Cell-ing future power – hydrogen and fuel cells
John Kilner of Imperial College London, and Peter Edwards and Vladimir Kuznetsov of the University of Oxford, all in the UK, discuss the issues surrounding hydrogen and fuel cells.
Many people believe that the combination of hydrogen and fuel cells has a pivotal role in a carbon-free energy future, generating clean power and securing future supplies. However, progress is dependent upon materials discovery and development advancing at an unprecedented rate. Herein lies both the challenge and the unique opportunity for the materials community.
In the UK, hydrogen and fuel cells are the subject of two EPSRC-funded Sustainable Power Generation and Supply (SUPERGEN) projects – the UK Sustainable Hydrogen Energy Consortium (UK-SHEC) and the SUPERGEN Fuel Cells Consortium. Both groups address the critical materials requirements for fuel cells, hydrogen generation and its storage, and were involved in the creation of the Energy Materials Strategic Research Agenda for Renewable Energy Technologies published by Materials UK at the end of 2007.
Fuel cell challenges
A fuel cell is akin to a continuously recharging battery, generating electricity by the electrochemical reaction of hydrogen and oxygen (usually from the air). Any hydrogen-rich fuel can be used, but a hydrocarbon-based fuel leads to CO2 emissions while hydrogen-powered cells emit only water. Both types are efficient, quiet and scalable they are suitable for distributed generation for combined heat and power, and for transport applications.
‘Fuel cell’ is a generic name for a number of related technologies with differing materials requirements and varying abilities to run on hydrogen and fossil fuels. Their common working principle (Figure 1) is similar to that of a battery, but here the fuel and oxidant are continuously replenished. The key components of the cell are the porous anode and cathode, which must allow the gases to penetrate, catalyse the electrochemical reactions and carry the electronic current; and the dense electrolyte membrane, which is a pure ionic conductor. All must be thermo-mechanically and chemically compatible under operating conditions, which, for solid oxide fuel cells (SOFCs), can be in excess of 800ºC in both oxidising and highly reducing atmospheres.
The first three cells, often known as the low temperature cells, need to be fuelled by pure hydrogen, especially the proton exchange membrane (PEM) cell as it relies on platinum catalysts at the electrodes. These can be poisoned by the presence of CO, a common contaminant of hydrogen derived from reformed fossil fuels. The molten carbonate fuel cell (MCFC) and the solid oxide fuel cell (SOFC) run on either hydrogen or a reformed hydrocarbon. Indeed CO is a fuel for SOFCs, which are thus fuel-flexible. This could be critical in the decarbonisation of the energy supply and the ultimate transition to a hydrogen economy.
To obtain significant power, cells are stacked together through an interconnecting high electronic conductivity material, which needs to be lightweight, compatible with cell components, stable in both oxidising and reducing atmospheres at the operating temperature, and to have no significant permeation rate for fuel or oxygen. This presents severe challenges for materials selection.
Material requirements are critically dependent upon the cell type. In the UK, the most intensive research is centred on PEM cells for motive power and SOFCs for distributed generation. Scientists are seeking to acheive the three key targets for R&D – cost reduction, performance and durability.
For the PEM cell, the electrolyte is a polymer which conducts by the movement of hydrated protons (eg Dupont’s Nafion). Management of liquid water in the cell is therefore a concern and materials developments include a high temperature membrane (>100ºC) to solve this and other problems. A method has also been sought to reduce the loading of platinum group metals for electrodes while maintaining or enhancing performance.
For SOFCs, the requirements are more stringent and the challenges depend upon the temperature of operation. The electrolytes are thin supported ceramic membranes, which, for the highest temperature cells (>800ºC) are based on yttria stabilised zirconia, and, for those operating at intermediate temperatures (~600ºC), ceria-gadolinia is used.
The processing of these thin layer ceramic SOFCs is challenging, especially with the requirements of cost and durability. For example, the cathodes being developed for SOFCs are based on mixed conducting complex perovskite oxides, such as La1-xSrxCo1-yFeyO3-δ. The materials must be good mixed conductors, but they also need to be processed as thin porous layers in intimate contact with the electrolyte, and be able to withstand thermal cycling. Similar comments can be made for the anode with the additional constraint of redox cycling.
Hydrogen is highly abundant, but invariably found in chemical combination with oxygen in water, and with carbon in living plants, natural gas, petroleum and coal. It is extracted from these sources using electricity or heat. Present global production of hydrogen amounts to 1010kg per year, with 95% produced using steam reforming of hydrocarbons, with inevitable large amounts of CO2 as a by-product. Genuinely sustainable or renewable hydrogen needs to be produced from non-fossil fuel feedstock.
The holy grail of hydrogen production will be high-efficiency water electrolysis using renewable or sustainable electricity (from solar, wind, wave or nuclear) or a photocatalytic process that uses solar energy to generate holes and electrons in a semiconductor to split water to its constituents. For ‘sustainable hydrogen’ to be an economically viable technology, much higher efficiencies and dramatically reduced costs are needed. This requires new electronic band-gap engineered materials, as well as integrated inexpensive and efficient solar cells. The development of next-generation redox catalysts and an enhanced understanding of interfacial electron and ion transfer must accompany this.
A recognised barrier to the widespread use and ultimate market success of hydrogen fuel cell vehicles is the lack of a safe, low-weight and economic method for onboard hydrogen storage. Although hydrogen contains more energy on a weight-for-weight basis than any other substance, it has a very low energy density per unit volume. The gravimetric (kWh/kg) and volumetric energy density (kWh/L) of storage options and energy storage materials are illustrated in the diagram. In terms of gravimetric and volumetric hydrogen storage density, two classes of storage materials – interstitial hydrides and light and complex hydrides – represent a potential alternative to compressed and liquid hydrogen.
Interstitial hydrides (for example LiBH4) have a number of advantages, particularly good reversibility and low temperature of hydrogen adsorption/desorp-tion, so are promising candidates for stationary applications. However, because of their low gravimetric storage density, they are deemed impractical for transportation applications.
Light and complex hydrides (for example LiBH4) have attractive gravimetric and volumetric storage densities, but do not meet other important operating requirements for transportation, including low temperature, fast kinetics, reversibility of hydrogen uptake and release, and effective heat transfer. No materials simultaneously meet all major criteria for onboard hydrogen storage. A breakthrough in solid-state storage requires revolutionary new materials.
Hydrogen and fuel cells in transportation, and energy storage systems, and distributed heat and power generation, provide an opportunity to shift our global energy economy from a carbon-base to a clean, renewable and sustainable economy based on hydrogen. The materials challenges are interdisciplinary and involve the whole of the materials community. They are indeed ‘grand challenges’, but the opportunities for commercial exploitation and the environmental payback are on such an unprecedented scale, they cannot be ignored.
Further information: John Kilner