Adjust the catalyst microenvironment to achieve efficient electrochemical conversion of CO2 under strong acid conditions

On February 9, 2023, David Sinton, Edward Sargent of the University of Toronto, Canada, and Li Fengwang’s research group at the University of Sydney in Australia published a report entitled “Conversion of CO2 to multicarbon products in strong acid by controlling the catalyst” in the journal Nature Synthesis microenvironment”.

This achievement reports a catalyst local microenvironment regulation strategy that can achieve efficient conversion of carbon dioxide under acidic conditions. In this strategy, a regulatory layer composed of covalent organic framework (COF) nanoparticles and cation exchange polymer (PFSA) is coated on the surface of the catalyst layer, which reduces the proton flow facing catalyst migration and enriches alkali metal cations under the condition of strong acid electrolyte, thereby inhibiting the hydrogen evolution side reaction in the electrolysis process and realizing the efficient conversion of carbon dioxide to multi-carbon products.

The corresponding authors of the paper are David Sinton and Li Fengwang. Yong Zhao, Long Hao, Adnan Ozden, and Shijie Liu are co-first authors of the article.

In recent years, the preparation of multi-carbon products (ethylene, ethanol, etc.) by carbon dioxide electroreduction has achieved rapid development, however, so far, most catalytic systems still rely on basic or neutral electrolytes to inhibit the side reactions of hydrogen evolution and promote carbon-carbon coupling in catalytic reactions. Under these reaction conditions, a strong alkaline environment is formed at the cathode catalyst/electrolyte interface, and a large amount of carbon dioxide gas is neutralized to form carbonate (which can combine with alkali metal ions to precipitate carbonate), resulting in a low unidirectional utilization rate of carbon dioxide (the upper theoretical utilization efficiency of conversion to multi-carbon products is 25%). If carbonate is regenerated to form carbon dioxide, about 50% of the total catalytic reaction energy is required. In neutral electrolytes, carbonate can migrate to the anode through the anion exchange membrane and decompose again to form carbon dioxide under the local acidity of the anode, but additional energy is required to separate the carbon dioxide from the oxygen produced by the anode.

CO2 electroreduction in acidic electrolytes is expected to solve the above problems. In order to achieve the synthesis of multi-carbon products (FE~48%) in acidic electrolytes, previous studies (Fengwang Li, Sinton, Sargent et al., Science, 2021, 372, 1074-1078) required the application of large currents (>1 A cm-2) to construct the desired local alkaline environment. However, driving such a high current density requires a high cell voltage (about 4.2 V), which results in extremely low energy efficiency (about 12%). An effective way to improve energy efficiency (proportional to product selectivity and inversely proportional to slot voltage) is to pursue high multicarbon product selectivity at moderate current densities (100-400 mA cm-2). However, under medium current density conditions, the proton migration flow from the acidic electrolyte (pH<1) is much greater than that consumed by local electrolysis, and the resulting local environment can only reach weak alkalinity (pH<8), which is not conducive to the formation of multicarbon products.

By modifying the outer structure of the catalyst, the proton concentration near the catalyst surface can be limited, which is expected to improve the local alkalinity. Previous studies have shown that the presence of alkali metal cations at the catalyst/electrolyte interface (i.e., the electric double layer on the catalyst surface) is a necessary condition for carbon-carbon coupling under acidic conditions. The use of cation exchange polymer coatings can meet the requirements of alkali cation cation migration to catalysts, but the hydrophilic region of the coating for cation conduction also promotes proton transport, which is not conducive to the improvement of local alkalinity. In order to achieve the purpose of limiting the proton flow and enriching alkali metal cations, it is necessary to break the large number of cation conduction networks in polymer coatings and extend the cation transport pathway. Homogeneous incorporation of organic nanofillers into cation exchange polymer coatings may be an effective method.

Based on this, the researchers designed a cation that can pass through the heteropolymer regulatory layer that also restricts the proton flow, coat it on the copper catalyst layer, and achieve efficient conversion of carbon dioxide to multi-carbon products (FE~75%) under strong acidic electrolyte (pH~1) and medium current density (100-400 mAcm-2). This regulatory layer consists of covalent organic framework (COF) nanoparticles with imine and carbonyl groups and perfluorosulfonic acid (PFSA) oligomers. Under acidic conditions, the imide group is protonated, and the positively charged COF surface can induce the uniform distribution of PFSA oligomers between COF particles, and promote the linear and regular arrangement of PFSA oligomers on the COF surface. This COF:PFSA composite structure can confine proton transport to locally ordered PFSA hydrophilic nanochannels, proton transport pathway, thereby reducing its migration to the catalyst. At the same time, the negatively charged sulfonic acid group on PFSA can adsorb a large number of alkali metal cations (potassium ions) to the catalyst surface, thereby providing a kinetic environment conducive to carbon-carbon coupling. Using this strategy, the authors achieved Faraday efficiencies of up to 75% for multi-carbon products (200 mA cm-2) under acidic conditions, reduced the full electrolytic cell voltage to 3.5 V, and achieved a total energy efficiency of 25% for the conversion of carbon dioxide to multi-carbon products, twice the highest results reported in the literature.

Figure 1: Modulating the ionopolymer layer to limit proton flux to optimize the catalyst interface microenvironment in acidic media.

Figure 2: COF:PFSA heterostructure and characterization of its properties limiting proton flux.

Figure 3: The COF:PFSA regulatory layer promotes efficient carbon-carbon coupling of copper in acidic electrolytes.

Figure 4: Efficient synthesis of multicarbon chemicals by electroreduction of acidic carbon dioxide in microporous flow cells.

In summary, the researchers achieved efficient electrolytic synthesis of multi-carbon products in highly acidic media by constructing a local microenvironment regulatory layer composed of covalent organic framework and cationic polymer. This strategy provides an effective way to create a local environment conducive to carbon-carbon coupling without the need for ultra-high current density and voltage, and provides a new perspective to solve the problem of reactant loss in the field of carbon dioxide electroreduction. (Source: Science Network)

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