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…
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Rising global CO2 levels underscore the urgent need for effective carbon capture and utilization (CCU) technologies to support a circular carbon economy. This study evaluates the techno-economic per- formance of a novel integrated CCU system that combines a K2CO3-based capture column with a bicarbonate electrolyser for syngas production, specifically targeting applications in the steel industry. An ASPEN PLUS model of the capture column was developed and integrated with a pH-dependent Faradaic Efficiency (FE) model of the electrolyser in Excel. Five cases were defined: (I) 90 wt% CO2 capture, (II) syngas production with a 2:1 H2/CO ratio for the Fischer-Tropsch process, (III) electrolyser operation with FECO > 50%, (IV) syngas composition suited as feedstock for electric arc furnaces (EAF) in the Energiron III process, and (V) an intermediate pH step. A techno-economic analysis (TEA) was conducted across worst, base, and best-case scenarios for each case.  Key findings reveal a trade-off between achieving high FECO at low pH levels and maximizing CO2 capture efficiency at high pH levels. Systems operating with large pH steps demonstrated a lower Lev- elized Cost of Syngas normalized to the Lower Heating Value (LCOSLHV ), due to increased hydrogen output. In contrast, systems with smaller and narrower pH steps incurred higher LCOSLHV due to their output’s lower LHV. The techno-economic analysis (TEA) indicates that the operational expenditure (OPEX) for the integrated CCU system is currently too high to be cost-competitive with alternative solu- tions. Sensitivity analysis reveals that the integrated CCU system is competitive with other electrolysis methods only under best-case conditions. Electricity costs and a low CO2 utilization ratio are identified as the primary drivers of OPEX. Improvements in these areas result in the most significant reduction in LCOSLHV, making them critical enablers for the integrated CCU system. Additionally, the cost per kilogram of CO2 saved is high compared to EU CO2 Emission Trading System (ETS) prices.  Current bicarbonate electrolysers are more costly than gas-fed CO2RR systems in terms of Unit Capital Cost (UCC) per kilogram of CO produced, largely due to reduced performance at higher current den- sities (>100 mA/cm2). Achieving CAPEX parity with gas-fed CO2RR systems would require increasing current densities while maintaining high FECO and sustaining these efficiencies at alkaline pH levels.  Future work should prioritize reducing both OPEX and CAPEX for the system, with a particular focus on improving the technical performance of the bicarbonate electrolyser. Key objectives include increasing current density while maintaining high FECO at alkaline pH levels, improving the CO2 utilization ratio, and enhancing the stability of the electrolyser.  Keywords: Carbon Capture and Utilization, Bicarbonate Electrolysis, K2CO3-based CO2 Capture, Ben- field Process, Integrated CCU System, Techno-economic Analysis ... Read more