The PEC conversion of CO2 into value-added chemicals is a sustainable method to efficiently produce fuels by combining photocatalysis and electrocatalysis [1]. Efficient reactor setups and photoelectrodes are needed to reduce external energy input. Our previous work showed the benefits of combining CaTiO₃, WO₃, and BiVO₄ in multi-layered photoanodes under visible light [2,3]. Additionally, Cu₂O blended with a bio-MOF as photocathodes achieved excellent PEC CO₂ reduction to alcohols, particularly methanol. This study marks our group's first attempt to create a bias-free PEC electrolyzer for CO₂ reduction into alcohols.

Hence, this work represents the first attempt in our group to develop an integrated, bias-free PEC electrolyzer with a photocathode/photoanode configuration for CO₂ reduction into value-added products such as alcohols.

 The different catalysts are coated onto the substrates [4] with different loadings and mass ratios, which are optimized to reach the best PEC performance. Precisely, the bio-MOF is synthesized from the reaction of copper(II) acetate with adenine, which prompted the formation of a 3D microporous coordination polymer depicted by the formula [Cu₂(µ-acetato)₂(µ₃-adeninato)₂]_n. On the other hand, the photoanodes include CaTiO₃ (1 mg cm^-²) as the bottom layer, WO₃ (0.75 mg cm^-²) as an intermediate conductive layer, and BiVO₄ (3 mg cm^-²) as a light-harvesting layer. They are characterized using on-off PEC techniques, such as LSV, cyclic voltammetry, and EIS. Subsequently, both photoanodes and photocathodes are individually evaluated in a divided filter-press reactor in continuous mode under visible light irradiation (100 mW cm^-2), with a platinized titanium plate as the dark anode (photocathode/dark anode) or dark cathode (dark cathode/photoanode), using 0.5 M KHCO₃ as the electrolyte.

This work evaluates the integration of the photoactive surfaces in a bias-free photocathode/photoanode PEC electrolyzer, paying special attention to: i) the photovoltage generated by the photoanode and the coupling of the band positions with those of the photocathode; ii) photoreactor configuration: gas diffusion electrodes/membrane electrode assembly, and zero-gap; iii) complementary absorption of solar light to maximize the usage of solar spectrum.

PEC characterization results on developing CaTiO₃/BiVO₄ photoanodes showed optimal catalyst loadings of 1 mg cm^-2 and 3 mg cm^-2 [2].  Notably, a competitive current density of -71 mA cm^-2 was achieved at -1.8 V (vs. Ag/AgCl). Subsequently, the BiVO₄/WO₃ ratio was independently optimized  with WO₃ as the bottom layer. Higher BiVO₄ loadings (80:20) exhibited the most favorable results in terms of photogenerated current density [3].

The results on photocathode development under visible light revealed an optimum Cu₂O/bio-MOF ratio of 9/1 for an enhanced methanol production (477 mmol m^-2 s^-1) and Faradaic Efficiency (90.4%), while reaction selectivity towards ethanol is favored at lower ratios. The results clearly overpass the performance of previous MOFs on CO₂-to-alcohols conversion, showing potential for their application as photocathode that may bring an improved energy use, thus helping to advance towards sustainable bias-free energy solutions driven by sunlight.

This study focuses on integrating a bias-free PEC system by analyzing light sources, intensity, and solar concentration. It aims to use dual absorbers to maximize solar energy and ensure compatible band structures, a challenge in tandem systems. The photoreactor design will be optimized by adjusting photoelectrode area ratios and developing a zero-gap reactor with low electrical resistance. These efforts aim to achieve the necessary photovoltage for efficient CO₂ reduction and water oxidation, enabling an unbiased electrolyzer.

This study emphasizes the potential of innovatively designed photocathodes and photoanodes to advance PEC CO₂ reduction in bias-free systems using sunlight. The design of both the photocathode for CO₂ reduction and the photoanode for oxygen evolution is crucial for system efficiency. Optimizing these photoactive surfaces supports greener chemical processes and reduces reliance on external energy.