Dynamic hydrogen electrode

Hydrogen electrode — A gas electrode where purified hydrogen gas is dissolved, usually in an aqueous solution, in which an inert electrode, preferably a - platinized platinum (- platinum black, -> electrode materials) electrode is inserted. The hydrogen electrode is of exceptional importance in electrochemistry because the -> standard hydrogen electrode (SHE) provides by convention a reference potential for all half reactions and thus the thermodynamic reference point for all energy calculations. An alternative form is the -> dynamic hydrogen electrode (DHE). 

For EIS measurements of a direct methanol fuel cell (DMFC), the anode is supplied with an aqueous solution of methanol at a concentration such as 1 M and using controlled flow rates. The cathode is operated on either air, oxygen, or hydrogen, with controlled flow rate and pressure [43], In order to measure the anode EIS, the DMFC is fed with hydrogen gas instead of air or oxygen, to eliminate the contributions of the cathode. This cathode is normally denoted as a dynamic hydrogen electrode (DHE). Thus, the anode impedance spectra between the anode and the DHE can be obtained in a complete fuel cell. 

In general, the reference electrode should be (1) a reversible electrode that obeys the Nemst equation, (2) stable, and (3) able to respond quickly to changes in environmental conditions [63], The most commonly used reference electrodes for measuring the AC impedance spectra for fuel cells are the DHE (dynamic hydrogen electrode) and the RHE (reversible hydrogen electrode) [64, 65],... 

W designates the cathode ( working ) electrode, A, the anode, and R and C designate the dynamic hydrogen and the counterelectrode, respectively, for the DHE (dynamic hydrogen electrode) reference. 

Figure 2. Performance of unsupported Pt-Ru anode catalyst as a function of atomic content of ruthenium in the bulk catalyst phase current density determined from the anode polarization plots at a potential of 0.35 V v . dynamic hydrogen electrode.

The WC/Ta electrode deposited by RF sputtering on glassy carbon revealed the enhanced cathodic onset potential, current density, and stability at various temperatures (161). The electrocatalyst showed an onset potential of 0.8 V (vs dynamic hydrogen electrode, DHE) and a cathodic current density of-33 /iA/cm (at 0.2 V vs DHE) for ORR due to the enhanced stability of WC by Ta addition. The onset potential and current density were higher than those of WC, Ta-WC, Ta,andTaC. 

The cell voltage Ucell is defined as the potential difference between the cathode and the anode. It is usually measured during fuel-ceU operation. The potential difference between the electrode and the electrolyte, which is caUed the anode or cathode potential in the following, is responsible for the electrochemical reaction occurring within the catalyst layers but cannot be measured directly. In the further text, we use electrolyte and membrane as equivalent expressions. While the electrode potential can be sensed from the bipolar plates, it is not feasible to sense the membrane potential directly, since each measurement equipment forms an interface between the membrane and the metal contact Two methods for the installation of a reference electrode within the ceU have been discussed in the Hterature, namely the reverse hydrogen electrode (RHE) [18] and the dynamic hydrogen electrode (DHE). In addition to ceU internal methods, a conventional... 

Figure 20.2 Dynamic hydrogen electrode (DHE) as reference electrode in sandwich configuration and edge configuration.

Roth, C., Moeller, S., Fuess, H., Poco, J.G.R., and Linardi, M. (2009) The use of a dynamic hydrogen electrode as an electro-chemical tool to evaluate plasma activated carbon as electrocatalyst support for direct methanol fuel cell. Mater-Res. Bull, 44, 51-56. 

The second option is to incorporate a reference electrode, as shown in Figure 11.4. A small electrode separate from the anode is bonded onto the membrane. Pure hydrogen is fed to this reference electrode so that it behaves like a dynamic hydrogen electrode (DHE) the potential of which is 0 V. The voltage measured between the anode or the cathode and this reference electrode is the Va or the Vc. Such a cell design enables the measurement of the individual Va and Vc in a single experiment, and thus it is very useful. 

Fig. 5 CO stripping cyclic voltammetry data of (a) a PtRu electrode and (b) a platinum electrode. The electrodes were exposed to lOOsccm CO /Hj (50 50v/v) for lOh before being flushed with Nj for 5min. Scan rate lOmV/s 100% RH at = 70°C. DHE dynamic hydrogen electrode. (Adapted from Gu et al. 2005, with permission)...

The effectiveness of EIS can be greatly enhanced with the use of a reference electrode, which has a stable potential at the time of measurement [3]. A suitable reference electrode allows discernment of the different electrode losses from the overall cell response, resulting in a more appropriate equivalent circuit. Ideally, the collective responses of the anode and cathode will add to the full cell resistance. Because the use of a stable reference electrode in many fuel cell systems is difficult, one common way to examine fuel cell behavior is the use of a dynamic hydrogen electrode (DHE). In this case, one of the electrodes is used as the DHE, with hydrogen flow at this location. It is assumed that the losses associated with the DHE are minor, and all polarizations measured can be attributed to the other electrode. This approach can be dubious and is not appropriate when there are phenomena at the DHE that can affect losses, such as anode dryout in a PEFC. Note that the DHE does not have to be the actual anode in the fuel cell but can be used at either electrode to examine the polarization of the opposing electrode. For example, a DHE can be used at the cathode of a DMFC to examine the polarization behavior of the anode in the DMFC. In this case, of course, the reaction does not galvanically proceed in the desired direction, and external power from a galvanostal/potentiostat system must be applied to drive the reaction in the desired direction. 

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