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Vacuum 80 (2006) 1053–1065 www.elsevier.com/locate/vacuum
Proton exchange membrane fuel cells V.M. Vishnyakov Dalton Research Institute, Manchester Metropolitan University, Manchester M1 5GD, UK
Abstract The desire to have compact, high power sources with low environmental impact has focused attention on proton exchange membrane fuel cells (PEMFC). The paper gives a brief overview of processes and technical challenges related to PEMFCs. The main goal is also to give a short overview of the underlying science and to show a feasible approach to implementation of these cells. The potential for use of vacuum-based techniques in PEMFC manufacturing is also discussed. r 2006 Elsevier Ltd. All rights reserved. Keywords: Proton exchange membrane; Fuel cell; Backplate; Catalyst; Hydrogen
1. Introduction It is said that the amount of energy used determines the technological sophistication of a society. There is a constant development of existing and a search for new energy sources. This search is always fuelled either by new energy-hungry processes, or by some energy crisis. In principle, fuel cells offer electrical and heat energies by combining fuel and oxygen without the drama of a hightemperature burning process. They were discovered a long time ago and have developed through a few phases of rises and falls of interest from academia and industry. In the past, technical problems and high production and exploitation costs have always managed to wipe them out of the focus of interest. It can be argued that the last rise in interest began with space exploration. This started the current stretch of development which is proving to be the longest and most intensive one. Partially, this can be explained by the potential energy density fuel cells promise to bring for critical, transportation and other portable applications and, partially, by the promise of ‘‘green’’, carbon-emission-free energy. While fuel cell technology by itself is very simple in principle and huge advantages have been made in many aspects of its implementation, it still can be considered to be in its infancy for applications in the wide consumer market. There are numerous reasons for the Tel.: +44 161 247 3385; fax: +44 161 247 3382.
E-mail address: [email protected]
0042-207X/$ - see front matter r 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.vacuum.2006.03.029
relatively slow development in ‘‘market-ready’’ fuel cells. The more general ones are generic to all new technologies: it always takes time to develop technology for a broad consumer market. Completed implementation at all times relies on combined areas of technology and they need to be advanced all together. This takes effort and a considerable investment. Contemporary society in developed countries enjoys, at the moment, an abundance of relatively cheap energy and this adds to a certain level of reluctance (and even resistance) to invest heavily in order to compete with developed market supplies. While in some areas fuel cells promise a revolution there is a tendency to stick with evolution up to the point of sharp crisis. At the very beginning of the current wave of interest, the cost of fuel cells was very high, but this was easily offset by their high energy/weight density as compared with other electrical energy sources. The mass market, on the other hand, needs supplies which are cheap, have high efﬁciency and are made using reliable technology. Considerable progress has been made to reduce costs and in the last 10–15 years, the price per energy unit generated by fuel cells dropped by a factor of almost 10–20. The development is still driven by the promise of new markets, security of energy supply and environmentally clean energy. Traditionally, there are ﬁve types of fuel cells: alkaline, proton exchange membrane (PEMFC), phosphoric acid, molten carbonate and solid oxide (see accompanying article in this issue). One can argue that new types have
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been developed recently. Generically, all cells consume hydrogen and oxygen to produce electrical current. Different fuel cell types have their strengths and weak points and, as a result, they have their application niches. It is possible to argue that the PEMFCs are the most simple ones and, as such, are very easy to understand, implement and use. The description of processes in PEMFCs and the challenges during development can be used to some extent as an example which will form a better understanding of all fuel cells. Their basic simple implementation and expected wide penetration of PEMFCs into the end user market have attracted much research and development effort of the PEMFC. One selling point of fuel cell technology is its high energy-conversion efﬁciency. In favourable circumstances this efﬁciency can be almost 60%. This seems not to be a high value for the ‘‘green economy’’. However, car engines at low power (.ysay 20% of a rated maximum) can achieve only 30% efﬁciency and the engine will only be more efﬁcient at high powers. Fuel cells on the contrary are more efﬁcient at low power loads. Thin ﬁlm technologies were always used to improve performance and to reduce cost of devices. Vacuum-based processes are known for creation of high-quality ﬁlms. Some very encouraging work has been published, mostly in the last decade on the application of vacuum techniques for various PEMFC components.
2. General considerations The principal construction of a contemporary fuel cell is shown in Fig. 1. In a very basic form it has only two catalyst layers and a proton exchange membrane. In this basic form, the proton exchange membrane is just a piece of thin special plastic and a catalyst layer would comprise around 100 g of catalyst per square metre (10 mg/cm2). The side supplied with hydrogen (or other hydrogen-containing fuel) is named the anode, the oxygen (or air) side is named the cathode. Products of the reaction (electrical current and water) must be taken away. From this practical point of view one needs immediately two rigid and rather complicated constructions named backplates. The backplates would also help to dissipate heat in a high-power cell. Fuel cell surface areas range from a few square centimetres for a very small cells to 0.1–1 square metres for large power cells. The backplates in all cases should make very good and close contact with the catalyst. It is very expensive to make backplates with high surface tolerances which can be maintained over a wide temperature range. The solution is to use a carbon cloth placed between a rigid backplate and the catalyst. The cloth allows gases and water to access the reaction area and it is a good electron current conductor. Being made of soft and pliable material it also helps to deal with imperfections in backplate size, lower the required tolerances for machining and, thus, to minimise costs of backplate production.
Fig 1. Proton exchange membrane fuel cell essential components.
For demonstration purposes only the components of the fuel cell are shown separately in Fig. 1, but in a working cell all parts should be pressed tightly together. In all fuel cells there are two contributors to current ﬂow. The usual, external to fuel cell, electrical current consists of moving negative electrons. The current of charged ions is conﬁned to the fuel cell itself. In the acid type of fuel cell the proton H+ moves between two catalyst layers. At the anode the hydrogen molecule is split into two protons and two electrons H2 ! 2Hþ þ 2e . At the cathode oxygen reacts with protons and consumes electrons to form water O2 þ 4Hþ þ 4e ! 2H2 O. The electrolyte, separating electrodes and reactions, should only allow protons to pass which means that electron current ﬂows outside the fuel cell to produce electrical energy. Some polymers can be prepared in such a way that they conduct protons and thus they act as solid electrolytes. The thin ﬁlm of polymer works in this case as an electrochemical membrane and PEMFCs are devices using such a polymer. Both reactions at the cathode and anode have potential barriers. The barriers are high enough to reduce reactions proceeding at low temperatures and pressures to unusable low rates. This can be overcome by the use of catalysts and/ or high temperatures. There is a limited number of
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materials which can catalyse reactions and survive for a long time without degradation in the very stringent environment of the fuel cell. The choice of catalysts is essentially narrowed to platinum and some platinumcontaining alloys. The volt–ampere characteristic of fuel cells can be, in principle, understood from basic thermodynamics and electrochemistry. If one is to imagine and analyse a fully reversible process then the maximum voltage generated (denoted as E) should correspond to the electromotive force (EMF) generated at the output. The process is fully reversible if the process combining hydrogen (or any suitable fuel) and oxygen into water in the fuel cell does not make unrecoverable losses (for example does not produce heat). In the reversible case one can roughly assume that the energy change in the system (Gibbs free energy of formation) is converted into electrical energy. Then DG T; P; f n ¼ nNeE, Or E ¼ DG T; P; f n =nNe, Where DG is a change of Gibbs free energy of formation in the system, n is the number of electrons transferred during the reaction (2 in the case of hydrogen), N is Avogadro’s number, and e is the charge of an electron. Minus signs were omitted on both sides of the equation. The Gibbs free energy of formation depends on the temperature (T), pressure (P) and the phase state of the reactants and product (liquid or gas, fn). For all materials in a fuel cell in the gaseous state, at atmospheric pressure and temperature 353 K (around 80 1C, a quite typical temperature of operation for a PEMFC) one can easily calculate that E ¼ 1.17 V. This derived voltage can be treated as the maximum one can get under no-loss conditions. In an ideal case scenario this voltage should be independent of the electrical current drawn. In reality one cannot avoid losses and there are other limitations. The process is irreversible and therefore results in lower output voltages. A typical voltage and current characteristic of a cell is given in Fig. 2. There are at least four different processes responsible in a fuel cell for the observed behaviour. The ohmic resistance to both currents (electrons and protons) generates heat and results in the slow and linear drop of voltage when the current increases (middle part of the curve). This voltage drop can be presented as DV r ¼ i Re þ Rp and Re ¼ Rcont þ Rbackpl , where i is the current, Re is the resistance to electron current, Rp is the resistance to proton current through the PEM, Rcont is the contact resistance between catalyst and
Fig 2. Schematic dependence of output voltage and power density on electrical current density from a hydrogen fed PEM fuel cell.
backplate, Rbackpl is the resistance of backplates. The nature of the resistances will be discussed later in the paper. The voltage and current relationship during the electrochemical reaction itself on each catalyst layer are linked by a simple dependence named after J. Tafel, who is renowned for the formulation in 1905 of this electrochemistry law for irreversible reactions. The voltage drop occurring on the electrode is known as the over-voltage (overpotential) and reﬂects the fact that some energy is needed to generate a reaction product. The dependence has the simple form DV OV ¼
2:3RT ln i=i0 ðM; T; SÞ , anF
where R is the gas constant, T is temperature, a is a symmetry coefﬁcient (usually around 0.5), n is the number of exchanged electrons, i0 is a exchange current which is dependent on the materials involved, temperature T and material active area S. The dependence on active area only springs from the fact that, while we can relate current from the cell to the projected membrane area, the exchange current is directly proportional to the chemically active area, which can be much bigger (or smaller, if all surface is not active) than this projected area since surface areas are nearly always bigger than projected areas. The chemical meaning of i0 can be understood from the fact that, even without any applied external current, the electrochemical reaction takes place with a certain probability, but the reaction products at the same time collapse back to form the initial reactants. We can say that the reaction does not have a direction. So i0 is a balance current of the fully reversible reaction. When we apply external current the balance is shifted and this is reﬂected by generating reaction products. Two different electrodes in a fuel cell with two different reactions on them lead to two different current/voltage dependences. Even with the most active catalysts known, the reaction on the anode, as a rule, is much faster than the reaction on the cathode and, as a result of this, we lose much more voltage at the cathode. The way to deal with this is to increase the chemically
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active area at the cathode. This can be done either by increasing the catalyst active area or, if this is not possible, by increasing the amount of catalyst. The amount of catalyst is usually called ‘‘catalyst loading’’ and, in SI units, would be measured in mg per square metre, but is normally quoted in mg/cm2. Some hydrogen in molecular form always passes through a PEM and ends at the cathode area. The catalyst on the cathode very effectively splits this hydrogen to protons. The protons then react with oxygen and this means that there is a small electrical load on the cathode side even without external current. Since the activity of the cathode is not particularly high, this leads to a noticeable over-voltage even without external current. Our open circuit voltage will be less than the ideal EMF by this over-voltage amount. The Gibbs free energy of formation and, as a result the EMF of the reaction, changes with the pressure, or, more accurately, with partial pressure and temperature of the reactants. In fact the dependence is a bit more complicated and involves also the amounts of active area on which the reaction takes place. In a certain situation it is usually found to depend on reactant activities. The dependence is known as the Nernst equation. In many cases it is simpliﬁed to the form of just pressure and temperature dependence. Without going into detail, the result to remember is that higher pressures and temperatures produce higher EMF values . Partial pressures of reactants and their variations across the membrane during the functioning of the fuel cell have a serious implication on functionality. While hydrogen supply and reactions on the anode side can be treated relatively simply in most cases, the situation on the cathode is much more complicated and special measures should be taken to avoid oxygen starvation at high currents. Since reactants should be supplied to the electrodes and starvation is associated with the reactant usage, the whole problem may usually be described in mass transport or concentration losses terms. Externally to the fuel cell those losses can be associated with a certain output voltage drop. While it is difﬁcult to describe all the mechanisms in detail and partial pressure variation on a point-to-point basis, it is possible to describe the external voltage drop by a simple empirical equation: DV los ¼ k expðniÞ, where k and n are some empirical constants and have typical values of around 3 105 V and 8 103 cm2 mA1, respectively. The equation was ﬁrst derived by Kim et al.  and describes voltage losses at high currents. Different constants apply to every speciﬁc cell construction. In summary, one can say that the voltage produced by the cell can be described as V cell ðiÞ ¼ E DV r DV OVAnode DV OVCathode DV los . More information about modelling and ‘‘real cell parameters’’ can be found for example in Refs. [3,4].
3. Proton exchange membranes Essentially, the function of the PEM should only be to conduct protons and separate catalysts. For the ﬁrst activity the membrane needs good proton conductivity, for the second function mechanical strength is required. Unfortunately, the list of requirements for the PEM is much longer. The membrane, for instance, should conduct protons, but not electrons. It should be as thin as possible, so that proton current is affected as little as possible and the voltage drop across the membrane is minimized. The PEM should survive in a highly acidic environment at elevated temperatures for thousands of hours. It also should have a reasonably low permeability for the fuel. There are few commercially available polymers developed by different companies which can satisfy those requirements. For temperatures below 100 1C Naﬁons developed by the Du Pont company is most probably widely known and used. Over the last 40 years Naﬁons properties have become a benchmark for comparison with other materials. On the molecular level Naﬁon is a polytetraﬂuorothylene (widely known as PTFE or Teﬂons) with a long side chain ended by a sulphonic acid + radical SO 3 H . Being highly polarised, the radical is bonded to the end of the side chain by an ionic force. The radicals are highly hydrophobic while PTFE itself is highly hydrophilic. As a result, the material becomes like a sponge for water and can absorb it in high quantities. Owing to undulation of hydrophilicity within the material, the water tends to be retained in clusters. In some proton-conductivity models it is believed that water clusters form a nearly continuous pathways for protons. From this simple model one can conclude that proton conductivity is proportional to water content. Highest conductivities of protons are only achieved for fully hydrated membranes. The best conductivities achieved for Naﬁons usually are in the region of 0.001 S/m. Simple calculation shows that at 500 mA/cm2 and a membrane thickness of 50 mm the voltage drop across the membrane would be 25 mV. Dehydrated membrane conductivity can drop by more than two-orders of magnitude and this would make the voltage drop across membrane a limiting factor for the cell performance. On the other hand, if too much water is present in the backplate channels, condensation occurs and electrode areas become ﬂooded. Accessibility of reactants to catalyst layers then diminishes and a situation of reactant starvation takes place with a signiﬁcant drop in output power . In many cases the hydrophobicity of the membrane surface plays a decisive role in the water balance . Hydration of the membrane is not helped by the process of the electro-osmotic drag, when up to ﬁve molecules of water are transferred from an anode to a cathode with each passing hydrogen atom . The anode side of the membrane can quickly become dehydrated and needs a source of water to function properly. Some water, produced on the cathode side, diffuses back and special
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measures can be taken to assist this diffusion . However, for high performance, external hydration of both sides is still necessary . Hydration of a membrane is accompanied by membrane size changes. Fuel cells usually work under external mechanical compression in order to optimise the contact area, reduce contact resistance between materials at the electrodes and also to optimise gas ﬂow and heat removal . When the membrane changes size this readjusts overall compression in the cell. Diffusion layers or backplates can accommodate part of this movement, but the new set of working parameters does not guarantee optimum performance and constant readjustments lead to a drop-off of system performance over time. This situation is not helped by the fact that it is almost impossible to keep the same level of hydration at all membrane surface points during operation under dynamic load . Cell design for undulated high-power applications is a very challenging task. Alternative membranes to Naﬁon have been developed. Perﬂuorinated ionomer (named Hyﬂon), with a side chain shorter than in Naﬁon, has been investigated for a considerable time and shows very promising properties especially for operations above 373 K. The reader is referred to one of the latest reports on this material by Ghielmi et al. . Sulfonated aromatic polymers (for example sulfonated polyetherketone [SPEK] and sulfonated polyetherketone [SPEEK]) have been widely investigated, but they also need high levels of hydration and, at the membrane composition optimised for conductivity, may suffer from low mechanical stability [13,14]. PEM fuel cells usually function at temperatures below 360 K. Many fuel cell parameters would be more optimised if the PEM can be made to function at higher temperatures. For instance, heat dissipation from the fuel cell becomes a smaller problem for high temperatures as the cooling system grows to be more weight and power efﬁcient. Poisoning of contemporary catalysts by CO can be almost fully avoided at temperatures somewhere above 430 K as it is well known, that, if catalyst can tolerate 10–20 ppm levels of CO at 350 K, then it can tolerate a few 1000 ppm at 430 K. Attempts to signiﬁcantly raise the temperature above 430 K are limited by the stability of the catalyst and its support, which start to degrade rapidly at the higher temperatures. Another barrier to a temperature rise is a ‘‘hydration price’’. Unfortunately, the complexity and associated costs of appropriate hydration levels for standard membranes are very high at temperatures above 373 K. There is also the problem of the Naﬁon glass transition . All this indicates that new PEM materials should be developed to work at lower levels of hydration, or even, without it at elevated temperatures . Encouraging results are achieved by impregnating Naﬁon with mineral materials, for instance silicon oxide . One way to achieve this is to use proton conductors in the PEM which do not require water. Some progress has been seen during the use of
polymers doped with imidazole or N-methyl imidazole (see for example Refs. [18,19]. Unfortunately, the proton conductivity of the membranes was still not as high as for fully hydrated Naﬁons and there is a loss of dopant during operation. Polybenzimidazone (PBI), widely used for durability in many applications, has a high glass transition temperature at above 480 K and can be doped with strong acids (phosphoric or sulphuric) to provide proton conductivity . The PBI derivative membranes allow humidiﬁcationfree operation at temperatures reaching 470 K . High proton conductivity for PBI derivatives of almost 5 104 S/m has been reported , but practical implementation for these materials has been slow, probably owing to the gradual loss of doping acids during operation. Complex mixtures of basic and acidic polymers have been shown to exhibit very promising properties in the midrange of operational temperatures . Solid acid membranes are in the development stage and have some promising properties though much work is needed before they can be used in practice . Membranes containing fullerenes and carbon nanotubes are but another example [25–27]. More details can be found in recent reviews on polymer membranes by Savadogo  and on polymer composite PEMs [29,30]. Despite these developments Naﬁon still retains its lead position. Water management in fuel cell is still very important and the current way forward is to use selfhumidiﬁcation. In this case much of the water management equipment would become redundant and the Naﬁon cells could be made much lighter if this can be made to work with high efﬁciency. It would also make the cell selfregulating and has been shown to signiﬁcantly prolong a cell lifespan. Two approaches are possible for this purpose: to promote water diffusion back from cathode to anode and to produce water inside the membrane. The ﬁrst approach will be discussed later in the paper. The second approach is implemented by impregnating the membrane with a catalyst to promote oxygen diffusion into the membrane. Both hydrogen and oxygen cross-over are then bound to interact with each other and generate water inside the membrane. This was ﬁrst demonstrated by Watanabe et al.  and it is now under further development [32,33]. 4. Catalysts The development of catalyst layers for the PEM has a long history. In order to appreciate the amount of work done and achievements made it is advisable to look at one of the ﬁrst reviews on fuel cells . One can measure the progress from the simple fact that, during the last 40 years, the catalyst load (amount of catalyst on the membrane) has dropped from 30–40 mg/cm2 to well below 1 mg/cm2 whilst performance has increased a few times. As mentioned earlier the role of the two catalysts is to split the hydrogen molecule on the anode side and combine proton with oxygen on the cathode side. Let us ﬁrst
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concentrate on the anode side. As was mentioned earlier, under the given conditions there is a certain concentration of protons which is produced over each unit area of a catalyst. Some protons will recombine back to form a hydrogen molecule and some will diffuse into the available proton-conducting volume. In the situation, when there is a net drain of protons some equilibrium would be reached deﬁned by proton production, recombination, diffusion and drainage. In the case of a fuel cell the PEM plays the role of the drain. Protons pass through the membrane and play their part in electricity and water production. To increase the overall reaction rate one needs to make sure that the maximum number of protons is produced by the catalyst, diffusion is as fast as possible, or, the diffusion path to the drain is as short as possible. The number of protons produced is deﬁned by the catalyst activity and the amount of active catalyst surface area (related to catalyst load). The desire to reduce the weight of the catalyst leads to the development of materials with the highest possible active surface area. Long searches for the best catalysts have produced very limited results so far. Platinum in pure form or alloyed with other elements is regarded as the only choice so far for high and stable performance . Earlier pure metals and alloys (so named unsupported metal blacks) have been used as catalysts , but later it was discovered that catalyst particles could be attached to a ﬁnely dispersed carbon (the system is usually denoted as metal on carbon, Me/C). This not only increases the catalyst surface area, since small catalyst particles are separated, but also provides good electrical contact between the catalyst and the diffusion layer and, ultimately, the current collector. It is almost possible to spread the catalyst as a mono-atomic layer over the carbon particle in a quest for the highest catalyst area. However, it seems that there is a catalyst minimum thickness limit at around few nanometres below which there is no gain in catalyst activity [36,37]. This is thought to be related to some surface reconstruction which is affected by the thickness. The reconstructed surface signiﬁcantly affects catalyst activity. It also has been shown that only catalyst particles connected to the PEM surface contribute to the power production. This has led to a development where a catalyst layer is usually impregnated with an ionomer (for example perﬂuorosulfonic acid) in order to create three phase contact (gas/catalyst/membrane) for each particle in the catalyst layer. This arrangement provides easy enough gas access to the catalyst and a readily available diffusion path for a proton. The importance of this short diffusion path manifests itself in the fact that the catalyst layer thickness, from an overall performance point of view, has an optimum value at below 100 mm. A thicker layer, produced by loading more catalyst, does not improve performance signiﬁcantly. Methods for catalyst loading have been recently reviewed by Lister . Operation of a fuel cell in a real-life situation means that the purity of the gases cannot be maintained at very high levels at a reasonable cost. Some impurities would come
from the air intake and some would come with the hydrogen. It seems that most concerns are centred on carbon monoxide and carbon dioxide. Both gases are produced during hydrogen extraction from fossil fuels. It is possible to reduce their concentration in the hydrogen stream to a very low level, but this usually means additional costs. The acceptable concentration of carbon monoxide in reformatted hydrogen is usually in the region of 10–100 ppm, whereas the concentration of carbon dioxide can often be as high as 40%. This does not mean, that these 10–100 ppm levels do not affect the performance at all, but it is an acceptable compromise. The overall strategy in this case is either to produce catalysts with high impurity tolerance levels or to develop techniques for catalyst activity restoration. The poisonous inﬂuence of carbon monoxide on platinum catalyst layers is well researched (e.g. [39–42]). It results from the attachment of a CO molecule onto the catalyst surface thereby reducing the surface available for hydrogen reactions. Linear or bridge-bonded CO species are formed on the catalyst surface [39,42,43]. Externally, this leads to a signiﬁcant reduction of cell voltage under load. At current densities around 0.5 A cm2 the voltage can be reduced by as much as 0.5 V for a pure Pt catalyst. Losses arising from carbon dioxide presence are smaller but still can be signiﬁcant depending on the load and feed gas composition . To reduce poisoning and the associated losses, Pt is usually combined with ruthenium in different proportions approaching a 50/50 at% value. Much work has been done on alloying Pt with other metals and oxides to increase poisoning resistance. However, only molybdenum and, probably, gold  seems to work well during long term trials and the performance of PtMo/C (PtMo on carbon) catalysts can be comparable or better than PtRu/C. Some reported work and critical reviews on this topic have been published recently by Ralph [45–47] and Urian et al. . Another way to approach the poisoning issue is to clean the catalyst surface during fuel cell operation. It is possible to add some oxidant (such as air, oxygen or hydrogen peroxide) into the hydrogen stream [45–47] in order to oxidise the carbon monoxide to dioxide which is bonded much more weakly to the surface. The increase of temperature on the catalyst surface and accelerated membrane failure are drawbacks of this method if moderation is not observed. On the other hand, it is possible to pulse periodically the load current , which reduces the potential on the cell to a value low enough to promote electro-oxidation of CO on the surface. In fact, if the cell is kept at constant current in certain conditions it would start oscillations by itself . The problem arises in accommodation of both of those techniques into a fuel cell stack. It is most likely that some blend of using a catalyst with high resistance to poisoning and a dynamic cleaning technique could be used in future cells. Catalyst activity on the cathode is a few times lower than on the anode. This usually means that higher catalyst loads
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at the cathode should be employed to equalise performances of catalysts on both sides of a cell. In many cases the composition of the cathode catalyst is the same as that of the anode one or is of pure Pt black. There is an abundance of literature on the search for new catalysts and on the activity of different catalysts, including platinumfree layers. A critical review has been published on this work recently by Gasteiger et al. . The techniques for preparation of catalyst layers vary signiﬁcantly. Nevertheless, there seem to be a few steps which can be taken to increase performance. It was shown lately that oxidation of carbon paper is beneﬁcial and probably works by improving contact properties and/or electrochemical activity. The oxidation for instance can be done by electrochemical etching. The process presumably forms some acidic groups on the carbon surface. In a second step, the catalyst supporting layer is impregnated with a proton conducting ionomer such as polyperﬂuorosulphonic acid. The aqueous catalyst ink is then applied to the carbon cloth after the ionomer has dried. Both anode and cathode carbon cloth-supported catalysts are then sandwiched with a PEM and hot pressed together at pressures as high as 10 MPa and temperatures up to almost 430 K. An ubiquitous way of catalyst preparation and deposition is the ‘‘wet-chemistry’’ approach. However, it has been shown recently that vacuum deposition is very beneﬁcial for catalyst deposition . Very small catalyst loads at around 0.005 mg/cm2 have been also shown to have an exceptional activity for low energy applications . It is also possible to use chemical vapour deposition grown carbon nanoﬁbres for efﬁcient catalyst support . In the same work it has been demonstrated that the nanoﬁbres also have an advantage of creating lower mass transfer losses.
5. Bipolar plates External electrodes/diffusers are commonly referred to as ‘bipolar plates’ or sometimes as backplates. Bearing in mind the number of tasks they have to perform, they should be viewed as a multifunctional device. For a fuel cell performing at its best the backplates are as essential as the PEM and catalyst layers. The main functions of backplates are to uniformly distribute fuel and oxygen, to remove water and to collect and transmit electrical current. The need to connect a number of cells into a stack (e.g. battery) leads to a requirement of structural toughness and the ability to contain reactants without leakage during possible stack vibration and temperature cycling. Weight minimisation is also an important factor as backplates usually account for more than three quarters of the stack total mass. To satisfy their main functionality the plates should be also chemically stable in a highly reduction/oxidation environment. All this also should be achieved at an acceptable price.
At the ﬁrst glance gas distribution seems not to be a big problem. The task is very straightforward and is to deliver hydrogen and oxygen to the catalyst layers and to remove excess water. This is done by making ﬂow channels on the plate surfaces. The intricate surface structure which is formed by channels is usually referred to as the ﬂuid (gas) ﬂow-ﬁeld. If one looks at a simple laboratory-scale fuel cell, the ﬂow ﬁeld looks very uncomplicated and most likely would be in the form of a series-parallel ﬂow ﬁeld or pin-type ﬂow ﬁeld. Unfortunately, there are problems associated with gas handling, water management, current collection and membrane compression in both designs. The gas pressure is uneven along the channels and it is very difﬁcult to design for even pressure distribution along the stack. At the same time, on the cathode side, water tends to form droplets attached to the pin or channel walls. Water droplets formed at the beginning of the channel limit, or completely block, oxygen ﬂow and stop reactions in the surrounding area, thus causing output power oscillations. Current collection depends on contact surface area and usually increases with the pressure applied to the contact. There are two opposing plates compressing a carbon cloth/ catalyst/PEM/catalyst/carbon cloth assembly and compression should be as even as possible across the surface. Small misalignment can lead to a situation when ridges on one side can align with valleys on the other and this would lead to high local compressions and fast cell failure. To tackle these problems numerous approaches have been proposed (see for example Ref. ). Designs range from simple modiﬁcation of the straight channel pattern to development of channels by copying designs in nature  and fractal models . New design developments are aided by direct observation of water distribution in the channels  and computer simulations of overall water management in a PEM fuel cell . Some general ideas and patent activity on the ﬂow-ﬁeld design has been recently reviewed by Li and Sabir . From the industrial point of view the production of intricate channel patterns at a low cost represents a signiﬁcant technological challenge. Gold is probably the best material for the backplates for small production volumes and when the price is not important. To achieve chemical stability it is also possible to fabricate the plates from graphite and composites of graphite but, unfortunately, the ﬁrst are heavy and expensive in production and the latter are still far from perfect electrical conductors. Carbon steels and stainless steels are much more promising in terms of electrical conductivity and cost saving [61,62]. The deﬁnitive setback with these alternatives is surface corrosion, which leads to a loss of performance due to an increase of contact resistance over time and, in some cases, catalyst poisoning. Certain types of stainless steel perform relatively well anticorrosivewise by developing passivated surfaces. Unfortunately interfacial contact resistance also increases during passivation [63–66]. Thin ﬁlm coatings can be applied to a steel
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surface and, arguably, the best performance is produced again by gold. Unfortunately, this is still rather expensive in mass production and alternative materials have to be found. Titanium nitride (TiN) has a high chemical stability and exhibits high electrical current conductivity. These properties are very promising and indeed the use of TiN is already exploited in fuel cell backplates [67,68]. Not much has been revealed in the open press about TiN behaviour in a fuel cell environment. It is known that TiN has a low interfacial contact resistance and itself exhibits good corrosion resistance in the cell electrochemical environment. However, defects in TiN coatings, such as pores, pinholes and cracks, open up paths for corrosion of the underlying steel. Fortunately, even when corrosion through these defects occurs, coated stainless steel performs better than carbon steel. Steady slow corrosion proceeds but, owing to the fact that TiN now forms the electrical contact, the overall contact resistance stays unaffected for much longer [69,70]. It should be stressed that TiN coating corrosion behaviour itself in a fuel cell environment is not well understood due to the fact that it is masked by processes which take place as a result of coating defects and corrosion of the base steel. The roughness of a surface and crystallography of the coating do inﬂuence the corrosion resistance. Surface roughness can also change electrical contact resistance , but existing data does not point to a clear trend. The surface roughness of a coating is always higher or equal to the roughness of the substrate. It is extremely difﬁcult to maintain low surface roughness in intricate ﬂow ﬁelds. This surface roughness can lead to the creation of micropotentials across the surface which, in turn, can promote corrosion. TiN has, so far, attracted the greatest extent of attention for backplate coatings. Other binary and ternary compounds are known to have high corrosion resistance (see list of materials in, for example, Ref. ). Nitrides such as CrN and TaN have been tried in a fuel cell environment [73–76] and show very promising performance. All those materials are vacuum deposited on a suitable support material. The way forward is in the improvement of existing coatings and the development of new materials. For instance it would be very interesting to look at the corrosion resistance of CrN/NbN superlattice coatings grown by the various physical vapour deposition techniques created by Hovsepian . The quality of coatings should be viewed from the point of low or zero porosity and ability to deposit on an intricate backplate surface without compromise of coating uniformity. High-density coatings without pores can be created by Pulsed Magnetron Sputtering . It is generally accepted that such coatings should follow the intricate surface features, but achieving the coating quality in this case can still be challenging. It has been pointed out already that a PEM fuel cell produces a signiﬁcant amount of heat, which is roughly
equal to the amount of electrical energy it produces. This heat should be removed from the cells in order to maintain the fuel cell temperature in the relatively narrow optimum temperature range. Convectional or forced air cooling is only suitable for relatively low power fuel cell stacks. Liquid cooling becomes a necessity for high power batteries as the need for heat dissipation growths. From the bipolar plates point of view this means that some coolant structure should either be added to the plate itself or special cooling plates should be added for every so many fuels cells in the stack. While both approaches have their merits, the ﬁrst one seems to be the preferred option. The resulting bipolar plate structure becomes even more complicated. 6. Hydrogen for PEM fuel cells Generally speaking the question about fuels for fuel cells is a very broad ﬁeld and deserves a special paper. It will be outlined here very brieﬂy and the reader should appreciate that there is always a gap between the desired future cells and what is currently available and feasible. Progress on the short historical scale never follows a straight line, it is rather a Brownian motion in the stream of time. PEM fuel cells can utilise different types of fuel. Hydrogen is very attractive since running on hydrogen will produce only water and the system can perfectly be labelled as ‘‘zero carbon emission’’. There is also the fact that hydrogen has a very high-energy density. A small car with a hydrogen fuel cell would be able to travel more than 200 km on just 1 kg of hydrogen. There are factors in hydrogen use, unfortunately, which are technically challenging. In any form (gas or liquid) hydrogen has a very small density. In liquid form at 253 1C one kilogram of hydrogen occupies almost 13 litres. This means that if a hydrogen tank of normal capacity is used on a car, it will hold only 4–5 kg of hydrogen. The car range on one tank would be acceptable and a properly heat insulated tank would weigh around 30 kg. It is, however, impossible to make thermal insulation absolute and some hydrogen would continuously evaporate slowly from the tank. Arguably, the biggest problem with liquid hydrogen is the high-energy losses associated with its production. Depending on scale and the effectiveness of equipment more than one-third of the useful energy provided by hydrogen would be needed for its liquefaction. More information on hydrogen storage can be found in paper by D.K. Ross in this issue. In nature, hydrogen is always present in a bound form. Some process is needed to extract it. Either hydrocarbon fuels or water could be used. The ﬁrst way is the cheapest one, but involves generation of carbon dioxide and the use of diminishing resources of fossil fuels. It can be viewed as an acceptable temporary solution, which will allow us to develop a hydrogen economy. For the second source, electrolysis, is expensive at current prices and requires large amounts of electrical energy. It can be used with ‘‘zero
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carbon emission’’ by utilising energy from nuclear power stations or by renewable energy generating sources. Use of wind generated electricity is particularly promising especially as it can even help to resolve some issues associated with the whole process of wind generated energy: conditioning, distribution and consumption . Another way to address the hydrogen supply is to produce it ‘‘on board’’, mostly from hydrocarbons either in form of motor fuels or alcohols. An additional possibility is to use fuels specially produced for this purpose. All this has an advantage of possible utilisation of existing fuel distribution networks and partially resolves the problem of hydrogen storage. Small hydrogen producing devices (reformers) have been developed specially for this purpose. It should be pointed out that reforming of alcohols is rather less technically challenging. There is also another advantage which unites alcohols with specially developed fuels—they all have much less impurities. The main impurity in motor fuels is sulphur which is detrimental to the whole reformer/fuel cell chain. Some research groups have recently claimed that they have developed a sulphur tolerant process, but long-time stability of the proposed systems is still to be proven. The standard method for fuel reforming proceeds through a common set of steps. If sulphur content is high then it should be reduced by a desulfurisation step [80,81]. The level of sulphur should be reduced to below 1 ppmw. There are different approaches to achieve this level ranging from selective absorption to hydrodesulfurisation. Clean fuel then can be reformed either by one of these main methods: steam reforming, partial oxidation and autothermal reforming. One needs to bear in mind that the reactions by themselves are endothermic even with catalysts and some energy should be supplied to the reactants in order to free hydrogen. Depending on the process, process parameters and fuels, three major ‘‘side species’’, apart from hydrogen, are produced during reforming: carbon, carbon monoxide and carbon dioxide. Carbon production might be interesting for carbon sequestration , but if it is not desired then one needs take into account that the process reduces performance and creates problems by accumulating carbon in the downstream system and blocking wanted reactions. Carbon dioxide, while detrimental to a PEM fuel cell as mentioned earlier, is usually left in the stream and passed to the fuel cell. Carbon monoxide will have to be further oxidised to carbon dioxide in a so-named water-gas-shift reaction. This is usually done over some catalytic layers containing nonprecious and precious metals [83–85]. Despite current advances it is still difﬁcult to remove all the CO completely at this stage. Additional ﬁltering with a Pd membrane can remove all unwanted atomic species, but the process is still regarded as too expensive for small fuel cell installations. Automotive applications of fuel cells are traditionally required to operate under frequent load variation and start-ups. This presents certain difﬁculties for longestablished reformer assemblies which are traditionally
quite inert. New designs have been implemented for faster response time and additional devices could be needed to ‘‘smooth’’ transient conditions and to improve performance. On the other hand constant power applications of PEM cells without this additional complexity from reformers are feasible and can show signiﬁcant beneﬁts. Special interest for small power applications is in microreformers running on methanol  and there is potential in development of the porous silicon technology for reformer application . More detailed information on different aspects of fuel reforming technology can be found in current reviews [88–93]. 7. Direct fuel PEM fuel cells Some hydrogen-containing fuels can be directly used in PEM fuel cells. This in principle resolves many problems associated with production and delivery of hydrogen. Most of the research and development is concentrated on the methanol fuel cells, but there are other fuels to be exploited. Methanol attractiveness for fuel cells is in its potentially high-energy density, which reaches approximately 5.6 kWh/kg and its widespread availability. Direct methanol fuel cells (DMFC) are not very efﬁcient for converting their inherit energy into electricity but, even with low efﬁciency, a signiﬁcant amount of energy can be produced and this makes the technology very competitive in the energy market. The overall reaction on the anode in a DMFC proceeds with the presence of water and can be described with the equation CH3 OH þ H2 O ! CO2 þ 6Hþ þ 6e . The reaction is perceived to proceed through sequential hydrogen stripping and a lot of intermediate compounds. The exact reaction kinetics and its intermediates are still under investigation . At certain stages some reactions lead to the creation of carbon monoxide . The effect of this misfortune is the same as described before, in that carbon monoxide bound to the catalytic layer reduces the active surface area available for reaction and creates a signiﬁcant over-voltage. The solution is also the same and is to add other metals to the Pt catalyst in order to promote monoxide oxidation. Despite a long list of possible tried candidate elements, only Ru is widely used in the DMFC. In many cases the proportion of Pt to Ru is rather different compared with the hydrogen-fed fuel cells. There is an opinion that the best working catalysts have between onethird and one-ﬁfth of atomic percent of ruthenium. The trend is thought to be related to the mechanisms of absorption of methanol on a catalyst surface. Even the best known anode catalysts for the DMFC have much lower efﬁciency and this is reﬂected in the high catalyst loading required. The amount of catalyst for a reasonable performance is usually at least one-order of
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magnitude higher for the DMFC than for hydrogen-fedPEM cells. Recent results show that catalyst activity can be signiﬁcantly increased by using carbon nanotubes as a support material . As discussed earlier the EMF produced in a methanol cell at room temperature and atmospheric pressure is around 1.2 V. Unfortunately, the open circuit voltage as a rule is only in the region of 0.7 V. The big difference is explained by the high cross-over of methanol though the standard Naﬁon membrane and the creation of mixed potential. Semi-empirical models have been developed to describe cell performance and to account for all the phenomena involved in the process . On the other hand, while the effect of cross-over is well known, there is some evidence that it is not well understood and its models need further development . Reduction of methanol cross-over is a signiﬁcant challenge for the DMFC. Few different approaches can be used and combined together in order to reduce the phenomena to a satisfactory level. The most common approach is the use of a diluted fuel solution at concentrations around of 1 M methanol in water. If at the same time the catalyst is made as active as possible, then almost all methanol under operational conditions would react at the anode and not so much of it would be left to diffuse through the PEM. It is also possible to modify the PEM surface itself and to apply a thin Pd coating to the anode side [99,100]. It was shown that, either plasma treatment or deposition of just a 20 nm thick layer of Pd can signiﬁcantly reduce methanol cross-over . At the same time the cell performance signiﬁcantly increased as compared with that for an untreated membrane system. Some polymeric materials have been synthesised which have a cross-over much lower than that of Naﬁon [23,101]. Cross-over of methanol also can be reduced by modiﬁcation and tailoring of membrane material by grafting . A thicker membrane would have a lower level of crossover. This is a widely used approach but, unfortunately, a thick membrane has also high resistance to proton permeation which is performance limiting. It is possible to change membrane properties by using ﬁllers. The inﬂuence of ﬁllers on membrane properties is far greater than just the modiﬁcation of the methanol crossover. Proton conductivity, oxygen diffusion, water swelling, water retention, mechanical strengths all change with the introduction of an additive. Some changes to the properties (not all at the same time) follow percolation phenomena: introduction of higher and higher amounts of ﬁller change a property slowly at ﬁrst and then, at concentrations around the 20–30% region, properties change suddenly by a step function. The hydrophilic ﬁllers signiﬁcantly increase water retention and water re-absorption on the cathode side. This may be not so important for DMFC, but it signiﬁcantly simpliﬁes water management issues for a hydrogen fed assemblies. Different ﬁllers have been used starting from colloidal silica [31,103] and simple metal oxides to zeolites and aluminosilicate clays.
Reviews of current activities in this direction can be found elsewhere . An interesting attempt to reduce methanol cross-over effects is to use cathode catalysts with high oxygen activity but low activity to methanol . This approach is being developed in mixed reactant fuel cells, where catalysts may be very selective either to fuel or oxygen [106–108]. The results in this direction so far are very promising. Carbon dioxide is produced in the anode area during operation of a DMFC. The gas can form bubbles and block fuel access to the catalyst if left unmanaged in certain situations. Many current issues for DMFC and international activities have been summarised recently by Dillon et al.  Other direct alcohol fuel cells (DAFCs) are possible. The energy content of higher alcohols is in excess of 8 kWh/kg and the electromotive force produced under normal conditions is again in the region between 1.1 and 1.2 V. Unfortunately, oxidation is more difﬁcult and this leads to higher overvoltages. In reality, only ethanol is considered at the moment as another viable fuel for the DAFC. Compared to methanol it has a few clear advantages. For instance it is not toxic, can be produced by fermentation and is thus regarded as a ‘‘renewable’’ fuel. Some countries, like Brazil, already use ethanol for internal combustion engine cars and have developed a production and delivery infrastructure. At low temperatures, normal pressure and pure platinum catalyst performance of the ethanol fuel cell is not impressive and the delivered power is usually below 10 mW/cm2. Low temperature cells beneﬁt from Pt/Ru or Pt/Sn catalysts. Latest data suggest that, for a Pt/Sn catalyst at 363 K, power densities of 60 raW/cm2 can be reached [110–112]. This was achieved using platinum loadings just above 1 mg/cm2, but with pure oxygen on the cathode at 2 bar pressure. High-temperature cells with silica-ﬁlled Naﬁon or a phosphoric acid-doped polybenzimidazole and high reactant pressure have been reported to achieve power densities reaching 140 mW/cm2 [113–115]. It is not clear at the moment if the earlier reported presence of not fully oxidised ethanol products in the anode exhaust  can be easily minimised or fully avoided. Carbon emission from hydrocarbon fed fuel cells cannot be avoided but there are some alternatives. It is possible to avoid this emission while utilising direct fuel cells if hydrazine (N2H4) (known also as diamine, diamide and nitrogen hydride) is used as a fuel. Hydrazine is a clear, water-white, hygroscopic liquid with almost water-like physical properties: melting point 2 1C, boiling point 113 1C. It is ﬂammable, has caustic properties and can be used as a strong reducing agent. One also can regard it as toxic, while effects of long-term toxicity for humans are mostly unknown. On the positive side one can consider that it does not accumulate in the environment: when released it slowly oxidises to ammonia. The price of hydrazine in big quantities is unknown, but probably can be estimated as a few US dollars per litre.
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Recently Yamada et al. have published reports on a direct hydrazine fuel cell [117,118]. The material has a high reversible thermodynamic potential of 1.56 V. Utilising Naﬁon 117 as a PEM and different unsupported single element powder catalysts (Pt, Ru, Rh, Pd) powers between 80 and 100 mW/cm2 at 0.5 V and anode loading 2–3 mg/cm2 were produced. Water solution at 10 wt% was used. The anode exhaust contained nitrogen, ammonia and hydrazine and it is clear that work to make this exhaust reach an acceptable quality remains an essential task and can be challenging by itself. Some fuel cross-over and mixed potential were also observed. 8. Delivering sustainable and affordable power Fuel cell implementation and development can be viewed through the achievement of certain targets. In many cases fuel cells replace existing technology and as such should show superior performance. This is a very tricky process since benchmarking is performed against well-developed and mature technology. Existing technology has already, in many cases, delivered its dividends on development and can itself be relatively cheap. Fuel cell technology on the contrary still needs to pay for research and development in many areas. One has also take into account such difﬁcultto-quantify parameters as environmental impact, future price and availability of hydrocarbon fuels. It is easy to get confused about what are exactly our current goals and targets in fuel cell performance. While it is possible to search the USA Department of Energy information releases for general performance benchmarks and use them as a guidance, it is also possible to talk about speciﬁc fuel cell use niches and implementations. An example of a general goal is, for instance, to produce a fuel cell with the support structures for automotive application with the cost of around 30–40 US dollars per kilowatt power and lifespan in the region between 5000 and 40,000 h (lower for cars and higher for buses). This is needed to match cost/lifespan and replace the current generation of internal combustion engines. Signiﬁcant cost savings, for instance, are impossible without replacing the Naﬁon, as its price alone can amount up to few US hundred dollars per kilowatt. In order to reach the general goal the whole membrane electrode assembly should cost below $10 kW1. The lifespan of the fuel cell stack is affected by such factors as membrane humidiﬁcation (with internal humidiﬁcation so far showing the best results), number of stop/ start cycles, span/amplitude/number of power cycles and number of starts from the frozen state. The volumetric power density is very important for critical applications (best result published at around 1.4 kW/l at 0.65 V) but this is probably not so important for other uses. Practical implementations of the DAFC can be roughly divided into two areas. The ﬁrst is low (below 373 K) temperature and atmospheric pressure air-fed fuel cells. This area mostly covers small batteries for mobile
applications. Published data on devices suggests that, at reasonable catalyst loads of around 30 g /kW, powers between 50 and 80 mW/cm2 can be reliably achieved. The second area covers high density high power applications. Working pressure and temperature are increased so that the cell performance usually increases 2–5 times at the comparable catalyst loads. The hope is that more active catalysts on high temperature stable supports and high temperature membranes will increase performance even further. Complex fuel cell systems depend on many types of sensor: power, temperature, ﬂow rate, pressure, gas chemical composition. All of them can be manufactured in thin ﬁlm form by vacuum deposition techniques, which could allow signiﬁcant cost reduction. Some sensors are very speciﬁc for fuel cell operation. For instance, CO sensing in the pure hydrogen stream [119,120], sulphur sensors [121,122] in reformers. 9. Conclusions PEM fuel cells are very simple in principle, but real life implementation of fuel cells is very challenging, mostly owing to the vast number of requirements the cells and technology need to satisfy to be acceptable for mass production. Requirements do not exclusively focus on performance and price but take into account availability of fuels, distribution and storage networks, environmental and human health impacts. Balancing this complicated equation with many futuristic unknowns is a challenging task. On the positive side, given enough hydrogen, PEM fuel cells can deliver low to zero pollution power. Production and delivery of the hydrogen to feed them is a major task by itself. One of the biggest challenges is still the price which is currently around $1000 kW1. The major part of this price is almost equally split between cost of the membrane and the cost of the assembly. The catalyst cost is relatively low, especially for hydrogen-fed PEMFCs. Mass production will bring down the price for assembly parts and there is strong indication that new polymers will be much cheaper than Naﬁon. Lately Ballard Power Systems announced that $100 kW1 is feasible for mass production with the current technology, but it is not clear how exactly this can be achieved. A signiﬁcant amount of knowledge has been accumulated relating to all aspects of fuel cells. Part of it has been published, countless aspects have been patented, but a signiﬁcant amount of it is treated as proprietary and is held by organisations on the frontiers of development. In appreciation of resources spent on accumulation of this information and possible future gains, the latter is very understandable. The thing to bear in mind is that in many situations when words ‘‘it is difﬁcult’’ are used, it does not mean ‘‘impossible’’ it means ‘‘still too expensive’’. Vacuum-based material production and treatment techniques can be used for all PEM fuel cell component
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production and for device monitoring. They also have use in hydrogen generation, puriﬁcation and storage. The overall potential is essentially at least four fold. First, it is deﬁnitely possible to increase performance of a fuel cell by use of vacuum deposited materials and offer further reduction of production costs. Secondly, it is possible to create materials which would be difﬁcult or impossible to produce by ‘‘wet technology’’. Thirdly, it is possible to reduce the environmental impact of fuel cell assembly manufacturing, as vacuum-based techniques are known to produce much less pollution as compared to traditional solvent-based techniques. It is also potentially possible to use plasma treatments (e.g. ) to improve, not only surfaces of a catalyst and a backplate, but also, diffusion layers and surfaces for a fuel cell assembly. References  Baranowski B. J Membr Sci 1991;57:119.  Kim L, Lee S-M, Srinivasan S, Chamberlin CE. Journal of Electrochemical Society 1995;142:2670.  Rowe A, Li X. J Power Sources 2001;102:82.  Busquet S, Hubert CE, Labbe J, Mayer D, Metkemeijer R. J Power Sources 2004;134:41.  Voss HH, Wilkinson DP, Pickup PG, Johnson MC, Basura V. Electrochim Acta 1995;40:321.  Zawodzinski JTA, Springer TE, Uribe F, Gottesfeld S. Solid State Ionics 1993;60:199.  Zawodzinski TA, Davey J, Valerio J, Gottesfeld S. Electrochim Acta 1995;40:297.  Ge S, Li X, Hsing I- M. Electrochim Acta 2005;50:1909.  Eckla R, Zehtnera W, Leub C, Wagnera U. J Power Sources 2004;138:137.  Lee W, Ho C, Van Zee JW, Murthy M. J Power Sources 1999;84:45.  Benziger J, Chia E, Moxley JF, Kevrekidis IG. Chem Eng Sc 2005;60:1743.  Ghielmi A, Vaccarono P, Troglia C, Arcella V. J Power Sources 2005;145:108.  Silva VS, et al. J Power Sources 2005;140:34.  Silva VS, et al. J Power Sources 2005;140:41.  Miura Y, Yoshlda H. Thermochim Acta 1990;163:161.  Mallant RKAM. J Power Sources 2003;118:424.  Adjemian KT, Srinivasan S, Benziger J, Bocarsly AB. J Power Sources 2002;109:356.  Li S, Zhou Z, Liu M, Li W, Ukai J. Electrochim Acta 2006;51:1351.  Schechter A, Savinell RF. Solid State Ionics 2002;147:181.  Wainright JS, Wang J-T, Weng D, Savinell RF, Litt M. J Electochem Soc 1995;142:121.  Jensen JO, Li Q, He R, Pan C, Bjerrum NJ. J Alloys Compd 2005;404–406:653.  Asensio JA, Borros S, Gomez-Romero P. Electrochem Commun 2003;5:967.  Jorissen L, Gogel V, Kerres J, Garche J. J Power Sources 2002;105: 267.  Matsuo Y, Saito K, Kawashima H, Ikehata S. Solid State Commun 2004;130:411.  Focus Catal 2003;2003:6.  Focus Catal 2004;2004:6.  Gupta RK, Dwivedy I. Curr Appl Phys 2005;5:163.  Savadogo O. J Power Sources 2004;127:135.  Mikhailenko SD, Zaidi SMJ, Kaliaguine S. Catal Today 2001;67:225.  Qingfeng L, Ronghuan H, Jensen JO, Bjerrum NJ. Chem Mater 2003;15:4896.
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