Hydrogen oxidation, exchange current density

Electrocatalytic Oxidation of Hydrogen The rate constant of the hydrogen oxidation reaction (HOR), as measured by the exchange current density jo (i.e. the current... 

Given that the rates of oxidation and reduction of the half-reactions are controlled by activation polarization only, that = 4-0.07 and = —0.08, and that the exchange current densities for both the oxidation of Fe and reduction of hydrogen in acidic solution are identical, use the data in Tables 3.3 and 3.4 to determine the following quantities. Recall that the potential for each half-cell is the sum of the equilibrium potential and the corresponding overpotential, in this case, r]a-... 

According to the different exchange current densities, i0, for hydrogen oxidation and hydrogen evolution on Ni and Pt, the catalytic activity of platinum is by a factor of several hundred to a thousand higher than that of nickel. Therefore, if the utilization of Raney-nickel particles below 10 jum size approaches 100%, it is clear that Pt-activated porous soot particles must be by a factor of from 10 to 30 smaller than Raney-nickel particles to achieve full utilization, that is, vanishing fuel starvation of the catalyst. This happens to be the case with soot agglomerates that are by their very nature of correct size (dv < 0.1 /im) (150, 151). 

The experimental data presented in this paper demonstrates the potential of CuCl/HCl electrolysis for nuclear hydrogen production. The CuCl/HCl electrolysis reaction requires a cation exchange membrane in order to produce hydrogen at a current density that exceeds 0.1 A-cm-2. In order to carry out the hydrogen production reaction a platinum electro-catalyst is required. The copper(I) oxidation reaction, on the other hand, does not require a Pt catalyst. This reaction proceeds quite readily on Pt-free graphite electrodes. Methods to mitigate the passage of the copper ion species across the membrane need to be developed to maintain the performance of the cell at the desired level. 

Electrode — Electrocatalytic electrode — Figure. Plots of the exchange current density of the hydrogen electrode reaction (left) as a function of the enthalpy of the metal-hydrogen bond and the current density of the electrochemical oxidation of ethylene as a function of the enthalpy of sublimation of various metals and alloys (right). These displays are also called Volcano plots... 

As discussed above, if the exchange current density of anode hydrogen oxidation is i nMc =0,1 Acni 2 and that of the cathode oxygen reduction reaction (ORR) is... 

The major dehciency of the oxygen electrode reaction is its low exchange current density (about 10 A/cm on a smooth surface) in acid electrolytes on even the best-known electrocatalyst (a platinum-chromium alloy). This value is about six orders of magnitude lower than that for the hydrogen electrode reaction in the same electrolyte. The reaction is about three orders of magnitude faster on smooth platinum or nickel oxide surfaces in an alkaline medium as compared to acid. The... 

This particular volcano plot is indeed nearly symmetric and demonstrates that Pt, Rh, Ir, and Re have the optimal M—H bond strength for maximum activity (exchange current density) for the hydrogen evolution/hydrogen oxidation reaction (HER/HOR). 

Figure 16. Plots of the exchange current density Jq for hydrogen oxidation (in humidified H2, p[H2O] = 0.04 atm) at Pt anode against ionic conductivity Oi of zirconia solid electrolyte measured. Solid line is the least square fitting for all the data. Reprinted from Ref. 44, Copyright (1995) American Chemical Society.

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