The sub-technologies which will be explored for this area include different types of fuel cells spanning transport and stationary applications, as well as hydrogen production and storage (EC, 2011c):
• solid oxide fuel cells (SOFC)
• polymer electrolyte membrane fuel cells (PEMFC) • molten carbonate fuel cells (MCFC)
• direct methanol fuel cells (DMFC) • alkaline fuel cells (AFC)
• phosphoric acid fuel cells (PAFC) • hydrogen production
• hydrogen storage • refuelling infrastructure Solid oxide fuel cells
In solid oxide fuel cells (SOFC) the term solid oxide refers to the electrolyte material used, which is typi- cally made of zirconium oxide stabilised with yttria. The fuel cells belong to the group that operates at high temperatures, in this instance between 500˚C and 1,000˚C (EC, 2011c). The high temperatures used in SOFCs promote increased reaction rates, which negate the need for noble metal catalysts such as platinum. SOFCs can use natural gas, propane or liquid petroleum gas as their fuel, increasing the flexibil- ity of their use. They have a range of possible applications in power units for automotive vehicles, sta- tionary power generators and combined heat and power units.
Polymer electrolyte membrane fuel cells
PEMFCs, also known as proton exchange membrane fuel cells, have the distinguishing characteristic of an electrolyte layer made of a polymer membrane, most commonly Nafion, which is made by DuPont. They operate at temperatures below 100˚C and consequently require noble metal catalysts to accelerate electrochemical reactions. Platinum is most commonly used, and is required for both the anode and cathode catalysts.
Molten carbonate fuel cells
As their name implies, molten carbonate fuel cells (MCFC) operate at high temperatures; between 600˚C and 700˚C. They can be fuelled with coal-derived fuel gas, methane or natural gas. The molten carbonate is generally either a combination of lithium carbonate with potassium carbonate, or lithium carbonate with sodium carbonate. When molten, these carbonates are highly corrosive and so MCFCs face technical difficulties providing a working lifetime of 40,000 hours (EC, 2011c). The anode is typically made of a low surface area porous nickel doped with chromium; the cathode may be made of lithiated nickel oxide. Since these fuel cells operate at high temperature, there is no requirement for noble metal catalysts. Direct methanol fuel cells
The design of a direct methanol fuel cell (DMFC) is similar to that of a PEMFC except that the fuel is a liquid solution of methanol and water, rather than hydrogen gas. DMFCs operate at low temperature and so have catalysts made of platinum. The anode catalyst often also includes ruthenium, which belongs to the platinum group metals and is of interest to this study.
Alkaline fuel cells
Alkaline fuel cells (AFCs) have been in use since the 1960s when they served as auxiliary power units on the Apollo missions and later on the space shuttle. These fuel cells use an alkaline electrolyte such as potassium hydroxide (KOH) in water. Hydrogen oxidation in these fuel cells occurs at the anode, rather than at the cathode as occurs in PEFC fuel cells.
Phosphoric acid fuel cells
As its name suggests, the electrolyte used in PAFCs is phosphoric acid. They are used for stationary and residential power applications and have also been used in buses, but their automotive application has largely been superseded by lighter PEMFCs. A PAFC - the PC25 developed by ONSI - was the first com- mercially available fuel cell for stationary applications and was introduced in 1992. Such fuel cells’ power units are more expensive per kW than other sources of power generation, but they have applications where low noise level operation is essential. PAFCs operate at temperatures between 160˚C and 210˚C but still require the use of platinum as their catalysts, and may also employ chromium, vanadium or cobalt (EC, 2011c).
Hydrogen storage
Hydrogen storage covers many different technological approaches appropriate to different applications. For light duty vehicles using hydrogen fuel cells, between 5 kg and 13 kg of hydrogen are needed to achieve a driving range of around 480 km. To fulfil this requirement, hydrogen can be stored either in cryogenic liquid hydrogen tanks or in compressed gas tanks, both of which require energy to achieve these states from the gaseous state. Liquid hydrogen tanks can store more hydrogen than compression tanks for the same volume. However, large amounts of energy - approximately 30% of the heating value of hydrogen - are required for hydrogen liquefaction (EC, 2011c). Additionally, hydrogen may be lost as a consequence of boil off, so storage methods must employ efficient insulation to minimise this effect. Compression tanks have been certified that contain hydrogen at pressures 350 (35) and 700 (70) bar (MPa), which requires materials and design that can safely hold hydrogen at these pressures (EC, 2011c). Currently under development are hydrogen tanks that use carbon-fibre as a reinforcement. Such tanks have an inner liner made of a high molecular weight polymer, a second layer made of a carbon fibre / epoxy resin composite shell, and an outer shell to protect against impact.
Hydrogen may also be stored in a number of chemical forms including as metal hydrides or carbohy- drates, in ammonia and in methanol. Portable fuel cells that use metal hydride refillable storage cartridg- es are already commercially available. For non-mobile purposes, hydrogen can be stored in caverns, depleted oil and gas fields, or in salt domes - in either liquid or gaseous form. Hydrogen can also be transported along networks of pipelines.
Hydrogen production and refuelling infrastructure
The production of hydrogen and the infrastructure necessary for a hydrogen economy were examined but detailed information on scenarios, technology requirements and material requirements was not found. However, in the case of an infrastructure using pipelines available data suggests that such pipe- lines are unlikely to operate under high pressure and so will not require unusual alloys.