Recent Progress in Surface-Defect Engineering Strategies for Electrocatalysts toward Electrochemical CO₂ Reduction: A Review

Abstract

Climate change, caused by greenhouse gas emissions, is one of the biggest threats to the world. As per the IEA report of 2021, global CO₂ emissions amounted to around 31.5 Gt, which increased the atmospheric concentration of CO₂ up to 412.5 ppm. Thus, there is an imperative demand for the development of new technologies to convert CO₂ into value-added feedstock products such as alcohols, hydrocarbons, carbon monoxide, chemicals, and clean fuels. The intrinsic properties of the catalytic materials are the main factors influencing the efficiency of electrochemical CO₂ reduction (CO₂-RR) reactions. Additionally, the electroreduction of CO₂ is mainly affected by poor selectivity and large overpotential requirements. However, these issues can be overcome by modifying heterogeneous electrocatalysts to control their morphology, size, crystal facets, grain boundaries, and surface defects/vacancies. This article reviews the recent progress in electrochemical CO₂ reduction reactions accomplished by surface-defective electrocatalysts and identifies significant research gaps for designing highly efficient electrocatalytic materials.

1. Introduction

The continuous combustion of fossil fuels for energy production has led to the release of extreme concentrations of carbon dioxide (CO₂) into the atmosphere. Furthermore, CO₂ traps solar radiation within the atmospheric surface of the Earth (i.e., the greenhouse effect) and causes severe climate alterations (i.e., global warming). According to the 2022 United Nations Environment Programme (UNEP) report, the global temperature will be raised by 1.8 °C by the end of this century due to the greenhouse effect. According to the latest report (November 2022), the global average CO₂ level has reached a new record of 417.3 ppm, which is about 1 ppm (0.24%) higher than the previous year.

The conversion of abundant CO₂ into chemical and fuel energy using renewable energy sources is considered a promising alternative method for utilizing and converting renewables and environmental protection [1,2]. Several conversion techniques, including thermo-chemical, photo-chemical, electrochemical, and biological methods, have been used to convert CO₂ into valuable feedstock chemicals. Among them, the heterogeneous electrocatalytic/electrochemical CO₂ reduction reaction (EC-CO₂-RR) has attracted keen interest owing to its enormous advantages, as it leads to the direct conversion of fuels and valuable chemical products [3,4]. Figure 1A presents a simplified pictorial representation of the anthropogenic carbon cycle. The CO₂ emissions are controlled by capturing and converting CO₂ gas by the abovementioned technical pathways. This efficient carbon cycling strategy solves energy and environmental issues and creates a highly profitable carbon economy platform [5]. In the presence of intermittent renewable electricity and water (proton donor), the oxygen evolution reaction (OER) occurs at the anode compartment, and the EC-CO₂-RR takes place at the cathode compartment, converting CO₂ into the various carbon products, such as carbon monoxide (CO), formic acid (HCOOH), methane (CH₄), methanol (CH₃OH), ethanol (CH₃CH₂OH), acetic acid (CH₃COOH), and ethylene/olefins (CH₂=CH₂) (Figure 1B). Owing to these numerous advantages, the EC-CO₂-RR has received keen attention from researchers. Among the different products, the EC-CO₂-RR produces a large amount of gaseous CO due to its low overpotential of formation or multi-electron transfer reaction kinetics (i.e., two-electron transfer process) compared to that of other hydrocarbon products (i.e., multi-electron processes). However, the EC-CO₂-RR has some practical downsides/challenges, such as the high overpotential requirements for the multi-electron transfer reaction, low current density, product selectivity, and poor Faradaic efficiency (FE). Furthermore, the current densities are still limited to <30 mA/cm² owing to the longer diffusion length and low solubility of CO_2(g) in aqueous/electrolyte media. Due to the high stability of the C=O bond (750 kJ/mol), the complexity of the charge transfer and the interference of HER overpotentials cause sluggish EC-CO₂-RR kinetics and a low FE in the desired product, respectively.

Several efforts have been made to enhance electrochemical CO₂ reduction and large-scale viability, including the design of catalysts and reactors, electrode modification, and electrolyte optimization, followed by the pioneering research conducted by Hori and his co-workers [7]. Much work is needed to develop novel technologies for efficiently capturing and converting CO₂ into value-added chemicals. Thus, this mini-review discusses the main practical problems, challenges, and mechanistic pathways of the EC-CO₂-RR and presents a clear overview. In particular, the recent electrocatalyst design processes/modifications via surface-defect engineering approaches in significant metallic, metal oxide/sulfide, and carbon-based alloy catalysts are discussed. Furthermore, this review presents several electrocatalyst design techniques for attaining higher current densities, low overpotentials, and higher Faradaic efficiencies, with the aim of obtaining the desired value-added products through the electrochemical CO₂-RR.

2. Electrocatalytic CO₂ Reduction Reaction (CO₂-RR) Mechanism on Electrode Surface

Many research groups have published exciting results with novel electrocatalytic materials and are trying to understand the exact reaction mechanism of electrochemical CO₂ reduction and the catalytic performance of various metal/metal-mediated catalysts. According to the thermodynamic prospects, the electrochemical reduction of CO₂ into mono-carbon (C₁) and multi-carbon (C₂) products is generally a multi-electron transfer process associated with a proton-coupling reaction [8]. Furthermore, CO₂ electroreduction depends on the electrocatalytic materials, electrolytes, pH, and related experimental conditions. Regardless of the carbon products formed, the CO₂-RR is generally initiated by a single-electron injection to form unstable anionic ^•CO₂⁻ radicals. This entails a high overpotential, and, more importantly, the designated reaction pathways (i.e., CO or formate) are determined by the coordination/adsorption of ^•CO₂⁻ radicals on the electrode surface [9,10], since ^•CO₂⁻ has a high energy (−1.9 V vs. SHE) and readily reacts with adsorbed water molecules, including other CO₂ molecules in the solution, producing CO, formate, and other reduction products. The standard redox potentials (vs. SHE) of electrochemical CO₂ reduction reactions for the formation of various products are shown in Figure 2B [11].

The transition-metal electrocatalysts are categorized into four groups according to their binding tendency (available orbitals) and active d-orbital electrons for binding with ^•CO₂⁻ intermediate species (Figure 2A). The first group of metals (i.e., Ni, Pt, Fe, and Ti) are less active towards CO₂-RR but mostly favor hydrogen (H₂) generation due to their strong hydrogen binding tendency. The second group consists of metals such as Zn, Au, Ag, and Pd. These metals can bind with CO₂^•− intermediates to different degrees, producing carbon monoxide (CO) as a significant product, and are not able to reduce further owing to their lower binding affinity for surface CO. The group-three metals, including In, Sn, Pb, Hg, Tl, Cd, and Bi, can bind with intermediates and generate formic acid/formate species because of the greater negative formation potential of HCOO⁻ and the weak adsorption of the intermediates on the surface [12,13]. The fourth group consists of copper (Cu), which has been widely used as a CO₂ electrocatalyst to produce various (C₁) hydrocarbons and alcohol products owing to its excellent binding tendency with intermediates, including ^•CO₂⁻ and CO [9,10,14]. In general, the heterogeneous electrocatalytic CO₂-RR involves the following three significant steps: (i) the chemisorption of CO₂ on the electrode surface, (ii) the electron injection process to cleave C–O bonds and the proton transfer reaction to form C–H bonds (hydrocarbons), and (iii) the desorption of the final carbon products from the electrode surface and their diffusion into the electrolyte medium.

Consequently, forming C₂ reduction products is tedious compared to forming C₁ products such as CO and formate, since it is a multi-electron transfer process. Additionally, in the formation of C₂ hydrocarbons, such as ethyl alcohol and ethylene, the competing C–C and C–H bonds during the electrochemical CO₂-RR depend on the electrolyte conditions and applied potential [15]. In the CO₂-RR, the C–C bond formation is considered the rate-determining step (RDS) for the generation of C₂ species [16]. Thus, the formation of C₂ reduction species can be classified into the ethylene and ethanol pathways (Figure 2C). The ethylene pathway starts with the dimerization of two surface-adsorbed CO intermediates, generating a C₂O₂⁻ intermediate in a bridging mode. After that, the C₂O₂⁻ intermediate undergoes protonation reactions (*CO + H = *CHO; *CHO + CO = *COCHO; *COCO + H = *COCOH) and thus produces stable CO–COH and COCHO intermediates. Previous studies have stated that the formation of C–C through the two surface-adsorbed CO intermediates occurs at a low overpotential. At the same time, the reaction between the surface-bound CO and *CHO intermediate requires a high overpotential due to the enormous activation barrier to form a CO dimer. Furthermore, the subsequent reduction of CO to *COCHO, followed by CO to *CHO conversion, is more favorable than the CO dimerization reaction [17]. When comparing the tautomeric *COCOH and *COCHO structures, *COCHO is more stable and preferred due to its lower involvement in generating double bonds on the surface or forming free radicals on the carbon atom. Therefore, the *COCHO intermediate can be protonated or reduced into COCH₂OH and undergoes electron injection and the proton transfer reaction to produce the final H₂C=CH₂ (ethylene) products.

On the other hand, the ethanol pathway starts with the intermediates (i.e., *COCHO and *COCOH), resulting in the ethylene pathway. However, forming *COCHO intermediates is considered a pivotal step in the selectivity of the C₂ reduction products on the Cu surface. C–C bond formation is significant for generating C₂H₅OH and other C₂₊ reduction products [17]. The *COCHO further undergoes a reduction/protonation reaction for ethanol production and generates *C₂H₂O₂ (cis-glyoxal). At this stage, the consumption of glyoxal is thermodynamically more favorable owing to the stable oxygen bonds of cis-glyoxal on the catalyst surface. Then, the cis-glyoxal is further reduced to form acetaldehyde (at a low potential) and ethanol (at a high potential). The cis-glyoxal to ethanol conversion consists in the formation of several intermediates, such as C₂H₆O₂ (ethylene glycol), C₂H₄O (vinyl alcohol), and CH₃CHO (acetaldehyde), in which the vinyl alcohol is quickly reduced to generate ethanol rather than that of the ethylene glycol owing to its negative –OH groups and sp²-hybridized oxygen atoms bound with the copper surface [18]. Moreover, a recent study conducted by Varandili et al. presented the concept of forming C₂ products over a CuZn catalytic system for electrochemical CO₂ reduction reactions. The as-reported catalysts, Cu-derived ZnO@Cu and ZnO-derived Cu@ZnO, showed selectivity towards ethanol and methane, respectively. The higher concentration of Zn immensely increased the concentration of CO and shifted the selectivity from CH₄ to C₂H₅OH through a tandem conversion mechanism [19]. Additionally, Iyengar et al. established a high ethanol selectivity over a Cu–Ag tandem electrocatalyst. Their results suggested that the *CH_x–*CO coupling pathway drastically enhanced the production of C₂H₅OH. This indicated that under a high CO concentration, the bifurcation reactions through *CH_x–*CO coupling occurred prior to the –CO dimerization reaction on Cu(111), leading to ethanol production over ethylene [20]. The overall electrochemical process involving the reactor cell is expressed below (Equations (1) and (2)) [21]:

$$\mathrm{At} \mathrm{anode}:{\mathrm{H}}_2\mathrm{O}-\mathrm{electrons}\left({\mathrm{e}}^{-}\right)\to {\mathrm{O}}_2+{\mathrm{H}}^{+}$$

(1)

$$\mathrm{At} \mathrm{cathode}:{\mathrm{CO}}_2+\mathrm{electrons}\left({\mathrm{e}}^{-}\right)+{\mathrm{H}}^{+}\to \mathrm{Various} \mathrm{reduction} \mathrm{products} ({\mathrm{C}}_1,{\mathrm{C}}_2)$$

(2)

2.1. Advantages and Economic Benefits of Electrochemical CO₂ Conversion

Electrochemical CO₂ reduction (CO₂R) is considered one of the most effective methods of reducing CO₂ emissions [22,23]. Electrochemical methods work in moderate conditions and may be tailored to specific products, enable modular systems, and allow the combination of renewable energy and CO₂ conversion in energy-intensive industrial sectors, including iron and steel production [24]. The corresponding advantages of the electroreduction of CO₂ for society are illustrated in Figure 3A. Previous research has largely found short-term, local, and provincial advantages for lowering greenhouse gas emissions, while global warming is a long-term, worldwide problem [25]. Because the advantages of lowering emissions will not be realized in their entirety for 50 to 100 years, the world has been sluggish in addressing climate change [26]. The CO₂ electrochemical reduction reaction (CO₂-RR) converts the greenhouse gas CO₂ into value-added goods, reducing carbon emissions and the use of fossil fuels and feedstocks to manufacture fuel/chemical products [27]. This offers the possibility of combining electrocatalytic procedures using renewable energy to store renewable energy. The electrochemical reduction of CO₂ seems to be a way to contribute to a less-carbon-based economy by providing value-added compounds from CO₂ whilst employing minimal renewable energy [28]. Renewable electricity costs have dropped dramatically in recent years, so that it is now competitive with conventional power generation methods and may shortly be the lower-priced source of power [29]. By employing particular transition metals for electrocatalysts, synthetic chemicals may be produced from the electrochemical reduction of CO₂. The products generated depend on the metal’s potential to absorb CO, the key step in the CO₂ reduction cycle [30]. In this context, employing aqueous electrolytes and CO₂ conversion to fuels and feedstocks using renewable energy is a highly appealing alternative to closing the carbon cycle [31]. The liquid electrolyte–gas diffusion electrode system also has several practical drawbacks, related to system stability and energy efficiency. The membrane electrode assembly (MEA)-based electrolyzer system for the EC-CO₂-RR was recently introduced to overcome these issues. Moreover, the enhanced performance of the MEA system is associated with eliminating ohmic losses in the electrolyte, the fouling of electrolyte impurities with catalysts, and flooding through gas diffusion electrodes. Thus, the combined hybrid system strategies enhance the production of gaseous and liquid products with long-term (>100 h) uninterrupted operation stability. Several recent studies have reported the MEA-based electrolyzer system’s enhanced performance resulting in ~80% and ~50% FE for C₂₊ and ethylene products, respectively, with current density and operation stabilities over 100 h [32,33,34].

Conversely, due to its low cost and environmental compatibility, heterogeneous electrocatalysis has recently attracted great interest. In the past decade, these electrocatalysts have been widely explored with a combination of three strategies: the first is the electroreduction of CO₂ to syngas, a combination of CO and H₂, employing Ag and Au electrodes, which approximates closely to the real-world applications [35,36]; the second approach is to convert CO₂ to formate on Sn electrodes with Faradaic efficiencies (FE) of more than 70% at current densities ranging from 100 mA/cm² to 300 mA/cm² [37]; and the third and most challenging approach is to reduce CO₂ to C₂ and C₃ hydrocarbons such as ethylene, ethanol, and n-propanol. Among the reduction products, formate (formic acid) is widely used by several industries, including in the production of medicine, dyes, rubber, leather, and pesticides. Nevertheless, the industrial manufacturing of formate involves tedious methanol carbonylation and hydrolysis processes, which are costly processes, higher energy demanding, and yielded with unfavorable intermediates [38]. Likewise, the production cost of CO₂-RR products such as CO, CH₃OH, CH₂=CH₂, C₂H₅OH, CH₄, and HCOOH is more viable than commercial and industrial manufacturing and operational costs (Figure 3B).

The commercial and economic viability of CO₂-RR products (kg) is easily calculated, since the production is majorly depended on the input of electricity (kWh⁻¹). Furthermore, life-cycle assessment (LCA) is considered an important evaluation method to quantify the environmental impacts and performance of a process or technology. A recent article published by Ong et al. described a prospective LCA of the EC–CO₂-RR for the production of selective formic acid and ethylene products based on a standard laboratory-scale H-cell reactor design. The analysis found that the average production of formic acid and ethylene by the electrochemical CO₂ reduction method (ECR) was around 57% and 52%, respectively. In addition, the formic acid production via the ECR process reduced the consumption of petrochemical resources by 24% and the ethylene production via the ECR processes significantly decreased the human health damage (67%), petrochemical consumption (110%), and ecosystem damage (94%) [39]. Khoo et al. presented a unique LCA case study assessing the potential environmental impacts of small-scale and large-scale models for the electrochemical reduction of CO₂ to ethylene. The authors conducted an LCA for the CO₂-RR with 1 g ethylene production for the small-scale experimental setup and 1 ton ethylene production for the large-scale industry-standard experiment with a similar CO₂ capture and separation set up. Producing 0.98~3.7 g CO₂ on a small scale was equivalent to producing 0.65~3.0 tons of CO₂ in the large-scale model [40]. Formic acid consumed more electrical energy than the other C₂–C₄ products. Currently, the selectivity of the maximum electrocatalytic materials is as follows: CO or formic acid ~100% [41,42,43,44], ethylene ~60% [45,46,47], acetate ~40% [48,49,50], and n-propanol ~15% [51]. This necessitates additional purification costs. Thus, electrochemical reduction (ECR) often occurs under favorable reaction conditions at ambient temperatures and pressures, which substantially advantages large-scale applications.

2.2. Challenges of CO₂ Electroreduction

Generally, the CO₂ reactants are supplied to the active sites (CO₂ mass transport) of the catalyst surface (i.e., electrode surface) by diffusion via the non-flowing bulk electrolyte medium. Hence, as discussed before, the reduction reaction begins with the formation of intermediate products. Additionally, the reaction rate or efficiency for obtaining the desired products depends on the binding tendency, electron–proton coupling reactions, and the solubility/mass-transportation efficiency of CO₂. In brief, HCOO⁻ products are formed by electron transfer and proton injection. If the catalyst surface has only adsorbed protons, this leads to the formation of ^•COOH and CO. In the case of the strong adsorption of CO on the metal surface (i.e., Cu), it will further reduce into the C₁ and C₂ products; otherwise, the weak adsorption (i.e., Zn, Ag, Au, Pd) of CO will lead to desorption from the catalyst surface without any further reduction processes [52]. Since the mass transportation of CO₂ in the bulk electrolyte is limited, and the gas reactant at the catalyst surface is insufficient, several recent studies have tried to overcome this issue by designing gas-diffusion electrode (GDE) materials to achieve high and flawless CO₂ flux at the cathode surface [11,53]. The GDE maintains a high CO₂ (gas) concentration in the vicinity of the electrocatalyst through the porous-structured electrodes that can enhance gas mass transport and current densities. When compared to the static non-flowing CO₂-RR cell, the GDE consists of the following electrode segments: a gas diffusion layer (GDL), a carbon paper/cloth or polytetrafluoroethylene (PTFE) membrane, and a catalyst layer (CL), followed by a microporous carbon/PTFE layer (MPL). In general, the catalyst inks are prepared and coated on the GDL during the fabrication of the GDE [34,54]. The electrode configuration of the GDE is relatively complicated and more efficient than that of laboratory electrochemical H-cell and flow-cell planar electrode systems. In addition, the other technological challenges related to the commercialization of the electrochemical CO₂-RR pertain to its efficiency, which is mainly hindered by the voltage efficiency (η), Faradaic efficiency (FE), and electrode stability (Figure 4).

Even though the kinetics of the CO₂-RR are thermodynamically more favorable than the HER, the electrocatalytic efficiency is mainly affected by sluggish reaction kinetics; high overpotentials (i.e., voltage efficiency); and poor selectivity, owing to the similar reduction potentials of the various reduction products [8]. The overpotential (η) is defined as the extra energy (i.e., potential/voltage) required to drive CO₂ reduction in addition to its thermodynamically required energy. Previous studies have suggested that the standard overpotential for the CO₂-RR is −1.90 V (vs. SHE), and the concept of overpotential originated from the sluggish reaction kinetics of the formation of ^•CO₂⁻ intermediate products [9,55]. Furthermore, the problems related to voltage efficiency can be overcome by catalyst modifications (surface defects) to enhance the stabilization of the intermediate products on metal surfaces. Secondly, FE is defined as the efficiency with which charges (i.e., electrons) are transferred in a system facilitating an electrochemical reaction for the generation of a desired product. Usually, in electrochemical CO₂ conversion reactions, the current is an essential component that is consumed to generate reduced products; in most reported cases, the loss of current (i.e., energy loss) was due to the occurrence/formation of undesirable reactions or products (e.g., H₂). Thus, electrocatalytic materials with high H₂ overpotentials characteristically present a promising FE for the CO₂-RR [52]. The product selectivity of the different catalytic materials is measured by the generated products’ Faradaic efficiency. The FE of the desired products can be calculated by the following expression: FE = nFN/Q × 100%, where n is the number of electrons transferred per mole (products), N is the number of moles (products), Q is the total charge consumed during the electrochemical reaction, and F is the Faradaic constant [56].

The electrocatalyst stability or electrode stability is also considered a key performance indicator for evaluating a catalyst’s long-term applicability via the produced cathodic current density and FE of the desired products during the CO₂-RR [5]. Due to the generation of byproducts, the electrolyte medium is considered a primary source of impurities in the working electrode. In some cases, the produced intermediates or byproducts are adsorbed on the electrode surface, restricting further reactions and affecting the electrocatalyst’s performance. In this regard, more research must be carried out to fabricate impurity-resistance working electrodes for long-term stability and to decrease the operational cost of the CO₂-RR [52,57]. Figure 4 (right side) depicts various catalyst modification processes and designs for an enhanced electrochemical CO₂ reduction reaction. The overpotential and selectivity of the CO₂-RR are significantly influenced by the electronic structure (i.e., binding energies with intermediate species) of the catalytic materials. On the other hand, the geometric properties, such as the structure/morphology, surface roughness, porous nature, and particle size, are directly related to the number of catalytic active sites for catalyst–CO₂ interactions. Thus, a large grain boundary can be created by highly nanoporous materials, quantum confinement can be achieved through a confined geometry, and electronic properties can be directly modified by alloying and doping processes between metals.

2.3. Factors Affecting CO₂ Electroreduction

The efficiency of the CO₂-RR is affected by several factors, mainly including the nature or type of electrocatalyst, the electrochemical cell parameters, and the electrolyte. Among these factors, the physicochemical properties of the electrocatalytic materials have a significant role in the CO₂-RR efficiency and selectivity. Many strategies have been put forward for the rational design of novel electrocatalytic materials. Nanoscale material synthesis [58], specific facet-oriented material preparation [59], the construction of porous materials [60], the formation of alloys [61], doping [62], defect engineering [63], and surface modification [64] have been implemented to enhance CO₂-RR efficiency through increased adsorption sites, enhanced intermediate product formation/binding capabilities, and altered geometric and electronic properties [5].

The activity or catalytic performance of electrocatalysts can be further confirmed through the total current density produced via an anodic CO₂-RR. The current density (J) is determined via the normalization of the catalytic current (i.e., anodic current) by the geometric area or by the electrochemical active surface area (ECSA) of the working electrode [65]. On the other hand, the rate of the CO₂-RR is indicated by the turnover frequency (TOF), which is derived from the active sites of the catalytic materials [66]. In addition to the above factors, the efficiency of the CO₂-RR is mostly influenced by the potential-dependent structure of the solvent, the concentration, the potential-dependent behavior of the ions present in the electrolyte, the impact of the double-layer (adsorbed) species, and electrode–electrolyte ion interactions [5,67].

3. Effect of Surface Defect Sites on CO₂-RR Electrocatalysts

The physicochemical properties and performance of all ‘solid-state’ materials are controlled by defects. In general, synthesizing a perfect crystal without any defect is difficult. The surface and electronic performance of catalysts are tuned according to several intrinsic properties, such as the morphology, particle size, crystal facets, lattice distortion, surface defects, surface functionalization, grain boundaries, doping, oxygen (O_v) or copper (Cu_v) vacancies, and surface states of the element [9,68,69,70,71,72]. Surface defect sites or structural disorder can be rationally introduced via several engineering strategies, influencing site-specific activities in the CO₂-RR [73,74]. Surface/crystal defects can be identified or characterized by several techniques, such as Raman spectroscopy [75], X-ray photoelectron spectroscopy (XPS) [76], X-ray absorption spectroscopy (XAS) [77], electron spin resonance (ESR) [78], aberration-corrected transmission electron microscopy (AC-TEM) [79], and density functional theory (DFT) methods [80,81], based on the changes in the vibrational modes of the crystal, the changes in the bonding energy, the electronic structure of the crystal or local geometric measurements, the determination of unpaired electrons, atomic-scale defect counting, and computational simulations, respectively.

Among the above-mentioned surface properties of the catalytic materials, the defect and interface properties influence the efficiency, selectivity, and performance. Electrochemical reactions (i.e., CO₂-RR) mostly depend on the catalysts’ adsorption, charge distribution, and surface structure properties [73,82]. Moreover, defects or lattice imperfections are consistently observed in materials such as crystalline (low-dimensional) and amorphous solids [83,84]. Crystal defects can be classified into four types: line defects, point defects, bulk defects, and planar defects. However, most materials have native defects, which are intensely involved in catalytic activities. In addition to the native defects, other defects can be introduced to a catalyst (i.e., composition, etching, pretreatment, and doping) to increase the catalytic performance [85,86,87,88].

In general, the defects and interfaces have diverse functional scales. In brief, the defects at the edges can stabilize the interface, and the near-interface environment enables defect formation. Consequently, vacancies and doping focus on the 0D environment, while grain boundaries generally exist in 1D or 2D environments. Usually, the 2D and 3D boundaries are the most predominant active sites for electrochemical reactions [89]. Figure 5A shows the systematic strategies that have been applied for defect engineering and interface processes. Crystal defects and interfaces may have a robust synergistic effect on catalytic reactions by creating more active sites for CO₂ adsorption for the surface reactions [90]. Figure 5B depicts the effect induced by defects/interfaces on the catalyst surface. The most evident advantages of interface/defect engineering processes are: (i) the increase in the active sites on the catalyst surface, (ii) the facilitating of different types of surface charges (i.e., negative and positive) for CO₂ adsorption, (iii) the development of a vacancy-influenced unsaturated coordination environment, and (iv) the creation of a defect-localized non-uniform charge distribution over the catalyst surface. The performance of the EC-CO₂-RR over the electrocatalyst mostly depends on the number of active sites on the catalyst surface. Additionally, it can be modified through electronic structure and surface structure regulations. The presence of intrinsic and extrinsic defects in metal-free materials, carbon materials, metal oxides, metal sulfides, and single/bimetallic materials can efficiently facilitate the EC-CO₂-RR by modulating and improving the reaction active sites and the electronic structures for the effective adsorption and conversion of the reaction species via the enhancement of the local electron density, the binding energies of the reaction intermediate species, and the conductivity of the materials [91].

3.1. Single/Bimetallic Materials

Over the past several decades, numerous studies have focused on designing high-performance single and bimetallic materials (such as Sn, Zn, Fe, Cu, Bi, Tl, Pb, Au, Ag, Pd, and Pt and Cu-Zn, Sn-Cu, Au-Cu, PdCu, and In-Cu) for high product selectivity and stability [11,92,93]. Among the metallic electrocatalytic materials, copper (Cu) is widely reported as an excellent catalyst for CO₂ reduction into the various C₂ hydrocarbons [94,95,96], with appreciable current densities of up to 5~10 mA cm⁻² [96,97,98]. However, ‘selectivity’ is an important concept that needs to be considered to overcome the practical application and commercialization challenges. The selectivity of the desired hydrogenated products is influenced by the proton concentration near the electrode, the crystal structure, the morphology, and the size of Cu-based catalytic materials [99,100]. On the other hand, single-atom catalysts have attracted much interest owing to their many attractive features and benefits, such as their metal-supportive solid chemical interactions, large atom utilization, low-coordination metal atoms, and specific electronic configurations [101]. Additionally, they have an excellent CO₂ to CO selectivity, which indicates that the HER competition is extremely limited due to the large number of inherent isolated active sites, which facilitates the low adsorption of H₂O on the catalyst surface [102] and diminishes the H₂ generation. Jaramillo and his group reported the combined theoretical and experimental evaluation of the selectivity of different metal electrodes for electrochemical CO₂ reduction. According to the well-defined volcano plots of several metals [13], the noble metals, including Au, Ag, Pt, and Cu, have weak binding tendencies and weak interactions with *OCHO intermediates and are thus not able to produce formatted species (low selectivity).

In contrast, Zn and Ni have a strong binding tendency and interact strongly with *OCHO intermediates; however, they also have a low selectivity towards formate, owing to the easy desorption of intermediates from the surface. Sn is located at the top of the volcano plot, demonstrating that it has a sufficient binding tendency with *OCHO and thus produces formate species. Moreover, Cu, Bi, Sn, Sb, Pd, In, and Pb demonstrated high selectivity [103] for HCOO⁻ products (Table 1). Specifically, a key drawback of these catalysts is their high overpotentials (i.e., low catalytic activity), which can be overcome by several techniques, such as catalyst modification (i.e., doping, alloying, rational arrangements, porous structures, and atomically thin 2D-engineering and defect-engineering processes) [104,105,106].

In the electrochemical CO₂-RR, carbon monoxide is probably the most common intermediate species for producing formate, methanol, and methane. DFT calculations for electrochemical reduction on Cu (211) surfaces were conducted by Peterson et al., showing that the most thermodynamically favorable pathway involved the initial formation of *CO; the subsequent hydrogenation to *HCO, *H₂CO (desorbed as HCHO), and *H₃CO (methoxy) intermediates; and the formation of CH₃OH by *H₃CO reduction [107]. Liu et al. reported a new strategy to introduce defective sites by eliminating carbon deposition on the Cu electrode [108]. Since Cu is an excellent metal catalyst that converts CO₂ into high-value-added hydrocarbon fuels, it is limited by poor stability due to the deposition of carbon on the electrode surface following the production of CH₄, which leads to deactivation. Liu et al. reported the synthesis of 1D Cu-nanoneedles (CuNNs) and found that (1) crystal defects facilitated ethylene formation while suppressing methane production, and (2) crystal defects enhanced the catalytic stability, revealing that the ethylene formation pathway avoided the carbon deposition related to methane formation (Figure 6A,B). The carbon deposition on the Cu electrode favored H₂ generation rather than CO, CH₄, or C₂H₄ generation, which was confirmed by the FE% of the annealed (under pure Ar) CuNN electrodes (Figure 6C) and demonstrated that carbon deposition decreased the stability of the electrocatalytic reaction.

In contrast, the FE% of the C₂H₄ produced by pure CuNNs and OT-CuNNs (annealed under air) was 15% and 52% higher than that produced by the annealed CuNN electrode, respectively (Figure 6D,E). Additionally, the FE% of CH₄ decreased by around 4% (CuNNs) and 1.8% (OT-CuNNs) compared to the annealed CuNNs. Furthermore, the instability of the annealed CuNNs was revealed by i-t measurements (Figure 6F), thus proving the long-term stability of CuNNs and OT-CuNN electrodes. Figure 6G,H presents the defect densities vs. the catalytic stability and the Raman measurements. From these results, it can be quickly understood that the different annealing treatments of the Cu electrodes led to various reduction product (C₂H₄) distributions and enhanced the FE% with excellent stability. Recently, Mi et al. demonstrated the selective electroreduction of CO₂ to C₂ reduction products in the presence of Cu nanowires derived from Cu₃N. Compared to the Cu₃N nanowires, the Cu₃N-derived Cu nanowires exhibited many grain boundaries, which undoubtedly facilitated high selectivity, catalytic activity, and long-term durability. Furthermore, the Cu-NWs (single-metal catalyst) showed a high FE of 86% for C₂ products at −1.0 V (vs. RHE). Additionally, the stability test revealed 28h of flawless durability for continuous electrolysis [109]. Another work reported by Hoang et al. revealed the excellent catalytic performance of a bimetallic Cu-based catalyst (Cu-Ag) synthesized by the electrodeposition technique for the CO₂-RR. The obtained results showed that the Cu-Ag film containing 6% Ag exhibited a superior CO₂ reduction performance, with high FEs of 60% and 25% for ethylene (C₂H₄) and ethanol (C₂H₅OH), respectively, at −0.7 V (vs. RHE) with a total current density of 300 mA/cm² [110]. More recently, Fu et al. reported an atomically dispersed highly defective Ni-N₃/C catalyst for superior electrochemical CO₂ reduction reactions. Herein, the micro-mesoporous carbon substrate enabled high-charge transport and provided more active sites for CO₂ adsorption and conversion. The as-reported Ni-SAs/HMMNC-800 electrocatalyst achieved excellent selectivity, with a high FE of 99.5% for the formation of CO at −0.7 V (vs. RHE), and exhibited a high current density of 13 mA/cm² with long-term working stability. The electrochemical activity enhancement was mainly due to the Ni-N₃ coordination sites coupled with defects, which favored the generation of large amounts of *COOH intermediates and suppressed competing H₂ generation [111]. Moreover, Zhang et al. synthesized highly defective and amorphous Au-NCs@ MnO₂ nanosheet core–shells (Au NCs@a-MnO₂ NSs) for effective CO₂ reduction into CO. Herein, the Au NCs@a-MnO₂ NSs showed highly gas-permeable behavior due to their nanocrystal/defective nanosheet/core–shell structures, which helped to expand their CO₂ adsorption capabilities. Additionally, the Au NC core promoted high electron conductivity and boosted the electron transport from the catalyst to the CO₂ molecules. Thus, the Au NCs@ α-MnO₂ NSs core–shell electrocatalyst exhibited a high FE of 90.5% for CO conversion at −0.7 V (vs. RHE), with a high current density of 14.3 mA/cm². The above results revealed that the synergistic structural and electronic effect between Au and α-MnO₂ was the major reason for the excellent electrocatalytic activity, selectivity, and stability [112]. Another recent work reported by Wu et al. elucidated the construction of single-atomic Ni-sites in CNT for the CO₂-RR. The Ni/N-CNTs catalyst was constructed through a one-pot pyrolysis route, exhibiting many surface defects and single-atom Ni sites for highly effective CO₂ capture and reduction reactions. Furthermore, the as-prepared Ni/N-CNT electrocatalyst exhibited excellent electrocatalytic performance with a 98% FE for CO at −0.65 V (vs. RHE) and an excellent TOF of 304.5 h⁻¹ [113]. Furthermore, Ali et al. reported the synthesis of porous aza-doped unique 2D graphene analogs for the electrochemical conversion of CO₂ to formic acid. The single-layered porous aza-fused/pi-conjugated graphene exhibited highly ordered specific porous structures, and the dispersed N-atoms helped to stabilize the CO [114]. In addition to the abovementioned research, a recent study conducted by Ni et al. investigated in-plane defective metal-nitrogen-carbon catalysts for the electroreduction of CO₂ (CO₂-RR). Herein, the authors demonstrated that the intrinsic carbon defects could be substantially enhanced by coupling single-atom Fe-N₄ sites. The DFT calculations revealed that the energy barrier for CO₂ reduction could be reduced by the intrinsic defects associated with the Fe-N₄ sites, which could also suppress H₂ production. Thus, the resulting electrocatalyst exhibited an FE of around 90% for CO, with a high current density of 33 mA/cm² [115].

Table 1. Single- and bimetallic-material-based electrocatalysts for electrochemical CO₂-RR.

Single and Bimetallic Materials for CO2-RR
Catalyst	Type of Defect	Electrolyte/E vs. RHE	Product	* FE (%)	Ref.
BiOx@C	O2 vacancy	0.1 M KHCO3/−0.52 V	HCOO−	89.3	[116]
Bi-few layers	Exfoliation	0.5 M KHCO3/−0.7 V	HCOO−	85	[117]
Exfoliated Bi nanosheets	Increased edge sites/defects	0.5 M KHCO3/−1.1 V	HCOO−	86	[118]
Bulk Bi nanosheets				64.9
Sn foil	High native O2 content	0.1 M KHCO3/−1.36 V	HCOO−	49.1	[119]
Heat-treated Sn dendrites				71.6
Few-layer Sb nanosheets	Exfoliation/active edge sites	0.5 M NaHCO3/−1.07 V	HCOO−	84	[120]
Cu dendrite	Oxide-derived surface active sites	1.0 M Na2SO4/−0.8 V	C2H4	55	[121]
			C2H6
Cu (100)	Crystal orientations/ defects	0.1 M KHCO3/−5 mA/cm2	C2H4	40.4	[122]
			C2H5OH	9.7
			CO	0.9
			CH4	30.4
Cu mesocrystals	Facets/edge defects	0.1 M KHCO3/−0.99 V	C2H4	27.2	[123]
			HCOO−	4.3
			CO	0.55
			CH4	1.47
Bi/Cu	Surface oxide layer	0.1 M KHCO3/−1.69 V	HCOO−	91.3	[124]
Pd-Sn	Alloying/tuning surface electronic structures	0.5 M KHCO3/−0.26 V	HCOO−	99	[125]
Sn56.3-Pb43.7/ Carbon cloth	Alloying	0.5 M KHCO3/−2.19 V	HCOO−	79.8	[126]
Core–shell Ag-Sn NPs	O2 vacant sites	0.5 M NaHCO3/−0.81 V	HCOO−	87.2	[127]
Cu/Au	Core–shell structures	0.5 M KHCO3/−0.65 V	CO	30	[128]
Pd85Cu15	Metal doping/alloying	0.1 M KHCO3/−0.89 V	CO	86	[129]
Cu38Cd62	Alloying	0.05 M KHCO3/−1.12 V	HCOO−	10	[130]
			CO	43
Cu-In	Alloying/edge sites	0.1 M KHCO3/−0.7 V	CO	95	[131]
Ordered Cu-Pd	Alloying/tuned geometric effects	1.0 M KOH/−0.55 V	CO	80	[132]
Disordered Cu-Pd		1.0 M KOH/−0.60 V	CO	60
			CH4	1.0
			C2H4	4.0
			C2H5OH	2.0
Phase-separated Cu-Pd		1.0 M KOH/−0.71 V	CO	17
			C2H4	48
			C2H5OH	15

* FE% = Faradaic efficiency (%); HCOO⁻ = formate/formic acid; CO = carbon monoxide; C₂H₅OH = ethanol; CH₄ = methane; C₂H₄ = ethylene; H₂ = hydrogen.

Table 1. Single- and bimetallic-material-based electrocatalysts for electrochemical CO₂-RR.

Single and Bimetallic Materials for CO2-RR
Catalyst	Type of Defect	Electrolyte/E vs. RHE	Product	* FE (%)	Ref.
BiOx@C	O2 vacancy	0.1 M KHCO3/−0.52 V	HCOO−	89.3	[116]
Bi-few layers	Exfoliation	0.5 M KHCO3/−0.7 V	HCOO−	85	[117]
Exfoliated Bi nanosheets	Increased edge sites/defects	0.5 M KHCO3/−1.1 V	HCOO−	86	[118]
Bulk Bi nanosheets				64.9
Sn foil	High native O2 content	0.1 M KHCO3/−1.36 V	HCOO−	49.1	[119]
Heat-treated Sn dendrites				71.6
Few-layer Sb nanosheets	Exfoliation/active edge sites	0.5 M NaHCO3/−1.07 V	HCOO−	84	[120]
Cu dendrite	Oxide-derived surface active sites	1.0 M Na2SO4/−0.8 V	C2H4	55	[121]
			C2H6
Cu (100)	Crystal orientations/ defects	0.1 M KHCO3/−5 mA/cm2	C2H4	40.4	[122]
			C2H5OH	9.7
			CO	0.9
			CH4	30.4
Cu mesocrystals	Facets/edge defects	0.1 M KHCO3/−0.99 V	C2H4	27.2	[123]
			HCOO−	4.3
			CO	0.55
			CH4	1.47
Bi/Cu	Surface oxide layer	0.1 M KHCO3/−1.69 V	HCOO−	91.3	[124]
Pd-Sn	Alloying/tuning surface electronic structures	0.5 M KHCO3/−0.26 V	HCOO−	99	[125]
Sn56.3-Pb43.7/ Carbon cloth	Alloying	0.5 M KHCO3/−2.19 V	HCOO−	79.8	[126]
Core–shell Ag-Sn NPs	O2 vacant sites	0.5 M NaHCO3/−0.81 V	HCOO−	87.2	[127]
Cu/Au	Core–shell structures	0.5 M KHCO3/−0.65 V	CO	30	[128]
Pd85Cu15	Metal doping/alloying	0.1 M KHCO3/−0.89 V	CO	86	[129]
Cu38Cd62	Alloying	0.05 M KHCO3/−1.12 V	HCOO−	10	[130]
			CO	43
Cu-In	Alloying/edge sites	0.1 M KHCO3/−0.7 V	CO	95	[131]
Ordered Cu-Pd	Alloying/tuned geometric effects	1.0 M KOH/−0.55 V	CO	80	[132]
Disordered Cu-Pd		1.0 M KOH/−0.60 V	CO	60
			CH4	1.0
			C2H4	4.0
			C2H5OH	2.0
Phase-separated Cu-Pd		1.0 M KOH/−0.71 V	CO	17
			C2H4	48
			C2H5OH	15

* FE% = Faradaic efficiency (%); HCOO⁻ = formate/formic acid; CO = carbon monoxide; C₂H₅OH = ethanol; CH₄ = methane; C₂H₄ = ethylene; H₂ = hydrogen.

3.2. Metal Alloys

As seen above, numerous metal electrodes have been utilized/investigated for the electrochemical CO₂-RR. Furthermore, according the abovementioned studies, the binding energy/tendency of the metals strongly influences the selectivity of the CO₂ reduction products. Several recent studies demonstrate that alloying has received much attention due to its excellent catalytic performance, achieved by tuning the intermediate products’ binding energies and stabilization degrees [70] (

Table 2. Single-metal- and alloy-based electrocatalysts for electrochemical CO₂-RR.

	Single Metals and Alloys for CO2-RR
Catalyst	Type of Defect	Electrolyte/E vs. RHE	Product	FE (%)	Ref.
AuNPs/CNT	Metal doping	0.25 M Na2CO3/−0.5 V	CO	94	[142]
Cu NPs (solar-driven)	Grain boundary	1 M KOH/−1.0 V to −1.3 V	C2	73.1	[143]
Bi nanodendrites	Low-angle grain boundary	0.5 M KHCO3/−0.76 V	HCOO−	92	[144]
CuAg wire	Alloying/intrinsic defects (point, line, and plane)	1 M KOH/−0.7 V	C2H4	60	[110]
			C2H5OH	25
Au-Pd Core–shell	Core–shell structures	0.5 M KHCO3/−0.6 V	CO	96.7	[137]
AuCu3@Au	Oxidative etching/alloying	0.5 M KHCO3/−0.6 V	CO	97.27	[136]
Cu@CuEu NPs	Point (Eu) defect	0.5 M KHCO3/−1.2 V	CH4	74.7	[139]
Bi@Sn NPs	Compressive strain effect	0.5 M KHCO3/−1.1 V	HCOOH	91	[140]
Cu@Sn	Metal doping	0.5 M KHCO3/−0.93 V	HCOO−	100	[141]

). Investigations have also been conducted on ECR in Sn-based alloys through an aqueous medium of 0.5 M KHCO₃. The Sn-Pb alloy with a surface composition of Sn₅₆.3Pb_43.7 had the greatest FE of 79.8% for HCOOH and the highest current density of 45.7 mA/cm² [126]. A Cu alloy catalyst was also reported to be selective for CO in aqueous ECR, with a FE higher than 90% for CO [133]. According to a report by Xu et al., incorporating Au with highly stabilized Cu nanoparticles resulted in very low overpotentials in the CO₂-RR [134]. Surface strain refers to the force in the mismatched lattice when doping a metal into other metal components. It plays a significant role in influencing catalytic performance. The compressive strain effect in a CuAg alloy facilitates the reduction of alcohol and acid [48,110,135]. It is believed that core–shell structures can provide more active sites due to the surface strain. A nanoporous core–shell AuCu₃@Au electrode had an FE of 97.27% for CO production [136] (concave defect). Au-Pd core–shell nanoparticles reached a maximum FE of 96.7% for CO production [137]. The tensile strain in the core–shell structures makes the adsorption of *COOH easier than *CO [138]. A study on a heterolayer slab model of Cu-M (M = Rh, Ir, Pd, Pt) concluded that both the strain and ligand effects impacted the adsorption energies of the intermediates of the CO₂-RR, and the adsorption of *COOH and *CHO was more strain-sensitive than that of *CO and *COH on the Cu surface [138]. Core–shell Cu@CuEu nanoparticles were found to have 3.4 times the CH₄ FE of Cu nanoparticles due to the vacancy in the CuEu layers [139]. The compressive strain in core–shell Bi@Sn nanoparticles boosted the electroreduction of CO₂ into formic acid, with an FE of 91% [140]. Furthermore, the micro-morphology of a core–shell porous-structured Cu@Sn electrode reached an FE of 100% for formate production [141]. The grain boundary is the interface between two crystallites. Grain boundaries in gold nanoparticles have been applied to control the product selectivity of the CO₂-RR [142]. For example, Au nanoparticles rich in grain boundaries have an FE of 94% for CO production. Copper rich in grain boundaries was found to have a high FE of 73% for ethylene, ethanol, and propanol production in a wide potential range, which exceeded that of most copper electrocatalysts [143]. Bismuth grain boundaries were formed by galvanic-cell deposition to improve formate selectivity, with an FE of more than 92% [144].

3.3. Metal Oxides

The success of electrochemical CO₂ reduction to value-added C₁ and C₂ chemicals/fuels may resolve the environmental issues brought on by excessive fossil fuel usage. An effective method for lowering the amount of metal used and for electro-reducing CO₂ is to load transition metals onto metal oxides (Figure 7). Numerous recent publications in peer-reviewed journals have documented the adjustment (and general enhancement) of the selectivity and activity of materials employed in the electrochemical conversion of CO₂ to value-added chemicals following oxide formation. Such species are known as oxide-derived (0D) materials, and improved CO₂ reduction activity has been demonstrated for 0D-Cu [145], 0D-Au [94], and 0D-Pb [146] when compared to corresponding metal substrates. The other catalysts investigated for CO₂ electroreduction can be divided into three categories. The first category, consisting of gold, silver, zinc, and (to a lesser degree) copper, has a higher Faradaic efficacy in producing CO [147]. The second group, which includes metals such as Pt, Ni, Fe, Ti, and Al, mainly forms hydrogen [148], while Pt may also produce hydrocarbons containing up to nine carbon atoms, though with a low yield [149]. Finally, the third category consists of metals with a higher hydrogen overpotential, such as Hg, Cd, Pb, In, and Sn, which convert CO₂ exclusively to formic acid and formate salts. This third set of catalysts has good specificity, since they primarily create aqueous formate with hydrogen gas. As per Oloman and Li, these might provide the foundation for industrial formate synthesis by rapid CO₂ electroreduction [37]. Lead and tin have already been widely explored in this regard; nonetheless, their stability under extended electrolysis remains restricted [150,151]. Recently, a bismuth oxide (BiO_n) cluster catalyst was developed for an effective CO₂-RR to formate. This BiO_n cluster catalyst displayed good activity, selectivity, and stability during formate synthesis, with a formate Faradaic efficiency of more than 90% at current densities of up to 500 mA/cm² in an alkaline membrane electrode assembly electrolyzer, equivalent to a mass action of 3750 AgBi. Tin (Sn) [152], bismuth (Bi) [43], indium (In) [153], lead (Pb) [154], and mercury (Hg) are common metal catalysts used for the CO₂-RR to produce formate.

A fast-heating approach was used to create 1.72 and 3.51 nm thick Co₃O₄ films as a prototype. The atomic thickness provided the Co₃O₄ with an abundance of active sites, allowing the adsorption of a substantial quantity of CO₂. The greater and more scattered charge densities around the Fermi level enabled improved electron transport. The electrocatalytic activity of the 1.72 nm thick Co₃O₄ layers was just 1.5 and 20 times greater than that of the 3.51 nm thick Co₃O₄ layers and the bulk analog. In addition, the 1.72 nm thick Co₃O₄ films demonstrated a formate Faradaic efficiency of more than 60% in 20 h [155]. Zn-based compounds were also investigated as ECR catalysts. ECR on such a highly purified Zn electrode is often preferential for producing HCOOH and CO in an aqueous environment [156]. Previously, the creation of a mixed metal-oxide CuInO₂ nanostructure during CO₂ electroreduction stabilized the active catalytic phase of copper(I). The introduction of a nanoporous Sn:In₂O₃ interlayer to a Cu₂O pre-catalyst system resulted in the creation of CuInO₂ nanoparticles with a much greater efficacy for CO₂ electroreduction and a lower overpotential than traditional Cu nanoparticles obtained from solitary Cu₂O [157]. Metal oxides may also be utilized directly as catalysts for the reduction of CO₂ in addition to metal-based catalysts. Tin oxide nanocrystals were studied by Meyer et al. as effective electrocatalysts for the production of formate [158]. These nanostructured tin oxide catalysts had optimum Faradaic efficiencies of more than 93% and current densities of 10 mA/cm² at overpotentials as low as 340 mV. Supporting this research, improved active CO₂ electroreduction catalysts, including decreased SnO₂ porous nanowires [159], hierarchical mesoporous SnO₂ nanosheets [160], and hybrid SnO_x/carbon nanotubes [161], have been produced. Thorough research by Bijandra et al. revealed that the presence of several grain boundaries inside this porous structure was the cause of the enhanced CO₂ reduction performance on decreased SnO₂ porous nanowires [159]. In a two-step procedure, Zhang et al. created multilayer mesoporous SnO₂ nanosheets using carbon cloth after synthesizing SnS₂ hydrothermally [162]. Subsequently, after further calcination, highly porous SnO₂ nanosheets with a surface area of 93.6 m²/g were produced [160]. To further understand the mechanisms behind the higher electroreduction activity of Pt oxides, the species adsorbed onto the surface of Pt and Pt oxide were examined using SEIRAS. Following CO₂ electroreduction, the primary adsorption species were methanol and HCOO on the Pt oxide, whereas they were and methanol and linear-CO on the Pt. It has been shown that HCOO on Pt oxide and CO on Pt are necessary for the CO₂ electroreduction process. Although CO aggressively adsorbs on the active site and hinders subsequent reactions, the adsorption species significantly impact the CO₂ electroreduction efficiency [163]. The Fermi level of ZnO may be tuned by substituting heteroatoms with various outer electron configurations (Mo and Cu) for the Zn site [164].

Recently, Zhong et al. synthesized V_o-CuO(Sn) nanosheets through a simple one-pot synthesis route (Figure 8A), and the as-synthesized V_o-CuO(Sn) consisting of oxygen vacancies with Sn dopants was utilized for CO2-RR applications [165]. The structural/morphological investigations confirmed that the Sn atom slightly affected the CuO nanosheet morphology. The amount of Sn dopants was measured using ICP-OES, indicating 2.97% atomic Sn. The experimental results showed that the V_o-CuO(Sn) exhibited higher selectivity for the CO₂-RR in the CO₂-saturated electrolyte compared to the Ar-saturated electrolyte medium and exhibited a higher current density of 38.65 mA/cm². The CO₂-RR resulted in CO as the reduction product, which was confirmed by the GC-MS results. V_o-CuO(Sn) presented a 42.1% FE for CO at −0.23 V (vs. RHE) and a 99% FE at −0.53 V (vs. RHE); thus, the selectivity of CO was around 95% over V_o-CuO(Sn). The substantial enhancement in the catalytic activity of V_o-CuO(Sn) was mainly due to the synergistic effect of Sn doping and enriched oxygen vacancies.

Furthermore, Huang et al. reported the preparation of O-vacancy-engineered indium oxide (InO_x) nanoribbons (NRs) for a highly efficient CO₂-RR (Figure 8B). They synthesized three InO_x NRs with different V_O concentrations by simply calcinating them under different atmospheres. High V_O concentrations endowed the H–InO_x NRs with an excellent FE% (HCOO⁻) of over 80% at broad potential values and a total FE of 91.7% with a higher current density. The H–InO_x NRs exhibited long-term stability for the conversion of CO₂ to formate without substantial decay during 20 h of continuous reduction treatment. The enhanced CO₂ adsorption, activation, and reduction performance of H–InO_x were owed to the high V_O concentrations, thus showing the importance of defect engineering on the catalyst surface [166]. Additionally, a recent work reported by Sun et al. described the enhanced CO₂ electroreduction activity of oxygen vacancies/defects (V_o) on a 1D In-SnO₂ hollow-nanofiber catalyst [167]. The authors prepared SnO₂ hollow nanofibers through the electrospinning technique and with In as a deponent. The oxygen vacancies/defects were confirmed through XPS and EPR analyses (Figure 8C). As can be seen from the schematic diagram (e), the V_o in the In–SnO₂ was considered an anion vacancy (n-type semiconductor) and could act as a quasi-free electron donor. The free electron readily available in the V_o on the surface contributed to the high electronic conductivity and enhanced the local surface electron densities for the electrocatalytic reactions. These electron-rich domains also served as highly efficient active sites to stabilize the reduction intermediates such as ^•CO₂⁻ for O–bonding, which further protonated to generate *OCHO for HCOO⁻ production. Hence, the In–SnO₂ V_o hollow-nanofiber electrocatalyst resulted in an FE of 86.2% in the conversion of CO₂ to formate species at −1.34 V (vs. RHE).

More recently, Wang et al. reported the synthesis of Cu–ZnO electrocatalyst by adopting the low-valance Cu-doping and oxygen vacancy engineering techniques for electrochemical CO₂ reduction reactions. The production or the selectivity of CO can be optimized by weakening the adsorption sites of *H through oxygen vacancies (V_os). Thus, the as-synthesized Cu-ZnO (V_os) resulted in a >80% FE for the conversion of CO₂ to CO products within the potential range of −0.76 V to −1.06 V (vs. RHE), with a high current density of 45 mA/cm² [168]. Geng et al. prepared ZnO nanosheets rich in oxygen defects through H₂ plasma treatment for the enhanced electrochemical conversion of CO₂ to CO. The Gibbs free energy for the CO₂ activation process in the presence of oxygen-vacancy ZnO nanosheets decreased by 0.20 eV compared to the ZnO nanosheets without oxygen vacancies. The resultant V_O-ZnO nanosheets produced a current density of −16.1 mA/cm² with a high FE of 83% for the generation of CO from CO₂. This proved that Vos facilitated the activation of CO₂ with superior reaction kinetics [169]. Recently, Zhang et al. prepared porous indium oxynitride nanosheets with oxygen vacancies and N deponents (V_o-N–InON) to produce formic acid from CO₂. The V_o-N–InON electrocatalyst exhibited 95.1% formate selectivity at −0.8 V (vs. RHE) in the flow cell, producing a current density of 121.1 mA/cm² at −1.13 V (vs. RHE). In addition, the DFT calculations demonstrated that the activation energy for forming an intermediate product (*OCHO) was drastically reduced by about −0.812 eV after introducing V_o. This revealed the synergistic effect of oxygen vacancies and N-doping in the In₂O₃ catalyst during the CO₂-RR [170]. Consequently, Du et al. reported the introduction of high-energy plasma-assisted oxygen vacancies (V_o) in In₂O₃ (V_o-In₂O₃) films for the electrocatalytic CO₂ reduction reaction. The DFT calculation results indicated that the CO₂ reduction reaction proceeded by forming *COOH as an intermediate product, and the Gibbs free energy for the *COOH decreased with the increase in the Vos. The oxygen richness inherited inside the In₂O₃ facilitated the efficient formation of HCOOH, and resulted in 70% and 83% FEs for the HCOOH and C₁ reduction products, respectively [171]. A recent study by Zong et al. explored the synthesis of morphology-controlled ZnO by the electrospinning method with enriched O-vacancies to enhance the electrochemical reduction of CO₂ to CO. The obtained p-ZnO-800 electrocatalyst resulted in a high current density of 150 mA/cm² and an 82.93% FE for CO in a flow-cell configuration reactor. The authors concluded that the enhanced electrocatalytic activity of the p-ZnO-800 catalyst was due to several reasons, including the increased active sites from the ZnO clusters, the enhanced electron transfer process, and the high selectivity towards CO by the introduction of O-vacancies [172]. Additionally, Ren et al. demonstrated cerium-doped ZnO catalysts (Ce_0.016Zn_0.984O) for the enhancement of the CO₂ electroreduction (CO₂-RR) process. The oxygen vacancies were achieved by doping the Ce³⁺ deponent at different concentrations. The Ce_0.016Zn_0.984O with the highest V_o exhibited a high current density of 24 mA/cm² at −1.0 V (vs. RHE), with a high FE of 88% for CO [173]. This study revealed that an increase in the electron transfer kinetics and a reduction in the thermodynamic energy barrier of the metal-oxide catalysts could be achieved through defect (vacancy) engineering approaches. Furthermore, EC-CO₂-RR activities of various metal-oxide catalysts were shown in

Table 3. Metal-oxide-based electrocatalysts for electrochemical CO₂-RR.

Metal-Oxide-Based Electrocatalysts
Catalysts	Type of Defect	Electrolyte/E vs. RHE	Products	* FE%	Ref.
SnOx	Thermal treatment/ annealing in O2 atm	0.1 M KHCO3/−0.76 V	COOH	64	[174]
Cu2O/ZnO	Oxygen vacancies	0.5 M KHCO3/−0.3 V	Alcohols	64.27	[175]
7.0 nm SnOx layer of Sn nanoparticles	Core–shell structures/ alloying	0.5 M KHCO3/−0.7 V	CO	35	[176]
CuO/ZnO/MoS2	Grain boundaries	0.5 M KHCO3/−0.6 V	CH3OH	24.6	[177]
3.5 nm SnOx layer of Sn nanoparticles	Layer thickness	0.1 M KHCO3/−1.2 V	COOH	64	[178]
Sn-Cu/SnOx	Core–shell structures	1 M KOH/−0.7 V	COOH	98	[152]
Au-CeOx nanosheet	Metal doping	0.1 M KHCO3/−0.5 V	CO	90.1	[179]
Co2FeO4 nanosheets	Metal (Co) incorporation	0.1M KHCO3/−1.0 V	CO	92	[180]
Sn-Ti–O	O2 defect sites	0.5M KHCO3/−0.54 V	CO	94.5	[181]
SnOx/AgOx	Plasma oxidation	0.1M KHCO3/−0.80 V	C1	91	[182]
Cu/In2O3	Core–shell structures	0.5M KHCO3/−0.40 V to −0.90 V	Syngas	90	[183]
ZnO shell/CuO core	Core–shell structures	1M KOH/−0.68 V	CH3CH=OH	49	[184]
Cu2O-Ag	Grain boundaries	0.2M KHCO3/−0.60 V −1.2 V	C2H4	52	[185]
In-doped Cu@Cu2O	Metal doping/core–shell structures	0.1M KHCO3/−0.45 V to −0.84 V	CO	2.2	[186]
Cu-CeO2–4%	O2 vacancy	0.1M KHCO3/−1.8 V	CH4	58	[187]
Ag-Bi-S–O-decorated BiO	Bimetal (Ag, Bi) doping/decoration	0.5M KHCO3/−0.45 V	HCOOH	94.3	[188]

* FE% = Faradaic efficiency (%); HCOOH = formate/formic acid; CO = carbon monoxide; C₂H₅OH = ethanol; CH₃CH=OH = isopropyl alcohol.

.

3.4. Metal Sulfides

Due to their favorable electronic band gaps, energy-band location exposed active sites, and potential catalytic activity, metal sulfides have gained much interest. The significant advances made possible by interfacial engineering are primarily attributable to its remarkable capacity for controlling electron and mass transport, adjusting intermediate adsorption, preventing severe catalyst aggregation, and offering sophisticated promoters for the logical design of highly effective catalysts [189]. Meanwhile, defect engineering in metal sulfides involving vacancies and metal/heteroatom doping enhances the electrocatalytic efficiency of the catalytic materials, owing to their increased current densities, Faradaic efficiencies, and active reaction sites and decreased work function compared to pristine catalyst materials [190,191]. Changes to the composition, morphology, and electrical structure of bimetallic sulfides, on the other hand, may be accomplished by metal doping and the fabrication of heterostructures that have significant promise in the electrocatalytic CO₂-RR [192]. A hierarchical CuS hollow polyhedron (CuS-HP) was developed for the electrochemical CO₂ reduction (E-CO₂R) in neutral pH aqueous environments. The CuS-HP experienced structural rebuilding to form a sulfur-doped metallic Cu catalyst under E-CO₂R conditions, encouraging formate synthesis with a Faradaic efficiency of >90% throughout a broad potential range [193]. Compared to E-MoS₂ and other previously reported transition metal sulfide electrocatalysts, the improved N-MoS₂@NCDs-180 for CO₂ electroreduction demonstrated a high CO Faradaic efficiency of up to 90.2% and a low onset overpotential of 130 mV for CO production [194]. CuInS₂ hollow nanoparticles were created by Xiong et al. with homogeneous bimetallic mixing, combining the benefits of hollow nanostructures with the synergy of bimetallic sulfides and exhibiting an outstanding electrochemical CO₂-RR capability [195]. At −0.7 V, the FE for HCOOH was 72.8%, and at −1.0 V, the FE for CO was 82.3%. Amorphous Ag–Bi–S–O-altered Bi nanoparticles were produced by Zhang et al. via the electrocatalytic processing of Ag_0.95BiS_0.75O_3.1 nanorods [188]. The catalysts produced a high FE for HCOOH of up to 94.3% and a significant fractional current density of 12.52 mA/cm² when the overpotential was just 450 mV. Additionally, another recent report suggested that adding Mn atoms to In₂S₃ nanosheets enhanced the FE and the current density of the carbon products [196]. Mn–In₂S₃ nanosheets had a high FE of 92% and a significant current density of 20.1 mA/cm² at −0.9 V [39]. In the CO₂-RR process, Zn-based catalysts exhibited a high selectivity for CO. However, they often present a large overpotential and slow reaction kinetics. Due to Zn’s poor adsorption onto the chemical intermediate *COOH in the CO synthesis route caused by its electron-rich properties, a significant energy barrier must be overcome during the CO₂-RR procedure [197]. In comparison to a reversible hydrogen electrode (RHE), a cadmium sulfide (CdS) nanoneedle grid catalyst could reduce CO₂ to CO with a Faraday efficiency of up to 95.5:4.0% at an exceptional exchange rate of 212 mA/cm² at @1.2 V [198]. A schematic illustration of metal oxides and metal sulfides and their structures for the electroreduction of CO₂ is depicted in Figure 3. Furthermore, EC-CO₂-RR activities of various metal-sulfide catalysts were shown in

Table 4. Metal-sulfide-based electrocatalysts for electrochemical CO₂-RR.

Metal-Sulfide-Based Electrocatalysts
Catalysts	Type of Defect	Electrolyte/ E vs. RHE	Products	* FE%	Ref.
Ag-SnS2	Chemical-induced defective sites	0.5M KHCO3/−1.0 V	HCOOH	65.5	[199]
5%Ni-SnS2	Metal (Ni) doping	KHCO3/−0.9 V	CO	93	[191]
Mn-In2S3	Metal (Mn) doping	0.1M KHCO3/−0.9 V	HCOOH	86	[196]
Nitrogen-doped MoS2	The heteroatom (N) doping	EMIM-BF4/−0.9 V	CO	90.2	[194]
ZnS/ZnO	Oxide–sulfide interface	1M KOH/−0.73 V	CO	91.9	[197]
In2S3-rGO	Layer thickness	0.1M KHCO3/−1.2 V	-	91%	[200]
SnS	The heteroatom (In) doping	1M KOH/−0.60 V	HCOOH	96.6	[201]
Ag2S/N, S-rGO	Heteroatom (N, S) co-doping	0.1M KHCO3/−0.76 V	CO	87.4	[202]
	Lattice defect/metal (Cu) vacancy	0.5M KHCO3/−0.84 V	COOH	-	[203]
CuS/CuPor	Surface structural defects	0.5M NaHCO3/−2.0 V	C2H5OH	74.4	[204]
MoSeS	Alloying (TMD alloy)	−/−1.15 V	CO	45	[205]
SnS2/rGO	O2 defects	0.5M NaHCO3/−0.75 V	CO	84.5	[206]

* FE% = Faradaic efficiency (%); HCOOH = formate/formic acid; CO = carbon monoxide; C₂H₅OH = ethanol; CH₃CH=OH = isopropyl alcohol.

.

3.5. Carbon-Based Materials

In terms of defect-engineered materials, carbon-based nanomaterials (CBNs) have been identified as an excellent choice for the electroreduction of CO₂ under liquid- and gas-phase reactions [1,207]. From the family of carbon nanostructures, all the allotropes, such as graphene, carbon nanotubes (CNTs), and carbon nanodiamond materials, have obtained paramount importance in many catalytic activities owing to their remarkable properties [208]. This is mainly due to the unhinged electron distribution and defect-assisted electronic structural distortion in the catalytic materials. Introducing defects can alter the carbon skeleton’s charge state and, thereby, the catalytic material’s conductivity and stability [209,210]. Defect engineering includes intrinsic effects (topological and edge defects, vacancies, and holes made without any dopants) and extrinsic defects (such as metal atom sites and non-metal heteroatom doping) [211,212,213]. Each of these techniques has its advantages for catalytic reactions in terms of catalytic stability, product selectivity, electron transfer, increasing the active reaction sites, product stability, and the stabilization of intermediates, which contribute to enhancing CO₂ reduction reactions (CO₂-RR) [214,215,216].

Dai’s research group was the first to shed light on the use of defect-induced CBNs in catalysis via their ground-breaking work on oxygen reduction using vertically aligned nitrogen-containing carbon nanotubes (VA-NCNTs) published in 2009 [217]. The authors demonstrated this metal-free catalyst for high-performance oxygen reduction reactions (ORRs) in fuel cells with better electrocatalytic activity and long-term stability. Following this, numerous methodologies and techniques have been employed to formulate CBNs as an efficient catalytic material for the electrocatalytic CO₂-RR. The main techniques that have been employed to modify intrinsic defects, i.e., point defects and topological defects, include mechanical ball milling [218], chemical oxidation or etching [219], plasma etching [220,221], nitrogen removal [222], and in situ etching [223]. On the other hand, extrinsic defects such as heteroatom doping with varied electronegativity [85,224] and metal-atom dispersed active sites [225,226,227] are primarily implemented using various chemical vapor deposition (CVD) [228,229,230] and pyrolysis [231] methods for heteroatom doping and pyrolysis synthesis [232,233], with defect engineering [234], spatial confinement [235,236] and coordination design [237] used for the metal-atom dispersive site processes. The appropriate synthesis method yields distinctive electrocatalytic materials that can be employed in efficient CO₂-RRs in unique ways to provide carbon fuels and other value-added products. Figure 9 depicts the various defect engineering methods, including the modification of intrinsic and extrinsic defects, applied to synthesize highly active electrocatalysts.

The utilization of CBNs in the electrocatalysis of carbon dioxide mainly results in the formation of CO, which is the primary intermediate (*CO) in most CO₂ reduction pathways. For instance, Ajayan et al. [230] investigated the reduction of CO₂ to CO using N-doped 3D graphene foam. In this work, pyrrole and pyridinic-N defects were modified to reduce the barrier height of the *COOH intermediate formation. This was supported by density functional theory (DFT) calculations. Meanwhile, Zhou et al. [85] synthesized an N-doped CNT array with more accessible pyridinic-N sites for the CO₂ reactant and *COOH intermediate in the electroreduction of CO₂ to CO. In another work by Meyer et al. [241], N-doped carbon nanotubes were employed as a robust catalytic material for the electroreduction of CO₂ in aqueous media to generate formate as an end product. This metal-free catalyst was supported by a polyethyleneimine (PEI) co-catalyst to enhance the CO₂ absorption, thereby increasing the Faradaic efficiency to 87% with a decreased catalytic overpotential for the formation of CO₂^●− from CO₂ reactant. Wang et al. [242] synthesized another N-doped graphene from melamine and graphene oxide to catalyze CO₂ into formate; the presence of over 5 atomic % nitrogen atoms triggered the CO₂ conversion with a small overpotential of 0.24V, involving graphitic, pyrrolic, and pyridinic-N atoms. Nitrogen is the most commonly explored heteroatom with high electronegativity to carbon atoms, e.g., N-doped graphene, quantum dots (QDs), and CNTs [49,243]. Other heteroatoms, such as B, F, and S, have also been doped with various carbon materials and employed for electrocatalysis, showing unusual activity compared to metal catalysts [238,244,245]. These heteroatom-doped electrocatalysts present enhanced catalytic activity for the electroreduction process compared to pristine graphitic carbon materials, which have trouble interacting with CO₂ molecules due to the inactive neutral carbon atoms in the sp²-conjugated structures (Figure 10).

It is a well-established fact that metal catalysts or the presence of metal atoms in composites substantially enhance catalytic activity [246,247]. Introducing metallic active sites on CBNs with uniform dispersion boosts the electrocatalytic activity. Recently, single-metal-atom electrocatalysts have attracted much attention due to their high atomic utilization rate [248,249,250]. Similar to the carbon effect, these metal atoms can strongly bind with C–N atoms in the network, achieving outstanding catalytic performance and stability. In particular, the M–N–C (metal-nitrogen-carbon) system not only has a high atomic utilization rate but also can effectively reduce the hydrogen adsorption on metal atom sites, thereby enhancing the selectivity and efficiency of the CO₂-RR by greatly reducing the HER [251,252,253]. Among the works on the ORR using metal catalysts, Strasser et al. [225] first explored electrocatalytic reduction (ECR) using Fe–N–C, Mn–N–C, Fe, and Mn–N–C catalysts and showed high performance. Following this, many noble metals and other transition metals were paired in M–N–C systems; for instance, Kattel et al. [254] prepared N-doped carbon-assisted Pd single-atom catalysts possessing well-dispersed Pd-N₄ sites that greatly stabilized the adsorbed CO₂ reduction intermediates, enhancing the ECR at lower overpotentials. However, considering the expensiveness and rarity of noble metals, low-cost single-atom transition metals have also been extensively explored. Co-N-C, Fe-N-C, and Ni-N-C are the most commonly investigated catalysts for selective ECR reactions. Among these, compared to Co-N-C and Fe-N-C, Ni-N-C is considered to be an effective electrocatalyst due to its higher CO selectivity and improved catalytic activity. Jiang et al. [251] synthesized a highly porous microwave-peeled graphene oxide (MPGO) with major surface defects and single-atom Ni anchoring (Ni-N-MPGO) with a loading of about 6.9% Ni. The improved ECR performance of the Ni-N-MPGO was established by forming edge-anchoring unsaturated Ni–N active structures in the catalyst with a Faradaic efficiency of 92.1%. Various modifications and designs of CBNs with intrinsic and extrinsic defects have presented catalytic abilities with exceptional performance, selectivity, and stability for ECR and the CO₂-RR (

Table 5. Carbon-based electrocatalysts for electrochemical CO₂-RR.

Carbon-Based Nanomaterials for CO2-RR
Catalyst	Type of Defect	Electrolyte/E vs. RHE	Product	* FE (%)	Ref.
GNR AuNPs	Metal doping	0.5M KHCO3/−0.2 V	CO	>90	[255]
BND	B, N co-doping	0.5M KHCO3/−1.0 V	C2H5OH	93.2	[256]
B-graphene	B doping	0.1M KHCO3/−1.1 V	HCOOH	-	[245]
N-graphene	N doping	0.5M KHCO3/-	HCOO−	-	[242]
F-CPC	F doping	0.5M KHCO3/−1.0 V	CO	88.3	[257]
DPC-NH3-950	Thermal N removal, topological effects	0.1M KHCO3/−0.6 V	CO	95.2	[258]
N-GRW	N doping	0.5M KHCO3/−0.49 V	CO	87.6	[239]
NCNTs-ACN-850	N doping	0.1M KHCO3/−1.05 V	CO	80	[85]
P-OLC	P doping	0.5M KHCO3/−0.9 V	CO	81	[228]
FC	F doping	0.1M KHCO3/−1.22 V	CO	93.1	[259]
Ni-SAs/N-C	Metal (Ni) doping	0.5M KHCO3/−1.0 V	CO	71.9	[236]
N, P-FC	N, P co-doping	0.5M KHCO3/−0.8 V	CO	83.3	[260]
D-NC-1100	Heteroatom doping, porous carbon	0.1M KHCO3/−0.6 V	CO	94.5	[238]
DHPC	Thermal treatment	0.5M KHCO3/−0.5 V	CO	99.5	[261]
NG-800	N doping	0.1M KHCO3/−0.58 V	CO	85	[230]
NCNTs	N doping	0.1M KHCO3/−0.78 V	CO	80	[262]
NPC-1000	N doping	0.5M KHCO3/−0.55 V	CO	98.4	[263]
NDD	N doping	0.5M NaHCO3/−1.0 V	Acetate	91.8	[49]
NPCA	N, P co-doping	0.5M KHCO3/−2.49 V	CO	99.1	[264]
NGQDs	N doping	1M KOH/−0.75 V	(Total)	90	[265]
		−0.75 V	C2H4	31
		−0.86 V	CH4	15
		−0.74 V	C2H5OH	11.8
Co-N5/HNPCSs	N doping	0.2M NaHCO3/−0.79 V	CO	99.4	[248]
Ni-N-C	Metal (Ni, Fe) and N co-doping	0.1M KHCO3/−0.78 V	CO	85	[240]
Fe-N-C		−0.55V		65
Co-N2	Metal (Co) and N co-doping	0.5M KHCO3/−0.63 V	CO	94	[252]
Ni-N-MPGO	Metal (Ni) doping	0.5M KHCO3/−0.70 V	CO	92.1	[251]
Zn/Co-N-C	N doping	0.5M KHCO3/−0.50 V	CO	93.2	[266]
Fe-N/CNT@GNR	Fe, N co-doping	0.5M KHCO3/−0.76 V	CO	96	[267]
C-Zn1Ni4ZIF-8	Ni, N co-doping	0.5M KHCO3/−1.03 V	CO	98	[249]
Ni/Fe-NC	Ni, Fe, and N doping	0.5M KHCO3/−0.70 V	CO	98	[268]
NiSA-N2-C	Metal (Ni) doping	0.5M KHCO3/−0.80 V	CO	98	[269]

* FE% = Faradaic efficiency (%); HCOOH = formate/formic acid; CO = carbon monoxide; C₂H₅OH = ethanol; CH₃CH=OH = isopropyl alcohol.

).

3.6. Metal Nitrides

Transition metal nitrides (TMNs) have shown excellent catalytic performance in electrochemical CO₂ reduction reactions due to their high mechanical/electrochemical stability and metal-like properties [270]. Jiang et al. reported low-cost, metal-free, C-doped, line-defect embedded boron nitride (BN) nanoribbons for electrocatalytic CO₂ reduction reactions. The boron and carbon dopants acted as dual active sites. These two active sites could strongly bind with CO intermediates and thus promote the selective conversion of CO₂ to CH₄. Additionally, the edges of B and C atoms enabled the coupling of *CH_x and CO intermediates, which led to the formation of C₂ reduction products such as ethylene and ethanol [271]. Yin et al. reported the synthesis of a new perovskite-type Cu(I) nitride (Cu₃N) nanocube (NC) electrocatalyst at different sizes (10, 20, and 25 nm NCs) for the electrochemical CO₂-RR. The experimental results showed that the 25 nm Cu₃N-NCs exhibited significant catalytic activity and selectivity for the conversion of CO₂ to ethylene, with an FE of ~60% at −1.6 V (vs. RHE) [272]. Moreover, Mi et al. achieved the selective electrochemical reduction of CO₂ to C₂ products using Cu₃N-derived Cu nanowires (NWs) with high-density grain boundaries. The reported grain-boundary-induced Cu₃N-derived Cu-NWs exhibited FEs for the C₂ products of ethanol, ethane, and ethylene of around 8%, 12%, and 66% at −1.0 V (vs. RHE), respectively [109]. Furthermore, Liang et al. synthesized a Cu–Cu₃N core–shell-like electrocatalyst to produce multi-carbon products from CO₂. The Cu–Cu₃N electrocatalyst exhibited an FE of around 64 ± 2% for C₂₊ reduction products, with an enhanced working stability over 30 h. The Cu₃N acted as an active center and reduced the energy barrier of the CO dimerization reaction [273]. Conversely, Liu et al. prepared Pd-modified niobium nitride (Pd/NbN) and vanadium nitride (Pd/VN) electrocatalysts for the EC-CO₂-RR for syngas production. The experimental and computational calculations resulted in a higher FE% for CO achieved by the Pd/NbN than by the Pd/VN and commercial Pd/C catalytic materials. This work suggested that NbN could be a good substrate for Pd modifications, and the 5 wt.% Pd/NbN exhibited higher CO₂-R activities than the 10 wt.% Pd/C commercial electrocatalyst for the production of syngas [270]. Li et al. constructed unique isolated FeN₄ active sites on a disordered carbon substrate (FeN₄/C) for enhanced electrochemical CO₂ reduction reactions. The FeN₄/C catalyst achieved an FE of around ~93% for CO at −0.6 V (vs. RHE) with a high current density of 2.5 mA/cm², improving upon that of pristine N/C (46%) and Fe/C (23%). Additionally, the FeN₄/C demonstrated 24 h of excellent working stability in the EC-CO₂-RR [274].

4. Conclusions and Perspectives

Over the next few decades, renewable energy may remain the foremost energy source due to the over-consumption of fossil fuels. At the same time, the impact of CO₂ emissions is still a severe issue in our modern technological society. The electrochemical CO₂ reduction reaction (CO₂-RR) is an emerging alternative technique that provides a fascinating way to reduce CO₂ into C₁, C₂, and various value-added chemicals/fuels. Numerous electrocatalysts are used for the CO₂-RR, among which several metals with surface imperfections or defects have been identified as exceptional catalysts that can achieve the effective conversion of CO₂ into valuable hydrocarbon products. Controlling, modulating, and tuning metal/catalyst surfaces is essential to achieve stable and highly selective CO₂ reduction. With the ultimate purpose of establishing a carbon-neutral economy, various strategies have been proposed and executed at the laboratory level over the past decade to uncover techniques with technical and economic feasibility. Amongst others, defect engineering has found its place at the top of the list owing to its exceptional catalytic activity and increased Faradaic efficiency for the electrocatalytic conversion of CO₂ into desired value-added carbonaceous fuels and chemicals that can be stored for future use. Defects in the catalytic surface provide more room to accommodate more CO₂ molecules, with great increases in the active sites on the surface and porous levels. In this mini-review, we meticulously summarized the relevant aspects of this subject, beginning with the mechanistic pathways of the electrochemical CO₂-RR and the advantages and economic benefits of the various conversion reactions, before moving onto the challenges surrounding the processes and the possible factors affecting the conversion of CO₂ into the various value-added C₁ and C₂ reduction products.

Over the past several decades, numerous electrode materials (i.e. electrocatalysts) have been utilized/investigated for the electrochemical CO₂-RR. More importantly, the binding energy/tendency of the metals strongly influences the selectivity of the CO₂ reduction products. Thus, an alternative catalyst modification strategy is necessary to overcome the practical complications of the electrochemical CO₂-RR. This mini-review discussed the main practical problems, challenges, and mechanistic pathways of the EC-CO₂-RR and presented clear interpretations. In particular, the recently developed electrocatalyst design processes/modifications via surface-defect engineering approaches using significant metallic, metal-oxide/sulfide, carbon-based, alloy, and metal-nitride electrocatalysts were discussed. Furthermore, this review presented past, present, and future perspectives on catalyst design for the electrochemical CO₂-RR with the aim of producing desired fuels/products.