Photochemical Diodes for Simultaneous Bias-Free Glycerol Valorization and Hydrogen Evolution

Finding sustainable and renewable energy is currently one of the most urgent challenges facing society today. Artificial photosynthesis offers a route to using solar energy to produce fuels such as hydrogen, a fundamental component for building a carbonfree economy. One such approach to realizing artificial photosynthesis is presented through the photochemical diode. Conventional approaches targeted the optimization of the overall water splitting (OWS) reaction in which hydrogen and oxygen are produced. However, the sluggish kinetics of the oxygen evolution reaction (OER) and the high thermodynamic potential requirement of 1.23 V for the OWS limit the current performance of bias-free photoelectrochemical (PEC) systems. Here, we offer an alternative approach by replacing the OER with the glycerol oxidation reaction (GOR) for value-added chemicals using Si photoanode. By employing a low overpotential catalyst such as PtAu for GOR, which shows a low onset potential of 0.4 V vs RHE electrochemically, the voltage requirements to couple with hydrogen evolution reaction (HER) become substantially lowered compared to OWS. As a result, the PtAu/Si photoanode exhibits a low GOR onset potential of −0.05 V vs RHE and a photocurrent density of 10 mA/cm2 at 0.5 V vs RHE. Coupled with a Si nanowire photocathode for the HER, the integrated system yields a high photocurrent density of 6 mA/cm2 with no applied bias under 1 sun illumination and can run for over 4 days under diurnal illumination. The demonstration of the GOR-HER integrated system provides a framework for designing bias-free photoelectrochemical devices at appreciable currents and establishes a facile approach to artificial photosynthesis.