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

Enhanced Stability and Efficiency for Photoelectrochemical Iodide Oxidation by Methyl Termination and Electrochemical Pt Deposition of n-Si Microwire Arrays

Arrays of Si microwires doped n-type (n-Si) and surface-functionalized with methyl groups have been used, with or without deposition of Pt electrocatalysts, to photoelectrochemically oxidize I–(aq) to I_3–(aq) in 7.6 M HI(aq). Under conditions of iodide oxidation, methyl-terminated n-Si microwire arrays exhibited stable short-circuit photocurrents over a time scale of days, albeit with low energy-conversion efficiencies. In contrast, electrochemical deposition of Pt onto methyl-terminated n-Si microwire arrays consistently yielded energy-conversion efficiencies of ∼2% for iodide oxidation, with an open-circuit photovoltage of ∼400 mV and a short-circuit photocurrent density of ∼10 mA cm^(–2) under 100 mW cm^(–2) of simulated air mass 1.5G solar illumination. Platinized electrodes were stable for >200 h of continuous operation, with no discernible loss of Si or Pt. Pt deposited using electron-beam evaporation also resulted in stable photoanodic operation of the methyl-terminated n-Si microwire arrays but yielded substantially lower photovoltages than when Pt was deposited electrochemically

Comparison of the Photoelectrochemical Behavior of H‑Terminated and Methyl-Terminated Si(111) Surfaces in Contact with a Series of One-Electron, Outer-Sphere Redox Couples in CH_3CN

The photoelectrochemical behavior of methyl-terminated p-type and n-type Si(111) surfaces was determined in contact with a series of one-electron, outer-sphere, redox couples that span >1 V in the Nernstian redox potential, E(A/A^−), of the solution. The dependence of the current vs potential data, as well as of the open-circuit photovoltage, V_(OC), on E(A/A^−) was compared to the behavior of H-terminated p-type and n-type Si(111) surfaces in contact with these same electrolytes. For a particular E(A/A^−) value, CH_3-terminated p-Si(111) electrodes showed lower V_(OC) values than Hterminated p-Si(111) electrodes, whereas CH_3-terminated n-Si(111) electrodes showed higher V_(OC) values than H-terminated n-Si(111) electrodes. Under 100 mW cm^(−2) of ELH-simulated Air Mass 1.5 illumination, n-type H−Si(111) and CH_3−Si(111) electrodes both demonstrated nonrectifying behavior with no photovoltage at very negative values of E(A/A^−) and produced limiting V_(OC) values of >0.5 V at very positive values of E(A/A^−). Illuminated p-type H−Si(111) and CH_3−Si(111) electrodes produced no photovoltage at positive values of E(A/A^−) and produced limiting V_(OC) values in excess of 0.5 V at very negative values of E(A/A^−). In contact with CH_3CN-octamethylferrocene^(+/0), differential capacitance vs potential experiments yielded a −0.40 V shift in flat-band potential for CH_3-terminated n-Si(111) surfaces relative to H-terminated n-Si(111) surfaces. Similarly, in contact with CH_3CN-1,1′-dicarbomethoxycobaltocene^(+/0), the differential capacitance vs potential data indicated a −0.25 V shift in the flat-band potential for CH_3-terminated p-Si(111) electrodes relative to H-terminated p-Si(111) electrodes. The observed trends in V_(OC) vs E(A/A^−), and the trends in the differential capacitance vs potential data are consistent with a negative shift in the interfacial dipole as a result of methylation of the Si(111) surface. The negative dipole shift is consistent with a body of theoretical and experimental comparisons of the behavior of CH_3−Si(111) surfaces vs H−Si(111) surfaces, including density functional theory of the sign and magnitude of the surface dipole, photoemission spectroscopy in ultrahigh vacuum, the electrical behavior of Hg/Si contacts, and the pH dependence of the current−potential behavior of Si electrodes in contact with aqueous electrolytes

Photoelectrochemical Behavior of n‑Type Si(111) Electrodes Coated With a Single Layer of Graphene

The behavior of graphene-coated n-type Si(111) photoanodes was compared to the behavior of H-terminated n-type Si(111) photoanodes in contact with aqueous K_3[Fe(CN)_6]/K_4[Fe(CN)_6] as well as in contact with a series of outer-sphere, one-electron redox couples in nonaqueous electrolytes. The n-Si/Graphene electrodes exhibited stable short-circuit photocurrent densities of over 10 mA cm^(–2) for >1000 s of continuous operation in aqueous electrolytes, whereas n-Si–H electrodes yielded a nearly complete decay of the current density within 100 s. The values of the open-circuit photovoltages and the flat-band potentials of the Si were a function of both the Fermi level of the graphene and the electrochemical potential of the electrolyte solution, indicating that the n-Si/Graphene did not form a buried junction with respect to the solution contact

Enhanced Stability and Efficiency for Photoelectrochemical Iodide Oxidation by Methyl Termination and Electrochemical Pt Deposition of n-Si Microwire Arrays

Arrays of Si microwires doped n-type (n-Si) and surface-functionalized with methyl groups have been used, with or without deposition of Pt electrocatalysts, to photoelectrochemically oxidize I–(aq) to I_3–(aq) in 7.6 M HI(aq). Under conditions of iodide oxidation, methyl-terminated n-Si microwire arrays exhibited stable short-circuit photocurrents over a time scale of days, albeit with low energy-conversion efficiencies. In contrast, electrochemical deposition of Pt onto methyl-terminated n-Si microwire arrays consistently yielded energy-conversion efficiencies of ∼2% for iodide oxidation, with an open-circuit photovoltage of ∼400 mV and a short-circuit photocurrent density of ∼10 mA cm^(–2) under 100 mW cm^(–2) of simulated air mass 1.5G solar illumination. Platinized electrodes were stable for >200 h of continuous operation, with no discernible loss of Si or Pt. Pt deposited using electron-beam evaporation also resulted in stable photoanodic operation of the methyl-terminated n-Si microwire arrays but yielded substantially lower photovoltages than when Pt was deposited electrochemically

Photoanodic behavior of vapor-liquid-solid–grown, lightly doped, crystalline Si microwire arrays

Arrays of n-Si microwires have to date exhibited low efficiencies when measured as photoanodes in contact with a 1-1′-dimethylferrocene (Me_2Fc^(+/0))–CH_3OH solution. Using high-purity Au or Cu catalysts, arrays of crystalline Si microwires were grown by a vapor-liquid-solid process without dopants, which produced wires with electronically active dopant concentrations of 1 × 10^(13) cm^(−3). When measured as photoanodes in contact with a Me_2Fc^(+/0)–CH_3OH solution, the lightly doped Si microwire arrays exhibited greatly increased fill factors and efficiencies as compared to n-Si microwires grown previously with a lower purity Au catalyst. In particular, the Cu-catalyzed Si microwire array photoanodes exhibited open-circuit voltages of ~0.44 V, carrier-collection efficiencies exceeding ~0.75, and an energy-conversion efficiency of 1.4% under simulated air mass 1.5 G illumination. Lightly doped Cu-catalyzed Si microwire array photoanodes have thus demonstrated performance that is comparable to that of optimally doped p-type Si microwire array photocathodes in photoelectrochemical cells

Internally coupled ears in living mammals.

It is generally held that the right and left middle ears of mammals are acoustically isolated from each other, such that mammals must rely on neural computation to derive sound localisation cues. There are, however, some unusual species in which the middle ear cavities intercommunicate, in which case each ear might be able to act as a pressure-difference receiver. This could improve sound localisation at lower frequencies. The platypus Ornithorhynchus is apparently unique among mammals in that its tympanic cavities are widely open to the pharynx, a morphology resembling that of some non-mammalian tetrapods. The right and left middle ear cavities of certain talpid and golden moles are connected through air passages within the basicranium; one experimental study on Talpa has shown that the middle ears are indeed acoustically coupled by these means. Having a basisphenoid component to the middle ear cavity walls could be an important prerequisite for the development of this form of interaural communication. Little is known about the hearing abilities of platypus, talpid and golden moles, but their audition may well be limited to relatively low frequencies. If so, these mammals could, in principle, benefit from the sound localisation cues available to them through internally coupled ears. Whether or not they actually do remains to be established experimentally.This is the final version of the article. It first appeared from Springer via http://dx.doi.org/10.1007/s00422-015-0675-