The Advanced Materials group within NREL's Materials and Computational Sciences Center develops novel and optimized materials for energy-related applications that include sorption-based hydrogen storage, fuel cells, catalysts, photovoltaics, batteries, electrochromics, electronics, sensors, electricity conduction, and thermal management. These R&D efforts use first-principle models combined with state-of-the-art synthetic and characterization techniques to rationally design and construct advanced materials with new and improved properties.
In addition to creating specific material properties tailored for the application of interest by understanding the underlying chemical and physical mechanisms involved, the research focuses on developing materials and processes that are scalable, cost-competitive, and durable. The Advanced Materials group focuses on:
- Photoelectrochemical Materials for Direct Solar Hydrogen Production
- Nanostructured Materials for Hydrogen Storage
- Fuel Cell Component Materials
- Thin Film Hydrogen Sensors.
Photoelectrochemical Materials for Direct Solar Hydrogen Production
A photoelectrochemical (PEC) hydrogen production system integrates a photovoltaic (PV) semiconductor with an electrolyzer into a single, monolithic device that can produce hydrogen directly from water, using sunlight as the only energy input. The challenge is to find a material that can drive this process efficiently. To be viable, the material must have the correct interfacial energetics to drive the electrolysis reactions, absorb visible wavelengths of light, and be stable in an aqueous environment. So far, no single semiconductor has been identified that simultaneously satisfies these criteria. Researchers are now evaluating novel materials that emerged from PV systems to determine if they satisfy the demanding requirements of PEC applications. Recent candidate semiconductor materials include amorphous and single-crystal nitrides, carbides, oxides, and transition metal chalcogenides.
Fundamental photoelectrochemical research is focused on the ways in which reactions at the junction of a semiconductor material and a liquid convert solar irradiance into stored chemical energy or electricity. NREL is also evaluating the performance, stability, and conversion (sunlight to hydrogen) efficiency of photoelectrochemical cells. Applying both theoretical and experimental techniques, NREL can then explain and manipulate the processes occurring at the semiconductor and in the electrolyte. Interfacial photochemical work involves studies of various charge-transfer and catalytic agents that alter the surface properties of semiconductor electrodes and particles; this work enables NREL to develop more efficient water-splitting and electricity-generating technologies.
Nanostructured Materials for Hydrogen Storage
NREL leads the DOE Hydrogen Sorption Center of Excellence that uses focused collaborative efforts to study and optimize materials for hydrogen storage. Developing safe, reliable, cost-effective ways to store hydrogen is critical to the commercialization of non-polluting, renewable hydrogen and fuel cell technologies. Current hydrogen storage technologies include compressed gases (e.g., the way propane and natural gas are transported), which occupy large volumes, and metal hydrides, which are very heavy and substantially increase the vehicle weight to maintain a driving range similar to that of a gasoline fueled car. Scientists at NREL are using state-of-the-art nanoscience and nanotechnology to engineer materials that will adsorb hydrogen gas reversibly at high efficiencies and energy densities.
Seeking to construct designed nanostructures with a maximum number of adsorption sites that have a high affinity for hydrogen, NREL's materials development aims to create nanostructures that are evenly distributed and accessible. By increasing the adsorption affinity for hydrogen through structural and chemical surface modifications, more hydrogen can be stored at temperatures approaching ambient. The closer to ambient conditions that hydrogen can be stored by the sorbents, the less engineering, components, and cost will be required for the storage system. NREL is developing a number of materials that use physical/chemical vapor and solution-based chemical processes.
NREL has also championed the use of lightweight and coordinated—yet electronically unsaturated—metal centers that adsorb multiple dihydrogen molecules. This work has helped NREL identify unique materials that can be synthesized to store dihydrogen in a stable manner with capacities greater than 10 wt% and 100 g/L at ambient temperatures well in excess of liquid hydrogen, which occurs at approximately 20 K. This capability will enable substantial hydrogen storage system-cost reductions because it requires no insulation and much lower pressures compared to compressed storage.
Fuel Cell Component Materials
Polymer electrolyte membrane (PEM) fuel cells deliver high power density and offer the advantages of low weight and volume, low-temperature operation (~80°C), and rapid start-up time compared to other fuel cells, making them particularly suitable for use in passenger vehicles. Unfortunately, PEM fuel cell components—solid polymer electrolyte and porous carbon electrode plates containing a metal (typically platinum) catalyst—are expensive. The development of new materials for improved PEMFC stack and system performance and reduced cost is central to commercializing fuel cells. Fuel cell materials development efforts at NREL are currently focused on developing and testing lower cost corrosion-resistant metal bipolar plates and identifying new solid electrolyte membrane materials for high-temperature PEMFC operation.
The use of a solid electrolyte system that can operate in an intermediate temperature range (ambient to 350°C) is one way to reduce catalyst loading requirements, increase impurity tolerance, and reduce the cost of PEMFCs. Inorganic solid state proton conducting systems, such as heteropoly acids (HPAs) and their salts, are under investigation as a promising option for high temperature (ambient to 200°C) fuel cell membranes. The ultimate goal of this research is to develop HPA-based composite materials that can be combined with polymers and other potential supports to manufacture thin films as membrane materials for use in higher temperature fuel cells.
Thin metallic bipolar plates are a promising light-weight alternative that could be mass produced in high-speed, high-volume manufacturing processes, at significantly lower cost than the machined graphite bipolar plates used in PEM fuel cells today. However, the bipolar plates in a PEMFC stack are exposed to a corrosive environment that can attack the metal plates and lead to corrosion and significantly degrade stack performance. NREL scientists are investigating the use of conducting oxide coatings, originally developed for solar cells, for corrosion protection. Researchers are also investigating commercially-available stainless steel alloys to determine the best combination of alloy composition and additives that provides acceptable corrosion performance.
Thin Film Hydrogen Sensors
Like all fuels, hydrogen can be handled and used safely with appropriate sensing, handling, and engineering measures. Hydrogen safety sensors must be inexpensive enough to allow the use of multiple sensors on a hydrogen powered vehicle, and sensitive and fast enough to provide early leak detection so that action can be taken before the explosive limit in air is reached. NREL researchers are developing a fiber-optic sensor configuration that will meet these criteria. Fiber-optic sensors are inherently safe because they do not use electrical wiring, which represents a possible ignition source in the presence of hydrogen, as other sensor systems do.
In NREL's fiber-optic sensor system, thin films of chromogenic materials, such as WO3, NiOx, V2O5, are deposited on the end of a fiber-optic cable and used to indicate the presence of hydrogen. At concentrations above 0.02% hydrogen in air, these materials undergo optical changes, either changing color or changing the transmittance through the film as atomic hydrogen is incorporated. When a beam of light is propagated down the cable, the intensity of either the reflected beam or the transmitted beam is monitored to indicate the presence of hydrogen gas. Research is focused on developing a better understanding of the service lifetime and performance issues that will enable the commercialization of thin film hydrogen sensors.
Contact: Carl Rivkin 303-275-3839