Ionic conductivity PEMFC

Li, G., and Rickup, P. G. Ionic conductivity of PEMFC electrodes. Journal of the Electrochemical Society 2003 150 C745-C752. 

Li G, Pickup PG (2003) Ionic conductivity of PEMFC electrodes effect of Nafion loading. JElectrochem Soc (ll) C745-52... 

It has been demonstrated that EIS can serve as a standard analytical diagnostic tool in the evaluation and characterization of fuel cells. Scientists and engineers have now realized that the entire frequency response spectrum can provide useful data on non-Faradaic mechanisms, water management, ohmic losses, and the ionic conductivity of proton exchange membranes. EIS can help to identify contributors to PEMFC performance. It also provides useful information for fuel cell optimization and for down-selection of the most appropriate operating conditions. In addition, EIS can assist in identifying problems or predicting the likelihood of failure within fuel cell components. 

Guo Q, Cayetano M, Tsou Y, De-Castro ES, White RE (2003) Study of ionic conductivity profiles of the air cathode of a PEMFC by AC impedance spectroscopy. J Electrochem Soc 150(ll) A1440-9... 

Figure 6.27. Nyquist plots (65000 to 0.82 Hz, 22 + 2°C, 1.00 V vs. H2) for nitrogen-bathed cathodes with various Nafion loadings. The inset shows an expansion of the high-frequency region of the plots [25], (Reproduced by permission of ECS—The Electrochemical Society, and the authors, from Li G, Pickup PG. Ionic conductivity of PEMFC electrodes.)...

This chapter has examined a variety of EIS applications in PEMFCs, including optimization of MEA structure, ionic conductivity studies of the catalyst layer, fuel cell contamination, fuel cell stacks, localized impedance, and EIS at high temperatures, and in DMFCs, including ex situ methanol oxidation, and in situ anode and cathode reactions. These materials therefore cover most aspects of PEMFCs and DMFCs. It is hoped that this chapter will provide a fundamental understanding of EIS applications in PEMFC and DMFC research, and will help fuel cell researchers to further understand PEMFC and DMFC processes. 

Taylor et al.8 were the first to report an electrochemical method for preparation of MEAs for PEMFCs. In their technique, Pt was electrochemically reduced and deposited at the electrode membrane interface, where it was actually utilized as an electrocatalyst. Nation, which is an ion exchange polymer membrane, is first coated on a noncatalyzed carbon support. The Nafion-coated carbon support is then immersed into a commercial acidic Pt plating solution for electrodeposition. Application of a cathodic potential results in diffusion of platinum cations through the active Nation layer. The migrated platinum species are reduced and form Pt particle at the electrode/membrane interface only on the sites which are both electronically and ionically conductive. The deposition of Pt particles merely at the electrode/membrane interface maximizes the Pt utilization. The Pt particles of 2-3.5 nm and a Pt loading of less than 0.05 mg cm-2 were obtained employing this technique.8 The limitation of this method is the difficulty of the diffusion of platinum... 

The ionic conductivities of sulfonated polyetherketone/imidazole (pyrazole) systems were first studied by Kreuer et al. [161]. The intercalation of imidazole (pyrazole) into the polymer with Bronsted add functions was shown to produce a high protonic conductivity (ca. 10 S cm ). However, the volatility of these heterocycles hampers their application for high-temperature PEMFCs. In order to improve the thermal stability, the immobilization of these heterocyde systems was evaluated [162-164]. When Schuster et al. characterized the conductivity of imidazole-terminated ethylene... 

Recent intensive studies have, however, been reported to lead to AEMs with high ionic conductivities, reportedly comparable to Nalion . These promising AEMs [11, 45-51] are still to be evaluated in AMFCs. Most hydrocarbon AEMs are soluble in various solvents, which is potentially useful for the formulation of alkaline ionomers required for the preparation of high-performance membrane electrode assemblies (MEAs). If the conductive properties reported can be translated into high power outputs, then AMEC performances comparable to those of PEMFCs can be expected in the near future. 

Figure 23.4. Finite transmission-line equivalent circuit describing the impedance behavior of a PEMFC electrode [29]. (Reproduced by permission of ECS— The Electrochemical Society, from Lefebvre M, Martin RB, Pickup PG. Characterization of ionic conductivity profiles within proton exchange membrane fuel cell gas diffusion electrodes by impedance spectroscopy.)...

Ammonia (NH3) or ammonium (NH4+) can exist in both the fuel and air streams. The diffusion of ammonium is fast, therefore, the ammonium entering the fuel cell from either side can quickly diffuse to the other side causing the contamination effect on both sides. For instance, for a typical membrane with a thickness of 10 to 100 jim, the estimated characteristic time constant for diffusion is 1 to 100 sec [149]. Ammonia may affect the PEMFC performance in different ways (1) by the reduction of the ionic conductivity of the membrane, which in its ammonium form is a factor of 4 lower than in the protonated form [149-151] (2) by poisoning the cathode catalyst [151] and (3) by poisoning the anode catalyst [149]. Recently, fuel cell tests have shown that the reduced membrane conductivity is not the major reason for performance losses induced by ammonia [149,150]. The effect of ammonia on the HOR was found to be minor at current densities below 0.5 A cm", but would increase with increasing current densities. The current density did not exceed 1 A cm in the presence of ammonia [149]. 

The performance of a HT-PEMFC depends mainly on the amount of phosphoric acid in the polymer membrane and in the porous catalyst layer as well as on the temperature. Furthermore, it is well known that phosphoric acid dehydrates at low water vapour partial pressure and rehydrates with increasing partial pressure, effects which can be observed in HT-PEMFCs under operating conditions [1, 2]. The composition change of phosphoric acid results in a variation of the ionic conductivity as well as of the viscosity. 

As already explained, IL exhibit numerous advantages as candidates for electrolyte in high temperature PEMFC. The introduction of IL into acid-doped PBI membranes significantly increases the ionic conductivity in the polymeric electrolyte and the mechanism of conduction seems to be the same as that of H3PO4/PBI. Protons are conducted by both hopping and vehicular mechanism, but hopping is the main mechanism of conduction. Vehicular mechanism is dependent of IL content due to the availability of free ions [64]. Ye et al. [66] found that the... 

The basic setup of polymer electrolyte membrane fuel cells (PEMFC) as well as direct methanol fuel cells (DMFC) is in line with the standard fuel cell setup the two electrodes are separated by means of an electrically insulating but ionically conductive membrane (polymer electrolyte). In a PEM fuel cell, the anodic reactant is hydrogen, which is oxidized to form protons on the cathode, oxygen is reduced and forms water with protons that are transported through the proton conductive membrane. The overall reaction is... 

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