As the primary greenhouse gas, CO₂ contributes to worldwide ecological disruption, climate instability, and economic risk. Part of the mitigation includes the increased attention on carbon capture and utilization (CCU) technologies. Among these, electrochemical CO₂ reduction (eCO₂R) has emerged as a highly promising approach for producing carbon-neutral chemicals and fuels, especially when using renewable energy. While up until now most lab-scale CO₂ reduction experiments are conducted at ambient temperatures (20-25°C), maintaining such low temperatures at an industrial scale will become challenging due to joule heating. In our study, we pushed the boundaries further than the already existing systems, opting for a higher temperature of 85°C for CO₂ reduction. This deliberate choice was made to accentuate and evaluate the changes in reaction kinetics, mass transport, and product distribution from ambient conditions and gain understanding of the thermal effects in practical electrolyzer configurations. Here, we explore the interplay between temperature, catalyst behavior and reactor design. 

We have synthesized a series of bismuth-related nano catalysts, including oxidized, metallic and carbon containing nanoparticles, and have quantitatively evaluated their performance at 85°C. The observations suggest that catalysts without carbon exhibit high initial selectivity but demonstrate diminished stability over a 24-hour timeframe compared to catalysts with a protective carbon layer. Indeed, over a 24-hour timeframe, the carbon-free catalyst showed a 20% decrease in FE, while for the carbon-containing catalyst this drop was more than halved in the same timeframe. By evaluating the role of carbon additives our results yield valuable insights into the impact of carbon additives on stability and overall performance, especially at elevated temperature. These findings uncovered important critical aspects for catalyst design, providing essential knowledge for future larger-scale applications. 

Besides catalyst design, also reactor optimization is required. Specifically, we focused on the three-phase boundary, where the eCO₂R reaction actually takes place. Logically, it is important that this boundary is at the exact location of the catalyst layer (CL). To this end, gas diffusion layers (GDLs) are specifically designed to align this boundary to the right location through variations in hydrophobic additives, thickness, porosity, etc. However, we have found that all these efforts to perfect the GDL properties can easily be forfeited if the differential pressure across it were to change as it shifts the three-phase boundary. A shift inward the GDL will result in a flooded CL, lengthening the diffusion path of the gaseous CO₂in the electrolyte to the active sites of the CL resulting in increased hydrogen evolution. Changing the temperature of the system will affect the location of the boundary layer and by optimizing the differential pressure we can shift its location back to its optimal position and increase the performance of the CO₂ electrolyzer. By optimizing the differential pressure, i.e. elevating the backpressure at gas side, it was possible to increase the system’s durability with a factor of 1.6. Besides the differential pressure, also the GDL type and composition plays a crucial role and was investigated in this work by evaluating different commercially available GDLs with/without microporous layer and with different porosity and wet proofing. By re-evaluating and optimizing the operational conditions and GDE composition, we are getting closer to making the electrochemical CO₂ reduction efficient also at elevated temperatures.