Electrochemical reduction of CO₂ is a promising method for converting a greenhouse gas into value-added products, utilizing renewable energy. Novel catalysts, electrode assemblies, and cell configurations are all necessary to achieve economically appealing performance. In this talk, I am going to talk about two of these aspects, targeted by our laboratory.

First, I am going to present a zero gap electrolyzer cell, which converts gas phase CO₂ to products without the need for any liquid catholyte. This is the first report of a CO₂ electrolyzer cell, where multiple stacks are connected, thus scaling up the electrolysis process (patent pending, PCT/HU2019/095001). The operation of the cell was validated using both silver nanoparticle and copper nanocube catalysts, and the first was employed for the optimization of the electrolysis conditions. Upon this, CO formation with partial current densities above 250 mA cm⁻² were achieved routinely, which was further increased to 300 mA cm⁻² (with ~95 % Faradaic efficiency) by pressurizing the CO₂ inlet. Evenly distributing the CO₂ gas among the stacks (parallel connection), the operation of the multi-stack cell was identical to the sum of multiple single-stack cells. When passing the CO₂ gas through the stacks one after the other (serial gas connection), the CO₂ conversion efficiency was increased remarkably. Importantly, the presented electrolyzer simultaneously provides high partial current density, low cell voltage (−3.0 V), high conversion efficiency (up to 40 %), and high selectivity for CO production; while operating at up to 10 bar differential pressure.

In the second part of my presentation I will shed light on the importance of catalyst morphology, using N-doped carbon (N–C) catalysts as a model system.[1]  We found that CO2R activity, selectivity, and stability of N–C electrodes are highly dependent on their porosity. The presence of mesopores was demonstrated to be beneficial in achieving high CO selectivity and current density, with an optimal pore size around 27 nm. Even after convoluting factors other than morphology (e.g., surface chemistry, level of graphitization, surface area), the reasons behind the observed trends are complex. CO₂ adsorption properties, wetting characteristics, and geometric effects are jointly responsible for the massive difference in the CO2R performance. All these properties must be taken into consideration when we aim to understand the reduction mechanism on different catalysts and while improving the performance further to a technologically relevant level (as alternatives to precious metal catalysts).