bluuebunny/crossref_metadata_2025 huggingface dataset:
Hydrogen/oxygen (air) fuel cells with alkaline electrolytes
Authors: Cifrain M. , Kordesch K.
Published: December 2010

Alkaline fuel cells (AFCs) operate well at room temperature, yield the highest voltage at comparable current densities, but the use of potassium hydroxide requires the elimination of carbon dioxide from the fuel and oxidants. The electrodes can be built from low‐cost porous materials (e.g., carbon, nickel‐foam) with small amounts of catalysts. The components and accessories for a system with a circulating electrolyte are fully developed. AFCs excel in operating at intermittent duty cycles, for example in mobile service. But AFCs can also be used in stationary applications, such as houses (solar power) and on farms (with ammonia cracker). Military applications need sizes ranging from 150 W to several kilowatts for charging accumulators in the field.

Very many different AFC systems have been developed, such as the bacon fuel cell, the Apollo/NASA space shuttle and the Russian systems, the Allis/Chalmers (static water control) H2/O2tractor, the Union Carbide Corporation H2/O2(150 kW) Electrovan, the Austin city car (H2/air and lead battery hybrid), the ELOFLUX system, the ELENCO system and the efforts to build an European Space Agency fuel cell power plant for the HERMES manned space vehicle, the Dornier/Siemens System and the efforts of the Swedish navy, Olle Lindström's work in Stockholm and his small bio‐fuel cells in India, the bipolar AFC system developed in Graz and the falling film fuel cell by Hoechst. Whereby, compressed and liquid hydrogen was used or hydrogen was produced using hydrocarbon, methanol or ammonia reformers.
Phosphoric acid electrolyte fuel cells
Authors: King J. M. , Kunz H. R.
Published: December 2010

Phosphoric acid electrolyte fuels cells were the first fuel cells to be commercialized with the 200 kW ™PC‐25 power plant of UTC Fuel Cells (formerly International Fuel Cells). The technology base was developed over many years and included input from the technology of other types of fuel cells. Improvements in fuel processing which converts hydrocarbon fuel to hydrogen, the cell stack that converts hydrogen to direct current power, the inverter that converts direct current to alternating current and controls ancillaries, were all required. The overall power plant design and ancillary component technology required substantial development to meet requirements for practical application. Fuel cell power plants were evaluated for various applications as the technology evolved. This section describes the applications and evolution of the components. The cell technology evolution is then described in more detail. All of the materials in the stack required modification to increase cell performance and allow operation at higher temperatures. Anode catalysts were developed with improved tolerance to the presence of carbon monoxide in the hydrogen fuel gas. Cathode catalysts were improved to reduce the performance loss associated with the oxygen reduction reaction, the main contributor to the cell polarization. In both cases, the quantities of precious metals used in the electrodes were reduced by more than an order of magnitude. The separator plates, electrode gas diffusion layers, and porous matrix for holding the electrolyte between the electrodes were all improved to provide higher cell performance and durability.
Aqueous carbonate electrolyte fuel cells
Authors: Cairns E. J.
Published: December 2010

Aqueous carbonate electrolytes for fuel cells represent another class of invariant electrolytes for use with carbonaceous fuels and/or air (containing carbon dioxide). This section reviews the development of aqueous carbonate electrolytes, their properties, and their use in fuel cells with organic fuels such as methanol. The demonstration of invariance for extended operating times is discussed. The improved oxygen reduction kinetics in these electrolytes is shown, and partially compensates for the somewhat slower kinetics of organic fuel oxidation. Reduced corrosion rates of electrocatalysts and cell construction materials are an attractive feature of these electrolytes.
Other direct‐alcohol fuel cells
Authors: Lamy C. , Belgsir E. M.
Published: December 2010

The state‐of‐the‐art of direct combustion fuel cells using oxygenated organic compounds with more than one carbon atom as fuels is reviewed. After introducing the basic principles of a direct combustion fuel cell in a friendly environmental context, thermodynamic and kinetic data concerning the electro‐oxidation of these fuels are discussed. Detailed reaction mechanisms together with the catalytic role of the electrode material are presented. Then the state‐of‐the‐art of some direct‐alcohol fuel cells, namely ethylene glycol and ethanol fuel cells, is discussed. Finally, some emphasis is put on future fuels, such as ethers and hydrated aldehydes.
Biochemical fuel cells
Authors: Katz E. , Shipway A. N. , Willner I.
Published: December 2010

Biofuel cells transform abundant raw materials into electrical power in the presence of biocatalysts, enzymes, or whole cell organisms. Biomaterials may participate in the biofuel cell activity by either producing fuel substrates or by catalyzing the electron transfer chain between the fuel substrates and oxidizers and the electrodes. Two types of biofuel cell elements are discussed in this review article: microbial‐based biofuel cells and enzyme‐based biofuel cells.

Microorganisms may act as microreactors in fuel cells for the generation of the fuel products such as H2or H2S. These fuel products may be generated apart from the biofuel cell and transported to its anodic compartment or, alternatively, may be directly generated in the anodic compartment of the biofuel cell. Different microbial‐based biofuel cells are reviewed in the account.

Enzymes are employed as catalysts for the activation of electron transfer chains between the fuel substrate and the anode in the anodic compartment, and between the oxidizer and the electrode in the cathodic compartment. In order to activate the electron transfer cascades between the enzymes and the electrodes, native electron carriers (co‐factors) and artificial electron transfer mediators must be coupled to the biocatalytic transformations. By the nanoengineering of the electrode surfaces with co‐factor/electron‐relay/enzyme assemblies, integrated, electrically contacted, bioelectrocatalytic anodes and cathodes are tailored. Different enzyme‐based biofuel cell configurations are described. The different parameters controlling biofuel cells efficiencies are discussed. The efficiencies of the different biofuel cell configurations are evaluated in terms of the limiting factors and kinetic features of the systems.
Metal/air batteries: the zinc/air case
Authors: Haas O. , Holzer F. , Müller K. , Müller S.
Published: December 2010

Metal/air batteries provide high specific energy and consist of inexpensive and environmentally benign materials. They have begun to penetrate the market of portable power sources and offer great promise for transportation applications. In this review, we first discuss the basic aspects of metal/air batteries, then concentrate on the Zn/air system as the most promising metal/air battery. The Zn/air systems have important advantages due to the excellent electrochemical properties of metallic zinc, which include a high overpotential towards hydrogen evolution. This implies that Zn/air batteries with aqueous electrolytes can be electrically recharged, that zinc from spent batteries can be regenerated by electrolysis in aqueous systems, and that mechanically rechargeable Zn/air batteries (arrangements resembling fuel cells) can be realized with aqueous electrolytes.

Advances in the development of Zn/air battery components are reviewed and the state of the art of electrically and mechanically rechargeable Zn/air systems is described in detail. An overview of industrial developments in zinc/air battery systems and the nominal performance of available products is included. Practical specific energies in the range of 90–120 Wh kg−1have been reported for electrically rechargeable Zn/air batteries, about twice as much for mechanically rechargeable systems. Significant progress has been made concerning the cyclability of the electrically rechargeable Zn/air system, by utilizing pasted zinc electrodes and graphitized carbon for the bifunctional oxygen diffusion electrodes. Cycle lives in the range of 200–450 cycles have been achieved.
The role of adsorption
Authors: Sánchez C. , Leiva E.
Published: December 2010

In this section different aspects of adsorption and its role in the electrochemical environment are discussed. The methods employed to describe chemisorption from a theoretical point of view are enumerated.
Electrode potential as parameter
Authors: Sánchez C. , Leiva E.
Published: December 2010

In this section the role of the changes in the potential difference across the metal/solution interface is discussed, especially concerning electron transfer reactions. The predictions of phenomenological approaches are analyzed, as well as models based on quantum mechanics.
The
NEMCA
effect
Authors: Sánchez C. , Leiva E.
Published: December 2010

A brief introduction to the non‐faradaic electrochemical modification of catalytic activity (NEMCA) effect is given.
Adsorption — volcano curve
Authors: Trasatti S.
Published: December 2010

This chapter gives a theoretical description of electrocatalysis, providing fundamental aspects not related to particular experimental details. The relationship between intermediate adsorption and reaction rate is discussed. This includes the nature of the solid as well as its surface crystallographic orientation. Primary and secondary effects of the electrode material are illustrated in terms of double layer and chemisorption effects. The origin of volcano curves is demonstrated and their relation to the reaction mechanism pointed out. It is shown that no absolute evaluation of electrocatalysis is possible since the conditions at which electrical effects at the interface vanish are not known.
Theoretical aspects of some prototypical fuel cell reactions
Authors: Leiva E. , Sánchez C.
Published: December 2010

In this section we discuss the application of different theoretical models to two types of prototypical fuel cell reactions. On the one hand, we discuss the electrocatalysis of the electrooxidation C1 (CO, HCOOH and CH
3
OH) molecules. On the other hand, we consider theoretical aspects of the oxygen reduction reaction, with special emphasis in the most recent developments in the theoretical field.
Theory of the
NEMCA
effect
Authors: Leiva E. , Sánchez C.
Published: December 2010

Theoretical aspects of the NEMCA effect are discussed.
Cyclic voltammetry
Authors: Vielstich W.
Published: December 2010

Cyclic voltammetry (CV) is the most commonly used method in electrocatalysis. The technique offers a first view on electrode/electrolyte systems in the potential window of interest. For most metals and alloys, reproducible results can be obtained after cycling the surface in a base electrolyte and over suitable potential ranges. Cyclic voltammograms of some metal/electrolyte interfaces and some selected charge transfer reactions, of interest in fuel cell electrocatalysis, are presented. The use of CV for investigations, e.g., with ultramicro‐electrodes, rotating electrodes (disc and ring/disc), as well as in combination with online mass spectroscopy and Fourier‐transform infrared radiation spectroscopy, is outlined.
Product analysis
Authors: Torresi R. M. , Wasmus S.
Published: December 2010

Considering interfacial reactions, during the last three decades, surface scientists and electrochemists have made an important effort to develop instrumental techniques in order to study not only the structure and composition of solid interfaces but also to identify product reactions. The need for measuring interfacial events at solid surfaces under ambient conditions has induced the developing of direct in situ methods. In the case of electrochemical processes, which involve mass changes, one of the new techniques introduced in the last two decades is the electrochemical quartz crystal microbalance (EQMB). Results related to anion adsorption and oxidation of small organic molecules are discussed. Other technique which has become a powerful method in electrochemistry is the electrochemical mass spectroscopy, now also directly applicable to fuel cell studies. The technology necessary to design the instrumentation for these investigations is described in depth. Results related to anode oxidation processes are discussed. Initial concerns that the methanol oxidation in a direct methanol fuel cell (DMFC) may not be complete have been shown to be unsubstantiated. Using a Pt/Ru anode and a sufficient excess of water, a yield of CO
2
near 100% is achieved. In contrast, the use of ethanol as a fuel leads to a predominant aldehyde formation under all conditions. The mechanisms of the processes involved are analyzed in the light of these results. Cathode studies showed that the methanol crossover effect in a DMFC is likely a dual mechanism consisting of a non‐electrochemical reaction with additional cathode poisoning. Product identification of oxidation of small organic molecules by using liquid micro‐chromatography or fluorimetry it is also discussed.
Electrochemical impedance spectroscopy
Authors: Ivers‐Tiffée E. , Weber A. , Schichlein H.
Published: December 2010

The fundamentals of electrochemical impedance spectroscopy and its application to the identification of dynamical processes in fuel cell operations are described in this chapter. The scope of the method is seen in the fact, that by applying a small signal perturbation to the cell under test conditions, the relaxation times and relaxation amplitudes of the processes governing cell performance can be identified and targeted for cell optimization. Based on the working principle and the basic measurement set‐up of impedance spectroscopy, measurement arrangements for fuel cells are discussed and specific problems of the method are addressed. Various impedance data analysis techniques are described, including the well‐known complex non‐linear least squares curve fit to equivalent electrical circuit modes as well as the direct deconvolution of impedance data into a distribution of relaxation times. The importance of suitable physical models for the identification of microscopic cell parameters is stressed and an example for the identification procedure is given.
Ex‐situ surface preparation and analysis: transfer betweenUHVand electrochemical Cell
Authors: Hoster H. E. , Gasteiger H. A.
Published: December 2010

The further optimization of electrode materials for fuel cells in laboratory studies offers two main approaches: (i) improving the understanding of the microscopic electrode processes by observation with in‐situ and ex‐situ methods; and (ii) the systematic compositional and structural variation of surfaces to create model electrodes for electrocatalytic measurements.

The ultra high vacuum (UHV) environment offers unique possibilities for both surface preparation and analysis and has thus been used extensively in recent electrocatalytical research work. At the same time, methods allowing the in‐situ observation of electrochemical processes has also gone through impressive advances in recent years, and in many cases it has been found that earlier ex‐situ results obtained in UHV needed to be revised due to changes of the electrode surfaces during the transfer into UHV. In the following we will discuss some of the most frequently applied ex‐situ methods in electrocatalytic research and review some of the important aspects concerning the reliability of the resulting data. Utilizing a number of examples, we will demonstrate how such combined UHV electrochemistry experiments can be technically realized, including a discussion of commonly encountered practical problems.

We will further point out the advantages offered by the electrocatalytical characterization of UHV designed and UHV characterized model electrodes. In our opinion, this is a very promising approach to UHV/electrochemistry experimentation: on the one hand, the uncertainties due to the transfer between UHV and electrochemical environment are less significant compared to emersion experiments of adsorbed adlayers; on the other hand, well‐established UHV preparation techniques combined with structural characterization tools (atomic force microscopy (AFM)/scanning tunneling microscopy (STM)) provide the possibility of precisely tailoring defined model electrodes. Some examples from research on anode catalysts are given to illustrate this unique capability of UHV electrochemistry setups as powerful tool for the systematic tailoring of model electrodes.
Structure sensitive methods:
AFM
/
STM
Authors: Kibler L. A. , Kolb D. M.
Published: December 2010

In this chapter, the principles of atomic force microscopy (AFM) and mainly of scanning tunneling microscopy (STM) are described. For applications in electrochemistry, in situ operation and the potentialities of STM and AFM are addressed.

STM, a powerful technique for obtaining local structure information of model electrodes on an atomic level and in real space, is highlighted by several case studies, relevant to fuel cell systems: (i) preparation and characterization of well‐ordered noble metal surfaces; (ii) noble metal deposition on foreign substrates; and (iii) nanostructuring of surfaces by the STM tip.
Rotating thin‐film method for supported catalysts
Authors: Schmidt T. J. , Gasteiger H. A.
Published: December 2010

In this chapter, the thin‐film R(R)DE (TFR(R)DE) method is described, i.e., a method for the quantitative evaluation of the electrocatalytic activity of high‐surface area catalysts in a true R(R)DE configuration. The method offers complete catalyst utilization combined with a high reproducibility with respect to the catalyst loading on the electrodes. It is further proven that both catalyst agglomerate and film diffusion effects, respectively, are negligible in the thin catalyst/Nafion layers. That means the thin‐film electrodes can be treated like smooth rotating electrodes with the well‐known hydrodynamics and defined mass‐transport conditions. In other words, kinetic parameters such as kinetic current densities, activation energies, Tafel slopes etc. can directly be determined from the measured current densities
without
applying any mathematical modeling of the mass transport in the catalyst/Nafion layer. Furthermore, this method allows the determination of electrode kinetics under fuel cell relevant mass‐specific current densities of 1–2 A

. The suitability of the TFRDE method is demonstrated by discussing results from different kinetic studies on the oxidation of CO and CO/H
2
mixtures on high‐surface area Pt, PtRu, PtSn and PdAu electrocatalysts. Additionally, it is illustrated how thin‐film electrodes can be successfully used for measurements with a differential electrochemical mass spectrometry (DEMS) setup for the mass‐spectrometrical determination of the electrochemically formed products. In the last section, the adaptation of the TFRDE method for ring‐disk measurements is described. The potential‐ and rotation‐rate independent, well‐defined collection efficiency for the thin film RRDE configuration enables the quantification of the molar fraction of H
2
O
2
produced during oxygen reduction. In general, the TFR(R)DE method can be seen as an easy‐to‐use tool for the screening and characterization of high‐surface area electrocatalysts under low temperature fuel cell relevant conditions.
Reaction mechanisms of theH2oxidation/evolution reaction
Authors: Breiter M. W.
Published: December 2010

The mechanisms of the H2oxidation/evolution are discussed on basis of the classical pathways (Volmer–Tafel reactions and Volmer–Heyrovsky reactions). For simplicity the discussion is given for acidic systems under the assumption of Langmuirian adsorption of the atomic hydrogen, adsorbed on the electrode surface. The modifications necessary for alkaline systems are indicated.

The conditions are outlined when one of the pathways is predominant. Control by a single step is then discussed. The possible effect of mass transport is considered. The dependence of the exchange current densities of the individual reactions upon the coverage with atomic hydrogen is derived.

The experimental observation that relationships between the logarithm of the exchange current density and the MH bond strength appear to possess a volcano shaped curve is discussed in detail under consideration of how the hydrogen atoms are bonded. The question of which type of hydrogen atoms is involved in the H2oxidation/evolution on platinum metals is briefly considered.

Finally, a more complex reaction is treated in which either one or several products are formed. The differences in the kinetics of such reactions to that of the simpler H2oxidation/evolution are pointed out.
Hydrogen evolution reaction
Authors: Lasia A.
Published: December 2010

This chapter presents a review of electrode materials used for the hydrogen evolution reactions and a comparison of their electrocatalytic activities. The introduction presents a brief definition of various thermodynamic water electrolysis potentials. In general, there are two ways to improve the performance of electrode materials: (1) use of electrode materials characterized by higher intrinsic activity i.e., higher exchange current density and (2) use of electrodes characterized by large real surface area. Both methods are used in practice. In this chapter various electrode materials are reviewed: smooth metals, alloys, intermetallic compounds and composites, Raney type materials (Raney Ni, Zn, etc.), oxides, carbides, sulfides, borides, phosphides, amorphous and nano‐crystalline materials etc., Among the most active materials are noble metals, doped Raney‐type alloys, IrO2/Ru2O, sulfides, borides, and Ni/Mo based alloys. Unfortunately, noble metals are very expensive and easily poisoned and their activity decreases with time. Although many materials may still be improved and optimized there is a need to study the detailed mechanism and kinetics of the HER on these materials and the relation between the geometric (surface roughness) and intrinsic electrocatalytic properties.
O2reduction on thePt/polymerelectrolyte interface
Authors: Ota K. , Mitsushima S.
Published: December 2010

In order to attain the high performance of polymer electrolyte fuel cells (PEFCs), the cathode overpotential as well as the resistance of the membrane electrolyte should be reduced. In this paper, the oxygen reduction reaction on Pt has introduced in relation to the quality of the polymer electrolyte. The oxygen overpotential was measured by three electrodes using the mini‐cell method. The mini‐cell has a Pt microelectrode for the working electrode and a dynamic hydrogen electrode for the reference electrode. The contact force at the Pt electrode/membrane interface was measured and controlled by a strain gauge. The solubility and the diffusion coefficient of oxygen in the polymer electrolyte depended on the ion‐exchange capacity of the membrane electrolyte. The diffusion coefficient increased with the decrease of the ion‐exchange capacity. The solubility increased with the increase of the capacity. The dependence of the diffusion coefficient was larger than that of the solubility. The Tafel plot of oxygen reduction current on the Pt showed the two distinct mechanisms for the low and the high overpotential regions. At the low overpotential region, the oxygen reduction reaction proceeded by a 4–electron reaction, the exchange current density had a range from 10−9to 10−8 A cm−2. The dependence of the exchange current density on the ion‐exchange capacity was small.
O2reduction and structure‐related parameters for supported catalysts
Authors: Mukerjee S. , Srinivasan S.
Published: December 2010

Supported electrocatalysts starting with the initial patent by Petrow and Allen opened up a new horizon in the field of fuel cell electrocatalysis. Choice of new chemical dispersion methods and choice of carbon support materials led to a quantum jump in the performance of low and intermediate temperature acid and alkaline fuel cells. Among the advantages realized early were: (i) lowering overpotential losses in the activation, mass transport and ohmic polarization regions; (ii) mitigating corrosion and sintering problems; and (iii) increased catalyst utilization and hence decreased cost of noble metals by one or two orders of magnitude. This chapter reviews the fundamental and technological progress in the research on supported electrocatalysts for both phosphoric acid fuel cells (PAFCs) and polymer electrolyte fuel cells (PEMFCs). Early developments in the realization of particle size effects and the interplay between electronic and short range atomic order has been chronicled for the more kinetically difficult oxygen reduction reaction, until the advent of Pt alloy electrocatalysts. The effect of carbon support is also examined from the perspective of possible catalyst‐support synergy both in terms of imparting improved electrocatalysis but also in terms of increased stability (resistance to sintering). Methods of dispersing metals on carbon support are chronicled along with some new developments. In addition, newer techniques involving synchrotron X‐ray techniques are presented in terms of elucidating the true nature of electrocatalysis of oxygen reduction reaction. Some new perspectives on the continued use of these supported electrocatalysts are presented, especially in the emerging field of new synthesis routes for making tailored Pt alloy electrocatalysts.
Oxide‐based
ORR
catalysts
Authors: Goodenough J. B. , Cushing B. L.
Published: December 2010

The reaction pathways on Pt for the oxygen reduction reaction (ORR) in acid and alkaline solution are compared with those on metallic transition‐metal oxides. These reactions are distinguished from those that occur at high temperature at an oxide/air interface of an oxygen‐permeation membrane or the cathode of a solid oxide fuel cell (SOFC). Distinctions are made between polaronic, vibronic, and metallic electronic conductivity in oxides, and the influences of counter cation on the conductivity are pointed out for oxides like the AMO
3
perovskites and A
2
M
2
O
6
O′ pyrochlores, where M is the transition‐metal atom responsible for electron transport. Mixed oxide‐ion and electronic conductors (MIECs) are used as permeation membranes or as cathodes of a SOFC. The criteria for oxide‐ion conductivity in an oxide‐ion electrolyte are reviewed; the same criteria apply for MIECs. The ORR takes place at the cathode of an electrochemical cell, and gas‐diffusion cathodes operating with aqueous electrolytes contain as catalyst Pt or an oxide dispersed on teflon‐bonded, stabilized carbon. Dispersed Pt is compared with Pb
2
[Ir

2−
x

Pb

x

]O
6
O′

1−
y

pyrochlore for an acidic electrolyte. Perovskites are used in alkaline solution, and a bifunctional ORR and oxygen evolution reaction (OER) is used in secondary metal/air batteries. Performance curves for perovskites and pyrochlores are compared. Finally, the criteria for choosing a MIEC as a SOFC cathode or an oxygen‐permeation membrane are given and the performances of several of them are compared.
Chevrel phases and chalcogenides
Authors: Alonso‐Vante N.
Published: December 2010

The underlying background and present development of methanol tolerant electrocatalysts, selective for the oxygen reduction reaction (ORR), are presented. These catalysts are essentially metal‐centered (ruthenium) clusters or cluster‐like compounds. These clusters, partially embedded in a chalcogenide matrix (e.g., selenium) constitute the active center for the electrochemical process ORR. On well‐defined cluster materials, as for example, the so‐called Chevrel phases, based on octahedron clusters [(Mo2/3Ru1/3)6Se8], the ORR electrochemical activity clearly indicated that this phenomenon can be selectively sustained with a four‐electron charge transfer (water formation). This is due to their electronic and geometric characteristics. Hence, clusters work as a pool of charges that can be engaged in a multi‐electron process. Therefore, this family of materials served as a model for the development of novel ones. In this respect, e.g., RuxSey(x ≈ 2,y ≈ 1), tailored from chemical precursors in organic solvents, under mild conditions, shows a high dispersiveness in the nanoscale range. The metallic center shows similar electrochemical properties to those of well‐defined Chevrel phases, thus behaving as a cluster‐like material. The ability of the catalytic center to provide coordination chemistry for reactivity and selectivity via cluster, or cluster‐like compounds is discussed, as well as the trends in the synthesis and the results of tests in half‐cells and in direct methanol fuel cell systems.
Macrocycles
Authors: Zagal J. H.
Published: December 2010

The catalysis of the reduction of oxygen on supported metal macrocyclics is discussed in terms of electronic interactions of the dioxygen molecules with metal centers that lead to the weakening of the OO bond and to a lowering of activation energy. Correlations between electronic properties of the macrocyclic complexes and activity are also discussed, such as the influence of the redox potential of the metal center in the electrocatalysis. The overall 2‐ versus 4‐electron reduction mechanisms on metallo‐macrocyclics are discussed including the different approaches used in the literature to achieve 4‐electron reduction mechanisms by designing face‐to‐face binuclear complexes, polymerized metal complexes or macrocycles substituted with peripheral redox groups. The systems that are more promising for practical applications, such as the heat‐treated carbon supported catalysts, are discussed in detail and with explanations on how these pyrolyzed catalysts have higher activity and better stability compared with other supported macrocyclic systems. The requirement for the existence of the “burned‐in” MN4moiety also known as the “super‐site” in heat‐treated catalysts is emphasized. Heat‐treated carbon supported macrocyclics catalysts are not poisoned by methanol and in principle they can be suitable for low‐temperature acid electrolyte fuel cells. The methods of preparation of supported catalysts include: (a) modified electrodes by adsorption or surface modification; (b) heat‐treated carbon supported catalysts; (c) influence of carbon surface modification and electrodes prepared by polymerized macrocyclics; and (d) methanol tolerance and comparison of the activities with platinum‐based electrocatalysts.
O2‐reduction at high temperatures:SOFC
Authors: Ivers‐Tiffée E. , Weber A. , Schichlein H.
Published: December 2010

In this chapter the general requirements for solid oxide fuel cell (SOFC) cathodes and several ways of implementing them are described. The transport properties of cathode structures based on different perovskite type metal oxides are the main focus. Various possible reaction pathways for the two classes of cathode material, i.e., electronic conducting and mixed ionic/electronic conducting cathodes, are discussed. The importance of the microstructural properties of the cathode/electrolyte interface is illustrated. The dependence of cathode performance on the interaction between cathode composition and interface structure is demonstrated by i/v‐characteristics of different cathode/electrolyte configurations. A literature review of models for the high temperature oxygen reduction is given. The elementary reaction steps used in the different models are classified into three categories: surface reaction, surface and bulk diffusion, and incorporatied into the electrolyte. A general approach for modeling and simulation of the electrical response of the oxygen reduction process is presented by using a two‐step model accounting for surface adsorption of oxygen and charge transfer of oxygen into the electrolyte in the vicinity of the three phase boundary.
Formic acid oxidation
Authors: Feliu J. M. , Herrero E.
Published: December 2010

Formic acid oxidation on platinum is a structure‐sensitive reaction that takes place via a mechanism with two paths. One of the paths leads to the formation of CO (poison intermediate), which adsorbs strongly on to the surface and hinders the oxidation of formic acid through the active intermediate. Both paths are structure sensitive. Activities for both paths are parallel; a high activity for the path through the active intermediate is usually associated with a high activity for poison formation. The modification of the surface with adatoms has been used to increase catalytic activity and/or diminish the poisoning of the platinum surfaces. The behavior of the adatoms on the different platinum surfaces is discussed and a model for the actuation of the adatoms is given.
Methanol effects on theO2reduction reaction
Authors: Gilman S. , Chu D.
Published: December 2010

The present most advanced technology for direct methanol fuel cell systems is based on proton exchange membrane technology. The membrane/electrode assembly is normally comprised of a sulfonated perfluorinated hydrocarbon membrane that acts as both the electrolyte and mechanical support for the anode and cathode electrodes. The cathode is usually fabricated using Pt black or Pt supported on carbon as the electrocatalyst. The operating temperature is usually well below 100 °C, to minimize vaporization of fuel and water. While this construction offers many advantages, including an invariant, immobile high conductivity electrolyte and very compact cells and cell stacks, it presents disadvantages relating to high permeability of methanol through present available membranes. Methanol permeating through the membrane is usually reported as completely oxidized at the Pt‐based cathode. Permeation of methanol occurs by a combination of thermal diffusion and osmosis and can be equivalent to upwards of 100 mA cm−2of methanol oxidation current. From the practical point of view, methanol permeation is objectionable because it both wastes fuel and depresses the cathode potential.

At the oxygen/air cathode, oxygen reduction and methanol oxidation occur simultaneously. For a simple “mixed potential” effect, the two partial currents would have the same magnitude at any potential as they would if fed to the electrode separately. That is, the cathodic overvoltage would increase to an extent corresponding to an increase in current density corresponding to the partial methanol oxidation current. In the actual practical situation, there is an additional increase in overvoltage of ∼50 mV that appears to correspond to poisoning of the electrocatalyst by methanol adsorbate.

There are a number of approaches that have been pursued to reduce the “methanol effect”. Using dilute fuel and operating a fuel cell at high anodic current densities (causing some concentration polarization) can greatly reduce permeation, but with an increase in system complexity. A number of new or composite ion membranes have been explored. So far, reduced permeation has also been accompanied by lower conductivity, poorer mechanical strength or higher electrode polarization. Perhaps the most promising approach involves the development of methanol tolerant electrocatalysts, that are based on porphyrins, chevrel‐phase compounds and other transition metal chalcogenides, some of which appear to approach the catalytic efficiency of Pt. Establishment of long‐term chemical/electrochemical stability of such materials could provide the best solution to the problem of “methanol effects”.
Oxidation ofC2molecules
Authors: Petrii O. A.
Published: December 2010

The nature of processes responsible for open‐circuit potentials in solutions containing C2 compounds is considered. The peculiarities of adsorption of C2 compounds on electrode surfaces are reviewed, and also the specific features of electrocatalytic oxidation of these reactants under nonsteady state and steady state modes. The principles of choice of electrode materials for effective electrooxidation of C2 molecules are formulated, and the prospects of practical applications of these reactions are discussed.
Oscillations and other dynamic instabilities
Authors: Krischer K. , Varela H.
Published: December 2010

Virtually any electrochemical reaction exhibits temporal or spatial instabilities, such as bistable reaction rates, spontaneous rate oscillations and the formation of propagating potential waves, in some ranges of the external parameters. Most of these instabilities are associated with a negative real faradaic impedanceZF(ω), whereby two cases are distinguished: Either the stationary polarization curve possesses a negative differential resistance, i.e.,ZF(ω = 0) < 0, orZFis negative only in a certain interval of finite perturbation frequencies ω. The first type of systems may oscillate if a fixed voltage is applied to the cell (i.e., under potentiostatic control), the second class exhibits oscillations under potentiostatic and galvanostatic conditions. Most of the oxidation reactions of small organic molecules belong to the second class. We first discuss general properties of systems falling in either of these two categories. Then we present a survey on temporal and spatial nonlinear phenomena observed in fuel cell relevant electrocatalytic reactions and deduce the reaction steps essential for the instability in the individual reactions.
Hydrogenation reactions
Authors: Pastor E. , Rodríguez J. L.
Published: December 2010

Hydrogenolysis reactions (at alcohols) and hydrogenation of unsaturated groups (double and triple bonds in hydrocarbons and alcohols) are analyzed by combining data obtained applying traditional electrochemical methods, such as long‐term electrolysis followed by chromatographic studies, with results from differential electrochemical mass spectrometry (DEMS). This technique allows the detection not only of the gases evolved in these processes, but also of the volatile products formed. Thus, for example, the formation of propanol and allyl alcohol during the electroreduction of propargyl alcohol at platinum was established from DEMS, while previous studies only suggested this possibility as an explanation for the current efficiency of about 60–70% calculated for the gas production. Moreover, the formation of each reduction product can be recorded independently with time, choosing appropriate mass to charge values for these compounds, as shown for benzyl alcohol at platinum and palladium. Finally, the introduction of single crystals to the electrochemical systems provides a new tool for the study of the influence of the structure on the reactivity towards the reduction of these molecules. The hydrogenation of ethene, ethyne or benzene at different oriented surfaces serves as illustration of this effect.
CO2‐reduction, catalyzed by metal electrodes
Authors: Hori Y.
Published: December 2010

The electrochemical reduction of CO2proceeds in aqueous electrolytes with large overpotential, since the first step of the CO2reduction, i.e., CO2−anion radical formation, requires highly negative potential. The product distribution from the CO2reduction depends greatly on the electrode metal. Cu yields CH4, C2H4and alcohols; Au, Ag and Zn give CO as the main product; Pb, Hg, In, Sn, Cd and Tl form HCOO−; Ni, Fe and Pt do not reduce CO2whereas H2evolves from the electrolysis of H2O. The reason is rationalized in terms of the adsorption of CO2−anion radical on the electrode metals. The CO2reduction at the single crystal electrodes of Cu and Pt metals is summarized, and the mechanism of the hydrocarbon formation at Cu electrode is discussed in detail. The rate of CO2reduction is enhanced by increased transport of CO2to the electrode; such as the solid polymer electrolytes, the gas diffusion electrodes, the electrolyses under high pressure or in nonaqueous electrolytes.
Natural gas for power generation and the automotive market
Authors: Theisen T.
Published: December 2010

This article demonstrates that natural gas must be counted among the sustainable energy sources. What is more, natural gas acts as a bridge to a supply system based on hydrogen. The significance of natural gas in the primary energy mix of industrialized countries will only increase as time goes on. Increasing competition in the energy markets under the heading of liberalization will lead on the one hand to the development and use of highly efficient energy converters like fuel cells, for example, and on the other hand political pressure with an eye to the European CO
2
reduction targets will give clear impulses in this direction. This article also demonstrates the advantages and opportunities of using natural gas as a low carbon fossil fuel in order to achieve these targets. Above and beyond this, the role of natural gas in the energy intensive domestic, industrial and transportation sectors will be fully examined. In particular, the influence of fuel cell technology in these areas of application will be presented and the significance of natural gas as a fuel examined from technical and economical points of view.
Synthetic hydrocarbons as long‐term fuel option
Authors: Price J. G. , Jager B. , Dancuart L. , Keyser M. J. , van der Walt J.
Published: December 2010

While debate still rages over what the long term primary fuel source for fuel cells will be, there is no doubt that hydrocarbon fuels have a major advantage in terms of infrastructure and energy density compared to all other options. The disadvantage of these fuels lies in the fact that oil‐derived fuels are relatively expensive to convert into hydrogen and tend to be aggressive towards the fuel processor (reformer) and fuel cell catalysts. Synthetic hydrocarbon fuels, produced via the Fischer–Tropsch (FT) synthesis provide a sulfur‐ and aromatic‐free alternative to conventional hydrocarbons which are consequently more benign towards fuel cell catalysts. Currently, the technology is on a fast pace towards full commercialization, with the expectation that as much as 1 million barrels per day of capacity could be on line by the end of this decade, contingent on world economic conditions. Fuels produced using the gas‐to‐liquids technology, particularly when utilizing associated gas sources, provides an environmentally sound method of producing liquid fuels. Gasification of solid and liquid waste products may also be beneficially used to produce the synthesis gas required for the FT synthesis.
High pressure storage
Authors: Funck R.
Published: December 2010

The use of hydrogen as an automotive fuel requires storage systems that have inherent safety as well as volumetric and gravimetric efficiency. While the market for composite pressure vessels is continuously growing, metallic and hoop wrapped metallic pressure vessels are more and more substituted by fully wrapped metal liner and all‐composite pressure vessels, which bring weight savings of up to 75%.

Different types of composite pressure vessels are described in the following. Also equations for the estimation of the density of hydrogen gas and its energy content at different pressure levels are given. The description of design principals of pressure vessels is followed by the overview of manufacturing techniques and filament winding theory underlined by some typical examples taken from actual manufacturing. Future development of high pressure storage is also given within this contribution.
Liquid hydrogen technology for vehicles
Authors: Wolf J.
Published: December 2010

The paper provides information about the advantages of using of liquid hydrogen (LH
2
) compared with compressed gaseous hydrogen gas. The necessity of specially designed tank systems to fit the restricted available space within the vehicles is highlighted. The hermetic clean break coupling for the filling/refilling of cryogenic fuel systems, which enables safe and easy handling, extremely short filling and coupling times as well as a high rate of filling sequences, is presented. The indispensable basic element of the world's first public LH
2
filling station in Munich airport — the robot application of this coupling — is illustrated.
Aqueous borohydride solutions
Authors: Suda S.
Published: December 2010

This chapter describes the aqueous borohydride solution as a hydrogen storage system. Its applications for supplying gaseous hydrogen to the polymer electrolyte membrane fuel cell are introduced. The contribution describes the borohydride system as a liquid hydride and consists of the following four main topics: (i) borohydride systems as a gaseous hydrogen source; (ii) production of borohydride from borax or a mixture of borate and sodium hydroxide; (iii) hydrogen generation system; and (iv) protide (H
−
) in BH
4
−
complex ions as the source of two electrons per protide and its application to a new type of fuel cell named the borohydride fuel cell.
Catalyst development and kinetics for methanol fuel processing
Authors: Peppley B. A. , Amphlett J. C. , Mann R. F.
Published: December 2010

The reactions involved in methanol fuel processing are discussed. It is stated that the direct reaction of methanol and steam to form carbon dioxide and hydrogen is the key hydrogen producing reaction. Catalysis by Cu/ZnO/Al
2
O
3
is described in general terms. The literature on the kinetics of methanol–steam reforming is described in detail. Early kinetic models that were adequate for a limited range of operating conditions are discussed. The importance of understanding the surface reaction mechanism for developing models that are valid over wide ranges of conditions is described, followed by a description of the evolution of surface mechanisms for the process leading to a comprehensive kinetic model. The usefulness of this mechanistically‐based model is described in detail. The relative importance of the water–gas shift reaction, in particular, is revealed. Subsequently, the limitations and operating problems associated with Cu‐based catalysts are discussed. Both deactivation and pyrophoric behavior are cited as major problems. Finally, alternatives to Cu‐base catalysts are discussed. These include Ni‐hydrotalcites and Pt on ceria. Although these catalysts have lower activity than Cu‐based catalysts at temperature below 300 °C, their thermal stability at temperatures as high as 390 °C makes them more practical in fuel processors for methanol‐fuelled fuel cell systems.
MixedPOX/steam‐reforming reactor design considerations
Authors: Reinkingh J. , Petch M.
Published: December 2010

Proton exchange membrane fuel cell (PEMFC) system developers for automotive applications are looking to generate hydrogen on‐board a vehicle. Methanol is relatively easy to reform into a hydrogen‐rich stream at moderate temperatures. Since methanol is a liquid, it is easy to handle and it has a good energy density.

The two processes applied to generate H2from methanol are steam reforming and mixed partial oxidation (POX)/steam reforming (SR). In a mixed POX/SR reformer system reactor the oxidation and steam reforming reactions all take place in the same reactor. This type of reforming is also often referred to as autothermal reforming.

A mixed POX/SR reformer system reactor holds several advantages over a SR reformer system reactor. These include shorter start‐up time, excellent transient performance and a compact reactor design. The efficiency of a mixed POX/SR fuel processor, if designed properly, should be similar to that of a steam reformer system.

Mixed POX/SR fuel processors have similar challenges to SR systems, which are heat integration, catalyst optimization and durability. The disadvantages are the dilution effect of the nitrogen from the air and the parasitic power loss of supplying the air when operated at pressure. Mixed POX/SR reformers are more suitable for near ambient pressure operation.
Steam reforming,ATR, partial oxidation: catalysts and reaction engineering
Authors: Rostrup‐Nielsen J. R. , Aasberg‐Petersen K.
Published: December 2010

This chapter presents steam reforming and partial oxidation as a means for converting a hydrocarbon feedstock into a hydrogen containing gas for fuel cells. The different reactor concepts, choice of catalyst and causes of deactivation are described together with reaction mechanisms. The advantages and disadvantages of the alternatives are discussed with emphasis on efficiency, operating conditions, type of fuel cell, and simplicity.