CO Selectivity and Stability in Bicarbonate Electrolysis

Abstract

As global warming proceeds with increasing consequences, new and improved solutions that can mitigate the increasing CO2 emissions are becoming ever more important. Renewable energy technologies are advancing and along with carbon capture technologies, they promise to lower global emissions and help combat climate change. Renewable energy sources can be used to form value-added chemicals from captured CO2 through a method known as CO2 electrolysis. A novel approach to CO2 electrolysis is bicarbonate electrolysis, which uses carbon capture solutions directly to make products. By integrating the capture and conversion process this way, effectively bypassing the energy intensive steps of CO2 recovery required for conventional gas fed operation, CO2 conversion to products can become even more sustainable.

The objective of this thesis is to explore ways to improve the selectivity of carbon monoxide (CO) in a bicarbonate electrolyser. In this experimental study, focus is also placed on the stability of the process, characterising the selectivity over time. Causes of selectivity decline are examined as well as methods of improvement. Furthermore, the effect of pH on CO selectivity and stability is given special consideration. Literature in the field of (bi)carbonate electrolysis was reviewed to gather understanding on the process, to clarify recent advances made and to find areas for improvement. Based on the findings from the literature review the experimental study was designed.

Experiments were conducted in a membrane electrode assembly (MEA) flow-cell in constant current fashion, applying a current of 100 mA/cm2. The membrane chosen was a bipolar membrane (BPM) as it offers the possibility of operating with distinct electrolyte environments, separating the 3M bicarbonate catholyte from the 1M potassium hydroxide anolyte. Gas diffusion electrodes (GDE) were prepared by spray-coating silver nanoparticles on the surface, using Nafion ionomer as binding material. An interdigitated catholyte flow plate was used which ensured the bicarbonate would pass through the GDE due to its discontinuous channels forcing the flow through.

By introducing a catalyst-membrane gap through inserting a hydrophilic porous spacer between the GDE and the BPM, CO selectivity was improved from 50% to 78% in peak production, recording 55%
averaged over 3 hour operation. This enhancement in selectivity can be explained by the defined pH gradient resulting from the gap, permitting a low pH at the BPM for protons to react with the bicarbonate, liberating i-CO2; and a higher pH at the catalyst for CO2 conversion to CO while suppressing the hydrogen evolution reaction (HER). An optimum gap was found to be 135 - 270 μm. These results compare with the previously highest reported CO selectivity values from the literature at ambient conditions and 100 mA/cm2. While improving the selectivity, the stability of CO was not improved by the catalyst-membrane gap. The pH was found to affect both the selectivity and stability of CO, with higher bicarbonate pH leading to reduced selectivity but improved stability. This behaviour is explained by the reduced i-CO2 liberation and increased carbonation reactions taking place at higher pH levels…