A Complete Study of the Effects of Ultrasound on the Electrochemical CO2 Reduction Reaction on Copper

Abstract

Conversion of CO2 into useful fuels and chemicals is particularly interesting due to the synergetic benefits of obtaining value added products and contribution towards mitigating climate change and global warming. Various methods such as chemical, radiochemical, thermochemical, photochemical, electrochemical and photochemical techniques are currently employed for the conversion of CO2 [1]. Electrochemical conversion of CO2 into hydrocarbon is a promising method in which various short and long chain hydrocarbon compounds can be produced, depending upon the type of electrode material, the applied potential and electrolyte used [1], [2].The electrochemical CO2 reduction reaction (CO2RR) includes a series of steps involving dissolution, transport, adsorption and reaction of CO2. Moreover, the CO2RR rate is limited by the electron transfer at low overpotential whereas at high overpotential, it is governed by mass-transfer of CO2 or proton (H+) at the electrode surface. The first CO2RR product is the CO which is adsorbed at the electrode surface and subsequently reduced to hydrocarbons. Surendranath et al. [3], [4], Lates et al. [5] and Goyal et al. [6] have experimentally shown that CO2RR is not limited by mass transfer caused by hydrodynamic agitation. For example, increasing rotation rate (w) of the rotating disc electrode does not increase the CO2RR product yield. They concluded that CO2RR is far more resistant to mass-transfer limitations than the hydrogen evolution reaction (HER).However, it was recently observed that specific agitation on CO2RR at copper (Cu) electrode induced by power ultrasound (24 kHz) increases the production of CO and CH4 significantly [7]. Moreover, in the presence of ultrasound, C2H4, HCOOH, C2H5OH were found whereas under silent conditions these products were not formed. In addition, under ultrasonic conditions, it was found that the HER was suppressed significantly. It is well known that ultrasound in an electrochemical cell impart some particular advantages such as degassing, cleaning and activation of the electrode surface, solution degassing, disruption of the Nernst diffusion layer, enhanced mass transfer of ions through the double layer [7], [8]. It was also found that ultrasound could aid in initiating new electrochemical CO2RR reaction pathways yielding new products. Therefore, it can be assumed that the above-mentioned effects could be attributed to the enhanced Faradaic efficiencies of electrochemical CO2 reduced products.