Introduction & Overview

Fuel cells were originally developed as efficient and compact on-board electrical sources for space missions, and are used today in the shuttle. A fuel cell does precisely the reverse of an electrolyzer. Instead of supplying electrical energy to breakdown compounds into simpler elements or products, a chemical fuel and air [or oxygen] are supplied to opposite poles of the cell chamber. The fuel is oxidized electrochemically at the anode, i.e., it loses electrons, to produce positively-charged intermediates or protons, H⁺ , in the case of hydrogen fuel. At the cathode, oxygen is reduced, accepting electrons to make oxide anions, O^2-. Thus, a unidirectional [DC] flow of electricity is created in an external circuit and passed through a load to do useful work. To perpetuate the process, the cation or anion must combine by one of these diffusing across a solid membrane separator. A neutral product is then released and the electrode surfaces are free to adsorb and convert more fuel and oxidant. Chemical energy has been transformed into electrical energy.

Several types of fuel cell exist, including PEM, alkali, phosphoric acid, molten carbonate, and solid oxide variants. Except for the last two, they operate at temperatures below 200 °C. As the waste product is identical with that obtained by burning the fuel in air, the fuel cell process is sometimes termed ‘cold combustion’.

The reasons for such serious interest in fuel cells as future clean power systems are manifold:-

First, their theoretical efficiencies are exceptionally high, approaching or even exceeding 100%, with practical efficiencies already greater than 50%. The best combustion processes can reach about 45%. The reason for this is that fuel cells avoid the Carnot cycle, a fundamental thermodynamic limitation in all heat engines.

Second, emissions of pollutants are very low or vanishingly-small in the case of the PEM hydrogen fuel cell, producing essentially pure water.

Third, unlike batteries the chemical energy is external to the unit and limited only by the capacity of the fuel storage tank. Instead of electrical recharging, which renders the battery unavailable for extensive periods, the fuel cell tank can simply be topped up or switched, thereby permitting continuous operation in principle.

Fourth, the power-to-weight ratio and capacity of a fuel cell is far superior to batteries. Transportation based on fuel cells will be clean and have better efficiency, economy, and range than battery-powered versions. Auto-manu-facturers are now urgently developing prototype fuel cell vehicles as the next generation of transport beyond the gasoline era. In 1999 alone, Japan’s four largest car makers have invested nearly $ 1 billion in their development, and large-scale commercialization is envisaged by AD 2004. Early prototypes use methanol as fuel for reasons of convenient on-board storage.

Fifth, fuel cells are assembled by stacking individual cells in series into compact modules of any specified power rating. Thus, they are versatile systems for decentralized and stand-alone power applications up to 500 kW. Even portable units of 25W are under consideration in the leisure market.

Since the polymer electrolyte membrane fuel cell [PEMFC] is arguably the type with the greatest potential and that favoured by car-makers, we will now consider this in more technical detail.

The PEM Hydrogen Fuel Cell

The layout and operation of the membrane-electrode-assembly [MEA], at the core of the PEM fuel cell is represented in simple terms above. It is very similar to the PEM electrolyzer scheme but with one fundamental difference, and more complexity, linked to the composition and physical structure of the electrode. Whereas H₂ is produced at the cathode of the electrolyzer, it is fed as fuel to the anode of the fuel cell where it is split into atoms and electro-oxidized, i.e., electrons are removed, to yield protons [H⁺ ] which pass into the membrane. Once again, a sulphonated solid fluoropolymer such as nafion^® serves quite well although membrane development is an intensive field of research, driven not least by nafion’s high cost. The electrons are drawn externally through a load to do useful work before arriving at the cathode. Here, molecular oxygen [O₂] in the air feed is activated, i.e., split into two O species, by 4 successive electro-reduction steps. The mechanism is complex and must pass through various meta-stable molecular intermediate forms, e.g., superoxide, O_2-, and peroxide, O₂^2-, before splitting to give a pair of stable oxide anions, O^2-, bound to the catalyst surface. To complete the cycle, protons "hop" across the membrane separator, reach the cathode, encounter the oxide and bind to it. After a second proton binds with the HO^- anion, the product is a neutral water molecule, which is free to diffuse into the exit stream as water vapour.

In electrochemical terms, the half-cell reaction at the anode is:-

2 H₂ -> 4 H⁺ + 4 e^-

At the cathode:-

O₂ + 4 H⁺ + 4 e^- -> 2 H₂O

Summing these two half-cells is equal to the overall chemical process of two molecules of hydrogen and one molecule of oxygen combining to produce two molecules of water. Because this is energetically favoured by thermodynamics, i.e., water is much more stable than hydrogen, energy is made available in electrical form in the fuel cell. The working efficiency of the fuel cell is governed by two separate factors, viz., thermodynamics and kinetics. The first defines the value you could expect in a perfect process where the only consideration is the overall gain in chemical stability by forming water from hydrogen. The second governs the practical performance, and is beset with greater technical problems as compared to the electrolyzer.

According to thermodynamics, the ideal voltage for this process is 1.23 V. Just as the electrolyzer must supply at least this value to break down water, so the fuel cell can produce this value but only if the kinetics are fast enough. However, reality is never perfect and so the practical voltage at useful currents, or power, is almost always below 1 V. Nevertheless this still translates into high efficiency when compared to burning the H₂ and utilizing the reaction heat to do mechanical work even in the most sophisticated turbine [see Carnot cycle and thermodynamic efficiency].

Unlike the electrolyzer, one of the main practical problems of running the fuel cell effectively lies in the water management. As seen in the scheme above, H₂ fuel must be supplied to the anode with substantial humidity entrained. This is because proton migration through the membrane also carries water with it in a process known as "electro-osmotic drag". If this water is not constantly replenished, the membrane will dehydrate, leading to very low conductivity. The supply of protons to the cathode will then be said to be "rate-limiting" in the overall kinetics. In contrast, water is produced at the cathode such that running at high power creates a danger of "flooding" of the electrode, i.e., the rate of O₂ supply is seriously impeded. For these reasons, diffusion layers of fine particles are interposed between the electrodes and gas flows, hydrophilic [water-absorbing] at the anode, and hydrophobic [water-repellant] at the cathode. These are usually composed of treated carbons, possibly with teflon also impregnated in the cathode layer. The operating temperature is also kept low (~ 60 ° C) to minimize water evaporation at the anode.

The electrodes consist of electrocatalysts like platinum supported on fine carbon, which is sufficiently conductive for electrons. This is for reasons of water management and economy. The platinum loading is typically 0.4 milligrams per square centimetre, and so well-dispersed that it has an efficiency equivalent to ten or a hundred times this amount if unsupported. A recent development is to prepare a paste with nafion solution before press-bonding the electrodes to the membrane. This further improves the protonic conductivity through the active electrode layer, which may be several tens of microns thick. None of the platinum should be redundant because it is too expensive.

In common with all power devices running on gaseous fuel and oxidant, there can be a lack of power in the fuel cell simply due to the low rate of molecular supply as compared with, say, carburetion in a gasoline engine. For this reason, the PEM fuel cell "flow-field" is carefully engineered [not shown], and usually operated at elevated pressure (~ 3 atm.).

The PEM fuel cell is not economically competitive with conventional power sources, costing up to ten times that of the internal combustion engine, which is only ~ $ 30 per kilowatt. However, intensive research is underway to bring down this cost, the greatest factor in which is actually not the precious metal, but the membrane material! Graphite must also be replaced as the bipolar plate material. Nevertheless, the benefits offered by such systems in terms of versatility, environmental impact, and sustainability, make them an attractive goal as we enter the Third Millenium, heralding as it does a new dawn in our global consciousness.

written by Jim Highfield
last modified Sunday,  12th December 1999
 
 

Glossary

Polymer Electrolyte Membrane [PEM]

Polymer electrolyte membranes [PEMs] were first developed for the chlor-alkali industry. These specialized materials, also known as ionomers, are solid fluoropolymers which have been chemically altered to make them electrically conductive. The fluorocarbon chain or backbone has typically a repeating structural unit, such as - [CF_2¾CF₂]_n- , where n is very large. Treatments like sulphonation or carboxylation, insert ionic or electrically-charged pendant groups, statistically spaced, into some of these base units giving, e.g., - [CF( SO_3-H+)¾ CF2]_m-, where m can range from n/5 to n/20 depending on the properties most suited for the application. Nafion^Ò , made by Du Pont, is the most commercially successful of such materials, and is available as thin pre-formed membranes in various thicknesses, or as a 5 % solution which may be deposited and evaporated to leave a polymer layer of customized shape.

By coincidence, the letters PE in PEM can stand for "Polymer Electrolyte" or "Proton Exchange" because the last process is linked to the conductivity. Unfortunately, these terms are used interchangeably leading to some confusion for the layman. What is worse, PEM and SPE (Solid Polymer Electrolyte) are also used interchangeably, but the key point is that they all refer to the nafion-type ionomer or its family of compounds.
 

Solid Fluoropolymer

Normal polymers found in the amazing variety of plastics so useful today are hydrocarbon-based, i.e., the repeating unit or monomer consists of carbon (C) and hydrogen (H) atoms. However, for some specialized applications the hydrogen component is not ideal and is replaced by fluorine (F). Perhaps the best-known example is teflon, the non-stick coating used in cooking utensils. Teflon is polytetrafluoroethylene (PTFE), which can be written in chemical shorthand as -[CF_2¾CF₂]_n- , where n can range typically from 100 to 100,000. Fluoropolymers are more chemically and thermally inert than conventional polymers, but are more expensive to manufacture. Their chemical stability is the key to their usefulness as base materials in the manufacture of polymer electrolyte membranes for electrolyzers and fuel cells.