As part of the efforts to mitigate climate change, the electrochemical reduction of CO2 into valuable chemicals and fuels has been identified as a pivotal technology due to its potential for CO2 utilization and ability to store excess electricity as chemical energy. While significant progress has been made in optimizing various aspects of the electrochemical CO2 reduction system, an unexplored area pertains to the improvement of the anodic reaction. The conventional anodic reaction, namely the oxygen evolution reaction (OER), is constrained by kinetic and thermodynamic unfavorability, reliance on precious metal catalysts, and the need for costly downstream gas separation.
To address these limitations, a novel approach has gained traction within the research community: the paired electrolysis of CO2 reduction reaction (CO2RR) and glycerol oxidation reaction (GOR). How- ever, these studies mostly employ expensive platinum group metal (PGM) catalysts in flow cells, failing to address cost dependency or scalability for future industrial applications due to inefficient energy use in this electrolyzer configuration. Therefore, this study intends to understand and optimize the paired electrolysis of CO2RR with GOR in zero-gap electrolyzers while comparing the performances of Pt (a relatively rare PGM catalyst) and Ni (an abundant non-PGM catalyst). The effects of applied cell po- tential, glycerol concentration, active surface area, and ion exchange membrane type on GOR product selectivity and the system’s energy demand are evaluated through potential and current controlled ex- periments. Gas chromatography (GC) and proton nuclear magnetic resonance spectroscopy (H-NMR) are used for product analysis.
This thesis demonstrates the viability of paired electrolysis in zero-gap electrolyzers, yielding major products like formate and lactate alongside minor byproducts such as acetate, glycerate, and dihydrox- yacetone. The results reveal Ni’s superior performance over Pt at current densities below 200 mA/cm2 in zero-gap electrolyzers. The negative influence of increasing applied potentials on faradaic efficien- cies (FEs) is presented, particularly in Ni, likely due to side reactions like OER or formate oxidation. The study also illustrates that increased glycerol concentrations decrease FEs and system activities due to heightened viscosity-related diffusivity issues. Moreover, the tests conducted using the Ni anode in zero-gap electrolyzers utilizing bipolar membranes (BPM) show a minor reduction in product selectivity likely caused by the increased amounts of OH– ions near the anode coming from the water dissociation reaction (WDR).
The study also uncovers that the anticipated significant reduction in the cell’s energy demand with the replacement of OER with GOR is not observed in zero-gap electrolyzers. No conclusive improve- ments are observed for either catalyst when anion exchange membranes (AEM) are employed, and only marginal improvements in the cell’s energy demand are achieved when bipolar membranes are used with Ni. Although this behavior is speculated to be a consequence of the absence of a flowing electrolyte near the anode, further investigations are needed to identify the cause of this unexpected lack of improvement in the energy demand.