Electrochemical reduction of carbon dioxide (CO₂) to chemicals is considered to be a sustainable strategy in preventing our  planet from global warming while keeping the growth of our economy. So far, commercially available CO₂ electroreduction reaction (CO₂ER) technologies are almost nonexistent [1]. A cost-competitive CO₂ electrolysis requires a high current density (>200 mA·cm⁻²), a high selectivity, a low overpotential (<1 V), and a long-term operation (>8000 h or 1 year) ^[2][3][2,3]. Among these factors, the current density is a key indicator for evaluating the catalytic performance, because a higher current density represents a higher reaction rate. Therefore, technical developments regarding electrocatalyst, electrolyzer, electrolyte, and operational condition are greatly demanded in order to realize high current density.

CO₂ electroreduction involves different numbers of electrons and protons to produce specific products. Elemental metallic catalysts can be classified into the following three categories according to their major products: (1) Au, Ag, Pd, and Zn to produce CO ^[4][5][6][4,5,6]; (2) Pb, Bi, Sn, In, and Hg to produce formic acid/formate ^[7][8][9][7,8,9]; (3) Cu to produce various hydrocarbons ^[10][11][12][10,11,12]. Although the above catalysts have demonstrated remarkable selectivity towards different products, they are still far away from industrial application, especially regarding the aspect of the current density. However, in order to have a sustainable impact on the environment and climate, industrially relevant research is urgently required [13].

Therefore, the integration of catalyst innovations and reactor designs is desirable. Traditional CO₂ERs are conducted in an H-type cell, which is limited to relatively low current densities due to the limited solubility of CO₂ in aqueous solution. To meet the requirements for industrialization, various reactor configurations such as the GDE-based flow cell, microchannel reactor, membrane electrode assembly, and high-pressure cell have been developed. Although perovskite catalysts in solid oxide electrolyzer cells (SOECs) can electrolyze CO₂ at the gas–solid interface with a high current density ^[14][15][16][14,15,16], they are not discussed within the scope of this article, due to their restricted operational conditions and products. In an economically practicable electrolyzer, variables related to electrodes, electrolytes, and operations should also be taken into account.

2. Mechanisms of CO₂ER

CO₂ electrochemical reduction can proceed through reduction pathways involving two to eighteen electrons to produce various products including formic acid (HCOOH)/formate (HCOO⁻), carbon monoxide (CO), methane (CH₄), methanol (CH₃OH), acetic acid (CH₃COOH), ethanal (CH₃CHO), ethanol (CH₃CH₂OH), ethylene (CH₂CH₂), and others. The most commonly reported reactions are listed below in Table 1. During CO₂ER, a CO₂ molecule is first adsorbed onto a vacant catalyst site, electron transfer is carried out to form various intermediates [17], and then the corresponding products are obtained [18]. Figure 1 shows the proposed reaction mechanisms for various CO₂ER products. The generally accepted first step in CO₂ER is the protonation process of the CO₂ molecule on different atoms (C or O atom) to form *OCHO or *COOH. In order to produce HCOOH, a further proton and electron transfer is needed. The production of CO requires the formation of *COOH in the first step, followed by H⁺/e⁻ transfer to the hydroxyl group and then the loss of H₂O [19]. The weak bond of *CO with the surface of a metal catalyst such as Au or Ag promotes the desorption of CO. In order to obtain C₂₊ products, either *CO dimerization [20] or *CHO formation [21] is required. The generation of *CHO is thought to be the potential-determining step for generating methane and ethylene [22]. However, the rate-determining steps for methane and ethylene are distinct, with the first e⁻ transfer for ethylene, and the second e⁻ transfer for methane [23].

3. Electrocatalysts for CO₂ Electroreduction

Elemental metallic catalysts for CO₂ electroreduction are traditionally classified into four distinct groups depending on their major products, according to experimental data produced by Hori et al. [29] (Figure 2). In aqueous electrolytes, Au, Ag, and Zn catalysts mainly produce CO, whereas Cd, In, Sn, Hg, Tl, Pb, and Bi [30] catalysts favor the production of HCOOH. Cu is unique, and only Cu-based catalysts are able to yield large amounts of hydrocarbons, as they facilitate the formation of C-C bonds ^[31][32][31,32].