Transparent thin film structured catalysts for electrochemical CO2 reduction via wet-chemical deposition methods

To address the anthropogenic climate challenges in a sustainable and efficient manner, developing innovative technological solutions is essential. Solar-driven electrochemical CO2 reduction represents a promising approach that harnesses the abundant solar energy to valorize CO2. The BMBF-funded project DEPECOR (Direct Efficient Photo-Electrocatalytic CO2 Reduction) aims to achieve a monolithically integrated, fully solar-driven electrolyser capable of driving electrochemical CO2 reduction (CO2R) without external energy input. Accomplishing this objective necessitates advances in materials science across multiple domains, including the enhancement of solar cell performance, the effective integration of protection layers, and the development of highly efficient and sufficiently optically transparent catalyst layers. The last is the main objective of this PhD work at HZB. Accordingly, nano-structured thin Ag layers of wires and particles were prepared on conductive substrates (i.e., gas diffusion layer, glassy carbon, and fluorine-doped tin oxide glass) via wet-chemical and electrochemical deposition techniques. Their electrochemical CO2 reduction to CO activity was examined by standard electrochemical methods, including linear sweep voltammetry (LSV), cyclic voltammetry (CV), and chronoamperometry (CA), while the product formation was quantified by on-line gas chromatography (GC), mass spectrometry (MS), and ultra-high-performance liquid chromatography (UHPLC). Understanding the dependence of layer transparency and CO2R activity on mass loading and particle morphology enables rational selection of the optimal catalyst layer and permits estimation of the performance achievable in Ag-integrated photovoltaic (PV) electrolyser systems. Finally, silver layers were integrated onto the front side of the TiO2-protected PV cell, and the CO2R catalytic performance of the Ag-integrated PV electrolyser was examined and evaluated. In particular, Ag nanowires (AgNWs) were successfully synthesized via polyol synthesis and were proven to be highly active for CO formation. The optimal AgNWs layer for maximizing the CO production rate, when integrated onto a PV cell, exhibits a transmittance of 42% and a CO faradaic efficiency of 34%. Nevertheless, the poor mechanical stability of the AgNWs layers precludes their experimental investigations in an AgNWs-integrated PV electrolyser. In contrast, electrodeposited Ag (ED-Ag) particles demonstrated enhanced stability with comparable CO faradaic efficiency and transparency. Given the demand for a transparent, stable, and conductive substrate in large quantity to optimize the parameters of electrodeposition, a fabrication method was successfully developed to protect commercial FTO glass against corrosion under CO2-electrolysis conditions. An Ar ion beam was first employed to smooth the morphology of FTO glass, then a Nb-doped TiO2 thin layer was deposited by direct-current (DC) magnetron sputtering. The ED-Ag deposition parameters were first optimized on this substrate. Finally, the optimized ED-Ag layers were deposited on the TiO2-protected (3-junction) PV cell. The CO2R activity of the ED-Ag/PV photocathode was demonstrated and evaluated in a light-assisted electrolysis experiment. Nonetheless, the performance of the integrated PV systems is significantly limited by low catalyst loading and partial shadowing of the solar cell. Furthermore, the stability of both the TiO2 protection layer and the ED-Ag particles remains an unsolved challenge and requires significant improvement.