Perspectives on the photoelectrochemical storage of solar energy

Abstract

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.

General Characterization Methods for Photoelectrochemical Cells for Solar Water Splitting

“…Generally,t of orm an ohmic contract, the work function of the substrate needs to be smaller than that of an n-type photoanode or larger than that of ap -typep hotocathode. [21,22] Typically,t ransparent conductive oxides, such as fluorine-doped tin oxide (FTO) and indium tin oxide (ITO), have been used as conductive substrates for n-type photoanodes, [23] and high work function metals,s uch as gold andp latinum, have been used for p-type photocathodes. [24] The CE provides the reaction site for the other half-reaction of water splitting, which should have al arge surfacea rea and fast kinetics, so as not to limit the circuit current.…”

“…In publications on PECw ater splitting, the appliedp otential measured with respect to aS CE or Ag/AgClR Ei sn ormally converted into the reversible hydrogen electrode (RHE) scale for easy comparison between reported data in the literature. [21] The conversion processi sbased on their potential with respect to the standard hydrogen electrode (SHE). For instance, Ag/ AgCl in a3 .0 m solution of KCl has ap otentialo f0 .210 Vw ith respectt oS HE, whereas the potentialo nt he RHE scale is equal to 0.000-0.0591 pHw ith respect to the SHE.…”

“…[30,31] Third, the saturated photocurrent density represents the maximum H 2 or O 2 production rate that the photoelectrode can achieve through PEC water splitting,w hich is mainly constrained by the band gap and light absorption efficiency of the semiconductor. [21,22] Plotting the photocurrentd ensity at af ixed applied potential as af unctiono ft ime will generate the J-t plot. [32] This J-t plot is usually used to evaluate the stability performance of the photoelectrode.…”

“…APCE measures the percentage of absorbed incident photons that are converted into electrons collected by the PEC cell as af unction of the wavelength of the illuminating light. [21,22] APCE can be derived from IPCE through Equation (2):…”

“…[5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20] However, most of these papers have focused mainly on mechanistic investigationsa nd/or materials of the working electrode (WE;o r tandem cell in some cases) to explain the performance of PEC cells. [5][6][7][8][9][10][11][12][13][14][15][16][17][18] Af ew papers have discussed methodsa nd configurations, but they are either not comprehensive (only focusingo n one or several sections) or slightly esoteric and lack detailed operation processes; [2,11,[19][20][21] this makes them unsuitable for beginners in the PEC field to learnt he procedures correctly.…”