Homogeneous and heterogeneous molecular catalysts for electrochemical reduction of carbon dioxide

Carbon dioxide (CO2) is a greenhouse gas whose presence in the atmosphere significantly contributes to climate change. Developing sustainable, cost-effective pathways to convert CO2 into higher value chemicals is essential to curb its atmospheric presence. Electrochemical CO2 reduction to value-added chemicals using molecular catalysis currently attracts a lot of attention, since it provides an efficient and promising way to increase CO2 utilization. Introducing amino groups as substituents to molecular catalysts is a promising approach towards improving capture and reduction of CO2. This review explores recently developed state-of-the-art molecular catalysts with a focus on heterogeneous and homogeneous amine molecular catalysts for electroreduction of CO2. The relationship between the structural properties of the molecular catalysts and CO2 electroreduction will be highlighted in this review. We will also discuss recent advances in the heterogeneous field by examining different immobilization techniques and their relation with molecular structure and conductive effects.


Introduction
Carbon dioxide (CO 2 ), as a greenhouse gas, is a signicant contributor to climate change. The global average atmospheric CO 2 level in 2019 was 409.8 ppm, much higher than the previous highest concentration of 300 ppm, with levels projected to keep increasing unless immediate measures are taken. 1,2 These emission levels have raised serious environmental concerns and have translated to noticeable, aberrant meteorological changes.
Recent strategies that convert CO 2 into value-added materials using photochemically 3 or electrochemically 4-6 powered reduction reactions have shown promise in recent years. However, this task is challenging due to the high energy required (750 kJ mol À1 ) to break the C]O bond 7,8 and the molecule's stable linear geometry, which makes CO 2 reduction reactions (CO 2 RRs) sluggish and challenging. 3,9 Additionally, the electrocatalytic CO 2 reduction mechanism is a complex process that involves multiple proton-coupled electron transfer steps and may include several side-reactions and intermediates. [10][11][12][13][14] The rst step of many CO 2 RRs is the one-electron reduction of CO 2 to a CO 2 c À radical anion which has a more reactive, bent geometry (Table 1). 15,16 Although most CO 2 RRs describe twoelectron reduction to carbon monoxide (CO) and formaldehyde, products of multi-electron transformations such as methane, 17 methanol 18 and ethanol 19 are highly coveted. Table 1 shows the theoretical potentials required to form various multielectron reductions. Although the theoretical potentials required to form the target products shown in Table 1 appear relatively low, because the products formed are oen either thermodynamically similar or more stable than CO 2 , more negative potentials are required for practical applications to obtain reasonable reaction rates. 9 In order to facilitate CO 2 RR, the use of catalysts is essential and serves several purposes including lowering activation energy barriers, improving reaction rates, and increasing product selectivity. [20][21][22][23] Electroreduction of CO 2 RR can be completed using either homogeneous or heterogeneous catalysts. Although homogeneous catalysis have shown high selectivity, with near product unity for the production of CO and other reduction products, 18,[25][26][27][28][29] these systems are dependent on the solubility constraints of the catalysts and are limited by low current densities and instability. 30 On the other hand, heterogeneous

Electroreduction of carbon dioxide
Electrochemical capture and reduction of CO 2 60 has received extensive interest in the last decade because of the: (1) controllable nature of the technique (e.g. potential and temperature); (2) exibility between organic and aqueous media; (3) relative scalability of bench-side reaction setups to industrial application. 61 Typical electrochemical cells consist of a cathode, anode, electrolyte and a membrane (Fig. 1). CO 2 RR occurs at the cathode, while reciprocal oxidation or oxygen evolution reactions (OERs) occur at the anode. The cathode and the anode are separated by a membrane which maintains charge balance and separates the respective redox products. The electrolyte carries the charge between the electrodes and delivers dissolved CO 2 to the catalytically active surface.

Quantifying catalytic performance
Several factors are used to quantify catalytic performance. Selectivity is measured by the faradaic efficiency (FE), and the catalyst activity is determined by the current density (j) as a function of the electrode area. The current density can be used to describe either the total current density of all reduced products or the partial current density of one particular product. In the context of CO 2 RR, current density can be used to describe the rate of reaction. The robustness of the catalyst is calculated with the turnover number (TON) which is determined by dividing the mole of reduced product with the mole of catalyst. The turnover frequency (TOF, s À1 ) is dened as the mole of reduction product divided by the mole of active catalysts per unit of time.

Homogeneous amine molecular catalysts and electrochemical CO 2 RR
Homogeneous studies of amine-based molecular electrocatalysts have been identied their utility for CO 2 RR. Using Scheme 1 Carbamate formation using primary and secondary amines. meso-substituted amino groups on metallated porphyrins, we were able to achieve selective reduction of CO 2 to CO and methanol (Fig. 2a). 18 Comparing the cyclic voltammograms of Co-TPP and Co-TPP-NH 2 in the presence of CO 2 clearly highlights the importance of the amino group and its role in reducing CO 2 (Fig. 2b). The inuential presence of the cobalt center in CO 2 RR, can be seen in Fig. 2d. In this project, H 2 O was used as an extra proton source to facilitate the C-O bond cleavage (Fig. 2e). To further understand the electroactivity of the amino group, a comparison with nitro porphyrins (TPP-NO 2 ) shows a slightly better performance of the amino group (Fig. 2f).
Chapovetsky et al. 94 also reported a cobalt aminopyridine macrocycle with amine substituents selectively reducing CO 2 to CO. From electrochemical experiments, they found that the catalytic activity is strongly dependent on the number of secondary amines (Fig. 3). 95 Subsequent studies showed how those amine groups could act as hydrogen bond donors to enhance catalytic performance.
The identity of the electrode used has been found to have a large inuence on the catalytic activity of homogeneous amine solutions, with different electrodes such as glassy carbon, copper, and silver each eliciting their own distinctive response. 18,46,55,63,[96][97][98][99][100][101] Lue et al. 55 reported a systematic study on electrochemical CO 2 reduction with 30% (w/w) MEA on, Sn, Pb, Pd, Ag, Cu and Zn metal electrodes. Schmitt et al. 100 used in situ surface-enhanced Raman spectroscopy to study 3,5-diamino-1,2,4-triazole (DAT) exposed-Ag electrodes, nding that the amine treated electrode increased FE CO due to a weakening of the CO bonding strength.
Many studies of copper (Cu) electrodes have characterized their ability to reduce CO 2 to multi-carbon products, 14,33,55,87,102-110 whereas when exposed to molecular catalysts it is more common to see CO 2 selectively converted to CO, 57 formate, 111 and formic acid. 13,104 We have also investigated the ability of primary amines to selectively reduce CO 2 to CO using Cu electrodes (Fig. 4). 57 In these studies, ethylenediamine (EDA) proved to be the most effective absorbent for CO 2 capture and subsequent reduction to CO among MEA and decylamine (DCA), with a current density of À18 mA cm À2 , TON of 252 and a FE of 58% at À0.78 V vs. RHE. Compared to glassy carbon electrodes, the cathodic current was dramatically enhanced when Cu was used as a working electrode ( Fig. 4f and g).
Our recent studies on the electrochemical reduction of CO 2 in various fractions of MEA solutions at smooth and nanodendrite (ND) Cu, Ag and Au showed that a 0.05 M fraction of MEA exhibited the highest catalytic activity for each surface. 112 CO 2 electroreduction to HCOO À . The ND electrodes exhibited much higher current efficiencies for CO 2 to HCOO À conversion compared to the smooth metal electrodes, revealing the critical role of surface morphology in enhancing catalytic activity.

Heterogeneous amine molecular catalysts and electrochemical CO 2 RR
Heterogeneous electrocatalysts have benets over homogeneous electrocatalysts for CO 2 RR application due to the catalytically active site being either located directly on the electrode surface or the electrode itself. As a result, catalytic loading concentrations can be lower. Molecular catalysts can be   This journal is © The Royal Society of Chemistry 2020 RSC Adv., 2020, 10, 38013-38023 | 38015 attached to solid, conductive surfaces via covalent/non-covalent immobilization techniques 29,113,114 or using polymers and metal-organic frameworks. [115][116][117] This strategy offers higher stability and catalytic efficiency 56 with a greater potential of reaching the necessary current densities for industrial implementation. 118 Due to its simple preparations, one of the most popular immobilization techniques involves depositing conjugated organic ligands onto carbon surfaces which are stabilized by the non-covalent p-p interactions between the catalyst and solid surface. 17,56,119 The molecular catalysts can be also deposited on electrode surfaces through covalent bond. 120,121 Previous reports on CO 2 RR selectivity involved either the use of a metal electrode surface, where the electron-transfer efficiency was largely dependent on the material's conductivity, or the incorporation of small inactive molecules 39 on the surface of the metal electrode to maximize interaction between the electrode and the molecular catalysts. 56,61,[122][123][124][125] An example of this are electrograing techniques which produce a direct chemical bond between the catalyst and a solid substrate. 98,126 The direct connections that arise from these methods are believed to be the primary factor in increasing the reaction rate of CO 2 RR relative to hydrogen evolution reactions (HERs) and lowering overpotentials. 125,[127][128][129][130][131] Using this technique, immobilization of terpyridine onto glassy carbon electrodes has been previously reported. 66 Marianov et al. 121 have also successfully electrograed amino porphyrins via electro reduction of diazonium salt onto glassy carbon (Scheme 2). By introducing a conjugated linker between the porphyrin and the electrode, they proved that the Co I /Co II redox couple facilitates the CO 2 electroreduction process (Fig. 5a). With the covalently linked catalyst an increase to the current density (4.7 mA cm À2 ) was seen, compared to the unlinked catalysts (1.4 mA cm À2 ) (Fig. 5b). In addition to the covalent linkage facilitating electrode-to-catalyst charge transfer, the current density was also observed to be dependent on the catalyst loading concentration and the total active surface area ( Fig. 5b and c).
Gold (Au) has been also found to exhibit catalytic activity towards CO 2 RR. [132][133][134] Mikoshiba et al. 135 showed that imidazolium ions immobilized on Au electrodes suppress H 2 generation and accelerate CO 2 RR. In their study, imidazolium salts with small methylene units (IL-2, Fig. 7A) exhibited greater current densities compared to longer chained units with FEs up to 87% (Fig. 7B).
In another study, Au electrodes functionalized with 4-pyridinylethanemercaptan (PEM) thiols showed similar increases in product selectivity and catalytic activity (Fig. 8a). 136 The proposed mechanism for formate production shows the pyridine H-atom abstracted by reduction of the aqueous solution   and adsorbed onto the Au surface (Fig. 8a). HCO 2 is formed through electrophilic attack of CO 2 onto the adsorbed proton. The FE of the electroreduction products in this system were observed to be potential dependent. Fig. 8b-g shows the potential-dependent product distribution (formate, CO and H 2 ) of functionalized Au and bare Au surfaces.
A 2-fold increase in FE formate and a 3-fold increase in current density were achieved and attributed to enhancement of proton and electron transfers using Au foil (Fig. 8b and c). 137 This increase in current density is due to the amine's ability to make a complex with CO 2 near the Au surface. 138 Cystemine modied electrodes saw a 2-fold increase in both CO and H 2 production (Fig. 8d-f), while electrodes with 2-mercaptopropionic acid (MPA) ligands reported nearly 100% selectivity for H 2 (Fig. 8g).
In another study, it was found that immobilization of Au nanoparticles using N-heterocyclic carbenes facilitated electron transfer from Au to CO 2 (Fig. 9a). 139 The electrochemical reduction of CO 2 to CO catalysed by a Au-1,3-bis(2,4,6 trimethylphenyl)imidazol-2-ylidene nano particle (Au-Cb NP) was found to be greater than that of bare Au nanoparticles (Au NP). Oleylamine-capped Au NPs (Au-Oa NP) were rst loaded onto carbon black to make a Au-Oa NP/C mixture. 140 The active surface area for Au NP/C and Au-1,3-bis(2,4,6 trimethylphenyl) imidazol-2-ylidene nano particle (Au-Cb NP) electrode were evaluated using Pb underpotential deposition (upd). [141][142][143] The current density increased substantially (Fig. 9c and e) and the FE CO increased from 53% to 83% in when the Au nanoparticles were deposited onto CB (Fig. 9d). The kinetics of the CO 2 reduction were examined using Tafel analysis (Fig. 9f) which shows a decreasing slope from 138 mV dec À1 to 72 mV dec À1 .
Various strategies, such as morphologynanostructuring have been paired with these electrodes. 148,149 Hwang and co-workers 100 prepared three different types of Ag nanoparticles with different surface capping agents. These included oleylamine (OLA), having an amine functional group; oleic acid, having a carboxyl functional group; and dodecanethiol (DDT) with a thiol functional group. They discovered that the amine substituent was highly effective in enhancing the electrochemical reduction of CO 2 to CO with high selectivity (FE ¼ 94%) at low overpotentials (À0.75 V vs. RHE) due to an exceptional suppression of HER.
Comparing the mass activities of the CO and H 2 products in Fig. 10d and e, HER suppression was observed at more negative potentials (lower than À0.9 V vs. RHE). DDT showed the highest CO partial mass activity compared to both OLA and the oleic acid (OA) at À0.4 V to À0.9 V vs. RHE (Fig. 10e). They also compared the immobilization of ethylenediamine (EDA) to cysteamine onto Ag nanoparticles and found that EDA showed a higher selectivity toward CO production due to the presence of the additional amine group.
Carbon-based materials such as CNTs have proven to be a promising conductive solid support for heterogenization of molecular catalysts toward electrochemical CO 2 RR. This is due to their ability to form a strong noncovalent p-p interactions with aromatic ligands such as pyrene 150 and porphyrin. 41,151 Hu  FE of formate formation (AE2.5% at 95% confidence level (CL)), (c) FE of CO formation (AE6.2% at 95% CL), (d) FE of H 2 formation (AE25% at 95% CL); (e) partial current density of formate formation; (f) partial current density of CO formation, and (g) partial current density of H 2 formation. 136 Copyright (2020) American Chemical Society. This journal is © The Royal Society of Chemistry 2020 RSC Adv., 2020, 10, 38013-38023 | 38017 et al. 152 reported reduction of CO 2 to CO with an efficiency of over 90% using immobilized cobalt-tetraphenylporphyrins (CoTPPs) onto CNT in aqueous solution. Likewise, previous work by our group demonstrates selective reduction of CO 2 to CO with a FE of 90% upon immobilization of iron-porphyrindimers onto CNTs. 56 This proved to be twice as efficient as when the same catalyst was applied in a homogenous medium.
Similar enhancements to the reduction of CO 2 to methane (CH 4 ) and CO with both metalled and non-metallated ironporphyrin-pyridine (Fe-TPPy) catalysts were seen when noncovalently immobilized onto CNTs. 17 Among the synthesized catalysts shown in Fig. 11a, Fe-cis (2b)-pyridine-porphyrin catalysts, exhibited the highest current density (1.32 mA cm À2 ) and FE (76%) in reducing CO 2 to CH 4 and CO. Current density and product selectivity were remarkably enhanced to 30 mA cm À2 with the total FE of 92% aer immobilization onto CNTs, comparable or higher than that of similarly reported catalysts.
Comparing the CV of non-metallated 2a/GCE in Fig. 11b under argon and CO 2 , an enhancement to the current density can be seen in the CO 2 saturated solution stating at $À0.8 V vs. RHE. This increase in current density seen aer purging 2a/GCE with CO 2 demonstrates the important role of pyridine in the capture and electroreduction of CO 2 to methane. Metallated isomers increased the number of available capture sites and led to a direct increase in current density for all studied compounds (Fig. 11c). As seen in Fig. 11c, the broad CO 2 reduction peak at $À1.3 V vs. RHE aligns with the potential range observed for iron-cantered porphyrins.
Another report suggests using polyethylenimine (PEI) (Fig. 12a) will stabilize the electroreduction of CO 2 to HCOO À through hydrogen bonding interactions (Fig. 12b). 153 As shown in Fig. 12c and d, PEI-NCNT had the highest current density (9 mA cm À2 ) compared to nitrogen doped carbon nanotubes (NCNT) and bare CNT with a high FE of 87%.

Enhanced heterogeneous amine molecular catalysts using ow cells
In addition to the aforementioned solid supports and immobilization techniques for heterogeneous molecular catalysis, use of ow cell electrolyzers is another technique that has been proven to enhance overall catalytic performance. This emerging system minimizes the distance between the electrode and the catalytic layer; combining efficient electrode-to-catalyst electron transfers with a continuous, single-pass directional CO 2   delivery. These optimizations ultimately result in high energy efficiencies, product selectivities, and a reduction to operational costs. [154][155][156][157][158][159][160] An additional benet of ow cell electrolyzers is the translatability of their results to modern industrial practices. Generalized ow cell setups include a gas diffusion layer (GDL) which is directly exposed to the electrolyte solution (Fig. 13). [161][162][163] The catalyst layer is typically deposited directly onto the GDL, allowing for a greater effective catalyst surface area.
Recent studies of molecular catalysts operated in ow cells nd signicant gains to both product selectivity and reaction conversion rate. Cobalt and iron porphyrin and phthalocyanine complexes deposited onto a gas diffusion electrode through non-covalent bonding in a ow cell have been reported to achieve high current densities and selectivities. 164,165 An example of immobilized cobalt and iron amino molecular catalysts on carbon paper supports report current densities up to 165 mA cm À2 while maintaining high product selectivity (up to 94%). 161,165 These results, conrm the importance of state--of-the-art noble molecular based catalysts for electrochemical CO 2 RR.

Conclusions and future prospects
A wide range of amine-based molecular catalysts has been explored for the electrochemical reduction of CO 2 over the years and the contributions of small molecule catalysis to nding insights into the mechanism of electrochemical CO 2 RR is instrumental to the intelligent design of new catalysts. In particular, the role of amine-based ligands and functional groups were found to play an important role in capturing CO 2 itself and being used as covalent linkers for direct immobilization.
Although insights into the intricacies of CO 2 RR have been garnered thanks to thorough studies of immobilization techniques, the inuence of metal electrodes, and the role of different metal centers in organometallic compounds, further improvements to catalytic activity and stability are still needed before large-scale application can be realized. As described in this review, noncovalent and covalent immobilization can be achieved through various techniques to positive effect. Expanding on this new approach, many renewed studies on both homogeneous and heterogeneous systems are gaining greater traction with promising bounds being made every year.
Various strategies can be considered to overcome the current limitations in the electrochemical reduction process for CO 2 using amine-based molecular catalysts. For homogeneous electrocatalysis; (i) synthesizing small amine molecules that have a high affinity towards CO 2 but have a weaker amine-CO 2 bond; (ii) developing new nanostructured catalysts with large electrochemically active surface areas to facilitate the reduction process of the amine-CO 2 at lower potentials and high catalytic activity and selectivity would be promising next steps. For heterogeneous systems: (i) developing facile synthetic approaches to amine-functionalized MOFs; (ii) preparing high amine content MOFs with improved chemical stability; and (iii) improving immobilization strategies with nanostructured materials instead of the smooth metal surfaces are recommended to achieve higher catalytic performance.
Therefore, further investigations are required to achieve high stability and catalytic activity of the amino electrocatalysts to understand the fundamental kinetics of CO 2 reduction, and the effectiveness of the catalysts.

Conflicts of interest
There are no conicts to declare.