A Disquisition on the Active Sites of Heterogeneous Catalysts for Electrochemical Reduction of CO₂ to Value‐Added Chemicals and Fuel

Abstract

Renewable‐electricity‐powered electrocatalytic CO2 reduction reactions (CO2RR) have been identified as an emerging technology to address the issue of rising CO2 emissions in the atmosphere. While the CO2RR has been demonstrated to be technically feasible, further improvements in catalyst performance through active sites engineering are a prerequisite to accelerate its commercial feasibility for utilization in large CO2‐emitting industrial sources. Over the years, the improved understanding of the interaction of CO2 with the active sites has allowed superior catalyst design and subsequent attainment of prominent CO2RR activity in literature. This review tracks the evolution of the understanding of CO2RR active sites on different electrocatalysts such as metals, metal‐oxides, single atoms, metal‐carbon, and subsequently metal‐free carbon‐based catalysts. Despite the tremendous research efforts in the field, many scientific questions on the role of various active sites in governing CO2RR activity, selectivity, stability, and pathways are still unanswered. These gaps in knowledge are highlighted and a discussion is set forth on the merits of utilizing advanced in‐situ and operando characterization techniques and machine learning (ML). Using this technique, the underlying mechanisms can be discerned, and as a result new strategies for designing active sites may be uncovered. Finally, this review advocates an interdisciplinary approach to discover and design CO2RR active sites (rather than focusing merely on catalyst activity) in a bid to stimulate practical research for industrial application.

Carbon‐Supported Single Metal Site Catalysts for Electrochemical CO2 Reduction to CO and Beyond

“…Among them, ZnN 4 on microporous N-doped carbon (SA-Zn/MNC) derived from ZIF-8 exhibited the highest activity and selectivity for CH 4 production (Figure 11). [14] The FE for CH 4 For heterogeneous molecular catalysts, Weng et al [107a] revealed the catalytic mechanism of Cu phthalocyanine supported on CNT involves the reversible structural and oxidation state changes of Cu atoms to form ≈2 nm Cu clusters as the active sites for methane production. As confirmed by in situ XAS analysis, a similar phenomenon was not observed on the Cu based MOF or complexes with a nonconjugated ligand.…”

“…[ 3 ] Based on different catalysts and reaction pathways, more than 16 different products have been obtained, such as CO and HCOOH via a two‐electron process, HCHO via a four‐electron process, CH 4 via an eight‐electron process, as well as multicarbon (C 2+ ) products (e.g., ethylene and ethanol) via CC coupling. [ 4 ] The reduction process is further complicated due to the competing hydrogen evolution reaction (HER), which has similar equilibrium potentials in aqueous electrolytes to the CO 2 RR (Equation ). As a result, the synthesis of an ideal catalyst with high selectivity for a specific product except CO is still a significant challenge.…”

Recent Progress in MXene‐Based Materials: Potential High‐Performance Electrocatalysts

“…[114] This renewable-electricity-powered CO 2 RR can reduce CO 2 into value-added fuels and chemicals such as formic acid (HCOOH), methanol (CH 3 OH), methane (CH 4 ), ethylene (C 2 H 4 ), and carbon monoxide (CO). [115] MXenes, as a class of promising electrocatalyst materials, have been used in CO 2 RR because of their high conductivity, tunable surface groups, and multiple binding sites. [25,48,56,116] Hiring theoretical calculations to predict the performance of transition metal (Group IV, V, VI) carbide M 3 C 2 , Li et al [25] found that Mo 3 C 2 and Cr 3 C 2 were the two best catalyst candidates for the highly selective conversion of CO 2 to CH 4 .…”

Recent Progress in Electrocatalytic Methanation of CO2 at Ambient Conditions

“…In addition, CO 2 methanation provides an alternative way to H 2 storage by formation of synthetic natural gas, which is also attractive in the power‐to‐gas technology. [ 5 ] The approaches of CO 2 conversion can be classified as four categories: biochemical method, [ 6 ] thermochemical method, [ 7 ] photochemical method, [ 8 ] and electrochemical method. [ 1,9 ] Among them, electrochemical reduction reaction (CO 2 RR) is treated as promising technology because it can be conducted under mild conditions in aqueous solution, in which water instead of molecular H 2 gas is employed as hydrogen source.…”

“…In addition, CO 2 methanation provides an alternative way to H 2 storage by formation of synthetic natural gas, which is also attractive in the power-to-gas technology. [5] The approaches of CO 2 conversion can be classified as four categories: biochemical…”