Production and use of hydrocarbon energy result in enormous release of carbon dioxide (CO2) into the atmosphere, eventually causing the global climate change. Furthermore, the world is now facing a dual energy challenge, which is to minimize use of fossil fuels to decrease the impact of CO2 on climate change while meeting the growing global energy demand. To solve these challenges, development of an innovative technology capable of reducing or reusing CO2 as well as producing environmental-benign and sustainable energy using renewable energy sources is urgently required. As such, a technology that can produce solar fuel energy using water oxidation and CO2 reduction that mimics natural photosynthesis has been attracting extensive interest (Figure 1). Many approaches for solar-driven fuel production using artificial photosynthesis have been suggested and developed. Among them, a promising one is to use semiconductor-based photoelectrochemistry (PEC) because there are much potential to meet the requirement for sustainable, selective, and efficient production of solar-driven fuels and the proton-coupled electron transfer to CO2 by enhancing light absorption, charge generation, and separation through construction of a junction between semiconductors. For CO2 reduction, bare and Cu-electrodeposited p-Si electrodes were immersed in 0.1 M borate (pH 9.2) or 0.1 M bicarbonate (pH 7.2) electrolytes, to which AM 1.5 light of 100 mW/cm2 (1 Sun) was irradiated. Chopped linear sweep voltammograms showed that the onset potential is anodically shifted by 0.5 V and the photocurrent is significantly enhanced by Cu deposition. To examine the competitive reaction between water reduction (hydrogen evolution) and CO2 reduction occurring on p-Si/Cu, three different gases (O2, N2, CO2) were purged into the bicarbonate electrolyte solution. The purged gases change little the onset potentials. A high photocurrent with N2 compared to O2 indicates that water reduction becomes a primary electron-quenching process in the absence of dissolved oxygen. At low potential range, CO2 is most effective whereas N2 becomes more effective with increasing the negative potentials. Because the two-electron reduction potentials of H2O (H2 evolution) and CO2 (CO evolution) are very similar (only ~100 mV difference), the high photocurrent with CO2 at low potential range implies that the CO evolution may be the primary process. Instead of p-Si, a novel photocathode, p-CuFeO2 was also synthesized and its PEC performance was tested. The onset potential was around -0.2 V vs. SCE and the photocurrent increased with increasing the film thickness. Due to its transparency, this photocathode can be coupled with other transparent photoanodes for CO2 conversion.


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