Hydrogen Evolution with Minimal Parasitic Light Absorption by Dense

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Hydrogen Evolution with Minimal Parasitic Light Absorption by Dense Co-P Catalyst Films on Structured p-Si Photocathodes Paul Andrew Kempler, Miguel Angel Gonzalez, Kimberly Papadantonakis, and Nathan S. Lewis ACS Energy Lett., Just Accepted Manuscript • DOI: 10.1021/acsenergylett.8b00034 • Publication Date (Web): 08 Feb 2018 Downloaded from http://pubs.acs.org on February 9, 2018

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ACS Energy Letters

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Hydrogen Evolution with Minimal Parasitic Light Absorption by Dense Co-P Catalyst Films on Structured p-Si Photocathodes Paul A. Kempler,† Miguel A. Gonzalez,† Kimberly M. Papadantonakis,† Nathan S. Lewis*,† Division of Chemistry and Chemical Engineering†, California Institute of Technology, 1200 East California Blvd, Pasadena, California

Abstract Planar and three-dimensionally structured p-Si devices, consisting of an electrodeposited Co-P catalyst on arrays of Si microwires or Si micropyramids, were used as photocathodes for solar-driven hydrogen evolution in 0.50 M H2SO4(aq) to assess the effects of electrode structuring on parasitic absorption by the catalyst. Without the use of an emitter layer, p-Si/Co-P microwire arrays produced a photocurrent density of -10 mA cm-2 at potentials that were 130 mV more positive than optimized planar p-Si/Co-P devices. Champion p-Si/Co-P microwire array devices exhibited ideal regenerative cell solar-to-hydrogen efficiencies of > 2.5%, and were primarily limited by the photovoltage of the p-Si/Co-P junction. The vertical sidewalls of Si microwire photoelectrodes thus minimized effects due to parasitic absorption at high loadings of catalyst for device structures with or without emitters.

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TOC Figure (2” x 3”)


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ACS Energy Letters

3 Fully integrated, membrane-embedded solar fuels systems1-2 require that the catalyst for the fuel-forming reaction and/or the catalyst for the oxygen-evolving reaction are in the path of incident sunlight.3 Increasing the density of the catalyst increases the electrolyzer efficiency by reducing the kinetic overpotential required to drive the reaction at a given rate, allowing the system efficiency to be limited by the properties of the semiconductor.4-6 However, high loadings of catalyst also attenuate the light reaching the semiconductor. Sparse loadings of highly active noble-metal catalysts for the hydrogen-evolution reaction (HER), such as Pt, lead to efficient devices based on p-type Si or p-InP photocathodes, because the catalyst films can be structured as small islands that are effectively transparent.7 Transparent or semitransparent amorphous thin films of earth-abundant HER electrocatalysts have also recently been reported.8-9 Specifically, MoSxCly has been used in conjunction with planar p-Si and anti-reflective n+pp+-Si photocathodes and was reported to exhibit a cathodic current density >38 mA cm -2 for up to 2 h at 0 V vs the reversible hydrogen electrode (RHE) potential.10 HER catalysts that contain metal phosphides and metal phosphorous alloys (M-Ps) have exhibited absolute current densities > 10 mA cm-2 at overpotentials less than 100 mV, but require higher catalyst loadings than when highly active noble-metal catalysts are used, due to the lower turnover numbers, and thus generally result in increased parasitic optical absorption losses.11-14 Low filling fractions of catalyst islands on planar photocathodes minimize optical losses but yield increases in overpotential due to the concentration of current density at the catalystcoated regions of the electrode.6 Moreover, the approach either requires an emitter layer for efficient carrier transport to the catalyst-coated regions, or requires that the islands are separated by less than the minority-carrier collection length and optimally operate in the pinch-off regime to avoid excess majority-carrier recombination.15 Microstructured electrode morphologies also



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4 have been used, and films of opaque catalyst nanoparticles (NPs) can yield minimal photocurrent losses when deposited strategically on emitter-containing p-Si microwire arrays.16-19 Specifically, high-lifetime n+p-Si microwire-array (W) cathodes with CoP NPs loaded at the base of the Ws yield an ideal regenerative cell efficiency (IRC) of 1.9% under 1 Sun illumination, but these devices exhibit low fill factors.18 Designs using n+p Si µW-array cathodes with NiMo loaded at the base of the µWs, beneath a layer of TiO2 NPs, or directed to the tips by an insulating, antireflective SiO2 layer, yielded IRC = 2.9% and 10.8% respectively.17, 19 These systems require a radial emitter to facilitate carrier transport axially from the site of photogeneration to the location of the catalyst.

We demonstrate herein that three-dimensionally structured

semiconductors allow dense films of Co-P to be integrated with photocathodes in designs that place the opaque catalysts in the path of incident sunlight, but produce minimal parasitic absorption because the catalysts are primarily placed on surfaces that are orthogonal to the incident rays. This strategy is beneficial in the presence or absence of an emitter layer, and thus is a promising design for efficient photoelectrodes for solar fuel production. Detailed experimental procedures are provided in the Supporting Information. Randomly textured micropyramidal, µP, Si structures were formed via anisotropic etching of a P-type Si wafer with resistivity 10-20 Ω-cm for 1 h at 80 °C in an aqueous solution of 2% wt potassium hydroxide/20% v isopropyl alcohol. Silicon microwire arrays 30 µm in height and 3 µm in diameter were formed via deep reactive-ion etching in an SF6/O2 plasma, with a photolithographically patterned alumina mask used to define the microwire dimensions. Si samples were coated with a Co-P catalyst via low-temperature, scalable photoelectrochemical deposition.20 The cathodic charge density passed during deposition, normalized to the projected area of the individual electrode, was used to determine the catalyst loading (Figure S1). 4 ACS Paragon Plus Environment

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ACS Energy Letters

5 Immediately after deposition of the catalyst, electrodes were tested for hydrogen evolution in a cell filled with 0.50 M H2SO4(aq) (TraceMetals grade, Fisher) that was continuously purged with 1 atm of H2(g) and illuminated with 100 mW cm-2 of simulated AM1.5 sunlight. The acidic testing environment leached excess Co from the films, reduced the absolute overpotential for hydrogen evolution until a stable value was reached within 30 min, and left islands of Co-P with an elemental Co/P ratio of ~2:1 on the substrates (Figure S2). The ~2:1 Co/P ratio obtained after the catalyst-activation step is in close agreement with values measured for electrochemically deposited Co-P.20-21 Figure 1 shows scanning-electron micrographs (SEMs) of each of the p-Si/Co-P cathode structures examined in this work: planar p-Si, as well as p-Si microwires (W) or p-Si micropyramids (P on a planar, photoactive substrate). After etching, the height of the vertical Si microwires was 30 µm and the heights of the Si micropyramids were ≤ 3 µm (Figure S3). Photoelectrodeposition of Co-P resulted in discontinuous islands of Co-P distributed over the planar or microstructured p-Si surface. The vertical sidewalls of the µWs comprised the majority of the geometric surface area for the p-Si µW-on-planar substrate. Electrodeposition directed the opaque catalyst preferentially to the surface of the µWs and away from the planar portion of the substrates, which remained mostly bare. Notably, deposition at high, masstransport limited, current densities directed the catalyst deposits towards the tips of the microwires (Figure 1d).

Electrodeposition of Co-P onto n+-Si µW substrates in the dark

produced a similar distribution of catalyst along the length of the µW as was observed for p-Si µW substrates. Top-down comparisons of microwires and planar substrates with equal amounts of charge passed for Co-P electrodeposition showed that a substantially larger portion of the



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6 light-absorbing face of the semiconductor remained exposed than for deposition on planar Si electrodes (Fig 1 a,c). Figure 2 compares the current density vs potential (J-E) behavior for representative planar p-Si, P p-Si, and W p-Si/Co-P cathodes for a 400 mC cm-2 loading of electrodeposited and activated Co-P HER catalyst. Planar p-Si loaded with 400 mC cm-2 of Co-P exhibited substantial parasitic absorption as well as low fill factors, and these devices yielded short-circuit (i.e. at 0 V vs RHE) current densities of -13.6 ± 0.6 mA cm-2 and cathodic current densities > 10 mA cm-2 at an average potential of 74 ± 10 mV vs RHE. Structured p-Si/Co-P devices exhibited a larger cathodic current density at all potentials relative to planar devices, or equivalently, a more positive potential vs RHE at all operational current densities. For example, µP p-Si/Co-P and µW p-Si/Co-P devices exhibited cathodic current densities > 10 mA cm-2 at potentials of 181 ± 19 mV and 211 ± 11 mV, respectively, vs RHE. The difference in Voc between µW and µP pSi/Co-P compared to planar p-Si/Co-P electrodes cannot be explained solely by the decrease in light-limited photocurrent densities, and hence must be due to some interfacial phenomena, including possibly oxide formation or other interfacial kinetic processes differentially affecting the anodic and cathodic current densities that determine the potential of net zero current for the electrode/electrolyte system of interest. A full comparison of these figures-of-merit is provided in the Supporting Information. Prior to deposition of the catalyst, the photocathode structures exhibited similar limiting current densities while in contact with 0.50 M H2SO4(aq) and under 100 mW cm-2 of simulated AM1.5 illumination. The limiting current density was 26.7 ± 0.5 mA cm-2 for planar p-Si, 31 ± 2 mA cm-2 for p-Si P, and 28 ± 1 mA cm-2 for bare p-Si W devices (Figure S4a). These values represent the maximum photocurrent densities, |Jmax|, obtainable for each device type before losses due to parasitic absorption by the catalyst are introduced. The 6 ACS Paragon Plus Environment

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ACS Energy Letters

7 measured |Jmax| was consistent with the theoretical maximum photocurrent density simulated for a planar Si slab, after correcting for losses due to reflection at the Si/0.50 M H2SO4(aq) interface and absorption in the ~1 cm of electrolyte between the cell window and the surface of the sample.22-23 For each structure, |Jsc| was lower than |Jmax| for the respective bare p-Si structured electrode, indicating that in all cases addition of the Co-P catalyst introduced some parasitic optical absorption. However, equivalent catalyst loadings impeded light absorption differently depending on the structure of the Si substrate. The 400 mC cm-2 loading of Co-P blocked more light for the planar p-Si/Co-P and µP p-Si/Co-P devices, which yielded Jsc / Jmax values of 0.51 and 0.58, respectively, than for the µW p-Si/Co-P devices, which yielded Jsc / Jmax = 0.78 (Table 1). Thus, the Si µW structure reduced parasitic optical absorption due to deposited Co-P deposition by a factor of 2 relative to the planar surface. The inclusion of an emitter layer further increased the tolerance of µW devices to high catalyst loadings (Fig S3 c,d). For example, increasing the loading of Co-P from 400 to 800 mC cm-2 reduced Jsc / Jmax from 0.58 to 0.39 for planar devices, but did not have a substantial effect on n+p-Si P or W devices, producing Jsc / Jmax of 0.49 and 0.76, respectively (Supporting Information, Table S1). Figure 3 illustrates the effect of electrode structuring on obscuration by opaque films, by displaying

the

photocurrent

obtained

from

each

of

the

device

structures

during

photoelectrodeposition of the opaque Co-P catalyst-precursor film, i.e. prior to the activation step that removed excess Co and structured the film as islands, leaving exposed areas on the substrate. The planar and P p-Si devices both exhibited a rapid rate of decrease in photocurrent as the loading of the opaque film was increased, whereas the p-Si µW photocathode showed a much lower rate of decrease in photocurrent with increased loading of the opaque film than the other 7 ACS Paragon Plus Environment


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8 types of Si photocathodes. The decrease in |Jsc| after catalyst deposition for planar and P Si devices was due to masking of the light-absorbing surfaces by the catalyst film. For Si µW devices, the high-aspect-ratio structure of the substrate provided a geometric surface area that was several times larger than the projected surface area, allowing the total mass of catalyst supported on the electrode to be increased without proportionately increasing shadowing of the planar surfaces of the structure. Most of the geometric surface area of the Si µW-on-planar devices is on the sidewalls of the Si Ws and is oriented normal to the direction of illumination, so additional catalyst deposition did not substantially increase the parasitic optical absorption, and large catalyst loadings can be used without substantial losses of |Jsc|. Activation of the catalyst-precursor films resulted in a catalyst structured as islands, allowing more light to reach the semiconducting substrate after activation of the catalyst than during electrodeposition.

Upon activation, |Jlim| remained stable for > 30 h of continuous

operation (Figure S5). Higher photocurrents were thus obtained from devices with activated catalyst islands (Figure 2) than from devices with an equivalent loading of catalyst structured as a continuous film (Figure 3b). When catalysts with low Tafel slopes are used, structuring opaque catalyst films as islands has been shown to lead to improved efficiencies in planar devices relative to structuring the catalyst as a continuous film,6 but the catalyst-island strategy for minimizing parasitic absorption is limited for most catalyst-deposition methods other than lithography, because increases to the surface area of the catalyst typically result in lateral growth of the islands and therefore increased shadowing of the semiconductor by the catalyst. The structuring of the Co-P films as islands improved the performance of these devices relative to devices having continuous films, but the strategic placement of the catalyst onto a threedimensionally structured electrode is the predominant factor responsible for the improvement in 8 ACS Paragon Plus Environment

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9 performance of the W Si devices relative to P and planar Si cathodes observed herein. To assess the effect of the increased surface area available for catalyst loading provided by the Si µW structure, the dark catalytic behavior was compared for electrodeposited Co-P on planar n+Si and µW n+-Si electrodes (Figure 4a). The overpotential required to reach -10 mA cm-2 was reduced to 170 mV from 190 mV for the n+-Si µW sample relative to a planar n+-Si substrate. Figure 4b presents a comparison of the photoelectrochemical behavior of planar p-Si and µW pSi electrodes at increased loading, 800 mC cm-2, of Co-P. When the illumination intensity was increased to 125 mW cm-2 on the planar p-Si sample, Jlim increased to a value comparable to that of the µW device, but the increase in Jsc did not proportionally reflect this change. Furthermore, the potential at which -10 mA cm-2 was produced increased slightly, but remained over 100 mV more negative, for the planar p-Si sample relative to that of the µW p-Si electrodes. This behavior demonstrates that the improved light absorption by the µW device is not sufficient to explain the increased photoelectrochemical performance of these devices relative to the planar controls. The potential vs RHE required to reach an absolute current density of 10 mA cm-2, E10, is a figure of merit that relates the performance of a component half-cell measured using a threeelectrode experiment, such as the photocathode described herein, to the performance that would be expected for a full cell with an operating current density of 10 mA cm-2 and containing the component half-cell.24-25 The average E10 obtained for a p-Si µW-array photocathode in this work, +211 mV versus RHE, is a substantial improvement relative to the +134 mV versus RHE reported for photoelectrochemically deposited Co-P on porous ‘black’ p-Si or +150 mV versus RHE for NiCoSex on nanopillar ‘black’ p-Si.8, 26 With the inclusion of an emitter layer, the champion E10 for µW n+p-Si/Co-P, +310 mV vs RHE (Figure S6), is slightly less positive than 9 ACS Paragon Plus Environment


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10 the E10 = +345 mV versus RHE value observed for crystalline, nanometer-thick films of CoP on planar n+p-Si cathode.27 The latter comparison is noteworthy because the n+p-Si µW-array photocathode exhibited a more rapid onset to -10 mA cm-2 relative to Voc, indicating that the catalyst film was able to compensate for the decreased photovoltage relative to the planar film in the earlier study. When utilizing earth abundant catalysts with low Tafel slopes, the photovoltage lost from increased junction area can be offset by the lower overpotential that results from the increased area available for catalyst loading. The improved E10 for µW Si relative to planar and µP counterparts, with and without a buried junction, is a direct demonstration of this design principle. For Si photocathodes decorated with earth-abundant catalysts, the device efficiency can thus be increased substantially by minimizing the geometric footprint of the opaque catalyst over the light-absorbing surfaces of the semiconductor. Structured substrates allow for the catalyst footprint to be minimized without a reduction in the active surface area of the catalyst. The performance of the silicon microwire devices reported herein relied on long carrier-collection lengths in the axial direction. Free-standing microwire arrays achieve optimal performance when minority carriers are collected radially, which requires that the catalyst loading be distributed along the length of the microwire to match the local carrier generation rate. The more general approach should be also useful for other semiconductor materials that can be structured such that the majority of the electrochemically active surface area is located on surfaces orthogonal to incident illumination,28 provided that optical carrier generation is maximized and is spatially comparable to the photogenerated charge-carrier collection distances and catalyst location.29-30 Structuring Si into an array of microwires improved the peak power conversion efficiency by 250% while shifting the maximum power point positive by 65 mV (Figure S7).


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11 The photoactive planar substrate supporting the Si W-on-planar device described herein is not desirable for a completely membrane-embedded water-splitting device, hence study of catalyst placement onto Si W arrays that have been removed from the planar Si substrate and embedded in ion-exchange membranes, and testing of the membrane-embedded Si W arrays under a variety of device conditions, e.g. back-side illumination through the membrane, will be needed to evaluate these approaches in the context of specific device designs.31 Such studies may include selective deposition of the catalyst onto selected regions of structured semiconductors, e.g. the tips of the Si Ws, using wavelength-dependent photoelectrochemical deposition to further reduce parasitic absorption by the catalyst.32 The bare Si µW-arrays in this study were not optimized to minimize reflection, absorbing < 65% of incident photons. Silicon photocathodes textured to minimize reflection have achieved measured photocurrents near the absorption limit of silicon under AM1.5 illumination8-9,

26, 33

. Texturing high aspect ratio structures with

antireflective surfaces could further improve the utilization of earth-abundant catalysts in photocathodes used for the HER. P-type Si photocathodes incorporating high-aspect-ratio microwire structures and supporting high mass loadings of an opaque earth-abundant catalyst showed substantially improved values of |Jsc| relative to planar p-Si devices or p-Si devices textured with micropyramids. The vertical sidewalls of the microwires provided surfaces that supported high surface areas of the catalyst in an orientation that minimized the area of the semiconductor shadowed by the catalyst, thereby reducing parasitic optical absorption by the catalyst and improving |Jsc| relative to the other photocathode structures. The potential required to reach -10 mA cm-2 of current density for hydrogen production using the microwire-based devices was +210 mV vs RHE for p-Si and +300 mV vs RHE for n+p-Si, both of which are among the most 11 ACS Paragon Plus Environment


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12 positive reported values for Si-based HER photocathodes utilizing earth-abundant materials, without and including a solid-state junction, respectively. Hence, catalyst placement on sidewalls of high-aspect-ratio structures is a viable strategy for incorporating large loadings of opaque catalysts directly on the surface of light absorbers.


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Figures and Tables

Figure 1: Scanning-electron micrographs of photoelectrochemically deposited Co-P particles after 400 mC cm-2 charge was passed. (A) Top-down image of planar p-Si, inset: magnified image of individual particle showing nanoscale roughness; (B) Top-down image of p-Si microwire array showing minimal deposition along the base and caps of the wires; (C) Image at 30-degrees tilt of p-Si micropyramid array; (D) Image at 20-degrees tilt of p-Si microwire array. The largest deposition occurred at the tips of the microwires.



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Figure 2: J-E behavior of p-Si photocathodes with 400 mC cm-2 of Co-P catalyst in 0.50 M H2SO4(aq) under simulated 1 Sun illumination. Scans were recorded at 50 mV s-1 at potentials negative of the open-circuit potential, to avoid oxidation of the Si/catalyst interface.


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15 Table 1: Average figures of merit for different p-Si structures loaded with Co-P and tested at 100 mW cm-2 of simulated sunlight in 0.50 M H2SO4(aq). The short-circuit current density, Jsc is the current density at the equilibrium potential for hydrogen evolution, RHE. The limiting current density, Jlim, was recorded at -0.3 V vs RHE and is proportional to the total number of photons collected by the underlying Si. These values are compared to the limiting current density recorded for the bare Si devices. The potentials at -10 mA cm-2 and 0 mA cm-2 are reported as V10 and Voc, respectively. Data points were recorded during scans at 50 mV s-1 after the device performance had stabilized. Cathode structure

|Jmax| (mA cm-2)

p-Si/Co-P planar p-Si/Co-P P p-Si/Co-P W

27 ± 0.5

Co-P loading = 400 mC cm-2 |Jsc| (mA cm-2) Voc (V vs V10 (V vs Jsc/Jmax RHE) RHE) 13 ± 1 0.265 ± 0.026 0.074 ± 0.010 0.51 ± 0.02

31 ± 2

18 ± 1

0.286 ± 0.011

0.181 ± 0.019 0.58 ± 0.05

28 ± 1

22 ± 2

0.342 ± 0.024

0.211 ± 0.012 0.78 ± 0.05



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Figure 3: (a) Loss in photocurrent during deposition of Co-P on various p-Si structures, expressed as the light-limited current density vs. passed cathodic charge density. For clarity, the current density was normalized to the initial current density recorded for the bare photocathode in the deposition cell. (b) The measured photocurrent density in the deposition cell was highly correlated with Jsc for hydrogen evolution prior to dissolution of excess cobalt during activation. The predicted values of Jsc under 100 mW cm-2 of simulated solar illumination in 0.50 M H2SO4(aq) were calculated based on the ratio of the initial photocurrent and final photocurrent during deposition, multiplied by the limiting photocurrent measured for the bare photocathode in 0.50 M H2SO4(aq) under nominally identical illumination conditions.


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Figure 4: (a) Dark catalytic behavior of Co-P on planar n+-Si and on an n+-Si µW array with 7 µm pitch and 3 µm diameter in 0.50 M H2SO4(aq). (b) Comparison at increased light intensity of the behavior of a high loading (800 mC cm-2 of cathodic charge density passed) of Co-P on planar p-Si with the behavior of Co-P loaded p-Si microwires, demonstrating that the observed increases in performance for the p-Si µW/Co-P array cannot be explained solely by an increase in the collection of photons.

Acknowledgements This material is based upon work performed by the Joint Center for Artificial Photosynthesis, a DOE Energy Innovation Hub, supported through the Office of Science of the U.S. Department of Energy under Award Number DE-SC0004993.

Supporting Information Available: Detailed experimental procedures for the fabrication of silicon photocathodes, photoelectrochemical deposition of Co-P, hydrogen evolution testing. Models of various photocathode structures with equivalent loadings of Co-P. Energy dispersive x-ray spectra and scanning electron micrographs of Co-P films and Si photocathodes. Current-voltage behavior for hydrogen evolution from different loadings of Co-P on silicon photocathode. Detailed tabulated figures of merit for the silicon photocathodes. Extended stability testing in 0.50 M H2SO4(aq) of p-Si/Co-P photocathodes. Champion device performance and comparison of power conversion efficiency for different photocathodes under 100 mW cm-2 of simulated AM1.5G illumination.



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References (1) Lewis, N. S. Developing a Scalable Artificial Photosynthesis Technology through Nanomaterials by Design. Nat. Nanotechnol. 2016, 11, 1010-1019. (2) Shaner, M. R.; Atwater, H. A.; Lewis, N. S.; McFarland, E. W. A Comparative Technoeconomic Analysis of Renewable Hydrogen Production Using Solar Energy. Energy Environ. Sci. 2016, 9, 2354-2371. (3) Sun, K.; Moreno-Hernandez, I. A.; Schmidt, W. C.; Zhou, X.; Crompton, J. C.; Liu, R.; Saadi, F. H.; Chen, Y.; Papadantonakis, K. M.; Lewis, N. S. A Comparison of the Chemical, Optical and Electrocatalytic Properties of Water-Oxidation Catalysts for Use in Integrated SolarFuels Generators. Energy Environ. Sci. 2017, 10, 987-1002. (4) Dominey, R. N.; Lewis, N. S.; Bruce, J. A.; Bookbinder, D. C.; Wrighton, M. S. Improvement of Photoelectrochemical Hydrogen Generation by Surface Modification of p-Type Silicon Semiconductor Photocathodes. J. Am. Chem. Soc. 1982, 104, 467-482. (5) Heller, A.; Aharon-Shalom, E.; Bonner, W.; Miller, B. Hydrogen-Evolving Semiconductor Photocathodes: Nature of the Junction and Function of the Platinum Group Metal Catalyst. J. Am. Chem. Soc. 1982, 104, 6942-6948. (6) Chen, Y.; Sun, K.; Audesirk, H.; Xiang, C.; Lewis, N. S. A Quantitative Analysis of the Efficiency of Solar-Driven Water-Splitting Device Designs Based on Tandem Photoabsorbers Patterned with Islands of Metallic Electrocatalysts. Energy Environ. Sci. 2015, 8, 1736-1747. (7) Heller, A.; Aspnes, D.; Porter, J.; Sheng, T.; Vadimsky, R. Transparent Metals Preparation and Characterization of Light-Transmitting Platinum Films. J. Phys. Chem. 1985, 89, 4444-4452. (8) Zhang, H.; Ding, Q.; He, D.; Liu, H.; Liu, W.; Li, Z.; Yang, B.; Zhang, X.; Lei, L.; Jin, S. A p-Si/NiCoSe x Core/Shell Nanopillar Array Photocathode for Enhanced Photoelectrochemical Hydrogen Production. Energy Environ. Sci. 2016, 9, 3113-3119. (9) Zhang, X.; Meng, F.; Mao, S.; Ding, Q.; Shearer, M. J.; Faber, M. S.; Chen, J.; Hamers, R. J.; Jin, S. Amorphous MoS x Cl y Electrocatalyst Supported by Vertical Graphene for Efficient Electrochemical and Photoelectrochemical Hydrogen Generation. Energy Environ. Sci. 2015, 8, 862-868. (10) Ding, Q.; Zhai, J.; Cabán‐Acevedo, M.; Shearer, M. J.; Li, L.; Chang, H. C.; Tsai, M. L.; Ma, D.; Zhang, X.; Hamers, R. J. Designing Efficient Solar‐Driven Hydrogen Evolution Photocathodes Using Semitransparent MoQxCly (Q= S, Se) Catalysts on Si Micropyramids. Adv. Mater. 2015, 27, 6511-6518. (11) McCrory, C. C.; Jung, S.; Ferrer, I. M.; Chatman, S. M.; Peters, J. C.; Jaramillo, T. F. Benchmarking Hydrogen Evolving Reaction and Oxygen Evolving Reaction Electrocatalysts for Solar Water Splitting Devices. J. Am. Chem. Soc. 2015, 137, 4347-57. (12) Warren, E. L.; McKone, J. R.; Atwater, H. A.; Gray, H. B.; Lewis, N. S. HydrogenEvolution Characteristics of Ni–Mo-Coated, Radial Junction, n+p-Silicon Microwire Array Photocathodes. Energy Environ. Sci. 2012, 5, 9653. (13) Popczun, E. J.; Read, C. G.; Roske, C. W.; Lewis, N. S.; Schaak, R. E. Highly Active Electrocatalysis of the Hydrogen Evolution Reaction by Cobalt Phosphide Nanoparticles. Angew. Chem. Int. Ed. 2014, 53, 5427-30. (14) Kibsgaard, J.; Jaramillo, T. F. Molybdenum Phosphosulfide: An Active, Acid‐Stable, Earth‐Abundant Catalyst for the Hydrogen Evolution Reaction. Angew. Chem. Int. Ed. 2014, 53, 14433-14437. 18 ACS Paragon Plus Environment

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ACS Energy Letters

19 (15) Rossi, R. C.; Lewis, N. S. Investigation of the Size-Scaling Behavior of Spatially Nonuniform Barrier Height Contacts to Semiconductor Surfaces Using Ordered NanometerScale Nickel Arrays on Silicon Electrodes. J. Phys. Chem. B 2001, 105, 12303-12318. (16) Bao, X.-Q.; Cerqueira, M. F.; Alpuim, P.; Liu, L. Silicon Nanowire Arrays Coupled with Cobalt Phosphide Spheres as Low-Cost Photocathodes for Efficient Solar Hydrogen Evolution. Chem. Comm. 2015, 51, 10742-10745. (17) Vijselaar, W.; Westerik, P.; Veerbeek, J.; Tiggelaar, R. M.; Berenschot, E.; Tas, N. R.; Gardeniers, H.; Huskens, J. Spatial Decoupling of Light Absorption and Catalytic Activity of Ni–Mo-Loaded High-Aspect-Ratio Silicon Microwire Photocathodes. Nature Energy 2018, [Online early access]. DOI: 10.1038/s41560-017-0068-x. Published Online: Jan 15, 2018. (18) Roske, C. W.; Popczun, E. J.; Seger, B.; Read, C. G.; Pedersen, T.; Hansen, O.; Vesborg, P. C. K.; Brunschwig, B. S.; Schaak, R. E.; Chorkendorff, I., et al. Comparison of the Performance of CoP-Coated and Pt-Coated Radial Junction n+p-Silicon Microwire-Array Photocathodes for the Sunlight-Driven Reduction of Water to H2(g). J. Phys. Chem. Lett. 2015, 6, 1679-1683. (19) Shaner, M. R.; McKone, J. R.; Gray, H. B.; Lewis, N. S. Functional Integration of Ni-Mo Electrocatalysts with Si Microwire Array Photocathodes to Simultaneously Achieve High Fill Factors and Light-Limited Photocurrent Densities for Solar-Driven Hydrogen Evolution. Energy Environ. Sci. 2015, 8, 2977-2984. (20) Saadi, F. H.; Carim, A. I.; Verlage, E.; Hemminger, J. C.; Lewis, N. S.; Soriaga, M. P. CoP as an Acid-Stable Active Electrocatalyst for the Hydrogen-Evolution Reaction: Electrochemical Synthesis, Interfacial Characterization and Performance Evaluation. J. Phys. Chem. C 2014, 118, 29294-29300. (21) Saadi, F. H.; Carim, A. I.; Drisdell, W. S.; Gul, S.; Baricuatro, J. H.; Yano, J.; Soriaga, M. P.; Lewis, N. S. Operando Spectroscopic Analysis of CoP films Electrocatalyzing the HydrogenEvolution Reaction. J. Am. Chem. Soc. 2017, 139, 12927-12930. (22) Döscher, H.; Geisz, J. F.; Deutsch, T. G.; Turner, J. A. Sunlight Absorption in Water – Efficiency and Design Implications for Photoelectrochemical Devices. Energy Environ. Sci. 2014, 7, 2951-2956. (23) McIntosh, K. R.; Baker-Finch, S. C. In OPAL 2: Rapid Optical Simulation of Silicon Solar Cells, Photovoltaic Specialists Conference (PVSC), 2012 38th IEEE, IEEE: 2012; pp 000265-000271. (24) Walter, M. G.; Warren, E. L.; McKone, J. R.; Boettcher, S. W.; Mi, Q.; Santori, E. A.; Lewis, N. S. Solar Water Splitting Cells. Chem. Rev. 2010, 110, 6446-6473. (25) Coridan, R. H.; Nielander, A. C.; Francis, S. A.; McDowell, M. T.; Dix, V.; Chatman, S. M.; Lewis, N. S. Methods for Comparing the Performance of Energy-Conversion Systems for Use in Solar Fuels and Solar Electricity Generation. Energy Environ. Sci. 2015, 8, 2886-2901. (26) Zhang, H.; Li, A.; Wang, Z.; Ma, W.; Li, D.; Zong, X.; Li, C. Decorating Mesoporous Silicon with Amorphous Metal–Phosphorous-Derived Nanocatalysts Towards Enhanced Photoelectrochemical Water Reduction. J. Mater. Chem. A 2016, 4, 14960-14967. (27) Hellstern, T. R.; Benck, J. D.; Kibsgaard, J.; Hahn, C.; Jaramillo, T. F. Engineering Cobalt Phosphide (CoP) Thin Film Catalysts for Enhanced Hydrogen Evolution Activity on Silicon Photocathodes. Adv. Energy. Mater. 2016, 6, 1501758. (28) Kayes, B. M.; Atwater, H. A.; Lewis, N. S. Comparison of the Device Physics Principles of Planar and Radial p-n Junction Nanorod Solar Cells. J. Appl. Phys. 2005, 97, 114302.



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20 (29) Fukami, K.; Kobayashi, K.; Matsumoto, T.; Kawamura, Y. L.; Sakka, T.; Ogata, Y. H. Electrodeposition of Noble Metals into Ordered Macropores in p-Type Silicon. J. Electrochem. Soc. 2008, 155, D443. (30) Yalamanchili, S.; Emmer, H. S.; Fountaine, K. T.; Chen, C. T.; Lewis, N. S.; Atwater, H. A. Enhanced Absorption and

Hydrogen Evolution with Minimal Parasitic Light Absorption by Dense

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