Pressure hydrogen overvoltage

Pressure at elevated pressures, the value of the hydrogen overvoltage changes only very slightly but at lower pressures, it increases sharply on certain metals, as for example in the case of copper, nickel, and mercury cathodes. 

SOME APPLICATIONS OF THE VARIATION OP HYDROGEN OVERVOLTAGE WITH THE PRESSURE... 

It next became interesting to investigate some chemical processes in which hydrogen overvoltage is intimately involved in order to see whether changes in these processes, produced by variations in the external pressure, are also in the direction predicted by the theory. Three such processes are (1) the corrosion of metals in acid solutions,... 

This equation reduces to Eh = -4.308 x 10 5T ln/n, i.e. Eh = -0.029 log/h at 20°C, where 0.98 bar hydrogen pressure would result from a hydrogen overvoltage of 29 mV. It is interesting to note that maximum embrittlement occurs here at a fugacity of 108 (i.e./fj = 104) compared with the much lower value of/h = 104 found for AISI 4340 steel by Scully and Moran,9 i.e. a martensitic microstructure is more readily embrittled by cathodic polarization than the duplex. 

Schuldiner, S. (1959) Hydrogen overvoltage on bright platinum.3. Effect of hydrogen pressure. Journal of the Electrochemical Society, 106, 891-895. 

Consider a hydrogen electrode in equilibrium with H ion at a concentration c and hydrogen gas at a pressure p. The equilibrium potential of this electrode is denoted by 00 The equilibrium is Hj 2H + 2e . If the potential of the electrode is increased (made more positive), this equilibrium will be disturbed. The reaction from left to right will predominate, H2 will be oxidized, and a positive current will flow into the solution. If the potential of the electrode is lowered (made more negative), the equilibrium will be disturbed. The reaction from right to left will predominate, H2 will be liberated, and a positive current will fiow into the electrode or a negative current will flow into the solution. The current that fiows to the electrode, therefore, depends on the departure of the potential from the equilibrium value, 0 — 0o This difference between the applied potential 0 and the equilibrium potential 0 is the overpotential, or overvoltage,... 

PEM fuel cell characteristics are generally described with polarization curves. The thermodynamic equilibrium potential of the hydrogen/oxygen reaction is reduced by various overvoltage terms that depend on mass transport, kinetic, and ohmic phenomena within cell. In other words, the output voltage of a single cell is attributable to different current, temperature, and pressure dependant factors [1]. 

Activation overvoltage at both electrodes is important in cells using fuels other than hydrogen, such as methanol. At higher temperatures and pressures the activation overvoltage becomes less important. 

The macroscopic modeling described for the PEMFCs can also be apphed to the case of an SOFC fed with hydrogen if we have tests which enable us to identify the different parameters. The activation overvoltage term is far smaller in the case of an SOFC. In fact, the open circuit voltage of a cell is nearest to the reversible potential of the cell. Because of the high temperature, all the species are in gaseous form, and their activity is equal to their partial pressure. The Ohmic resistance of the MEA (the electrodes and the membrane) is highly dependent on the temperature. 

In low- and medinm-temperature fuel cells, activation overvoltage is the most important irreversibility and cause of voltage drop, and occurs mainly at the cathode. Activation overvoltage at both electrodes is important in cells using fuels other than hydrogen, such as methanol. At higher temperatures and pressures the activation overvoltage becomes less important. 

---