UNDERSTANDING CARBON DIOXIDE ELECTROLYSIS USING GAS DIFFUSION ELECTRODES

Society relies heavily on the use of virgin fossil fuels as feedstock for chemical and plastics manufacturing. As pressure to decarbonise builds, this is not possible for all industries, leading to an increasing threat of financial penalties for high CO2 emitters that are unable to completely decarbonise, like cement, steel and glass producers.
Electrochemical CO2 reduction provides an opportunity to utilise intermittent renewable energy sources (wind, solar) to drive the conversion of CO2 into useful products. These products range from CO (component of syngas) to sustainable aviation fuel. Electrochemical CO2 reduction is a mature technology, however, due to the requirement for rapid development to achieve Net Zero by 2050, research into the challenges and opportunities for process scale-up has accelerated.
In this work, I show the use of a cheap Mn electrocatalyst in two common electrolysers, a flow-cell and a zero-gap cell, designed to achieve higher currents than a typical batch electrochemical experiment. I show that the Mn operates with increased partial current densities for CO (jCO) of 13.7 mA cm-2 than has previously been reported in near-neutral electrolyte in a flow-cell. However, device stability is poor with jCO dropping to <5 mA cm-2 after 5 hours. In basic environments, the Mn catalysts demonstrates low activity with jCO <10 mA cm-2, although in an acidic environment selectivity for CO is good with jCO ~35 mA cm-2.
As we move towards industrialisation for decarbonisation processes, it is critical to understand the importance of impure gas feeds on CO2 reduction. Typically, studies have focussed on an ultra-pure CO2 stream as feedstock for electrolysis, however, in reality both flue gas and captured CO2 contain impurities like O2. Therefore, it is imperative to understand the impact of even small concentrations on electrolysis selectivity and devices. Often, if the impact of O2 is explored, device failure mechanisms are not explained, leading to a knowledge gap.
Following on from my work using pure CO2, I explore the impact of small concentrations of O2 on both the Mn complex and a simple Au catalyst. I show that the presence of 5% O2 impedes CO2 reduction in acid, and I move on to study Au as a catalyst. I benchmark Au activity in pure CO2 showing high selectivity for CO with jCO ~90 mA cm-2, although when 0.25% O2 is introduced jCO decreased to 80 mA cm-2. Significantly, the introduction of O2 decreases device stability (5 hours to 3 hours operation) due to peroxide formation leading to increased (bi)carbonate formation. I show that providing an acidic environment has a small impact on improving O2-tolerance but sacrifices CO selectivity (45 mA cm-2 to 24 mA cm-2) and cell voltage (2.6 V to 3.4 V).
Finally, I screen polymer additives to suppress O2-reduction. I find that by selecting a polymer that restricts O2-transport to the catalyst (Nafion), we can suppress O2 reduction. However, CO2 transport is also impeding, leading to poor selectivity for CO formation.