The technology around generating efficient and sustainable energy is rapidly evolving; hydrogen and fuel cells are versatile examples within a portfolio of options. This article provides an overview of the early-stage materials R&D in hydrogen and fuel cells at the US Department of Energy (DOE) Fuel Cell Technologies Office within the Office of Energy Efficiency & Renewable Energy. The article highlights technology status and progress toward achieving DOE targets, discusses R&D needs and challenges, and provides specific examples where advanced materials research is relevant to addressing those challenges. For broader context, materials R&D advances are discussed in the context of DOE’s H2@Scale initiative, which is enabling innovations to generate cost-competitive hydrogen as an energy carrier, enabling renewables, as well as nuclear, fossil fuels, and the grid, to enhance the economics of both baseload power plants and intermittent solar and wind, enhancing resiliency and avoiding curtailment.

The U.S. Department of Energy (DOE) is supporting a wide range of research and development efforts that fall under the umbrella of low temperature electrolysis (LTE). These efforts range from early stage R&D on cell components to demonstrating the effectiveness of integrating MW-scale electrolyzer systems with the electric grid to provide ancillary services. LTE is included in multiple initiatives led by the Fuel Cell Technologies Office (FCTO) at DOE’s Office of Energy Efficiency and Renewable Energy (EERE). Low temperature electrolysis is one of four pathways being supported under DOE’s HydroGEN Energy Materials Network (EMN) Consortium on Advanced Water Splitting Materials (AWSM) for H2 production. The HydroGEN EMN offers an extensive collection of materials research capabilities at 6 core national laboratories for addressing AWSM R&D challenges in efficiency, durability, and cost. The LTE work supported under HydroGEN includes early stage R&D in membranes and catalysts for both PEM and AEM electrolysis. Low temperature electrolysis also has a role in DOE’s H2@Scale energy system vision. This initiative is bringing together diverse stakeholders to advance affordable wide-scale hydrogen production, transport, storage, and utilization to unlock revenue potential and value across multiple sectors. The use of low-cost electricity to affordably split water into hydrogen and oxygen is central to implementation of the H2@Scale concept. Work is also being carried out on electrolyzer manufacturing, benchmarking, protocol development, and technoeconomic analysis. An overview of FCTO-supported activities related to these initiatives and topics, and the role of LTE in them, will be provided.

Solar-powered electrochemical production of hydrogen through water electrolysis is an active and important research endeavor. However, technologies and roadmaps for implementation of this process do not exist. In this perspective paper, we describe potential pathways for solar-hydrogen technologies into the marketplace in the form of photoelectrochemical or photovoltaic-driven electrolysis devices and systems. We detail technical approaches for device and system architectures, economic drivers, societal perceptions, political impacts, technological challenges, and research opportunities. Implementation scenarios are broken down into short-term and long-term markets, and a specific technology roadmap is defined. In the short term, the only plausible economical option will be photovoltaic-driven electrolysis systems for niche applications. In the long term, electrochemical solar-hydrogen technologies could be deployed more broadly in energy markets but will require advances in the technology, significant cost reductions, and/ or policy changes. Ultimately, a transition to a society that significantly relies on solar-hydrogen technologies will benefit from continued creativity and influence from the scientific community.

Affordable, reliable, high performance materials and manufacturing technologies are key for most transformational technology advancements, including those in critical domestic energy applications. However, many materials discoveries in the laboratory either never reach widespread market deployment or spend decades in the development cycle at a cost of many tens or hundreds of millions of dollars. In response, the U.S. Department of Energy (DOE) has established the Energy Materials Network (EMN) to accelerate the materials-to-market process by better integrating the stages from materials discovery through manufacturing. The EMN is a network of national lab-led consortia that support both industry- and government-led research (e.g. CRADAs and FOA awards) on specific energy-related material challenges. The EMN framework facilitates industry stakeholder engagement to ensure that the research is immediately relevant, with high-impact commercial potential. To date, EMN consortia have been set up in the seven technology areas of: light-weight materials for transportation; precious-metal-free catalysts for fuel cells; water-splitting materials for hydrogen production; advanced catalysts for biomass conversion; new materials and designs for photovoltaic modules; advanced solid-state hydrogen storage materials; and advanced caloric materials for refrigeration. This presentation will describe the vision of the EMN as an enduring national resource, and discuss recent technical accomplishments in the current network of EMN consortia.

Photoelectrochemistry is a particularly rich field of study that combines principles and concepts from both semiconductor physics as well as electrochemistry. Although its intellectual depth provides incredible opportunities for technical development, it also presents a steep learning curve for scientists new to this field who wish to leverage PEC in their efforts to advance low-carbon energy conversion research. As a result, PEC studies are not always performed consistently from one laboratory to the next, limiting the ability to accurately compare results in the literature and amongst researchers. In order to address this challenge, the U.S. Department of Energy formed the PEC Standards Working Group to identify the most readily accessible experimental methodologies as well as the most meaningful metrics required to advance materials development. Herein, we discuss the path taken towards establishing benchmarks in PEC water splitting as well as the key concepts and methodologies identified as being the most pertinent within the field. These include various efficiency definitions in addition to techniques such as illuminated 3- and 2-electrode electrochemical measurements. Such measurements can elucidate important material properties including band structure and charge transport. Understanding the advantages and the limitations of these methods is a critical step towards enabling researchers to more accurately evaluate the performance capability of PEC devices and to advance progress in the field. Figure 1

For optimal performance, the intermediate window layer in multijunction photovoltaics should transmit as much light as possible to guarantee maximum device efficiency. In this work, we demonstrate that indium molybdenum oxide (IMO) is a more suitable intermediate layer, compared to indium tin oxide (ITO), as it would absorb significantly less infrared light with comparable electrical conductivity once integrated into a multijunction solar cell. In fact, we show that IMO optoelectronic properties are virtually unchanged by the typical thermal budgets used in solar absorber deposition processes used in low-cost high-performance multijunction photovoltaics (e.g. CuInGaSe2). Specifically, IMO and ITO thin films were reactively sputtered onto glass substrates at 150°C, then subjected to a vacuum annealing process (550°C, 2h) identical to that of co-evaporated copper gallium diselenide (CGSe), a candidate material for the top absorber in multijunction cells. We found that annealing substantially reduces the infrared transmittance of ITO starting at 900nm, reducing by 2.5% per 100nm, while IMO only started experiencing a reduction at 1400nm and decaying more slowly at 1.6% per 100nm. Furthermore, the resistivity of IMO was comparable to that of ITO after annealing. The resilience of IMO to such high temperature processes show that it has potential to enhance the performance of multijunction devices.

To facilitate the research in photocatalysts and solar fuel production, a symposium on "Renewable Fuels from Sunlight and Electricity" was held at the 222nd ECS Meeting in Honolulu, in October 2012. In this article the highlights from the symposium are summarized.

Photoelectrochemical (PEC) hydrogen production, using sunlight to directly split water, is one of the key enabling technologies for a future where hydrogen is widely deployed as an energy carrier. However, the “traditional” semiconductor-based PEC material systems studied to date, including simple metal oxides such as TiO2, WO3 and Fe2O3, have not been successful in meeting all the performance, durability and cost requirements for practical hydrogen production. Technology-enabling advances in the development of new, advanced PEC materials and systems have been needed. Toward this end, the International Energy Agency's Hydrogen Implementation Agreement (IEA-HIA) Task-26, working in close conjunction with “Working Group on PEC Hydrogen Production” in the Fuel Cell Technology Program at the U.S. Department of Energy, has brought together experts in materials theory, synthesis, characterization and analysis from research sectors across the world. This endeavor has resulted in exciting recent progress over a broad range of PEC materials classes, including high efficiency crystalline semiconductors (e.g., III-V materials), promising thin-film semiconductors (including Fe2O3-,WO3-, and CuGaSe2- based films), novel photocatalyst powders (such as Cs-Modified WO3) and innovative photocatalyst nano-particles (e.g., MoS2). The research and development progress in these important PEC materials classes will be summarized, and key implications discussed.

This presentation will investigate various parameters regarding the use of I-III-VI2 Copper Chalcopyrite-based materials for use in tandem-hybrid photocathodes capable of splitting water into hydrogen and oxygen gases in an acidic electrolyte. Constituent parts (fabricated at HNEI) of a proposed monolithically integrated hybrid photovoltaic/photoelectrochemical (PV/PEC) device were characterized separately and combined theoretically using electronic and optical models to simulate tandem operation to first indicate feasibility of matching existing materials. Robust CGSe2 photocathodes were focused on for the PEC cells and CIGSe2 and CISe2 devices were evaluated for the PV cells. Simulation suggested the hybrid PV/PEC system could pass enough light to produce up to 15.87mA/cm2, validating the feasibility and warranting the fabrication of stacked PV/PEC devices.

The control of sodium in CIGS solar cells is critical to achieve high efficiency devices, but to date composition measurement techniques either cannot detect the sub-one atomic percent levels (for example, x-ray fluorescence (XRF)) or are expensive, time consuming, and destructive (examples include SIMS, XPS/ESCA). We employed an inexpensive, fast, and minimally destructive method to measure the concentration of sodium in CIGS solar cells fabricated at the Hawaii Natural Energy Institute (HNEI). Laser induced breakdown spectroscopy (LIBS) was used to determine the relative concentration of sodium. Two different analysis methods of the LIBS data were explored: the first assumes local thermal equilibrium (LTE) of the plasma and is calibration-free while the second employs comparison of relative peak heights after calibration to determine the concentration. Analysis is presented for solar cells produced on thin titanium foils where sodium fluoride is included in the deposition process to incorporate sodium into the CIGS layer.

The US Department of Energy (DOE) hydrogen production research and development portfolio focuses on low-cost, highly efficient and environmentally friendly production technologies based on diverse, domestic resources. Within the DOE, work on hydrogen production technologies integrates basic and applied research, as well as technology development and demonstration. The integration of basic and applied research is of particular importance in “transformational” production technologies, such as photoelectrochemical (PEC) hydrogen production, where scientific advances are needed for achieving the long-term DOE performance and cost targets. In the case of renewable hydrogen production via PEC solar water splitting, high solar-to-hydrogen conversion efficiency has been demonstrated to date on the laboratory scale, but only with high-cost, low-durability material systems. In order to identify and develop the appropriate high-efficiency, low-cost, durable and scalable PEC material systems, research and development efforts in the DOE EERE (Energy Efficiency and Renewable Energy) Office have keyed in on specific focus areas, including: 1) the engineering of solar energy absorption properties in PEC semiconductor materials, such as the bandgap lowering in stable metal oxides as well as bandgap raising in nanostructured sulfide catalysts; 2) the engineering of PEC solid-liquid interfaces for optimal reaction rates and stability, such as surface nitrogenation in III-V semiconductor systems; 3) the standardization of PEC measurement and reporting methodologies, using national and international peer-review process, for facilitating research progress; and 4) the design and analysis of integrated PEC device and system configurations for scalable hydrogen production. As described in this presentation, all of these research and development areas rely heavily on collaborative efforts among academia, industry and national laboratory partners, utilizing state of the art resources in materials theory, synthesis, characterization and analysis. The collaboration extends nationally among research programs supported by the DOE EERE as well as Office of Science; and internationally via networking through the International Energy Agency’s Hydrogen Implementation Agreement Annex-26. Key and encouraging accomplishments resulting from the collaborative work are highlighted in this presentation.

Photoelectrochemical (PEC) water splitting for hydrogen production is a promising technology that uses sunlight and water to produce renewable hydrogen with oxygen as a by-product. In the expanding field of PEC hydrogen production, the use of standardized screening methods and reporting has emerged as a necessity. This article is intended to provide guidance on key practices in characterization of PEC materials and proper reporting of efficiencies. Presented here are the definitions of various efficiency values that pertain to PEC, with an emphasis on the importance of solar-to-hydrogen efficiency, as well as a flow chart with standard procedures for PEC characterization techniques for planar photoelectrode materials (i.e., not suspensions of particles) with a focus on single band gap absorbers. These guidelines serve as a foundation and prelude to a much more complete and in-depth discussion of PEC techniques and procedures presented elsewhere.

We report on an integrated photoelectrochemical (PEC) device for hydrogen production using amorphous silicon carbide (a-SiC:H) material as the photoelectrode in conjunction with an amorphous silicon (a-Si:H) tandem photovoltaic device. With the use of a-Si:H tandem solar cell, the flat-band potential of the hybrid PEC structure shifts significantly below the H2O/O2 half-reaction potential and is in an appropriate position to facilitate water splitting. Under reverse bias, saturated photocurrent of the hybrid device ranges between 3 to 5 mA/cm2 under AM1.5 light intensity. In a two-electrode setup (with ruthenium oxide counter electrode), which is analogous to a real PEC configuration, the hybrid cell produces photocurrent of about 0.83 mA/cm2at zero bias and hydrogen production is observed. The hybrid device exhibits good durability in pH2 buffered electrolyte for up to 150 hours (so far tested).