New world record for solar hydrogen production

Scientists at the U.S. Department of Energy’s (DOE) National Renewable EnergyRenewable energy is power generated from infinite sources, such as wind or solar power. Conventional energy is generated from finite sources, such as natural gas or fossil oil. Laboratory (NREL) recaptured the record for highest efficiencyUsing less energy/electricity to perform the same function. Programs designed to use electricity more efficiently - doing the same with less. in solar hydrogen production via a photoelectrochemical (PEC) waterClimate change is expected to exacerbate current stresses on water resources from population growth and economic and land-use change, including urbanisation. On a regional scale, mountain snow pack, glaciers and small ice caps play a crucial role in freshwater availability. Widespread mass ...-splitting process.

The new solar-to-hydrogen (STH) efficiency record is 16.2 percent, topping a reported14 percent efficiency in 2015 by an international team made up of researchers from Helmholtz-Zentrum Berlin, TU Ilmenau, Fraunhofer ISE and the California Institute of Technology. A paper in Nature Energy titled Direct Solar-to-hydrogen Conversion via Inverted Metamorphic Multijunction Semiconductor Architectures outlines how NREL’s new record was achieved. The authors are James Young, Myles Steiner, Ryan France, John Turner, and Todd Deutsch, all from NREL, and Henning Döscher of Philipps-Universität Marburg in Germany. Döscher has an affiliation with NREL.

The record-setting PEC cell represents a significant change from the concept device Turner developed at NREL in the 1990s.

Both the old and new PEC processes employ stacks of light-absorbing tandem semiconductors that are immersed in an acid/water solution (electrolyteAn electrolyte is a solid or liquid chemical substance that receives ions, gives them off and can transport ions. Accumulators with a solid electrolyte can be used as absorbent glass mat batteries or gel batteries.) where the water-splitting reaction occurs to form hydrogen and oxygen gases. But unlike the original device made of gallium indium phosphide (GaInP2) grown on top of gallium arsenide (GaAs), the new PEC cell is grown upside-down, from top to bottom, resulting in a so-called inverted metamorphic multijunction (IMM) device.

This IMM advancement allowed the NREL researchers to substitute indium gallium arsenide (InGaAs) for the conventional GaAs layers, improving the device efficiency considerably. A second key distinguishing feature of the new advancement was depositing a very thin aluminumAluminium is extracted from bauxite ore with considerable deployment of electricity. indium phosphide (AlInP) “window layer” on top of the device, followed by a second thin layer of GaInP2. These extra layers served both to eliminate defects at the surface that otherwise reduce efficiency and to partially protect the critical underlying layers from the corrosive electrolyte solution that degrades the semiconductor material and limits the lifespan of the PEC cell.

Turner’s initial breakthrough created an interesting new way to efficiently split water using sunlight as the only energy input to make renewable hydrogen. Other methods that use sunlight entail additional loss-generating steps. For example: Electricity generated by commercial solar cellsA solar cell transforms sunlight directly into electrical energy.A solar cell or photovoltaic cell is a wide area electronic device that converts solar energy into electricity by the photovoltaic effect. Photovoltaics is the field of technology and research related to the application of solar ... can be sent through power conversion systems to an electrolyzer to decompose water into hydrogen and oxygen at an approximate STH efficiency of 12 percent. Turner’s direct method set a long-unmatched STH efficiency record of 12.4 percent, which has been surpassed by NREL’s new PEC cell.

Before the PEC technology can be commercially viable, the costCost: The consumption of resources such as labour time, capital, materials, fuels, etc. as the consequence of an action. In economics, all resources are valued at their opportunity cost, which is the value of the most valuable alternative use of the resources. Costs are defi ned in a variety of ... of hydrogen production needs to come down to meet DOE’s target of less than $2 per kilogram of hydrogen. Continued improvements in cell efficiency and lifetime are needed to meet this target. Further enhanced efficiency would increase the hydrogen production rate per unit area, which decreases hydrogen cost by reducing balance-of-system expenditures. In conjunction with efficiency improvements, durability of the current cell configuration needs to be significantly extended beyond its several hours of operational life to dramatically bring down costsCost: The consumption of resources such as labour time, capital, materials, fuels, etc. as the consequence of an action. In economics, all resources are valued at their opportunity cost, which is the value of the most valuable alternative use of the resources. Costs are defi ned in a variety of .... NREL researchers are actively pursuing methods of increasing the lifespan of the PEC device in addition to further efficiency gains.

While an alternative configuration where the device isn’t submerged in acidic electrolyte and instead is wired to an external electrolyzer would solve the durability challenge, a techno-economic analysisA systematic approach to determining the optimum use of scarce resources, involving comparison of two or more alternatives in achieving a specific objective under the given assumptions and constraints. Economic analysis takes into account the opportunity costs of resources employed and attempts ... commissioned by DOE has shown that submerged devices have the potential to produce hydrogen at a lower cost.

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Materials provided by DOE/National Renewable Energy Laboratory. Note: Content may be edited for style and length.

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