Electrochemical characterisation of bimetallic materials for CO2 reduction

Abstract

The growth of CO2 concentration in the atmosphere is leading to an increasing global warming concern. Among the different approaches to reduce CO2 emissions, the application of Carbon Capture and Utilisation technologies is particularly interesting because our reliance on fossil fuels can be reduced, producing also value added chemicals. There are several methods to activate and convert CO2 into useful chemicals. The electrochemical reduction option seems to be one of the most interesting possibilities owing to the opportunity to carry out the CO2 reduction reaction at ambient conditions. However, there are some issues that limit the practical application of this technology in the short‐term: catalytic materials (involving lower activities, selectivities and stabilities) and mass transfer limitations issues. Therefore, the aim of this work is to prepare and characterize electrochemically gas diffusion electrodes based on bimetallic materials (Cu‐Au, Cu‐Mo, Cu‐Ru and Cu‐Pd) and bimetallic Metal Organic Frameworks (HKUST‐1, HPd10, HZn10, HRu10, HBi5, HBi20, HBi10mix and CAU‐17) that could be able to reduce reaction overpotentials and adsorption of reaction intermediates in the CO2 reduction to methanol. The working electrodes were manufactured by air‐brushing a microporous layer based on carbon black powder (2.6 mg/cm2) onto a carbon paper support. Then, different catalytic layers were airbrushed on it at different catalytic loadings (i.e. 0.5, 1 and 2 mg/cm2). The final bimetallic electrodes were characterised by cyclic voltammetry and Tafel plot analyses. The effect of the catalytic loading showed the highest activity for Cu‐Pd‐based electrodes at the 2 mg/cm2 loading, although only slight differences in activity can be observed between the electrodes tested. Higher catalytic activities were observed under N2 conditions due to the effect of the hydrogen evolution reaction, competing with CO2 conversion in the presence of this gas. Comparing the current‐voltage responses, Cu‐Pd electrodes showed the highest catalytic activity (‐21 mA/cm2) at 1 mg/cm2. The most active Metal Organic Framework was HBi5 (5 wt% of Bi in HKUST‐1, composed of Cu), which was interestingly stable after 40 cycles. Tafel plot analyses showed differences in the slopes for the activation of CO2 at Cu‐Pd, Cu and HBi5 materials. The lowest kinetic barrier for this step was achieved at Cu‐Pd‐based electrodes (83.7 mV/dec). The most active bimetallic and Metal Organic Framework materials were compared with Cu electrodes. In general, bimetallic electrodes showed higher activities than the bimetallic Metal Organic Framework‐based electrodes tested, and in particular Cu‐Pd‐based electrodes were able to surpass activity values achieved with Cu. Thus, Cu‐Pd‐based electrodes seem to be interesting for CO2 electroreduction processes, even though further electrochemical cell tests are needed to analyse the selectivity, productivity and efficiency of these materials for the continuous CO2 electroreduction.