Paving the way toward Photo-Electrochemical Carbon Monoxide Reduction

Human activities have led to an increase in CO₂ emissions, disrupting the global carbon cycle. This excess greenhouse gas in the atmosphere contributes to global warming, triggering a climate crisis that destabilized both the Earth's ecosystem and human livelihoods.
One promising strategy to decrease CO₂ emissions centers around the concept of artificial photosynthesis. This approach involves directly converting solar energy into the form of chemical bonds. Of particular interest is the conversion of CO₂ into multi-carbon compounds and sustainable fuels. This not only produces valuable resources but also reduces CO₂ levels in the atmosphere. However, the efficiency and selectivity of catalyzing this process pose challenges. These hurdles can be overcome by considering a sequential conversion pathway with carbon monoxide (CO) as central intermediate.
This research endeavors to create an artificial photosynthesis system by employing photo-electrochemical reduction of carbon monoxide. To accomplish this, a triple junction solar cell is adapted to catalyze electrochemical transformations on its surface, utilizing its inherent voltage to drive the process autonomously.
During the photo-electrochemical process, the solar cell's front side is the only part exposed to the electrolyte, as contact with other sides risks short-circuiting. Thus, a setup that shields the solar cell from the electrolyte is developed. Additionally, the system is designed to enhance the transport of carbon monoxide, crucial for efficient transformation, by promoting high electrolyte convection.
Both the light absorption and catalysis processes take place on the front side of the triple junction solar cell. Consequently, efforts shift towards finding the optimal balance between maximal light absorption, protection against electrolyte corrosion, and catalytic efficiency.
Part of this work concentrates on designing and characterizing a copper catalyst that combines selectivity for multi-carbon products with transparency to light. A refined pulsed electrodeposition technique yields copper nanoparticles in a cubic shape, outperforming thin-film polycrystalline copper in terms of efficiency. Various synthesis methods, based on current and potential controlled electrodeposition, guide the development of a deposition mechanism that allows the selective formation of cubic copper oxide particles. This deposition approach is successfully adapted to photo-electrochemical conditions, confirming particle deposition and shaping as proposed by the mechanism. Unfortunately, utilizing only copper catalysts on the photo-absorber surface proves to be unstable. Consequently, a significant portion of the research is dedicated to identifying and characterizing suitable protective layers for the process. Surprisingly, titanium dioxide, typically used as a protection layer in photo-electrochemical systems, exhibits hydrogen production when combined with a copper catalyst. Further investigations eliminate various hypotheses: The reduction of titanium dioxide to metallic titanium, as well as the intercalation of protons into the titanium dioxide lattice and the contamination with noble metals were found to be unlikely. No conclusive evidence for the dissolution and re-deposition of copper nanoparticles of few nanometer size could be demonstrated, either. This leaves hydrogen spillover from the metal oxide to the catalyst as the most plausible explanation. 
To circumvent this issue, tantalum oxide is explored as an alternative protection layer. While demonstrating good selectivity for carbon monoxide reduction, direct application to the solar cell's surface results in instability. This challenge is resolved by developing a dual electron transport layer composed of titanium dioxide and tantalum oxide. 
Ultimately, insights from all preceding investigations converge to realize the first photo-electrochemical CO reduction system. Various copper catalysts, in conjunction with photo-absorber protection layers, are evaluated based on their selectivity and stability. Notably, the configuration featuring copper cubes with a polycrystalline thin-film catalyst exhibits the highest selectivity for CO reduction products, motivating further exploration of this configuration.