Metal electrocatalysts

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. 

Establishment of carbon-supported Pt catalysts as a means to achieve higher and more stable dispersion of the precious metal electrocatalyst on an electronically conducting support [Petrow and Allen, 1977]. 

Attwood PA, McNicol BD, Short RT. 1980. Electrocatalytic oxidation of methanol in acid electrolyte—Preparation and characterization of noble-metal electrocatalysts supported on pretreated carbon-fiber papers. J Appl Electrochem 10 213-222. 

There is an increasing interest in the development of electrochemical sensors and microsensors for detecting and monitoring NO or N02, due to their importance in clinical and environmental analysis. It has been suggested that transition metal electrocatalysts active for NO or N02 coordination and reduction could be exploited for the development of metal-complex film electrodes for N02 and NO sensing. However, most of the sensory devices reported so... 

Based upon analogies between surface and molecular coordination chemistry outlined in Table 1, we have recently set forth to investigate the interaction of surface-active and reversibly electroactive moieties with the noble-metal electrocatalysts Ru, Rh, Pd, Ir, Pt and Au. Our interest in this class of compounds is based on the fact that chemisorption-induced changes in their redox properties yield important information concerning the coordination/organometallic chemistry of the electrode surface. For example, alteration of the reversible redox potential brought about by the chemisorption process is a measure of the surface-complex formation constant of the oxidized state relative to the reduced form such behavior is expected to be dependent upon the electrode material. In this paper, we describe results obtained when iodide, hydroquinone (HQ), 2,5-dihydroxythiophenol (DHT), and 3,6-dihydroxypyridazine (DHPz), all reversibly electroactive... 

The ideal performance of a fuel cell depends on the electrochemical reactions that occur with different fuels and oxygen as summarized in Table 2-1. Low-temperature fuel cells (PEFC, AFC, and PAFC) require noble metal electrocatalysts to achieve practical reaction rates at the anode and cathode, and H2 is the only acceptable fuel. With high-temperature fuel cells (MCFC, ITSOFC, and SOFC), the requirements for catalysis are relaxed, and the number of potential fuels expands. Carbon monoxide "poisons" a noble metal anode catalyst such as platinum (Pt) in low-temperature... 

Investigations of enzyme-catalyzed direct electron transfer introduce the basis for a future generation of electrocatalysts based on enzyme mimics. This avenue could offer new methods of synthesis for nonprecious metal electrocatalysts, based on nano-structured (for example, sol—gel-derived) molecular imprints from a biological catalyst (enzyme) with pronounced and, in some cases, unique electrocatalytic properties. Computational approaches to the study of transition state stabilization by biocatalysts has led to the concept of theozymes . " ... 

Another important aspect of electrocatalysis is the study of dispersed high specific area and supported, both metal and non-metal, electrocatalysts. A high degree of dispersion brings about enhancement of the catalytic activity because of the specific area and energetics of active sites [140] and decrease of susceptibility of poisoning because of the improved ratio of catalyst area to impurities in solution. 

It is necessary to discuss four scientific topics for phosphoric acid fuel cells. Those interconnected topics are the design of the precious metal electrocatalyst properties of the phosphoric acid properties of the matrix, and those of the carbon catalyst support. 

It has long been established that Pt is the most efficient singlemetal electrode for the catalysis of both reactions (1) and (2). In the case of ddiydrogen activation, no metal electrocatalyst performs better than platinum. However, aside from the fact that platinum is a precious metal, a major drawback is that commercial (fossil-based) hydrogen contains residual amounts of impurities (e g., carbon monoxide) that only serve to poison the catalyst surface." To address this particular problem, present research has focused on the employment of metal additives (e.g., Ru) or of molecular catalysts that mimic the impressive activity of biological materials (e g., hydrogenase enzymes) " the use of molecular catalysts appears to be the more attractive option since such com-... 

In the present work, CO2 electrochemical reduction was examined on higji area metal electrocatalysts supported on activated carbon fibers (ACF), which contain slit-shaped pores with widths on the order of nanometers. Such electrocatalysts were used in the form of gas difiusion electrodes (GDE), which are used in the fuel-cell field. The structure of this type of electrode is shown in Figure 1. The reaction takes places at the gas phase / electrolyte (liquid phase) / electrode interface, the so-called three-phase boimdary. 

Different electron-conducting polymers (polyaniline, polypyrrole, polythiophene) are considered as convenient substrates for the electrodeposition of highly dispersed metal electrocatalysts. The preparation and the characterization of electronconducting polymers modified by noble metal nanoparticles are first discussed. Then, their catalytic activities are presented for many important electrochemical reactions related to fuel cells oxygen reduction, hydrogen oxidation, oxidation of Cl molecules (formic acid, formaldehyde, methanol, carbon monoxide), and electrooxidation of alcohols and polyols. 

When it comes to the intrinsic activity of the metal electrocatalyst itself, both... 

Depending on potential, oxygen is bound on the surface of noble metal electrocatalysts as a chemisorbed species or a surface oxide (7, 99). The formation of these surface oxygen layers is independent of Oj presence (7,779) and irreversible, with the exception of iridium (97,120,121). Irreversibility becomes more pronounced in the order Os < Ru < Rh < Pd Pt... 

The adsorption of alcohols, aldehydes, and carbon oxides on metal electrocatalysts has been extensively studied because of the significance of their oxidation reactions for electrochemical energy generation (7,9,81,195). Particular attention has been payed to the surface intermediates of methanol oxidation on platinum. At least two adsorption states have been assigned to methanol, a weak one possibly associated with physisorption (196) and one or more states arising from dissociative strong adsorption of the reactant (797, 198). Breiter (799) proposed a parallel scheme for methanol oxidation... 

Metals and particularly the noble ones have always attracted attention because of their high catalytic activity. Although economic incentives prompted the examination of nonnoble metal electrocatalysts, few such efforts have proven fruitful. Electrocatalysts that show sufficient activity, but not longevity, include tungsten bronzes and solid organometallic complexes of transition metals, and are discussed in this section. 

Oxygen Electrocatalytic Properties Oxygen Reduction. Figure 8 compares steady-state polarization curves for the electroreduction of Op on a typical pyrochlore catalyst, Pb2(Rui.42Pbo.53)06.5 15 w/o platinum on carbon. The latter was considered representative of conventional supported noble metal electrocatalysts. The activities of both catalysts are quite comparable. While the electrodes were not further optimized, their performance was close to the state of the art, considering that currents of 1000 ma/cm could be recorded, at a relatively moderate temperature (75 C) and alkali concentration (3M KOH). Also, the voltages were not corrected for electrolyte resistance. The particle size of the platinum on the carbon support was of the order of 2 nanometers, as measured by transmission electron microscopy. 

Zhang, Y. Sethuraman, V. Michalsky, R. Peterson, A. A. Competition between CO2 Reduction and H2 Evolution on Transition-Metal Electrocatalysts. ACS Catal., 2014, 4, 3742-3748. 

J. P. Sauvage, J. P. Electrochemical Reduction of Carbon-Dioxide Mediated by Molecular Catalysts Coord. Chem. Rev. 1989, 93, 245. (c) Sullivan, B. P. Reduction of carbon dioxide with platinum metals electrocatalysts. A potentially important route for the future production of fuels... 

Polymer electrolyte fuel cells, also sometimes called SPEFC (solid polymer electrolyte fuel cells) or PEMFC (polymer electrolyte membrane fuel cell) use a proton exchange membrane as the electrolyte. PEEC are low-temperature fuel cells, generally operating between 40 and 90 °C and therefore need noble metal electrocatalysts (platinum or platinum alloys on anode and cathode). Characteristics of PEEC are the high power density and fast dynamics. A prominent application area is therefore the power train of automobiles, where quick start-up is required. 

The high concentration of the acid increases the conductivity of the electrolyte and reduces the corrosion of the carbon-supported electrodes. PAFC need platinum-based noble metal electrocatalysts. 

Kinoshita s review suggests that the 4-electron pathway is predominant on noble-metal electrocatalysts such as platinum and that the 2-electron peroxide intermediate mechanism is the primary pathway on graphite and most other carbons, oxide-covered metals, transition-metal oxides, and some macrocycles. Certainly, though, peroxide is formed to some degree in... 

---