Tuning electrochemical CO2 reduction pathways using pulsed potentials on silver

Abstract

Renewable energy powered electrochemical CO2 reduction (CO2ER) enables us to utilize the greenhouse gas and turn it into useful commodity chemicals, thus mitigating harmful carbon emissions and offering a more sustainable carbon source alternative to crude oil. Yet, a low selectivity, stability and activity hamper the industrial application. We have studied pulsed potential CO2ER (p-CO2ER), which serves as an elegant way of controlling the product selectivity simply by modulating the applied electrical input. In pulse potential CO2ER driven by square-wave voltammetry, four variables can be adjusted to drive the catalysts selectivity: the pulse potentials herein labeled “cathodic” (Ec) and “anodic” (Ea) (meaning that one is more negative than the other, while both are typically > 0 V) as well as the time of the applied cathodic pulse tc and of the anodic pulse ta. We have investigated the effect of p-CO2ER on silver and found the unique behavior of high methane formation. The catalyst underwent an “activation” period of 4-5 h to reach its CH4 selective state, which we have analyzed to be due to changes in the surface morphology as well as changes in adsorbates. We have systematically studied the effect of pulse potential and pulse time in a classical H-cell as well as in a flow-cell connected to a mass spectrometer. We found that the CH4 formation depends on the right combination of Ec and Ea and discovered significant rate enhancement towards CH4 when applying millisecond pulses. A thorough analysis of the product formation on a millisecond timescale by means of differential electrochemical mass spectrometry (DEMS) allowed us to gain a deeper understanding of the pulsing effect. The fast response product analysis by DEMS allowed us further to screen a plethora of pulse potential combinations, identifying new combinations for high CH4 activity. We provide additional information about adsorbed intermediates and products on the silver surface by in-situ Raman spectroscopy, and hence were able to show that pulsing leads to an increase of local CO2-concentration, which can promote CO2ER, while suppressing the formation of hydrogen.