Photoelectrochemical water splitting is a promising route for the renewable production of hydrogen fuel. This work presents the results of a technical and economic feasibility analysis conducted for four hypothetical, centralized, large-scale hydrogen production plants based on this technology. The four reactor types considered were a single bed particle suspension system, a dual bed particle suspension system, a fixed panel array, and a tracking concentrator array. The current performance of semiconductor absorbers and electrocatalysts were considered to compute reasonable solar-tohydrogen conversion efficiencies for each of the four systems. The U.S. Department of Energy H2A model was employed to calculate the levelized cost of hydrogen output at the plant gate at 300 psi for a 10 tonne per day production scale. All capital expenditures and operating costs for the reactors and auxiliaries (compressors, control systems, etc.) were considered. The final cost varied from $1.60-$10.40 per kg H 2 with the particle bed systems having lower costs than the panel-based systems. However, safety concerns due to the cogeneration of O 2 and H 2 in a single bed system and long molecular transport lengths in the dual bed system lead to greater uncertainty in their operation. A sensitivity analysis revealed that improvement in the solar-to-hydrogen efficiency of the panel-based systems could substantially drive down their costs. A key finding is that the production costs are consistent with the Department of Energy's targeted threshold cost of $2.00-$4.00 per kg H 2 for dispensed hydrogen, demonstrating that photoelectrochemical water splitting could be a viable route for hydrogen production in the future if material performance targets can be met. Broader contextAs global energy consumption continues to rise, it is imperative that we develop renewable alternatives to the fossil fuel energy sources that currently power our civilization, curb CO 2 emissions, and secure a permanent energy supply for the future. Although the solutions to these global challenges are likely to consist of many different energy storage and conversion technologies, sustainably produced chemical fuels will likely play an important role due to their high energy density. Hydrogen gas is an especially promising energy carrier, but current hydrogen production processes such as steam methane reforming are unsustainable. Photoelectrochemical (PEC) water splitting is an alternative process that enables sustainable hydrogen production from water using the energy from sunlight. PEC water splitting has been demonstrated on the laboratory scale, but it has never been implemented on a large scale relevant to the global energy demand, so the prospects for scaling up this process have remained controversial. The present paper addresses the technical and economic feasibility of plants producing hydrogen via PEC water splitting. We establish practical operating efficiencies for PEC reactors, detail four potential reactor and centralized plant designs, and discuss the...

The conversion of sunlight into fuels and chemicals is an attractive prospect for the storage of renewable energy, and photoelectrocatalytic technologies represent a pathway by which solar fuels might be realized. However, there are numerous scientific challenges in developing these technologies. These include finding suitable materials for the absorption of incident photons, developing more efficient catalysts for both water splitting and the production of fuels, and understanding how interfaces between catalysts, photoabsorbers and electrolytes can be designed to minimize losses and resist degradation. In this Review, we highlight recent milestones in these areas and some key scientific challenges remaining between the current state of the art and a technology that can effectively convert sunlight into fuels and chemicals.

Oxygen electrochemistry plays a key role in renewable energy technologies such as fuel cells and electrolyzers, but the slow kinetics of the oxygen evolution reaction (OER) limit the performance and commercialization of such devices. Here we report an iridium oxide/strontium iridium oxide (IrO/SrIrO) catalyst formed during electrochemical testing by strontium leaching from surface layers of thin films of SrIrO This catalyst has demonstrated specific activity at 10 milliamps per square centimeter of oxide catalyst (OER current normalized to catalyst surface area), with only 270 to 290 millivolts of overpotential for 30 hours of continuous testing in acidic electrolyte. Density functional theory calculations suggest the formation of highly active surface layers during strontium leaching with IrO or anatase IrO motifs. The IrO/SrIrO catalyst outperforms known IrO and ruthenium oxide (RuO) systems, the only other OER catalysts that have reasonable activity in acidic electrolyte.

Hydrogen production via electrochemical water splitting is a promising approach for storing solar energy. For this technology to be economically competitive, it is critical to develop water splitting systems with high solar-to-hydrogen (STH) efficiencies. Here we report a photovoltaic-electrolysis system with the highest STH efficiency for any water splitting technology to date, to the best of our knowledge. Our system consists of two polymer electrolyte membrane electrolysers in series with one InGaP/GaAs/GaInNAsSb triple-junction solar cell, which produces a large-enough voltage to drive both electrolysers with no additional energy input. The solar concentration is adjusted such that the maximum power point of the photovoltaic is well matched to the operating capacity of the electrolysers to optimize the system efficiency. The system achieves a 48-h average STH efficiency of 30%. These results demonstrate the potential of photovoltaic-electrolysis systems for cost-effective solar energy storage.

To develop active nonprecious metal-based electrocatalysts for the oxygen evolution reaction (OER), a limiting reaction in several emerging renewable energy technologies, a deeper understanding of the activity of the first row transition metal oxides is needed. Previous studies of these catalysts have reported conflicting results on the influence of noble metal supports on the OER activity of the transition metal oxides. Our study aims to clarify the interactions between a transition metal oxide catalyst and its metal support in turning over this reaction. To achieve this goal, we examine a catalytic system comprising nanoparticulate Au, a common electrocatalytic support, and nanoparticulate MnOx, a promising OER catalyst. We conclusively demonstrate that adding Au to MnOx significantly enhances OER activity relative to MnOx in the absence of Au, producing an order of magnitude higher turnover frequency (TOF) than the TOF of the best pure MnOx catalysts reported to date. We also provide evidence that it is a local rather than bulk interaction between Au and MnOx that leads to the observed enhancement in the OER activity. Engineering improvements in nonprecious metal-based catalysts by the addition of Au or other noble metals could still represent a scalable catalyst as even trace amounts of Au are shown to lead a significant enhancement in the OER activity of MnOx.

Photoelectrochemical (PEC) water splitting is a means to store solar energy in the form of hydrogen. Knowledge of practical limits for this process can help researchers assess their technology and guide future directions. We develop a model to quantify loss mechanisms in PEC water splitting based on the current state of materials research and calculate maximum solar-to-hydrogen (STH) conversion efficiencies along with associated optimal absorber band gaps. Various absorber configurations are modeled considering the major loss mechanisms in PEC devices. Quantitative sensitivity analyses for each loss mechanism and each absorber configuration show a profound impact of both on the resulting STH efficiencies, which can reach upwards of 25 % for the highest performance materials in a dual stacked configuration. Higher efficiencies could be reached as improved materials are developed. The results of the modeling also identify and quantify approaches that can improve system performance when working with imperfect materials.

Developing improved catalysts for the oxygen evolution reaction (OER) is key to the advancement of a number of renewable energy technologies, including solar fuels production and metal air batteries. In this study, we employ electrochemical methods and synchrotron techniques to systematically investigate interactions between metal oxides and noble metals that lead to enhanced OER catalysis for water oxidation. In particular, we synthesize porous MnOx films together with nanoparticles of Au, Pd, Pt, or Ag and observe significant improvement in activity for the combined catalysts. Soft X-ray absorption spectroscopy (XAS) shows that increased activity correlates with increased Mn oxidation states to 4+ under OER conditions compared to bare MnOx, which exhibits minimal OER current and remains in a 3+ oxidation state. Thickness studies of bare MnOx films and of MnOx films deposited on Au nanoparticles reveal trends suggesting that the enhancement in activity arises from interfacial sites between Au and MnOx.

An illustration of solar-driven synthesis of ammonia using nitrates with >10% solar-to-fuel efficiencies that can potentially decarbonize and decentralize ammonia production.