Influence of Water Molecules on CO2 Reduction at the Pt Electrocatalyst in the Membrane Electrode Assembly System

Marked CO2 Reduction to Generate C1-C3 Products Using Pt0.9Ru0.1/C-Based Membrane Electrode Assembly at Extremely Low Overpotentials

The electrochemical CO2 reduction reaction (CO2RR) over Pt electrocatalysts in an aqueous solution system yields mainly H2 and a slight amount of carbon-based products. This limitation can be overcome by using a membrane electrode assembly (MEA) containing a Pt/C electrocatalyst without any overpotential. However, the obtained CO2RR product was only CH4. In this study, we investigated the CO2RR using MEA containing a Pt0.9Ru0.1/C electrocatalyst. Consequently, not only methane as a C1 product but also ethanol as a C2 product and acetone as a C3 product were produced at extremely small overpotentials. This is the first time in the literature that ethanol and acetone were produced from the CO2RR over a Pt-Ru-based electrocatalyst. This fact was confirmed through mass spectrometry, gas chromatography, and isotope labeling experiments. The Faradaic efficiencies of CH4, C2H5OH, and CH3COCH3 were 20.6%, 5.9%, and 5.2%, respectively, and the total Faradaic efficiency was 31.7%. The three products were generated via the Langmuir-Hinshelwood mechanism involving adsorbed CO (COads) and H atoms on the electrocatalyst. The adsorption configuration of COads determines the generation of methane, ethanol, and acetone. The C-C coupling reactions occurred through the formation of COads clusters. Our findings promote the production of valuable C2+ compounds from the CO2RR, which is an important CO2 capture, utilization, and storage technology for realizing carbon neutrality.

Practical potential of suspension electrodes for enhanced limiting currents in electrochemical CO2 reduction

CO2 conversion is an important part of the transition towards clean fuels and chemicals. However, low solubility of CO2 in water and its slow diffusion cause mass transfer limitations in aqueous electrochemical CO2 reduction. This significantly limits the partial current densities towards any desired CO2-reduction product. We propose using flowable suspension electrodes to spread the current over a larger volume and alleviate mass transfer limitations, which could allow high partial current densities for CO2 conversion even in aqueous environments. To identify the requirements for a well-performing suspension electrode, we use a transmission line model to simulate the local electric and ionic current distributions throughout a channel and show that the electrocatalysis is best distributed over the catholyte volume when the electric, ionic and charge transfer resistances are balanced. In addition, we used electrochemical impedance spectroscopy to measure the different resistance contributions and correlated the results with rheology measurements to show that particle size and shape impact the ever-present trade-off between conductivity and flowability. We combine the modelling and experimental results to evaluate which carbon type is most suitable for use in a suspension electrode for CO2 reduction, and predict a good reaction distribution throughout activated carbon and carbon black suspensions. Finally, we tested several suspension electrodes in a CO2 electrolyzer. Even though mass transport limitations should be reduced, the CO partial current densities are capped at 2.8 mA cm-2, which may be due to engineering limitations. We conclude that using suspension electrodes is challenging for sensitive reactions like CO2 reduction, and may be more suitable for use in other electrochemical conversion reactions suffering from mass transfer limitations that are less affected by competing reactions and contaminations.