Photoelectrochemical water splitting: silicon photocathodes for hydrogen evolution

The development of low cost, scalable, renewable energy technologies is one of today's most pressing scientific challenges. We report on progress towards the development of a photoelectrochemical water-splitting system that will use sunlight and water as the inputs to produce renewable hydrogen with oxygen as a by-product. This system is based on the design principle of incorporating two separate, photosensitive inorganic semiconductor/liquid junctions to collectively generate the 1.7-1.9 V at open circuit needed to support both the oxidation of H_2O (or OH^-) and the reduction of H^+ (or H_2O). Si microwire arrays are a promising photocathode material because the high aspect-ratio electrode architecture allows for the use of low cost, earth-abundant materials without sacrificing energy-conversion efficiency, due to the orthogonalization of light absorption and charge-carrier collection. Additionally, the high surfacearea design of the rod-based semiconductor array inherently lowers the flux of charge carriers over the rod array surface relative to the projected geometric surface of the photoelectrode, thus lowering the photocurrent density at the solid/liquid junction and thereby relaxing the demands on the activity (and cost) of any electrocatalysts. Arrays of Si microwires grown using the Vapor Liquid Solid (VLS) mechanism have been shown to have desirable electronic light absorption properties. We have demonstrated that these arrays can be coated with earth-abundant metallic catalysts and used for photoelectrochemical production of hydrogen. This development is a step towards the demonstration of a complete artificial photosynthetic system, composed of only inexpensive, earth-abundant materials, that is simultaneously efficient, durable, and scalable

Kinetics and mechanism of heterogeneous voltage-driven water-dissociation catalysis

The water-dissociation (WD) reaction (H2O → H+ + OH−) affects the rates of electrocatalytic reactions and the performance of bipolar membranes (BPMs), but how WD is driven by voltage and catalyzed is not understood. We report BPM electrolyzers with two reference electrodes (REs) to measure temperature-dependent WD current and overpotential (ηwd) without soluble electrolyte. Using TiO2-P25-nanoparticle catalyst and Arrhenius-type analysis, we found Ea,wd of 25–30 kJ/mol, independent of ηwd, and a pre-exponential factor proportional to ηwd that decreases ∼10-fold in D2O. We propose a new WD mechanism where metal-oxide nanoparticles, polarized by the BPM-junction voltage, serve as proton (1) acceptors (from water) on the negatively charged side of the particle to generate free OH−, (2) donors on the positively charged side to generate H3O+, and (3) surface proton conductors that connect spatially separate donor/acceptor sites. Increasing electric field with ηwd orients water for proton transfer, increasing the pre-exponential factor, but is insufficient to lower Ea

Electrically Insulated Catalyst–Ionomer Anode Interfaces toward Durable Alkaline Membrane Electrolyzers

Anion-exchange-membrane water electrolysis (AEMWE) is an emerging technology for scalable hydrogen production. AEMWE has poor durability when operating without supporting electrolyte due to the oxidation of ionomers and membranes in contact with the anode oxygen evolution reaction (OER) catalyst. We report a new “passivated” anode architecture for AEMWE where the OER catalysts and ionomers are physically separated with a thin film amorphous oxide coating that is electrically insulating but conductive to hydroxide ions. We find that 2-3 nm of HfOx passivation layers show sufficient hydroxide ion transport to minimally limit the cell performance while suppressing ionomer degradation with both Ir (500 mA·cm-2 for 40 h) and CoOx (1.0 A·cm-2 for 100 h) model porous-transport-layer-supported catalysts in AEMWE. This interfacial engineering approach guides electrode design to improve the durability of AEMWE, particularly for systems operating with pure-water feed

pH-Independent, 520 mV Open-Circuit Voltages of Si/Methyl Viologen^(2+/+) Contacts Through Use of Radial n^+p-Si Junction Microwire Array Photoelectrodes

The effects of introducing an n^+-doped emitter layer have been evaluated for both planar Si photoelectrodes and for radial junction Si microwire-array photoelectrodes. In contact with the pH-independent, one-electron, outer-sphere, methyl viologen redox system (denoted MV^(2+/+)), both planar and wire array p-Si photoelectrodes yielded open-circuit voltages, V_(oc), that varied with the pH of the solution. The highest V_(oc) values were obtained at pH = 2.9, with V_(oc) = 0.53 V for planar p-Si electrodes and V_(oc) = 0.42 V for vapor−liquid−solid catalyzed p-Si microwire array samples, under 60 mW cm^(−2) of 808 nm illumination. Increases in the pH of the electrolyte produced a decrease in V_(oc) by approximately −44 mV/pH unit for planar electrodes, with similar trends observed for the Si microwire array electrodes. In contrast, introduction of a highly doped, n^+ emitter layer produced V_(oc) = 0.56 V for planar Si electrodes and V_(oc) = 0.52 V for Si microwire array electrodes, with the photoelectrode properties in each system being essentially independent of pH over six pH units (3 < pH < 9). Hence, formation of an n^+ emitter layer not only produced nearly identical photovoltages for planar and Si microwire array photoelectrodes, but decoupled the band energetics of the semiconductor (and hence the obtainable photovoltage) from the value of the redox potential of the solution. The formation of radial junctions on Si microwire arrays thus provides an approach to obtaining Si-based photoelectrodes with high-photovoltages that can be used for a variety of photoelectrochemical processes, including potentially the hydrogen evolution reaction, under various pH conditions, regardless of the intrinsic barrier height and flat-band properties of the Si/liquid contact

Hydrogen-evolution-reaction kinetics pH dependence: Is it covered?

Writing in Joule, Surendranath and coworkers report intrinsic hydrogen-evolution-reaction activity and kinetic parameters for Pt and Au electrodes using "innocent” buffers that don't substantially affect electrode kinetics. A pH- and potential-dependent coverage of metal-H species is proposed to modulate apparent activity, findings that are important for a range of electrochemical technologies

10 µm minority-carrier diffusion lengths in Si wires synthesized by Cu-catalyzed vapor-liquid-solid growth

The effective electron minority-carrier diffusion length, L_(n,eff), for 2.0 µm diameter Si wires that were synthesized by Cu-catalyzed vapor-liquid-solid growth was measured by scanning photocurrent microscopy. In dark, ambient conditions, L_(n,eff) was limited by surface recombination to a value of ≤ 0.7 µm. However, a value of L_(n,eff) = 10.5±1 µm was measured under broad-area illumination in low-level injection. The relatively long minority-carrier diffusion length observed under illumination is consistent with an increased surface passivation resulting from filling of the surface states of the Si wires by photogenerated carriers. These relatively large L_(n,eff) values have important implications for the design of high-efficiency, radial-junction photovoltaic cells from arrays of Si wires synthesized by metal-catalyzed growth processes

Solar Water Splitting Cells

No abstract

Nanoscale Measurements of Charge Transfer at Cocatalyst/Semiconductor Interfaces in BiVO4 Particle Photocatalysts

Semiconductor photocatalyst particles convert solar energy to fuels like H2. The particles are often assumed to provide crystalline-facet-dependent electron-hole separation. A common strategy is to deposit a hydrogen evolution reaction (HER) electrocatalyst on electron-selective facets and an oxygen evolution reaction (OER) electrocatalyst on hole-selective facets. A precise understanding of how charge-carrier-selective contacts emerge and how they are rationally designed, however, is missing. Using a combination of ex situ and in situ conducting atomic force microscopy (AFM) experiments and new ionomer/catalyst-semiconductor test structures, we show how heterogeneity in charge-carrier selectivity can be measured at the nanoscale. We discover that the presence of the water/electrolyte interface is critical to induce hole selectivity between the CoOx water-oxidation catalyst and the BiVO4 light absorber. pH-dependent measurements suggest that negative surface charge on the semiconductor is central to inducing hole selectivity. The work also demonstrates a new approach to control local pH and introduce water using thin-film ionomers compatible with conductive AFM measurements

Evaluation of Pt, Ni, and Ni–Mo electrocatalysts for hydrogen evolution on crystalline Si electrodes

The dark electrocatalytic and light photocathodic hydrogen evolution properties of Ni, Ni–Mo alloys, and Pt on Si electrodes have been measured, to assess the viability of earth-abundant electrocatalysts for integrated, semiconductor coupled fuel formation. In the dark, the activities of these catalysts deposited on degenerately doped p^+-Si electrodes increased in the order Ni < Ni–Mo ≤ Pt. Ni–Mo deposited on degenerately doped Si microwires exhibited activity that was very similar to that of Pt deposited by metal evaporation on planar Si electrodes. Under 100 mW cm^(−2) of Air Mass 1.5 solar simulation, the energy conversion efficiencies of p-type Si/catalyst photoelectrodes ranged from 0.2–1%, and increased in the order Ni ≈ Ni–Mo < Pt, due to somewhat lower photovoltages and photocurrents for p-Si/Ni–Mo relative to p-Si/Ni and p-Si/Pt photoelectrodes. Deposition of the catalysts onto microwire arrays resulted in higher apparent catalytic activities and similar photoelectrode efficiencies than were observed on planar p-Si photocathodes, despite lower light absorption by p-Si in the microwire structures