Carbon dioxide (CO2) electrolysis is a promising technology for producing carbon-neutral chemical feedstocks. However, further improvements in energy efficiency and system cost are required before large-scale deployment can be realized. Capillary-fed electrolysis (CFE), originally developed for water electrolysis, has revealed lower cell overpotentials by providing relatively bubble-free environments in which gas products can readily leave the cell. This study investigates the implementation of CFE for CO2-to-CO electrolysis and aims to identify the operating principles governing its effective application.

Our CFE cell, employing a porous polyethersulfone (PES) membrane and alkaline electrolyte, is successfully demonstrated. Comparison with an AEM-based hybrid-MEA cell confirms that the CFE cell operates at lower cell potential. However, an initial voltage increase not observed in conventional cell architecture is identified. The increase is attributed to CO2 bubbles forming inside the membrane through (bi)carbonate neutralization, which increase the ohmic overpotential by blocking ion transport and the kinetic overpotential by locally increasing the current density through bubble coverage of the cathode active area.

During longer-term operation, water management and electrolyte concentration determine the product selectivity of the CFE cell. The use of a porous membrane in a CFE cell enables control over the water supply unavailable in other architectures, which can be realized through selection of membrane pore size, cell compression, and CO2 feed humidification. A balance between cathode flooding and salt supersaturation is key to achieving optimal water supply. In addition, electrolyte concentration is found to contribute more strongly to salt precipitation than to promoting carbon-product formation. These findings establish key design principles for implementing capillary-fed CO2 electrolysis. Despite the need for further research to enhance stability and product selectivity, the proposed CFE cell has shown potential cost reduction through its simplified system setup and the use of lower-cost membrane materials. ... Read more

Electrochemical reduction of carbon dioxide (CO2) to carbon monoxide (CO) offers a promising pathway for sustainable chemical production, but its large-scale implementation is limited by challenges related to energy efficiency, selectivity, and membrane-mediated ion transport. Bipolar membrane (BPM) CO2 electrolyzers have emerged as an attractive architecture, as they enable independent control of cathodic and anodic pH environments, therefore allowing the use of earth-abundant anode materials. However, BPM operation in CO2 electrolysis is governed by complex transport phenomena and inherent trade-offs between performance metrics like cell voltage, faradaic efficiency (FE), water dissociation efficiency, and co-ion crossover. In this thesis, the effect of asymmetric bipolar membrane design on CO2-to-CO electrolysis performance is investigated. A series of Nafion/PiperION-based BPMs with systematically varied AEL and CEL thicknesses were fabricated and evaluated in a zero-gap membrane–electrode assembly CO2 electrolyzer. Key performance metrics including cell voltage, faradaic efficiency toward CO, crossover of species through the membrane and water dissociation efficiency (WDE), were quantified under relevant operating conditions. Additionally, results were compared to the commercially available state-of-the-art Fumasep bipolar membranes. The results demonstrate that membrane layer thickness strongly influences ion transport and charge-carrier distribution across the BPM. Increasing AEL thickness effectively suppresses co-ion crossover and improves WDE, but can limit cation transport to the cathode and thereby reduce FE toward CO. In contrast, variations in CEL thickness have a smaller impact on crossover and selectivity for the membrane chemistries studied. Co-ion fluxes across the membrane were shown to be dominated by K+ transport for the Nafion/PiperION membranes, which contrasted with the HCO3 – dominant behaviour found for the Fumasep membranes. This indicates that co-ion transport is also largely dependent on layer chemistry, rather than thickness alone, and the optimization strategy will differ with layer identity. Additionally, the first limiting current density was shown to correlate with co-ion transport behavior, specicically K+ crossover, indicating its potential as a diagnostic tool for evaluating asymmetric BPM performance. Along with these findings, important tradeoffs in BPM CO2 electrolysis were evaluated. Although layer thicknesses were found to enable decoupled control over ion fluxes, important tradeoffs between conductivity and FE to CO remain for Ag based configurations, regardless of layer asymmetry. Overall, this work elucidates how asymmetric BPM layer thicknesses shape transport phenomena and performance trade-offs in CO2 electrolysis. The insights obtained provide guidance for the rational design of bipolar membranes optimized for selective, energy-efficient CO2 conversion and highlight BPM layer thickness as a practical design lever for mitigating key challenges in BPM CO2 electrolyzers. ... Read more

A transition towards renewable energy sources is necessary to globally achieve net-zero carbon emissions. To achieve reliable renewable energy production, storage of sustainable energy is key. The utilisation of hydrogen, whether for long-term or short-term storage, could provide a vital solution in achieving reliability for sustainable energy demand. However, producing green hydrogen (water electrolysis) is not cost-competitive with fossil fuels and other hydrogen production methods. An original alkaline electrolyser design has been created that can help with lowering the CAPEX and OPEX when optimized. In common alkaline electrolysers, electrodes are completely immersed in liquid. However, the electrode surfaces of the novel design apply wetted surfaces through the use of capillary forces to suck up electrolytes. Compared to immersed electrode surfaces, the wetted surface shows a reduction in (bubble) resistance leading to lower voltage losses. Additionally, the overpotential can be further reduced with, for example, the use of a PES membrane instead of a common Zirfon membrane, better contact with the anode by welding it to the current collector, and adding PTFE to the anode for better gas removal. Prior to testing the assembled cell design, the electrodes were tested individually to understand the mechanics of each electrode. Here, we report that the electrodes perform well individually in alkaline media.
However, our findings reveal that the assembled cell performs poorly, mostly because of significant overpotentials brought on by increasing resistance, along with other drawbacks including salt formation and exceeding the legal gas crossover limit. Some of these issues regarding performance are still poorly understood and therefore need further investigation. ... Read more

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. ... Read more

The global concern of the increasing levels of CO2 is growing quickly in the recent years. Therefore, a lot of research is currently underway with respect to closing the carbon cycle. The electrochemical reduction of CO2 is a promising technology that could help utilize the CO2 as a feedstock to produce chemicals and fuels, while storing the excess energy generated from renewable energy sources in chemical bonds. Due to its simplicity and economic feasibility, the conversion of CO2 to CO has a high potential in the industrial market. Membrane Electrode Assembly (MEA) is an interesting electrochemical reactor configuration to produce CO on industrial scale due to the low ohmic losses and reduced risk of catalyst poisoning. Optimizing the catalyst and operating conditions are key steps towards the commercialization of the process. This research focuses on understanding the influence of different process parameters on the CO selectivity while analyzing the performance challenges. Multiple inlet flow rates of CO2 were tested at different current densities to evaluate its impact on the faradaic efficiency. The experiments were performed using KOH-exchange MEA cell with gas diffusion electrodes and Sustainion membrane. Since the product of interest is CO, Ag-based catalyst layer was sputtered on the gas diffusion electrode. The cathodic products were identified and quantified using gas chromatograph. The experimental results have shown that increasing current density resulted in lower CO selectivity, while the inlet flow rate did not have a significant effect. It was also shown that the cell could not achieve higher than 200mA/cm2 due to the accumulation of salts blocking the gas flow channel.
On top of that, a simple 2D model was developed in COMSOL Multiphysics to understand the mass transport and concentration distribution in the gas flow channel. The model was not able to simulate the complexities of the electrochemical process and represented an ideal plug flow reactor. It is understood that the incorporation of reaction kinetics and current distribution is necessary to replicate the real scenario.
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Electrolyzers for CO2 reduction can be used to synthesize renewable fuels, as a route to replace fossil fuels in our everyday economy with the purpose of minimizing the effects of climate change. For a high-scale implementation of electrolyzers, high operation current densities and low overpotentials are essential. Currently available options do not offer this and are limited by the low mass transfer of CO2 in the system. We envision using catalyst-coated capacitive particles (slurry electrodes) to be a solution: a system where the reaction site can be brought to the reagent, instead of the other way around. This thesis is a first step in the direction of a slurry electrode flow cell and aims to produce silver nanoparticles deposited on activated carbon particles to function as catalysts in such a system. The goal of the synthesis procedures was to produce a capacitive powder (activated carbon) with efficient deposition of silver nanoparticles spheres) ranging 5-20 nm in diameter. Based on literature, the most interesting methods for synthesis were selected to be: solution-based methods with only activated carbon, with added polyvinylpyrrolidone, with added sucrose, electrodeposition and impregnation. The resulting powders were analyzed using ICP-OES and TEM, showing that only impregnation and electrodeposition can yield the desired powders with a near-100% silver deposition efficiency and particles < 20 nm in size. Next, the impregnation and electrodepostion samples were prepared into a 15 wt% slurry, and a third slurry was made with bare AC as a blank. The catalysis experiments were executed in a flow cell (electrode area = 4 cm2), with currents ranging from -5 to -15 mA/cm2. Within this range of currents, extreme potentials up to -10 V (measured over working electrode versus counter electrode) were measured. The potentials on the anode and cathode side of the cell were also measured individually using reference electrodes, showing that the anodic side of the cell reached +5 V, whereas the cathodic side of this cell reached only -1 V. Additionally, CV scans before and after the experiments confirm that the cell’s conductivity decreased within an experiment, however, this degradation is reversed for a new experiment. These observations combined lead us to believe that the cause of the cell problems is located on the anodic side of the system, and is most likely resistance from oxygen bubbles, which needs to be solved to allow cell operation at higher current densities. Since the aim of the project was CO2 reduction on the slurry electrodes, the outgoing gases from the cathode were measured in a GC. The blank was included to demonstrate the catalytic effect of the silver nanoparticles, yet the highest CO flow (around 0.05 ml/min) was measured at the lowest current density during this blank experiment. Calculating the Faradaic efficiency (FE) suggests that 33% of the electrons in that experiment was used for this conversion. The other measurements, at higher currents or with silver present, did not reach an FE of 10%. This combination leads to the conclusion that the observed CO is most likely not produced by CO2 reduction. A hypothesis was made that this CO was already present at the powder’s surface and released during the experiment, however XPS data showed that there were no noticeable differences in the content of oxygen-bound carbon between the coated samples and the bare sample. Thus, the source of the CO is at this stage unknown. Future work towards proving CO2 taking place on the slurry electrode should first investigate the source of the observed CO, which requires testing the influence of the different carbon sources present in the system. Additionally, the blank experiment should be improved: using a bare carbon slurry that underwent the same procedure as the coated slurry (without the precursor present) eliminates any differences in the powder. ... Read more

In the last decade, many initiatives have been undertaken by governments, companies and other institutions to reduce the output and concentration of CO2 in the atmosphere. The current non-emitting energy sources with highest potential are solar and wind energy. Unfortunately, these energy sources vary in power production, while the demand fluctuates far less. Hence, a solution is needed for crossing the times when there is a mismatch in energy supply and demand. One potential solution is to store excess energy in chemical bonds by using electrochemical CO2 reduction. ... Read more

Rising CO2 levels in the atmosphere are becoming increasingly problematic, due to the effect of CO2 on climate change. CO2 capture and utilization has high potential as strategy to close the carbon cycle. An example of utilization of CO2 is the electrochemical reduction of CO2 to more valuable compounds. This thesis discusses the electrochemical reduction of CO2 to oxalic acid. Until now, oxalic acid as target product of the electrochemical CO2 reduction has not been studied in great depth, mainly because it only forms in non-aqueous solutions. The influence on several parameters, namely cathode material, applied potential,
anolyte, catholyte, membrane, supporting electrolyte, and temperature, on the electrochemical conversion of CO2 to oxalic acid has been studied. The first step in scaling-up has been taken, from a batch reactor (H-cell reactor) to a semi-continuous system (flow-cell reactor). In order to investigate the feasibility and its implementation in the industry, several options for the downstream processing of oxalic acid are discussed and a techno-economic analysis is performed on the proposed process design.

From the parametric study that was carried out, the parameters that had a considerable effect on the performance of the electrochemical reduction of CO2 to oxalic acid were cathode and anode material, catholyte and anolyte, membrane, applied potential and temperature. With the batch reactor optimal results in terms of faradaic efficiency and current density have been found, using lead as cathode, platinumas anode, propylene carbonate+0.7M tetraethylammonium chloride as catholyte and 0.5M H2SO4 as anolyte in which the cathodic and anodic compartment are separated by a Nafion 117 membrane. At higher temperatures, higher current densities were induced by reduction of mass transfer limitations. Within the range of -2.2V to -2.7V vs Ag/AgCl, increasing current densities and decreasing faradaic efficiencies were found with increasing applied potential. Semi-continuous flow-cell was investigated as a strategy to increase the mass transfer in the system. Although higher reduction currents were measured during CO2 reduction in a flow-cell compared
to the batch reactor, the faradaic efficiency towards oxalic acid was lower. The oxalic acid produced during CO2 reduction in the electrochemical reactor is dissolved in the liquid electrolyte. A further separation step of the oxalic acid from the liquid needs to be present to recover the product in a solid state. In order to assess the feasibility of the separation of oxalic acid, several technologies were addressed. Liquid-liquid extraction followed by crystallization is experimentally proved to be a suitable method for the separation and recovery
of oxalic acid from the electrolyte. Based on this separation method, a process design is proposed and a techno-economic analysis has been performed. The techno-economic analysis showed a favorable economic potential for this technology if certain key performance indicators can be achieved. However, the maturity level of this process is still in early stages. Some of those key performance indicators still need to be experimentally improved, further research should focus on increasing the obtained current densities and obtaining
stable faradaic efficiencies. ... Read more