Photoelectrochemical systems at the state-of-the-art for the sustainable H2 and syngas production: Literature Review and Proof-of-Concept.

The countdown on the use of conventional carbon-rich fossil fuels has begun; not only due to their future depletion but mainly due to the increasing concerns on their use as energy source and raw materials in the industries, which have been the main causes of the current environmental issues and climate change. Therefore, photoelectrochemical systems for H2 production and CO2 reduction into syngas or liquid fuels are promising approaches for the transformation of the abundant sunlight into renewable, green and sustainable energy carriers. Those technologies offer the perspective advantages of creating zero-emissions fuel and reducing the atmospheric CO2 levels, restoring the natural carbon cycle. For both purposes, state-of-the-art researches recognize photovoltaic-electrocatalytic (PV-EC) devices as the most established technology in terms of technology readiness levels (TRL) and solar-to-fuel efficiencies. In this work, noble metal-based catalysts (Pt/C and Pt3Co/C) have been tested for the alkaline hydrogen evolution reaction in different reactor designs. The catalytic activity of these catalysts has been evaluated in a rotating-disk-electrode (RDE) system, with which state-of-the-art overpotential values (i.e. 276 mV and 76 mV, for the Pt/C and Pt3Co/C, respectively) at 10 mA cm-2 of current density have been registered, demonstrating a higher performance for the Pt3Co/C material. These catalysts have been used to perform the proof-of-concept (PoC) of a novel design of electrochemical device for the sun-driven hydrogen production by the start-up Green Independence (GI). Gas diffusion electrodes (GDEs) of both Pt/C and Pt3Co/C catalyst with an active size of 1 cm2 and 0.5 mg cm-2 of catalyst loading have been tested in GI cell, confirming that Pt3Co/C provides the best performance in terms of a lower cell overpotential and 95% of H2 faradaic efficiency, measured during a controlled current electrolysis experiment at 10 mA cm-2. It was also demonstrated the feasibility of the sun-driven water splitting in a PV-EC system assembled by wiring a Silicon minimodule photovoltaic cell with a Pt3Co/C-based electrode into the GI cell. This configuration exhibited 5% of solar-to-hydrogen efficiency with 96% of H2 faradaic efficiency, which can be considered as a PoC of this technology. Moreover, a membrane-electrode assembly (MEA) made with the Pt/C catalyst (0.5 mg cm-2 catalyst loading) was tested in a commercial reactor design, which showed a lower cell overpotential than the one obtained in the GI cell with the same catalyst. Based on these results, different possibilities to improve the solar-to-hydrogen efficiency and H2 productivity are analyzed considering: a modification in the GI cell design to operate in flow configuration and with a MEA, the selection of the most appropriate PV cell to maximize the solar-to-hydrogen efficiency, and the appropriate coupling of the PV-EC system, in order to optimize the performance of the final device.