Electrocatalysts

The porous electrodes in PEFCs are bonded to the surface of the ion-exchange membranes which are 0.12- to 0.25-mm thick by pressure and at a temperature usually between the glass-transition temperature and the thermal degradation temperature of the membrane. These conditions provide the necessary environment to produce an intimate contact between the electrocatalyst and the membrane surface. The early PEFCs contained Nafton membranes and about 4 mg/cm of Pt black in both the cathode and anode. Such electrode/membrane combinations, using the appropriate current coUectors and supporting stmcture in PEFCs and water electrolysis ceUs, are capable of operating at pressures up to 20.7 MPa (3000 psi), differential pressures up to 3.5 MPa (500 psi), and current densities of 2000 m A/cm. ... 

Alkaline Fuel Cell. The electrolyte ia the alkaline fuel cell is concentrated (85 wt %) KOH ia fuel cells that operate at high (- 250° C) temperature, or less concentrated (35—50 wt %) KOH for lower (<120° C) temperature operation. The electrolyte is retained ia a matrix of asbestos (qv) or other metal oxide, and a wide range of electrocatalysts can be used, eg, Ni, Ag, metal oxides, spiaels, and noble metals. Oxygen reduction kinetics are more rapid ia alkaline electrolytes than ia acid electrolytes, and the use of non-noble metal electrocatalysts ia AFCs is feasible. However, a significant disadvantage of AFCs is that alkaline electrolytes, ie, NaOH, KOH, do not reject CO2. Consequentiy, as of this writing, AFCs are restricted to specialized apphcations where C02-free H2 and O2 are utilized. 

Phosphoric Acid Fuel Cell. Concentrated phosphoric acid is used for the electrolyte ia PAFC, which operates at 150 to 220°C. At lower temperatures, phosphoric acid is a poor ionic conductor (see Phosphoric acid and the phosphates), and CO poisoning of the Pt electrocatalyst ia the anode becomes more severe when steam-reformed hydrocarbons (qv) are used as the hydrogen-rich fuel. The relative stabiUty of concentrated phosphoric acid is high compared to other common inorganic acids consequentiy, the PAFC is capable of operating at elevated temperatures. In addition, the use of concentrated (- 100%) acid minimizes the water-vapor pressure so water management ia the cell is not difficult. The porous matrix used to retain the acid is usually sihcon carbide SiC, and the electrocatalyst ia both the anode and cathode is mainly Pt. 

Several activities, if successful, would strongly boost the prospects for fuel ceU technology. These include the development of (/) an active electrocatalyst for the direct electrochemical oxidation of methanol (2) improved electrocatalysts for oxygen reduction and (2) a more CO-tolerant electrocatalyst for hydrogen. A comprehensive assessment of the research needs for advancing fuel ceU technologies, conducted in the 1980s, is available (22). 

A viable electrocatalyst operating with minimal polarization for the direct electrochemical oxidation of methanol at low temperature would strongly enhance the competitive position of fuel ceU systems for transportation appHcations. Fuel ceUs that directiy oxidize CH OH would eliminate the need for an external reformer in fuel ceU systems resulting in a less complex, more lightweight system occupying less volume and having lower cost. Improvement in the performance of PFFCs for transportation appHcations, which operate close to ambient temperatures and utilize steam-reformed CH OH, would be a more CO-tolerant anode electrocatalyst. Such an electrocatalyst would reduce the need to pretreat the steam-reformed CH OH to lower the CO content in the anode fuel gas. Platinum—mthenium alloys show encouraging performance for the direct oxidation of methanol. 

One factor contributing to the inefficiency of a fuel ceU is poor performance of the positive electrode. This accounts for overpotentials of 300—400 mV in low temperature fuel ceUs. An electrocatalyst that is capable of oxygen reduction at lower overpotentials would benefit the overall efficiency of the fuel ceU. Despite extensive efforts expended on electrocatalysis studies of oxygen reduction in fuel ceU electrolytes, platinum-based metals are stiU the best electrocatalysts for low temperature fuel ceUs. 

The second form consists of Pt metal but the iridium is present as iridium dioxide. Iridium metal may or may not be present, depending on the baking temperature (14). Titanium dioxide is present in amounts of only a few weight percent. The analysis of these coatings suggests that the platinum metal acts as a binder for the iridium oxide, which in turn acts as the electrocatalyst for chlorine discharge (14). In the case of thermally deposited platinum—iridium metal coatings, these may actually form an intermetallic. Both the electrocatalytic properties and wear rates are expected to differ for these two forms of platinum—iridium-coated anodes. 

ELECTROPOLYMERIZED FLAVINS AND AZINES AS ELECTROCATALYSTS FOR NADH OXIDATION... 

A fuel cell is simply a device with two electrodes and an electrolyte for extracting power from the oxidation of a fuel without combustion, converting the power released directly into electricity. The fuel is usually hydrogen. The principle of a fuel cell was first demonstrated by Sir William Grove in London in 1839 with sulphuric acid and platinum gauze as an electrocatalyst, and thereafter there were very occasional attempts to develop the principle, not all of which were based on sound scientific principles , as one commentator put it. 

Redox flow Positive electrode, negative electrode substrate, electrocatalyst support, current collector, bipolar separator... 

Hydrogen/NiOOH Electrode additive, electrocatalyst support... 

Of practical importance is the contribution that is made by carbonaceous materials as an additive to enhance the electronic conductivity of the positive and negative electrodes. In other electrode applications, carbon serves as the electrocatalyst for electrochemical reactions and/or the substrate on which an electrocatalyst is located. In... 

In acid electrolytes, carbon is a poor electrocatalyst for oxygen evolution at potentials where carbon corrosion occurs. However, in alkaline electrolytes carbon is sufficiently electrocatalytically active for oxygen evolution to occur simultaneously with carbon corrosion at potentials corresponding to charge conditions for a bifunctional air electrode in metal/air batteries. In this situation, oxygen evolution is the dominant anodic reaction, thus complicating the measurement of carbon corrosion. Ross and co-workers [30] developed experimental techniques to overcome this difficulty. Their results with acetylene black in 30 wt% KOH showed that substantial amounts of CO in addition to C02 (carbonate species) and 02, are... 

Other experiments by Ross and co-workers [30] clearly indicate that the common metal (Co, Ni, Fe, Cr, Ru) oxides that are used for oxygen electrocatalysts also catalyze the oxidation of carbon in alkaline electrolytes. 

Carbon shows reasonable electrocatalytic activity for oxygen reduction in alkaline electrolytes, but it is a relatively poor oxygen electrocatalyst in acid electrolytes. A detailed discussion on the mechanism of... 

The overpotentials for oxygen reduction and evolution on carbon-based bifunctional air electrodes for rechargeable Zn/air batteries are reduced by utilizing metal oxide electrocatalysts. Besides enhancing the electrochemical kinetics of the oxygen reactions, the electrocatalysts serve to reduce the overpotential to minimize... 

In redox flow batteries such as Zn/Cl2 and Zn/Br2, carbon plays a major role in the positive electrode where reactions involving Cl2 and Br2 occur. In these types of batteries, graphite is used as the bipolar separator, and a thin layer of high-surface-area carbon serves as an electrocatalyst. Two potential problems with carbon in redox flow batteries are (i) slow oxidation of carbon and (ii) intercalation of halogen molecules, particularly Br2 in graphite electrodes. The reversible redox potentials for the Cl2 and Br2 reactions [Eq. (8) and... 

Electrocatalysis Again by definition, an electrocatalyst is a solid, in fact an electrode, which can accelerate a process involving a net charge transfer, such as e.g. the anodic oxidation of H2 or the cathodic reduction of 02 in solid electrolyte cells utilizing YSZ ... 

Most of the electrocatalysts we will discuss in this book are in the form of porous metal films deposited on solid electrolytes. The same film will be also used as a catalyst by cofeeding reactants (e.g. C2H4 plus 02) over it. This idea of using the same conductive film as a catalyst and simultaneously as an electrocatalyst led to the discovery of the phenomenon of electrochemical promotion. 

The concept of a promoter can also be extended to the case of substances which enhance the performance of an electrocatalyst by accelerating the rate of an electrocatalytic reaction. This can be quite important for the performance, e.g., of low temperature (polymer electrolyte membrane, PEM) fuel cells where poisoning of the anodic Pt electrocatalyst (reaction 1.7) by trace amounts of strongly adsorbed CO poses a serious problem. Such a promoter which when added to the Pt electrocatalyst would accelerate the desired reaction (1.5 or 1.7) could be termed an electrocatalytic promoter, or electropromoter, but this concept will not be dealt with in the present book, where the term promoter will always be used for substances which enhance the performance of a catalyst. 

Electrochemical promotion or NEMCA is the main concept discussed in this book whereby application of a small current (1-104 pA/cm2) or potential ( 2 V) to a catalyst, also serving as an electrode (electrocatalyst) in a solid electrolyte cell, enhances its catalytic performance. The phenomenology, origin and potential practical applications of electrochemical promotion, as well as its similarities and differences with classical promotion and metal-support interactions, is the main subject of this book. 

Solid electrolyte fuel cells have been investigated intensively during the last four decades.10,33 37 Their operating principle is shown schematically in Fig. 3.4. The positive electrode (cathode) acts as an electrocatalyst to promote the electrocatalytic reduction of O2 (g) to O2 ... 

Although several metals, such as Pt and Ag, can also act as electrocatalysts for reaction (3.7) the most efficient electrocatalysts known so far are perovskites such as Lai-xSrxMn03. These materials are mixed conductors, i.e., they exhibit both anionic (O2 ) and electronic conductivity. This, in principle, can extend the electrocatalytically active zone to include not only the three-phase-boundaries but also the entire gas-exposed electrode surface. 

The negative electrode (anode) acts as an electrocatalyst for the reaction of O2 with the fuel, e.g. H2 ... 

One can only admire the insight of the first researchers who used Ni as the active electrode material in the Ni/YSZ cermet anodes In addition to being a good electrocatalyst for the charge transfer reaction (3.8), Ni is also an excellent catalyst for the steam or C02-reforming of methane ... 

Thus indeed CH4 oxidation in a SOFC with a Ni/YSZ anode results into partial oxidation and the production of synthesis gas, instead of generation of C02 and H20 as originally believed. The latter happens only at near-complete CH4 conversion. However the partial oxidation overall reaction (3.12) is not the result of a partial oxidation electrocatalyst but rather the result of the catalytic reactions (3.9) to (3.11) coupled with the electrocatalytic reaction (3.8). From a thermodynamic viewpoint the partial oxidation reaction (3.12) is at least as attractive as complete oxidation to C02 and H20. 

Table 3.1 lists some of the anodic reactions which have been studied so far in small cogenerative solid oxide fuel cells. A more detailed recent review has been written by Stoukides46 One simple and interesting rule which has emerged from these studies is that the selection of the anodic electrocatalyst for a selective electrocatalytic oxidation can be based on the heterogeneous catalytic literature for the corresponding selective catalytic oxidation. Thus the selectivity of Pt and Pt-Rh alloy electrocatalysts for the anodic NH3 oxidation to NO turns out to be comparable (>95%) with the... 

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