Electrochemical CO2 reduction toward multicarbon alcohols - The microscopic world of catalysts & process conditions

Abstract

Tackling climate change is one of the undoubtedly most important challenges at the present time. This review deals mainly with the chemical aspects of the current status for converting the greenhouse gas CO₂ via electrochemical CO₂ reduction reaction (CO₂RR) to multicarbon alcohols as valuable products. Feasible reaction routes are presented, as well as catalyst synthesis methods such as electrodeposition, precipitation, or sputtering. In addition, a comprehensive overview of the currently achievable selectivities for multicarbon alcohols in CO₂RR is given. It is also outlined to what extent, for example, modifications of the catalyst surfaces or the use of bifunctional compounds the product distribution is shifted. In addition, the influence of varying electrolyte, temperature, and pressure is described and discussed.

Keywords: Catalysis; Electrochemistry.

Graphical abstract

Figure 1

Visualization of possible CO2 reduction pathways with ethanol and propanol as the target products Own representation based on Liu et al., 2019; Hasani et al., 2020; Todorova et al., 2020 and Cheng et al., 2021.

Figure 2

Proposed mechanism for CO₂RR to CO, followed by ethanol formation at bimetallic Cu-Ag foam Own representation based on Dutta et al., 2020.

Figure 3

Schematic hypothetical representation of CO insertion on a Cu/Ag catalyst and the influence of domain size on product distributions (A) With large domains of Cu/Ag catalysts and consequently low amount of biphasic boundaries. (B) With smaller domains of Cu/Ag catalysts and correspondingly pronounced biphasic boundaries, which favor C-C coupling. The target product ethanol is highlighted as ∗C₂. Own representation based on Lee et al., 2017.

Figure 4

Schematic illustration of the electrodeposition of porous copper on a copper substrate with H2 bubbles as geometric template Own representation based on Dutta et al., 2016.

Figure 5

Example of the impact of catalyst morphology and composition for catalyst systems based on Cu or Cu/Zn on the Faraday efficiencies of C₂₊ products at −1.1 V with simultaneous indication of the roughness factor (RF) (A) Influence of the morphology of pure Cu catalysts. (B) Influence of the morphology of Cu₉₀Zn₁₀ catalysts. (C) Influence of the morphology of Cu₇₅Zn₂₅ catalysts. (D) Influence of the Cu:Zn ratio for cubic catalysts. Adapted from Journal of Electroanalytical Chemistry (da Silva et al., 2020), applying terms of CC BY licens.

Figure 6

Influencing factors on FE_C2+ product formation using Cu-halide catalysts (Left) Influence of halide type and potential in 1 M KOH (Right) Influence of KOH concentration and potential using a Cu-F catalyst. Reprinted by permission from Nature Catalysis (Ma et al., 2020b).

Figure 7

Suggested mechanism for the formation of ethanol at Cu-Zn catalysts at different potentials Own representation based on Ren et al., 2019.

Figure 8

FEs using a poly(ionic liquid)-based Cu⁰-Cu^I tandem catalyst for CO₂RR varying the electrolyte and the concentration of KOH electrolyte; 400 mA cm⁻² Copyright Wiley-VCH. Reproduced with permission (Duan et al., 2021).

Figure 9

Comparison of FE_EtOH and FE_H2 depending on chosen electrolyte solution with different buffering effects; H-type cell, at −1.1 V_RHE Reprinted by permission from John Wiley and Sons Ltd. (Kim et al., 2021).

Figure 10

FE_H2 at different temperatures, CO₂ feeds, and current densities during CO₂RR Reprinted from Löwe et al., 2019, ChemElectroChem, applying terms of CC BY license.

Figure 11

Impact of temperature on MEA during CO₂RR on current density as well as on the FE of ethanol, recovered from both anode and cathode streams. Reprinted by permission from Elsevier (Gabardo et al., 2019).