Richard Rocheleau

Electrochemical behavior of reactively sputtered iron-doped nickel oxide

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.

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

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.

Photoelectrochemical production of hydrogen: Engineering loss analysis

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.

Electrochemical and Electrochromic Behavior of Reactively Sputtered Nickel Oxide

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.

The Multiprocess Degradation of PEMFC Performance Due to Sulfur Dioxide Contamination and Its Recovery

The effects of trace concentrations of SO 2 contaminant present in the cathode feed stream on proton exchange membrane fuel cell (PEMFC) performance are studied. Contaminant concentrations of 1, 2, and 10 ppm were exposed to the cell applying a total dosage of 160 μmol of SO 2 at 80°C and a current density of 0.6 A cm ―2 . All experiments show significant cell performance degradation before the steady-state poisoning state is reached. The performance degradation shows an inflection in the cell voltage, which is attributed to at least two different poisoning processes. The overall poisoning process is shown to consist of an irreversible part and a reversible part. While the performance loss of the reversible part is dependent on the SO 2 concentration and is recoverable during a H 2 /air operation, that of the irreversible part is greatly recoverable by potential cycling in the H 2 /N 2 mode. Evidence is also presented that cathode exposure to SO 2 results in a performance impact at the anode. Furthermore, sulfur species that remain in the membrane electrode assembly accelerate the cell performance degradation during a neat H 2 /air operation and subsequent SO 2 contaminant exposure.

Densification of plasma deposited silicon nitride films by hydrogen dilution

The effects of hydrogen dilution on the bonding characteristics, composition, and properties of SiN films deposited from a SiH4/NH3 mixture by r.f. plasma enhanced chemical vapor deposition were studied. The addition of relatively small amounts of hydrogen increased the Si/N ratio resulting in a corresponding increase in the SiH/NH bonding ratio. At higher hydrogen dilutions, the Si/N ratio decreased towards stoichiometric with significant changes in the hydrogen bonding characteristics. Changes in the physical properties are discussed in terms of the measured changes in bonding structure. Changes normally associated with changes in bulk film density were found to be well correlated to the SiN bond density. The effects of substrate temperature and NH3/SiH4 ratio on films deposited under conditions of high hydrogen dilution were similar to those widely reported in the literature for plasma-enhanced chemical vapor deposition films deposited without hydrogen. Films deposited by remote plasma using hydrogen as the excitation exhibited high SiN bond densities and low hydrogen. Experiments are planned to clarify the mechanism responsible for the observed changes in film properties.

Effect of hydrogen dilution on the properties and bonding in plasma‐deposited silicon nitride

The effects of hydrogen dilution on the properties and structure of silicon nitride films deposited by plasma‐enhanced chemical vapor deposition from NH3/SiH4 mixtures were studied. The addition of relatively small amounts of hydrogen at a fixed NH3/SiH4 ratio increased the excess Si in the film with a corresponding increase in the Si—H/N—H bonding ratio. At higher dilution [H2/(NH3+SiH4)] the films became more stoichiometric with significant changes in the hydrogen bonding. Decreases in the etch rate and refractive index with increasing hydrogen flow are discussed in terms of the changes in bonding structure and were found to be well correlated to changes in the Si—N bond density.

The Impact of Trace Carbon Monoxide / Toluene Mixtures on PEMFC Performance

Hydrogen fuel for proton exchange membrane fuel cells (PEMFC) may contain trace contaminants such as hydrogen sulfide (H2S), ammonia (NH3), and carbon monoxide (CO), all of which have been widely studied [1]. Various hydrocarbons may also be present, introduced by gas processing or handling. The majority of impurity studies have focused on investigating the impact of single contaminants while very few studies examined the effect of mixed contaminants. These exceptions include the work by Rockward et al. [2] and Shi et al. [3] who studied the effect of mixtures of H2S and CO. It is well known that CO adsorbs strongly on the platinum (Pt) catalyst, decreases the catalytically active Pt surface area and thereby significantly reduces fuel cell efficiency [1]. When H2S is added, the contaminant species compete for adsorption on Pt reaction sites. Rockward et al. showed that H2S replaces faster adsorbing CO over time using cyclic voltammetry experiments [2]. Shi et al. reported that the performance impact of H2S and CO mixtures is first greater and then less than the sum of the two individual performance impacts [3]. However, both studies were short term experiments ranging from a few minutes to six hours and were performed using high contaminant concentrations. The Hawaii Natural Energy Institute (HNEI) has developed a methodology for studying the long term effects of single and mixed contaminants in fuel and oxidant feed streams. The method uses a high resolution gas chromatograph which enables identification of all reaction products and closure of the contaminant species molar flow balances. We have demonstrated this technique with CO at contaminant levels as low as 1 ppm [4]. In this work we report the effect of trace level mixtures of CO and toluene (C7H8) on PEMFC performance at varying operating conditions. The results indicate that the impact of the contaminant mixture at steady state is greater than that of either of the species alone. Figure 1 shows the overpotential increase as a function of time caused by the introduction of (i) 2 ppm CO, (ii) 20 ppm C7H8, and (iii) a mixture of the two. Experiments were performed in a 50 cm test cell using Ion Power MEAs at a constant current density of 1 A/cm and a temperature of 60°C. As shown, the introduction of 2 ppm of CO results in an overpotential change of 247 mV. The injection of 20 ppm C7H8 has no significant impact on performance. However, the combination of C7H8 and CO results in an overpotential increase of 345 mV, significantly greater than the sum of both individual performance impacts. Figure 2 shows molar flow rates of C7H8 and methylcyclohexane (C7H14) entering and exiting the fuel cell during exposure to the contaminant mixture. Immediately following injection, significant hydrogenation of C7H8 into C7H14 occurs. As time increases, the extent of this reaction decreases becoming almost negligible 20 hours after injection (hour 30 in Figure 2). We attribute this to the stronger adsorption of CO inhibiting the hydrogenation of C7H8. With increasing CO coverage, the hydrogen reduction reaction and the hydrogenation of C7H8 compete for the remaining catalyst reaction sites resulting in greater overpotential than observed for CO alone. Results for varying operating conditions will be discussed.

Optimization of multijunction a-Si:H solar cells using an integrated optical/electrical model

An phenomenological model for multijunction amorphous silicon solar cells which integrates a detailed optical model and an equivalent circuit model with a voltage-dependent photocurrent was developed. The model equations accurately describe the light J-V curves for a-Si:H and a-SiGe:H single junction cells using carrier transport properties comparable to values reported in the literature and which agree closely with values derived from quantum efficiency measurements. Closed form expressions for collection efficiency are derived by approximating the carrier generation profile by an exponential distribution. This new model can be used to predict cell behavior with changes in i-layer thickness and bandgap, carrier mobility-lifetime, and illumination. The model has been combined with a powerful multivariable optimization routine to analyze alternative cell designs. The model is described and representative results are presented.

Effect of Hydrogen Bonding Characteristics on the Corrosion of Plasma‐Deposited Silicon Nitride Films

Silicon nitride (SiN[sub x]:H) films were deposited onto Al, Mo, Si, and Ti substrates by RF plasma-enhanced chemical vapor deposition from mixtures of SiH[sub 4] and NH[sub 3] with and without hydrogen dilution. Under conditions yielding the same Si/N ratio in the films (0.76); the addition of H[sub 2] to the gas mix increased total bonded H in the film, resulted in a shift from predominantly Si-H to N-H bonding, and yield higher density films. The anodic polarization behavior of coated and uncoated substrates were characterized in aqueous solutions of HCl and NaCl. Films deposited under conditions of hydrogen dilution exhibited up to three orders of magnitude lower dissolution rates than those deposited in the absence of hydrogen, affording greater protection to the underlying substrates. A high defect density in the SiN[sub x]: H films on aluminum, believed to be due to chemical interaction with the substrate during growth, resulted in high anodic current densities due to rapid dissolution to the underlying metal at the defect sites. The scratch resistance of the nitride films on aluminum was lower than on other substrates, apparently due to the softness of the pure aluminum.