Photocorrosion-Limited Maximum Efficiency of Solar Photoelectrochemical Water Splitting

Abstract

Photoelectrochemical (PEC) water splitting to generate hydrogen is one of the most studied methods for converting solar energy into clean fuel because of its simplicity and potentially low cost. Despite over 40 years of intensive research, PEC water splitting remains in its early stages with stable efficiencies far less than 10%, a benchmark for commercial applications. Here, we revealed that the desired photocorrosion stability sets a limit of 2.48 eV (relative to the normal hydrogen electrode (NHE)) for the highest possible potential of the valence band (VB) edge of a photocorrosion-resistant semiconducting photocatalyst. We further demonstrated that such limitation has a deep root in underlying physics after deducing the relation between energy position of the valence band edge and free-energy for a semiconductor. The disparity between the stability-limited VB potential at 2.48 V and the oxygen evolution reaction (OER) potential at 1.23 V vs NHE reduces the maximum STH conversion efficiency to approximately 8% for long-term stable single-bandgap PEC water splitting cells. Based on this understanding, we suggest that the most promising strategy to overcome this 8% efficiency limit is to decouple the requirements of efficient light harvesting and chemical stability by protecting the active semiconductor photocatalyst surface with a photocorrosion-resistant oxide coating layer.

Control of Band Gap and Band Edge Positions in Gallium–Zinc Oxynitride Grown by Molecular Beam Epitaxy

“…For comparison, a commercial n-type GaN template has been measured (black squares). The CB edge of GaN is located about 4 eV below the vacuum level; consequently, our results suggest that the CB edge of MBE-grown GZNO layers is located at −3.8 to −5.0 eV relative to the vacuum energy level. , …”

Photocorrosion at Irradiated Perovskite/Electrolyte Interfaces

“…Therefore, the “holy grail” would be to drive an industrially relevant redox reaction that simultaneously is able to suppress photocorrosion. The thermodynamic criteria toward this reaction (i.e., redox potential) is ultimately limited by (i) the band positions, (ii) the corrosion potentials, and (iii) the bandgap of the perovskite . At this point, it is important to distinguish between photo­(electro)­catalytic or photo­(electro)­synthetic processes. , Light-driven reactions that are thermodynamically “downhill” (Δ G < 0) can be termed photo­(electro)­catalytic, while thermodynamically “uphill” (Δ G < 0) reactions are considered photo­(electro)­synthetic processes.…”

High performance of the flow-type one-compartment hydrogen peroxide fuel cell using buckypaper and narrow fuel pathway under physiological conditions

“…9,10 Thus, photoelectrochemical water splitting (PECWS) is receiving attention as the adaptable solution of the above drawbacks of solar cells since the intermittency, and energy-saving issues can be overcome by converting solar energy to chemical energy, such as green hydrogen. 11 However, the efficiency of PECWS has been known to be about 10%, which is relatively lower than that of the solar cell (over 25%), 12 and the public receptivity and inconvenience in the use of hydrogen gas remain controversial in the usability of PECWS employing the solar-hydrogen energy system. To address these issues, several studies regarding H 2 O 2 generation using PECWS have been reported recently.…”