The work presented in this thesis addresses the challenges of upscaling microscopic models such as microkinetic model to mesoscopic models (mass transport models) for CO₂ reduction reaction (CO₂RR). It also bridges the gap between mass transport model and molecular modelling through a multiscale modelling approach of the discussed electrochemical system.

CO₂RR is a promising carbon utilisation technology which converts or recycles CO₂ into value-added hydrocarbons and fuels, powered by water and renewable energy (solar and wind). Gas diffusion electrodes (GDEs) are an important component of the electrochemical cell employed to conduct this reaction. Compared to other electrocatalysts used for CO₂RR, copper electrocatalysts have the advantage of producing several products in a single electrolytic process.

CO₂RR on Cu electrodes is a very complex reaction for which the entirety of the mechanistic reaction pathways to achievable products on the electrodes are still being studied till date. This is exacerbated by two parasitic reactions (hydrogen evolution reaction and homogeneous bicarbonate reaction) which compete and inhibit the high current density of valuable products like ethylene, ethanol, propanol etc.

Hydrogen gas will naturally evolve in a water rich environment requiring only two electron transfers. On the other hand, bicarbonate reaction will be favoured and compete with important products like ethylene since they both thrive in a high pH regime. The favorability of homogeneous bicarbonate reaction over CO₂RR is as a result of the non-requirement of any electron transfer for the same reaction. This makes them susceptible to occurring at any point in the system as long as CO₂ and participating ions like OH¯ ions are readily available. Therefore, conducting the CO₂RR on GDEs requires accurately capturing species mass transport and kinetic profiles of the participating species which would enable the optimisation of CO₂RR on the GDE assembly.

The outcome of the thesis is in two portions: One is the upscaling of microkinetic model that carries the kinetic signature of CO₂RR on a well-studied Cu facet: Cu(100), to a GDE mass transport model. Here, it was found that due to the complexity of the reaction and multiple kinetic regions observed, the kinetic profile defined by the Butler-Volmer’s exchange current density (i₀) and transfer coefficient (α) is inadequate in expressing the true kinetics of CO₂RR.

A new methodology for directly integrating icrokinetic model to mass transport model is developed by employing a well-calibrated microkinetic model of the reaction on Cu(100) facets. This eliminates the oversimplification of CO₂RR kinetics. The impact of local pH, CO₂ concentration and applied voltage on product selectivity can be observed as documented in experimental work in literature. This work also reveals certain limitations with existing experimental work on CO₂RR that warrant exploration in future mechanistic studies, particularly as the move towards commercial applications of CO₂RR gains momentum.

The microkinetic model also proves to carry richer information of the reaction kinetics which when combined with mass transport models will lead to better understanding of the reaction microenvironment and serve as an important step to optimising GDE performance for commercial applications.

In the second part of the thesis, Cu catalyst-ionomer interaction is investigated. Nafion®, a perfluorosulfonate ionomer, has been proposed in literature, as a shielding material against homogeneous bicarbonates reaction. Due to the residing negative sulfonate ions which deprotonates in water, Nafion® is able to repel the negative bicarbonate ion, thus tuning the reaction towards CO₂RR. This appears to be a potential resolution for addressing bicarbonate reactions and their competition with CO₂RR particularly in the presence of abundant CO₂.

This hypothesis is tested by modelling a one dimensional planar electrode with a Nafion® layer and conducting DFT calculations on the molecular structure of Nafion®. Important parameters such as the thickness of Nafion® layer in the thick film regime (200 – 1100 nm), the ionic strength of supporting aqueous electrolyte and the applied voltage were investigated to model the concentration profile of participating species within the Nafion® layer.

It was observed that the bicarbonate ion concentration and local pH decrease with decreasing thickness which makes thinner films better at mitigating bicarbonates at the cost of a relatively lower pH. There is also a breakdown of Donnan exclusion effects with higher ionic strength electrolytes which allows diffusion of both co-ions and counter-ions, promoting the bicarbonate reactions.

Further, an important relationship between the pKₐ of Nafion® and the local dielectric constant was found. It was observed that the pKₐ of Nafion® generally increases as the local dielectric constant of the reaction medium decreases. This is primarily due to the accumulation of K⁺ and production of low dielectric constant products such as ethanol, albeit their concentrations are low on planar Cu electrodes.

Lastly, the lower permittivity of the reaction medium weakens the exclusion forces and changes the morphology of Nafion® which ultimately impacts its ability to mitigate homogeneous bicarbonate reactions.

The findings from this study indicate that employing Nafion as a protective barrier to deter homogeneous bicarbonate formation is a provisional remedy, emphasising the need for a more robust/resilient solution. Perhaps, the solution proposed earlier in literature that involves a tandem GDE arrangements for which one GDE reduces CO₂ to CO using Ag electrocatalysts and the other GDE, comprising Cu electrocatalysts, further reduces the produced CO to hydrocarbons is still a better option at the expense of higher energy requirement from running two separate electrochemical cells.

This approach mitigates the bicarbonate curse and address the intricate challenge of tuning the reaction microenvironment to favour desired products and mitigate the bicarbonate curse.