Direct current proton conductivity measurements

If an electrical potential is applied to a conductor, a current / is produced 

By contrast, Au or Cu are blocking electrodes that can neither inject nor dissolve protons. Hence, carrying out identical experiments with Au or Cu and with H2-saturated Pd electrodes provides the means of separating protonic contribution, reducing Eqn 9.8 to = 

The current depends upon the number and the mobilities of the protonic charge carriers. In the intrinsic case, when (H ) = n(0 ), only the mobility n determines which species carries the majority current. In the extrinsic case, for instance when (H ) n(O ), r is determined by the product n-fi. 

Note that, in an electric field, the two partial currents carried by H and O run opposite to each other. However, the motion of O, say, from left to right depends upon a proton jump from right to left as illustrated in Fig. 9.4b. Therefore, under d.c. conditions, one actually measures two parallel proton fluxes moving on two different energy levels. Conductivity measurements cannot provide information about the nature of the charge carriers, in particular their signs. All that Eqn (9.11) provides is the sum of the H and O contributions. 

Conductivity measurements have still other limitations. Anodic proton injection requires high electric fields, and cathodic proton up-take requires dissolution of H in the electrode. If the anode does not supply enough protons, the current becomes injection-limited. If cathodic redissolution does not keep up with the arrival of protonic charge carriers, the cathode becomes polarized by the formation of Hj gas. The performance can be improved by using Pd-black electrodes with small Pd particles which give 

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One study specifically designed for PEFC was reported by Thompson et al.8 They used a direct current to measure the proton conductivity at low temperature. In conjunction with the DSC data, they found the dependency of crossover temperature (temperature where the activation energy changes) on water content and hysteresis between freezing and melting. 

Here, V and Eq are cell potentials or terminal voltage at arbitral current density I and 1 = 0. Usually, E(-) is called a tliermodynamic electromotive force, EMF. The notations of A and 5 are a surface area and thickness of the ceramic electrolyte, and s is an electric conductivity. Since measurement was carried out by a direct current method, the s value corresponded to an overall one including the contributions of anode and cathode polarities as well as the protonic conductivity of ceramic electrolyte. The dependence of Eq on temperature was very small and almost independent of it. On the other hand, the Eq values slightly depended on the input CH4/H2O ratio. 

The size of electroconductivity compressed samples MoClj j(C3(, jHgQ j), measured at a direct current at a room temperature-( 1.3 3.3) 10 Ohm -cm is in a range of values for a trans-polyacetylene and characterizes a composite as weak dielectric or the semiconductor. The positioned size of conductivity of samples at an alternating current tr = (3.1 4.7)-10 Ohm cm can answer presence of ionic (proton) conductivity that can be connected with presence of mobile atoms of hydrogen at structure of polymer. 

High frequency resistance measurements, usually done to measure the membrane resistance for direct current, will see this effective conductivity. At low frequency and DC current, only protons flow with the conductivity Oh-The effective conductivity is only uniform across the membrane when no current is flowing and X is uniform. The high and low frequency resistances are... 

On the same principle as the routine measurement of electronic resistivity, Ohm s law is used to analyze the resistivity of a proton-conductive membrane against the flow of either alternating current (AC) or direct current (DC). The proton conductivity can be calculated according to Eqn (5.1) ... 

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