High Performance and Stability of Micropatterned Oxide-Passivated

Dec 14, 2015 - Silicon is a promising material for efficient PEC cells because it is low-cost and earth-abundant and has a small bandgap (1.12 eV) whi...
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High Performance and Stability of Micropatterned Oxide-Passivated Photoanodes with Local Catalysts for Photoelectrochemical Water Splitting Seungtaeg Oh and Jihun Oh* Graduate School of EEWS (Energy, Environment, Water, and Sustainability), Korea Advanced Institute of Science and Technology (KAIST), 291 Daehak-ro, Yuseong-gu, Deajeon 350-701, Republic of Korea S Supporting Information *

ABSTRACT: A new passivation strategy for an oxygen evolving Si photoanode for efficient and stable photoelectrochemical (PEC) water splitting is presented. In our Si photoanode structure, to eliminate the Si/water interface, the Si is stabilized with a thick insulating and chemically inert SiO2 film and locally defined electrocatalysts on the Si surface. The stabilized p+n-Si photoanode with SiO2 film and micropatterned Ni catalysts produced photocurrent densities of 27 mA cm−2 at water oxidation potential without corrosion for 24 h under water oxidation conditions. In addition, we provide a device modeling of our Si cell, i.e., an integrated photovoltaics− electrolyzer cell, to quantitatively assess the interplay between optical shadowing and catalytic performance, as a function of the coverage and characteristics of electrocatalysts on the water oxidation reaction during PEC water splitting. Design principles for high performance oxygen evolving Si photoanodes are presented based on the device modeling.

1. INTRODUCTION Photoelectrochemical (PEC) cells are attracting intense interest as a viable technology for directly converting intermittent solar energy into storable and distributable chemical fuels and feedstocks, such as hydrogen and hydrocarbons. In a PEC solar energy conversion cell, the oxygen evolution reaction (OER), i.e., water oxidation reaction, provides electrons and protons for the reduction of water or CO2, to produce hydrogen or hydrocarbons, respectively. The OER has been regarded as one of the major obstacles to realizing an efficient PEC cell due to high overpotential.1−3 In addition, when a photoanode for OER is used in a PEC cell, the OER competes with the oxidation and/or corrosion of the semiconductor itself, and thus a semiconductor is required to be stable in a water oxidation condition.4 For this reason, metal oxides that are chemically stable in an aqueous solution, such as TiO2, WO3, BiVO4, and α-Fe2O3, have been widely used as photoanodes in PEC cells.5−11 However, metal oxides suffer from low energy conversion efficiency due to their high bandgap and poor electric properties. Silicon is a promising material for efficient PEC cells because it is low-cost and earth-abundant and has a small bandgap (1.12 eV) which enables it to absorb most of the solar spectrum.12,13 In addition, in tandem PEC cells silicon can serve as the bottom cell, and in combination with a semiconductor with a bandgap of 1.7−1.8 eV as the top cell, this configuration can reach more than 20% conversion efficiency.14 However, when silicon is in contact with water, it is easily oxidized or even corrodes in the water oxidation conditions found in an aqueous solution.15 To address this issue, corrosion protection strategies for Si-based © XXXX American Chemical Society

photoanodes have focused on eliminating the Si/electrolyte interface by depositing a corrosion protection layer on Si photoelectrodes. This layer must allow light absorption in the Si as well as the transport of photoinduced holes through the protection layer for the water oxidation reaction.16−25 For instance, the deposition of thin metal catalyst films, such as Ni and Ir, on n-type Si photoanodes has been demonstrated to improve stability under water oxidation conditions over 12 h.16−18 However, the thicknesses of the metal films must be carefully controlled to ensure efficient and stable OER, since thick metal films can obstruct light absorption in the underlying Si electrode, while thin metals can result in damage to the Si electrode due to delamination of the metal layer during longterm operation.26 Alternatively, metal oxide films with high bandgaps, such as TiO2, have been used as a protection layer for Si photoanodes due to their optical transparency and chemical stability in aqueous solutions with wide ranges of pHs.27 However, the unfavorable valence band edge alignment between TiO2 and Si blocks the transport of photoexcited holes from Si to an OER cocatalyst through the TiO2 protection layer: the valence band edge of TiO2 is more positive than that of Si in the electrochemical energy scale. As a consequence, the stabilization of an efficient Si OER photoanode with TiO2 has only been demonstrated by utilizing charge transport mechanisms in the TiO2, such as tunneling and defect-state-mediated transport Received: November 5, 2015 Revised: December 12, 2015

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implantation followed by annealing at 1000 °C. The secondary ion mass spectrometry (SIMS) indicates that the peak concentration of boron and the junction depth of the p+ layer are about 3 × 1020 cm−3 and 850 nm, respectively. Then, a 110 nm thick SiO2 passivation layer was grown by thermal oxidation. Then, photolithography and reactive ion etching (RIE) using CF4 were used to etch the SiO2 surface to locally expose the Si surface. Then, 40 nm thick Pt and Ni catalyst films were deposited by sputtering and an e-beam evaporator, respectively. For the Ni, a 40 nm thick Ti layer was used as an adhesion promoter. After a lift-off process using acetone for 30 min in a sonication bath, the p+n-Si photoanode with SiO2 film and micropatterned catalyst patches were formed. Si photoanodes having various catalyst coverages were obtained by using different photomasks. Scanning-electron microscopy (SEM) was performed using a NovaSEM 230 (FEI) with an accelerating voltage of 5 kV. 2.2. Photoelectrochemical (PEC) Measurements. A three-electrode measurement configuration was used in this work. A Pt wire and an Ag/AgCl (3 M KCl) were used as a counter electrode and a reference electrode, respectively. A 1 M NaOH aqueous solution (Sigma-Aldrich, pH 13.3) was used as an electrolyte. An ohmic contact to a Si wafer with a p+n buried junction was formed by painting gallium−indium eutectic alloy (Sigma-Aldrich) on the backside of the sample and attaching a copper wire. The active front surface was defined by sealing with an industrial epoxy (Loctite 9460), and its area was measured by scanning with a scanner having a resolution of 300 dpi and using an image processing program (ImageJ). A Pyrex glass vessel with a flat window was used as the PEC cell. A 300 W Xe lamp (Oriel Instrument, Model 6258) was used as an illumination source. Light intensity was set to AM 1.5G after calibrating with a Si reference (Oriel Instrument, 91150 V). All electrodes were subjected to linear sweep voltammetry (LSV) with a scan rate of 20 mV s−1 from −0.02 to 2.48 V versus the reversible hydrogen electrode (RHE). The stability test was conducted by a chronoamperometry for 24 h at a constant potential about 500 mV positive to the potential at the start of the photocurrent density plateau measured by the LSV. This stability test is intended to investigate corrosion of our Si electrodes at the maximum PEC OER reaction rate: the photocurrent at a potential near the photocurrent plateau during the 24 h stability test can be sensitive on slight changes of metal micropatches instead of the corrosion of Si itself. All electrochemical measurements were performed using a potentiostat (BioLogic, SP-150) without stirring.

of holes, by forming ultrathin and thick leaky TiO2 by atomic layer deposition, respectively.19,20 Other transparent conducting oxides (TCO) such as indium-doped tin oxide (ITO) also allow hole transport from Si during OER;21 however, ITO is not stable in high and low pH media and produces low photovoltage due to defect states at the Si and TCO interface.28,29 In this strategy, nano- and microscale OER catalysts are often deposited on top of protecting metal oxide films to drive the OER. More recently, multifunctional metal oxide films with high transparency and OER electrocatalytic properties have been successfully used to stabilize Si photoanodes in an aqueous solution.22−25,30 In particular, Sun et al. demonstrated that a reactive sputtered NiOx film, a highly active OER catalyst in a strong basic solution, is an effective protective coating against corrosion. Their NiOx film enabled more than 1000 h continuous operation of highly efficient NiOx/p+-n-Si photoanodes, producing a photocurrent density of 30 mA cm−2 at the water oxidation potential in 1 M KOH under simulated 1 sun illumination.22 Furthermore, this NiOx film was shown to successfully stabilize other low bandgap semiconductors, such as amorphous hydrogenated Si (a-Si:H), InP, and CdTe, during solar-driven water oxidation reactions.23 However, the OER catalytic activity of these multifunctional metal oxide coatings is pH-specific, which could limit the set of allowed catalysts and/or absorbers that could be used for lowcost, efficient, and stable hydrogen evolution reactions in an unassisted water splitting system.31 In addition, typical TiO2 and ITO coatings are known to have poor surface passivation properties, which is associated with the high surface recombination of photoexcited carriers, resulting in modest photovoltage in Si solar cells, while little is known about NiOx coatings.24,25 Here, we present a new Si stabilization strategy to mitigate corrosion during the PEC water oxidation reaction using an insulating SiO2 passivation layer and locally defined metal cocatalyst micropatches on a silicon photoanode. In addition, the new photoanode structure intends to provide high PEC performance because it can reduce surface recombination by using the passivating SiO2 layer, a well-known surface passivation layer in photovoltaics industry, and by minimizing Si/metal catalysts interface of high recombination velocity.32 The approach is based on a local contact scheme in an advanced Si solar cell technology with a power conversion efficiency of more than 20%. Our Si photoanode structure provided a photovoltage of about 500 mV and exhibited a photocurrent density of about 27 mA cm−2 without any noticeable corrosion or degradation, for 24 h at the water oxidation potential in 1 M aqueous NaOH under simulated 1 sun illumination. Dissolution of the SiO2 passivation layer in strong alkaline solutions such as NaOH would limit the stability of our Si photoanode during OER. In addition, we investigated the opposing roles of catalysts’ surface coverages on the PEC performance of our Si photoanode, in relation to the optical shadowing effect and electrocatalytic performance. This study could provide insights useful for designing an integrated photovoltaics−electrolyzer cell capable of highly efficient PEC water-splitting reactions.

3. RESULTS AND DISCUSSION 3.1. Micropatterned SiO2-Passivated Si Photoanode with Local Catalysts for Efficient PEC Water Splitting. Figure 1a shows the schematic diagram of our Si photoanode structure, designed to enable an efficient solar water oxidation reaction. As depicted in Figure 1a, the Si photoanode structure provides high OER performance by passivating high recombination Si surface with an SiO2 layer. The SiO2 layer serves as a surface passivation layer which can produce superior photovoltage in Si by minimizing active recombination centers, such as Si dangling bonds.32 Since SiO2 is an insulator, the OER catalyst micropatches are formed directly on the silicon surface by patterning the SiO2 film. The micropatches allow the injection of photoexcited holes to water. Other well-known surface passivation layers such as Si3N4 and Al2O3 can be also used in an electrolyte where the passivation layer is stable.

2. EXPERIMENTAL SECTION 2.1. Preparation of the Si Photoanode with SiO2 Film and Micropatterned Catalysts. A 675 μm thick n-type Si (100) wafer (0.5−3 Ω·cm) was used in this work. A p+n junction was formed on top of a wafer by boron ion B

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illumination, and a buried p+−n junction provides the electric field to separate and collect the photoexcited holes to the OER catalyst micropatches for OER. In addition, the catalyst micropatches on our Si photoelectrode structure behave as ultramicroelectrodes (UME) and can further enhance solardriven OER by reducing concentration overpotential.26 Note that a similar p-Si photoelectrode structure with an anodic SiO2 layer and Pt and Rh nanoparticles has been reported for the hydrogen evolution reaction, but its stability during HER or OER has not been investigated.33,34 Figure 1b shows a plan-view scanning electron microscope (SEM) image of a p+n-Si photoanode with SiO2 film and micropatterned Pt catalyst patches. The dimensions of the Pt micropatches are 6 × 6 μm2 in size, separated by 5.5 μm gaps, resulting in total Pt surface coverage of 27%. Additional photoanodes having catalyst surface coverages ranging from 17 to 41% were also prepared by controlling patch separation (see Figure S1 in the Supporting Information). Figure 1c shows a tilted SEM image of the corresponding Si photoanode: the silicon surface is completely covered by SiO2 and Pt layers. 3.2. Corrosion in Micropatterned SiO2 Passivated Si Photoanode during OER. Figure 2a shows the PEC current density−voltage (j−V) curves of our Si photoanode structure, passivated with thermal SiO2 film and with a 27% total Pt micropatch surface coverage in 1 M NaOH solution under simulated 1 sun illumination. Pt was used as a model OER catalyst due to its chemical stability in a strong basic electrolyte, despite its poor OER catalytic property. For comparison, a Si photoanode with Pt micropatches and without a SiO 2 passivation layer was also investigated. As shown in Figure 2a, both Si electrodes exhibit almost identical PEC OER performances, such as onset potential of about 1.2 V vs RHE and reduction of overpotential of 350−400

Figure 1. Structure of a passivated Si photoanode with an insulting dielectric layer and micropatterned catalysts. (a) Schematic diagram, (b) plan-view, and (c) 15°-tilted view scanning electron microscope (SEM) images of an SiO2-passivated Si photoanode with micropatterned Pt catalysts. The total coverage of the Pt microcatalysts is 27%.

In our Si photoanode structure, photoexcited electron−hole pairs are generated in the silicon beneath the SiO2 layer upon

Figure 2. Photoelectrochemical (PEC) performance and stability test of the p+n-Si photoanode with micropatterned Pt catalysts with and without SiO2. (a) PEC j−V curves of a Si photoanode passivated with (black) and without (red) an SiO2 layer for the oxygen evolution reaction (OER). The total coverage of micropatterned Pt is 27%. The electrochemical OER j−V curve of a continuous Pt film (i.e., 100% coverage) on a p++-Si substrate is also shown in pink. (b) Chronoamperometry of passivated and unpassivated Si photoanodes in 1 M NaOH (pH 13.3) under simulated 1 sun illumination. Constant voltage of 2.48 V (vs RHE) was applied to both Si photoanodes. (c) Plan-view and (d) 15°-tilted view SEM images of the unpassivated p+n-Si photoanode with Pt micropatterned catalysts after 24 h chronoamperometry. (e) Plan-view and (f) 15°-tilted view SEM images of the passivated p+n-Si photoanode with Pt micropatterned catalysts after 24 h stability test. The white arrow in (f) indicates locally dissolved Si at/ near the edge of Pt microcatalysts after the chronoamperometry. C

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The Journal of Physical Chemistry C mV, compared to a Pt film on heavily doped p+-Si. This suggests that in our Si photoanode charge separation occurs via the buried p−n junction, not by a semiconductor/electrolyte junction, since SiO2 in contact with an electrolyte does not alter PEC j−V curves. Without Pt, the oxidation of p+n-Si occurs at a potential as low as 0.1 V vs RHE with negligible photocurrent in the same condition (see Figure S2). Notably the passivated Si photoelectrode produces slightly higher maximum photocurrent density, as shown in Figure 2a, despite that both Si electrodes are anticipated to produce a similar maximum photocurrent density due to similar Pt coverage. The slightly higher photocurrent density of the Si photoelectrode with SiO2 is attributed partly to the refractive index difference between water/SiO2 and the water/Si interface. The refractive index of water, SiO2, and Si are 1.33, 1.45, and 3.5 in the visible range, respectively.35,36 In order to examine the role of SiO2 passivation on the stabilization of our Si photoanode structure during OER, structural investigation of the Si electrodes was conducted after chronoamperometry at a maximum photocurrent density in 1 M NaOH for 24 h under illumination. As shown in Figure 2b, both the Si photoanodes with and without the SiO2 passivation layer operated continuously for 24 h without noticeable photocurrent degradation under highly oxidative potential (i.e., 2.48 V vs RHE) in 1 M NaOH. The photocurrent fluctuation of the Si photoanodes in Figure 2b could be attributed to the O2 bubble evolution and detachment at the electrode. Difference in hydrophilicity between SiO2 and Si could result in larger current fluctuation of the SiO2-passivated Si photoelectrode. However, an SEM investigation of the Si photoanodes revealed that the Si photoanode without SiO2 film did not prevent corrosion of the exposed Si surface during OER, as shown in Figure 2c,d: about 200 nm of Si outside the Pt micropatches was dissolved (Figure 2d), and some Pt patches were detached from the Si surface (Figure 2c) after the 24 h stability test in 1 M NaOH. This delamination of Pt patches can enhance light absorption in the Si electrode by exposing the bare Si surface and can result in slight increase of photocurrent density of the unpassivated Si photoanode during the stability test for 24 h, as shown in Figure 2b. Note that most of the separated photoexcited holes were still collected to the Pt micropatches for OER due to the low Faraday efficiency of the silicon anodic etching (see detailed information in the Supporting Information). In addition, the exposed Si surface was more quickly etched in the dark, which corresponds to half of the duty cycle in an outdoor solar water splitting test (see Figure S4). The lower dissolution of the unpassivated Si during OER under illumination is believed to originate, partly, from the local reduction of hydroxyl ion concentration at the Si photoanode by consumption for water oxidation reaction. Si dissolution in an alkaline solution is known to occur by oxidation of Si with hydroxyl ions to silicate which further reacts with hydroxyl ions to form water-soluble silicate complex.37 In contrast, the Si photoanode passivated by a SiO2 film and Pt micropatches operated without degradation or delamination of the Pt patches for 24 h under the same conditions, as shown in Figure 2b,e. A cross-sectional SEM image obtained after the 24 h stability test, shown in Figure 2f, indicates that the SiO2 layer did stabilize the silicon outside the Pt micropatch against corrosion. However, Figure 2f also indicates local dissolution of silicon beneath the edge of the Pt patches. This local dissolution of Si could deteriorate the stable operation of the passivated Si photoanode after prolonged OER

time (>24 h). We believe that this local dissolution of Si under the edge of the Pt micropatches occurs by penetration of the aggressive solution through the interface/gap between the SiO2 and Pt patches, or through pinholes in the Pt, whose thickness becomes thinner toward the edge. Therefore, in order to enhance the stability of the passivated Si photoanode, modification of the photoelectrode structure was required to eliminate local Si dissolution paths. 3.3. Overlap of Catalysts on Passivation Layer To Eliminate Corrosion. Figure 3 shows a Si photoanode

Figure 3. Modified SiO2-passivated p+n-Si photoanode with Ni microcatalysts for enhanced PEC performance and stability during OER. (a) Schematic diagram of the local dissolution mechanism in the Si photoanode with the default microcatalysts, and the Si photoanode with the modified Ni OER microcatalysts, employed to eliminate the dissolution paths. (b) Low-magnification 15°-tilted view SEM image of the modified Si photoanodes with Ni microcatalysts. (c) Enlarged cross-sectional view SEM image showing the continuous coverage of the Ni microcatalyst at the SiO2/Si opening.

modified and passivated for enhanced stability as well as for OER efficiency under illumination. In the modified Si photoanode structure, the local Si dissolution paths under the edge of the OER patches were removed by extending the catalyst patch over the SiO2 layer outside the Si openings about a few micrometers, as shown in Figure 3a. With this modified photoelectrode structure, an aggressive electrolyte cannot penetrate into the Si surface since the catalyst patches perfectly cover the vulnerable catalyst patch edges at/near the SiO2−Si contact lines. Figure 3b shows a tilted-view SEM image of the Si D

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was protected by eliminating the dissolution paths with the modified catalyst patch structure. The plan-view SEM image also confirms undamaged electrode structures after the stability test (see Figure S5). Note in the inset that Ni(OH)2 was formed on the Ni micropatch after the stability test (see X-ray photoelectron spectroscopy (XPS) in Supporting Information). Also, it should be noted that the stability of our photoanode can be ultimately limited to the dissolution of thermal SiO2 in 1 M NaOH, although we did not observe noticeable degradation of our Si photoanode during the 24 h stability test. We also compared the j−V curves of the Si photoanode with the Ni micropatches before and after the stability test in 1 M NaOH. As shown in Figure 4b, our modified Si photoanode with Ni of 27.5 mA cm−2 and an onset of 1 V vs RHE before the stability test: the reduction of overpotential by more than 400 mV compared to the photoanode with the Pt catalyst originates from the higher OER catalytic activity of Ni. Interestingly, the j−V curves continuously improve in the cathodic direction during the stability test, reaching the maximum photocurrent density of 27 mA cm−2 at the water oxidation potential. This can be attributed partly to the increased catalyst area resulting from Ni(OH)2 on the Ni. It must be noted that the maximum current density in Figure 4b is slightly higher than that in Figure 2a due to the increased light absorption in Si after the reduction in total catalyst coverage from 27% (Figure 2a) to 20% (Figure 4b). 3.4. Device Modeling of a p+n-Si Photoanode with Micropatterned Electrocatalysts. The coverage of metal cocatalysts on a photoelectrode can significantly influence the PEC performance of the photoelectrode by restraining light absorption in the underlying photoabsorbers and by providing active sites for electrocatalysis. In order to quantitatively investigate the impact of the coverage of the electrocatalysts on PEC performance, we prepared p+-n Si photoanodes with various Pt coverages. Figure 5a shows the j−V curves of Si photoanodes with Pt coverages of 17, 27, and 41% in 1 M NaOH under simulated 1 sun illumination. As shown in Figure 5a, the Si photoanodes with higher Pt coverage produce smaller OER current density since light absorption in the Si is reduced due to increased shadowing from the Pt catalysts. For example, the Si photoanode with 41% Pt coverage produced about 36% less photocurrent density than the photoanode with 17% Pt coverage. Note that from the Pt coverage ratio only 29% photocurrent loss is expected. We attribute this slightly higher photocurrent loss than the Pt coverage ratio, partly, to the increased surface recombination at the Si/Pt interface. At the same time, the onset potential for the OER decreased for higher Pt coverage due to the increased number of active sites for water oxidation. Note also that the onset potential of the Si photoelectrode was further reduced by using Ni, which is a better OER catalyst than Pt, as can be seen in Figure 4b. This indicates that a careful PEC cell design considering cocatalyst coverage and catalytic properties is required to maximize light absorption and minimize the overpotential required for solardriven electrolysis. Recently, Lewis et al. performed a quantitative analysis of the efficiency of tandem water splitting cells with Pt microcatalysts for the hydrogen evolution reaction and concluded that Pt coverage of about 1% could provide the maximum solar-to-hydrogen (STH) efficiency due to the tradeoff between shadowing and electrochemical overpotential.26 However, the analysis by Lewis et al. only considered light absorption in an absorber restrained by bandgap and shadowing

photoanode with the modified catalyst patch structure. This Si photoanode was fabricated by first patterning SiO2 to expose Si openings by photolithography and RIE, and then the modified catalyst patches were formed by additional photolithography and lift-off. As can be seen in Figure 3c, Ni patches fully cover the SiO2 at/near the Si opening, eliminating the local Si dissolution paths. Note that to improve OER performance, we used Ni micropatches as OER catalysts since Ni is well-known to show high water oxidation catalytic activity in a highly basic aqueous solution.18,38−40 40 nm thick Ti was used to promote the adhesion of the Ni on Si and SiO2. Figure 4 demonstrates the enhanced stability and performance of our modified silicon photoanode for water oxidation

Figure 4. Enhanced PEC performance and stability of the modified SiO2-passivated p+n-Si photoanode with micropatterned Ni catalysts. (a) Chronoamperometry of the modified Si photoanodes in 1 M NaOH under simulated 1 sun illumination. Constant voltage of 1.78 V (vs RHE) is applied. The coverage of Ni microcatalysts is 20%. Inset shows a cross-sectional SEM image of the p+n-Si photoanode with SiO2 layer and modified micropatterned Ni catalyst after the stability test. After the stability test, a Ni(OH)2 layer was observed on top of the Ni. (b) PEC OER j−V curves of the modified p+n-Si photoanode with Ni microcatalysts before and after the 24 h stability test in 1 M NaOH under simulated 1 sun illumination. The electrochemical OER j−V curve of a continuous Ni film (i.e., 100% coverage) on a p++-Si substrate is also shown in green.

reaction in 1 M NaOH in simulated 1 sun illumination. As shown in Figure 4a, our Si photoanode with a total Ni coverage of 20% does not exhibit photocurrent degradation after prolonged operation of 24 h at V = 1.78 V vs RHE in 1 M NaOH, using chronoamperometry. The fluctuation of the photocurrent in Figure 4a is due to the oxygen bubble evolution on the electrode surface. The cross-sectional SEM image obtained after the 24 h stability test, shown in the inset of Figure 4a, confirms the Si at/near the edge of the Si opening E

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Figure 5. Design principles of a buried junction Si photoanode with OER microcatalysts for high performance solar-driven water splitting reaction. (a) PEC j−V curves of p+n-Si photoanodes with a SiO2 layer and Pt microcatalysts with various coverages in 1 M NaOH in the dark (dotted line) and under simulated 1 sun illumination (solid line). Pt coverages of the Si photoanodes are 17% (blue), 27% (black), and 41% (red). The electrochemical OER j−V curve of a continuous Pt film (i.e., 100% coverage) on a p++-Si substrate is also shown in pink. The inset shows the enlarged PEC j−V curves of the Si photoanodes around onset potential; the calculated PEC j−V curves of the Si photoanodes with micropatterned electrocatalysts (b) with various exchange current densities j0 from 10−3 to 1 mA cm−2 and a fixed charge transfer coefficient α of 0.5 and (c) with various α from 0.35 to 0.65 and fixed j0 of 0.1 mA cm−2. Electrocatalyst coverages of 17, 27, and 41% are used in the device modeling. (d) Calculated solar-driven OER half-cell efficiency of buried junction Si photoanodes with microcatalysts with various j0 and α as a function of catalysts’ coverage.

electrocatalysts with different j0 and α. As indicated in Figure 5a−c, our device modeling successfully reproduces the catalystcoverage-dependent PEC j−V behaviors, such as increased photocurrent density and overpotential, with decreasing electrocatalyst coverage. More importantly, our device modeling indicates that the photoanode’s PEC OER performance is dependent on not only the coverage of the catalysts but also (even more strongly) their electrochemical catalytic properties. For example, as the j0 of a electrocatalyst increases from 10−3 to 1 mA cm−2, the overpotential of a Si photoanode is reduced to about 400 mV with the given photovoltaic property of the Si photoanode, as shown in Figure 5b. In addition, as shown in Figure 5c, the decrease of α from 0.65 to 0.35 results in a reduction of overpotential of about 300 mV at a photocurrent above ∼2 mA cm−2 for an electrocatalyst with j0 of 0.1 mA cm−2. An α of 0.35 corresponds to that of the noble metal catalysts, such as IrO2 and RuO2 in acids (e.g., α = 0.35 is equal to the Tafel slope of 40 mV dec−1), and an α of 0.65 is similar to that of a Ni catalyst in alkaline electrolytes.38,41,42 Note that the onset potentials of the Si photoanodes with various αs are nearly identical when the j0 of the electrocatalysts is fixed. The j0 of an electrocatalyst determines the onset potential of the photoelectrode, while α governs the overall resistance of photoelectrolysis at low water oxidation current density. Moreover, the catalyst-coverage dependence on the reduction of overpotential is more pronounced for an electrocatalyst with higher α, while it is nearly identical for various j0s. Therefore, the PEC performance of a buried p−n junction photoanode with patterned OEC catalysts, i.e., an integrated PV−electro-

from the Pt microislands. In addition, kinetic modeling was not presented when constructing the j−V relationship of the photoelectrode with electrocatalysts having various catalytic characteristics, such as the exchange current density j0 and the charge transfer coefficient α which is closely related to the Tafel slope, as a function of the coverage of electrocatalysts. Here, we present a device modeling of a buried p−n junction photoanode with electrocatalysts having various coverages and catalytic characteristics to provide the design principles for a photoanode with optimized PEC OER performance. This modeling was established by considering the current matching of a series-connected OER electrolyzer, i.e., metal micropatches in our case, atop a photovoltaic device, i.e., a p+−n buried junction Si photoanode in our case. Then, the PEC j−V curves is found from the independent j−V curves of an electrolyzer and a PV device and identifying the overpotential η applied to the OER electrolyzer under illumination in a series-connected current matching device (see Figure S7). The overpotential η applied to the OER electrolyzer is provided by the external bias Vapp to the rear contact of a Si PV cell as well as the photovoltage generated in a Si PV cell Vpv, such that η = (Vapp + Vpv) − E o

(1)

In the modeling, an ideal PV cell with an open circuit voltage Voc of 550 mV was used, and nonidealities such as shunt and series resistances were not considered (see detailed information for the device modeling in the Supporting Information). Shown in Figure 5b,c are the calculated j−V curves of a buried junction Si photoanode with various coverages of F

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The Journal of Physical Chemistry C lyzer, is strongly correlated with the coverage, j0 and α of an electrocatalyst as well as the PV properties of the cell. In order to find the optimum configuration in our Si photoanode with various combinations of electrocatalysts and coverages, we defined the hypothetical efficiency ηhalf of a photoanode for an OER half-cell reaction under 1 sun illumination by ηhalf (%) =

(1.23V − Vop)jop P1 sun

× 100 (%)

our Si photoanode structure achieved a photocurrent density of 27 mA cm−2 at water oxidation potential and stability during more than 24 h in 1 M NaOH under 1 sun illumination. In addition, we have presented device modeling of an integrated PV−electrolyzer cell in which OER catalysts are patterned in various coverages. Our analysis identified the effect of the catalyst’s characteristics on the PEC j−V curves of the cell: the j0 and α of the catalysts dominate the onset potential and overall resistance of the water-splitting reaction, respectively. Our device modeling can provide the design principles for a high performance and low-cost integrated PV−electrolyzer cell, involving the careful design of catalysts’ coverage with high j0 and low α. Finally, our Si photoelectrode structure and device modeling can be applied to highly efficient and stable photoelectrodes for other solar-driven reactions, such as hydrogen evolution reaction and CO2 reduction.

(2)

where Vop and jop are the operating potential (V vs RHE) and the operating current density at maximum efficiency condition, respectively. P1 sun is the incident power under 1 sun illumination which is 100 mW cm−2. It must be noted that ηhalf is defined to diagnose the relative PEC performance of each of the cell configurations and does not indicate the overall solar-to-hydrogen (STH) efficiency. The STH efficiency must be measured using a two electrode configuration without applying external bias.43 Figure 5d shows the ηhalf of the Si photoanodes as a function of the coverage of catalysts with various electrocatalytic parameters. As can be seen in Figure 5d, the trade-off between the shadowing and overpotential of patterned OER catalysts on a Si photoanode results in the optimum catalysts’ coverage for maximum ηhalf. In addition, the optimum catalyst coverage for the maximum ηhalf decreases with increasing j0 and decreasing α, reaching the highest ηhalf of about 7.5% in our modeling. It is worth noting that only 10% of catalyst coverage is needed to obtain the maximum efficiency when a high performance electrocatalyst with j0 of 1 mA cm−2 and α of 0.35 is used. Since high performance catalysts are often costly noble metals,44,45 the low optimum coverage predicted by this modeling suggests the cost of an integrated PV−electrolyzer cell can be reduced. This is consistent with the conclusion of the analysis for a tandem PEC cell with Pt cocatalysts by the Lewis group.26 In addition, interestingly, our modeling indicates that a buried junction photoanode with a catalyst with moderate j0 and with excellent α could have a comparable or even better ηhalf than that with a catalyst with high j0 but with poor α. For instance, a cell in conjunction with a catalyst with a j0 of 0.1 mA cm−2 and α of 0.35 exhibits an absolute 1% higher ηhalf than that with a j0 of 1 mA cm−2 and α of 0.65, as shown in Figure 5d. A similar argument is also suggested by considering the overpotential of various electrocatalysts at low current density from simple current−potential curves.1 Therefore, the analysis based on our device modeling can provide design rules for high performance and low cost integrated PV−electrolyzer cells for water-splitting reactions, by reflecting the complicated interplay between light absorption and kinetic overpotential, although our analysis is based on ideal PV and electrocatalysis behaviors. For a more precise analysis, the effect of parasitic resistances of the PV cells and the difference of concentration overpotential between patterned and film catalysts26 will be incorporated in our device modeling in the future.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.5b10859. Faraday efficiency calculation of Si anodic etching under water oxidation, etching characteristics of p+n-Si in high pH in dark, XPS data of Ni microcatalyst, equivalent circuit of series-connected current matching PEC device, detailed device modeling, and additional SEM image (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (J.O.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge Dr. Woo Lee at Korea Research Institute of Standards (KRISS) and Professor Yeon Sik Jung at Korea Advanced Institute of Science and Technology (KAIST) for use of these facilities and Dr. Howard M. Branz at Branz Technology and Partners for helpful discussions. This work was supported by the Korea CCS R&D Center (KCRC) grant funded by the Korea government (Ministry of Education, Science and Technology) (No. NRF-2014M1A8A1049303).

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REFERENCES

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