ABSTRACT:
issues of efficiency and ecology converge at this time to renew interest in fuel cells as systems for electricity generation. In recent times fuel cell technology became significant for aerospace applications, but it is only recently that they attract serious attention in the utility industries, particularly in cogeneration of heat and power, and for automotive applications. The various types of fuel cell are presented, with evaluation relative to thermomechanical systems, but also noting the restrictions based on materials requirements and fuel specifications.

THE ENERGY ECONOMY AND ITS ENVIRONMENTAL IMPACT

A generation ago, in the context of the "first oil crisis" it was the common perception that a shortage of conventional fuel resources would be a constraint on social and economic progress. In particular, the oil reserves would be depleted, remaining coal deposits would be more difficult, and therefore more expensive to exploit, and even nuclear energy would be restricted by the small amount of fissile uranium to be found in nature. Increasing scarcity of resources, reflected economically in terms of energy pricing, was anticipated as the factor limiting growth in energy demand and consequently in living standards both in industrialised and developing societies.

Then, in the 1980's, awareness of planetary ecology came on the scene. The earth was no longer a reserve of resources to be mined and exploited, but a small vulnerable planet, demanding respect, understanding and conservation. There was the first widespread awareness that human activity now had the potential to impair, and even destroy, the interlocking systems of mutual protection for survival which make up our biosphere. In popular perspective the most obvious threat, after Chernobyl, was nuclear. However, invisible and seemingly innocuous gases - CFCs and carbon dioxide in particular - were also recognised to threaten the physical and chemical equilibrium of the atmosphere, perhaps not endangering our biological survival as a species, but certainly limiting our technology-based social organisation. At the same time significant new reserves of conventional fuels, particularly offshore oil and gas, were verified and exploited. Currently the as-yet unknown quantities of deep- ocean methane clathrates promise a further significant reserve. In consequence, rather than a resource-limited situation for conventional energy technologies in the 21st century, the present scenario is sink-limited, conditioned by the rate at which the earth can adapt to and accommodate the consequences of human economic activity, particularly the discharge of greenhouse gases. Hence the concern for efficiency and environmental compatibility in evaluating future energy technologies.

In practice, however, in our present situation, ecological values have yet to modify our behaviour significantly, and in particular our resource consumption pattern. The past year specifically has seen a surge of CO₂ emissions, not just from industry and transport but also from uncontrolled wildfires (Indonesia) and underground combustion of coal (China). Ten years ago, the "nuclear winter" analysis led to a profound reevaluation of geopolitical and military priorities. Can a similar reorientation be expected in energy and industrial policies? Some experts, speaking in the name of realism, would answer negatively. For example, an analysis from the US Electric Power Research Institute (EPRI) expects that even on a favourable scenario total carbon combustion will rise from the 5.5 gigatons of 1986 to at least 8.6 gigatons in 2060 despite the introduction of renewables and a relaunch of nuclear development¹. In welcome and promising contrast, a reorientation has occurred in some sectors, most notably in the automotive industry, where low emissions of nitrogen oxides and hydrocarbons have been mandated, and the community has accepted the higher prices and increased complexity of catalyser-equipped cars. The electricity utilities will equally be required to increase their fuel-use efficiency, reducing not just sulphur and nitrogen oxides emissions as sources of acid rain, but also discharges of carbon dioxide. This is where the high efficiency and low emission characteristics of fuel cells will impact.

EFFICIENCY AND EMISSION LEVELS WITH THERMOMECHANICAL SYSTEMS

Competing technologies will be briefly presented. The EPRI study already mentioned admits that a commercial coal-fired electric generation plant has a conversion efficiency of only 34%. Standard steam conditions at the turbine inlet are 540 C at a pressure of 17 MPa. As is standard thermodynamics, the limiting efficiency of such a system, following Carnot, is:

Improved efficiency is now being sought by combined cycle plants, particularly with liquid and gaseous fuels, since the flame temperature during combustion greatly exceeds the steam delivery temperature from the boiler superheater. In a combined cycle a gas turbine can preceed the steam turbine, effectively using the high-temperature exhaust of the "header" unit. With integrated gasification of coal, a combined cycle (IGCC) 100 MW demonstrator of Southern California Edison operates near 40% efficiency², and the U.S. Combustion 2000 programme aims at 47% in HIPPS (High Performance Power System) with a new high-temperature advanced furnace³. Natural gas- fired combined cycle plant can even attain efficiencies over 50%⁴, with reports of up to 58%. However, higher combustion temperatures can increase disproportionately the fraction of nitrogen oxides (NOx) in the flue gas, so that in turn combined cycle plants may require sophisticated combustion control, as well as flue gas treatment, to reach environmentally accepted standards of emissions.

EFFICIENCY AND EMISSION LEVELS WITH ELECTROCHEMICAL SYSTEMS

In contrast to the thermomechanical system, with operation by extracting energy from a heat source and discharging to a heat sink, electrochemical oxidation is essentially an isothermal process, although not necessarily at ambient temperature. This applies to all electrochemical energy conversion systems, the fuel cell differing from other primary or secondary batteries only in that the reagents enter continuously, rather than simply forming part of the electrodes themselves. The efficiency of the process will therefore be given by

Table 1 Maximum efficiency of fuel cell at 25^o C and 1 atm.

Fuel	Reaction	n	Voc	Efficiency(%)
H2	H2 + 1/2O2 H20	2	1.229	83
CO	CO + 1/2O2 CO2	2	1.07	91
CH4	CH4 + 2O2 CO2 + 2H2O	8	1.06	93

Evidently this thermodynamic calculation does not take into account ohmic and polarisation losses which cause a fall-off in cell voltage with increasing current density and hence evolution of heat. The calculated maximum efficiency therefore cannot be reached in actual fuel cell operation. Polarisation losses are particularly severe in lower-temperature fuel cells which require catalytic processes for adequate reaction kinetics to be attained, whereas in the molten carbonate (MCFC) and solid oxide (SOFC) systems are activated by their high operating temperatures. In addition, it is impossible to operate a fuel cell to stoichiometric depletion of the fuel stream, as the free energy calculation requires an activity of all involved reagents (Nernst relationship). Up to 10% of the incoming fuel must bypass the electrochemical process and is combusted in an afterburner. Taking account of these considerations the intrinsic efficiency advantage of fuel cells is difficult to maintain in practice, but the effort in fuel cell development is justified on the overall characteristics of the system including environmental considerations, and not fuel efficiency alone.

FUEL CELL PRINCIPLES AND OPERATION

Despite the modern, "high technology" image, the fuel cell concept was already demonstrated in the early 19th century. Fig.1 reproduces an engraving of the Grove "gas battery" from 1839, displaying a set of four fuel cells driving an electrolyser. In each cell the reaction of electrolysis of water is reversed: rather than an electrical energy input reducing water to produce hydrogen, in the fuel cell hydrogen is oxidised to water while generating an electric current. The figure also illustrates the problem which has ever since inhibited fuel cell applications - the inefficiency due to polarisation effects at the electrodes, with consequent losses so that four fuel cells were required to drive a single electrolysis cell. However, the efficiency of 25% compares more than favourably with that of steam engines of the period!

Modern fuel cells incorporate the three essential components of the Grove battery, a cathode for the reduction process of air or oxygen being the positive pole of the cell, with an electrolyte for selective low-resistance transport of ionised species, and the anode where the fuel is oxidised and which forms the negative pole of the cell. The major fuel cell configurations are identified by the type of electrolyte used in each, which in turn determines the mobile ionic species responsible for the cell reaction mechanism (Table 2). Those cells which are cation conductors have acid electrolytes and the mobile species is the hydrogen ion; in consequence these cells cannot directly oxidise carbon-containing fuels, and a preliminary fuel processing step is imperative. Despite this complication of the balance of plant, the phosphoric acid fuel cell (PAFC) is establishing itself commercially. Since 1991, for example, an 11 MW unit has been under test and demonstration by Tokyo Electric Power Co., Japan, and it has achieved a gross AC power output efficiency of 43.6%⁵. Smaller units, rated at 200 kW_e and with a fuel efficiency close to 40% are in series production⁶. Typical operating conditions of a PAFC are pressure 1 atm. and temperature of electrolyte 190^oC with close to a H₃PO₄ stoichiometry for the acid which is retained in a porous separator matrix. The advantage of phosphoric acid is the low vapour pressure at a relatively high temperature, combining thermally accelerated kinetics for hydrogen oxidation with a reduced susceptibility of the platinum catalyst to poisoning by residual carbon monoxide in the fuel stream. The principal limitation of lifetime is the slow corrosion of carbon in the electrodes in the presence of traces of water⁷ under the cell operating conditions.

Fig. 1: Grove "gas battery" of 1839; four fuel cells in series power an electrolysis cell. Efficiency 25%, but at that time no system to convert electricity into mechanical work existed, so competing thermomechanical systems prevailed.

Table 2. Principle fuel cell configurations.

Cell Configuration	Anode Reaction	Mobile Ion	Cathode Reaction
Polymer Exchange (Membrane): PE(M)FC	H2 2H+ + 2e-	H+	O2 + 4H+ 2H2O
Alkaline: AFC	H2 + 2OH- 2H2O + 2e-	OH-	O2 + 2H2O + 4e- 4OH-
Phosphoric Acid: PAFC	H2 2H+ + 2e-	H+	O2 + 4H+ + 4e- 2H2O
Molten Carbonate: MCFC	H2 + CO32- H2O + CO2 + 2e-	CO32-	O2 + 2CO2 + 4e- CO32-
Solid Oxide: SOFC	H2 + O2- H2O + 2e- CO + O2 CO2 + 2e-	O2-	O2 + 4e- 2O2-

The other established acid fuel cell configuration is the polymer electrolyte (PEFC), otherwise known as the proton exchange membrane (PEMFC) cell. These first came to prominence as the power (and water) source for the US "Gemini" manned spacecraft programme, though in more recent spacecraft technology they are replaced by alkali electrolyte devices. The original Gemini programme PEFC used a polystyrene-divinylbenzene sulphonic acid polymer membrane, unfortunately of limited lifetime, with hydrogen and oxygen from cryogenic storage as fuel and oxidant. With the introduction of perfluorocarbon sulphonate membranes (DuPont de Nemours, Dow) electrochemical stability of the polymer electrolyte is guaranteed up to 100^o C. Even higher temperature electrolytes are under development. These materials consist essentially of acid (sulphonate) groups immobilised on polymer chains, charge-compensated by mobile hydrogen ions. However, given the lower operating temperature, the platinum catalyst is more liable to poisoning than in the PAFC case, so the carbon monoxide level in the fuel feed must remain below 20 ppm. The PEFC is currently the favoured candidate for automotive applications, in part because of its ability to start rapidly from cold. A number of road demonstration vehicles have appeared over the past several years, particularly using Ballard PEFC stacks. However only in late 1997 has a road vehicle been announced - the NECAR lll from Daimler-Benz - which dispenses with hydrogen cylinders, the fuel being obtained by processing methanol. Similarly Auigust 1997 has seen the commissioning of a stationary Ballard PEFC demonstrator plant rated at 250kWe and operating on processed natural gas. Fuel systems will be discussed later.

The presently favoured technology for spacecraft is the alkali fuel cell (AFC), taking advantage of the intrinsically faster kinetics for oxygen reduction to the hydroxyl anion in an alkaline environment to achieve an impressive power to weight ratio. The specific power of the complete installation is 275W/kg, with an operating lifetime of 2000hr, and a rated power of 12kW with the individual cells operating at 0.86V and 470mA/cm². The use of any carbon-containing fuel is excluded because the oxidation product, carbonate, would bring about a chemical neutralisation of the alkali electrolyte. This is no disadvantage when both fuel and oxidant, hydrogen and oxygen, are available from cryogenic storage. At ground level, however, even the limited carbon dioxide component in the air would also gradually affect the electrolyte of the AFC. The pure oxygen requirement poses a severe restriction on the acceptability of the concept for terrestrial applications, so it will therefore not be further considered here.

The other anion-conducting electrolytes, in the molten carbonate (MCFC) and solid oxide (SOFC) systems, operate at high temperature so the use of noble-metal activating catalysts on the electrodes is avoided by thermal activation of the charge transfer reactions. There is the additional systems advantage that chemical processing of fuel, the feedstock reforming, is simplified and the carbon monoxide formed can be admitted to the anode for oxidation. The catalyst poisoning found in lower-temperature cells does not occur here. The key issue currently is whether the high temperature operation conveys sufficient advantages through rapid kinetics and carbon monoxide tolerance to outweigh the materials and operational problems incurred.

In the MCFC the electrolyte is a mixture of alkali metal carbonates entrained in a porous refractory tile, the mobile species being carbonate (CO₃^2-) ions formed by reduction of oxygen from the air in the presence of carbon dioxide which must also be fed to the cathode. The carbon dioxide can of course be obtained from the anode oxidation product stream, but its separation and return to the cathode is evidently a system complication. At this time the reputation of the MCFC configuration is in balance because of the difficulties of a major US demonstration project the 2 MW device in California which is out of action after a few months' service due to power instabilities.

In consequence, from the system viewpoint, the simplest of all fuel cell configurations is the solid oxide fuel cell. Here the electrolyte is an electroceramic, normally a yttrium-stabilised cubic zirconia (YSZ) in which, at temperatures over 700^o C, oxygen ions are mobile between defect sites induced by the presence of the heterovalent yttrium in the zirconia crystal lattice. Electrode materials compatible with the YSZ electrolyte in thermal stability and expansion have been found, for the cathode a perovskite-structure conducting oxide lanthanum-strontium manganite and for the anode a nickel-YSZ composite cermet⁸. Active development is being pursued to optimise electrode morphologies and interface properties. In operation, the YSZ lattice is in equilibrium with air, giving an oxygen partial pressure P_O2 of approximately 0.2 atm. On the anode side, on the other hand, lattice oxygen ions equilibrate with a gas environment of fuel and its combustion products, water and carbon dioxide, with an effective P_O2 of 10^-17 atm. This differential in oxygen activity across the solid electrolyte generates the output voltage of the SOFC element.

Electrode geometry and interconnection methods for series assembly of SOFC cell elements into stacks or batteries is also part of system design. Although some favour planar geometries⁹, the largest SOFC system currently under test and demonstration is that constructed by Westinghouse Electric (USA) for installation at the Nuon district heating system, Duiven, near Arnhem, Netherlands, which uses tubular cell elements¹⁰. In contrast the configuration presently favoured by Swiss industry, the heat-exchanger-integrated stack (HEXIS) of Sulzer Innotec AG, uses planar disc cell elements stacked as cylinders¹¹.

An additional advantage of the SOFC system is the high-temperature exhaust, which at over 800^o C exceeds that of the steam turbine inlet temperature considered earlier. For maximum generation efficiency, there is the option of a SOFC topping cycle replacing the gas turbine in the combined cycle plant. It is even suggested by Westinghouse that a pressurised SOFC stack could replace the combustion stage in a combined cycle with a gas turbine. In cogeneration systems, overall fuel efficiency can be further enhanced by recovering exhaust heat for process heat or space heating applications. It should also be noted that nitrogen does not have access to the oxidation zone in an SOFC element: nitrogen oxides can therefore be formed only by combustion of the anode bypass gases in the afterburner. These fuel cell systems are therefore characterised by levels of NO_x emission as low as 3 ppm.

FUEL PROCESSING

As mentioned several times, hydrogen is the fuel of preference for fuel cells, even for the high temperature devices for which it is not the only compatible fuel. However, hydrogen is not commonly available at a price suitable for use as a fuel, whatever the hopes for its future role as an energy vector when produced from sustainable sources. Also, particularly for mobile applications, storage and distribution of hydrogen poses problems. Cryogenic storage as liquid hydrogen is technically possible, but the liquid is of low density, one third of the energy content is lost in the liquifaction cycle, and a boil-off loss of up to 3% per day must be accepted, in sum a high- technology costly procedure. Storage in gaseous form - either compressed or adsorbed on alloys or perhaps in the near future on carbon nanotubes - must still demonstrate a sufficient energy density. Therefore the hydrogen supply must be synthesised from a conventional fuel.

The first step in the procedure is the steam reforming of the feedstock, for example natural gas:

CH₄ + H₂O CO + 3H₂

CH₄ + O₂ CO₂ + 2H₂

CO + H₂O CO₂ + H₂

Fig. 2a: Sulzer HEXIS stack repeat unit, with electrolyte and electrodes (PEN),

electrical contacts and gas distribution elements, with biploar interconnent plates

Fig. 2b: Sulzer HEXIS system schematic. 1: stack of repeat units as shown in Fig. 2a;

2: prereforming fuel treatment unit incorporating afterburner and air preheating;

3: heat recovery for combined heat and power (CHP) applications;

4. electrical output power conditioning unit - inverter.

Although thermomechanical and electrochemical systems have a common origin in the early 19th century, the first went on to provide the "age of steam and iron" a century ago, and remains the key to electricity generation and transport to this day. Electrochemical fuel cell technology remained in obscurity until identified as a priority for manned spacecraft applications. Only now, with significant advances in materials science and catalysis, and the imperative of environmentally-acceptable low emission operation, is the real potential for fuel cells being recognised in the utility industry, in cogeneration applications and on the road. At the very least, this clean efficient alternative will stimulate the thermomechanical engineers, despite their Carnot and Rankine limitations, to even greater efforts.

REFERENCES

Click here to return to Fuel Cell Group Homepage