|
Advanced materials R&D is one key to improving the cost-competitiveness of many of the hydrogen and fuel cell technologies now under development. Understanding the underlying chemical and physical mechanisms, investigating new materials and systems, and evaluating material durability are a few of the laboratory's on-going materials R&D activities.
Photoelectrochemical Materials for Direct Solar Hydrogen Production
 This multiple-band-gap photoelectrochemical cell can be used to split water to produce hydrogen at a 10% solar conversion efficiency.
A photoelectrochemical hydrogen production system integrates a photovoltaic (PV) semiconductor with an electrolyzer into a single, monolithic device that can produce hydrogen directly from water, using only sunlight. The challenge is to find a material that can drive this process. To be viable, the material must have the correct energetics to drive the electrolysis reactions and be stable in an aqueous environment. So far, no single semiconductor has been identified that simultaneously satisfies these criteria. Researchers are now evaluating materials originally developed for PV systems for PEC applications.
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. We are also evaluating the performance, stability, and conversion (sunlight to hydrogen) efficiency of photoelectrochemical cells. Applying both theoretical and experimental techniques, we 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 us to develop more efficient water-splitting and electricity generation technologies.
Contact: John Turner
Carbon Nanostructures for Hydrogen Storage
 Researcher uses temperature programmed desorption apparatus to measure hydrogen storage properties of carbon materials.
Developing safe, reliable, cost-effective ways to store hydrogen is critical to the commercialization of hydrogen and fuel cell technologies. Current hydrogen storage technologies include compressed gases (like propane and natural gas are transported), which occupy large volumes, and metal hydrides, which are very heavy and therefore result in a reduced driving range. Scientists at NREL are exploring carbon nanoscience and nanotechnology to engineer carbon materials that will adsorb hydrogen gas reversibly at high efficiencies and energy densities.
Pure carbon single-wall nanotubes (SWNTs) and SWNT-hybrid materials can be formed into well-packed nanoporous solids and have electronic properties that may be controlled through nanotube geometry, the introduction of defects, derivatization via chemical species, elemental substitution on the nanotube lattice, and the introduction of adventitious dopants or catalytic species. Work at NREL has shown that SWNTs can adsorb up to 8 wt% hydrogen when catalytic metal species are present. Other nanostructured carbon materials such as nanohorns and multi-wall nanotubes are under investigation because they provide additional systems in which the fundamentals governing the adsorption interactions with hydrogen may be understood.
Home to DOE's Virtual Center of Excellence for Carbon-Based Hydrogen Storage, NREL is leading the effort to study and optimize these materials for hydrogen storage.
For more information contact: Mike Heben
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.
Contact: John Turner
Thin Film Hydrogen Sensors
 Schematic of fiber optic cable with chemochromic hydrogen sensor deposited on end.
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: Roland Pitts
Aging and Durability of Photolytic Reactor Materials
Capturing energy from the sun for the production of hydrogen creates some challenges in materials of construction, including cost, the ability to transmit and concentrate light, gas permeation rates, UV stability, biocompatibility, and factors related to strength and impact resistance, to name a few. Researchers are evaluating and comparing materials for suitability in photolytic hydrogen production systems. The most promising materials are undergoing accelerated and outdoor testing of durability and to measure and confirm the properties of the materials. We will also begin work to identify potential barrier coats that reduce the hydrogen and oxygen permeation rates. Samples with promising barrier coats will be included in weathering tests.
Contact: Dan Blake
|
