Current Fuel Cell Technologies


Although fuel cells are attractive in principle, there are several barriers which must be overcome before they can become a cheap and economical alternative to the ICE. A key requirement is system size and the need to generate sufficient power within the available space to meet the demands of the particular vehicle. The system must also be able to start rapidly and respond quickly to changes in power demand. These requirements are not met by fuel cell systems based on molten carbonate and solid oxide electrolytes, since these operate at temperatures in the range 700 to 1000°C, and therefore require extended, high power consuming, periods to reach operating temperature. 

The use of methanol offers two distinct approaches. The currently favoured option involves an on-board reformer system which converts methanol into a hydrogen-rich gas which can then be consumed by the fuel cell. However, this fuel feed is diluted with carbon dioxide and possibly nitrogen, and may contain traces of carbon monoxide which acts as a catalyst poison. Carbon monoxide can be removed from the fuel feed with water gas shift and preferential oxidation reactors, but the removal can reduce the overall system efficiency and increase the weight, volume, start-up time and response to changes in power demand of the system.

In terms of system simplicity, the alternative and more desirable option is to use the direct methanol fuel cell - DMFC - where methanol fuel is supplied directly to the cell. However, there are drawbacks to this system which reduce its power output and efficiency. Therefore a number of research programmes have been undertaken to find both more active anode catalysts for promoting methanol oxidation and more advanced electrolyte materials to combat fuel cross-over. The latter is essentially a chemical ‘short circuit’ which results in a mixed potential at the cathode, greatly reducing its efficiency.

Principles of the DMFC


Methanol and water react electrochemically (methanol being oxidised) at the anode to produce carbon dioxide, protons and electrons as shown in Equation (i). The DMFC must contain an acidic electrolyte to aid carbon dioxide rejection, since insoluble carbonates form in alkaline electrolytes. The protons produced at the anode migrate through the polymer electrolyte to the cathode where they react with oxygen (usually from the air) to produce water, as shown in Equation (ii). The electrons produced at the anode carry the free energy change of the chemical reaction and travel through the external circuit where they can be made to do useful work, such as power an electric motor. The overall cell reaction, as shown in Equation (iii), is therefore the reaction of methanol and oxygen to produce water and carbon dioxide. In a practical system, these reactions are promoted by the incorporation of platinum-based elec-trocatalyst materials in the electrodes.
 

 
(i)
 
(ii)
 
(iii)
















In principle, methanol should be oxidised spontaneously when the anode potential is above 0.046 V, with respect to the reversible hydrogen electrode (RHE). Similarly, oxygen should be reduced spontaneously when the cathode assumes a potential below 1.23 V. In reality, and in common with all fuel cell types, poor electrode kinetics (kinetic losses) cause the electrode reactions to deviate from their ideal thermodynamic values so as to incur a practical reduction of the extremely high theoretical efficiency possible from the cell. 

Kinetic Limitations


The poor electrode kinetics at the anode and cathode result from the electrochemical processes being much more complex than Equations (i) and (ii) suggest. Each oxygen molecule requires the transfer of four electrons for complete reduction. The simultaneous transfer of these electrons is difficult to achieve, and in fact only partial electron transfer takes place, leading to the formation of surface intermediates, such as superoxide. Using a platinum electrocatalyst allows the stabilisation of these intermediates and permits the reaction to proceed at a reasonable and useful rate. The catalyst may also accelerate the reaction by opening up new reaction pathways. 

The electro-oxidation of methanol to carbon dioxide requires the transfer of six electrons, but it is highly unlikely that these electrons will transfer simultaneously. It is also unlikely that partial electron transfer will lead to the formation of a range of stable solution intermediates. Clearly, there must be surface adsorbed species present on the surface of the platinum electro-catalyst across its useful potential range, and it is these species which are responsible for the poor catalytic activity of platinum towards methanol electro-oxidation.
 

Advanced Materials


Some of the advanced materials that have been developed display enhanced activities, and there are a number of possible explanations to account for this. 
The most likely are :
1. The secondary metal (for example, ruthenium) modifies the electronic properties of the catalyst, weakening the chemical bond between platinum and the surface intermediate.

2. The secondary metal (for example, ruthenium, tin, lead and rhodium) is unstable and leaches out of the alloy leaving a highly reticulated and active surface. This results in a higher number of extended step sites associated with the methanol electrosorption process. In addition, these low co-ordinate sites may be much more easily oxidised, giving rise to Pt-OHads species at potentials far below that at which planar platinum is oxidised.

3. The secondary metal (such as ruthenium, tin and tungsten) is able to provide a site with -OHads, adjacent to the platinum, through a spill-over process. Hence, the catalyst activity is governed by the potential at which the secondary metal oxidises and donates -OHads to adjacent platinum sites. For materials such as ruthenium, this can occur at significantly lower potentials, below 250 mV, than is possible on a platinum surface (6–9), and indeed, at present the most active catalysts are based on platinum-ruthenium alloy materials.

Half Cell Data


The development of the DMFC was pioneered in the 1960s and 1970s by Shell and Exxon-Alsthom using liquid sulphuric acid and alkaline electrolytes, respectively. However, these programmes failed to produce stacks with sufficiently higher power densities, due to poor electrode kinetics and severe fuel cross-over between the electrodes. In sulphuric acid electrolyte, methanol cross-over was a particular problem, since both the anode and cathode catalysts were based on platinum. The performance of the cathode is reduced due to the ability of the platinum to electrochemically oxidise any methanol reaching it by diffusion from the anode.
 
In recent years, however, significant progress has been made in the development of the DMFC using solid polymer electrolyte materials. These polymer materials have extended the operating temperature of the cell from 60°C to close to 100°C, and this coupled with the possibility of enhanced intrinsic kinetics by use of the perfluorosulphonic acid electrolyte has led to the improved performances. Electrocatalyst developments have also continued and have centred around the need for stable materials with higher intrinsic activity for methanol electro-oxidation.
 

Single Cell Data


There are a number of engineering criteria associated with the design and construction of a DMFC The wide range of operating temperatures possible with a solid polymer electrolyte system means that methanol can be supplied either as a liquid or a vapour. Vapour systems, while offering higher performance and improved mass transport, are more complex as they require additional hardware to provide cooling. Fuel preheating is also necessary before injection; this carries a large energy penalty. The simplest systems from the engineering stand-point appear to be liquid-feed systems. Circulating the liquid fuel mixture prevents excessive heating of the cell, thus reducing the number of components and the size of the system. It is therefore not surprising that the majority of fuel cell research groups have chosen to construct liquid-feed systems.

In the U.S.A., the Advanced Research Projects Agency (ARPA) regards the DMFC as a potential mobile power source and also as a possible replacement for some of the primary batteries which are widely used by U.S. military forces. Several groups, funded by ARPA and the U.S. Department of Energy, have been collaborating to develop DMFC technologies. These groups include the Jet Propulsion Laboratory (JPL) and Giner Inc., Los Alamos National Laboratory (LANL) and International Fuel Cells (IFC). In Europe, the European Commission has actively funded DMFC projects for the past ten years under the framework of the Joule Programmes, and several groups have been active during this period, the most successful being Siemens (Germany) and Newcastle University. Johnson Matthey has recently been collaborating with Siemens and Innovision (Denmark) under the framework of ‘Joule 3’ to develop a fuel cell stack. This programme aims to develop highly efficient DMFC stacks operating with liquid fuel at ambient pressures.
 
Siemens have developed their single cell technology around highly loaded unsupported platinum-ruthenium black anodes (4 mg/cm2) and platinum black cathodes (4 mg/cm2), operating at high temperatures and pressures. Their best data show a high performance of 0.52 V at 400 mA/cm2 and 130°C with pressurised methanol/ water vapour and oxygen at 4.4 bar and 5 bar, respectively. This produces a respectable power density of about 200 mW/cm1 which meets the target for a practical device, although this was achieved with pure oxygen. Durability testing of the single cell shows that stability is not yet sufficient for practical applications. However, following the successful demonstration of such high performances, Siemens are currently working with Johnson Matthey and Innovision to develop cost effective cells that operate under more realistic conditions.
 

Summary and Future Opportunities


In the last few years there has been a considerable improvement in the activity of methanol electro-oxidation catalysts, through improved operating conditions and better dispersion and control of the composition of existing platinum-ruthenium materials.The single cell data presented by various groups demonstrate the influence of parameters such as temperature, pressure, concentration of reactants and electrode structure. The present level of technology requires high temperatures (130°C) and pressures before practical cell power densities can be obtained.

Most groups appear to use high noble metal loading of up to 4 mg/cm2 on the anode to increase the methanol turnover to a useful rate. This level of catalyst loading is too high for transportation applications and clearly indicates that the anode catalyst activity has still to increase, perhaps by a factor of at least ten, to reduce the noble metal loadings to more acceptable levels of below 0.5 mg/cm2. Optimisation of the electrode structure, leading to higher catalyst utilisation, will also contribute to increased cell performance.

Methanol cross-over from the anode to the cathode appears to be a major limitation at present. This is reflected in the high platinum catalyst loadings and the high gas pressure and flow rates which are necessary for reasonable cathode performance. The performance of the DMFC would be improved considerably if a methanol-impermeable electrolyte or a methanol-tolerant cathode existed. In order to minimise the effects of methanol cross-over, alternative membrane materials have been sought. Present electrolyte materials are restricted by poor water management and therefore can only operate at temperatures below 100°C at ambient pressures. Ifthe operational temperature could be increased to 150°C at ambient pressures, this would considerably enhance the kinetics of the anode reaction. However, this requires new materials which do not require humidification to maintain high conductivity.

An alternative to new membrane technology is to employ methanol tolerant cathode catalysts. A possible class of materials are high surface chevrel phase composites which consist of molybdenum, ruthenium and sulphur. Although these may not offer the same oxygen reduction performance as platinum-based materials, this may outweigh the performance loss attributable to methanol cross-over seen with platinum-based materials.
The DMFC has always been considered as the ideal fuel cell. Its simplified system design and direct use of liquid fuel have in the past been outweighed by the very low power densities achievable. The poor performance of the cell was due to the poor kinetics of the anode reaction and fuel cross-over. Although performance levels are not yet sufficient for commercial application, if the progress made over the past two to three years is continued, then this fuel cell could emerge from the shadows of its hydrogen-fuelled counterparts.