Eric L. Miller

Accelerating materials development for photoelectrochemical hydrogen production: standards for methods, definitions, and reporting protocols

Photoelectrochemical (PEC) water splitting for hydrogen production is a promising technology that uses sunlight and water to produce renewable hydrogen with oxygen as a by-product. In the expanding field of PEC hydrogen production, the use of standardized

Technical and economic feasibility of centralized facilities for solar hydrogen production via photocatalysis and photoelectrochemistry

Photoelectrochemical water splitting is a promising route for the renewable production of hydrogen fuel. This work presents the results of a technical and economic feasibility analysis conducted for four hypothetical, centralized, large-scale hydrogen production plants based on this technology. The four reactor types considered were a single bed particle suspension system, a dual bed particle suspension system, a fixed panel array, and a tracking concentrator array. The current performance of semiconductor absorbers and electrocatalysts were considered to compute reasonable solar-to-hydrogen conversion efficiencies for each of the four systems. The U.S. Department of Energy H2A model was employed to calculate the levelized cost of hydrogen output at the plant gate at 300 psi for a 10 tonne per day production scale. All capital expenditures and operating costs for the reactors and auxiliaries (compressors, control systems, etc.) were considered. The final cost varied from

Electrochemical behavior of reactively sputtered iron-doped nickel oxide

1.60–

Design considerations for a hybrid amorphous silicon/photoelectrochemical multijunction cell for hydrogen production

10.40 per kg H2 with the particle bed systems having lower costs than the panel-based systems. However, safety concerns due to the cogeneration of O2 and H2 in a single bed system and long molecular transport lengths in the dual bed system lead to greater uncertainty in their operation. A sensitivity analysis revealed that improvement in the solar-to-hydrogen efficiency of the panel-based systems could substantially drive down their costs. A key finding is that the production costs are consistent with the Department of Energys targeted threshold cost of

Photoelectrochemical production of hydrogen: Engineering loss analysis

2.00–

Electrochemical and Electrochromic Behavior of Reactively Sputtered Nickel Oxide

4.00 per kg H2 for dispensed hydrogen, demonstrating that photoelectrochemical water splitting could be a viable route for hydrogen production in the future if material performance targets can be met.

Low-Temperature Reactively Sputtered Tungsten Oxide Films for Solar-Powered Water Splitting Applications

Iron-doped nickel oxide films were deposited by reactive sputtering from elemental and alloy targets in a 20% oxygen/argon atmosphere and were characterized for use as oxygen evolution catalysts. The incorporation of iron reduced the overpotential required for oxygen evolution by as much as 300 mV at a current density of 100 mA/cm 2 compared to undoped nickel oxide deposited under similar conditions. Tafel slopes were reduced from 95 mV/dec in undoped NiO x films to less than 40 mV/dec for films containing 1.6 to 5.6 mole percent (m/o) iron, indicating a change in the rate-limiting step from the primary discharge of OH ions to the recombination of oxygen radicals. Resistivity, structural, and compositional measurements indicate that high oxygen content is necessary to gain the full benefit of the iron dopant. Initial tests in KOH indicate excellent long-term stability. A film deposited from the FeNi alloy target, which exhibited low oxygen overpotentials and a Tafel slope of 35 mV/dec, had not degraded appreciably following more than 7000 h of operation at an anodic current density of 20 mA/cm 2 . Taken together, the low oxygen evolution reaction overpotentials, the excellent stability in KOH, and the relative insensitivity to iron content indicate that reactively sputtered iron-doped nickel oxide is promising as an oxygen catalyst.

Amorphous silicon carbide photoelectrode for hydrogen production directly from water using sunlight

Abstract Triple-junction amorphous silicon (a-Si) solar cells demonstrating photovoltaic (PV) efficiencies up to 12.7% and open-circuit voltages up to 2.3 V have recently been deposited onto stainless-steel foil substrates by the University of Toledo for photoelectrochemical (PEC) tests conducted by the University of Hawaii. The fundamental design strategy for producing such high efficiency in multijunction amorphous silicon devices involves careful current matching in each of the junctions by adjustment of the absorption spectra through bandgap tailoring. Integrated electrical/optical models are frequently used to aid in the optimization procedure, as well documented in the PV literature. Typically, the top nip junction in an a-Si triple-junction cell is designed to absorb most strongly in the 350– 500 nm range. In principle, this top cell could be replaced by a PEC junction with strong absorption in a similar range to form a water-splitting photoelectrode for hydrogen production. This photoelectrode could be fabricated on SS with the back surface catalyzed for the hydrogen evolution reaction, and the front surface deposited with an a-Si:nipnip/ITO/SC structure. The top layer semiconductor (SC), which forms the PEC junction with an electrolyte, must have appropriate conduction band alignment for the oxygen evolution reaction, and the junction must be strongly absorbing in the 350– 500 nm region for current matching. Possible candidate SC materials include dye-sensitized titanium dioxide (TiO2), tungsten trioxide (WO3), and iron oxide (Fe2O3). This paper discusses the specific design considerations for high solar-to-hydrogen conversion efficiency in a hybrid solid-state/PEC photoelectrode, and describes the use of integrated electrical/electrochemical/optical models developed at the University of Hawaii for the analysis of such hybrid structures. Important issues include the bias-voltage and current-matching requirements in the solid-state and electrochemical junctions, as well as fundamental quantum efficiency considerations.

UV-Vis Spectroscopy

Abstract A method to analyze the hydrogen production potential (HPP) of photoelectrochemical systems, based on the lumped equivalent circuit model of a photocell driving a current dependent electrochemical load, is presented. Selection of the appropriate circuit and characteristic junction parameters allows analysis of single and multijunction photoelectrodes including those utilizing a semiconductor-electrolyte junction. In this paper, the HPP of single and multijunction photoelectrodes fabricated from high quality crystalline III–V materials is compared with that of the lower cost amorphous silicon and crystalline-amorphous silicon hybrid junction electrodes. The analysis shows that single junction photoelectrodes, even those with exceptional diode characteristics, require semiconductor bandgaps greater than 2.0 eV for efficient operation when realistic catalyst performance is considered. Multiphoton systems are shown to have higher HPP and exhibit greater operational stability when load mismatches, such as would occur from changes in junction characteristics or catalyst properties, are introduced.

Improved current collection in WO 3 :Mo/WO 3 bilayer photoelectrodes

Nickel oxide thin films were deposited by reactive sputtering in a 20% oxygen/argon atmosphere for use as oxygen evolution catalysts in the photoelectrochemical production of hydrogen. The optical properties of the films were also characterized to evaluate their application as window layers. The polycrystalline films deposited at residual gas pressures of 6 or 10 mTorr exhibited moderate activity for oxygen evolution in 1 N KOH and pronounced coloration and bleaching during alternating anodic/cathodic bias. Properties of these films were not sensitive to growth rate over the range studied, 0.5 to 4 {angstrom}/s. In contrast, films deposited at 2 mTorr exhibited poor activity for oxygen evolution and severely limited electrochromic behavior which the authors attribute to marked changes in the morphology and crystallinity in the low-pressure films. The films grown at 6 mTorr and higher tended to be more oriented, to have a higher degree of crystallinity, and higher oxygen content. Strong linkages between the electrochemical and optical behaviors observed in this work provide new insights into the processes involved in oxygen evolution reaction catalysis and electrochromism in reactively sputtered NiO{sub x} films. The results presented indicate that reactive sites located on or near grain boundaries are responsible for both behaviors.