In their pioneering work, Tang et al. presented the “lattice hydride mechanism” in 2017^[54] to explain the ECO₂RR over [Cu₃₂(H)₂₀{S₂P(OⁱPr)₂}₁₂] (Cu₃₂) nanoclusters at low overpotentials. Cu₃₂ features a hexacapped pseudo-rhombohedral metal core of 14 Cu atoms packed between two triangular cupola fragments of Cu atoms, and overall Cu₃₂ is stabilized by 1,2-dithiophosphate (S₂P(OⁱPr)₂) ligands and 20 hydride ligands with three kinds of bridging coordination patterns [Figure 2A]: 12μ₃-H (lattice hydrides), 6μ₄-H (interstitial hydrides), and 2μ₅-H (interstitial hydrides). The 12μ₃-H lattice hydrides are further sorted into three μ₃-H₁, two μ₃-H₂, four μ₃-H₃, and two μ₃-H₄. As per DFT calculations, CO₂ adsorbed at the most favored site at μ₃-H₁, followed by reduction by hydride addition (μ₃-H₁) to C of CO₂ to form adsorbed HCOO* (ΔG⁰ = 0.32 eV), owing to attraction between negatively charged hydride and positively charged C of CO₂ to form Cu₃₂H₁₉L₁₂-HCOO*. It is further succeeded by the addition of another hydride (μ₄-H₁) to finally form the HCOOH product (G = 0.07 eV) with FE of 90%. These two hydride vacancies of Cu₃₂H₁₈L₁₂ are regenerated by two back-to-back electrochemical proton reductions to resume the catalytic cycle [Figure 2B]. Furthermore, the first hydride transfer revealed that higher overpotentials of +0.81 V and +0.6 to +1 eV, respectively, are needed for competing CO formation and the hydrogen evolution reaction (HER) relative to HCOOH formation occurring at low overpotentials (+0.32 V) and thus cannot compete with HCOOH formation. The controlled potential electrocatalysis experiments using Cu₃₂ in 0.1 M KHCO₃ and 0.4 M KCl (pH = 6.8) on the Cu₃₂/C/GDL electrode supported the theoretical results. Cu₃₂ mainly produced HCOOH at low overpotentials (89% at +0.3 V and 83% at +0.4 V) with a small amount of CO and H₂ [Figure 2C]. At overpotentials above +0.5 V, H₂ was generated predominantly (85% at 0.5 V and 94% at +0.6 V).

To investigate the effect of ligands on ECO₂RR, Shingyouchi et al. prepared Cu₁₄ nanoclusters with compositions [Cu₁₄(CHT)₃(PPh₃)₈H₁₀]⁺ (Cu₁₄-CHT) and [Cu₁₄(PET)₃(PPh₃)₈H₁₀]⁺ (Cu₁₄-PET) protected by cyclohexane thiolate (CHT) and 2-phenylethane thiolate (PET)^[55]. Structure-wise, both nanoclusters have distorted fcc (face-centered cubic) structures equipped with an inner Cu₆ octahedral core and outer Cu₈ cube. These overall distorted structures are stabilized due to various intracluster interactions, e.g., C-H---π interactions in Cu₁₄-CHT and C-H---π interactions and T-shaped π-π interactions in Cu₁₄-PET [Figure 3A]. The smaller Cu-Cu and Cu-S average distances in Cu₁₄-PET resulted in a reduction in its size and inner cuprophilic interactions, enhancing its stability, which influenced the stability of reaction intermediates during ECO₂RR and determined the selectivity of the product.

HCOOH was the main product using both Cu₁₄ clusters in the potential range of -0.8 V to -1.4 V (vs. RHE). Briefly, for Cu₁₄-CHT/CB, FE_HCOOH and FE_CO were ≈31% and ≈8% and for Cu₁₄-PET/CB, FE_HCCOH and FE_CO were ≈39% and ≈2% at -1.2 V [Figure 3B and C], respectively, supported by higher current densities (J_COOH) displayed by latter. The higher stability of reaction intermediate HCOO* than *COOH in Cu₁₄-PET prefers production of highly selective HCOOH over CO. Cu₁₄-CHT clusters showed an increase in HER, whereas Cu₁₄-PET maintained HER and CO production while enhancing the HCOOH production over extended time^[55]. The increase in HER during prolonged electrochemical exposure is attributed to the aggregation of Cu₁₄-CHT.

To investigate effect of isomers for ECO₂RR, Liu et al. synthesized Cu nanoclusters with compositions Cu₈(H)(L1)₆PF₆ (Cu₈-1), Cu₈(^tBuS)₄(L1)₄ (Cu₈-2), and Cu₈(^tBuS)₄(L2)₄ (Cu₈-3) (L1: 9H-carbazole-9-carbodithiol; L2: o-ethyl carbonodithiol)^[56]. Cu₈-1 possesses a cubic kernel protected by L1-type ligands [Figure 4A], and upon partial replacement of L1 or L2 by ^tBuS^-, Cu₈-2 and Cu₈-3, possessing ditetrahedron kernels, were prepared, respectively [Figure 4B and C]. According to the space-filling model, all Cu₈ clusters with a low number of ligands covering Cu atoms act as active sites^[56].

Linear sweep voltammetry (LSV) curves of all Cu₈ clusters in CO₂-saturated solution exhibited high current density, indicating their activity for ECO₂RR. The lower onset potentials for Cu₈-2 and Cu₈-3 (-0.6 V and -0.48 V, vs. RHE, respectively) than Cu₈-1 (-0.71 V vs. RHE) suggested the lower energy barrier of ECO₂RR for ditetrahedron kernels^[56]. The electrochemical performance in the range of -0.8 to -1.2 V vs. RHE yielded H₂, CO, and HCOOH with total FE > 90%. The ditetrahedron Cu₈ clusters exhibited higher FE_HCOOH and selectivity than cubic Cu₈ in this potential range as shown in Figure 4D. In more detail, as potential tuned from -0.8 to -1.0 V, HCOOH selectivity for all Cu₈ clusters increased, accompanied by a notable decrease beyond -1.0 V, where H₂ became the primary product. The FE_HCOOH values for ditetrahedron Cu₈-2, shown in Figure 4E, were evaluated to be 90% and 92% at -0.9 and -1.0 V, respectively, which were 2 times higher than Cu₈-1 clusters (39.1% and 48.6% at -0.9 and -1.0 V, respectively), supported by higher J_COOH values for Cu₈-2 and Cu₈-3. Of note, the slight difference in activities between the ditetrahedron Cu₈-2 and Cu₈-3 owes to the ligand effect. The chronoamperometric measurements revealed the relatively high electrochemical stability of Cu₈-2 and Cu₈-3 under the operating conditions for 8 h^[56].

As per DFT calculations, the difference in CO₂RR activity for Cu₈-1 and Cu₈-2 is ascribed to distortion caused by the corner site in the latter [Figure 5A and B], supported by the potential energy diagram showing suppression of HER for Cu₈-2 [Figure 5C]. These results were consistent with the experimental findings shown in Figure 4E. The CO₂RR process shown in Figure 5D, where the conversion of *CO₂ to HCOO* occurs with a lower free energy for Cu₈-2 than Cu₈-1, further proves the former’s higher reactivity and selectivity.

Biswas et al. recently fabricated a highly stable [Cu₂₃H₄(SC₇H₇)₁₈(PPh₃)₆] (Cu₂₃) (SC₇H₇: p-toulenethiolate) nanocluster via fusion of two [Cu₁₂H₂(SC₇H₇)₇(PPh₃)₃] subunits sharing a Cu(0) atom [Figure 6A]^[43]. The remarkable stability of Cu₂₃ motivated the authors to investigate Cu₂₃/CB for ECO₂RR. FE data in Figure 6B indicated a maximum FE_HCOOH ~26% at -1.2 V along with FE_H2 ~42% and minor CO for Cu₂₃. While 30 nm Cu particles prepared with the same precursor via calcination displayed FE_H2 ~80% at -1.0 V, indicating uncontrolled competitive HER along with FE_HCOOH ~8% and FE_CO ~6%, underlining the superior HCOOH selectivity of Cu₂₃. DFT revealed antenna-like Cu₃ units on both sides of the overall geometry, forming the more favorable hollow triangular prism-like sites for H₂ adsorption in line with high HER at low overpotential [Figure 6C]. Additionally, these sites also act as potential active sites for CO₂RR [Figure 6C], and among their two investigated pathways, the *HCOOH (via HCOO* intermediate) pathway is preferred over the *COOH pathway as per their energy profiles [Figure 6D], in agreement with selective production of HCOOH on Cu₂₃.

Han et al. synthesized two Cu nanoclusters with the compositions [Cu₁₇H₆(NHC^H)₄(dppm)₄](PF₆)₃(MeCN)₃(CH₂Cl₂)(Et₂O) (Cu17a) [NHC^H: 1,3-diprop-2-ynyl-1H-imidazol-3-ium hexafluorophosphate; dppm: Bis(diphenylposphino)methane] with its crystal structure [Figure 7A] and Cu₁₇H₆(NHC^ph)₄(dppm)₄](PF₆)₃(MeCN)(CH₃OH) (Cu17b) [NHC^Ph: 1,3-bis(2-propyn-1-yl)-1H-benzoimidazol-3-ium hexafluorophosphate]. Both Cu17a and Cu17b feature a Cu₁₇H₆ square orthobicupola as a common core, shown in Figure 7B^[57]. The σ and π bonding between Cu and N-heterocyclic carbene (NHC) ligands [Figure 7C] provides high stability to both these Cu17 clusters, and moreover, the distinctive coordination pattern (μ₇-η_σ¹:η_σ¹:η_σ¹:η_σ¹:η_σ¹:η_π²:η_π²) of the NHC ligand exposes the neighboring Cu atoms as active sites for CO₂RR.

Comparing the structures of Cu17a [Figure 7A] and Cu17b^[57], a ligand effect was noticed on ECO₂RR activity due to minimal steric hindrance for CO₂ adsorption on Cu17a compared to Cu17b. The more positive onset potential and higher cathodic current density observed for Cu17a indicate their higher activity towards CO₂RR than Cu17b^[57]. For Cu17a, FE_CO reached approximately 91% at -0.37 V [Figure 8A], exceeding the values reported for the Cu nanocluster-based electrocatalysts^[58]. In contrast, Cu17b displayed poor catalytic activity, with CO being the main product across the potential range from -0.37 to -1.07 V, with maximum FE_CO ~36% [Figure 8B]. As more negative potential is applied, FE_C2H4 is enhanced initially and then decreased beyond -1.1 V for Cu17a [Figure 8C].

Due to the competitive HER, H₂ was the main product at high potentials [Figure 8A and B]. The attenuated total reflectance surface-enhanced infrared absorption spectroscopy (ATR-SEIRAS) showed the broad peak of adsorbed water molecules at 1,670 cm^-1, which was used to perform proton-coupled electron transfer. The peak at 1,366 cm^-1, owing to *COOH, suggested CO₂ hydrogenation followed by adsorption of *CO. Further, peaks of *OCCHO at 1,498 cm^-1 and 1,316 cm^-1 confirm its significance as an intermediate in the C-C coupling process. Thus, the formation of the C₂H₄ product is facilitated by Cu17a. The optimized space-filling model of these clusters revealed the cavity between two parallel NHC ligands, resulting in exposing two adjacent Cu atoms as potential active sites. The free energy diagram [Figure 8D] explains the thermodynamic favorability of CO₂ reduction by suggesting that surface ligands with less steric hindrance assisted the substrate adsorption. Hence, Cu17a displays more effective CO₂ adsorption with a free energy of -0.56 eV, which is less than -0.23 eV of Cu17b.

Biswas et al. reported ECO₂RR driven selective synthesis of methanol over defect-induced nanocluster [Cu₅₈H₂₀(SPr)₃₆(PPh₃)₇]²⁺ (Cu₅₈-I) (Pr: CH₂CH₂CH₃; PPh₃: triphenylphosphine)^[59]. Cu₅₈-I, lacking one PPh₃ ligand, was synthesized using the protocol reported for the synthesis of regular [Cu₅₈H₂₀(SPr)₃₆(PPh₃)₈]²⁺ (Cu₅₈) with nested Keplerian architecture^[59,60]. The further dissociation of a PPh₃ ligand from Cu₅₈ was found difficult; however, this was achieved upon replacing the SPr with SEt (Et: CH₂CH₃), resulting in another defect-induced nanocluster [Cu₅₈H₂₀(SEt)₃₆(PPh₃)₆]²⁺ (Cu₅₈-II)^[59]. The Keplerian architecture of all three above Cu₅₈ nanoclusters exhibits a Cu₈ cubic core confined by four concentric Cu(I) shells: the innermost Cu₆ octahedron shell, followed by the Cu₂₄ rhombicuboctahedron shell, followed by the Cu₁₂ cuboctahedron shell, followed by the outermost Cu₈ cubic shell^[59,60]. Notably, a key difference was found in the arrangement of PPh₃ ligands in Cu₅₈ and Cu₅₈-I. Unlike in Cu₅₈, where the eight PPh₃ ligands form a complete cubic ligand shell (P₈) around nanoclusters [Figure 9A], one and two phosphine ligand vacant sites were found in cubic P₈ ligand shells of Cu₅₈-I [Figure 9B] and Cu₅₈-II [Figure 9C], respectively, resulting in corresponding one and two exposed vertex Cu sites. The construction of the P₈ ligand shells depends on Cu-P bond formation involving Cu(I) atoms of the outermost Cu₈ cubic shell [Figure 9D-F]. As a result, the following distortions modify the internal cationic geometry, leading to strong cuprophilic interactions at defect sites, which in turn impacts the product selectivity.

Cu₅₈-II with more defects showed the highest reduction current, followed by less defect-induced Cu₅₈-I, followed by regular Cu₅₈ at -0.9 V. However, all Cu₅₈ nanoclusters yielded different selective products during ECO₂R. For instance, H₂, CO, and CH₃OH are key products for Cu₅₈-I [Figure 10A], while H₂, CO, and HCOOH for Cu₅₈-II [Figure 10B]. FE analysis revealed that HER was dominant (FE_H2 > 50%) for Cu₅₈-II, with a higher number of defects resulting in more exposed Cu vertices and edges. The Cu₅₈-I, with relatively fewer distortions and exposed Cu sites, selectively yielded CH₃OH with FE_CH3OH ≈54% at -0.7 V [Figure 10A]. On the other hand, regular Cu₅₈ produced only CO due to few active sites [Figure 10C].

The computational hydrogen electrode method revealed that the formation of *COOH is more favorable than *HCOO since it is less uphill for Cu₅₈-1 than Cu₅₈ in the energy level diagram [Figure 10D]. Subsequently, the formation of *CO from *COOH occurs, and after H₂O desorption, *CO converts into methanol on Cu₅₈-1. In contrast, *CO desorption is easier in the case of regular Cu₅₈ nanoclusters. Furthermore, the enhanced binding of *CO and *CHO intermediates at the edge Cu site i.e. next to the vertex site with a ligand vacancy in Cu₅₈-I [Figure 10E and F], is attributed to the upshift of d-states of the edge Cu site^[59].

Chen et al. synthesized a metastable thiacalix[4]arene-protected {NaCu₃₅(TC4A)₄(PhC≡C)₂₀} (Cu₃₅) cluster shown in Figure 11A^[46]. It has a semiring architecture composed of two angularly offset {Cu₁₇(TC4A)₂L₉} (L: alkynyl ligand) subunits [Figure 11B], connected by a linear CuL₂ bridging unit [Figure 11C]^[46]. Each {Cu₁₇(TC4A)₂L₉} subunit is composed of two {Cu₄-TC4A} units linked to {Cu₉L₉} core [Figure 11D]. Owing to the structural adaptability of TC4A, Cu₃₅ possesses plasticity, resulting in its two distinct occupancy states: one where it coordinates with Na⁺ and a second where it encapsulates a CH₂Cl₂ molecule, as shown in Figure 11A. Thus, the central void acts as a potential site for further metal incorporation. Cu₃₅ at elevated temperature can undergo fragmentation to form the thermodynamically stable [Cu₁₄(TC4A)₄(PhC≡C)₆] (Cu₁₄) cluster [Figure 11E]. Further, due to flexibility, a thermal etching of Cu₃₅ by Ag⁺ can also form bimetallic [Cu₁₄Ag₆(TC4A)₂(PhC≡C)₁₂(MeOH)] (Cu₁₄Ag₆) nanoclusters [Figure 11E]. Cu₁₄Ag₆ [Figure 11F] exhibits a dimer composed of two terminal shuttlecock-like {Cu₆(TC4A)L₄} units [Figure 11G] and a central {Ag₆Cu₂L₄} core [Figure 11H].

All three clusters displayed lower onset overpotentials and enhanced cathodic current densities under CO₂ than under N₂, but their different product distributions represent a clear structural dependence. The monometallic clusters Cu₁₄ and Cu₃₅ mainly yielded CH₄ and C₂H₄ gas-phase products [Figure 12A and B]. For instance, Cu₁₄ showed a FE_CH4 ~17.94% and FE_C2H4 ~27.50% at -1.1 V [Figure 12A]. Similarly, Cu₃₅ displays FE_CH4 ~34.01% and FE_C2H4 ~13.95% at -1.3 V [Figure 12B]. Notably, monoatomic Cu₃₅ and Cu₁₄ displayed a limited ethanol selectivity (FE_C2H5OH < 7%) across the full potential range. While across the potential range of -0.9 to -1.6 V, Cu₁₄Ag₆ exhibits FE_C2H5OH > 20% with a maximum FE_C2H5OH ~49.27% at -1.3 V [Figure 12C]. Thus, the incorporation of Ag⁺ guides the Cu cluster catalysis via stabilizing the key intermediates in the ethanol production pathway. Interestingly, both C₂H₅OH and C₂H₄ routes in ECO₂RR have the same *CO-*COH intermediate. Conventionally, this favors the formation of ethane if it coordinates with two Cu sites through its two carbon atoms. Whereas to obtain ethanol as a selective product, its coordination to a Cu site must be through its oxygen atom rather than its carbon atom [Figure 1].

In Cu₁₄Ag₆, the two surface-exposed Cu sites of the Ag₆Cu₂ subunit are the two active sites for ECO₂RR, which have oxyphilic properties that coordinate with CO₂ through the dual-O-bridged adsorption configuration of CO₂, supported by DFT calculations [Figure 12D]. This dual-O-bridged configuration at the two exposed Cu sites undergoes PCET to form bidentate *OCHO, which is further hydrogenated to produce *OCH₂ (* represents a Cu site). Simultaneously, at an adjacent Ag site, CO₂ follows conventional C-terminal adsorption and is reduced to ^#CHO via ^#COOH (# denotes an Ag site). The intermediates *OCH₂ and ^#CHO formed simultaneously undergo asymmetric coupling to generate *OCH₂-CHO, which is further hydrogenated to form CH₃CH₂O* before reduction to ethanol [Figure 12E]. Conversely, Cu---Cu dual sites in Cu₃₅ and Cu₁₄ favor C₂H₄ formation.

To discover the effect of complicated multiple surface shells on ECO₂RR, Li et al. synthesized [Cu₂₆(DPPE)₃(CF₃CO₂)₈(CH₃O)₂(^tBuC≡C)₄H₁₁]⁺ (Cu₂₆) nanocluster, which consists of 26 Cu atoms and five different kinds of ligands such as 1,2-bis(diphenylphosphino)-ethane (DPPE), trifluoroacetate (CF₃COO^-), 3,3-dimethyl-1-butyne (^tBuC≡C^-), methoxide (CH₃O^-), and hydride ligands^[61]. For comparison, other Cu nanoclusters such as Cu₂₅H₂₂((p-FPh)₃P)₁₂ (Cu₂₅)^[62], [Cu₅₃(CF₃COO)₁₀(^tBuCC)₂₀Cl₂H₁₈]⁺ (Cu₅₃)^[63], and [Cu₆₁(S^tBu)₂₆S₆Cl₆H₁₄]⁺ (Cu₆₁)^[64] with distinct interfaces were also prepared.

LSV curves suggested that the catalytic activity of CO₂R over different clusters varies substantially [Figure 13A]. The Cu₂₆ clusters showed FE_CO ~81% at -0.8 V [Figures 13B and C]. But Cu₅₃, in spite of having similar ligands, exhibited much lower FE_CO than Cu₂₆ at the same potential, indicating the complex surface structure of Cu₂₆ created by five ligands favors the formation of CO [Figure 13C]. The plausible reason for the high catalytic efficiency of the Cu₂₆ cluster is its strong capability to suppress the competitive HER. Moreover, FE_H2 for the Cu₂₆ was much lower than other clusters under investigation at -0.7 and -0.8 V [Figure 13D]. This suggests that under these potentials, a higher rate of CO₂ adsorption and CO desorption makes CO₂ to CO conversion more prominent on Cu₂₆.

DFT calculations compared the energy barrier for the formation of intermediates for two types of modeled Cu₂₆ clusters: Cu₂₆ having hydrides and Cu₂₆-H^- (i.e., Cu₂₆ without hydrides) [Figure 13E]. Here, the hydrides present in Cu₂₆ play an important role in the stabilization of the cluster as well as exposing active sites for selective reduction of CO₂ to CO. However, DFT-based free energy diagrams [Figure 13E and F] revealed that in the absence of hydrides, Cu₂₆ clusters can suppress even HER to a great extent and can significantly increase the yield of CO by decreasing the energy barrier for CO* formation.

Deng et al. synthesized bimetallic nanocluster with composition [Ag₁₅Cu₆(C≡CR)₁₈(DPPE)₂]^- (Ag₁₅Cu₆) [HC≡CR: 3,5-bis(trifluoromethyl)phenylacetylene; DPPE: 1,2-bis(diphenylphosphino)ethane]. Ag₁₅Cu₆ exhibits an Ag₁₁Cu₄ metal core with a bcc (body-centred cubic) structure capped by two Cu atoms, two Ag₂DPPE motifs, and eighteen alkynyl ligands. Such an unusual bcc metal core and accessible surface make it suitable for investigating ECO₂RR^[45]. For comparison, the [Ag₉Cu₆(^tBuC≡C)₁₂]⁺ (Ag₉Cu₆) cluster possessing a bcc metal core was synthesized. Both these clusters were supported on carbon black (Ag₁₅Cu₆/C and Ag₉Cu₆/C) to prevent conductivity problems and enhance dispersion^[45]. High current densities in LSV curves were noticed for both the samples in CO₂-saturated solution, indicating their catalytic activity for ECO₂RR^[45].

Interestingly, Ag₁₅Cu₆/C showed higher current density and lower onset potential than Ag₉Cu₆/C, suggesting the former’s higher catalytic activity. The data shown in Figure 14A revealed that Ag₁₅Cu₆/C exhibited high selectivity for CO (FE_CO > 85%) in the potential range of -0.72 to -0.87 V, with a maximum FE_CO ~91.3% at -0.81 V, whereas Ag₉Cu₆/C showed a maximum FE_CO ~48.5% at -0.89 V. In a membrane electrode assembly (MEA) electrochemical cell [Figure 14B], Ag₁₅Cu₆/C exhibited high selectivity towards CO, with FE_CO > 83% at all tested potentials with highest FE_CO ~91.3% at -3.50 V and a high partial J_CO ~-154 mA·cm^-2 at -4.00 V. Additionally, Ag₁₅Cu₆/C also demonstrated durability as FE_CO ~90% and partial current density of ~60 mA·cm^-2 maintained during 145 h [Figure 14C].

DFT simulations revealed two pairs of AgCu dual metals in Ag₁₅Cu₆/C as the most favorable active sites for ECO₂RR owing to the stripping of alkynyl ligands during catalyst preparation without any structural changes, resulting in more available space. Theoretical calculations shown in Figure 14D and E suggested that de-ligated Ag₁₅Cu₆ can reduce the exothermic energy barrier of CO₂ to *COOH by 0.21 eV, and the desorption of *CO to CO requires overcoming an endothermic energy barrier of 0.34 eV. However, in the competitive reaction pathway of HER, H⁺ desorption to form H₂ is a potential-determining step that needs to overcome a large energy barrier of 0.61 V. Thus, it suggested the increase of activity and selectivity of Ag₁₅Cu₆ towards CO formation.

Xie et al. recently presented tailored Cu nanoclusters to suppress HER while enhancing CO₂ to CH₄ transformation under acidic media^[65]. They deposited [Cu₈(H)(9H-carbazole-9-carbodithiolate)₆]PF₆ (Cu₈), [Cu₃₆H₁₀(PET)₂₄(PPh₃)₆Cl₂] (Cu₃₆), and [Cu₆₁(S^tBu)₂₆S₆Cl₆H₁₄]PF₆ (Cu₆₁) clusters onto carbon paper to make gas diffusion electrodes to test them in a bipolar membrane CO₂RR electrolyzer with an acidic cathode (1 M KCl, pH = 2) and an alkaline anode (1 M KOH). Here, an acidic cathode and basic anode controlled the carbonate formation and minimized ohmic losses to enhance CO₂RR activity. After pre-activation at 200 mA·cm^-2 for 600 s under saturated CO₂, FE_CH4 for Cu₃₆ was found to improve significantly. For instance, Cu₃₆ achieved FE_CH4 ~51.8% ± 1.1% at 300 mA·cm^-2 after activation [Figure 15A], which was higher than FE_CH4 ~35.3% ± 2% for Cu₃₆ without activation. On comparing the ECO₂RR performance of Cu₈, Cu₃₆, and Cu₆₁ across the current densities from 200 mA·cm^-2 to 700 mA·cm^-2 using an electrode area of 1.4 cm², the striking difference was noticed in their product distribution [Figures 15B-D]. Here, at a current density of 200 mA·cm^-2, the highest selectivity with FE_CH4 ~35.9% ± 1% was recorded for Cu₃₆, greater than for Cu₈ and Cu₆₁. Whereas, at 200 mA·cm^-2, Cu₈ primarily formed formate (FE_HCOOH = 37.7%) with substantial H₂ production (FE_H2 = 33.9% ± 1%) and minor CH₄ production (FE_CH4 = 7.0% ± 3.2%) [Figure 15B].

Cu₆₁ generated formate (FE_HCOOH = 69.4% ± 0.2%) and the lowest CH₄ (FE_CH4 < 1%) along with H₂ evolution [Figure 15D].

The stable Cu₃₆ maintained the methane selectivity (FE > 40%) across the current density from 300 mA·cm^-2 to 700 mA·cm^-2 [Figure 15E]. These results were supported by partial current density (J_CH4) curves of Cu₈, Cu₃₆, and Cu₆₁ nanoclusters. In contrast to Cu₃₆, an explicit transition in product distribution was observed from C₁ to C₂₊ for Cu₈ and Cu₆₁ across the 200 mA·cm^-2 to 700 mA·cm^-2 [Figure 15B-D]^[65]. Here, C-C coupling reactions responsible for C₂H₄ and C₂H₅OH formation were due to nanocluster aggregation owing to instability of thiolate ligands on Cu₈ and Cu₆₁ at high current densities, resulting in the formation of crystalline Cu_2-xS nanoparticles creating more favorable active sites to generate C₂ products. However, in case of Cu₃₆, the thiolate ligands prevented such agglomeration. The Cu₃₆ also maintained constant activity and selectivity for 70 h at -300 mA·cm^-2 [Figure 15F].

In another report, Han et al. demonstrated size effects on ECO₂RR for [Cu₁₃(SC₆H₃F₂)₃(P(PhF)₃)₇H₁₀] (Cu₁₃) and [Cu₁₄(SC₆H₃F₂)₃(P(PhF)₃)₈H₁₀]⁺ (Cu₁₄) nanoclusters with precise structures shown in Figure 16A and B, where even one atom alteration results in switching of catalytic reactivity^[66]. Here, Cu₁₄ has a Cu(PR₃) vertex at its defect site [Figure 16C], and this additional Cu atom at the top of Cu₁₄ allows the H atom bound to three core Cu atoms to move into the center of the same Cu₃ plane. As a result, the electronic structure of surrounding atoms is tuned to facilitate the CO₂ adsorption and H₂O dissociation simultaneously on Cu₁₄.

For Cu₁₃, H₂ was the main product with little CO across the whole potential range of -0.8 to -1.3 V [Figure 17A]. Notably, for Cu₁₄, with an increase of applied voltage from -0.8 to -1.2 V, FE_CO gradually decreases from 43% to 10.3%, while FE_CH4+C2H4 was found to be ~54.3% at -1.2 V with a corresponding current density of carbon-containing products (J_{carbon containing products}) of 60 mA·cm^-2 [Figure 17B]. Upon a further increase of potential to -1.3 V, H₂ evolution becomes extreme, resulting in a significant decrease in CO₂RR activity of Cu₁₄. It is worth noting here that the decreasing trend of FE_CO and increasing trend of FE of carbon-containing hydrocarbons in the potential range of -0.8 to -1.2 V indicate that CO₂ to CH₄ and C₂H₄ conversion occurs via a *CO intermediate. Interestingly, an increase in the rate of formation of carbon-containing products with an increase in pH indicates that Cu₁₄ speeds up H₂O activation even at higher pH^[66].

Furthermore, in situ FTIR (Fourier transform infrared spectroscopy) monitoring revealed that the *CO peak in Cu₁₄ becomes more intense with time, suggesting accumulation of *CO on Cu₁₄ leading to further deep hydrogenation^[66]. Next, Gibbs free energy calculations showed that the formation of the *COOH intermediate was more favorable in the case of Cu₁₄ (ΔG = 0.39 eV) than in Cu₁₃ (ΔG = 0.91 eV) [Figure 17C]. ΔG curves further indicate that H₂O dissociation is also more feasible on Cu₁₄ than on Cu₁₃ [Figure 17D]. But the *CO desorption on Cu₁₄ was found more difficult (ΔG = 1.15 eV), which promoted its deep hydrogenation to produce hydrocarbons [Figure 17E and F].

The coordination symmetry breaking of the metal center results in an increase in active sites. Wu et al. demonstrated this kind of breaking of coordination symmetry in thiolate-protected Cu₆(MBD)₆ (MBD: 2-mercaptobenzimidazole) nanoclusters having symmetry-broken CuS₂N₁ active sites and its impact on ECO₂RR^[58]. The Cu₆(MBD)₆ displayed a maximum current density of ~364 mA·cm^-2 at -1.5 V in saturated CO₂ [Figure 18A]. The product distribution analysis shown in Figure 18B revealed that at an applied potential of +0.7 V, CO was the main product (FE_CO > 31.8%) along with CH₄ (FE_CH4 ~2.1%) and C₂H₄ (FE_C2H4 ~3.5%). However, with an increase in applied potential, FE_CO gradually decreased, while FE_CH4 and FE_C2H4 continuously increased till FE_CH4 reached 42.5% at -1.4 V (J_CH4 = -119 mA·cm^-2), along with FE_C2H4 reaching 23% (J_C2H4 = -64.4 mA·cm^-2) [Figure 18C].

Further, DFT calculations were performed using optimized geometric models of Cu₆(MBD)₆ with CuS₂N₁ active sites and Cu₈(^tBuS)₄(L2)₄ (L2: o-ethyl carbonodithiolate) with Cu-S₃ sites. Unlike in Cu₈(^tBuS)₄(L2)₄, electron transfer from Cu₆(MBD)₆ to CO₂ was noticed owing to the higher polarity of the Cu-S₂N1 site relative to the Cu-S₃ site [Figure 18D], and the higher binding energy (0.19 eV) between Cu₆(MBD)₆ and CO₂ than between CO₂ and Cu₈(^tBuS)₄(L2)₄ (0.18 eV). Integrated projected density of states (IPDOS) revealed that for Cu₆(MBD)₆, the d_x2-y2 orbital of the CuS₂N₁ site is the highest occupied d-orbital [Figure 18E], and for Cu₈(^tBuS)₄(L2)₄, the d_xz of the CuS₃ site is the highest occupied d-orbital [Figure 18F]. Further, for Cu₆(MBD)₆, the symmetry and energy match of its highest occupied d_x2-y2 orbital with the lowest occupied π* orbital of CO₂ to form a π-complex via π-back bonding plays a role in binding the reactant and intermediate [Figure 18G]. In contrast, the highest occupied d_xz orbital of Cu₈(^tBuS)₄(L2)₄ would couple with the lowest occupied π* orbital of CO₂ to form a π-complex only when the adsorption of CO₂ occurs through the O atom [Figure 18H]. The key intermediate in the case of Cu₆(MBD)₆, was *COOH instead of *OCHO. On the basis of adsorption free energies of key intermediates at an applied potential of -1.4 V shown in Figure 18I, the CuS₂N₁ adsorbs the CO₂ with a free energy of -0.13 eV, leading to their strong interaction followed by hydrogenation through adding a proton and electron from the electrode to form the *COOH intermediate. This is followed by generation of *CO, which can either undergo desorption or hydrogenation to form CH₄ and/or C₂H₄ as products.

Li et al. illustrated the impact of the synergistic effect of dual sites, such as Cu⁺ and adjacent S sites, on ECO₂RR. To this end, they synthesised Cu₄(MMI)₄ (Cu₄) and Cu₈(MMI)₄(^tBuS)₄ (Cu₈) (MMI: 2-mercapto-1-methylimidazole) nanoclusters with well-resolved structures as shown in Figure 19A and B, respectively^[67]. LSV curves of both Cu₄ and Cu₈ exhibited high current densities in saturated CO₂, confirming their high activities for ECO₂RR. Cu₄ exhibited FE of carbon products > 85.8% across the potential from -0.9 to -1.6 V with a maximum FE ~94.0% at -1.2 V [Figure 19C]. The carbonaceous hydrocarbons formed on Cu₄ via ECO₂RR were CH₄ (FE_CH4 ~53.7%), C₂₊ products (FE_C2+ ~37.3%), and CO and HCOOH (FE_{CO + HCOOH} < 8.2%) at -1.2 V. This indicates that the Cu(I) site in Cu₄ effectively stabilizes the *CO, which is further reduced into high-value carbonaceous products. In contrast, FE of deep-reduced carbonaceous products for Cu₈ was found to be ~72.8% at -1.2 V [Figure 19D], along with FE of 2e-reduction products ~15.8% and FE_H2 ~11.4%. Notably, Cu₄ showed a high preference for CH₄ with FE_CH4 ~53.7% and J_CH4 ~89.1 mA·cm^-2 at -1.2 V (vs. RHE), whereas Cu₈ exhibits a preference for C₂₊ products with FE ~58.5% and J_C2+ of 152.1 mA·cm^-2 at -1.3 V (vs. RHE)^[67]. Interestingly, the continuous electrolysis revealed higher electrochemical stability of Cu₄.

Further, ATR-SEIRAS analysis displayed the peaks corresponding to the OH deformation and symmetric stretch of *COOH, which is key intermediate to form CO and CH₄ products in case of Cu₄^[67]. Furthermore, the peaks of *CH₂O and *OCH₃, which are crucial intermediates to form CH₄, were also noticed. Although most of the reaction intermediates on Cu₄ are similar to Cu₈, the appearance of peaks corresponding to the *COCHO intermediate indicates the formation of C₂₊ products in Cu₈. The Gibbs free energy diagram for Cu₄, shown in Figure 19E, revealed that the C-C coupling step of *CHOCO is uphill, with a barrier of 0.19 eV inhibiting the production of multicarbon products. Furthermore, the Gibbs formation energy of *CH₃OH from the *OCH₃ intermediate is 0.22 eV higher than that of CH₄ for Cu₄. The proposed mechanism of hydrogen bond interaction between H* and reaction intermediate is presented in Figure 19F along with relative free energies of initial configuration, transition state, and final configuration for protonation of *CO to *CHO [Figure 19G]. On the other hand, taking into account experimental findings that one tert-butyl group is removed, the energy barrier for *COCHO formation is lower than *CHO protonation on both CuS₂N and Cu-S₃ sites, indicating a stronger tendency of Cu₈ to form C₂₊ products.

The Faradaic efficiencies of products formed during ECO₂RR over various ligand-protected Cu nanoclusters mentioned in this review are summarized in Table 1.

The S, N atom-based coordination compounds are in more negative p-orbitals (lower in energy) relative to O atoms and can be coupled to produce more desirable band structures. With this understanding, Dong et al. synthesized Cu₆(HL₁)₂(L₁)₄(PF₆)₂ (Cu₆-NH), (L₁ = PymSH = 2-mercaptopyrimidine) nanocluster^[69]. The Cu₆-NH exhibited a distorted Cu₆ octahedron with 6 faces capped by six ligands with alternate alignment, as shown in Figure 21A. Interestingly, the two N atoms in Cu₆-NH are protonated to balance the two PF₆^- counterions [Figure 21B], which is contrary to Cu₆-N [Figures 21C and D], where all ligands are fully deprotonated. Structural analysis showed an identical metal core for both Cu₆-NH and Cu₆-N [Figure 21E]. The strong hydrogen-bonded network among clusters and between clusters and PF₆^- forms a stacking network in Cu₆-NH [Figure 21F] compared to weaker intermolecular interactions seen in Cu₆-N [Figure 21G].

UV-Vis (ultraviolet-visible) spectra indicated that Cu₆-NH and Cu₆-N have absorption in 400-550 nm. The conduction bands of both clusters were more negative (-1.08 and -1.11 V, respectively) than the redox potential of CO/CO₂ (-0.48 V, pH = 7), and the valence bands of both clusters are more positive than the redox potential of O/H₂O (+0.82 V, pH = 7), indicating both can catalyze CO₂ reduction and H₂O oxidation. DFT calculations revealed that both nanoclusters are direct band gap semiconductors with p-n heterostructures for efficient charge separation^[69]. After 6 h of visible light irradiation, the average rate of CO evolution was 24.8 μmol·g^-1·h^-1 for Cu₆-NH [Figure 21H], comparable with reported catalysts^[70-72]. Whereas Cu₆-N showed a low CO evolution rate of 4.3 μmol·g^-1·h^-1. Additionally, a minor amount of H₂ was detected, and O₂ was also generated with a molar ratio of 2:1 of CO and O₂. The sole difference between Cu₆-NH and Cu₆-N is the two protonated N atoms of the PymSH ligand in the former, as shown in Figure 21B, which play a key role as proton relay stations in improving its photocatalytic performance.

To gain further insights, in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) experiments were conducted, revealing relatively extra absorption bands of *COOH, mCO₃^2-, and b-CO₃^2- at 1,618, 1,517, and 1,342 cm^-1, respectively, for Cu₆-NH, ascribed to H-bonding between protonated N-H and different intermediates [Figure 21I and J]. This observation further validated the role of protonated N-H groups as proton relay stations in Cu₆-NH, beneficial for the formation of the *COOH intermediate, which was finally reduced to CO.

The efficiency of photocatalytic CO₂ reduction has still not reached its full potential due to inadequate utilization of the solar spectrum. Recently, Dong et al. presented an unprecedented near-infrared light-responsive [Cu₈(S^tBu)₄(PymS)₄] (Cu₈SN) nanocluster for effective photoreduction of CO₂^[73]. Notably, owing to C-H…π and C-H…N interactions between PymS and ^tBuS-ligands shown in Figure 22A, a 3D supramolecular network is formed in Cu₈SN [Figure 22B]. Further, thermal treatment of Cu₈SN at 240 ^oC under N₂ resulted in its transformation into a novel and structurally stable [Cu₈(S)₂(PymS)₄] (Cu₈SN-T) nanocluster [Figure 22C]. Comparison of the structures of Cu₈SN and Cu₈SN-T revealed that after heat treatment at 240 ^oC, the tert-butyl mercaptan uniting two Cu₄ sub-cores in Cu₈SN is replaced with S^2- in Cu₈SN-T, resulting in a novel Cu₈ Core in the latter [Figure 22A and C].

The estimated band gaps of 2.45 eV and 1.58 eV from Tauc plots for Cu₈SN and Cu₈SN-T, respectively [Figure 22D], confirmed their different band structures. Ultraviolet-visible-near infrared (UV-Vis-NIR) diffuse reflectance spectra (DRS) in Figure 22E revealed that Cu₈SN-T exhibited absorption bands across the UV-Vis-NIR region. Upon illumination with near-infrared (NIR) light (808 nm, 100 mW·cm^-2) [Figure 22F], the temperature of Cu₈SN-T increased from 24.2 to 87.2 ^oC within 25 s, which is much faster than for Cu₈SN (final temperature = 31.3 ^oC). Thus, Cu₈SN-T enables direct use of sunlight for high-temperature photocatalysis. Being a direct band gap semiconductor, the valence band of Cu₈SN-T at +0.90 V [vs. normal hydrogen electrode (NHE)] and the conduction band at -0.68 V (vs. NHE) make it possible to catalyze both CO₂ reduction and H₂O oxidation. Further, the quenching of PL intensity of Cu₈SN-T also confirmed its better charge separation ability.

The CO evolution rate of Cu₈SN-T was calculated to be 20.2, 17.92, and 7.1 μmol·g^-1·h^-1 under full-spectrum light, Vis-NIR light, and NIR light, respectively, as presented in Figure 23A, which was higher than that of commercially available Cu₂S^[73]. In contrast, the CO evolution rate of Cu₈SN was 3.1 and 2.6 μmol·g^-1·h^-1 under full-spectrum light and Vis-NIR light, respectively, which decreased with time, whereas no products were formed under NIR light^[73]. Thus, ligand removal improved the photocatalytic performance of CO evolution of Cu₈SN-T. Notably, the O₂ generation rate was nearly half that of CO evolution in these photocatalytic measurements.

In situ DRIFTS spectra shown in Figure 23B and C revealed that apart from the peaks owing to CO₂ adsorption on Cu₈SN-T at 2,335 and 2,360 cm^-1, additional peaks at 1,397, 1,558, and 1,649 cm^-1 were assigned to the *COOH intermediate responsible for CO₂ reduction to CO. The bands at 1,432 and 1,742 cm^-1 correspond to symmetric stretching of *HCO₃^-, highlighting that CO₂ and H₂O are co-adsorbed on Cu₈SN-T.

DFT calculations identified three types of Cu sites for CO₂ adsorption in Cu₈SN-T as shown in the inset of Figure 23D: A site (Cu-S₂), B site (Cu-N₁S₂), and C site (Cu-S₃) with adsorption energies of -0.31, -0.60, and -0.99 eV, respectively, indicating that any of them can act as a site for CO₂ reduction. However, CO₂ reduction was found more likely to occur at the C site (i.e., Cu-S₃) in the thermally treated Cu₈SN-T cluster as per their energy profiles [Figure 23D]. On the basis of DRIFTS results, authors proposed a plausible reaction mechanism of PCO₂RR over Cu₈SN-T shown in Figure 23E.