Since the industrial revolution of the 19th century, fossil fuels such as petroleum, natural gas, and coal have been used as the main source of energy to power economies and civilizations [1]. There is a need to reduce CO₂ emissions because the burning of these fossil fuels has resulted in excessive CO₂ emissions into the atmosphere, which have had significant negative effects on the environment and pose an immediate threat to human societies ^[2][3][4][2,3,4]. The swift transformation of the need of energy and chemical industries from fossil fuels to renewable energy resources, for example, solar and wind, can be identified as one of the solutions to achieve the closed-looped configurations on the carbon footprint ^[5][6][7][5,6,7].

Nonetheless, several artificial solutions to limit or reduce CO₂ emissions have been created, such as technological innovation to increase coal burning efficiency in boilers (reducing coal consumption) and carbon capture and sequestration (CCS) ^[8][9][10][8,9,10] though CCS is a costly and an energy-consuming technology. In fact, dangerous CO₂ leakage is a major concern that inhibits the commercialized large-scale deployment of CCS. As a result, fixation of CO₂ remains a significant concern on a global scale ^[11][12][13][11,12,13].

Hence, currently, the best strategy is to use atmospheric CO₂ as a renewable feedstock to create a few chemical products with added value, such as light olefins, urea, formic acid, methanol, syngas, and (poly)carbonate [14]. A technique such as this will reduce the atmospheric CO₂ levels while producing fuels and industrial chemicals, reducing the reliance on traditional fossil fuels ^[15][16][15,16]. Therefore, several CO₂ reduction strategies, such as photochemical, electrochemical, thermochemical, and biochemical procedures, have been developed and extensively researched ^[17][18][17,18].

Among these technologies, lowering CO₂ emissions using renewable power is especially tempting due to its enormous potential, simple reaction units, controlled selectivity, and modest efficiency for practical industrial applications [19]. Furthermore, it is possible to think of electrocatalytic carbon dioxide reduction (ECR) as a useful method for storing the renewable energy discussed above in chemical forms ^{[20][21][22][23][24]}[20,21,22,23,24]. ECR paired with renewable energy techniques as electricity sources are widely employed in the energy sectors and chemicals, and it may offer a promising route to create considerable amounts of chemicals and carbon-neutral fuels ^[25][26][25,26]. Electrochemical CO₂ conversion offers various benefits over other methods: (i) using renewable energy sources such as solar, wind, geothermal, and tidal; (ii) the mechanism is simpler and precise in terms of administering as it only requires the monitoring of reaction temperatures and the potential of electrodes; (iii) having scalable, compact and highly efficient on demand transmutation systems; (iv) hydrocarbons can be formed from water, carbon dioxide, and renewable electricity ^[27][28][27,28].

The main question is how to build a high-performance CO₂ conversion system that has all the desired qualities at the same time [29]. The main component of a high-performance CO₂ conversion system is a system that has higher operational current density and produces better faradaic and energy efficiency for CO₂R ^[30][31][30,31]. Many research efforts in ECR have been directed to the search for better electrocatalyst materials, because appropriate electrocatalysts have a better active site that ideally leads to the synthesis of desirable products at high rates and low overpotentials ^{[32][33][34][35]}[32,33,34,35].

Metals, metal oxides, two-dimensional materials, and functional microorganisms have all been investigated as CO₂ reduction electrocatalyst materials. Metals’ catalytic durability, selectivity, and activity could be improved by controlling their crystal faceting, morphology, and size [36]. There are activities for electrocatalytic CO₂ reduction in metal oxides such as Co₃O₄ ^[37][38][37,38], CuO ^[39][40][39,40], ZnO ^[41][42][41,42], and TiO₂ ^[43][44][43,44]. Contrary to pure metal catalysts, most CO₂ reduction process intermediates are expected to bind via their oxygen atoms and those of metal oxides. This criterion implies that metal oxides have higher oxygenate selectivity than pure metal catalysts ^[45][46][45,46].

Two-dimensional (2D) materials with nanosheets can exhibit unique features and great performance in catalytic processes when used as catalysts. Two-dimensional electrocatalysts decrease the energy barrier for CO₂ activation, improve electrical conductivity, and have a high surface-active site density, which makes them promising for highly efficient CO₂ conversion ^[47][48][47,48]. Because, as compared to ordinary bulk materials, they have a significantly higher percentage of bare surface atoms and higher specific surface areas, they might provide an abundance of active sites, enhancing catalytic processes ^[49][50][49,50]. It should be noted that highly exposed surface atoms might escape and create defect structures, resulting in lower coordination numbers of surface atoms, which are attractive locations for reactant or intermediate adsorption. Similarly, nanosheet edge atoms with low coordination numbers can display unique catalytic characteristics. As a result, 2D structures can boost reactant chemisorption and improve catalytic efficiency [51].

Bio-catalysis, which incorporates microbes and enzymes, has received a great deal of interest because the value-added products can be produced under mild circumstances with remarkable selectivity and without any undesirable byproducts ^[52][53][52,53]. Given previous research in bio-inspired molecular structure design, expanded and dynamic connections through the materials, biological, and chemical science domains will synergistically promote catalyst development ^[49][50][49,50]. Microbial electrosynthesis (MES) utilizes self-replicating bacteria as a catalyst at room temperature and pressure, which enables a more economical and ecologically benign process than traditional chemical catalyst-based conversion. To metabolize CO₂, bacteria in MES exchange electrons directly or indirectly using electron shuttle molecules [54]. To recycle anthropogenic CO₂, electroactive microorganisms are employed in MES as a biocatalyst on suitable electrode materials [55].

2. Concepts of Electrochemical CO₂ Reduction Reaction

Concepts of Electrochemical CO₂ Reduction Reaction

The electrochemical conversion of CO₂, a linear stable molecule with a powerful C–O bond (750 kJ mol⁻¹), is challenging. Multi-electron/proton transfer processes, a large variety of possible reaction intermediates, and an ECR in an aqueous electrolyte are all part of the extremely complicated process of ECR ^[56][57][56,57]. Electrochemical reduction has been researched in aqueous solutions with various metal cathodes, as well as in several organic solvents. Although the successfully documented six-electron and eight-electron conversions to methanol and methane exist, the commonly discussed reduction products are carbon monoxide, acetic acid, and formic acid ^[58][59][60][58,59,60]. The main ECR products’ half electrochemical thermodynamic reactions are shown in Table 1, ethanol (CH₃CH₂OH), ethylene (C₂H₄), formic acid (HCOOH), methanol (CH₃OH), methane (CH₄), carbon monoxide (CO), and acetate (CH₃COOH), with reporting of their standard redox potentials at acid and base electrolytes ^[61][62][61,62]. In an ECR process, CO₂ molecules adsorb on the catalyst surface and interact with the atoms there to produce *CO₂, which is then followed by many progressive transfers of electrons and/or protons toward different end products. For instance, methane is thought to originate via the pathways given below (Scheme 1) [63]: A multistep reaction process, electrochemical CO₂ reduction typically involves a different number of electron reaction pathways. The reaction frequently happens at the electrolyte–electrode interface for heterogeneous catalysts used in CO₂ reduction, where the electrode is typically a solid electrocatalyst and the electrolyte is typically an aqueous solution saturated with CO₂ through bubbling. Water is transformed to oxygen and CO₂ is reduced to the CO₂ anion radical at the anode in a single-electron ECR (CO₂⁻). The first step of converting CO₂ to reduced carbon species is difficult because the reaction rate is very slow. The single-electron CO₂ reduction to CO₂⁻ with a pH of 7 exhibits an unfavorable and energetic reaction, with a thermodynamic potential of roughly −1.90 V vs. SHE. Furthermore, the formation of the CO₂ intermediate is essential to the formation of the 2e⁻ reduction products and the initial process can be considered the rate-limiting step [64]. Several electron/proton transfer processes are involved in the electrochemical CO₂RR, and CO₂ can be reduced into a collection of gaseous and liquid products by diverse pathways, including hydrocarbons (CH₄ and C₂H₄), alcohols (CH₃OH and C₂H₅OH), carbon monoxide (CO), and formic acid (HCOOH) [65]. This depends on the electrolytic conditions and the electrocatalysts used (e.g., applied potential, electrolyte, etc.) ^[28][37][66][28,37,66]. Without a catalyst, it is challenging to complete the first stage of CO₂ activation, which produces the intermediate CO₂⁻ radical. However, with the aid of an electrocatalyst, the CO₂⁻ radical can be stabilized via a chemical link created between CO₂ and the electrocatalyst, leading to less negative redox potential. Moreover, proton-coupled electron transfer is advantageous at the likely range of 0.20 to 0.60 V vs. SHE. The end products are influenced by the electrocatalyst and electrolyte selections as well as the quantities of electrons and protons transferred [51]. Therefore, the activation routes of some typical products in CO₂RR are briefly shown in Figure 1 [64]. Molecule reactants may react with various CO₂RR intermediates at any phase since CO₂RR contains several reaction steps and intermediates, which greatly broadens the range of possible products. Consequently, potential products can be selectively derived through the adjustment of the adsorption and desorption capability of electrocatalysts to distinct reaction intermediates from coupled CO₂RR [64]. A laboratory electrochemical H-cell consists of oxygen evolution reaction (OER) happening at the surface of an anode that generates electrons (e⁻) and protons (H⁺) or consumes hydroxyl ions (OH⁻); a cathode in order to reduce CO₂ to produces such as HCOOH/HCOO⁻ or CO, and make OH⁻; an electrolyte with the intention of transporting CO₂ to the active cathode sites and conduct ions; a membrane that allows ion exchange to take apart the anode and cathode; and a bias with suitable value to move electrons from anode to cathode (Figure 2a). A few crucial steps in a CO₂R process are involved in such a system, including (1) movement of products into liquid phases or bulk gases from the cathode/electrolyte interface, (2) product desorption from the electrode, (3) transfer of electrons from the cathode to intermediates, (4) adsorption of CO₂ into adsorbed intermediates such as *CHO, *CO, and *COOH, (5) the surface of the cathode absorbing CO₂, (6) transport of dissolved CO₂ to the cathode/electrolyte interface from the bulk electrolyte, and (7) CO₂ mass transfer to the bulk electrolyte from the gas phase [67]. One of the most critical problems of the electrochemical CO₂R technologies to function at large-scale is to obtain a great CO₂ selectivity to desired value-added products to reduce product separation costs and complexity. High selectivity is difficult to achieve due to, as shown in Figure 2b, the majority of CO₂R reactions’ standard potentials (Eo) and the hydrogen evolution reaction (HER) all being within a limited variety (−0.250 V to 0.169 V vs. standard hydrogen electrode) (SHE) [36].

3. Product Selectivity Parameters

The applied potential, pressure, temperature, type of electrolyte (pH, concentration, and composition), and type of electrocatalyst (crystallographic structure, chemical state, composition, and morphology) are all variables that affect selectivity, FE, and ECR performance. In addition, the selectivity of catalysts for various products varies. The type and quantity of electrolytes also affect the catalyst’s activity and selectivity. While C₂ products (such as ethanol, ethylene, and acetic acid) have primarily been observed using copper-based catalysts, C₁ products (such as CO, methane, methanol, and formic acid) can develop in a variety of materials ^[68][69][68,69].