Anode electrocatalysts

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. 

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... 

The electrochemical oxidation of methanol occurs on the anode electrocatalyst (e.g., dispersed platinum), which constitutes the negative electrode of the cell ... 

Silver films and Ag-CaO-SragOg cermets were chosen as the anodic electrocatalysts because of their high electrical conductivity, which is necessary for electrocatalytic operation, and also because of their high (>95%) selectivity to Cg hydrocarbons at very low (<2%) CH conversions [9]. 

Roth C, Martz N, Buhrmester T, Scherer J, Fuess H. 2002. In-situ XAFS fuel cell measurements of a carbon-supported Pt-Ru anode electrocatalyst in hydrogen and direct methanol operation. Phys Chem Chem Phys 4 3555-3557. 

Viswanathan R, Hou GY, Liu RX, Bare SR, Modica F, Mickelson G, Segre CU, Leyarovska N, Smotkin ES. 2002. In-situ XANES of carbon-supported Pt-Ru anode electrocatalyst for reformate-air pol3mer electrol3de fuel cells. J Phys Chem B 106 3458-3465. 

An anodic electrocatalyst able to use this photocurrent for water electrolysis evolving 02 and producing four protons and four electrons. 

Tungsten carbide — WC, belongs to a class of Group IV B-VIB transition metal carbides and nitrides, often referred to as interstitial alloys, in which the carbon and nitrogen atoms occupy the interstitial lattice positions of the metal [i]. These compounds possess properties known from group VIII B precious metals like platinum and palladium [ii]. Thus, they show remarkable catalytic activities, attributed to a distinct electronic structure induced by the presence of carbon or nitrogen in the metal lattice. Tungsten carbide resembles platinum in its electrocatalytic oxidation activity (- electrocatalysis) and is therefore often considered as an inexpensive anode electrocatalyst for fuel cell [iii] and -> biofuel cell [iv] application. 

The selective oxidation or preferential oxidation of CO in hydrogen-rich stream is another important object for ceria based catalysts. The gas mixture from steam reforming/partial oxidation of alcohols or hydrocarbons, followed by the WGS reaction contains mainly FI2, CO2 and a small portion of CO, H2O, and N2. When such gaseous stream would be taken as input for hydrogen fuel cells, the CO has to be removed to avoid poisoning of the anode electrocatalysts. Ceria based nanomaterials, such as ceria/gold, ceria/copper oxide catalysts exhibit suitable catalytic activities and selectivities for CO PROX process. 

Sasaki, K. et al., Ultra-low platinum content fuel cell anode electrocatalyst with a longterm performance stability, Electrochim. Acta, 49, 3873, 2004. 

Enzymes have been considered in bio fuel cells as anode electrocatalysts since their use avoids the problem of poisoning the anode with carbon monoxide present in reforming gas, allowing the use of cheap hydrogen-containing fuels such as methanol. Even though enzymatic fuel cells have been reported to have power output and stability limitations, some of them are currently being used to produce electricity to power small electrical devices with power demands in the order of micro- and milh- Watts as power output limitations are overcome. 

Kuchynka et al. [125] studied the electrochemical oxidative dimerization of methane to C2 hydrocarbon species using perovskite anode electrocatalysts. Three designs of solid oxide fuel cells were used, including tubular and flat plate solid electrolytes. The maximum current density for the dimerization reaction at these electrocatalysts was related to the oxygen binding energies on the catalyst surface. The anodic reaction was ... 

A summary of some results for various anode electrocatalyst materials is given in Table III in relation to the d-electron configuration (357). It must be remarked that the linear dependence of several quantities on d-band vacancy, found by Arikado et al. in their CI2 evolution studies (357,434), will arise, in part, from the (probably doubtful) assumption that the d-band vacancy for alloys varies in linear proportion to the values for the elemental components, weighted by the composition ratio. A further factor is that the surface composition of alloys is rarely identical with that of the bulk (cf. Refs. 43 , 432). 

Design of Anode Electrocatalysts for the CO-Tolerant H2 Oxidation and CH3OH Oxidation... 

Since such high anodic overpotentials are prohibitive for practical applications, a very intense research effort has been launched during the last 20 years for finding more efficient anodic electrocatalysts. 

E25.17 Electrocatalysts are compounds that are capable of reducing the kinetic barrier for electrochemical reactions (barrier known as overpotential). While platinum is the most efficient electrocatalyst for accelerating oxygen reduction at the fuel cell cathode, it is expensive (recall Section 25.18 Electrocatalysis). Current research is focused on the efficiency of a platinum monolayer by placing it on a stable metal or alloy clusters your book mentions the use of the alloy PtsN. An example would be a platinum monolayer fuel-cell anode electrocatalyst, which consists of ruthenium nanoparticles with a sub-monolayer of platinum. Other areas of research include using tethered metalloporphyrin complexes for oxygen activation and subsequent reduction. 

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