Anode catalysts palladium

Figure 6.57 Polarization curve of a fuel operating at 21 and 30°C with a 10 M formic acid solution as fuel. Anode catalyst palladium, with platinum cathode catalyst. (Adapted from Ref. [55].)...

Fundamental anode catalyst research is imperative for improved direct formic acid fuel cell (DFAFC) performance and stability such that an intimate understanding of the interplay between structural, morphological, and physicochemical properties is needed. The primary base catalysts found to be active for formic acid electrooxidation are either platinum (Pt) or palladium (Pd). The cyclic voltammograms in Fig. 4.1 compare the activity of carbon-supported Pt to Pd towards formic acid electrooxidation. The anodic (forward) scan, relevant to DFAFC performance, is relatively inactive on Pt/C until the applied potential... 

The presence of both poisoning species and intermediate reaction products requires the development of new electrocatalysts able to break the C-C bond and to oxidize adsorbed CO at low temperatures [94]. Numerous studies have been carried out to develop more active electrocatalysts for ethanol eleetrooxidation. Besides Pt, other metals such as gold, rhodium, and palladium have been investigated as anode catalysts for ethanol eleetrooxidation and show certain activities [105]. On gold in an acid medium, the oxidation reaction leads mainly to the formation of acetaldehyde [94]. The oxidation of ethanol on rhodium proceeds mainly through the formation of acetic acid and CO. 

Figure 4.35. Chronopotentiometry of formic acid oxidation on Pd, PdPt (prepared by the polyol method), and Pt. 10 M HCOOH - 0.1 M H2SO4, 293 K, 100 mA cm l Catalyst load 6 mg cm . The Pd Pt atomic ratios are listed [174], (Reproduced by permission of ECS— The Electrochemical Society, from Blair S, Lycke D, lordache C. Palladium-platinum alloy anode catalysts for direct formic acid fuels.)...

Blair S, Lycke D, lordache C. Palladium-platinum alloy anode catalysts for direct formic acid fuels. ECS Trans 2006 3 1325-32. 

Since the reaction between hydrogen and oxygen is very slow at room temperature, catalysts are incorporated in the carbon electrodes. At the anode, suitable catalysts are finely divided into platinum or palladium at the cathode, cobaltous oxide, or silver. The two halfreactions shown above yield the overall result as ... 

The electrochemical Wacker-type oxidation of terminal olefins (111) by using palladium chloride or palladium acetate in the presence of a suitable oxidant leading to 2-alkanones (112) has been intensively studied. As recyclable double-mediatory systems (Scheme 43), quinone, ferric chloride, copper acetate, and triphenylamine have been used as co-oxidizing agents for regeneration of the Pd(II) catalyst [151]. The palladium-catalyzed anodic oxidation of... 

Tonkovich et al. [123] claimed a 90% size reduction due to the introduction of micro channel systems into their device, which made use of the hydrogen off-gas of the fuel cell anode burnt in monoliths at palladium catalyst to deliver the energy for the fuel evaporation. A metallic nickel foam 0.63 cm high was etched and impregnated with palladium to act as a reactor for the anode effluent It was attached to a micro structured device consisting of liquid feed supply channels and outlet channels for the vapor, the latter flowing counter-flow to the anode effluent... 

The resulting metal membrane has a structure similar to the bulk pores of the anodic alumina but without the dense "skin layer and has straight through-pores. Nickel and platinum membranes having pore diameters in the 15 to 200 nm range can be produced this way. One of the critical steps of this method is depositing palladium catalyst on the surface, but not inside the pores, of the anodic alumina. The catalyst is used to facilitate the deposition of metal from the bottom to the surface of the cylindrical pores. 

In a fuel cell, each electrode is a hollow chamber of porous carbon walls that allow contact between the inner chamber and the electrolyte surrounding it. The walls of the chamber also contain catalysts, such as powdered platinum or palladium, which speed up the reactions. These catalysts are similar to those in an automobile s catalytic converter, which you read about in Chapter 17. The following oxidation half-reaction takes place at the anode. 

The methods described in detail in Section 36.2, or only mentioned, have been used as follows for spectrophotometric determination of palladium the thio-Michler s ketone — in silver, copper, and anodic slime [32], in catalysts [31] with thiosemicarbazide derivatives — in water [44] and alloys [46] with palladium-carbon powder — with a-benzilmonoxime [48] with PAR — in catalysts and ores [58] with thiazolylazo derivatives — in Ni-Al catalysts [63] with 5-Br-PADAP — in titanium alloys with pyridylazo derivatives - in nickel alloys [68] with sulphonitrophenol - in silver alloys [70] with Arsenazo III — in iron and meteorites and with Palladiazo — in catalysts, minerals, silica gel, and calcium carbonate [78]. 

Fodisch et al. [238] deposited palladium by electroplating on a porous alumina layer, which had been obtained by anodic oxidation of aluminum. Electroplating can be seen as an alternative to impregnation. The authors observed a rather inhomogeneous (bimodal) deposition within the pores of the alumina layer palladium was mainly concentrated at the pore mouths and at the end of the pores, whereas no palladium was detected in the middle section of the pores. Possibly as a result of this distribution, the performance of the catalyst was found to be rather poor in comparison with that of a catalyst prepared by impregnation. 

With the experience gathered in the development of direct methanol fuel cells, platinum-ruthenium catalysts were used for the anodic process in the first studies on direct formic acid fiiel cells. Then, it was shown that much better electrical characteristics can be obtained with palladium black as the catalyst. Importantly, with this catalyst, one can work at much lower temperatures. In particular, at a temperature of 30°C power densities of 300 mW/cm were obtained with a voltage of 0.46 V, and about 120 mW/cm with a voltage of 0.7 V. Considering all these special features, it will be very convenient to use formic acid as a reactant in fuel cells of small size, for power supply in portable equipment, ordinarily operated at ambient temperature. 

O2. More recently, performances with formic acid as fuel were increased up to 28 mW cm with electrodeposited palladium (Pd)-containing catalyst at the anode [39], An allpassive system (passive 10 M formic acid and quiescent air) was also recently announced with interesting first results (12.3 mW cm ) [40],... 

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