Synthesis and interfacial integration of transition metal phosphide catalysts on high-performance solar cells for direct solar-driven water splitting
Creators: name: Gu, Hengfei
Publisher: name: No Publisher Supplied
Year: 2022

description: Photoelectrochemical (PEC) splitting of water into hydrogen and oxygen molecules using an integrated cell composed of semiconductor solar cells and electrocatalysts has been considered a promising pathway to convert solar energy to chemical energy. However, it has remained challenging for commercialization mainly due to the limited durability of integrated photoelectrodes owing to corrosion of semiconductor solar cells and failure of the interfacial connection between the photoabsorber(s) and catalyst(s). Although multijunction III-V semiconductors (amongst the best photovoltaics) have been successfully used as photoabsorbers in PEC devices and have yielded a high solar-to-hydrogen (STH) efficiency &gt;19 % using platinum group metal (PGM) catalysts, they rapidly corrode in aqueous electrolytes. Another new class of semiconductors that can be used to create high-performance solar cells are hybrid organic-inorganic perovskite (HOIP) thin-films. Their increasing solar power conversion efficiency (PCE) has triggered interest in their use for direct solar-driven water splitting to produce hydrogen. However, application of these low-cost, electronic-structure-tunable HOIP tandem photoabsorbers has been hindered by the instability of the photovoltaic-catalyst-electrolyte (PV+E) interfaces. In addition to poor device durability, low-cost catalyst thin films or nanoparticles for solar-driven hydrogen evolution reactions (HER) from water remain unavailable commercially, owing to multiple technical challenges.In Chapter 2, we used a bifunctional interface comprised of a titanium dioxide (TiO₂) corrosion barrier and titanium nitride diffusion barrier (TiN) to protect the interface between a n⁺GaInP₂-coated GaInP₂/GaAs III-V tandem photocathode and a PGM-free nickel phosphide catalyst. This strategy enables use of elevated temperatures to co-crystallize both the TiO₂ protection barrier and the Ni₅P₄ catalyst particles on TiO₂ without damaging the buried III-V tandem photoabsorbers. The combination of ultrathin catalyst and protective bilayers (Ni₅P₄/TiO₂/TiN) produces negligible optical loss. The resulting III-V tandem photocathode exhibits an initial STH efficiency in a range of 11.4 - 13.2% in sodium phosphate electrolyte at neutral pH 7. It operated continuously for over 200 hours at a STH efficiency &gt; 10%, exceeding the previous benchmarks. The TiO₂/TiN bilayers expand the synthesis temperatures for surface modification of, and catalyst integration on, III-V multijunction semiconductors without compromising the device photocurrent densities. The Ni₅P₄ catalyst can replace costly PGM catalysts with comparable HER activity and can operates at neutral pH in less corrosive electrolytes than either acid or alkaline electrolysis.In Chapter 3, photolytic water splitting is demonstrated using an integrated configuration consisting of an HOIP/n⁺silicon single junction photoabsorber and a platinum (Pt) thin film catalyst. An extended electrochemical (EC) lifetime in alkaline media is achieved using titanium nitride on both sides of the Si support to eliminate formation of insulating silicon oxide, and as an effective diffusion barrier to allow high-temperature annealing of the catalyst/TiO₂-protected-n⁺silicon interface necessary to retard electrolytic corrosion. Halide composition was examined in the lead halide ABX₃-type perovskite system with A-site mixtures of formamidinium (FA) and cesium (Cs): (FA₁₋ₓCsₓ)PbI3 system with a bandgap suitable for tandem operation. A fill factor of 72.5% was achieved using a Spiro-OMeTAD-hole-transport-layer (HTL)-based HOIP/n⁺Si solar cell, and a high photocurrent density of -15.9 mA/cm² (at 0V vs reversible hydrogen electrode (RHE)) was attained for the HOIP/n⁺Si/Pt photocathode in 1M NaOH under simulated one-sun illumination. While this thin-film design creates stable interfaces, the intrinsic photo- and electro-degradation of the HOIP photoabsorber remains the main obstacle for future HOIP/Si tandem PEC devices.In Chapter 4, we apply the electro-synthesis approach to the co-deposition of Ni and P to produce mono-phasic crystalline Ni₃P after annealing. The mechanism of thin film formation by reduction of Ni (II) and hypophosphite in aqueous solution as a function of pH was investigated by cyclic voltammetry (CV). Pulsed voltammetry was used to co-deposit elemental Ni and either red phosphorous or PH₃ selectively from solution by adjusting the concentration of precursors, the pH and the applied potential. Interference from H₂ bubble formation was suppressed by agitation between current pulses, leading to fewer defects. Subsequent gas phase annealing produced phase-pure Ni₃P films. Both direct and indirect P incorporation mechanisms were observed via formation of red P and PH₃ at different applied potentials, respectively. In the direct mechanism, Ni and P atoms co-electrodeposit together as uniform particles that form at defects and grow to make conformal films on 3D-structured substrates. By contrast, the indirect mechanism involves random nucleation of Ni-P spherical particles that do not grow as conformal films. The Ni₃P films can also form on top of conducting silicon (n⁺Si) coated with TiO₂/TiN overlayers to prevent interdiffusion of layers during annealing. The Ni₃P/TiO₂/TiN/n⁺Si interfaces exhibited excellent stability during continuous electrolysis at a current density of -10 mA/cm² for over 160 h without noticeable degradation. The TiO₂/TiN top layer was essential to this performance, serving as a protection layer and diffusion barrier. The electrolytic performance of the thin film Ni₃P in HER is on par with that observed by nanocrystalline Ni₃P microparticles.