Making of An Industry-Friendly Artificial Photosynthesis Device - ACS

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Making of An Industry-Friendly Artificial Photosynthesis Device Xiangjiu Guan, Faqrul Alam Chowdhury, Yongjie Wang, Nick Pant, Srinivas Vanka, Michel L. Trudeau, Liejin Guo, Lionel Vayssieres, and Zetian Mi ACS Energy Lett., Just Accepted Manuscript • DOI: 10.1021/acsenergylett.8b01377 • Publication Date (Web): 21 Aug 2018 Downloaded from http://pubs.acs.org on August 23, 2018

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ACS Energy Letters

Making of An Industry-Friendly Artificial Photosynthesis Device Xiangjiu Guan1,2 #, Faqrul A. Chowdhury1 #, Yongjie Wang3, Nick Pant1,3, Srinivas Vanka1,3, Michel L. Trudeau4, Liejin Guo2, Lionel Vayssieres2, and Zetian Mi1,3* 1

Department of Electrical and Computer Engineering, McGill University, 3480 University Street, Montreal, Québec H3A 0E9, Canada

2

International Research Center for Renewable Energy, State Key Laboratory of Multiphase Flow

in Power Engineering, Xi'an Jiaotong University, 28 Xianning West Road, Xi’an 710049, China 3

Department of Electrical Engineering and Computer Science, University of Michigan, Ann Arbor, MI 48109, USA

4

Center of Excellence in Transportation Electrification and Energy Storage (CETEES), HydroQuebce, 1806 Boul. Lionel Boulet, Varennes, Québec J3X 1S1, Canada #

These authors contributed equally to this work.

Corresponding Author E-mail: [email protected]

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ABSTRACT:

The distinct, as well as the synergistic effect of the water oxidation co-catalyst (Co3O4), the proton reduction co-catalyst (Rh/Cr2O3), and the optimum surface properties of Ga(In)N nanowire-arrays can significantly enhance the photocatalytic performance (STH ~2.7%) and long-term stability (>580 hours) of bias-free overall pure water splitting.

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ACS Energy Letters

Hydrogen generation from the two most abundant free resources available on earth, i.e. sunlight and water via photocatalytic water splitting, is a very appealing approach for the crucial societal transition to a clean and sustainable energy resource future1-3. Numerous semiconductor catalysts have been investigated during the last several decades but still suffer from low efficiency, poor long-term stability and are not consisting of industry-friendly compounds and processing techniques4,5. High-efficiency devices for photovoltaic-assisted photoelectrochemical water splitting6 and electrolysers7 had been reported in conductive electrolytes with selective pH adjustments. However, direct splitting of pure or sea water with significantly enhanced devicelongevity at concentrated sunlight holds enormous promise for hydrogen generation at pH neutral condition without any external bias, sacrificial reagent or conductive electrolytes. Here, we report on an artificial photosynthetic device using standard materials widely used in the industry, i.e. gallium nitride (GaN) on silicon wafers, which produces hydrogen efficiently by directly splitting pure water with long-term stable operation (584 hours, under concentrated illumination at 27 Suns) without applied bias or sacrificial agents. Each year, over 10,000 million square inches (or 6.5 million square meters) of Si wafers are produced, which largely drives the nearly US$3 trillion consumer electronics market. Similarly, GaN, which is the second most invested semiconductor material only next to Si, has been widely used industrially in solid-state lighting, blue/green laser diodes, and power electronic devices with a combined market value exceeding $100-billion. We have synthesized GaN nanostructures and its alloys, e.g. InGaN on commercial Si wafers, shown in Fig. 1a, which have up to 25% indium content and can thus very efficiently absorb UV, blue, and green parts of the sunlight8. The surfaces of the nanowires are engineered at the atomic-scale to be nitrogenrich to powerfully protect against photo-corrosion and oxidation even under concentrated


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irradiation and harsh aqueous environment9. The nanowires are doped p-type using Mg for optimum surface charge properties. Details about the design and growth are discussed in Supplementary Methods.

Figure 1. (a) A 45o-tilted scanning electron microscopy image of GaN/InGaN nanowire-arrays on a Si wafer (bottom). Scale bar, 500 nm. HRSTEM-ZC lattice fringe image from hyper-abrupt InGaN/GaN interface (top, scale bar 5 nm), illustrating defect-free single crystalline layers. (b) TEM image of a cocatalyst-decorated GaN nanowire, which depicts simultaneous loading (not optimized) of Rh/Cr2O3 and Co3O4 nanoparticles. Scale bar, 20 nm. The inset shows STEM-BF (bright field) and STEM-HAADF (high angle annular dark field) image of Rh/Cr2O3 (core-shell) nanostructures (top, scale bars 5 nm) and Co3O4 nanoparticles (bottom, scale bar 5 nm and 2 nm, respectively) on GaN nanowire surface. (c) The rate of H2 and O2 evolution in overall pure water splitting on Ga(In)N nanowires for single and dual-cocatalyst loading. The derived AQE and ECE is also elucidated. (d) Average rate of H2 and O2 production (7.14 and 3.54 mol h-1g-1, respectively) from each cycle during long-term repeated course of water splitting under concentrated sunlight illumination. To facilitate water splitting reaction, Co3O4 and Rh/Cr2O3 nanoparticles are deposited on the nanowire surfaces which serve as water oxidation and proton reduction co-catalysts, respectively. The chemical form of cobalt oxide was confirmed as Co3O4 using X-ray photoelectron spectroscopy (XPS) and high-resolution transmission electron microscopy


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ACS Energy Letters

(HRTEM) analysis (Fig. 1b). The dual-cocatalyst decorated double-band GaN/InGaN nanowires exhibit a solar-to-hydrogen (STH) conversion efficiency of ~2.7% for overall pure water splitting without bias or sacrificial reagent under concentrated sunlight on a sample with ~3 cm2 surface area. As shown in Fig. 1c, the apparent quantum efficiency (AQE) of the dual-cocatalyst loaded nanowires was boosted up to ~37% in the wavelength range of 200–490 nm with corresponding energy conversion efficiency (ECE) of ~14%, measured under full arc illumination with AM1.5G filter (see Supplementary Notes). With ~2.7% efficiency and using concentrated irradiance of 27 suns, the system can produce 7.14 mol h-1 of H2 gas per gram of photocatalyst (equivalent to 1.14 mmol h-1cm-2), and potentially 11.42 mol h-1 (equivalent to 256 L h-1 at STP) of H2 gas if a 1 m2 module can be successfully employed. Figure 1d illustrates the rates of hydrogen (H2) and oxygen (O2) evolution, showing practically stoichiometric (2:1) gas production of balanced overall neutral pH water splitting reaction. No performance degradation was observed in 584 hours of unassisted water splitting reaction in pure water. Considering the average roof in the United States has access to usable sunlight for nearly 5.5 hours10 per day, this artificial photosynthetic system can thus generate clean hydrogen under concentrated illumination for up to over 100 days so far and most likely much more as the efficiency did not drop when the stability test was concluded after 584 h. Significantly, the measurements were performed under concentrated sunlight (~27 suns), which is of utmost interest for economically viable industrial production of hydrogen using limited land area and employing minimum amount of photocatalyst1,11,12. Such remarkable long-term stability, which is attributed to the unique presence of inherent N-terminated surfaces9, is the longest ever measured for any semiconductor photocatalysts/photoelectrodes without protection/passivation layers in unassisted solar water


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splitting with STH >1%. The substantial enhancements in photocatalytic activity and devicelongevity reveal not only the crucial roles of band-flattening due to optimum Mg-dopant incorporation and high-intensity excitation, but more importantly, the synergistic/tandem effect along with the loading of both reductive and oxidative co-catalysts. The proximity of the nanowire photocatalysts, including the integrated nature of micro-/nanoelectrodes (co-catalysts) greatly eliminates the problem of ionic diffusion for balanced water splitting reaction. Loading of Co3O4 nanoparticles facilitate accumulation of photogenerated holes from the nanowire surface to simultaneously drive complex water oxidation process, the sluggish and major rate-limiting step for overall water splitting. Moreover, water molecules can be effectively adsorbed and dissociated on both the GaN and Co3O4 surfaces13. Consequently, this improves the stability of the photocatalyst and the longevity of the artificial photosynthetic system, as - in the absence of Co3O4 nanoparticles, the inefficient accumulation/trapping of holes on the nanowire surfaces and the long-awaiting time for water adsorption, dissociation and subsequent oxidation can accelerate the self-oxidation of the material by photogenerated holes under concentrated sunlight14. This wireless artificial photosynthetic system is also capable of direct hydrogen and oxygen generation from harsh aqueous environments such as seawater15. Moreover, its efficiency could be further optimized by synthesizing InGaN with higher indium incorporation (up to 50%). The band gap of as-obtained InGaN solid solution can be tuned to ~1.5 eV while straddling the redox potentials of water splitting under visible and near-infrared light irradiation (up to λ = 1 µm). Compared to conventional PEC water splitting, this wireless device does not require the use of any electrical components or connection, nor does it require the use of conductive electrolyte. Moreover, the ability to reproducibly fabricate wafer-scale industry-friendly artificial photosynthesis devices of high performance and long-term stability and without any electrical


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ACS Energy Letters

connection can consequently lead to commercially viable large-scale clean hydrogen generation from unassisted solar-driven pure (sea)water splitting.

ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publications website at DOI: XXXX Supplementary methods for material synthesis and characterization, and supplementary notes for efficiency measurement and comparison. AUTHOR INFORMATION Corresponding Author E-mail: [email protected] Notes The authors declare no competing financial interests.

ACKNOWLEDGMENT The authors gratefully acknowledge research support from Emissions Reduction Alberta and the HydroGEN Advanced Water Splitting Materials Consortium, established as part of the Energy Materials Network under the U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy, Fuel Cell Technologies Office, under Award Number DE-EE0008086. Electron microscopy images and analysis were carried out at IREQ of Hydro-Quebec and at Facility for Electron Microscopy Research (FEMR), McGill University. L.G. is thankful for the financial support from the National Natural Science Foundation of China (Grant No. 51236007).


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X.G. is thankful for the grant support awarded by the China Scholar Council (CSC) for his 1year stay at McGill University as a visiting scholar under the supervision of Z.M. F.A.C. acknowledges the Vanier Canada Graduate Scholarships (VCGS) for their support.

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ACS Energy Letters

(8) Kibria, M. G.; Chowdhury, F. A.; Zhao, S.; AlOtaibi, B.; Trudeau, M. L.; Guo, H.; Mi, Z. Visible Light-Driven Efficient Overall Water Splitting using p-type Metal-Nitride Nanowire Arrays. Nat. Commun. 2015, 6, 6797 (9) Kibria, M. G.; Qiao, R.; Yang, W.; Boukahil, I.; Kong, X.; Chowdhury, F. A.; Trudeau, M. L.; Ji, W.; Guo, H.; Himpsel, F. J. et al. Atomic-Scale Origin of Long-Term Stability and High Performance of p-GaN Nanowire Arrays for Photocatalytic Overall Pure Water Splitting. Adv. Mater. 2016, 28 (38), 8388-8397. (10) Turner, J. A. A Realizable Renewable Energy Future. Science 1999, 285, 687. (11) Lewis, N. S. Developing a Scalable Artificial Photosynthesis Technology through Nanomaterials by Design. Nat. Nanotechnol. 2016, 11, 1010. (12) Khaselev, O.; Turner, J. A. A Monolithic Photovoltaic-Photoelectrochemical Device for Hydrogen Production via Water Splitting. Science 1998, 280, 425. (13) Schwarz, M.; Faisal, F.; Mohr, S.; Hohner, C.; Werner, K.; Xu, T.; Skála, T.; Tsud, N.; Prince, K. C.; Matolín, V. et al. Structure-Dependent Dissociation of Water on Cobalt Oxide. J. Phys. Chem. Lett., 2018, 9 (11), 2763-2769. (14) Su, J.; Wei, Y.; Vayssieres, L. Stability and Performance of Sulfide-, Nitride-, and Phosphide-Based Electrodes for Photocatalytic Solar Water Splitting. J. Phys. Chem. Lett. 2017, 8 (20), 5228-5238. (15) Guan, X.; Chowdhury, F. A.; Pant, N.; Guo, L.; Vayssieres, L.; Mi, Z. Efficient Unassisted Overall Photocatalytic Seawater Splitting on GaN-Based Nanowire Arrays. J. Phys. Chem. C 2018, 122 (25), 13797-13802.


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Making of An Industry-Friendly Artificial Photosynthesis Device - ACS

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