Stable Unassisted Solar Water Splitting on Semiconductor

http://pubs.acs.org/journal/aelccp

Stable Unassisted Solar Water Splitting on Semiconductor Photocathodes Protected by Multifunctional GaN Nanostructures Yongjie Wang,† Jonathan Schwartz,‡ Jiseok Gim,‡ Robert Hovden,‡ and Zetian Mi*,†

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Department of Electrical Engineering and Computer Science, University of Michigan, 1301 Beal Avenue, Ann Arbor, Michigan 48109, United States ‡ Department of Materials Science and Engineering, University of Michigan, 2300 Hayward Street, Ann Arbor, Michigan 48109, United States S Supporting Information *

ABSTRACT: Producing hydrogen by unassisted solar water splitting is one essential step to make direct solar fuel conversion a viable energy source. To date, however, there has been no demonstration of stable photoelectrodes for high-efficiency photoelectrochemical water splitting. In this work, we report that a GaInP2/GaAs/Ge triple-junction (3J) photocathode protected by multifunctional GaN nanostructures can enable both efficient and relatively stable solar water splitting. A 12.6% solar-to-hydrogen (STH) efficiency is measured without any external bias. Of particular importance, we demonstrate relatively stable solar water splitting for 80 h in three-electrode configuration and 57 h in twoelectrode measurement at zero bias. This is the best reported stability for multijunction III-V semiconductor photocathodes in two-electrode configuration to our knowledge. The multifunctional GaN nanostructure significantly reduces the charge transfer resistance at the semiconductor/electrolyte interface and protects III−V materials against corrosion. Such multifunctional GaN photocatalytic nanostructures provide a new pathway to improve the performance of conventional photoelectrodes to achieve both efficient and stable unassisted solar water splitting.

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solar water splitting both ef f iciently and stably has remained one of the Holy Grails in solar fuels and artificial photosynthesis.13,14 Stabilizing the surfaces of III−V semiconductor photocathodes by employing an extra protection layer has been intensively studied as a potential alternative to address these critical challenges.15−17 The surface protection layer should exhibit extreme chemical stability and resistance to photocorrosion in harsh photocatalysis environments.18 It is also critical that the protection layer can cover the semiconductor surface conformally, be synthesized with precise chemical composition and electronic properties, and be manufactured at a large scale.19 To maintain high efficiency, it is essential that the surface passivation material possesses a large bandgap to have negligible absorption of solar photons.20 It must also have a small or negligible conduction band offset with the underlying semiconductor light absorber for the extraction of

eveloping efficient unassisted solar water splitting devices, with long-term stability, is one critical step to produce solar fuel by directly converting solar energy to chemicals without an electrical bias.1−4 The resulting hydrogen fuel can be generated directly on site, readily stored, and distributed to meet the increasing energy demand. Solar water splitting is also necessary for the artificial photosynthesis conversion of CO2 to hydrocarbon fuels,5,6 which has the potential to address environmental challenges such as replacing conventional fossil fuels. Recently, relatively high solar-tohydrogen (STH) efficiencies have been demonstrated on III− V semiconductor photocathodes, e.g., ∼19% for a TiO2/AlInPprotected GaInP/GaInAs double-junction,7 ∼16% for inverted metamorphic GaInP/AlInP and GaInP/GaInAs doublejunctions,8 and ∼13% for a metal-protected GaInP/GaInAs/ Ge triple-junction (3J).9 However, these photoelectrodes typically have a short lifetime due to detrimental corrosion in a harsh water splitting environment,7−9 preventing any practical application. While recent reports on semiconductor photoelectrodes may have shown improved stability,10−12 these devices generally have extremely low efficiency. Achieving semiconductor photoelectrodes that can drive © 2019 American Chemical Society

Received: March 13, 2019 Accepted: June 3, 2019 Published: June 3, 2019 1541

DOI: 10.1021/acsenergylett.9b00549 ACS Energy Lett. 2019, 4, 1541−1548

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Cite This: ACS Energy Lett. 2019, 4, 1541−1548

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ACS Energy Letters

Figure 1. Design of a GaN/3J photocathode for stable and efficient solar water splitting. (a) Schematic energy band diagram showing the efficient extraction of photogenerated charge carriers (electrons) for proton reduction by the GaN nanostructures. (b) Schematic illustration of the GaInP2/GaAs/Ge 3J structure protected by multifunctional GaN nanostructures.

protect the surface of photocathodes.20 A GaN protection layer can induce negligible absorption of sunlight due to its large bandgap (∼3.4 eV).37 Studies have shown that GaN nanostructures grown by molecular beam epitaxy (MBE) can exhibit N-terminated surfaces to make it resistant against oxidation and photocorrosion.16,20,38 For example, stable photocatalytic solar water splitting (>500 h) has been demonstrated on InGaN nanowire arrays without any extra surface protection.38 The relatively small conduction band offset between GaN and commercial photocathodes, e.g., Si, GaAs, and InGaP semiconductors,20,39−41 enables efficient charge carrier (electron) extraction. On the other hand, the deep valence band of GaN can effectively block photogenerated holes and suppress surface recombination.42 Industry standard manufacturing processes such as metal− organic chemical vapor deposition (MOCVD) and MBE can grow crystalline GaN while controlling its structural, electronic, and optical properties with high precision.43 To date, however, there has been little to no studies of GaN protection layers on conventional III−V photocathodes. One challenge in growing GaN is due to the difference in crystal structures with III−V materials (wurtzite and zinc-blende, respectively). Furthermore, GaN is often synthesized at temperatures (∼1000 °C) that would easily damage the underlying III−V substrate.44,45

photogenerated charge carriers (electrons). To date, there has been no demonstration of any surface protection layer that can simultaneously meet these essential requirements.21−23 Surface protection schemes, including the use of various metal oxides,10,15,24−29 metal contact films,30−32 and two-dimensional (2D) transition metal dichalcogenides (TMDs) such as MoS2 or MoSe2,12,33,34 have been intensively studied but have shown limited success. Although improved stability has been reported for various surface protection schemes, e.g., 100 h for TiO2-protected Si,10 8 days for metal-passivated GaAs,32 and 70 h for MoS2-protected GaInP2,12 these measurements were performed only in a three-electrode configuration. The threeelectrode configuration, however, does not accurately simulate the conditions in unbiased solar water splitting and therefore is largely irrelevant for stability analysis. To assess the stability for practical solar hydrogen production, the two-electrode configuration35 should be utilized. Current high-efficiency III−V semiconductor photoelectrodes can only yield 50 h in a two-electrode configuration, which is much better than that for previously reported III−V photocathodes, e.g., 50 h unassisted solar water splitting.

transport through GaN nanostructures and enormous smallsize Pt cocatalyst nanoparticles on the textured GaN surface. It is important to note that Pt nanoparticles play a key role in catalyzing water reduction reaction efficiently, see the Supplementary Experimental Section, and such GaN nanostructures provide enormous catalytic sites for Pt deposition that significantly facilitate water reduction reaction of platinized GaN/3J samples. Considering the small photovoltage of ∼0.3 V provided by the bottom Ge junction,59−61 compared to the large photo voltage in the presented device, we can reasonably conclude that a GaN-protected GaInP2/GaAs double-junction tandem structure can drive unassisted solar water splitting with significantly improved (light-limited) efficiency. In addition, the use of RuOx, IrOx, or other highperformance counter electrodes can lead to improved performance.11,37,62−64 The pivotal role of epitaxial GaN nanostructures is to protect the buried 3J structure against corrosion, which has remained challenging for III−V photoelectrodes.7 Previous reports have shown that MBE-grown GaN nanostructures are stable in harsh solar water splitting conditions due to the formation of N-terminated surfaces.16,20 Figure 4d shows that without a GaN passivation layer, significant corrosion of the bare III−V 3J occurs in a short time. Moreover, the measured fill factor and onset potential are much worse than those using a GaN passivation layer. This further demonstrates the multifunctional properties of GaN nanostructures to achieve efficient electron transport and enhance catalytic proton reduction. The reduced charge transfer resistance is attributed to the large surface area of the GaN nanostructure and the

ABPE > 13% in this work is among the best reported stability for multijunction III−V semiconductor photocathodes that can drive unassisted solar water splitting, compared to 10 h for inverted metamorphic TiO2-protected GaInP/GaInAs,8 16 h for AlInPOx-passivated AlInP/GaInP/GaInAs,9 and 50 h for TiO2-protected AlInP/GaInP/GaInAs7 (see Table S1). The decreasing current during the long-term stability test also leads to a reduction of the maximum ABPE. While most stability studies for photoelectrochemical water splitting are performed in a three-electrode configuration, it is important to note that these measurements do not accurately represent the conditions for true unbiased solar water splitting. Stability measurements under an unbiased two-electrode configuration are necessary to precisely evaluate the longterm stability of photoelectrodes. In this regard, we tested the monolithic GaN/3J photocathode in a two-electrode configuration versus a Pt counter electrode in 0.1 M H2SO4 electrolyte solution under AM 1.5G 1 sun illumination, illustrated in Figure 4a (see the Supplementary Experimental Section). Spontaneous H2 and O2 gas generation is clearly observed from the working and counter electrodes, respectively (see Movie S1). LSV measurement of the GaN/3J photocathode under chopped illumination is plotted in Figure 4b, which shows a light-limiting photocurrent at zero bias. The measured photocurrent in LSV measurements originates from solar hydrogen conversion with a negligible dark current. This can be further confirmed in Figure 4c, at zero bias in a twoelectrode configuration, corresponding to an STH efficiency ∼12.6% (Note S2). Important factors that contribute to the relatively high STH efficiency include efficient electron 1545

DOI: 10.1021/acsenergylett.9b00549 ACS Energy Lett. 2019, 4, 1541−1548

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ACS Energy Letters small conduction band offset between GaN and GaInP2. We have also investigated the effect of GaN thickness on the performance of the GaN/3J photocathode (see Supplementary Experimental Section). It was observed that a very thin GaN layer may not effectively protect the GaInP2 surface, whereas a too thick GaN layer leads to a worse fill factor. The degraded performance of the 3J sample with thick GaN is possibly due to increased charge carrier trapping and resistance. The growth conditions could be further optimized for relatively thick GaN to achieve a better fill factor, which can potentially lead to better stability without compromising efficiency. In addition, the deposition of Pt cocatalyst nanoparticles needs further optimization to achieve potentially higher efficiency. H2 production through unassisted solar water splitting on the GaN/3J photocathode is evaluated at zero bias in the twoelectrode configuration (see Movie S1). Shown in Figure 4e is the course of H2 gas production (red dot) vs time, compared to its theoretical value (solid red line) calculated from the number of photogenerated electrons flowing through the circuit (Note S3). It is seen that the measured and calculated hydrogen gas production agree fairly well with a nearly unity Faradaic efficiency. Long-term stability evaluation of the GaN/ 3J photocathode for unassisted solar water splitting was further performed in a two-electrode configuration, shown in Figure 4f. The GaN/3J photocathode can operate stably for unassisted solar water splitting at >10% STH efficiency for >50 h. Such long-term stability for unassisted solar water splitting, i.e., measured at zero bias in two-electrode configuration, has not been reported previously (see Tables S1 and S3). The 10% STH efficiency was suggested to be commercially viable,65 if long-term stability can be maintained. For comparison, the recently reported TiO 2-protected GaInP2/GaAs photocathode exhibits ∼19% STH efficiency but can drive unassisted solar water splitting for only ∼100 min.7 Figure 5 and Table S3 compare the STH efficiency and stability of previously reported high-efficiency photocathodes for unassisted solar water splitting. In particular, previously reported GaInP2/GaAs tandem structures were compared to our GaN/3J photocathode. To date, there has been no demonstration of semiconductor photocathodes with a STH efficiency of >10% and stability of >50 h, even by depositing an extra surface protection layer (see Table S3). A TiO2 thin film can protect photoelectrodes for >100 h in three-electrode measurements15,25,66 but not for unassisted photoelectrochemical solar water splitting devices operated in a two-electrode configuration, which is required for any practical application. The important advantages of using GaN nanostructures as a surface protection layer include (1) a large bandgap of ∼3.4 eV to have negligible light absorption, (2) negligible conduction band offset between GaN and GaInP2 surfaces for efficient electron transfer, (3) a large surface area for Pt cocatalyst deposition driving efficient proton reduction reaction, (4) extreme chemical stability due to the N-terminated surface, (5) nontoxic, (6) controlled and scalable epitaxial growth, and (7) earth-abundant and large-scale manufacturability. In summary, we report on the demonstration of a GaN/3J photocathode for efficient, stable solar water splitting by using multifunctional GaN nanostructures as a novel surface protection layer. When measured in two-electrode configuration, the GaN/3J photocathode can exhibit an STH efficiency of ∼12.6% at zero bias. Moreover, long-term stability has been demonstrated on a GaN-protected 3J photocathode

Figure 5. Performance comparison of STH efficiency and twoelectrode stability for unassisted solar water splitting for some previously reported high-efficiency photocathodes compared to this work. For literature [3], the two-electrode stability test was not reported.

for >50 h in two-electrode configuration with an STH efficiency of >10%. Such multifunctional GaN nanostructures can be monolithically integrated on commercial GaInP2/ GaInAs tandem structures to potentially achieve a semiconductor photocathode with STH efficiency > 20% for stable unassisted solar water splitting 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/acsenergylett.9b00549. Detailed experimental section, flat band diagram, photovoltaic measurement of the 3J wafer, SEM, EDX, and photoluminescence of the GaN/3J sample, efficiency calculations, and performance comparison tables (PDF) Video of spontaneous H2 and O2 gas generation observed from the working and counter electrodes (MP4)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Yongjie Wang: 0000-0002-9268-837X Zetian Mi: 0000-0001-9494-7390 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge research support from the HydroGEN Advanced Water Splitting Materials Consortium, established as part of the Energy Materials Network under the 1546

DOI: 10.1021/acsenergylett.9b00549 ACS Energy Lett. 2019, 4, 1541−1548

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U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy, Fuel Cell Technologies Office, under Award Number DE-EE0008086, the National Science Foundation (Grant CBET 1804458), and Emissions Reduction Alberta. R.H. acknowledges support from the DOE Office of Science (DE-SC0011385). This work was performed in part at the University of Michigan Lurie Nanofabrication Facility. The authors acknowledge financial support from the University of Michigan College of Engineering and technical support from the Michigan Center for Materials Characterization.

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DOI: 10.1021/acsenergylett.9b00549 ACS Energy Lett. 2019, 4, 1541−1548