While photoelectrochemical (PEC) solar-to-hydrogen efficiencies have greatly improved over the past few decades, advances in PEC durability have lagged behind. Corrosion of semiconductor photoabsorbers in the aqueous conditions needed for water splitting is a major challenge that limits device stability. In addition, a precious-metal catalyst is often required to efficiently promote water splitting. Herein, we demonstrate unassisted water splitting using a non-precious metal molybdenum disulfide nanomaterial catalytic protection layer paired with a GaInAsP/GaAs tandem device. This device was able to achieve stable unassisted water splitting for nearly 12 hours, while a sibling sample with a PtRu catalyst was only stable for 2 hours, highlighting the advantage of the non-precious metal catalyst. In situ optical imaging illustrates the progression of macroscopic degradation that causes device failure. In addition, this work compares unassisted water splitting devices across the field in terms of the efficiency and stability, illustrating the need for improved stability. TOC GRAPHICThe primary strategy that has emerged to mitigate semiconductor surface corrosion is depositing thin films, such as titanium dioxide, that can act as protective barriers to prevent the electrolyte from coming into contact with the semiconductor surface. 7,8 These films have to be stable, thin enough to prevent significant light blocking, conformal, and conductive to create a stable and functional device. 9-11 Furthermore, if these films do not demonstrate intrinsic catalytic activity, an additional hydrogen and/or oxygen evolution catalyst is needed to promote efficient water splitting. Molybdenum disulfide nanomaterials have been shown to stabilize a variety of singlejunction Si and III-V PEC systems, functioning as a hydrogen evolution reaction (HER) catalyst and protection layer. [9][10][11][12][13][14][15] Because of the promising performance in single-junction photocathodes, it is of interest to use MoS2 with tandem semiconductor systems to improve the stability during unassisted solar water splitting.While most III-V-based unassisted water splitting devices to date have incorporated a Ga0.51In0.49P (hereafter GaInP2) (1.8 eV) top cell, device lifetimes have been limited to <100 h. 7,16 GaxIn1-xAsyP1-y (1.7 eV) has shown promise as a PV material and has been paired with a GazIn1-zAs bottom cell (1.1 eV) for efficient tandem PV systems, motivating efforts to incorporate GaxIn1-xAsyP1-y into PEC systems and investigate the stability of this quaternary top cell. [16][17][18][19] The composition of GaxIn1-xAsyP1-y (hereafter GaInAsP), nominally x ~ 0.68 and y ~ 0.34, gives the desired bandgap of 1.7 eV and a lattice constant matching that of GaAs. 16,18 A GaInAsP/GaAs (1.7/1.4 eV) pairing has a predicted maximum STH efficiency of ~12%, 16,19 far from the ideal combination of absorbers to achieve the highest of efficiencies, however sufficiently high to perform durability studies on active, unassisted water splitting systems, guiding t...

Strain-balanced GaInAs/GaAsP quantum wells were incorporated into the classical GaInP/GaAs tandem photoelectrochemical water splitting device to increase the range of photon absorption and achieve higher solar-to-hydrogen efficiencies.

Photoelectrochemical (PEC) water splitting provides a pathway to generate sustainable clean fuels using the two most abundant resources on Earth: sunlight and water. Currently, most of the successful models of PEC cells are still fabricated on small scales near 1 cm2, which largely limits the mass deployment of solar‐fuel production. Here, the scale‐up to 8 cm2 of an integrated PEC (IPEC) device is demonstrated and its performance compared to a 1 cm2 IPEC cell, using state‐of‐the‐art iridium and platinum catalysts with III–V photoabsorbers. The initial photocurrents at 1 sun are 8 and 7 mA cm−2 with degradation rates of 0.60 and 0.47 mA cm−2 day−1, during unbiased operation for the 1 and 8 cm2 devices, respectively. Evaluating under outdoor and indoor conditions at two U.S. National Laboratories reveals similar results, evidencing the reproducibility of this design's performance. Furthermore, the emerging degradation mechanisms during scale‐up are investigated and the knowledge gained from this work will provide feedback to the broader community, since PEC device durability is a limiting factor in its potential future deployment.

GaInP2 has shown promise as the wide bandgap top junction in tandem absorber photoelectrochemical (PEC) water splitting devices. Among previously reported dual-junction PEC devices with a GaInP2 top cell, those with the highest performance incorporate an AlInP2 window layer (WL) to reduce surface recombination and a thin GaInP2 capping layer (CL) to protect the WL from corrosion in electrolytes. However, the stability of these III–V systems is limited, and durability continues to be a major challenge broadly in the field of PEC water splitting. This work provides a systematic investigation into the durability of GaInP2 systems, examining the impacts of the window layer and capping layer among single junction pn-GaInP2 photocathodes coated with an MoS2 catalytic and protective layer. The photocathode with both a CL and WL demonstrates the highest PEC performance and longest lifetime, producing a significant current for >125 h. In situ optical imaging and post-test characterization illustrate the progression of macroscopic degradation and chemical state. The surface architecture combining an MoS2 catalyst, CL, and WL can be translated to dual-junction PEC devices with GaInP2 or other III–V top junctions to enable more efficient and stable PEC systems.

No abstract