. Ni@NC-Cl outperformed the other catalysts in terms of CO₂ reduction, including CO yield and total J. At −1.16 V, 472.9 μmol yield of CO was obtained, and −31.3 mA cm⁻² was the total J with an FE of 93.3% at −0.76 V. Experimental control demonstrated that an amalgam of N and Ni, as well as adsorbed chlorine on the surface, can accelerate the reduction (Fig. 7).⁹⁰

Fig. 7 (a) Computational generic model; (b) ECO₂RR performance; (c) HER performance on Ni@NC700 and Ni@NC-X catalysts; (d) activity and selectivity comparison between Ni@NC-700 and Ni@NC-X catalysts; and (e) Ni@NCT catalyst preparation method illustration. Reproduced with permission.⁹⁰ Copyright 2023, Elsevier.

Tsujiguchi et al. fabricated reduced graphene oxide-supported Sn composites Sn/rGO. The study found that Sn/rGO adsorbed CO₂ approximately four times more effectively than bare Sn catalysts. The electrochemical reduction of CO₂, facilitated by enhanced adsorption, exhibited improved selectivity for the production of formate, a valuable chemical. The key role of formate formation is due to the interface between Sn and rGO. The oxidized functional groups on rGO helped capture CO₂, while Sn provided the catalytic site for its reduction.⁹¹ Zhong et al. used the Triton X-100-assisted electrodeposition approach to create new cone-structured Sn on carbon paper, OCSn. The technique provided a high FE of 92% in 0.5 M NaHCO₃ electrolyte, which remained static for 30 hours with no apparent activity change. The ordered cone structure and direct growth on carbon paper were the key factors for the improved catalytic efficiency compared to traditional tin catalysts (Fig. 8).⁹²

Fig. 8 (a) XRD patterns for several catalysts. (b) XPS of OCSn-60. (c) A low-magnification TEM picture. (d) HRTEM picture of OCSn-60 from the Sn cone side. (e) HRTEM picture of OCSn-60 near the Sn tip. (f) The equivalent SAED patterns for the inner region of (e). (g) Optimizing the OCSn's synthesis procedure. Reproduced with permission.⁹² Copyright 2021, Elsevier.

By altering the ZnO content to maximize performance, Lourenco and associates studied ZnO-biochar materials made from brewed waste coffee (CBC) and pyrolyzed chitosan (CTO). The optimal composition, ZnO/CTO₂ (40% carbon), displayed stable electrocatalytic behavior, retaining high CO selectivity and activity at −1.1 V, with current densities in the 50–60 mA cm⁻² range, indicating promise for real-world use.⁹³

Ma et al. developed a hybrid material, Cu-(Co + Cu)PMOF/CNT-COOH, combining two-dimensional bimetalloporphyrin-based CuMOFs with carboxyl-functionalized CNTs. This functionalization improved stability and increased surface area compared to the non-functionalized (Co + Cu) PMOF/CNT, which is stabilized by weaker π–π stacking. The hybrid material demonstrated a high FE of 95.98% for CO production at an overpotential of −0.9 V, achieving a J of −3.48 mA cm⁻². This performance surpasses that of analogous monometalloporphyrin-based Cu-MOFs and is comparable to that of the RHE, suggesting a favorable mechanism for developing productive electrocatalysts for the reduction of CO₂ gas (Fig. 9).⁹⁴

Fig. 9 Schematic illustration of the Cu-(Co + Cu)PMOF/CNT-COOH synthesis process. Reproduced with permission.⁹⁴ Copyright 2023, Elsevier.

Liu et al. reported successfully synthesizing CuNPs@N–C-2, a composite material of 64 nm Cu NPs derived from a Zn/Cu bimetallic ZIF-8 precursor and decorated with N-doped purple spherical carbon nanostructures. The CuNPs@N–C-2 modified electrode exhibited higher ECSA, faster conversion, and faster ECO₂R values, indicating good electrocatalytic ECO₂R activity and selectivity. The CuNPs@N–C-2 modified electrode has a peak potential varying between −0.4 V and −0.7 V with a 99% FE for CO. The chronoamperometric testing over 12 hours revealed that the CuNPs@N–C-2 electrode maintained approximately 100% of its initial J, confirming its long-term electrochemical stability. This work provides a useful approach for investigating copper-based nanostructures for the electrochemical reduction of CO₂ to CO, and there is proof that the performance of CuNPs@N–C-2 is significantly dependent on the size of Cu NPs and the number of Cu–N_x sites within the N-doped carbon matrix (Fig. 10).⁹⁵

Fig. 10 (a–c) TEM, HRTEM, and enlarged lattice image of CuNPs@N–C-2. (d) XRD patterns of CuNPs@N–Cs and N–C. (e–h) TEM with elemental maps for C, Cu, and N. (g) Schematic of CO₂ reduction. (h) J-time curves at −0.7 V. (i) LSV curves in CO₂-saturated 0.1 M KHCO₃. (j) Faradaic efficiencies of CO₂RR products for CuNPs@N–C-2. Reproduced with permission.⁹⁵ Copyright 2022, American Chemical Society.

Gu et al. studied amino-substituted cobalt porphyrin (CoTAP) supported on CNTs for its electrocatalytic CO₂RR, focusing on the impact of the immobilization structure. The covalent attachment strategy employed in CoTAP-cov improved electrocatalytic performance for CO₂ reduction. Notably, CoTAP-cov exhibited higher TOF_co and FE_co compared to CoTAP-noncon across a potential range of −1.20 to −1.40 V. CoTAP has strong catalytic activity, with a TOF_co of 6.0 s⁻¹ and over 100% FE_co at a potential of −1.30 V (compared to RHE). The study demonstrated that incorporating an –NH₂ donor group into the porphyrin structure enhanced the electron density at the cobalt center, significantly improving catalytic activity, as evidenced by a TOF_co of 36.6 s⁻¹. The findings suggest that the amide bond eases the transfer of electrons from the CNTs to the cobalt center, functioning as a molecular metal. Tafel analysis demonstrated that strong surface binding at high CoTAP loadings increases degradation, resulting in a reduced size limit. Therefore, covalent immobilization offers a promising avenue for future electrocatalyst design due to its ability to promote the ECO₂RR.⁹⁶

Zhang et al. investigated porphyrin-COF nanosheets on CNTs for enhanced electrocatalytic CO₂ to CO conversion. The combination of Por-COF and CNTs enhanced electron distribution and controlled the transfer of electrons during the process. Electrochemical investigation (LSV) in CO₂-saturated KHCO₃ demonstrated that the cobalt-based composite (MWCNT-Por-COF-Co) outperformed its nickel (MWCNT-Por-COF-Ni) and iron (MWCNT-Por-COF-Fe) counterparts, with a higher onset potential and J. The composite materials (MWCNT-Por-COF-M, where M is Co, Ni, or Fe) have higher electrocatalytic and FE than comparable pure Por-COF-M materials. The MWCNT-Por-COF-Co catalyst had a maximum FE for carbon monoxide (FE_CO) of 99.3% at 0.6 V of applied potential. At 1.0 V, the J was +18.77 mA cm⁻², with a turnover frequency of 70.6 s⁻¹. The improved electron transport allowed by the effective integration of components in MWCNT-Por-COF-Co is proposed as the secret to its higher performance. Further investigation using MWCNT-Por-COF-Cu in flowing water with 1.0 M KOH revealed a maximum FE of 71.2%, exceeding that of MWCNT-Por-COF. Post-CO₂ reduction analysis using HRTEM and Auger spectroscopy revealed the formation of copper-based nanoclusters within the MWCNT-Por-COF-Cu composite, suggesting a correlation with its enhanced electrocatalytic activity. This study introduces a novel design approach for COF-based composites aimed at improving CO₂ reduction (Fig. 11).⁹⁷

Fig. 11 (a) LSV curves; (b) partial J for CO; (c) predicted FE of CO from −0.5 to −1.0 V; (d) TOFs at various potentials for Por-COF and MWCNT-Por-COF-M (M = Co, Ni, Fe); (e) schematic of MWCNT-Por-COF-M (M = Co, Ni, Fe, Cu) synthesis. Reproduced with consent.⁹⁷ Copyright 2022, Elsevier.

Using different weights of Ag and carbon black carriers (Ag is Ag₂₀/C, Ag₅₀/C and Ag₇₅/20, 50 and 75 wt%), the electrolytic reduction of CO₂ to CO by the Ag/C composite catalyst produced by spray pyrolysis was studied. The Ag₇₅/C catalyst exhibits extreme ECO₂R performance and high stability compared to pure Ag particles.⁹⁸ Li et al. designed a 3D hybrid carbon nanocatalyst co-doped Ni and N, Ni–N-CNS/CNT-X, for the CO₂RR via the single step chemical vapor deposition method. In an H-cell configuration, the electrocatalyst produced 91% FE at −0.74 V and a partial J of 28.9 mA cm⁻². At an applied voltage of −2.0 V vs. RHE, a similar catalyst generated a viable J of +600 mA cm⁻² and CO selectivity exceeding 85% in a flow cell. The hierarchical layout and synergistic action of Ni and N are responsible for the increased activity (Fig. 12).⁹⁹

Fig. 12 (a–c) Scan rate-dependent FE of CO, CO partial J, and Δj for NiO-pC₃N₄, Ni-N-CNS-Y, and Ni-N-CNS/CNT-X. (d) CO₂RR stability of Ni-N-CNS/CNT-20 at −0.66 V vs. RHE. (e) Schematic of catalyst synthesis and the experimental setup. Reproduced with permission.⁹⁹ Copyright 2021, American Chemical Society.

Ha et al. developed CQD@MOF where carbon quantum dots (CQDs) are surrounded by metal–organic frameworks (MOFs) using natural resources involving eco-friendly methods for the CO₂RR. Furthermore, by optimizing parameters like synthesis and deposition temperatures, pH, time, electrolyte type, and scan rate, binder-free CQD@MOF/nickel foam (NF) electrodes are fabricated. This novel technique provides a long-term and efficient solution for electrochemical CO₂ conversion, with 91% FE and 10 mA cm⁻²J at a potential of 0.637 V vs. RHE.¹⁰⁰ With a 98% selectivity at −1.3 to −1.5 V vs. RHE, Zhang et al. used a solvothermal technique to support Bi NPs on polymeric carbon nitride (Bi/CN) for the electrocatalytic reduction of CO₂ to HCOOH. The catalyst was stable for more than 20 hours at −1.4 V vs. RHE, and the product yield was approximately 8 times higher than that of pure Bi.¹⁰¹

Ma et al. developed ZIS NSAs/NDCC via the hydrothermal route where ZIS nanosheet arrays (ZIS NSAs) were grown on N doped carbon cloth (NDCC). The as-prepared material aided electrocatalytic reduction of CO₂ to C₂H₅OH with FE = 42% at an applied potential of −0.7 Vs RHE. The higher electrocatalytic CO₂RR was attributed to the NDCC substrate, which improved electrochemically active surface area of the electrode, enhanced charge transfer, increased CO₂ adsorption capacity, activated *CO, and facilitated the coupling reaction of OC-CO.¹⁰² The Ni-NZC@NC catalyst for the CO₂RR was produced by Gao et al. using the pyrolysis of nickel-doped metal–organic framework precursors. This catalyst was created by combining nickel and bimetallic NiZn carbide that was encased in nitrogen-doped carbon during the process. In H cells with a voltage ranging between −0.6 V and −0.9 V (relative to RHE), this catalyst outperforms both Ni@NC and NC catalysts, with a FE of more than 93%. This is because the nickel carbide contact stabilizes the CO intermediate, whereas the nitrogen-doped carbon layer allows CO to depart the system. The Ni-NZC@NC catalyst performed well in an industrial alkaline membrane assembly (MEA) electrolyzer, with a FE of 97.1% and a total cell energy efficiency of 48.4% at 300 mA cm⁻². At 350 mA cm⁻² in 0.1 M CsOH, the maximum CO production rate was 9.13 mL min⁻¹.¹⁰³

Xu et al. created highly distributed Ni nanoclusters (NCs) on nitrogen-doped carbon using a precursor Ni/Zn bimetallic metal–organic framework (MOF). Adjusting the Ni : Zn ratio in the MOF precursor allows for fine control over the loading and size of the Ni catalyst. The –NH₂ group in the MOF ligand is important because it affects the size of the Ni catalyst and the characteristics of the carbon material. The catalyst has a high FE for CO (FE_CO = 98.7%) and a considerable partial J (J_CO = −40.4 mA cm⁻²) at −0.88 V contrary to RHE. The catalyst is very stable over a long time because the small Ni clusters work well together and interact effectively with the carbon support.¹⁰⁴

Wang et al. successfully synthesized a low-coordination Ni–N–C₃ single-atom catalyst (SAC) anchored on curved CNTs. The CO₂ to CO conversion efficiency is achieved through optimized Ni–N–C coordination and electronic states, with an FE of 97% at 2890 h⁻¹ at −0.9 V. The J is boosted to over 200 mA cm⁻² without impacting the mobile phone's operation. COOH and CO are important intermediates in the process of converting CO₂ to CO, according to research using in situ Raman spectroscopy. Furthermore, Density Functional Theory (DFT) calculations indicate that reducing the number of N coordination sites minimizes the energy impact from *COOH, thereby improving CO₂ reduction efficiency.¹⁰⁵

Hu et al. developed a highly efficient Pd electrocatalyst by adjusting the pH value during microwave-assisted ethylene glycol (EG) reduction, supported by unique multiphase nitrogen-doped carbon NPs (Pd/hNCNCs) for the CO₂RR (Fig. 13). The optimal Pd/hNCNCs-9 catalyst demonstrates a high FE of over 95% in the zero potential range, with a minimum value of −0.25 V. A remarkable J of 10.3 mA cm⁻² was achieved, outperforming both hCNC and N-doped ACET catalysts. The hierarchical structure of the catalyst, with its unique features, is key to the enhanced performance. This improvement is attributed to the highly dispersed Pd NPs, optimized intermediate adsorption/desorption behavior of Pd, and enhanced mass/charge transfer kinetics.¹⁰⁶

Fig. 13 CO₂RR results for Pd/hNCNCs-x. (a) LSV curves of Pd/hNCNCs-9 in Ar/CO₂-saturated 0.5 m KHCO₃ solution. (b) FE_formate at various voltages. (c) J_formate with various potentials. (d) A comparison between the J_formate for Pd/hNCNCs-9 and the findings of the literature. Reproduced with permission.¹⁰⁶ Copyright 2023, John Wiley & Sons.

Angeresa and co-workers developed an innovative dual-atom catalyst, Fe/Cu-NC, for the efficient and selective electrochemical conversion of CO₂ into ethanol.¹⁰⁷ This catalyst achieved a remarkable FE of 67.4% for CO₂ conversion, operating at a current density of 73 mA cm⁻² and a potential of −0.8 V. To unravel the catalyst's mechanism, in situ spectroscopic analysis confirmed the formation of critical intermediates, specifically *CO and *CHO, which are vital for ethanol synthesis. Additionally, DFT studies revealed a synergistic interaction between the Fe–N₄ and Cu–N₃ active sites within the catalyst. This collaboration not only reduces the energy barriers for the reaction but also increases the local concentration of CO. This enriched CO then effectively migrates to the Cu–N₃ sites, significantly enhancing overall ethanol production.

Tariq et al. fabricated Ag/CoO/NCNS, an innovative silver-based hybrid electrocatalyst, for highly efficient electrochemical CO₂ reduction.¹⁰⁸ This catalyst features cobalt oxide (CoO) and silver NPs uniformly dispersed across nitrogen-doped carbon nanosheets (NCNSs). The researchers employed ZIF-12 during synthesis to ensure the even distribution and prevent the aggregation of these NPs. The most effective iteration, Ag/CoO/NCNS1 catalyst, proved to be the most effective version, exhibiting outstanding performance with 87% FE at −0.5 V (vs. RHE), a minimal overpotential of just 130 mV, and a current density of 53 mA cm⁻². This superior activity is attributed to efficient charge transfer from the highly conductive CoO/NCNS support to silver. This charge transfer enables the active sites on the Ag NPs to effectively activate CO₂ molecules and stabilise the crucial *COOH, which is vital for the reduction process. Furthermore, the catalyst showcased remarkable stability and an increased electrochemical active surface area.

Xu and co-workers developed a copper-doped carbon–nitrogen shell catalyst (CNCu shell), which was synthesized using a silicon dioxide template, allowing for precise control over the copper doping morphology to tailor product selectivity.¹⁰⁹ The investigation was pursued to address prevalent challenges in the electrochemical CO₂ reduction reaction (ECO₂RR), such as catalyst structural instability and particle agglomeration, which frequently impair catalytic efficiency. Specifically, when copper was incorporated as isolated single atoms (CNCu_2.5), the catalyst exhibited an impressive FE of 85% for formate production at a potential of −0.9 V. In contrast, a catalyst featuring a coexistence of both copper clusters and single atoms (CNCu₂₅) demonstrated a preference for multi-carbon products, achieving a 45% Faraday efficiency for ethanol and 23% for acetic acid. Insights from DFT calculations explained these observations: copper single atoms provided an optimal binding energy for formate, facilitating its rapid formation, while the presence of both copper clusters and single atoms enhanced the adsorption of formate, consequently favoring the generation of multi-carbon products, accelerating its formation, while the combination of Cu clusters and single atoms increased formate adsorption, thereby promoting the formation of multi-carbon products.

Sun et al. investigated the combined action of nickel single atoms (SAs) and NPs which improves the electrocatalytic CO₂RR.¹¹⁰ The research team created Ni, N co-doped carbon materials, Ni-NC-T, through the pyrolysis of a nickel-loaded porous poly(ionic liquid) (PIL) precursor, carefully managing the ratio of Ni SAs to NPs by varying the annealing temperature. Crucial findings indicated that the Ni-NC-1000 catalyst, featuring an optimal balance between Ni SAs and NPs, demonstrated remarkable efficiency in converting CO₂ into CO, achieving a FE of 80% at a potential of −1.07 V vs. the RHE, with a current density of 140 mA cm⁻². In-depth mechanistic insights, provided by comprehensive analyses including operando Ni K-edge X-ray absorption near-edge structure (XANES) spectroscopy and DFT calculations, revealed that electron-enriched Ni SA active sites were formed due to charge transfer from the Ni NPs to the adjacent carbon layer and then to the Ni SAs. This synergistic presence of both Ni SAs and NPs markedly improved CO₂ activation and strengthened the binding of the critical *COOH intermediate, thereby reducing the energy barrier for the rate-limiting step and substantially enhancing catalytic efficiency.

Zhong and coworkers fabricated Sn/nitrogen-doped carbon (Sn@NC) composites to enhance the electroreduction of CO₂ specifically towards formate.¹¹¹ Their research addresses the pressing demand for lowering atmospheric CO₂ concentrations and producing useful chemicals via the CO₂ reduction reaction (CO₂RR). The synthesized Sn@NC composites proved to be highly effective electrocatalysts. Specifically, the Sn@NC-700 type achieved an outstanding 93.2% FE for formate production at a potential of −1.23 V, along with a current density of 140 mA cm⁻². Moreover, it exhibited excellent stability, maintaining its performance consistently for over 50 hours. This elevated activity is ascribed to multiple factors by the study: the distinctive 3D architecture of the nitrogen-doped carbon, the existence of Sn–C interfaces, and the composite's excellent electrical conductivity that enables swift charge transfer. These characteristics are critical for stabilizing essential *OCHO intermediates and speeding up formate generation.

Zhu et al. developed a synergistic combination of an ideal concentration of pyrrolic nitrogen (N) doped with stable N-doped carbon nanotube/graphene (NCNT/Gr) hybrid structures, Ni@NCNT/Gr-R and Ni@NCNT/Gr-800, in which an expansive electrochemically active surface area, abundant active sites, and minimized nickel nanoparticle (NP) dimensions collectively boost the electrocatalytic efficiency for CO₂ reduction.¹¹² The electrocatalyst Ni@NCNT/Gr-R demonstrated a remarkable FE_CO exceeding 90% across a wide range of potentials (−0.71 to −0.91 V vs. RHE) when tested in an H-cell configuration. Furthermore, its stability was exceptional, exhibiting a nearly undetectable loss in current density over 24 hours at −0.71 V (vs. RHE). This research provides vital data and understanding that can guide approaches for fine-tuning pyrrolic N doping levels and reducing nanoparticle sizes within durable NCNT/Gr hybrid support materials, thus aiding the development of highly effective CO₂ reduction catalysts.

Yan and coworkers employed DFT to examine one-dimensional heterostructures, specifically carbon nanotube/boron nitride nanotube arrangements, as single-atom catalysts (SACs) for the electrochemical conversion of CO₂ (CO₂RR).¹¹³ The researchers systematically analyzed heterostructures containing various doped transition metals (Cr, Mn, Fe, Co, Ni, and Cu), covering both embedded (TM-CNT@BNNT) and hybrid (TM-CNT/BNNT) configurations. A primary discovery indicated that electron transfer from the metal core to the substrate, alongside changes in the d-band center, was critical in controlling catalytic activity. Notably, reduced electron transfer within the metal core corresponded with ideal binding strengths for vital intermediates, which proved essential for high performance. All evaluated SACs exhibited robust stability. The research identified nine promising catalysts for the CO₂RR, attributed to their efficient CO₂ activation and significant suppression of the competing hydrogen evolution reaction. Fe-CNT/BNNT stood out as exceptionally effective, achieving the lowest limiting potential of −0.16 V, given its heterostructure's ability to optimise intermediate binding and improve the reaction pathway. Furthermore, the study introduced charge transfer (ΔQ) as a predictive indicator for catalytic activity, demonstrating its linear relationship with CO₂ adsorption energy and changes in the free energy of intermediates.

3.2. Porous carbon materials

Porous carbon materials with three-dimensional porous structures are more appealing electrocatalysts for CO₂ reduction than conventional graphite and diamond structures because they have many porous structures and large specific surface areas, which improves the active sites for CO₂ reduction and capture. There are three types of porous carbon: macroporous (>50 nm), microporous (<2 nm), and mesoporous (2–50 nm). The systematic porous structure, pore distribution, and pore width can be used to determine CO₂ accessibility and enrichment at the electrolyte's reactive sites.¹¹⁴ Tuning meso-, macro-, and microporous 3D carbon structures can produce unique surface characteristics and electrical properties. Furthermore, porous carbon contains bonded and sp² carbon, offering the benefits of sp²- and sp³-hybridized carbon atoms.¹¹⁵ Xu et al. synthesized Ni-doped 3-D ordered meso-/macroporous carbons (Ni–N-OMMCs) and studied CO₂ reduction performance using Ni–N-OMMC precursors of varying Ni concentrations. The ideal catalyst (Ni–N-OMMC-0.6) has a broad potential range of −0.65 to −1.0 V with a FE of >60% and good electrocatalytic performance with a maximum FE of ∼98% for CO production at −0.7 V vs. RHE (Fig. 14).¹¹⁶

Fig. 14 (a and b) SEM images of silica opal and Ni–N-OMMC-0.6 (inset: high-magnification with the (111) facet); (c) EDS mapping of Ni–N-OMMC-0.6; (d) schematic of Ni–N-OMMC synthesis. (e) LSV curves in CO₂-saturated 0.1 M KHCO₃ (inset: comparison in N₂/CO₂ atmospheres); (f) CO FE vs. potential; (g) CO partial current densities; (h) Tafel plots; (i) Nyquist plots at −0.7 V; (j) stability test of Ni–N-OMMC-0.6 (−0.75 V); inset: LSV before/after 5000 CV cycles. Reproduced with consent.¹¹⁶ Copyright 2020, American Chemical Society.

Li et al. synthesized a hydrophobic porous carbon scaffold loaded with Au NPs to study the impact of the porous structure on CO₂RR activity. With a 20% Au/FPC-800 electrocatalyst that has been tuned for use, up to 92% FE of CO and a J value of 10.4 are obtained in the applied potential range of −0.7 to −1.1 V vs. RHE.¹¹⁷

Zhuhang et al. employed a nickel phosphorus trisulphide 2D plate, CNS-NiSA, to create a sandwich-like microporous polymer with N and S co-coordinated Ni sites. The electrocatalyst demonstrated high-performance CO₂ reduction with a J of −7.8 mA cm⁻² (−0.8 V vs. RHE) and a 95% CO selectivity rate. The remarkable performance of the electrocatalyst was facilitated by the ionic, single N/S-coordinated Ni and the sheet-like geometries of the nanosheets with long-distance conduction (Fig. 15).¹¹⁸

Fig. 15 Preparation of CNS-NiSA. (i) 4-Bromobenzenediazonium tetrafluoroborate, 25 °C, overnight; (ii) M1, M2, Pd(PPh₃)₄, CuI, Et3N, DMF, Ar, 80 C, 3 days; (iii) 1000 C, 2 h; (iv) 1 M HCl, 24 h. Reproduced with permission.¹¹⁸ Copyright 2021, Elsevier.

Through a pyrolysis method, Fang and his colleagues were able to create a very reliable single-atomic nickel catalyst that was attached to porous hollow carbon spheres, specifically Ni–N-HCS. This catalyst demonstrated remarkable performance in CO₂ electro-reduction with a FE for CO of 95.04%, −0.70 V vs. RHE and a J of 10.88 mA cm⁻², thanks to its single-Ni active sites and sturdy hollow carbon structure (Fig. 16).¹¹⁹

Fig. 16 (a) Fabrication of Ni–N-HCS. Reproduced with permission.¹¹⁹ Copyright 2024, American Chemical Society. (b) Illustration for the fabrication of K-defect-C-1100 via a K⁺-assisted strategy. Reproduced with permission.¹²⁰ Copyright 2022, John Wiley & Sons. (c) Schematic illustration of the synthesis process for CNT@mC/Ni-T0.5. Reproduced with permission.¹²¹ Copyright 2022, John Wiley & Sons.

Ling et al. realized the production of non-carbon-based bio-MOF-1 precursors, and a K⁺-assisted synthesis strategy was developed. During the pyrolysis process, K⁺ ions in MOF remove N dopants and carbon atoms in the carbon matrix, promote the etching process, and increase the number of topological problems in the gas carbon matrix. K-defect-C-1100, with many V12 defects, exhibits good CO₂RR performance with CO FE up to 99% at ±0.45 V, which is far superior to that of K-defect-C-900, K-defect-C-1000, and N–C-1100, respectively. Theoretical calculations show that the V12 defect in K-defect-C-1100 can easily and rapidly capture electrophilic CO₂ molecules and form charge-determining COOH* intermediates, thus improving the CO₂RR activity well. Also, as a cathode catalyst for Zn–CO₂ batteries, K-defect-C-1100 exhibits ultra-high CO FE and excellent stability during discharge. This work establishes a new synthetic material for creating defects in carbon materials by successful heteroatom removal and provides insight into understanding carbon defects to achieve superior catalysis.¹²⁰

By coordinating Fe(NO₃)₃ and Na₂C₂O₄, Chen et al. created [Fe(C₂O₄)₃]³⁻, which is utilized as a ZIF-8 abrasive and calcined to create hollow low-carbon nanocages. The carbon nanocages' FeN₄ sites are identical to those of their counterparts, according to STEM, XPS, and EXAFS. The hollow structure of the carbon nanocage was verified by TEM. The hollow carbon has a more mesoporous structure, according to the results of the nitrogen adsorption–desorption isotherm. The Kirkendall effect in the pitting process was verified by numerous controlled studies. With a FE of 84% at an applied potential of −0.9 V, the hollow porous structure provides good CO₂RR activity and selectivity by speeding up the diffusion of CO₂.¹²²

Chen et al. modified CNTs by coating them with a mesoporous carbon shell anchored with Ni NPs. The Ni species facilitate carbon deposition from the pyrolysis of the surfactant 1-hexadecyl trimethyl ammonium bromide, leading to the formation of the mesoporous carbon shell. Simultaneously, the confinement effect enables the embedding of Ni nanoparticles within the carbon shell. Due to the well-dispersed Ni nanoparticles and the presence of N-doped active sites within the mesoporous carbon matrix, the synthesised electrocatalyst (CNT@mC-Ni-T_0.5) demonstrates excellent catalytic activity for the selective electroreduction of CO₂ to CO, achieving a maximum FE of 98% at a moderate overpotential of −0.81 V vs. the RHE, along with a high partial current density of 60 mA cm⁻² in an H-cell using an aqueous electrolyte.¹²¹

Luo et al. developed a novel approach to create single-site catalysts of various metals such as Ni, Fe, Co, Cu, Rh, Ru, Pt, and Pd supported on mesoporous carbon. The powerful coordination of the metal cations with ethylenediamine allows the metal to be atomically dispersed and enclosed in the carbon support at high loadings. The properties of M-NAC catalysts include a regular mesoscopic structure, high metal loading, and high surface area. These features enable CO₂RR catalysts to have high efficiency and high current. Ni-NAC shows the highest CO selectivity among all M-NAC catalysts, producing an FE of more than 95% for CO at moderate capacity.¹²³

A number of simple electrospinning and pyrolysis techniques were employed to successfully generate the atomic bridge structure of the double nuclear nickel solution of channel-rich porous carbon fibers. The in situ synthesis of new bridge structures between nickel, nitrogen, and carbon atoms, also referred to as Ni₂–N₄–C active sites, is demonstrated by the combination of structure-sensitive X-ray absorption spectroscopy (XAS) and high-angle atomic force microscopy (HAADF-STEM). These bridge structures allow nickel atoms to permeate throughout the carbon matrix. Thus, in an H-type cell, the ideal Ni–CNC-1000 electrocatalyst has an appealing TOF value of 4606.5 h⁻¹ at −1.0 V and a high FECO of 96.6% at −0.8 V. Using a basic electrolyte, a sufficient cell achieved a maximum FE of 97.8% CO at a voltage of 0.5 V and an exceptionally low starting capacity of 0.2 V (Fig. 17).¹²⁴

Fig. 17 (a) Proposed CO₂RR pathway on the Ni₂–N₄–C₂ model (C: grey, O: red, N: blue, H: white, and Ni: green); (b) free energy diagram for CO formation; (c) PDOS of Ni 3d orbitals in Ni₂–N₄–C₂, Ni–N₃–C, and Ni₄–N–C models; (d–f) VCDD maps for CO₂ adsorption on Ni–N₃–C, Ni₂–N₄–C₂, and Ni₄–N–C (red: electron accumulation and blue: depletion). Electrochemical ZCEC performance using a Ni–CNC-1000 cathode and Zn anode: (g) schematic setup, (h) polarization and power density plots, (i) discharge curves and corresponding FE_co, (j) LED lighting using three ZCECs in series, and (k) charge–discharge cycling at 0.5 mA cm⁻² for 130 cycles. Reproduced with permission.¹²⁴ Copyright 2021, John Wiley & Sons.

3.3. Heteroatom doped materials

Heteroatom-doped carbon-based materials have become up-and-coming candidates for CO₂ reduction because of their ability to enhance the properties of carbon materials. Researchers increased the efficiency, selectivity, and stability of CO₂ reduction processes by adding heteroatoms including nitrogen, phosphorus, and boron to the carbon matrix. These doped materials are more effective in catalyzing CO₂ conversion due to modifications in the electronic structure, new active sites, and increased surface reactivity. Kim et al. fabricated quasi-graphitic carbon shell-protected Cu NPs doped with N, Cu(N) as an electrocatalyst for selective reduction of CO₂. Cu(N) acquired 82.3% of C₂⁺ selectivity at −0.55 V vs. RHE with a partial J of 329.2 mA cm⁻². The reason for this improvement is the N-doped copper atoms within the carbon shell, which encourage the formation of C–C bonds and lower the number of unwanted byproducts produced (Fig. 18).¹²⁵

Fig. 18 (a) Schematic of the CO-mediated Cu–C–O reaction: inner pores form via C combustion and Cu NPs are confined within a quasi-graphitic C shell; solid and dashed lines represent the Boudouard reaction and nanocarving, respectively. (b) Cu–O–C ternary phase diagram at 800 °C showing the stable Cu–CO–C region. (c) Thermodynamically favoured Boudouard reaction product at 1 atm. (d) ΔG vs. temperature plot for the Boudouard reaction under varying pressures (10⁻⁸–1 atm); upper/lower regions indicate favourable forward/reverse reactions. (e) Proposed mechanism for quasi-graphitic C shell formation on Cu via CO disproportionation. Reproduced with permission.¹²⁵ Copyright 2021, Springer Nature.

Jin et al. synthesized pipet-like bismuth (Bi) nanorods semi-filled in N-doped CNTs, Bi-NRs@NCNTs, via coating of a polymer layer on Bi₂S₃ nanowires (Bi₂S₃ NWs) followed by thermal treatment. When reducing CO₂ to formate, the electrocatalyst demonstrated greater selectivity. The SEM shows that the pipet-like Bi-NR@ NCNTs contain a substantial empty space and internal cavities. The electrocatalyst has a high FE of 90.9% at a potential of −0.9 V vs. RHE. The spatial confinement and segregation provided by the NCNT shells minimize surface oxidation and self-accumulation of Bi-NRs, hence improving electrocatalyst stability and dispersion. As a result, the catalyst exhibits significant activity (Fig. 19).¹²⁶

Fig. 19 (a) Schematic of Bi-NRs@NCNTs synthesis. (b–d) SEM images of Bi₂S₃ NWs, polymer-coated Bi₂S₃ NWs, and Bi-NRs@NCNTs; insets: corresponding TEM images. (e) SEM and EDX elemental mapping of Bi-NRs@NCNTs. Reproduced with permission.¹²⁶ Copyright 2021, American Chemical Society.

Liu and his co-authors produced a series of Ni/nitrogen-doped graphite (Ni-NG) catalysts with various prepotent Ni species, including nitrogen-doped carbon-embedded Ni NPs, a single Ni atom, or both (Ni_1.11-NG-H, Ni_0.37-NG-H, and Ni_0.037-NG-H), and analyzed the CO₂ electroreduction capabilities of these catalysts. With a predominance of single Ni atoms, the Ni_0.037-NG-H catalyst demonstrated exceptional stability over a period of 64 hours, exhibiting a maximum FE of 97.3%, a mean FE of about 95%, and a J of 15 mA cm⁻² at −0.8 V (vs. RHE).¹²⁷

Zhou and co-workers synthesized a series of Ni particles condensed in N-doped CNTs on carbon paper, Ni@N-CNTs/CP composites, of which 2.5Ni@N-CNTs/CP-700 gave the highest FE of 57% at −0.76 V vs. RHE. The improved performance was attributed to surface N species being the active sites for the adsorption and activation of CO₂ molecules. This is because the Ni particles covered in N-CNTs improve the electron structure of the N species on the surface through Ni–N–C bonds (Fig. 20).¹²⁸

Fig. 20 Schematic illustration of the in situ CVD fabrication process for the Ni@N-CNTs/CP electrode. Reproduced with permission.¹²⁸ Copyright 2020, Elsevier.

Chen et al. used embedded Ni to create Ni0.87-NC-1-AE, an N-doped carbon. The approach for the CO2RR is based on the hard template assisted sacrificial template method. The SEM, TEM, HRTEM, and SAED pictures all reveal a hierarchical nanoplate shape with a rich porous structure.

At −1.05 V, the electrocatalyst had a J of 21.81 mA cm⁻² and a good FE of 97.6%. It's possible that this is due to the fact that the mesoporous structure boosts catalytic activity by providing a large number of active sites and making it easier for reactants to move around.

Furthermore, adding Ni encourages the formation of sensitive nitrogen classes such as graphitic-N and pyridinic-N. When this synergistic effect is paired with the dynamic charge transmission that occurs between the metal and the NC components, it readily triggers reactants and enhances the activities that involve proton-coupled electron transfer (Fig. 21).¹²⁹

Fig. 21 (a) Illustration of the production of Ni_0.87-NC-1-AE. Ni_0.87-NC-1-AE was analyzed morphologically and structurally. (b–d) SEM pictures at various magnifications. (e) TEM picture. (f) High-resolution TEM picture. (g) An SAED picture. (h) SEM picture with matching elemental mapping. Reproduced with permission.¹²⁹ Copyright 2024, Elsevier.

Boppella et al. synthesized Ni NPs encased in Ni–N–C catalysts at low temperature. In order to achieve a conversion from CO₂ to CO, the catalysts contain a large number of single-atomic Ni–N₄ sites and uncoordinated N-doped sites. At −0.7 V, the improved catalyst showed a great performance in the CO₂RR, with an FE of 97.4% and J of 58 mA cm⁻² at −1 V. The porous CNT network architecture made active sites plentiful and easy to reach, and it also made electron transport more efficient. Furthermore, the total catalytic activity was increased by the synergistic interaction between N-doped sites and Ni–N₄, which was affected by Ni NPs (Fig. 22).¹³⁰

Fig. 22 (a) Schematic of Ni-NP-encapsulated Ni–N–C nanotube catalysts and their CO₂RR activity/selectivity. (b) LSV curves in CO₂-saturated 0.5 M KHCO₃, (c) CO faradaic efficiencies, (d) capacitive J vs. the scan rate, and (e) j_(CO) correlated with Ni–N₄ and atomic N content across Ni–NCNT catalysts. Reproduced with permission.¹³⁰ Copyright 2024, Elsevier.

Applying a one-step CVD strategy, Zhuo et al. succeeded in manufacturing nickel NPs and produced a Ni@N–C catalyst that was encapsulated in N-doped CNTs (Fig. 23). With the addition of nitrogen, the mass limit of CO₂ is decreased and the kinetics of CO₂ intermediates in the Ni@N–C ECR process become more relevant. It was found that the N-doped CNTs that are coated on Ni NPs improve the ECR performance. The Ni NPs enclosed by N-doped CNTs stop the Ni from coming into direct contact with H⁺ protons in the electrolyte, which slows down the HER process. The Ni@N–C catalyst demonstrated up to 90% FE at −0.77 V. Compared with the RHE in the cell, the RHE in the H-type cell showed more than 93% FE in the range of −0.37 to −0.77 V.¹³¹

Fig. 23 Schematic illustration of the synthesis routes for (a) Ni@C, (b) Ni@N–C, and (c) N–C materials. Reproduced with permission.¹³¹ Copyright 2023, Elsevier.

For the purpose of achieving a considerable electrochemical reduction in CO₂, Li et al. developed atomically dispersed Fe–N–C catalysts and tuned them with silicon. In order to demonstrate remarkable catalytic activity for the conversion of CO₂ to CO, the Fe–N/P–C catalyst was able to obtain a high-quality normalized time of flight (TOF) of 508.8 h⁻¹ at a low overpotential of 0.34 V and a high overpotential. Ex situ XAS measurements and density functional theory calculations were carried out in order to demonstrate additional atomic tuning of the Fe-NC catalyst with P. This was done in order to enhance the conversion of CO₂ to CO by the inhibition of the HER and the reduction of the oxidation state of the Fe center (Fig. 24).¹³²

Fig. 24 (a) Schematic of Fe–N/P–C synthesis. (b–d) SEM, TEM (inset: HRTEM), and HAADF-STEM images. (e) Elemental composition from XPS; (f) high-resolution P 2p and (g) Fe 2p XPS spectra of Fe–N/P–C. Reproduced with permission.¹³² Copyright 2022, American Chemical Society.

Yang et al. successfully developed a bimetallic alloy CuNi/NC catalyst for the CO₂ reduction to CO. Notably, the concentrations of graphite-N and pyridine-N in CuNi/NC are higher than those in Cu/NC and Ni/NC, which improves the effectiveness of the electroreduction. It was found that Cu_0.543Ni₁/NC shows exceptional electrocatalytic performance in reduction of CO₂ and achieves high selectivity of CO in the potential range of −0.3 V to −0.8 V with the highest efficiency of 99.7% at – 0.6 V. The electronic interactions and repulsion due to the overlapping d-bands of Ni and Cu are responsible for the high CO selectivity.¹³³

3.4. Metal-free materials

Metal-free carbon-based materials for CO₂ reduction have emerged as an important field of research because they potentially provide alternatives to current metal-based catalysts that are more cost-effective, sustainable, and environmentally friendly. CNTs, CDs, activated carbon, and their heteroatom-doped variations are some of the materials that offer various advantages in the CO₂ reduction reaction. These advantages include high stability, cheap cost, and reduced environmental effect. Graphene is one of these materials. Wu et al. created N-doped graphene quantum dots (NGQDs) for electrocatalytic reduction of CO₂ to CO and HCOOH at −0.26 V vs. RHE. CO₂ can be transformed into hydrocarbons (C₂H₄ and CH₄) and oxygenates (C₃H₇OH, C₂H₅OH, and CH₃COO⁻) when the applied potential is negative. At a potential of −0.78 V, the maximum FE reaches 26%, with C₂H₅OH making up the majority at 16%. The partial J for creating C₂H₅OH is 21 mA cm⁻², and the formation rate is 0.7 mol h⁻¹ m⁻² at −0.86 V (Fig. 25).¹³⁴

Fig. 25 (a–c) Faradaic efficiencies for CO₂ reduction products at varying cathodic potentials for NGQDs, pristine GQDs, and NRGOs, respectively. (d) Tafel plots of partial J vs. potential for the three carbon nanocatalysts. Error bars indicate standard deviation from three replicate measurements. Reproduced with permission.¹³⁴ Copyright 2022, Springer Nature.

Quan and co-authors prepared a B- and N-co-doped nanodiamond (BND) over the Si material utilizing a filament-assisted chemical vapor deposition method. The BND reduces CO₂ in HCOOH, HCHO, C₂H₅OH, CH₃OH, and CH₃COOH. The major product is C₂H₅OH, with the highest FE of 93.2% (−1.0 V vs. RHE). The addition of additional N to the BND facilitates the synthesis of C₂H₅OH. DFT calculations demonstrate that the synergy between N and B dopants is the key source of the reasonable capacity of producing C₂H₅OH on the BND. By forming a connection with an O atom, B atoms can enhance CO₂ adsorption. *H transfer uses N atoms as active sites to speed up hydrogenation during CO₂ reduction (Fig. 26).¹³⁵

Fig. 26 (a) Free energy diagram for CO₂ reduction on the (111) facet of the BND. (b) Optimised structures of key intermediates on BND (111) during CO₂ reduction (C: grey, B: pink, N: blue, O: red, and H: white). Reproduced with permission.¹³⁵ Copyright 2017, Wiley.

Zhang et al. used melamine as a nitrogen source, which was encapsulated into a cellulose-based aerogel skeleton via an in situ synthesis method. There is a steady formation of a rich pore structure from the interior to the outside of the samples as the temperature of the pyrolysis process increases. This structure provides sufficient transport routes for CO₂ with a maximum surface area of N₃-750 of 1038.02 m² g⁻¹, and the FE of N₃-750 for CO products is as high as 75.90%. The high-pressure N/C balance plays an important role in reducing the CO₂ reaction. Pyridine N can increase the onset potential and become an active site for the formation of *COOH intermediates. In contrast, the electrochemical performance of pyrrole N is poor. For eight hours, the FE content of CO decreased from 75.90% to 68.26%, indicating superior catalyst stability.¹³⁶

Zhang et al. synthesized a set of N,P co-doped carbon (CNP) with distinct N and P contents and arrangements at diverse pyrolysis temperatures to reveal the actual active sites. CNP-900 showed an FE_CO of 80.8% at 0.44 V potential and also displayed the highest CO partial CD. CNP-900 maintained the maximum FE for 8 h at −0.55 V and exhibited a small Tafel slope (128.3 mV dec⁻¹), indicating a fast catalytic rate (Fig. 27).¹³⁷

Fig. 27 (a) FE_(CO) of CNP catalysts at different temperatures. (b) FE of CO and H₂ for CNP-900. (c) CO partial J across various electrocatalysts. (d) Comparison of CO and H₂ FE at −0.55 V vs. RHE; CN-900 shows low J and sub-100% FE. (e) Stability of CNP-900 at −0.55 V. (f) Tafel plots of different samples. Reproduced with permission.¹³⁷ Copyright 2022, American Chemical Society.

Tian et al. successfully prepared an N,P dual-doped carbon nanosheet catalyst (NPC) using the method of sacrificial templating for the CO₂RR. The attained NPC achieved an FE_CO of 88% in the CO₂RR at a low voltage of 300 mV and displayed excellent stability within 27 h. The synergistic effects of nitrogen and phosphorus on carbon frameworks, as well as the increased surface area of the 2D carbon nanosheet structure, are responsible for the excellent activity.¹³⁸

Fan et al. developed N-doped carbon spheres (NCSs) from metals and without substantial pyridinic N sites for the CO₂RR. The pyrrolic N in NCSs showed a direct-positive relation with the performance for the CO₂RR, representing the active centre with superior activity. According to the results of the DFT calculations, the intrinsic activity of pyrrolic nitrogen is far higher than what is required for effective CO₂RR. Nevertheless, the activity of pyrrolic nitrogen is severely suppressed as a result of the interaction with the pyridinic nitrogen sites that are close to it. The optimal NCSs attain a greatest FE_CO = 71% at −1.25 V and an extensive series of syngas CO/H₂ in ratios of 0.09 to 12 by alteration of applied potentials (−1.1 to −1.5 V).¹³⁹

Li and co-authors utilized phytic acid and aniline to develop N,P co-doped carbon, NPCM-1000, as an electrocatalyst for CO₂ reduction. The catalyst exhibited a high FE of 92% and −0.55 V vs. RHE and demonstrated improved selectivity towards CO production, minimizing the formation of unwanted byproducts.¹⁴⁰ Li and co-authors fabricated a metal-free nitrogen-doped carbon (NG-1000) using a composite of phthalocyanine and dicyandiamide for the electrocatalytic CO₂RR. Using NG-1000, up to 95% FE of CO and J = 9.07 mA cm⁻² were obtained (−0.72 V vs. RHE). Based on the analysis of original Pc/CNTs and NG-T (T = 700 to 1000) catalysts, it was found that the C atoms adjacent to graphitic-N species in NG-1000 act as the active component for the CO₂RR (Fig. 28).¹⁴¹

Fig. 28 (a) LSV curves of Pc/CNTs and NG-T catalysts. (b) FE for the CO₂RR at various potentials. (c) Product selectivity comparison at −0.72 V vs. RHE. (d) CO partial J vs. potential. (e) Schematic of NG-T catalyst synthesis. Reproduced with permission.¹⁴¹ Copyright 2021, Elsevier.

Wang et al. developed a set of metal-free porous Se, B, and N ternary-doped porous carbons (SeBN-C) using particular sources of precursors by the ball-milling method as electrocatalysts for the CO₂RR (Fig. 29). After 10 h of constant electrolysis, the J and FE_CO were sustained at 84.7% and 97.6% of their primary values, respectively, demonstrating that they are suitable for long-term operation.¹⁴²

Fig. 29 Schematic illustration of the synthetic route for SeBN-C-x electrocatalysts. Reproduced with permission.¹⁴² Copyright 2022, American Chemical Society.

4. Challenges, design principles and recommendations

4.1. Key challenges in industrial CO₂ reduction

The advancement of electrolytic CO₂ reduction to value-added carbon products using carbon-based catalysts faces substantial challenges that must be addressed to improve performance and enable industrial application. A primary challenge is achieving adequate catalytic selectivity and activity, as carbon-based catalysts often demonstrate lower activity than traditional metal catalysts and frequently require higher overpotentials to drive reactions. This limitation can result in inefficient processes and unwanted by-product formation, complicating product separation and purification. Additionally, carbon-based catalysts suffer from deactivation during continuous operation, leading to reduced effectiveness and poor stability and durability. The scalability of manufacturing methods presents another significant hurdle, as the synthesis of high-performance catalysts commonly involves expensive and complicated processes that may not be feasible for large-scale production applications. Furthermore, the limited understanding of reaction pathways for CO₂ reduction on carbon-based catalysts restricts the ability to design catalysts with features specifically tailored for particular reactions.

Achieving high energy efficiency and current density without compromising product selectivity represents one of the most critical challenges in the industrial CO₂RR. Energy efficiency depends on minimizing cell voltage and concentration overpotentials while effectively directing reaction pathways toward high-value carbon products. However, reaching industrially relevant current densities is often hindered by mass transport limitations, primarily due to the low solubility of CO₂ in aqueous electrolytes. GDEs are widely employed to enhance CO₂ delivery to the catalyst interface, but while they successfully improve CO₂ mass transport, they are susceptible to electrolyte flooding—a phenomenon where liquid infiltrates the gas diffusion layer, disrupting the crucial gas–liquid–solid interface. This intrusion restricts CO₂ access to active sites, lowers FE, promotes unwanted hydrogen evolution, and ultimately compromises system performance. These issues are exacerbated at high current densities and extended operation, making GDE flooding a significant bottleneck for scaling CO₂RR technologies.

Membrane electrode assembly (MEA) configurations offer an alternative approach by eliminating catholyte flow, thereby reducing ohmic losses and overall cell voltage while enabling operation over a broader pH range with improved product selectivity.^143,144 However, MEA electrolyzers face their own challenges, notably membrane degradation. Prolonged operation, particularly under alkaline conditions or at high current densities, can cause chemical degradation of membrane materials, mechanical delamination of electrode–membrane interfaces, and a decline in ion exchange capacity. These degradation processes lead to CO₂ crossover, local pH fluctuations, reduced ionic conductivity, and gradual performance decay, posing a critical barrier to the long-term stability and commercial viability of MEA-based systems.

Operating CO₂ reduction reactions in alkaline electrolytes offers distinct advantages in enhancing reaction kinetics and product selectivity, particularly for multi-carbon product formation. However, these benefits are counterbalanced by a critical limitation: significant carbon loss due to bicarbonate (HCO₃⁻) and carbonate (CO₃²⁻) species formation. In high-pH environments, dissolved CO₂ readily reacts with hydroxide ions (OH⁻) through non-electrochemical pathways, diverting CO₂ from the catalytic surface and diminishing its availability for electrochemical conversion. This issue is especially severe in open or single-pass systems, where carbon losses can reach up to 70%, drastically reducing overall carbon utilization efficiency. Moreover, the accumulation of carbonate species in the electrolyte can lead to their crossover into the anode compartment, disrupt pH balance, and impair membrane performance in MEA-based systems. These effects necessitate frequent electrolyte regeneration or replacement, introducing operational complexity, increased maintenance demands, and added energy costs. Therefore, despite the catalytic advantages of alkaline media, carbonate formation remains a major bottleneck to the scalability and economic feasibility of CO₂ electrolysis technologies.^59,145

Finally, metal–carbon catalysts, although promising, are prone to sintering and leaching, which reduce the number of active sites over time. Sintering leads to particle agglomeration, while leaching dissolves metal atoms into the electrolyte. These degradation pathways compromise durability and must be addressed through alloying, strong metal–support interactions, and protective encapsulation strategies.

4.2. Design principles and structure–activity relationships

Rational catalyst design must integrate fundamental insights into structure–activity relationships, with several key design principles having emerged from recent research. Doping chemistry plays a crucial role in product selectivity, as heteroatom doping significantly alters the electrical structure and catalytic characteristics of carbon frameworks. Nitrogen doping is particularly significant, with nitrogen atoms in systems like Cu(N) and Ni–N-CNTs modifying local electron density and facilitating C–C bond formation, thereby enhancing the generation of multi-carbon (C₂⁺) products such as ethylene and ethanol. The incorporation of nitrogen atoms promotes the stabilization of *CO intermediates and subsequent coupling processes necessary for multi-carbon species production. Moreover, co-doping with boron, nitrogen, and phosphorus, as demonstrated by the BND and CNP-900, facilitates synergistic modification of charge distribution and local binding sites. These dopants alter the adsorption energies of critical intermediates like *COOH and *CHO, enhancing selectivity for carbon products based on their spatial arrangement and electronic influences.^135,137

Defect engineering represents another essential approach for enhancing CO₂ activation and creating active sites. Structural defects such as pyridinic-N and graphitic-N within the carbon lattice function as principal active sites for *COOH production and CO desorption, which are rate-limiting steps in the CO₂RR. Catalysts such as NG-1000 and Ni₀.₀₃₇-NG-H demonstrate enhanced activity due to elevated nitrogen functionality density.^127,141 Furthermore, the intentional creation of topological defects, exemplified by K-defect-C-1100,¹²⁰ significantly enhances CO₂ capture and reaction kinetics. These defect-rich carbons exhibit enhanced localized charge density and binding affinity for electrophilic CO₂ molecules, facilitating favorable reaction energetics and elevated faradaic efficiencies.

Support morphology critically influences mass and electron transport properties, with nanostructured carbon frameworks including carbon nanotubes, carbon nanofibers, and hollow spheres markedly affecting catalytic performance. These materials provide elevated surface areas, effective mass transport pathways, and enhanced electrical conductivity. For instance, CNT@mC/Ni-T₀.₅ and Ni–N-HCS utilize these morphological characteristics to deliver elevated current densities and ensure reaction stability.¹²¹ Additionally, nanoconfinement effects within pores augment intermediate stabilization and the local CO₂ concentration, thereby elevating turnover frequency.

4.3. Recommendations

To address the aforementioned challenges, several strategic recommendations can be implemented as potential solutions. The development of hybrid catalysts that combine carbon materials with metals or metal oxides represents a promising approach to enhance catalytic activity and selectivity by leveraging the advantageous characteristics of each component. Nanostructured carbon-based compounds such as graphene and CNTs can be incorporated to significantly increase surface area and active site density, resulting in improved catalytic performance.

The development of protective coatings and encapsulation techniques could improve the longevity of active sites and shield them from degradation, thereby addressing stability concerns. To suppress GDE flooding and improve MEA durability, it is essential to develop enhanced hydrophobic and hierarchical structures resistant to electrolyte permeability. Flow-field engineering can optimize CO₂ distribution and preserve the gas–liquid–solid interface, increasing reaction efficiency and reducing the HER. For MEA stability improvement, utilizing chemically resilient ionomer membranes, interfacial adhesion layers, and mechanical reinforcements is crucial to prevent degradation under severe operating conditions, enhancing durability and ensuring consistent performance at elevated current densities.

Mitigating carbonate losses in alkaline systems can be achieved by shifting to acidic CO₂RR systems or implementing CO₂ recovery and recycling processes to reduce carbonate formation and minimize CO₂ loss. Optimizing electrolyte configurations to limit side reactions with OH⁻ can improve CO₂ utilization efficiency. Catalyst stability can be enhanced through approaches such as metal–carbon alloying, single-atom dispersion, and encapsulation within porous substrates, which can mitigate catalyst degradation through sintering and leaching while enhancing long-term stability and performance at high current densities. Furthermore, pulsed electrolysis is a promising strategy to enhance CO₂ reduction by dynamically modulating the applied potential, which improves C₂⁺ product selectivity, suppresses competing hydrogen evolution, and regenerates active catalyst sites. This approach also helps mitigate carbonate formation and catalyst degradation, enabling more efficient and stable operation at high current densities.

The integration of multi-scale modeling and in situ characterization techniques, utilizing DFT and operando spectroscopy methods, will provide valuable insights into reaction pathways, intermediate formation, and catalyst stability, facilitating rational catalyst design and reaction condition optimization. By systematically implementing these recommendations, future catalysts and reactor designs can be optimized to deliver industrially scalable and economically viable CO₂RR systems. Fig. 30 illustrates the key challenges and solutions for carbon-based materials in CO₂RR applications.

Fig. 30 Challenges and solutions for carbon-based materials for the CO₂RR.

5. Conclusions and outlook

CO₂ conversion is a transformative approach for tackling both global climate change and energy security by lowering atmospheric CO₂ levels and reducing reliance on finite fossil fuels. The CO₂RR has become a promising method, allowing waste CO₂ to be turned into high-value fuels and chemicals through sustainable electrochemical processes. Among the many potential products, multi-carbon (C₂⁺) compounds are especially promising because they have higher energy density and better economic value than single-carbon (C₁) products, making them more suitable for industrial use and energy storage. Producing these valuable C₂⁺ products depends heavily on achieving efficient carbon–carbon (C–C) coupling reactions, which requires advanced catalysts that can offer ideal adsorption energies for key reaction intermediates while maintaining high surface coverage of C₁ species to enable coupling. Carbon-based catalysts are particularly attractive for promoting C–C coupling due to their excellent electrical conductivity, highly adjustable surface properties, and strong chemical stability in aqueous electrolytic media. Their versatility allows for various enhancement strategies, including heteroatom doping to alter electronic properties, metal/carbon hybridization to create synergistic active sites, and nanoconfinement engineering to establish favorable microenvironments. These strategies collectively improve CO₂ adsorption, suppress competing HER, and favor the selective formation of multi-carbon products. However, fully realizing the potential of these systems requires overcoming several critical challenges that currently hinder practical implementation.

The transition from laboratory-scale demonstrations to commercially viable industrial applications demands a comprehensive approach targeting multiple interconnected objectives. Enhancing selectivity and faradaic efficiency represents a paramount priority, requiring the development of sophisticated catalysts and electrode architectures that maximize C₂⁺ product yields while effectively minimizing undesirable side reactions, particularly the competitive hydrogen evolution that reduces overall process efficiency. Simultaneously, improving catalyst stability through robust design principles, enhanced corrosion resistance, and effective mitigation of surface poisoning effects is essential for ensuring long-term operational performance under industrial conditions. The optimization of reaction conditions, including standardization of electrolyzer configurations, electrolyte compositions, and critical operational parameters such as pH, temperature, and pressure, will be crucial for successful scale-up operations.

Advancing mechanistic understanding through the integration of sophisticated operando spectroscopic techniques with comprehensive theoretical modeling approaches will be instrumental in identifying key reaction intermediates and elucidating the fundamental C–C coupling pathways, thereby enabling rational catalyst design strategies. The development of scalable fabrication methods for carbon-based catalysts, coupled with rigorous techno-economic analyses, will be essential for evaluating and ensuring commercial viability. Furthermore, the integration of renewable energy sources, including solar, wind, and waste electricity, with CO₂RR systems will be vital for ensuring sustainable and cost-effective operation while maintaining environmental benefits.

Addressing reactor-level challenges represents another critical frontier, where advanced designs such as GDEs and MEAs must overcome persistent operational challenges including electrolyte flooding and membrane degradation. The development of hydrophobic interfaces and chemically robust membranes will be fundamental to ensuring sustained long-term operation. To minimize CO₂ losses in alkaline media, innovative strategies such as CO₂ capture and recycling systems, optimized electrolyte compositions, or strategic transitions to acidic operating systems should be actively pursued. Concurrently, catalyst fabrication processes must embrace environmentally friendly and scalable synthesis approaches utilizing renewable resource feedstocks to maintain overall process sustainability. The acceleration of catalyst optimization will benefit significantly from combining advanced in situ characterization techniques with sophisticated DFT simulations and machine learning algorithms to identify optimal catalyst compositions and structures. Most importantly, fostering interdisciplinary collaboration among materials scientists, electrochemists, chemical engineers, and industrial partners will be critical for successfully translating these scientific innovations into practical, commercially viable CO₂ conversion technologies that can meaningfully contribute to global carbon neutrality targets. By systematically addressing these strategic directions through coordinated research and development efforts, carbon-based electrochemical CO₂ reduction can evolve from a promising laboratory concept into a transformative commercial solution for carbon mitigation and sustainable energy production, ultimately contributing to a more sustainable and carbon-neutral future.

Abbreviations

BND	B- and N-co-doped Nanodiamond
CBC	Brewed Waste Coffee
CDs	Carbon Dots
CNCu	Copper-doped Carbon–Nitrogen
CNP	N,P co-doped Carbon
CNTs	Carbon Nanotubes
CoTAP	Amino-substituted Cobalt Porphyrin
CO2RR	Co2 Reduction Reaction
CQD	Carbon Quantum Dot
CTO	Pyrolyzed Chitosan
DFT	Density Functional Theory
ECO2RR	Electrochemical CO2 Reduction Reaction
EG	Ethylene Glycol
FE	Faradaic Efficiency
GC	Gas Chromatography
GDE	Gas Diffusion Electrode
HER	Hydrogen Evolution Reaction
HPLC	High-Performance Liquid Chromatography
HRTEM	High-Resolution Transmission Electron Microscopy
IC	Ion Chromatography
LSV	Linear Sweep Voltammetry
MEA	Membrane Electrode Assembly
MOFs	Metal–Organic Frameworks
NCs	Nanoclusters
NCNSs	Nitrogen-doped Carbon Nanosheets
NCNT	N-doped Carbon Nanotube
NDCC	N-Doped Carbon Cloth
NGQDs	N-doped Graphene Quantum Dots
NMR	Nuclear Magnetic Resonance
NPC	Carbon Nanosheet Catalyst
NPS	Nanoparticles
OCSn	Sn on Carbon Paper
OMMCs	Ordered Meso-/Macroporous Carbons
ORR	Oxygen Reduction Reaction
PHI	Na-Polyheptazine Imide
PIL	Poly(Ionic Liquid)
PTI	Li-Polytriazine Imide
QDs	Quantum Dots
RHE	Reversible Hydrogen Electrode
RGO	Reduced Graphene Oxide
SAs	Single Atoms
SACs	Single-Atom Catalysts
SAED	Selected Area Electron Diffraction
SeBN-C	Se, B, and N ternary-doped porous Carbons
SHE	Standard Hydrogen Electrode
TEM	Transmission Electron Microscopy
TOF	Turnover Frequency
XAS	X-ray Absorption Spectroscopy
XANES	X-ray Absorption Near-Edge Structure
XPS	X-ray Photoelectron Spectroscopy
XRD	X-Ray Diffraction
ZISNSAs	ZIS Nanosheet Arrays

Conflicts of interest

The authors declare no conflicts of interest.

Data availability

No new data was generated.

Acknowledgements

The authors would like to express their gratitude to the institutions and colleagues who provided valuable insights and feedback throughout the preparation of this review article. Also, Garima Rathee expresses her profound gratitude to Marie Skłodowska-Curie Actions (MSCA) for the invaluable support through providing the Postdoctoral Fellowship Grant (HORIZON-101109383).

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