English

• 1. Introduction

  The significant growing demand for fossil fuel energy resources exacerbates the energy crisis and environmental problems [1-3]. In particular, the global warming and seawater acidification due to the indiscriminate CO₂ emissions are becoming more and more serious [4]. To address these energy and environmental problems, use of clean fossil energy with low carbon content, and development of non-fossil energy, such as nuclear energy, hydropower, wind energy, and solar energy would be an alternative route. In addition, converting CO₂ gas to high value-added chemicals and/or fuels through development of carbon capture, utilization, and storage (CCUS) techniques has been proposed as one of the most promising strategies to deal with the energy crisis and further achieve carbon neutrality [5].

  At present, various techniques have been studied for CO₂ conversion, such as photochemistry [6-8], electrochemistry [9-11], thermochemistry [12,13], and biological reduction methods [14]. Among these techniques, electrocatalytic CO₂ reduction reaction (CO₂RR) has the following advantages: (ⅰ) Wide range of renewable electric energy sources, mainly include solar energy, wind energy, tidal energy, and biomass energy [15]; (ⅱ) Mild and controllable reaction conditions; (ⅲ) Compact and modular electrolyzers could be designed for mass production [16]. Using CO₂ as a raw material and converting it into value-added fuels or valuable chemicals (CO, HCOOH, CH₄, C₂H₄, CH₃CH₂OH, etc.), driven by renewable electricity, is a promising approach to reduce CO₂ emission [17-20]. In addition, electrocatalytic CO₂RR is gradually becoming mature in terms of fundamental research, theoretical predictions, in-situ characterization, and electrolyzer design [21]. In addition, it is in line with the goals of green economy and sustainable development.

  However, electrolytic CO₂RR on most catalysts usually suffers from unsatisfactory catalytic activity due to the sluggish kinetics of the CO₂ activation. In addition, the accompanying hydrogen evolution reaction (HER) is a competing reaction and will decrease the target product selectivity and energy efficiency of CO₂RR. Up to now, the low activity, poor product selectivity, undesirable product conversion rate, and catalyst degradation after long-term electrolysis are still the major challenges for the commercialization of electrocatalytic CO₂RR [22]. Designing highly efficient and stable electrocatalysts for CO₂RR could be a promising route to address these problems. Recently, p-block metal catalysts, including Bi [23-25], Sn [26-28], In [29-35], and Pb [36,37], have been widely studied for electrocatalytic CO₂RR, especially for the production of formic acid/formate. Among these p-block metal-based catalysts, indium-based catalyst has attracted great attention due to the following advantages: (ⅰ) Indium has abundant resources with relatively low cost; (ⅱ) Indium is a non-toxic element with easy processing feature; (ⅲ) Excellent physical and chemical properties, such as good electrical conductivity and the property of easily forming alloys with other metals [38]; (ⅳ) High selectivity to the production of formic acid: In has a high binding energy to ^*HCOO, which favors the conversion of CO₂ to HCOOH [39]. Up to now, various indium–based catalysts including metal structures, alloys, oxides/hydroxides, nitrides, sulfides, clusters, single-atoms, and their composites with carbon materials have been intensively synthesized and used for electrolytic CO₂RR (Table 1). The catalyst types, dimensions, morphologies, defects, surface properties, and electrochemical cells all show significant influences on the final performance of CO₂RR. To date, a few review articles have been focused on the development of metal–based CO₂RR electrocatalysts [40-43]. However, a timely and comprehensive review on the design and development of indium-based electrocatalysts for electrolytic CO₂RR is still lacking.

  Table 1

  

  Table 1.  Catalytic performances of reported indium-based catalysts for CO₂RR.

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  Herein, the recent development of indium-based electrocatalysts in electrolytic CO₂RR to C1 products is systematically summarized and discussed. The rational design of indium-based catalysts, including structure, composition, morphology, and single-atom structure, has been proven to be effective in promoting the electrolytic CO₂RR performance (Fig. 1). We first briefly introduce the possible reaction pathways for the CO₂RR on indium-based electrocatalysts surface. Then we present the design strategies of various types of indium-based electrocatalysts with high performance, including the modulation of dimension, size, morphology, composition, single-atom structure, defect, heterostructure, and some other catalysts. Finally, the remaining challenges and future development in this field are envisioned.

  Figure 1

  

  Figure 1.  Summary of In-based catalyst design strategies for highly efficient electrolytic CO₂RR.

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  2. Reaction pathways of CO₂RR on indium-based electrocatalysts

  It is generally known that the formation of products in electrolytic CO₂RR mainly includes three steps: CO₂ adsorption on catalysts surface, the transfer of electrons/protons to the absorbed CO₂ molecules, and desorption of product molecules from catalyst surface. The reaction products of CO₂ on indium-based catalysts are mainly formate or CO, which is a typical two electrons/protons transfer process. The product selectivity largely depends on the first proton coupling process on the catalyst surface. In other words, whether the first proton is added to the C atom or the O atom in the activated CO₂⁻, determining the formation of different products. At present, a large number of studies suggest that the CO₂RR on indium-based catalysts surface usually has three pathways (Fig. 2) [44-47], which including (ⅰ) the two oxygen atoms in CO₂⁻ are bound to the catalyst surface, and the carbon atom is protonated to generate the HCOO^* intermediate, which follows by a second electron transfer and protonation process to form HCOOH [4]; (ⅱ) unlike the former, ^*OCHO intermediate should be formed through an electron transfer process of the as-formed HCOO^* intermediate, which will be further protonated to form HCOOH [47]; (ⅲ) different from the previous two pathways, the C atom in CO₂⁻ may also be bonded to the catalyst surface. In this case, one of the oxygen atoms will be protonated to form a COOH^* intermediate, following by an electron transfer and protonation process and a further dehydration to form CO [11].

  Figure 2

  

  Figure 2.  Possible reaction pathways of electrolytic CO₂RR on the surface of indium-based electrocatalyst.

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  3. Advanced indium-based catalysts for high-performance electrolytic CO₂RR

  3.1 In-based alloys

  Alloying is a common and efficient approach to improve the activity and selectivity for electrolytic CO₂RR. Attributing to the altered composition, electronic structure, and surface properties (such as varied electronegativity, undercoordinated active sites, surface atom distribution, and lattice mismatch), alloys show tunable catalytic activity compared to pure metals [48-50]. Until now, numerous studies have shown that the combination of binary metal elements with different electronic properties can significantly adjust the surface chemical environment and the adsorption energy of reaction intermediates during CO₂RR. Many indium-based alloys have been reported for electrolytic CO₂RR, however, unlike Sn and Bi, indium-based alloys are more likely to enhance the selectivity of CO [51-54].

  3.1.1   Cu-In alloy

  Among the numerous bimetallic catalysts, Cu-In alloys have attracted great attention due to the unique electronic structure of Cu. Rasul et al. electrodeposited In on rough Cu surface to a Cu-In alloy catalyst, exhibiting a Faraday efficiency of 95% for CO₂-to-CO conversion at a potential of ‒0.7 V vs. RHE. The Faraday efficiency of CO can be maintained at 85% after electrolysis for 7 h (Figs. 3a and b) [55]. The theoretical calculations show that the introduction of In does not change the electronic properties of Cu, but In preferentially locates at the edge of Cu, which can stabilize the reaction intermediate (COOH^*) and further promote the reduction of CO₂ to CO (Fig. 3c). Moreover, Rasul and his group members reported the construction of Cu-In alloy interface during electrolysis process of CO₂RR when using CuInO₂ as the starting catalyst (Figs. 3d and e) [56]. The as-formed Cu-In catalyst shows a formate selectivity of 31% selectivity and a CO selectivity of 63% selectivity with suppressed HER. Theoretical calculations reveal that In occupies the stepped sites of Cu, and does not show a preferential adsorption of H^* or CO^* intermediates. While the vicinal Cu affects the adsorption of H^* or CO^* intermediates, and the synergistic effect of In and Cu finally attributes to the selectivity of electrolytic CO₂RR (Figs. 3f and g). These studies indicate that the introduction of two metals with different properties at the two-phase interface can change the adsorption strengths of reaction intermediates, thereby changing/enhancing the selectivity of target products.

  Figure 3

  

  Figure 3.  (a) Faradaic efficiencies for CO and H₂ on Cu-In alloys at different potentials. (b) Long-term stability test for Cu-In alloy at a potential 0.6 V vs. RHE. (c) Site preference of In in Cu₅₅-Icosahedron cluster and Cu₅₅-Cubooctahedron cluster. The energies relative to Cu₅₅-Icosahedron are shown. Reproduced with permission [55]. Copyright 2015, Wiley-VCH Verlag GmbH & Co. KGaA. (d) Chronopotentiometric test of CuInO₂ catalyst at a current density of ‒1.67 mA/cm². (e) SEM images of Cu-In electrodes before and after electrolysis. Optimized geometries and relative adsorption energies of H (f) and CO (g) on Cu (100), Cu (211), and In-modified (211) Cu facet. Reproduced with permission [56]. Copyright 2015, The Royal Society of Chemistry.

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  Apart from the formation of alloy interface, recent studies revealed the promotion of electrolytic CO₂RR by tuning the structure and composition of Cu-In alloy. For example, by controlling the annealing temperature, Shen et al. prepared various Cu-In bimetallic catalysts with different structures (hollow spheres and double-layer hollow spheres) and different phases (CuO and Cu₂O) for CO₂ reduction to syngas (CO and H₂) [57]. The authors found that the structure and phase of catalysts during the CO₂RR process determines the formation of syngas with different compositions (Fig. 4a). After the introduction of In into Cu, the Faradaic efficiency of CO and the syngas yield increase at all potentials with increasing annealing temperature in Ar (Figs. 4b–d). In addition, the atomic composition of Cu-In alloy directly affects product selectivity during CO₂RR. Xie and colleagues synthesized Cu-In hydroxides (Cu_xIn_y-OH) with tunable compositions by a simple hydrothermal method, and they found that the selectivity of products shifted from CO to formic acid when the In content was increased (Figs. 4e–h) [58]. Similarly, Hoffman's group also demonstrated that the selectivity of target product could be controlled by altering the alloy composition [59]. Therefore, by controlling the structure and composition of the catalyst for CO₂RR, the selectivity of the product can be directionally improved or regulated [60,61].

  Figure 4

  

  Figure 4.  (a) Illustration shows the surface evolution of Cu-In based catalysts during CO₂RR and the varying product selectivity. (b-d) Evaluation of CO₂RR performance on various catalysts. Reproduced with permission [57]. Copyright 2021, Springer. Faradaic efficiencies for various products on Cu_xIn_y hydroxide catalysts: (e) Cu₇₆In₂₄-OH, (f) Cu_43.5In_56.5-OH, (g) Cu_11.5In_88.5-OH, (h) Cu_8.5In_91.5-OH. Reproduced with permission [58]. Copyright 2021, Elsevier.

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  3.1.2   Sn-In alloy

  Sn and In exhibit similar features in electrocatalytic CO₂RR with high selectivity for formate, and the Sn-In alloy is expected to enhance this selectivity to some extent. Pardo Pérez and his co-workers reported the formation of metal/metal oxide core-shell nanoparticles via an in situ reduction of the amorphous SnInO_x film during electrocatalytic process [62]. Their results show that the in situ reduced Sn_1–XIn_X@In_1–Y was composed of Sn-rich SnIn alloy nanocore and InO_x-rich SnInO_x shell (Figs. 5a–d). Consequently, the in-situ formed catalyst had a Faradaic efficiency of 80% for formate without significant degradation under electrolysis for up to 10 h (Fig. 5e). Wang group prepared Sn-In core-shell nanoparticles with an InSn₄ core and an amorphous In-doped tin oxide shell via a one-step method [63]. They found that the content of indium in the Sn-In alloy plays an important role in determining the activity and Faradaic efficiency of CO₂ reduction to formate. The best CO₂ reduction performance was achieved when the content of In was 30% (Fig. 5f). Theoretical calculations and X-ray absorption near edge structure (XANES) spectra show that In doping in SnO₂ shell results in the generation of oxygen vacancies, promoting the production of formic acid while suppressing hydrogen evolution (Fig. 5g). This core@shell structure can tune CO₂RR selectivity through synergistic effect between core-shell components, which offers potential advantages for electrocatalysis [26,64].

  Figure 5

  

  Figure 5.  (a) Schematic illusation of the in situ conversion of SnInO_x film to Sn_1-XIn_X@In_1-YSn_YO_Z core@shell nanoparticles. (b) SEM image of synthesized SnInO_x planar film (inset is a cross-sectional SEM image). (c) SEM image of the SnInO_x film after CO₂RR for 10 min. (d) TEM image of SnInO_x_CAT@-1V_10 min (inset shows the corresponding FFT image). (e) Long-term stability of untreated SnInO_x after electrolysis at −1.0 V for 10 h. Reproduced with permission [62]. Copyright 2021, Wiley-VCH GmbH. (f) The dependence of FE_formate and j_formate on the indium content in Sn-In alloy nanoparticles at −1.0 V vs. RHE. (g) Sn K-edge XANES spectrum of SnIn alloy nanoparticles at different potentials. Reproduced with permission [63]. Copyright 2021, Elsevier.

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  3.1.3   Bi-In alloy

  Bi-In alloy has also been recognized efficient catalyst for the electroreduction of CO₂ to formic acid with high selectivity. For instance, Yao et al. reported the synthesis of BiIn alloy nanoparticles using bimetallic metal-organic frameworks as precursors, which exhibited a HCOOH Faradaic efficiency of 92.5% at the current density of 300 mA/cm². Remarkably, long-term stability for HCOOH production was achieved with the FE > 80% at the current density of 120 mA/cm² in the MEA system for 25 h [39]. Tan et al. prepared the indium−bismuth nanosphere (In₁₆Bi₈₄ NS) via a facile liquid-polyol technique. The as-synthesized In₁₆Bi₈₄ NS exhibited ~100% formic acid faradaic efficiency at ‒1.4 V vs. RHE, and the FE could be maintained > 90% in a wide potential window (−0.84 to −1.54 V vs. RHE) [65]. The excellent catalytic performance was ascribed to the modulated electronic properties from the synergistic effect of In and Bi, enhancing the adsorption of the intermediate (HCOO^*) on the catalyst surface to favor the formation of formic acid. Xiong et al. presented the controlled synthesis of bimetallic Bi_xIn_y nanofiber catalysts with 3D network structure via an electrospinning process followed by electrochemical reduction. The obtained Bi₅In₅ nanofiber catalyst achieved a maximum FE_formate of 96.8% with a production rate of ~4.55 mmol h⁻¹ cm⁻². The enhanced CO₂RR performance was attributed to the large specific surface area and exposure of the InBi (200) plane, which has high reactivity and selectivity for formate [38].

  3.1.4   Other In-based alloys

  Apart from the In-based alloy with p-block metals and transition metals, some noble metals can also form alloys with In for electrocatalytic CO₂RR. It is known that Ag-based catalysts show good selectivity to CO in CO₂RR, and combining Ag with In to form a bimetallic alloy can further improve the CO₂RR performance. Larrazábal et al. deposited Ag nanoparticles on In₂O₃ and In(OH)₃ which was further transformed into Ag-In alloy during the electrolysis process [66]. Their experiments show that the Ag contents and the support catalysts both could affect the selectivity of CO, depending on the exposure of real active sites during electrolysis. It's also noting that the introduction of In into Pd catalyst to form an In-Pd alloy can suppress the CO poisoning problem in the pure Pd catalyst to a certain extent and further improve the CO selectivity [67].

  Compared with monometallic catalysts, bimetallic alloy catalysts are composed of two different elements, which, to a certain extent, could change or modulate the electronic structure and geometry of active sites on catalyst surface. The catalyst electronic structure directly affects the binding with reaction intermediates and the reaction pathways. Changes in the geometry affects the local atomic arrangement of the active site, which could favor the formation of certain intermediates and effectively alter the reaction pathway. Thus, the alloying process could enhance the adsorption and activation capabilities of the catalyst, further improving the overall performance of the catalyst [50,68]. On the other hand, the introduction of another element into the indium-based catalyst to form an In-based alloy could realize the regulation of product selectivity. Although alloying can increase product diversity, the Faradaic efficiency for individual product may decrease compared to bare indium catalysts. Thus, the future research in this field needs rational design of the structure, components, and composition of the In-based alloy to exert the role of the two components to maximize the synergistic effect to produce the target product.

  3.2 Atomically dispersed catalysts

  3.2.1   In single atomic catalysts

  Single-atom catalysts (SACs) have attracted extensive attention due to their full atom utilization efficiency and excellent catalytic performance. P-block metals (Bi [69], In [70], Sn [71]), transition metals (Fe [72], Ni [73], Zn [74]), and noble metals (Ag [75], Pd [76]) all have been used to construct single-atom catalysts. Ni SACs were the most widely used catalyst in CO₂RR research. Recently, In SACs have also been reported for electrocatalytic CO₂RR due to their excellent CO₂ catalytic activity.

  Zhang et al. prepared a series of In-N_xC_4−x (1 ≤ x ≤ 4) single-atom catalysts via a pyrolysis process for CO₂ reduction to CO [77]. It was found that the dominate product on In₂O₃ NPs catalyst is formate with a small amount of CO. While the reduction product on In SACs is transformed into CO, indicating the regulation of product selectivity on indium-based catalysts (Fig. 6a). Theoretical calculations simulated the adsorption configurations and evolution processes of catalysts with small molecules, and the results showed that the change of the product from formate to CO on the In SACs is due to the transfer of the real active site from pure metallic In center to the adjacent carbon atom coordinated by the In atom (Fig. 6b). Beside, In SACs can also affect the formation energies of reaction intermediates by adjusting the hybridization energy levels. Li and his collaborators proposed two types of In SACs: four nitrogen atoms-coordinated (In-N₄) SAC and three nitrogen atoms-coordinated with a vacancy (In-N₃-V) SAC (Fig. 6c) [78]. Notably, In-N₃-V catalyst exhibits a CO Faradaic efficiency (FE_CO) of 95%, which is much higher than that of In-N₄ catalyst with a maximum FE_CO of 80% (Fig. 6d). Theoretical calculations were further performed to investigate the intrinsic relationship between SAC structure and performance. Compared with In-N₄, the In atoms in In-N₃-V are much easier to enrich electrons, and the vacancy makes the s and p_z orbital energies of In atoms closer to the Fermi level, thereby reducing the formation energy of COOH^* and improving catalytic performance (Figs. 6e–g). The selectivity of CO₂RR products can be effectively tuned by regulation of the catalytic active sites and atomic structure of In SACs. In this regard, revealing the intrinsic relationship between the structure and catalytic property of SACs provides guidance for rational design of electrocatalysts.

  Figure 6

  

  Figure 6.  (a) Product selectivity of In₂O₃ and InSA catalysts at different potentials. (b) Proposed reaction pathways of CO₂RR on pr-In-N₄-(In) and pr-In-NC₃-(C2). Reproduced with permission [77]. Copyright 2022, Springer Nature. (c) Schematic illustration showing the prepration of In-N₃-V SACs. (d) FE_CO at various potentials for In@NC-900 and In@NC-1000. (e, f) Calculated charge density for In−N₄ and In−N₃−V catalytic centers (yellow iso-surfaces represent depletion and blue iso-surfaces represent gain of electron density. An iso-surface value was set at 0.001 e/ų). (g) PDOS of In (s and p_z orbitals In−N₄ and In−N₃−V) with C (s and p_z orbitals in COOH^*). Reproduced with permission [78]. Copyright 2022, American Chemical Society.

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  Even CO was the mostly reported reduction product on In SACs, formic acid/formate has also been achieved on the In-based SACs. For example, Shang et al. synthesized In^δ+-N₄ SACs through wet-impregnation and pyrolysis, exhibiting a FE_formate of 96% at the potential of ‒0.65 V vs. RHE and a turnover frequency (TOF) of 12500 h⁻¹ at ‒0.95 V vs. RHE [79]. In addition, Liu and his collaborators demonstrated that In single atoms supported on carbon substrates (In-N-C) can effectively improve the electrocatalytic performance of CO₂ reduction reaction, and the ^*OCHO intermediate formed at atomically dispersed In sites directly determine the formation of formate [80]. Due to the relatively simple coordination environment and well-defined active sites of In single atoms, the adsorption strength of intermediates can be effectively modulated by adjusting the coordination environment, thereby determining the product selectivity.

  3.2.2   In-based diatomic catalysts

  Although single-atom catalysts have high atom utilization efficiency, they still suffer from simple structures and lack of cooperative active sites, which limits their application in some complex reactions [81]. To solve this problem, single-atom catalysts with dual metals (diatomic catalysts) have been rationally designed. Yuan et al. prepared Cu-In diatomic catalyst by the pyrolysis of CuIn metal-organic framework (MOF). The Cu-In diatomic catalyst shows high selectivity for CO₂RR to methanol, which is higher than that of pure copper single-atom catalysts [82]. The Cu-In dual atomic sites were proved to effectively hinder the hydrogen evolution reaction and promote the conversion of CO₂ to methanol due to the synergistic effect between Cu and In. Fan and coworkers designed a dual-site atomic In-Ni catalyst anchored on nitrogenated carbon (InNi DS/NC). The InNi DS/NC has an excellent CO selectivity > 90% over a wide potential range (−0.5 to −0.8 V vs. RHE). In situ ATR-SEIRAS and theoretical calculations reveal that the In-Ni diatomic active sites and the O atom bridge cannot only lower the reaction energy barrier of ^*COOH but also suppress hydrogen evolution, promoting the CO₂RR performance [83]. In addition, atomically dispersed In-Ce diatomic catalysts on nitrogen-doped carbon (In-Ce/CN) have also prepared for CO₂ reduction to formic acid [84]. The introduction of Ce single atomic sites cannot only significantly promote the electron transfer during the reaction process but also optimize the In-5p orbitals, favoring the formation of. It is apparent that the two adjacent separated active sites in diatomic catalysts can provide two absorption sites and act as synergistic sites to improve catalytic activity as compared to the bare single-atom catalysts [85].

  3.3 Indium oxide-based catalysts

  3.3.1   In₂O₃

  In₂O₃ is also a typical catalyst in CO₂RR, and many studies have shown that the structure engineering of In₂O₃ is an effective way to enhance the CO₂RR performance [86,87]. Understanding the correlation between structure and catalytic performance of catalysts is of great importance. Firstly, the size or dimension of catalyst shows a significant influence in the catalytic reaction. The size engineering of the catalyst not only can affect the specific surface area but also can tune the local electronic structure of the catalyst, showing a critical role in determining the catalytic performance. To prove this point, Huang et al. prepared 5 nm In₂O₃ nanoparticles (NPs) and 15 nm In₂O₃ nanocubes by a solvothermal reaction [88]. Compared with 200 nm In₂O₃ cubes and 5 nm In₂O₃ NPs, 15 nm In₂O₃ nanocubes possessed highest formate Faradaic efficiency (> 95%) and negligible side reactions with a smallest overpotential (Figs. 7a and b). Fourier-transform extended X-ray absorption fine structure (EXAFS) spectroscopy reveals that the coordination numbers of In decreases with the decrease of the size (Fig. 7c). Moreover, theoretical calculations simulated the In₂O₃ models with three different particle sizes. The free energy diagrams for the formation of reaction intermediates show that size reduction (coordination deficiency) is beneficial for the adsorption of all three intermediates (^*OCHO, COOH^*, and H^*) (Fig. 7d), but the H^* adsorption free energy gradually approaches the thermoneutral as the catalyst size shrinks down, indicating that HER becomes more competitive at smaller sizes. This also agrees with the experimental result that 5 nm In₂O₃ shows higher H₂ selectivity (lower formate selectivity) than 15 nm In₂O₃. This study provides a new perspective on the structure-property relationship of indium-based catalysts: the size engineering of the catalyst does not necessarily mean that the smaller the better.

  Figure 7

  

  Figure 7.  Faradaic efficiencies for H₂ (a) and Faradaic efficiencies for formate (b) of different catalysts at various potentials. (c) EXAFS spectra and corresponding fitting curves for 5 nm-NPs and 15 nm-nanocubes (inset is a table showng the fitting parameters). (d) Optimized configurations of ^*OCHO and ^*COOH absrobed on In (101), In₁₆₅, and In₈₅ surface, and the Gibbs free energy diagrams for the formation of HCOO^–, CO and H₂. Reproduced with permission [88]. Copyright 2021, Wiley-VCH GmbH.

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  Except the size engineering, combining indium-based oxides with conductive carbon substrates to form hybrid materials (with potential chemical coupling between different components) is another effective route to modulate the CO₂RR performance. For example, Zhang and coworkers revealed the role of chemical interactions between In₂O₃ and reduced graphene oxide (rGO) in promoting the CO₂RR (Fig. 8a) [89]. In their work, porous In₂O₃ nanobelts were uniformly grown on reduced graphene oxide (Fig. 8b). Compared with the physically mixed In₂O₃-rGO catalyst, the Faradaic efficiency and partial current density of chemically bonded In₂O₃-rGO hybrid catalyst for formate formation were increased by 1.4 times and 3.6 times, respectively. Further, charge distribution and Gibbs free energy diagrams revealed that the strong chemical coupling between In₂O₃ and rGO could enhance electric conductivity and reduce the formation energy of reduction intermediates (HCOO^*), thus promoting the electrolytic CO₂RR activity (Figs. 8c and d). In addition, In₂O₃ nanoparticles dispersed on conductive carbon nanorods achieved a FE_formate of 90% and the catalyst also delivered a high current density of 300 mA/cm² when applied to a gas-diffusion electrode-based flow cell [90].

  Figure 8

  

  Figure 8.  (a) Schematic shows the preparation of porous In₂O₃−rGO hybrid structure. (b) TEM image of porous In₂O₃−rGO hybrid structure. (c) Calculated differential charge diagram for In₂O₃−rGO hybrid catalyst. (d) Gibbs free energy diagrams for CO₂-to-formate on different catalysts. Reproduced with permission [89]. Copyright 2019, American Chemical Society. (e) Electron paramagnetic resonance (EPR) spectra of three InOx samples. Partial current density (f) and Faradaic efficiency (g) of HCOO⁻ on three InO_x samples. Reproduced with permission [91]. Copyright 2019, Wiley-VCH Verlag GmbH & Co. KGaA.

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  Oxygen vacancy engineering to modulate the electronic structure of In₂O₃ is also an effective strategy to improve the CO₂RR performance. Zhang and his team synthesized InO_x nanobelts with three different concentrations of oxygen vacancies, in which the highest oxygen concentration leads to the best electrolytic CO₂RR performance (Figs. 8e–g) [91]. Cheng et al. synthesized In₂O₃ nanorods with different oxygen vacancy concentrations through an annealing atmosphere-assisted strategy for efficient electrocatalytic CO₂ reduction to produce formate (Figs. 9a and b) [92]. A combination of XPS, EPR, and XAFS techniques revealed the successful tuning of oxygen vacancy concentration in In₂O₃ (Figs. 9c–e). Notably, the introduction of oxygen vacancies significantly enhanced the conversion activity and selectivity of CO₂RR to formate (Fig. 9f). DFT calculations show that oxygen vacancies can significantly promote the interaction between HCOO^* intermediates and In atoms and lower the energy barrier for the formation of HCOO^*, leading to a high selectivity to CO₂ conversion to formate (Figs. 9g–j). Oxygen vacancies in the electrocatalyst can promote the adsorption and activation of CO₂ molecules and accelerate the electron transfer, thus improving the CO₂RR performance.

  Figure 9

  

  Figure 9.  (a) A typical SEM image of oxygen vacancy-rich In₂O₃ catalyst (R-In₂O₃)_. (b) HRTEM image of R-In₂O_3. (c) O 1s XPS spectra of In₂O₃, P-In₂O₃, and R-In₂O₃ nanorods. (d) Corresponding EPR spectra of three In₂O₃ samples. (e) In K-edge XANES spectra of In₂O₃, P-In₂O₃, R-In₂O₃ nanorods, and reference samples. (f) Faradaic efficiencies of formate on three In₂O₃ catalysts. (g–i) Electronic localization function (ELF) of ^*HCOO on three In₂O₃ catalysts surfaces. (j) Gibbs free energy diagrams of CO₂-to-HCOOH over three catalysts. Reproduced with permission [92]. Copyright 2023, American Chemical Society.

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  Various strategies have been used to enhance the CO₂RR performance on In₂O₃ catalysts, however, the real active site under electrocatalytic conditions is not well understood. Could the active site experience reversive transformation by the reaction intermediates or the O atoms during the reaction? The catalytic mechanism is kept or changed during the dynamic evolution of the active site is still unsolved. Therefore, it is essential to monitor the evolution of the catalyst and probe reaction intermediates and/or products on the catalyst surface by combining in situ characterization techniques and theoretical calculations.

  3.3.2   Doped In₂O₃

  As a method to change the electronic structure of materials, element doping has been widely used in the field of semiconductors. The regulation of electronic structure can always tune the semiconductor performance by altering its band structure [93]. In the past decades, element doping has also received extensive attention in the field of electrocatalysis. It is generally believed that HER is a competing reaction of CO₂RR, and the promoted activation of H₂O will reduce the Faradaic efficiency of CO₂RR products. However, Ma and his team broke this understanding and proposed that the activation of H₂O molecules plays an important role in the reduction of CO₂ on their sulfur-doped In catalyst (Figs. 10a and b) [94]. In a wide range of 25–100 mA/cm², the FE_formate on this catalyst can be maintained above 85%. The highest production rate can reach 1449 µmol h⁻¹ cm⁻² at a FE_formate of 93% (Fig. 10c). DFT calculations indicated that the S doping can significantly reduce the formation energy of ^*HCOO on the catalyst surface (Fig. 10d). This doping effect is also effective when S was changed to other chalcogen elements such as selenium and tellurium or the metallic In was changed to other p-block metals (Bi, Sn), indicating that the doping is an effective route to promote the H₂O molecules activation and further improve the electrocatalytic CO₂RR performance (Figs. 10e and f). In addition to non-metal doping, metal doping of In₂O₃ catalyst for electrocatalytic CO₂RR has also been reported. Kim et al. found that monodispersed V-doped In₂O₃ nanocrystals with a staged octahedral shape exhibited unusual selectivity for CO₂ reduction to methanol, which was achieved by previously reported In₂O₃ catalysts [95]. Their results indicate that the V doping has a remarkable effect on the stabilization of intermediates, moving the reduction further and resulting in the generation of more reduced species (such as methanol).

  Figure 10

  

  Figure 10.  (a) Schematic illustration showing the role of S²⁻ in enhanced CO₂RR to formate via promoting the water dissociation and the fo rmation of H^*. (b) SEM image of S−In catalyst (inset shows the HRTEM image). (c) Formation rates of different products, and FEs of formate on In foil and S−In catalysts. (d) Gibbs free energy diagrams for CO₂ reduction to formate on In (101) and S−In (101) surface. (e) Formation rates of formate on S−In, Se−In and Te−In catalysts. (f) Formation rates of formate over S−Bi and S−Sn catalysts. Reproduced with permission [94]. Copyright 2019, Springer Nature.

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  Doping is a simple and outstanding method to modulate the electronic structure of electrocatalyst, and the CO₂RR performance can be significantly enhanced by a small amount of element doping. In the enhancement of the CO₂RR performance, doping works in two ways: (ⅰ) Accelerate electron transfer and improve the electric conductivity of electrocatalyst to improve catalytic activity; (ⅱ) Adjust the adsorption energies of CO₂ molecules and the key reaction intermediates on the catalyst to improve the product selectivity. Therefore, the rational doping of the In₂O₃ catalyst could highly improve the CO₂RR performance.

  3.3.3   In₂O₃-based heterojunction

  It is well known that the construction of heterojunction interface can significantly alter the electron distribution of electrocatalyst and adjust the adsorption strength of reaction intermediates to improve the CO₂RR performance. Zhao et al. designed an In₂O₃/InN heterojunction and used it in electrolytic CO₂RR for the efficient production of formate for the first time [96]. They prepared the In₂O₃/InN heterojunction catalyst through a hydrothermal reaction together with a further annealing in N₂/NH₃ atmosphere. Their high-resolution TEM images showed that the as-synthesized heterojunction interface has well-defined lattice fringes, corresponding to (101) plane of hexagonal InN and (222) plane of cubic In₂O₃, respectively (Fig. 11a). They applied this In₂O₃/InN heterojunction catalyst for CO₂ reduction and found that the catalyst had a maximum FE_formate of 95%. The FE_formate could be maintained > 90% in a wide potential range between ‒1.37 V and ‒1.59 V (Fig. 11b). Electron localization function (ELF) analysis showed that the O atom in In₂O₃ and the N atom in InN could form a strong O-N coupling at the interface and offer a charge transfer channel for electron enrichment on InN surface (Fig. 11c). The Gibbs free energy change for CO₂RR to formate was further calculated to explore the role of the In₂O₃/InN heterojunction interface in the CO₂RR. The energy diagram shows that the In₂O₃/InN heterojunction can significantly strengthen the combination with CO₂ molecules and reduce the formation energy of the ^*HCOO intermediate, leading to the enhancement of the formate selectivity (Fig. 11d).

  Figure 11

  

  Figure 11.  (a) High-resolution TEM image of In₂O₃/InN heterojunction. (b) Faradaic efficiencies for formate, CO and CH₄ over In₂O₃ and In₂O₃/InN at different potentials. (c) Calculated electron localization function of In₂O₃/InN heterojunction. (d) Gibbs free energy diagram for CO₂RR to formate on the In₂O₃/InN heterojunction. Reproduced with permission [96]. Copyright 2022, Elsevier. (e) Faradaic efficiencies of formate on In/In₂O₃-800, In/In₂O₃-1600, In/In₂O₃-2400 catalysts. (f) Operando Raman spectra of In/In₂O₃-1600 with diffeerent applied potentials. Reproduced with permission [97]. Copyright 2022, Wiley-VCH GmbH.

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  Apart from directly synthesizing electrocatalysts with heterojunction interfaces, in situ formation of heterojunction during electrolytic reactions is another effective way to enhance the catalytic activity of In-based catalysts. For example, Wulan et al. prepared the In₂O₃ catalyst by a fast Joule heating method and such In₂O₃ catalyst was in situ transformed into an In/In₂O₃ heterojunction catalyst during the CO₂ reduction to formate [97]. The introduction of the heterojunction interface can tune the electronic structure of In and accelerate the protonation process of the adsorbed bicarbonate species, leading to a high FE_formate of 94%. The in situ experimental characterizations and theoretical calculations show that the In/In₂O₃ heterojunction catalyst can always maintain stable In-O species as real active sites under different electrolysis conditions (Figs. 11e and f).

  The rational design of heterojunction catalyst through a feasible strategy to modulate the interfacial structures and properties is promising strategy to further improve the CO₂RR performance on In₂O₃ catalysts. However, revealing the structural evolution and real active sites of electrocatalysts during electrocatalytic reduction is challenging and of great significance to build the structure-activity relationship to further synthesize high-performance catalyst and to deeply understand the CO₂RR reduction mechanism.

  3.4 Other indium-based catalysts

  In addition to the catalysts mentioned above, indium sulfide or selenide-based composites have also been fabricated for electrocatalytic CO₂RR. Ning et al. prepared a GO supported In₂S₃ composite (In₂S₃-GO), which showed a high formate selectivity due to the high specific surface area and sufficient exposure of the (440) plane of In₂S₃ nanosheets [98]. Similarly, Liu et al. prepared the In nanoparticles-decorated In₂S₃ during the electrolytic CO₂RR [99]. The In–In₂S₃ hybrid catalyst showed a remarkable current density (for carbonaceous product) of 62.1 mA/cm² with a maximum FE_formate of 76%. The improved catalytic activity was due to the increased carrier density and the lower work function after decoration of In nanoparticles, benefiting the CO₂ activation. In₂Se₃ catalyst has also been used for electrocatalytic CO₂RR, and the Faradaic efficiency of CO reaches 89% (Fig. 12a) [100]. DFT calculations suggested that the exposed Se atoms in In₂Se₃ could stabilize ^*COOH intermediates and thus boost the electrochemical performance of the CO₂-to-CO conversion (Fig. 12b). Moreover, Zhang et al. synthesized a highly efficient InN electrocatalyst which showed a FE of 95% for carbonaceous product (Figs. 12c and d) [101]. It was found that the InN was converted to an In-rich catalyst during the CO₂RR process. They proposed that the surface reconstruction resulted in a surface charge redistribution, significantly facilitating the adsorption of HCOO^* and thus favoring the CO₂ reduction to formate (Figs. 12e and f).

  Figure 12

  

  Figure 12.  (a) Faradaic efficiencies of CO on ultrathin In₂Se₃ nanosheet and bulk In₂Se₃ at different potentials. (b) Gibbs free energy diagrams of the formation of CO and HCOOH on In₂Se₃ (001) surface. Reproduced with permission [100]. Copyright 2019, Elsevier. (c) Faradaic efficiencies for different products over InN and In nanosheets at different applied potentials. (d) Faradaic efficiencies for different products over InN nanosheets at −0.9 V vs. RHE for 3 h. (e) Gibbs free energy diagrams of the formation of HCOOH on InN and In@InN slabs. (f) Projected density of states (PDOS) of HCOO^* intermediates adsorbed on InN and In@InN slabs. Reproduced with permission [101]. Copyright 2020, American Chemical Society.

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  4. Summary and outlook

  In this review, we briefly reviewed the possible reaction pathways on In-based catalysts and the recently reported various In-based electrocatalysts were comprehensively and systematically summarized with a focus on their synthetic strategies, structures, catalytic performance, and reaction mechanisms. Strategies such as alloying, creating single-atom sites, element doping, heterojunction interface engineering, and defect engineering have significantly improved the catalytic performance of In-based catalysts for electrolytic CO₂RR. Nevertheless, there is still plenty of room to further develop highly efficient In-based catalysts for a wide range of practical and commercial applications. In future, more research focus should be paid on the economical production of stable In-based electrocatalyst, the deep understanding of dynamic active sites, and the potential industrialization of the electrolytic CO₂RR.

  Firstly, most of the In-based catalysts suffered from complicated preparation processes. The development of a simple and scalable synthetic route to synthesize highly efficient In-based electrocatalysts is challenging but of significance for commercialization. The preparation strategies such as electroplating/electrodeposition, hydrothermal/solvothermal reaction, fast Joule-heating or the combination of these techniques might be helpful for large-scale production of stable In-based catalysts with low cost. Moreover, very few works have evaluated the long-term stability (> 100 h) of In-based electrocatalysts; therefore, the chemical stability of In-based catalysts also needs further exploration to meet the industrial requirement for long-term electrolysis.

  Secondly, the structure and chemical states of In-based electrocatalysts always change during the CO₂RR reduction process due to the applied large negative potentials and the possible transformation in the used electrolytes. To exactly identify the real active sites and correctly unveil the intrinsic reaction mechanism of electrolytic CO₂RR, in situ and/or operando characterizations techniques such as X-ray diffraction (XRD), electron paramagnetic resonance (EPR), Raman spectroscopy, attenuated total reflectance-Fourier transform infrared spectroscopy (ATR-FTIR), and X-ray absorption spectroscopy (XAS) should be used to monitor the phase/chemical state changes of In-based catalysts, the absorption/desorption of reaction intermediates during the electrolytic CO₂RR process. Meanwhile, the combination of theoretical calculation and in situ technique could be an effective way to deeply understand the reaction mechanism at the molecular level, and to provide a guidance for the rational design of highly-efficient electrocatalysts.

  Finally, optimizing the cathode/anode reactions to improve energy utilization efficiency should also be considered. Specifically, the reactor design, the local environment, and CO₂ gas source should be optimized to improve the catalytic performance and reduce the cost. The current H-type cells always show limit current densities, and thus new types of electrolyzers, such as flow cells and membrane electrode assembly (MEA) systems, are needed to be further developed for practical applications. In addition, for liquid-phase products such as formic acid, the use of solid-state electrolytes is of significance because they can directly and effectively separate the liquid products during the CO₂RR process, greatly reducing the cost of product separation. Finally, the integration of CO₂-based battery devices to fulfill the high efficiency CO₂ utilization and conversion should also be developed.

  Declaration of competing interest

  The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

  Acknowledgments

  This work was supported by National Science Foundation of China (Nos. 22225606, 22261142663 and 22176029), the Excellent Youth Foundation of Sichuan Scientific Committee (No. 2021JDJQ0006).

  
  
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  Figure 1  Summary of In-based catalyst design strategies for highly efficient electrolytic CO₂RR.

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  Figure 2  Possible reaction pathways of electrolytic CO₂RR on the surface of indium-based electrocatalyst.

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  Figure 3  (a) Faradaic efficiencies for CO and H₂ on Cu-In alloys at different potentials. (b) Long-term stability test for Cu-In alloy at a potential 0.6 V vs. RHE. (c) Site preference of In in Cu₅₅-Icosahedron cluster and Cu₅₅-Cubooctahedron cluster. The energies relative to Cu₅₅-Icosahedron are shown. Reproduced with permission [55]. Copyright 2015, Wiley-VCH Verlag GmbH & Co. KGaA. (d) Chronopotentiometric test of CuInO₂ catalyst at a current density of ‒1.67 mA/cm². (e) SEM images of Cu-In electrodes before and after electrolysis. Optimized geometries and relative adsorption energies of H (f) and CO (g) on Cu (100), Cu (211), and In-modified (211) Cu facet. Reproduced with permission [56]. Copyright 2015, The Royal Society of Chemistry.

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  Figure 4  (a) Illustration shows the surface evolution of Cu-In based catalysts during CO₂RR and the varying product selectivity. (b-d) Evaluation of CO₂RR performance on various catalysts. Reproduced with permission [57]. Copyright 2021, Springer. Faradaic efficiencies for various products on Cu_xIn_y hydroxide catalysts: (e) Cu₇₆In₂₄-OH, (f) Cu_43.5In_56.5-OH, (g) Cu_11.5In_88.5-OH, (h) Cu_8.5In_91.5-OH. Reproduced with permission [58]. Copyright 2021, Elsevier.

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  Figure 5  (a) Schematic illusation of the in situ conversion of SnInO_x film to Sn_1-XIn_X@In_1-YSn_YO_Z core@shell nanoparticles. (b) SEM image of synthesized SnInO_x planar film (inset is a cross-sectional SEM image). (c) SEM image of the SnInO_x film after CO₂RR for 10 min. (d) TEM image of SnInO_x_CAT@-1V_10 min (inset shows the corresponding FFT image). (e) Long-term stability of untreated SnInO_x after electrolysis at −1.0 V for 10 h. Reproduced with permission [62]. Copyright 2021, Wiley-VCH GmbH. (f) The dependence of FE_formate and j_formate on the indium content in Sn-In alloy nanoparticles at −1.0 V vs. RHE. (g) Sn K-edge XANES spectrum of SnIn alloy nanoparticles at different potentials. Reproduced with permission [63]. Copyright 2021, Elsevier.

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  Figure 6  (a) Product selectivity of In₂O₃ and InSA catalysts at different potentials. (b) Proposed reaction pathways of CO₂RR on pr-In-N₄-(In) and pr-In-NC₃-(C2). Reproduced with permission [77]. Copyright 2022, Springer Nature. (c) Schematic illustration showing the prepration of In-N₃-V SACs. (d) FE_CO at various potentials for In@NC-900 and In@NC-1000. (e, f) Calculated charge density for In−N₄ and In−N₃−V catalytic centers (yellow iso-surfaces represent depletion and blue iso-surfaces represent gain of electron density. An iso-surface value was set at 0.001 e/ų). (g) PDOS of In (s and p_z orbitals In−N₄ and In−N₃−V) with C (s and p_z orbitals in COOH^*). Reproduced with permission [78]. Copyright 2022, American Chemical Society.

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  Figure 7  Faradaic efficiencies for H₂ (a) and Faradaic efficiencies for formate (b) of different catalysts at various potentials. (c) EXAFS spectra and corresponding fitting curves for 5 nm-NPs and 15 nm-nanocubes (inset is a table showng the fitting parameters). (d) Optimized configurations of ^*OCHO and ^*COOH absrobed on In (101), In₁₆₅, and In₈₅ surface, and the Gibbs free energy diagrams for the formation of HCOO^–, CO and H₂. Reproduced with permission [88]. Copyright 2021, Wiley-VCH GmbH.

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  Figure 8  (a) Schematic shows the preparation of porous In₂O₃−rGO hybrid structure. (b) TEM image of porous In₂O₃−rGO hybrid structure. (c) Calculated differential charge diagram for In₂O₃−rGO hybrid catalyst. (d) Gibbs free energy diagrams for CO₂-to-formate on different catalysts. Reproduced with permission [89]. Copyright 2019, American Chemical Society. (e) Electron paramagnetic resonance (EPR) spectra of three InOx samples. Partial current density (f) and Faradaic efficiency (g) of HCOO⁻ on three InO_x samples. Reproduced with permission [91]. Copyright 2019, Wiley-VCH Verlag GmbH & Co. KGaA.

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  Figure 9  (a) A typical SEM image of oxygen vacancy-rich In₂O₃ catalyst (R-In₂O₃)_. (b) HRTEM image of R-In₂O_3. (c) O 1s XPS spectra of In₂O₃, P-In₂O₃, and R-In₂O₃ nanorods. (d) Corresponding EPR spectra of three In₂O₃ samples. (e) In K-edge XANES spectra of In₂O₃, P-In₂O₃, R-In₂O₃ nanorods, and reference samples. (f) Faradaic efficiencies of formate on three In₂O₃ catalysts. (g–i) Electronic localization function (ELF) of ^*HCOO on three In₂O₃ catalysts surfaces. (j) Gibbs free energy diagrams of CO₂-to-HCOOH over three catalysts. Reproduced with permission [92]. Copyright 2023, American Chemical Society.

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  Figure 10  (a) Schematic illustration showing the role of S²⁻ in enhanced CO₂RR to formate via promoting the water dissociation and the fo rmation of H^*. (b) SEM image of S−In catalyst (inset shows the HRTEM image). (c) Formation rates of different products, and FEs of formate on In foil and S−In catalysts. (d) Gibbs free energy diagrams for CO₂ reduction to formate on In (101) and S−In (101) surface. (e) Formation rates of formate on S−In, Se−In and Te−In catalysts. (f) Formation rates of formate over S−Bi and S−Sn catalysts. Reproduced with permission [94]. Copyright 2019, Springer Nature.

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  Figure 11  (a) High-resolution TEM image of In₂O₃/InN heterojunction. (b) Faradaic efficiencies for formate, CO and CH₄ over In₂O₃ and In₂O₃/InN at different potentials. (c) Calculated electron localization function of In₂O₃/InN heterojunction. (d) Gibbs free energy diagram for CO₂RR to formate on the In₂O₃/InN heterojunction. Reproduced with permission [96]. Copyright 2022, Elsevier. (e) Faradaic efficiencies of formate on In/In₂O₃-800, In/In₂O₃-1600, In/In₂O₃-2400 catalysts. (f) Operando Raman spectra of In/In₂O₃-1600 with diffeerent applied potentials. Reproduced with permission [97]. Copyright 2022, Wiley-VCH GmbH.

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  Figure 12  (a) Faradaic efficiencies of CO on ultrathin In₂Se₃ nanosheet and bulk In₂Se₃ at different potentials. (b) Gibbs free energy diagrams of the formation of CO and HCOOH on In₂Se₃ (001) surface. Reproduced with permission [100]. Copyright 2019, Elsevier. (c) Faradaic efficiencies for different products over InN and In nanosheets at different applied potentials. (d) Faradaic efficiencies for different products over InN nanosheets at −0.9 V vs. RHE for 3 h. (e) Gibbs free energy diagrams of the formation of HCOOH on InN and In@InN slabs. (f) Projected density of states (PDOS) of HCOO^* intermediates adsorbed on InN and In@InN slabs. Reproduced with permission [101]. Copyright 2020, American Chemical Society.

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  Table 1.  Catalytic performances of reported indium-based catalysts for CO₂RR.

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