Potential sweep technique, with hydrogen

In the linear sweep technique, a recording of the current during the potential sweep (say, from 0.0 V on the normal hydrogen scale to 1.2 V positive to it in a 1 M H2 S04 solution) completes one act of the basic experiment. However, and hence the title of this part of the chapter, the electronics can be programmed so that when the electrode potential reaches 1.20 V, it begins a return sweep, going from 1.2 to 0.00 V, NHS. Completion of the two sweeps and back to the starting point is one act in what is called cyclic voltammetry.16 The current is displayed on a cathode ray oscilloscope screen on an X Y recorder, and it is normal to cany out not one but several and often many cycles. Much information is sometimes contained in the difference between the second and other sweeps in comparison with the first (Fig. 8.10). 

Cyclic voltammetry is commonly used to study fuel cell electrodes and hydrogen crossover. In this technique, a linear sweep potential is applied to one electrode, while the other is held constant. The potential is cycled in a triangular wave pattern, while the current produced is monitored. The shape and magnitude of the current response provides useful quantitative and qualitative information regarding the amount of catalyst that is electro-chemically active, the double layer capacitance, hydrogen crossover, and the presence of oxide layers and contaminants. Wu et al. provide a description of this technique with example voltammograms [29]. 

The formation of oxygen layers on nickel electrodes in alkaline solutions has been investigated [71—90] with different techniques. Anodic charging curves taken with small currents on electrodeposited [75] or smooth [74, 85] nickel from a potential of hydrogen evolution display two well defined arrests before the oxygen evolution starts. Fine details are also observable in the i — U curves measured during anodic potential sweeps. The curves in Fig. 31 resulted [83] during the first sweep... 

Potentiodynamic Technique. Adsorption of methanol on Pt in acid solution was studied by Breiter and Gilman (3) using a potentiostatic technique. The anodic sweep, with a sweep rate of 800 V/s, was started at rest potential and extended to 2.0 V with respect to a hydrogen reference electrode in the same solution. As shown in Figure 10.8, the current was recorded as a function of potential (time) in the absence (curve A) and in the presence (curve B) of methanol. The increase in current in curve B is due to oxidation of the adsorbed methanol on the platinum electrode. Thus, shaded area 2 minus shaded area 1 (Fig. 10.8) yields the change 2m (C/cm ) required for oxidation of the adsorbed methanol ... 

The generation of hydrous films on platinum at low potentials on the anodic sweep seems quite feasible from a thermodynamic viewpoint—especially in base. The inhibition here is evidently related to the need for six hydroxide ions to have access to coordination sites at the same platinum atom—an improbable condition for a metal atom in a regular surface site. Evidence will be presented later indicating that such hydrous oxide formation can, to a very limited extent, precede monolayer formation in the case of gold— the atoms of the latter involved in formation of the hydrous material are presumably at low lattice coordination sites on the surface. There is some evidence from recent single-crystal studies (see Section XIV) for this type of behavior in the case of platinum—it is obviously difficult to detect with polycrystalline substrates as with only a small fraction of a monolayer involved optical techniques would need to be extremely sensitive and electrochemical procedures are hampered by the fact that the redox behavior of the hydrous material coincides with that for adsorbed hydrogen. 

LSV should be performed after characterization of the fuel ceU with polarization curves and EIS technique. Thereby, the fuel cell is firstly characterized at normal operation conditions in terms of reactant flow rates and then, cathode reactant gas can be switched from oxygen or air to the inert gas. The gas flow rates used to record the LSV are 0.3 NL/min of hydrogen and nitrogen for anode and cathode, respectively. Thus, the cathode is the working electrode and the anode is counter and reference electrode. Before starting the LSV measurement, the rest potential must reach a pseudo-steady-state value. After that, the LSV must start from the rest potential until 0.5 V, sweeping the potential with a rate of 2 mV/s. LSV from Fig. 17.6 has been recorded following the mention test procedure. 

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