Electronic and geometric modulations of catalysts for electrochemical CO₂ reduction reaction

Abstract

The electrocatalytic CO₂ reduction reaction (CO₂RR) converts CO₂ into high-value-added chemicals using clean and renewable energy, making it one of the most promising strategies for addressing present-day energy and environmental crises. Electrocatalysts play a crucial role in electrocatalytic CO₂RR systems because they determine catalytic activities and selectivities. Electronic and geometric structures are two important factors that affect the electrocatalytic performances of electrocatalysts, in which the electronic structures are related to the adsorption strengths of substrates/intermediates while the geometric structures are associated with the microenvironments of catalytic reactions. Consequently, modulating the electronic and/or geometric structures of electrocatalysts can significantly improve the catalytic performances. To better understand the roles and importance of electronic and geometric structural modulations, this review systematically summarizes and discusses the latest progress of these two strategies that apply to electrocatalytic CO₂RRs. First, the methods for electronic and geometric structural modulations of electrocatalysts are introduced. Then, the trends and mechanisms of electronic structural modulation of various metal elements are explored in detail based on element partitioning across the periodic table, starting with the s- and p-block elements and ending with broad coverage of the d- and f-block metals, and the advantages and functions of geometric structural modulation of catalyst supports and the impact mechanism on performance are discussed according to types of supports. Finally, some prospects are proposed to provide suggestions for designing more efficient electrocatalysts for the electrocatalytic CO₂RR based on these two strategies.

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Shilin Wei

Shilin Wei received his BEng degree from China University of Mining and Technology in 2020 and is now a doctoral student at the same university. His research interests focus on the reaction mechanisms of carbon dioxide reduction reaction via multi-physical field coupling and the modelling of electrochemical energy conversions.

Weiqi Liu

Weiqi Liu received his bachelor's and master's degrees from Shandong University of Technology and his doctoral degree from China University of Mining and Technology in 2023. He is now a postdoctoral researcher associate at China University of Mining and Technology. His major research interests include preparations of coal-based functional materials and electrocatalytic reduction of carbon dioxide.

Chuangchuang Yang

Chuangchuang Yang received his BSc degree from China University of Mining and Technology in 2019. At present, he is pursuing his doctoral degree at the same institute. His current research interests concentrate on the construction and exploration of electrocatalysts toward CO2 and O2 reduction reactions.

Peiyao Bai

Peiyao Bai received his BSc degree from China University of Mining and Technology in 2018. After that, he continued to pursue his doctoral degree at the same university. Now his research interests are mainly in the green synthesis of value-added chemicals via electrochemical routes, advanced energy-conversion system designs, and a deep understanding of electrochemical processes in multi-phase interfaces.

Xiao Kong

Xiao Kong received his bachelor's and master's degrees from Shandong University of Technology and is now a doctoral student at China University of Mining and Technology. His research interests involve carbon and nitrogen fixations using electrochemical methods.

Wenbo Sun

Wenbo Sun graduated from Henan University of Technology and is currently a master student at China University of Mining and Technology. His main research interests include the synthesis and characterization of biomass-based metal-free electrocatalysts for the carbon dioxide reduction reaction.

Lang Xu

Lang Xu graduated with a BSc degree from Nanjing University and a DPhil degree from University of Oxford under the supervision of Professor Fraser A. Armstrong FRS, and is now a Professor of Chemistry at China University of Mining and Technology. He is mainly engaged in research in the fields of green and low-carbon energy molecular science and engineering, including carbon dioxide resource utilization, fuel cells, coal/biomass-based electrocatalytic materials, and molecular modelling.

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1. Introduction

The fast development in modern industry accelerates the progress of society, but at the same time, it also consumes a lot of fossil fuels (such as natural gas, petroleum, and coal), resulting in an enormous amount of CO₂ being emitted into the atmosphere.^1–4 At present, the concentration of CO₂ in the atmosphere has reached 420 ppm, which is higher than the safe level of 350 ppm.^5,6 The excessive CO₂ has triggered the global greenhouse effect, leading to glacial ablation, rising sea levels, climate anomalies, species extinction, and other serious issues.^7–11 Therefore, it is crucial to overcome the CO₂ issues.^12–15

Among numerous schemes, the electrochemical CO₂ reduction reaction (CO₂RR) is particularly attractive, since it can not only directly use the electricity generated by renewable energy to drive the entire electrocatalytic system but also flexibly regulate the types and yields of reduction products by adjusting external parameters.^16–22 In addition, the electrochemical systems can be operated in aqueous solutions (providing protons) at normal temperature and pressure, which is beneficial to improving energy efficiencies and therefore providing sustainable development paths for a low-carbon society.^23–26 In recent decades, researchers have made great efforts to expand and optimize CO₂RR technologies. A substantial number of high-performance catalysts have been developed, such as single-atom catalysts,^27,28 hierarchical nanosheet catalysts,²⁹ 3D nanosheet arrays,³⁰ nano-polyaniline catalysts,³¹ porous hollow Cu microspheres catalysts,³² and metallic oxide catalysts.³³ Not only have various structures and types of electrocatalysts been prepared, but also the conversion mechanisms of CO₂ molecules on a vast array of catalytically active sites have been intensively studied using state-of-the-art characterization and theoretical calculation methods.^34–39 Since heterogeneous catalysts can be directly coated on electrically conducting catalyst supports, the efficient charge transfer between catalysts and supports enables the former to exhibit relatively high activities. At the same time, heterogeneous catalysts are ready for reuse, which greatly improves economic efficiencies.^40–43 As heterogeneous catalysts are advantageous in the electrocatalytic CO₂RR, we limit the research scope to this class of electrocatalysts in this review.

Traditional electrocatalysts show relatively low product selectivities and current densities for the CO₂RR, which are mainly attributed to their weak adsorptions (including adsorption strengths and capacities) to CO₂/intermediates.^44,45 The electronic structures of active sites of these electrocatalysts are quite stable, making it difficult to couple the outer orbitals of the active sites with the substrate or intermediate molecules and thus resulting in the weak adsorption strengths between electrocatalysts and CO₂/intermediates.^6,46 On the other hand, the catalyst supports that possess smooth surfaces and lack porous structures would have low adsorption capacities for CO₂/intermediates, which impedes the prompt supply of reaction substrate molecules to active sites and therefore greatly limits the resulting electrocatalytic activities.^45–47

Consequently, the electronic structures of metal sites and the geometric structures of supports are the two key factors in determining the CO₂RR performance of electrocatalysts, and the electrocatalytic activities can be therefore strengthened form these two aspects: electronic structural modulation of active sites and geometric structural modulation of supports. Electronic structural modulation commonly utilizes foreign active species to disturb the electronic configurations of host metal species.^48–51 The resulting overlap and distortion of electron densities can change the binding strengths between electrocatalysts and CO₂/intermediates.⁶ Accordingly, the precise electronic structural modulation enables electrocatalysts to obtain optimized adsorption strengths for CO₂/intermediates, thereby capable of improving the catalytic activities and selectivities. Moreover, the geometric structural modulation usually optimizes the porous structures and morphologies of catalyst supports, which allows them not only to adsorb a large amount of CO₂/intermediates but also to expose as many active sites as possible.^52–55 In addition, the size effects of the geometric structures of supports can further optimize the catalytic rates.^24,56,57 This is why electronic and geometric structural modulations are the two effective strategies to synthesize efficient CO₂RR electrocatalysts.

Many reviews regarding the CO₂RR have been published according to different emphases and topics to date, such as fundamentals and industrialization of the CO₂RR,⁵⁸ CO₂RR pathways,⁵⁹ single-atom catalysts,⁶⁰ copper-based catalysts,⁶¹in situ dynamic studies,⁶² CO₂RR selectivity,⁶³ and catalyst interface engineering.⁶⁴ However, an overview of electronic and geometric structural modulations in terms of CO₂RR is lacking. In this sense, it is timely to make a comprehensive and systematic summary regarding the electronic and geometric structural modulations toward the CO₂RR. Because the electronic and geometric structural modulation can regulate the electrocatalytic performances from microscopic perspectives, it is convenient for researchers to explore the structure–activity relationships and reveal the underlying reaction mechanisms. Moreover, the electronic and geometric structural modulations are applicable to various types and scales of CO₂RR electrocatalysts, and this broad application capability can facilitate the development of catalyst diversity. Therefore, in view of the importance of electronic and geometric structural modulations for the enhancement of catalyst performance as well as the lack of systematic reviews about these two strategies, we now summarize the recent progress of CO₂RR electrocatalysts whose electronic and geometric structures have been modulated.

In this review, the recently reported electronic and geometric structural modulations toward the improvement in electrocatalytic CO₂RR performances are comprehensively summarized. Scheme 1 illustrates a variety of paths that can be used to optimize CO₂RR behaviors via the two strategies. Moreover, we combine physicochemical characterization with theoretical calculations to probe the mechanisms of action for the effects of electronic and geometric structural modulations on electrocatalyst performance. Finally, an outlook for the future development of CO₂RR electrocatalysts is presented in light of these two modulation strategies. We hope that this review proves to be stimulating and provides inspiration for researchers to develop high-performance electrocatalysts toward the CO₂RR and other related reactions.

Scheme 1 Strategies of electronic and geometric structural modulations.

2. Electronic structural modulation

According to the Sabatier principle, electrocatalysts with excellent CO₂RR performance should have optimum adsorption strengths between their active sites and CO₂/intermediates.⁴⁴ It is found that the electronic structures of active sites strongly determine the adsorption states of CO₂/intermediates. Therefore, modulating the electronic structures of active sties can optimize the adsorption strengths of the reaction substrates and intermediates.^6,44,46 Because of the difference in electronegativity among various elements, atoms with higher electronegativity tend to attract electrons of those with lower electronegativity, resulting in the delocalization of valence electrons. Thus, introducing a second metal species into the host metal can effectively modulate the electronic structure of the latter.^65–67 More importantly, the electronegativity difference between the second metal species and the host can regulate the density of states in the vicinity of the Fermi level, which is conducive to enhancing the activity of electrons in the outer orbitals of active sites.^68–70 Therefore, utilizing the second species to precisely modulate the electronic structure of the host enables electrocatalysts to obtain the optimum adsorption strengths for CO₂/intermediates, thereby achieving high activities and selectivities.^48,50 Furthermore, the electronic structural modulation can break the linear scale relationship (i.e., the adsorption energy of one intermediate has a linear scaling relationship with that of its subsequent intermediate on the same active site), contributing to the diversity of products.^44,46Table 1 summarizes the performances of typical CO₂RR electrocatalysts whose electronic structures were modulated. We now give detailed descriptions of the electronic structural modulations, which are arranged in the order of s-, p-, d-, and f-blocks.

Table 1 Summary of typical CO₂RR electrocatalysts whose electronic structures were modulated

	Product	Electrocatalyst	Reactor	Electrolyte	FE^a (%)	j (mA cm^−2)	E (V vs. RHE)	Ref.
a Faradaic efficiency (FE). b Partial current density of products. c Applied potential at the highest FE, versus reversible hydrogen electrode (RHE, noting that all potentials in this review have been converted to the RHE unless otherwise specified).
s-Block	Formate	s-SnLi	Flow cell	1.0 M KOH	92	1000	–1.2	72
	C2+	Mg–Cu	Flow cell	1.0 M KOH	80	650	–0.69	73
p-Block	Formate	Basal-oriented Bi NSs	Flow cell	1.0 M KOH	98	90	–0.68	75
	Formate	In-SAs/NC	H-type cell	0.5 M KHCO3	96	8.87	–0.65	77
	Formate	Bi–SnO/Cu form	H-type cell	0.1 M KHCO3	93	12	–1.7	80
	Formate	2.5% Sn-doped Bi2O3 NSs	H-type cell	0.5 M KHCO3	93.4	24.3	–0.97	81
	Formate	BOC NSs	H-type cell	0.5 M KHCO3	98.3	8	–0.9	82
	Formate	Bi–Sn/CF	H-type cell	0.5 M KHCO3	96	58	–1.1	83
	Formate	Sn–Bi interface	H-type cell	0.5 M KHCO3	96.4	40	–0.84	84
	Formate	BixSn1−x	H-type cell	0.1 M KHCO3	78	8	–1.1	85
	Formate	Bi–Sn	H-type cell	0.1 M KHCO3	93.9	9.3	–1.0	86
	Formate	SnIn-3	Flow cell	1.0 M KOH	94	236	–0.98	87
	Formate	BiIn5-Y@C	H-type cell	0.5 M KHCO3	97.5	13.5	–0.86	88
d-Block	CO	Ni–N–C	Flow cell	1.0 M KOH	91	726	–1.18	8
	CO	Fe–N/P–C	H-type cell	0.5 M KHCO3	98	2	–0.45	10
	CO	NiPACN-low	H-type cell	0.1 M KHCO3	99	8	–0.8	21
	CO	Fe3C/Fe1N4	Flow cell	1.0 M KHCO3	88	10	–0.75	39
	Formate	Pb1Cu	Flow cell	0.5 M KHCO3	92	1000	–1.0	48
	CO	Au3Cu	H-type cell	0.1 M KHCO3	98	12.5	–0.7	49
	CO	CoCp2@MOF-545-Co	H-type cell	0.5 M KHCO3	97	15	–0.7	54
	CO	Zn–Cu (5 s)	Flow cell	1.0 M KOH	84	50	–0.54	65
	Formate	ZnIn2S4	Flow cell	1.0 M KHCO3	99.3	300	–1.2	66
	CO	Cu2O/CuO@Ni	H-type cell	0.5 M KHCO3	95	30	–1.2	68
	Formate	Ag–In–S	Flow cell	1.0 M KOH	94	560	–0.95	70
	CO	A–Ni–NSG	H-type cell	0.5 M KHCO3	97	36.5	–0.5	92
	CO	Ni–N–MEGO	H-type cell	0.5 M KHCO3	92.1	53.6	–0.7	94
	CO	Fe^3+–N–C	Flow cell	0.5 M KHCO3	95	94	–0.45	96
	CH4	Cu2O@CuHHTP	H-type cell	0.1 M KCl/0.1 M KHCO3	73	10.8	–1.4	98
	C2H4	KB@Cu3(HITP)2	Flow cell	1.0 M KOH	51	305	–0.93	99
	CO	5 nm Ag/C	H-type cell	0.5 M KHCO3	84.4	4	–0.75	105
	CO	3D AgAu	Flow cell	1.0 M KHCO3	90	280	–1.05	107
	CH4	t-Cu2O	H-type cell	0.1 M KHCO3	71	12	–1.6	110
	C2+	Ag65–Cu35 LNS-100	H-type cell	0.1 M KHCO3	72	3.6	–1.2	112
	C2+	CuNi-2	Flow cell	1.0 M KOH	62	227	–0.88	113
	C2–4	a-CuTi@Cu	H-type cell	0.1 M KHCO3	49	1.2	–0.8	114
f-Block	CO	CBNNiGd-700	Flow cell	1.0 M KOH	97	308	–0.91	116

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2.1 s-Block metals

s-Block metals, including alkali metals and alkaline earth metals, are a class of metals in which the last electron is filled into the s orbitals. In addition, they are abundant in the Earth's crust, but all exist in the oxidized states (specifically, cations). Because the dissociation energies of electrons in the outermost s orbitals of their metallic states are quite small, the s-block metals tend to lose these electrons.⁷¹ Fortunately, this electron-donating ability gives them the property of modulating electronic structures of other metal species. For example, Yan et al. developed an electrochemical surface lithiation strategy to modulate the electronic structure of a Sn electrode (Fig. 1a).⁷² By combining spectral characterization with theoretical calculations, they found that the Li intercalation induced the strains of surface Sn lattices and the localization of negative charges of Sn atoms. As shown in Fig. 1b, the photon energy of the Sn–Li catalyst was decreased, indicating the electron transfer from Li to Sn. The increased local electron density of the Sn–Li catalyst resulted in the stronger adsorption strength for HCOO* (the key intermediate of formate/formic acid) compared to the pure Sn catalyst (Fig. 1c), signifying the enhanced intrinsically catalytic activity of Li-modified Sn atoms. Thus, the Sn–Li catalyst displayed a high formate selectivity of 92% and a high partial current density of 1.0 A cm⁻² at a potential of –1.2 V in a flow cell. Xie et al. successfully modified a Cu-based catalyst with Mg. The magnesium ions enhanced the electropositivity of Cu through the interaction with O atoms, significantly increasing the electrocatalytic CO₂RR activity (Fig. 1d).⁷³ Compared with the unmodified catalyst, the Mg-modulated catalyst presented an excellent C₂₊ faradaic efficiency, and its C₂₊ product generation rate was increased by more than 5 times at –0.7 V (Fig. 1e). In addition, Xu et al. found that Cu catalysts decorated with alkaline earth metal oxide could improve the FE of CO₂-to-ethanol by selective hydrogenation.⁷⁴ In particular, the FE can reach 61% after BaO modification.

Fig. 1 (a) Electrochemical lithiation preparation, localization of negative charges, and lattice strains of s-SnLi. (b) Dependence of dμ(E)/dE on photon energy for s-SnLi and Sn nanoparticles, alongside SnO₂ and Sn powders. (c) Gibbs free energy of conversions of CO₂ to CO and HCOOH on s-SnLi and Sn.⁷² Reproduced with permission from Wiley-VCH Verlag GmbH & Co. KGaA, copyright 2021. (d) A reaction mechanism of CO₂ to C₂₊ products on the surface of Mg–Cu. (e) Formation rates of C₂₊ products at different potentials for Mg–Cu and Cu.⁷³ Reproduced with permission from Wiley-VCH Verlag GmbH & Co. KGaA, copyright 2022.

At present, there are very few reports on the use of s-block metals to modulate the electronic structures of host metal atoms. The electrons in the outermost s orbitals of their metallic states are extremely active, making it difficult for the s-block metals to be incorporated into other metal species in a stable state. Nevertheless, the strong electron-donating property of s-block metals makes them possess great development space in the field of electronic structural modulation of metal species.

2.2 p-Block metals

p-Block metals with the last electron located in the p orbitals are now widely used for the electrocatalytic CO₂RR, such as Sn, In, Bi, and Pb, because both p-block metals and their oxides exhibit relatively high selectivities for formate/formic acid.^75–78 Mechanistic studies have shown that the s and p orbitals of the outer layers of p-block metals can be hybridized with the 2p and 2s orbitals of oxygen, which makes p-block metals present the characteristic oxophilicity. Therefore, the p-block metals tend to adsorb CO₂ molecules in the form of oxygen coordination, thus favoring the generation of oxygen-bond intermediates as exemplified by *OOCH.^79,80 The electronic modulation of p-block metal species can further enhance their selectivities for formate/formic acid. The favorable orbital interactions between two metals (host and guest) can promote the upshift of density of states for host metal active sites, which is beneficial to reducing the bonding states and increasing the antibonding states and thereby enhancing the adsorption of active sites to CO₂/intermediates.^81,82 For example, Wen et al. used Bi nanoparticles to modify Sn nanosheets for highly selective CO₂RR toward formate (Fig. 2a).⁸³ The incorporation of Bi not only reduced the local electron density of Sn but also pulled the p and d orbitals of Sn atoms upward from the Fermi level (Fig. 2b and c). Hence, this interaction allowed the electron density of O atoms with the high electronegativity to easily enter the p and d orbitals of Sn atoms, which in turn boosted the bonding of Sn atoms with CO₂ and HCOO* and improved the selectivity of formate production. Ren et al. employed in situ electrodeposition to construct Sn–Bi bimetallic interface-rich materials (Fig. 2d).⁸⁴ The abundant interfaces between Sn and Bi were highly favorable for the flow of electron density from Sn to Bi. Furthermore, the strong electrostatic attraction of Bi to Sn successfully induced a suitable upshift of the center of the outermost p orbitals of Sn, which not only weakened the Sn–C hybridization of *COOH but also promoted the Sn–O hybridization of HCOO* (Fig. 2e). Accordingly, the Sn–Bi catalyst achieved an improved formate partial current density of 140 mA cm⁻². Tang et al. adopted a liquid-phase ultrasonication technique to synthesize a Bi–Sn nano-alloy.⁸⁵ Benefiting from the close contact between Bi and Sn, the highly electronegative Bi atoms drove the p and d orbitals of Sn atoms to undergo an upshift out of the Fermi level. This effect promoted the adsorption of active sites to reactants as well as the electron transfer between the two, thus enabling the catalyst to exhibit the enhanced performance. Wu et al. employed an aerogel method to prepare a Sn@Bi bimetallic catalyst, which showed a better selectivity for formate than pure Bi and Sn.⁸⁶ Because the strong hybridization between the p orbitals of Sn and Bi atoms increased the density of states of Sn@Bi(012) planes across the Fermi level, the resulting highly active Sn@Bi(012) planes showed the markedly reduced energy barrier for the generation of formic acid.

Fig. 2 (a) Schematic diagram of Bi–Sn formed on a porous carbon matrix and an interface obtained by the deposition of Bi nanoparticles on Sn nanosheets. PDOS (projected density of states) of (b) p orbital and (c) d orbital of Sn atoms on the surfaces of Bi–Sn and Sn without HCOO*.⁸³ Reproduced with permission from Wiley-VCH Verlag GmbH & Co. KGaA, copyright 2018. (d) High-resolution transmission electron microscopy (HRTEM) image of the nano-crumples. (e) PDOS of the Sn 5p orbitals of alloy Sn, interface Sn, and pure Sn, and weighted band centers of the models without adsorbate, a scheme (right) for PDOS overlapping areas of the s, p, and d orbitals of Sn on the Sn–Bi bimetallic and Sn–Bi alloy interfaces with the C 2p orbitals of *COOH.⁸⁴ Reproduced with permission from Springer Nature, copyright 2022. (f) Bi–SnO(001) surfaces with adsorbed *OOCH.⁸⁰ Reproduced with permission from American Chemical Society, copyright 2019. (g) Density of states of *COOH on In-doped Bi₂O₃(010).⁸⁹ Reproduced with permission from Elsevier, copyright 2022.

p-Block metal oxides are also catalytically active for the CO₂RR. However, the reduction potentials required to generate formate/formic acid are far more negative than the standard potentials for the p-block metal oxides, leading to their poor electrochemical stabilities.⁷⁹ On the other hand, modulating the electronic structures of p-block metal species can not only stabilize the valence states of p-block metal oxides in the operating environment but also further optimize their adsorption strengths for reactants. For instance, An et al. introduced metallic Bi into SnO nanosheets by using a hydrothermal method to enhance the electrocatalytic activity.⁸⁰ They found that the doped Bi element successfully stabilized the divalent Sn (Sn²⁺), preventing it from being reduced to metallic Sn (Sn⁰) in the electrocatalytic CO₂RR process. In addition, the 6s orbital of Bi was hybridized with the O 2s and 2p orbitals (Fig. 2f). The orbital hybridization strengthened the adsorption capability of SnO surfaces toward CO₂ and HCOO*, which thereby promoted the formation of formic acid. Wu et al. also found that Sn–Bi bimetallic catalysts could lower the energy barrier for *OOCH and promote the production of HCOOH.⁸⁷ Wang et al. employed a wet-chemical method to prepare Sn–In alloy nanoparticles, consisting of an InSn₄ core and an In-doped SnO₂ shell.⁸⁸ The element indium in the SnO₂ shell led to the generation of O vacancies, which not only stabilized the structure of SnO₂ but also reduced the formation energy of HCOO* on the surface of SnO₂. Moreover, the metallic InSn₄ core inside the nanoparticle have the material high electrical conductivity. Consequently, the unique core–shell structure enabled the Sn–In alloy to obtain a current density of 236 mA cm⁻² at –0.98 V and a formate selectivity of 94% in a flow cell. Guan et al. utilized In to improve the electrocatalytic stability and activity of Bi₂O₃.⁸⁹ The electrochemical performance and crystal phase of the In–Bi₂O₃ catalyst did not change during the 15 hour potentiostatic measurement, indicating its excellent stability. Moreover, the hybridization of the 5s and 5p orbitals of In with the 2s and 2p orbitals of HCOO* promoted the formation of strong binding states between In and HCOO*, which significantly reduced the free energy required for the generation of HCOO* and hence boosted the generation of formic acid (Fig. 2g). According to Liu et al., the S-modulated copper nanosheet also contributed to improved formate production from CO₂, maintaining a selectivity of 89.8% at an ultrahigh formate current density of 404.1 mA cm⁻².⁹⁰

2.3 d-Block metals

d-Block metals (i.e., transition metals) are a class of metals in which the last electron is filled into the d orbitals. They have relatively more valence electrons, allowing them to possess a variety of stable electron configurations.⁹¹ Because the outermost d orbitals of d-block metals are easily hybridized with the C 2p orbitals, they tend to adsorb CO₂ molecules in the form of carbon coordination during the electrocatalytic CO₂RR processes.⁹² Therefore, the d-block metal active sites are conducive to converting CO₂ into the *COOH intermediate, which is subsequently transformed into CO after gaining an additional electron and proton.^93–95 If the adsorption strength of active sites to CO is relatively weak, CO will easily desorb from the active sites and become the main reduction product.^96,97 On the other hand, if the active sites stably adsorb CO, then CO can continue to undergo the proton-coupled electron transfer (PCET) reaction on the active sites and be converted into some deep-reduction products as exemplified by methane, ethylene, ethanol, etc.^98–100

The electronic structures of transition metals are crucial for their electrocatalytic CO₂RR performances. For example, the metallic states of the first-row transition metals (e.g. Fe, Co, Ni, and Zn) have high activation energy barriers for CO₂ molecules, enabling them to be more inclined to activate protons.^101,102 Especially for the metallic Zn, its fully occupied 3d orbital hinders electron transfer and reduces its catalytic activity for the CO₂RR. Interestingly, converting the metallic Zn to the low-valence Zn^δ+ (0 < δ < 2) could significantly enhance its CO₂RR performance, because unsaturated Zn sites had more spaces to couple CO₂/intermediates.¹⁰³ Density functional theory (DFT) calculation results demonstrated that Zn in the low-valence state could greatly reduce the reaction energy barrier of *COOH (Fig. 3a and b). In addition, perturbing the originally stable electronic configurations of Zn atoms resulted in enhancing the activity of electrons in their 3d orbitals, which in turn allowed more electrons to participate in the stabilization of *COOH (Fig. 3c). Accordingly, the low-valence Zn sites showed a higher CO selectivity than the saturated Zn sites in terms of the CO₂RR. Yang et al. found that electrons from the 3d orbitals of oxidized Ni(I) could spontaneously delocalize and enter the C 2p orbitals of CO₂ molecules, which greatly boosted the adsorption of CO₂ and intermediate species at the electrocatalyst surfaces.⁹² The CO selectivity of monovalent Ni(I) active sites could reach 97%, while that of metallic Ni was only about 75% (Fig. 3d and e). A negative shift in the valence band edge and a sharp decrease in the electron density of Ni 3d orbitals indicate the transfer of electrons from 3dx²−y² of Ni(I) to 2π_u of C, as exhibited in Fig. 3f and g, after the formation of CO₂^δ− species on the surface of the catalyst A–Ni–NG.⁹² Although the metallic states of the first-row transition metals have relatively poor catalytic activities toward the CO₂RR, modulating their valence states enables them to catalyze the reduction of CO₂ into CO efficiently because the electrons of unfilled d orbitals of the low-valence metal sites are quite active. These d orbitals can easily hybridize the electronic orbitals of intermediates and thereby stabilize the species.

Fig. 3 (a) Gibbs free energy for the conversion of CO₂ to CO based on the graphitic Zn-N₃-V, Zn-N₃ and Zn-N₄ models. (b) Comparison of active sites between Zn-N₃ and Zn-N₄. (c) PDOS of the Zn-N₃-V, Zn-N₃, and Zn-N₄ models before and after the *COOH interaction.¹⁰³ Reproduced with permission from Wiley-VCH Verlag GmbH & Co. KGaA, copyright 2021. (d) X-ray photoelectron spectroscopy (XPS) high-resolution Ni 2p spectra. (e) Faradaic efficiency of CO at various applied potentials. (f) Valence band spectra of A–Ni–NG before (black) and after (red) the CO₂ exposure, and after the desorption of CO₂ by thermal processing in vacuum for twenty minutes at 500 °C (dark blue). (g) Activation routes of CO₂ on the Ni(I) site.⁹² With permission from Springer Nature, copyright 2018.

Noble metals in the second and third rows, such as Pd, Ag, and Au, also tend to generate CO because of their small CO desorption energies.^104,105 Nevertheless, the slow activation rates of CO₂ molecules as well as the weak adsorptions of *COOH on these metals require additional modulations toward their electronic structures to ensure an accepted level of activity and selectivity. As such, Zhu et al. adopted a self-assembly strategy to prepare Pd–Au bimetallic nanowires comprising the Pd–Au core and the pure Pd shell.¹⁰⁶ The doping of Au atoms into Pd lattices did change the local electronic properties and the d-band centers of Pd atoms, which greatly lowered the activation energy barrier of CO₂ molecules. Moreover, the synergistic effect between Au and Pd evidently promoted the adsorption of *COOH on the Pd/PdAu(110) facet. Thus, compared with the pure Pd nanoparticles, the electrocatalytic activity and selectivity of Au-modified Pd nanowires were significantly enhanced. Ozden et al. utilized Au adparticles to modulate the lattice and electronic structures of Ag, which allowed the electrocatalyst to efficiently convert CO₂ to CO at low overpotentials.¹⁰⁷ Since the strongly electronegative Au drove the electrons in the Ag 3d orbitals to move to the high energy levels outside the nuclei, the resulting highly active Ag sites significantly reduced the free energy required to form *COOH. In addition, the two adjacent sites of Au and Ag could simultaneously adsorb different atoms in one *COOH, which significantly enhanced the adsorption strength of *COOH at the electrocatalyst surface. The above studies have demonstrated that the electronic structural modulation of noble metals is able to decrease the activation energy barrier of CO₂ molecules through strengthening the activity of outer electrons from host metals. Moreover, the synergistic adsorption of dual metals toward the key intermediate can greatly promote the efficient conversion of *COOH to CO.

Since adsorption strengths to CO by the first-row transition metals such as Zn and Ni or noble metals such as Ag and Au are weak, it is easy for CO to desorb from active sites. In contrast, Cu has a moderate adsorption energy for CO, which enables this d-block element to continue transforming CO into deep-reduction products.^61,100,108 However, because the low activity of metallic Cu often results in small current densities, alongside poor product selectivities, it is necessary to modify the electronic configuration of Cu.¹⁰⁹ Modulating the electronic structures of Cu species can not only optimize the adsorption of CO at Cu-related sites, promoting the CO protonation or the CO–CO coupling, but also enhance the binding strengths of these sites toward key intermediates in terms of a certain deep-reduction product, thus achieving high selectivity. For example, Cu₂O with monovalent Cu(I) showed high methane selectivity in the CO₂RR, because it could readily protonate CO to CHO* (the key intermediate for the production of methane). At the same time, Cu₂O also had high adsorption strengths for other intermediates in the methane pathway, such as CH₂O* and OCH₃*, thereby avoiding the formation of other by-products.⁹⁸ Yi et al. used a facile electroreduction process to synthesize Cu₂O single-type sites, which showed an enhanced methane selectivity of 73% at –1.4 V.⁹⁸ The theoretical calculation results demonstrated that the energy barrier for the protonation of CO to CHO* on the Cu₂O(111) facet was only 0.43 eV. In addition, the PCET processes required the conversion of CHO* to methane to be exothermic, which greatly promoted the formation of methane. Deng et al. prepared Cu₂O microparticles using an electrochemical reconstitution method, and they systematically studied the free energy changes (ΔG) of CO to CHO* and CO to OCCOH* on the Cu₂O surfaces.¹¹⁰ Notably, the ΔG_CO→CHO* values were much smaller than ΔG_CO→OCCOH* on Cu₂O with different facets, reflecting the high affinity of Cu₂O for CHO*. Accordingly, the Cu₂O surfaces with different facets all showed the high selectivities to methane. When other valence Cu species (such as CuO) were present in the Cu₂O system, the selectivity of methane was significantly reduced.¹¹¹ According to the above works, it can be found that the high selectivity of Cu₂O for methane is not related to its exposed facets, but benefits from the unique electronic structure of its Cu species.

Employing a second metal to enhance the local electron density of metallic Cu can improve the selectivity for multi-carbon products, on the grounds that the high electron density is beneficial to the adsorption of a large amount of CO at the Cu sites and thus promoting the CO–CO coupling and the subsequent protonation. Furthermore, the optimization of density of states of Cu d orbitals near the Fermi level by the foreign metals can strengthen the activity of electrons in the d orbitals, thereby reducing the reaction energy barriers. For example, Ma et al. utilized confined growth of Cu on Ag nanocubes to synthesize Janus nanostructures of Ag–Cu with (100) facets (Fig. 4a).¹¹² Because the intimate electronic interaction between Cu and Ag induced the upshift of the centers of Cu 3d orbitals toward the Fermi level (Fig. 4b), the surface Cu sites exhibited a high electroactivity for the adsorption of OCCO*. At the same time, the Ag 4d orbitals also presented this upshift, which facilitated the transfer of electrons from silver to copper and further to the adsorbates. In this way, the Ag–Cu catalyst displayed an enhanced C₂₊ selectivity of 72% at –1.4 V (Fig. 4c). Zhang et al. developed a coordination enabled galvanic replacement approach to evenly distribute atomic Ni clusters over Cu surfaces that were rich in defects (Fig. 4d).¹¹³ The appearance of Ni–Cu bridges allowed Cu atoms to acquire extra electrons from the adjacent Ni atoms, which increased the local electron densities of Cu atoms. In addition, the intrinsically catalytic activity of Cu atoms was significantly enhanced, identified by the increased adsorption strength of CO and the decreased energy barrier of the CO–CO coupling on the Cu atoms (Fig. 4e). Thus, the highly active electrocatalyst CuNi-2 exhibited a seven-fold increase in the selectivity for C₂₊ products compared to pristine Cu (Fig. 4f). Moreover, a Cu–Ti alloy was prepared through an etching dealloying process and used for electrocatalytic CO₂RR toward multi-carbon (C_2–4) liquid fuels.¹¹⁴ As shown in Fig. 4g, the subsurface Ti atoms with the relatively low electron affinity could donate their electrons to the surface Cu atoms, and the resulting Cu active sites with high electron densities were able to both facilitate the adsorption of CO and reduce the activation energy barriers of dimerization and trimerization of CO (Fig. 4h and i). As a consequence, the Cu–Ti alloy yielded ethanol, acetone, and n-butanol as its major products. Different types of metals can be utilized to optimize the electronic structure of Cu to improve the catalytic selectivities for multi-carbon products. The precise modulation of electronic structures can efficiently tune the intermediate coupling and protonation processes, thereby expanding the product diversity of electrocatalytic CO₂RRs.

Fig. 4 (a) TEM and energy-dispersive X-ray spectroscopy (EDX) elemental mapping images of Ag₆₅–Cu₃₅. (b) PDOS comparison of Cu-3d. (c) FE of major CO₂RR products formed by Ag₆₅–Cu₃₅ JNS-100.¹¹² Reproduced with permission from Wiley-VCH Verlag GmbH & Co. KGaA, copyright 2022. (d) EDX elemental mapping images of Ni-CuNW. (e) Free energy for CO₂ activation by Ni-decorated Cu surfaces and pristine Cu(111). (f) FE comparison of C₂₊ by Ni-CuNW and CuNW.¹¹³ Reproduced with permission from Elsevier, copyright 2021. (g) Charge-density distributions of Cu_vac–Ti and Cu. Free energy profiles of (h) dimerization and (i) trimerization of *CO at the Cu_vac–Ti and Cu sites.¹¹⁴ Reproduced with permission from Wiley-VCH Verlag GmbH & Co. KGaA, copyright 2021.

2.4 f-Block metals

f-Block lanthanoid metals with the last electron being filled into the f orbitals possess a strong lanthanide contraction effect due to their loose 4f and unfilled 5d orbitals, giving them the capability to modulate the local electronic structures of other metal species.¹¹⁵ However, how to coordinately combine lanthanoids with other metals is a huge challenge due to their relatively large atomic radii. Fortunately, the Xu group successfully synthesized a Ni–Gd–N ternary doped carbon black catalyst through a two-step method of self-assembly and pyrolysis.¹¹⁶ Uniformly dispersed Gd atoms modulated the local electronic structure of Ni(I) active sites exposed outside the carbon layers and tailored the size of Ni nanoparticles wrapped inside the carbon matrix (Fig. 5a). Specifically, the Gd atoms drove the electrons in the outermost Ni 3d orbitals to higher energy levels because of their remarkable electron-withdrawing capability, thereby enhancing the electronic activity of Ni(I) active sites (Fig. 5b). Moreover, the doping of Gd atoms into the lattices of Ni nanoparticles reduced the particle size of the latter, which was beneficial to inhibiting the tendency of the hydrogen evolution reaction (HER) for the Ni nanoparticles. The incorporation of Gd modified the adsorption energy of *COOH at the Ni(I) active sites from –1.2 to –1.6 eV, indicating the enhanced adsorption strength of this key intermediate (Fig. 5c). Thus, the Gd-modified Ni-based electrocatalyst exhibited a high CO selectivity of 97% along with a high current density of 308 mA cm⁻² at –0.91 V (Fig. 5d). After that, Feng et al. doped Gd atoms into copper catalysts to modulate the electronic structure of the latter and significantly enhanced the activity of CO₂-to-C₂₊ products.¹¹⁷ Zhao et al. reported that the oxophilic metal (La) was able to modulate the catalytic activity of Cu, improving the CH₄ selectivity (up to 64.5%), which was approximately 7.8 times higher than for Cu.¹¹⁸ Infrared spectroscopy and DFT theoretical calculations showed that the presence of La would lead to stronger *CH_xO adsorption and promote the cleavage of C–O for *CH₃O, thus increasing the proportion of CH₄ in the products.

Fig. 5 (a) Charge-density redistributions of CBNNiGd-700 (left) and CBNNi-700 (right). (b) Total density of states (TDOS) and PDOS of Ni 3d and (c) XPS high-resolution Ni 2p_3/2 spectra of CBNNi-700 (upper) and CBNNiGd-700 (lower). (d) Adsorption energies of *CO and *COOH for various electrocatalysts.¹¹⁶ Reproduced with permission from Wiley-VCH Verlag GmbH & Co. KGaA, copyright 2022.

Currently, utilizing the f-block metals to modulate the electron structures of other metal species is seldom reported. However, the unique f orbitals of lanthanoid metals can indeed give rise to the strong hybridization with outer orbitals of other metal species, allowing the f-block elements to easily modulate the electronic structures of the latter. Consequently, the f-block metals possess the broad development prospects in terms of electronic structure regulation.

From the above studies, the electronic structures of active sites of electrocatalysts play a crucial part in the CO₂RR performance. As mentioned earlier, for instance, the p-block metal species tend to convert CO₂ to formate/formic acid, noble metals selectively generate CO, and Cu is able to generate deep-reduction products. It can therefore be summarized that the modulation of electronic structures of catalytically active sites could be achieved by introducing a second metal species (‘guest’) into host metals. The incorporation and doping of guest metal species into the host would give rise to a series of changes in the electronic configurations of active sites as well as the properties of the outermost orbital electrons, which can strengthen the orbital hybridization between electrocatalysts and CO₂/intermediates and thereby improve their adsorption strengths. Since different atoms possess distinct electronic structures, certain combinations of atoms can lead to desired electronic structures, which means that atoms with appropriate electronic configurations can be selected and integrated via the precise modulation of electronic structures to achieve the required results of the CO₂RR. It is noteworthy that electronic structural modulation is a highly sophisticated means of regulation, involving a set of microscopic processes such as electron delocalization and orbital hybridization. Therefore, a deep understanding of the underlying mechanisms of electronic structural modulation is necessary to prepare better electrocatalysts for CO₂RRs.

3. Geometric structural modulation

In addition to electronic structures of metal species, geometric structures of catalyst supports can also exert considerable influence on CO₂RR behaviors.^45,56,57 The Cu catalyst possessing a cavity structure exhibited an high current density and an outstanding FE for the reduction of CO₂ to C₂₊ products as the cavity structure could increase the local concentration of CO.¹¹⁹ Nevertheless, because metals can be directly used as current collectors and catalytically active sites for the CO₂RR, they are prone to electrochemical corrosion and performance degradation.¹²⁰ Therefore, it is necessary to anchor metal species within geometric structures of corrosion-resistant supports.^121,122 In this way, catalyst supports can protect these metal species against agglomeration, migration, and loss during electrochemical reactions. Modulating the geometric structures of supports can not only increase pores and specific surface areas of electrocatalysts, providing increased attachment points for metal species, but also enhance the adsorption capacities for CO₂/intermediates, supplying sufficient reaction-related molecules to metal species.¹²³ Moreover, some unique geometric structures of catalyst supports can exert their size effects, facilitating the efficient transfer of electrolyte, CO₂/intermediate molecules, and products and thereby enhancing the electrocatalytic performance.⁵⁶ In this review, carbon supports are divided into three subcategories: synthetic carbon matrix, biomass matrix and coal matrix.

The synthetic carbon matrix provides a variety of options for electrocatalysts because of their controllable porous structures. For instance, Li et al. prepared a hollow porous urchin-like microparticle carbon support anchored with Ni atoms (Fig. 6a and b), denoted as Ni-NC(HPU).¹²⁴ The two-sided available surfaces and the protruding nanothorns of the carbon support greatly increased the number of catalytically active sites and electrochemical surface areas. In addition, the abundant mesoporous structures of the urchin-like microparticles and the rich voids between them could effectively promote the adsorption and diffusion of CO₂ molecules, thereby achieving efficient mass transfer. To verify the advantages of the urchin-like geometric structure, they also prepared other hollow porous catalysts with similar components but different geometric structures, including wire-like and sphere-like geometric structures. Electrochemical measurements showed that Ni-NC(HPU) had similar CO selectivity to the two control samples, but the current density of Ni-NC(HPU) was much higher (Fig. 6c). These results demonstrated that modulating the geometric structure of the catalyst support via its size effect can significantly enhance the electrocatalytic activity. Additionally, the geometric structure of the support also plays a major role in the electrochemical performance of metal-free catalysts.^125–127 Because modulating the geometric structures of supports can optimize the local properties of materials, the resulting favorable microenvironments in the supports become conducive to enhancing the electrocatalytic performance of nonmetallic active sites. Ni et al. prepared a cage-like porous carbon doped with fluorine (F-CPC) through the template approach (Fig. 6d).¹²⁸ The cage-like carbon support with abundant micropores and moderate mesopores facilitated the capture and diffusion of CO₂ molecules, conducive to the improvement in mass transfer. More importantly, these opening circular pores on the support surface could accumulate charges, thus enhancing the local electric field intensity within the porous domains. The locally enhanced electrostatic field in the catalyst support was able to accumulate surface-adsorbed K⁺ ions (Fig. 6e), which not only promoted the rapid stabilization of CO₂ molecules on the F sites but also reduced the energy barrier. And the current density of F-CPC was significantly higher than that of undoped catalysts (Fig. 6f). Therefore, F-CPC showed a considerable current density of 37.5 mA cm⁻² and a high CO selectivity of 88.3% at –1.0 V. In contrast, the CO selectivity of the control sample obtained by directly incorporating F into commercial carbon black could reach 90%, but its maximum current density was as low as 0.3 mA cm⁻².¹²⁹

Fig. 6 (a and b) Field-emission scanning electron microscopy (FESEM) images of Ni-NC(HPU). (c) CO partial current density of the electrocatalysts at various potentials.¹²⁴ Reproduced with permission from Wiley-VCH Verlag GmbH & Co. KGaA, copyright 2022. (d) TEM image of F-CPC. (e) Current-density distributions and surface K⁺-ion concentrations of F-CS and F-CPC. (f) Current-density distributions of F-CS and F-CPC in terms of the CO₂RR.¹²⁸ Reproduced with permission from American Chemical Society, copyright 2020. (g) SEM image of NDC-700. (h) N₂ adsorption–desorption isotherms and pore size distribution (inset) for NDC-700.¹³⁰ Reproduced with permission from Elsevier, copyright 2017. (i) SEM image of N3-850. (j) Pore size distribution of N3-850.¹³³ With permission from Elsevier, copyright 2023. (k) SEM image of BCF. (l) Chronoamperometric results of the catalysts at –0.9 V.¹³⁵ Reproduced with permission from Springer Nature, copyright 2021.

Aside from the synthetic carbon matrix, renewable biomass is a high-performance carbon source and support. As a natural support, biomass is often employed as a catalyst precursor due to its unique porous structure. Li et al. prepared nitrogen-doped biochar with high surface area and hierarchical porosity by the carbonization of wheat flour using KOH, achieving CO selectivity of 83.7% at –0.82 V (Fig. 6g and h).¹³⁰ In a similar way, Yao et al. regulated the carbonization conditions of Typha to obtain nanoporous nitrogen-doped carbon with a high surface area and electric conductivity, which presented high CO selectivity (90%) at −0.31 V.¹³¹ Moreover, a biomass-based electrocatalyst with a specific surface area of 1673.6 m² g⁻¹ was also prepared by using cedar as the carbon source, with the CO selectivity of 91% at low potentials.¹³² As a common biomass material, the carbon aerogel derived from cellulose exhibited a large specific surface area, and its pore size distribution was quite uniform, within the range between 1 and 3 nm (Fig. 6i and j), providing good transfer channels for CO₂ and its products.¹³³ Chen et al. mixed an N-rich silkworm cocoon with ZnCl₂ and then pyrolyzed them to acquire the biochar with abundant intrinsic defects.¹³⁴ Some of the N-containing groups in the biomass carbon matrix were removed during the pyrolysis process, resulting in vacancy defects and greatly improving the electrocatalytic CO₂RR behaviors. Using insect wings as precursors, Qi et al. synthesized a porous electrocatalyst with excellent electrochemical performance via pyrolysis processing at high temperatures (Fig. 6k).¹³⁵ As shown in Fig. 6l, the abundant ordered pores gave these biomass-based electrocatalysts with high current densities. According to the above works, modulating the geometric structures of catalyst supports, derived from either synthetic carbon matrix or biomass matrix, can effectively promote the adsorption and conversion of CO₂ molecules and therefore optimize the electrocatalytic CO₂RR performance from both kinetic and thermodynamic aspects.

The carbon supports mentioned above were all prepared by calcining organic precursors, which inevitably caused some heteroatoms to be wrapped deep inside the supports. These deeply buried heteroatoms not only fail to play a catalytic role but also limit the overall activities of electrocatalysts.⁹⁴ In view of this, the Xu group have focused on natural carbon materials and found that coal has great application potential as the high-quality carbon support.^136–139 The robust carbon frameworks of coal enable the activation and doping treatments to introduce porous structures and heteroatoms only on the surface layers of coal particles, which can fully expose active sites and effectively guarantee the stability of catalysts. In addition, modulating the porous structures of coal-based supports can intensify the electron/substrate interaction, which significantly improved the electrocatalytic activity. As such, the Xu group prepared N-doped porous coal catalysts using the solvent evaporation induced self-assembly approach.¹³⁶ It can be seen from Fig. 7a that the surface layers of coal were modified by activation and doping during the high-temperature processing whereas the internal structures of coal particles were dense and remained intact. In other words, the internal robust framework of coal was retained (Fig. 7b and c). As shown in Fig. 7d, the high electrochemical surface area of NPC-900 showed that a great number of microporous structures and nitrogen sites could provide abundant reaction sites for the CO₂RR. The elevated temperature-induced pore shrinkage effect resulted in the formation of a large amount of micropores on the surface layers of the coal support, and N atoms were firmly anchored in these micropores. The spatial confinement effect of micropores in the coal support made the N atoms in the pores generate strong polarization, which significantly promoted the charge transfer from N atoms to CO₂ molecules. Therefore, the coal-based electrocatalyst showed a small onset potential for the CO product (−0.16 V), and its maximum CO selectivity could reach 95% at −0.67 V. On the basis of this work, the Xu group co-anchored a transition metal Ni and a non-metal N to micropores of robust coal-based carbon support (CNNi-700) by the self-assembly strategy to further enhance the electrocatalytic CO₂RR performance.¹³⁷ The physicochemical characterization data revealed that the spatial confinement growth effect in the coal support gave rise to the immobilization of Ni–N active sites in the micropores, which could effectively inhibit the agglomeration of Ni species and increase the number of active sites. Additionally, these abundant microporous structures significantly elevated the CO₂ capacity of CNNi-700, which efficiently supplied CO₂ to the active sites, thus providing ample reaction substrates for the Ni–N active sites anchored within the micropores (Fig. 7e). Electrochemical tests showed that the coal support possessed a lower charge-transfer resistance than the organic precursor-derived carbon support, indicating the higher conductivity of the former (Fig. 7f). Thus, the highly efficient interaction between electron and substrate at the Ni–N active sites enabled CNNi-700 to maintain a CO selectivity of greater than 90% over a wide potential range, and its maximum CO selectivity and partial current density could reach 97% and 223 mA cm⁻², respectively, as shown in Fig. 7g. It is noteworthy that the electrocatalytic CO₂RR activity of CNNi-700 remained stable within a long-term potentiostatic measurement for 120 h, reflecting its outstanding stability and great potential for industrial application.

Fig. 7 (a) FESEM and (b and c) TEM images of NPC-900. (d) Dependence of charging current density differences on scan rates of the catalysts.¹³⁶ Reproduced with permission from Elsevier, copyright 2020. (e) CO₂ adsorption isotherms of the catalysts. (f) Nyquist plots of the catalysts. (g) Long-term stability test of CNNi-700 at −0.93 V in a flow cell.¹³⁷ Reproduced with permission from Elsevier, copyright 2021.

It can be summarized from the above-mentioned discussion that modulating the geometric structures of carbon supports derived from coal, biomass, and synthetic carbon matrixes is important for improving the adsorption capacities of electrocatalysts for substrates and promoting the size effect of support structures to facilitate the CO₂RR performance. The porous structures and heteroatom distributions of biomass or synthesized carbon supports prove to be ubiquitous and uniform. Despite the fact that they are capable of obtaining good catalytic activities, their soft structures can also lead to a large number of heteroatoms being trapped within the internal materials, unable to exert their intrinsically catalytic capabilities. In contrast, the geometric structures of coal-based supports are so dense that doping treatments can allow extrinsic heteroatoms to be only doped and distributed on the surfaces, which to a large extent exposes the catalytically active sites and enhances the catalyst utilization. It is worth mentioning that the robust carbon skeletons inside the coal-based supports can provide strong support and protection for active sites, not only enabling the electrocatalysts to achieve high selectivities and large current densities in wide potential windows but also significantly improving the long-term catalytic stabilities. Whether it is biomass or coal, the exploration of natural carbon materials as catalyst supports can decrease the usage of organic reagents and reduce the production cost of electrocatalysts, which also proves to be environmentally friendly and conducive to the industrialization of the CO₂RR. The representative CO₂RR electrocatalysts whose geometric structures were modulated are summarized in Table 2.

Table 2 Summary of representative CO₂RR electrocatalysts whose geometric structures were modulated

Matrix	Product	Electrocatalyst	Reactor	Electrolyte	FE (%)	j (mA cm^−2)	E (V vs. RHE)	Ref.
Synthetic carbon	CO	A–Ni@CMK	H-type cell	0.5 M KHCO3	95	51	−0.8	56
	CO	C–Au-500	H-type cell	0.5 M KHCO3	94	150	−0.35	120
	CO	Fe–N/CNT@GNR-2	H-type cell	0.1 M KHCO3	98	12	−0.76	121
	CO	Ni–NC(HPU)	H-type cell	0.5 M KHCO3	91	24.7	−0.8	124
	CO	F-CPC	H-type cell	0.5 M KHCO3	88.3	37.5	−1.0	128
	CO	NSHCF	H-type cell	0.1 M KHCO3	94	100	−0.7	140
Biomass	CO	CB-NGC-2	H-type cell	0.1 M KHCO3	91	6.5	−0.76	141
	CO	COF@MOF800-Co	H-type cell	0.5 M KHCO3	92.6	9	−0.8	142
	Formate	N–BiNSs	H-type cell	0.5 M KHCO3	92.25	30	−0.95	143
	CO	PPy@MOF-545-Co	H-type cell	0.1 M KHCO3	98	10	−0.8	144
	CO	AgNNs@Zn15	H-type cell	0.5 M KHCO3	91.05	6.5	−0.86	145
	CO	Fe–N–PC	H-type cell	0.5 M KHCO3	90	11.44	−0.49	146
	CO	FeSA–S/N–C-9	H-type cell	0.5 M KHCO3	96.3	4.9	−0.48	147
Coal	CO	NPC-900	H-type cell	0.5 M KHCO3	95	2.3	−0.67	136
	CO	CNNi-700	Flow cell	1.0 M KOH	97	223	−0.93	137

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4. Conclusions and outlook

The utilization of renewable energy for electrochemical conversions of CO₂ to high-value-added chemicals presents a highly promising research area for sustainable development. As the electronic structures of active sites and the geometric structures of supports are the two critical factors that influence the performance of CO₂RR electrocatalysts, this review summarizes the recent research progress of enhancing the catalytic activities and selectivities through electronic and geometric structural modulation strategies, and systematically analyzes the impacts of these two strategies on catalyst performance and structure–activity relationships. Despite the significant progress made regarding the electronic and geometric structural modulations of CO₂RR electrocatalysts in recent years, there are still many important areas of research that are primitive or unclear. For example, roles of lanthanoid metals in electrocatalytic behaviors are yet to be well understood; heteroatoms can hardly be doped precisely to the desired sites; there are still unsolved issues in the utilization of staple carbon sources from nature; the combined effects of electronic and geometrical modulations need to be deeply explored; and the complexities of electrochemical interfaces and electrocatalytic conversions make calculations and modelling quite challenging. Our suggestions can now be summarized as follows:

(1) Using f-block lanthanoid elements. The strong spin–orbit coupling effect induced by the unique electronic configurations of f-block lanthanoids can be utilized for enhancing the performance of CO₂RR electrocatalysts. Doping lanthanoid elements into the host metal is indeed an effective way to regulate the electronic configurations of the host active species, which is conducive to accelerating electron transfer and optimizing the adsorption strengths of reactants/intermediates, thereby improving product selectivities. In addition, the large atomic radii of lanthanoid metals can cause lattice defects in the host active species if the f-block elements are successfully doped into the host. The decrease in crystallinity of the host metal is therefore beneficial for exposing large quantities of low-coordination high-activity sites, helping to further enhance electrocatalytic activities.

(2) Precisely regulating the doping states of heteroatoms. The doping level of heteroatoms, regardless of whether they are metals or nonmetals, is an important parameter for modulating the electronic structures of active sites of CO₂RR electrocatalysts. Generally speaking, the more the doping level, the greater the change in the intrinsically electronic structures of active sites. However, as far as the catalytic performance is concerned, the doping level of heteroatoms is not necessarily the more the better. The optimal catalytic behaviors may only require a small but suitable quantity of heteroatoms to tune the electronic structures. Thus, how to precisely regulate the doping level and how to accurately dope specific heteroatoms into correct sites are very challenging problems that need to be solved urgently, because they directly affect the electrocatalytic performance.

(3) Exploiting natural carbon sources as catalyst supports. As mentioned earlier, geometric structures of catalyst supports play an indispensable role in the CO₂RR performance. Carbon supports derived from natural materials, particularly biomass and coal, are worthy of attention. The geometric structures of biomass, which are naturally formed and difficult to synthesize artificially, provide excellent channels for mass and charge transfers as well as favorable distribution space for catalytically active sites. The diverse geometric structures of biomass include but are not limited to vascular bundle structures, hierarchical pores, wrinkled rough textures, and loose spongy porous structures. These natural structures provide excellent platforms for electrocatalytic reactions without the need for complex and cumbersome artificial construction, which also lay an important foundation for further modification and optimization. In addition, coal, which is formed by long-term geological processes from biomass, possesses robust structures that make it an excellent candidate for addressing the stability problem of electrocatalysts working in electrochemical environments. Meanwhile, the microporous structures of coal can give rise to the spatial confinement effect that promotes electrocatalytic reactions, which has great scientific research values and exploration space. It is worth noting that biomass and coal, as widely available natural carbon materials, still have countless unique geometric structures that are yet to be discovered or utilized. As rich natural treasures, they urgently need to be explored by researchers.

(4) Combining electronic modulation with geometric modulation. The modulations of both electronic and geometric structures give rise to the performance enhancement of CO₂RR electrocatalysts. Thus, a synergistic combination of electronic and geometric structural modulations of electrocatalysts should pave promising avenues for promoting the CO₂RR performance, by optimizing the electrocatalytic CO₂RR processes through the geometric structural modulation of catalyst supports to provide abundant active sites and facilitate mass transfer, and also by optimizing the electronic structural modulation of active sites to improve the adsorption of CO₂/intermediates. To acquire a high-performance electrocatalyst that is both highly catalytically active and stable, there are still many research gaps that need to be filled. Indeed, interdisciplinary research in physics, chemistry, biology, materials science, and other related fields will undoubtedly be a promising approach to achieve the research goals.

(5) Pushing the limits for calculations and modelling. Finding suitable descriptors that can accurately reflect the structure–performance relationship in terms of electrocatalytic CO₂RR is key to high-throughput screening of potential catalysts. Due to the enormous computational workload, it is necessary to utilize artificial intelligence (AI) technologies as exemplified by machine learning. Of course, the basis is to establish complete databases of catalysts. Furthermore, calculation methods that can efficiently and accurately obtain electronic structures need to be further developed, together with advanced approaches to modelling electrochemical interfaces based on density functional theory molecular dynamics (DFTMD). In short, a truly deeper understanding of fundamentals and mechanisms of the electronic and geometric modulations for optimizing the CO₂RR requires continuous improvement and crossover between experimental, theoretical, and computational research studies.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This study was financially supported by the National Key Research and Development Program of China (2021YFB4001502), the National Natural Science Foundation of China (51702358), the Natural Science Foundation of Jiangsu Province (BK20170281), and the Fundamental Research Funds for the Central Universities (2019ZDPY02). L. X. holds the Jiangsu Specially-Appointed Professorship.

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