Photo-Electrochemical Water Splitting to Generate Hydrogen Fuel: The Importance of Accurately Benchmarking Efficiency, Challenges with System Scale Up and Protective Barriers for Corrosion Mitigation

Solar-to-hydrogen (STH) conversion efficiency a common figure of merit for evaluating and comparing research results, and it largely establishes the prospect for successfully introducing commercial solar water-splitting systems. Present measurement practices do not follow well-defined standards, and common methods consistently overestimate performance. To remedy this need we confirmed underestimated influence factors and proposed experimental strategies for improved accuracy.<br/>Our focus was tandem (dual absorber) devices that have the prospect for greater STH efficiency, but increased complexity that requires more careful consideration of characterization practices. We performed measurements on an advanced version of the classical GaInP₂/GaAs design while considering (i) calibration and adjustment of the light source, (ii) validation of results by incident photon-to-current efficiency (IPCE), and (iii) definition and confinement of the active area of the device.<br/>We initially measured 21.8% STH efficiency using a tungsten broadband light source, a calibrated GaInP₂photovoltaic reference cell, and epoxy-encased photocathodes. In contrast, integrating experimental IPCE over the AM 1.5G reference solar spectrum showed that less than 10% STH conversion is possible. We performed a set of on-sun measurements that gave 16.1% STH efficiency before eliminating indirect light coupled to the sample by using a collimating tube and 13.8% STH efficiency. However, the value still significantly exceeded the current density expected according to the quantum efficiency measured via IPCE. Finally, suspecting that the illuminated area is poorly defined by epoxy, we use a compression cell for an epoxy-free area definition, resulting in 9.3% STH efficiency – a number corroborated by our IPCE results.<br/>The second part of this talk will identify the challenges encountered while scaling the inverted metamorphic multijunction (IMM) III-V absorber areas of from ~0.15 cm² up to 16 cm² and incorporating them in a photoreactor capable of generating 3 standard liters of hydrogen in 8 hours under natural sunlight. To successfully scale photo-electrochemical water-splitting technologies from bench to demonstration size requires addressing predictable and unpredictable complications. Despite using Comsol multiphysics to model our photoreactor and identify suitable specifications for a prototype, several practical issues were uncovered during testing that led to multiple iterations of photoreactor design between the initial and final generation. Several bottlenecks that ranged from counter electrode composition and orientation to bubble removal needed redress in order to meet our performance targets. Ultimately, the demonstration-scale system was able to generate nearly twice the target volume of hydrogen in an 8-hour outdoor trial.<br/>While III-V semiconductors have achieved high photo-electrochemical STH conversion efficiencies, they are remarkably unstable during operation in a harsh electrolyte. The final part of this talk will focus on the degradation mechanism of IMM III-V cells and surface modification strategies aimed at protecting them from photocorrosion. We applied noble metal catalysts, oxide coatings by atomic layer deposition, and MoS₂ in an effort to protect the GaInP₂ surface that was in contact with acidic electrolyte. We also grew epitaxial capping layers from III-V alloys that should be more intrinsically stable than GaInP₂. The ability of the various modifications to protect the IMM’s surface was evaluated by operating at each electrode at short circuit for extended periods of time. We will elaborate on the challenges of this mode of protection and new methods of protecting perovskite photo absorbers will be introduced.