Low coordination number copper catalysts for electrochemical CO2 methanation in a membrane electrode assembly

Abstract

The electrochemical conversion of CO2 to methane provides a means to store intermittent renewable electricity in the form of a carbon-neutral hydrocarbon fuel that benefits from an established global distribution network. The stability and selectivity of reported approaches reside below technoeconomic-related requirements. Membrane electrode assembly-based reactors offer a known path to stability; however, highly alkaline conditions on the cathode favour C-C coupling and multi-carbon products. In computational studies herein, we find that copper in a low coordination number favours methane even under highly alkaline conditions. Experimentally, we develop a carbon nanoparticle moderator strategy that confines a copper-complex catalyst when employed in a membrane electrode assembly. In-situ XAS measurements confirm that increased carbon nanoparticle loadings can reduce the metallic copper coordination number. At a copper coordination number of 4.2 we demonstrate a CO2-to-methane selectivity of 62%, a methane partial current density of 136 mA cm−2, and > 110 hours of stable operation.

Dynamic Evolution of Active Sites in Electrocatalytic CO2 Reduction Reaction: Fundamental Understanding and Recent Progress

“…Higher activity and lower CO selectivity for smaller particles Molecular catalysts CuPc [175] No catholyte Ex situ XPS and operando XAS Transition to metallic Cu with diverse coordination number CH 4 FE max of 62% when coordination number is 4.2 NNU-33(S) [176] 1 m KOH XRD, in situ XAS, and Raman Anion exchange of sulfate by hydroxyl moiety CH 4 FE increased from 55% to 82% after anion exchange HKUST-1 [144] 1 m KOH SEM, operando XAS Conversion to Cu cluster C 2 H 4 selectivity 45% Bi-MOFs [182] 0.1 m KHCO 3 In situ Raman, ex-situ TEM First conversion to Bi 2 O 2 CO 3 , then to metallic Bi Formate FE of 92% within 10 h FeF 20 TPP [195] 0.5 m NaHCO 3 Operando UV-vis and XAS Fe(III) conversion into Fe(II) first, then electron localization at the ligand is changed CO FE of 93% SACs Cu-N-C [177] 0.1 m CsHCO 3 Operando XAS Reversible conversion to metallic Cu…”

“…D) CO 2 RR selectivity for initial catalyst with different ratio between CNP and CuPc. Adapted with permission [175]. Copyright 2021, Springer Nature.…”

The Interactive Dynamics of Nanocatalyst Structure and Microenvironment during Electrochemical CO2 Conversion

“…By driving low-coordination formation, the authors achieved a catalytic selectivity enhancement otherwise unattainable from the original catalyst. 39 We envision that this type of strategy that generates nanocatalysts with improved catalytic performance via structural evolution during CO 2 electrolysis will open up new avenues for catalyst innovations.…”

Construction of single-atom copper sites with low coordination number for efficient CO2 electroreduction to CH4

“…These low coordinated metal sites have more vacant coordination sites and less steric hindrance, which might favour the CO 2 adsorption and activation. 40,41 Besides, the current densities are presented against the corresponding overpotentials to obtain a Tafel plot (Fig. 4a).…”