Platinum-based electrodes

ELECTROCHEMICAL OXIDATION OF METHANOL ON PLATINUM AND PLATINUM BASED ELECTRODES... 

The goal of this thesis is to achieve a deeper understanding of the mechanisms and their features of electrochemical oxidation of methanol on platinum and platinum based electrodes. This understanding will be helpful as a guide when searching for new catalysts and trying to improve the activities. 

Several added metals were investigated to improve the kinetics of ethanol oxidation at platinum-based electrodes, including ruthenium [27, 28], lead [29] and tin [22, 30]. Of these, tin appeared to be very promising. Figure 1.11 shows the polarization curves of ethanol electro-oxidation recorded at a slow sweep rate (5 mV s ) on different platinum-based electrodes. Pt-Sn(0.9 0.1)/C displays the... 

It was reported that cobalt-tetraphenylporphyrin complex (CoTPP) coated on an electrode catalyzes electrocatalytic proton reduction,215 but the activity was not very high. We have found that metal porphyrins and metal phtahlocyanines when incorporated into a polymer membrane coated on an electrode show high activity in electrocatalytic proton reduction to produce H2.22,235 Some data are summarized in Table 19.2. It was shown that this catalyst is more active than a conventional platinum base electrode. 

In contrast to these promising thermodynamic data, the kinetics of the electrochemical mineralization is very slow and in practice it can be achieved close to the thermodynamic potentials only in very limited cases. In fact, only platinum-based electrodes can allow EM of simple Ci organic compounds. A typical example is the use of Pt-Ru catalyst in the electrochemical mineralization of methanol. 

Figure 5 Schematics of a typical proton-exchange membrane fuel cell, indicating the platinum-based electrodes used for hydrogen oxidation and oxygen reduction, the interconnecting electrolyse, and the electrical circuit used to harvest the energy produced. (Reprinted from Ref. 35, 1998, with permission from Elsevier)...

The electrocatalytic oxidation of ethanol has been investigated for many years on different platinum-based electrodes, including Pt/X alloys (with X = Ru, Sn, Mo, etc ), and dispersed nanocatalysts. Pme platinum smooth electrodes are rapidly poisoned by some strongly adsorbed intermediates, such as carbon monoxide, resulting from the dissociative chemisorption of the molecule, as shown by the first experiments in infrared reflectance spectroscopy (EMIRS). Both kinds of adsorbed CO, either linearly-bonded or bridge-bonded to the platinum surface, are observed. Besides, oth-... 

Figure 10 Comparison of the behavior of different platinum-based electrodes dispersed in a PAni film toward the oxidation of CO from a saturated aqueous 0.1 M HCIO4 solution (100pgcm metal loading 5mV/s sweep rate). 

The materials for PEMFC electrodes should have good electrical conductivities and be stable in contact with electrolyte. Platinum-based electrodes have shown excellent electrochemical activities for PEMFC. 

Belgsir, E. M., H. Huser, J. M. Leger, and C. Lamy. 1987. A kinetic analysis of the oxidation of methanol at platinum-based electrodes by quantitative determination of the reaction products using liquid chromatography. 

Other additives to platinum-based electrodes, such as tin-oxide (SnOx) [71], have also been shown to significantly improve the catalytic activity of the oxygen reduction reaction. Using a PPA-Processed w-PBI membrane with a 7 wt% SnO in Pt/Sn02/C catalyst under unhumidified H2/O2 at 180°C, a voltage of 0.58 V under a load of 0.2 A cm was produced. Under the same conditions, a w-PBI MEA using a Pt/C catalyst produced only 0.4 V at 0.2 A cm . ... 

PBI has also been investigated as an additive to platinum-based electrodes. It is thought that incorporation of PBI in the catalyst layer would provide a better interface for proton conduction between the electrode and membrane. Qian [72] incorporated 6F-PBI into the electrodes by four different methods formation of a PBI bilayer inserting a thin 6F-PBI membrane between an E-TEK cathode and p-PBI membrane, casting 6F-PB1/PPA directly onto the E-TEK electrodes and hydrolyzing to form the gel, spraying a 6F-PBI/DMAc solution onto the electrodes. 

Kim, H., Park, J.-N. Lee, W.-H. Preparation of platinum-based electrode catalysts for low temperature fuel-cell. Catal. Today 87 (2003), pp. 237-245. 

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. 

This method involves very simple and inexpensive equipment that could be set up m any laboratory [9, 10] The equipment consists of a 250-mL beaker (used as an external half-cell), two platinum foil electrodes, a glass tube with asbestos fiber sealed m the bottom (used as an internal half-cell), a microburet, a stirrer, and a portable potentiometer The asbestos fiber may be substituted with a membrane This method has been used to determine the fluoride ion concentration in many binary and complex fluondes and has been applied to unbuffered solutions from Willard-Winter distillation, to lon-exchange eluant, and to pyrohydrolysis distil lates obtained from oxygen-flask or tube combustions The solution concentrations range from 0 1 to 5 X 10 M This method is based on complexing by fluonde ions of one of the oxidation states of the redox couple, and the potential difference measured is that between the two half-cells Initially, each cell contains the same ratio of cerium(IV) and cerium(tll) ions... 

Similarly to quantitative determination of high surfactant concentrations, many alternative methods have been proposed for the quantitative determination of low surfactant concentrations. Tsuji et al. [270] developed a potentio-metric method for the microdetermination of anionic surfactants that was applied to the analysis of 5-100 ppm of sodium dodecyl sulfate and 1-10 ppm of sodium dodecyl ether (2.9 EO) sulfate. This method is based on the inhibitory effect of anionic surfactants on the enzyme system cholinesterase-butyryl-thiocholine iodide. A constant current is applied across two platinum plate electrodes immersed in a solution containing butyrylthiocholine and surfactant. Since cholinesterase produces enzymatic hydrolysis of the substrate, the decrease in the initial velocity of the hydrolysis caused by the surfactant corresponds to its concentration. Amounts up to 60 pg of alcohol sulfate can be spectrometrically determined with acridine orange by extraction of the ion pair with a mixture 3 1 (v/v) of benzene/methyl isobutyl ketone [271]. 

Quantitative analysis can be carried out by chromatography (in gas or liquid phase) during prolonged electrolysis of methanol. The main product is carbon dioxide,which is the only desirable oxidation product in the DMFC. However, small amounts of formic acid and formaldehyde have been detected, mainly on pure platinum electrodes. The concentrations of partially oxidized products can be lowered by using platinum-based alloy electrocatalysts for instance, the concentration of carbon dioxide increases significantly with R-Ru and Pt-Ru-Sn electrodes, which thus shows a more complete reaction with alloy electrocatalysts. 

Another convenient way to disperse platinum-based electrocatalysts is to use electron-conducting polymers, such as polyaniline (PAni) or polypyrrole (PPy), which play the role of a three-dimensional electrode.In such a way very dispersed electrocatalysts are obtained, with particle sizes on the order of a few nanometers, leading to a very high activity for the oxidation of methanol (Fig. 10). 

This system was subsequently investigated by Christensen et at. (1990) also using in situ FTIR, who observed identical product features (see Figure 3.48). In order first to compare directly the IR spectrum of oxalate generated in situ, the authors took advantage of the surface reactivity of Pt and the poor diffusion of species to and from the thin layer. Thus, a solution of oxalic acid in the electrolyte was placed in the spectroelectrochemical cell, the potential of the platinum working electrode stepped to successively lower values and spectra taken at each step. The spectra were all normalised to the reference spectrum collected at the base potential of 0 V vs. SCE. As a result of the deprotonation of adventitious water ... 

This method in some ways resembles the technique for ASV [321,322]. The analytical device is based on a three-electrode system (1) a glassy carbon electrode, which serves as a cathode (2) a saturated calomel electrode (SCE), which is the reference electrode and (3) a platinum counter-electrode during electrolysis. 

Saha, M. S., Li, R., Cai, M., and Sun, X. High electrocatalytic activity of platinum nanoparticles on Sn02 nanowire-based electrodes. Electrochemical and Solid-State Letters 2007 10 B130-B133. 

Platinum-based catalysts are widely used in low-temperature fuel cells, so that up to 40% of the elementary fuel cell cost may come from platinum, making fuel cells expensive. The most electroreactive fuel is, of course, hydrogen, as in an acidic medium. Nickel-based compounds were used as catalysts in order to replace platinum for the electrochemical oxidation of hydrogen [66, 67]. Raney Ni catalysts appeared among the most active non-noble metals for the anode reaction in gas diffusion electrodes. However, the catalytic activity and stability of Raney Ni alone as a base metal for this reaction are limited. Indeed, Kiros and Schwartz [67] carried out durability tests with Ni and Pt-Pd gas diffusion electrodes in 6 M KOH medium and showed increased stability for the Pt-Pd-based catalysts compared with Raney Ni at a constant load of 100 mA cm and at temperatures close to 60 °C. Moreover, higher activity and stability could be achieved by doping Ni-Al alloys with a few percent of transition metals, such as Ti, Cr, Fe and Mo [68-70]. 

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