Thin-Film Materials for the Protection of Semiconducting Photoelectrodes in Solar-Fuel Generators

Hu, Shu; Lewis, Nathan S.; Ager, Joel W.; Yang, Jinhui; McKone, James R.; Strandwitz, Nicholas C.

doi:10.1021/acs.jpcc.5b05976

Cited by 262 publications

(267 citation statements)

References 227 publications

(457 reference statements)

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“…The reverse bias minority hole current experiences a different impedance Z I,h+ due to hole tunneling into trap states in the TiO 2 and trap-mediated conduction to the metal. This pathway has been proposed based on reported experimental results 4,5,9 for the TiO 2 -thickness and temperature dependence of Z I,h+ .…”

Section: The Pmentioning

confidence: 97%

“…3 These two prior reports illustrate the indirect nature of half-cell efficiency calculations in pointing toward the ultimate solar-to-hydrogen efficiency possible. As recommended in a recent review of the field, 4 the photovoltage at a reasonable operating current such as 10 mA/cm 2 likely represents the best alternative performance metric to half-cell efficiency because it more directly states the ultimate utility of the device. Elsewhere in this paper, various literature results will be compared using this photovoltage metric.…”

mentioning

confidence: 99%

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Understanding Photovoltage in Insulator-Protected Water Oxidation Half-Cells

Scheuermann

Chidsey

McIntyre

2015

J. Electrochem. Soc.

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In the pursuit of a photosynthetic and efficient water splitting device, detailed investigations of individual aspects of the whole device are necessary: catalysis, electronic conductivity, cathode and anode stability, and kinetics among other aspects. Improvement in one aspect can, however, often require fundamental tradeoffs affecting others. High solar-to-hydrogen efficiency of the overall system is the ultimate goal of water splitting research. When optimizing half-cells, either the anode or cathode, the photovoltage required to achieve a current density of interest is an especially important metric. This report investigates the photovoltage in insulator-protected water oxidation anodes using ALD-TiO 2 protected silicon devices as a case study and looks in depth at how photovoltage is correctly determined from typical electrochemical analysis and how this relates to the underlying solid-state carrier transport. Finally, the photovoltage at 10 mA/cm 2 referenced to the thermodynamic potentials is reviewed from several leading research reports for various photoanodes and photocathodes, providing a direct comparison of cell performance. Half-cell efficiency and photovoltage metrics.-The solar-tohydrogen (STH) efficiency is the most important metric for characterizing water splitting cells, and can be calculated in a variety of ways. When optimizing an anodic or cathodic half-cell, the impliedefficiencies are often reported as the product of the photovoltage and current, divided by the input solar power. The power point of an actual water splitting device, however, will be determined by current matching among the components, and also necessitates a minimum voltage to drive the reactions of interest. As such, implied half-cell efficiency plots as a function of voltage can include values that are not meaningful for full cell operation. As will be shown at the end of this report, most reported anodes and cathodes do not yet provide sufficient photovoltage for photosynthetic operation of full water-splitting cells, meaning the current stand-alone full cell efficiency is still zero.The closest metric to efficiency that can be easily compared from report to report and used to best imply final device operation is the photovoltage at a relevant current, namely the voltage given to or required from the rest of the device, including the other half cell, at a certain operating current. When the photovoltage is measured close to zero current, an approximation of the open-circuit photovoltage is obtained. When it is measured at a reasonable operating current like 10 mA/cm 2 , an approximation of the power given or needed at the max power point is obtained. The tables at the end of this report show the voltage provided from a given half-cell and needed from the other at 10 mA/cm 2 to illustrate these comparisons. Comparing experimental results to theory helps illustrate the difference between half-cell and full-device efficiency calculations and how to transform values between the two. Pinaud et al. have attempted to calculat...

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Section: The Pmentioning

confidence: 97%

mentioning

confidence: 99%

Understanding Photovoltage in Insulator-Protected Water Oxidation Half-Cells

Scheuermann

Chidsey

McIntyre

2015

J. Electrochem. Soc.

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“…25 Surface modification of photoanodes with OER catalysts is an effective approach to reduce the photocurrent onset potential because it can enhance water oxidation kinetics and reduce external bias. [25][26][27][28][29] For BiVO4, the surface modification should perform OER in the appropriate pH range and also reduce the hole recombination at the BiVO4/catalyst interface. There has been extensive work on the integration of BiVO4 with OER catalysts (see Table S1 for details, Supporting Information).…”

Section: Introductionmentioning

confidence: 99%

Undoped and Ni-Doped CoO_x Surface Modification of Porous BiVO₄ Photoelectrodes for Water Oxidation

et al. 2016

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ABSTRACT:Surface modification of photoanodes with oxygen evolution reaction (OER) catalysts is an effective approach to enhance water oxidation kinetics, to reduce external bias, and to improve the energy harvesting efficiency of photoelectrochemical (PEC) water oxidation. Here, the surface of porous BiVO4 photoanodes was modified by the deposition of undoped and Ni-doped CoOx via nitrogen flow assisted electrostatic spray pyrolysis. This newly developed atmospheric pressure deposition technique allows for surface coverage throughout the porous structure with thickness and composition control. PEC testing of modified BiVO4 photoanodes show that after deposition of an undoped CoOx surface layer, the onset potential shifts negatively by ca. 420 mV and the photocurrent density reaches 2.01 mA·cm -2 at 1.23 vs. VRHE under AM 1.5G illumination.Modification with Ni-doped CoOx produces even more effective OER catalysis and yields a photocurrent density of 2.62 mA·cm -2 at 1.23 VRHE under AM 1.5G illumination. The valence band X-ray photoelectron spectroscopy and synchrotron-based X-ray absorption spectroscopy results show the Ni doping reduces the Fermi level of the CoOx layer; the increased surface band bending produced by this effect is partially responsible for the superior PEC performance.3

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“…26 The former represents the traditional semiconductor/electrolyte junction where the charge separating junction forms spontaneously upon immersion into the electrolyte, whereas the latter is essentially immersing a solar cell into the electrolyte since the charge/separating junction is isolated from the electrolyte. The authors would prefer that the term photoelectrochemical device be reserved for systems with a true semiconductor/electrolyte interface.…”

Section: Research Needs For Photoelectrochemical Fuel Productionmentioning

confidence: 99%

Perspectives on the photoelectrochemical storage of solar energy

Krol

Parkinson

2017

MRS Energy & Sustainability

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Direct photoelectrochemical water splitting offers several advantages over PV-powered electrolysis and may become the technology of choice in the future. However, significant R&D efforts and breakthroughs are needed to accomplish this goal.The sustainable production of hydrogen would be an important first step for both powering fuel cells and for enabling large-scale and technologically mature gas phase processes to reduce CO 2 and nitrogen to get desired products. Specifically, the central challenge is to produce hydrogen from water using sunlight. Photovoltaics and wind-powered electrolysis are likely to be the technology of choice to produce renewable hydrogen for the next few decades. However, the integration of light absorption and catalysis in 'direct' photoelectrolysis routes offers several advantages, such as lower current densities and better heat management, and may become technologically relevant in the second half of this century. This article discusses the research and development efforts and needed breakthroughs to achieve this goal. New chemically stable semiconductors with a band gap between 1.5 and 2.0 eV and long carrier lifetimes are urgently needed to make efficient tandem devices.Scale-up of these research level devices beyond a few cm 2 introduces mass transport limitations that require creative electrochemical engineering solutions. Last but not least, standardized methods for measuring efficiencies and stabilities need to be implemented and should lead to official benchmarking and certification laboratories to guide commercial scale up efforts. Keywords: photochemical; energy storage; energy generation RevIew DISCUSSION POINTS• Water splitting will be a central challenge for any future fossil fuel-free energy infrastructure that relies on liquid or gaseous chemical fuels.• While the main materials challenge for solar-and wind-driven electrolysis is the development of better catalysts, the main challenge for photoelectrochemical water splitting is to find new chemically stable optimal-bandgap semiconducting light absorbers.• Further progress in the development of photo-driven water splitting generators requires significant additional efforts in electrochemical engineering and the development of standardized methods for benchmarking device performance and stability.

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Thin-Film Materials for the Protection of Semiconducting Photoelectrodes in Solar-Fuel Generators

Cited by 262 publications

References 227 publications

Understanding Photovoltage in Insulator-Protected Water Oxidation Half-Cells

Understanding Photovoltage in Insulator-Protected Water Oxidation Half-Cells

Undoped and Ni-Doped CoO_x Surface Modification of Porous BiVO₄ Photoelectrodes for Water Oxidation

Perspectives on the photoelectrochemical storage of solar energy

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Thin-Film Materials for the Protection of Semiconducting Photoelectrodes in Solar-Fuel Generators

Cited by 262 publications

References 227 publications

Understanding Photovoltage in Insulator-Protected Water Oxidation Half-Cells

Understanding Photovoltage in Insulator-Protected Water Oxidation Half-Cells

Undoped and Ni-Doped CoOx Surface Modification of Porous BiVO4 Photoelectrodes for Water Oxidation

Perspectives on the photoelectrochemical storage of solar energy

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Undoped and Ni-Doped CoO_x Surface Modification of Porous BiVO₄ Photoelectrodes for Water Oxidation