Impedance spectroscopy assessment of catalyst coated Nafion assemblies for proton exchange membrane fuel cells
Introduction
Systematic assessment of parameters that influence the performance of proton exchange membrane fuel cells (PEMFC) is one key step in the development of fuel cell technology [[1], [2], [3], [4], [5], [6], [7], [8]]. One important component of such understanding lies within the often problematic transport of ions through the membrane electrode assembly (MEA) [[8], [9], [10], [11]]. In a PEMFC [12], H2 enters the anode to be oxidized, producing protons and electrons. Electrons exit the MEA off to the external circuiting, while protons travel through the membrane to the cathode, combining with O2 to produce H2O (i.e. oxygen reduction reaction; ORR). The PEMFC can be operated at various pressures, temperatures, and relative humidity (RH), each of which can influence the electron/mass transport properties of the MEA [1,3,11,13,14]. Since most processes inside the PEMFC involve some sort of electron/mass transport, changes in those characteristic should dramatically affect the operational performance of the PEMFC.
Over the past many years, a significant body of literature has been bestowed to study the use of EIS for diagnosis and characterization of MEA's in PEMFCs, among which durability studies, degradation mechanism, and obtaining information on charge transfer and mass transport conductivities have attracted special attentions [9,10,[15], [16], [17]]. However, impedance assessment of PEMFC's has mostly been conducted in the presence of the cathodic reactant (i.e. O2 or Air) [11,15,18], whereas such assessment with no cathodic reactant (i.e. under N2 flow) has been documented limitedly [19,20]. While running EIS under H2/O2 is helpful in mimicking the actual operation of the fuel cell [14,15], the interference from the O2 reduction limits study of the inherent electron/mass transport properties of the catalyst layer (CL). As a result, such studies should be conducted under the H2/N2 feed [19].
To our knowledge there has been no systematic investigation of catalyst coated membrane (CCM) CL's consisting of ketjenblack type carbon supports (e.g. TKK), as a potential catalyst for PEMFC, for intrinsic electron/mass transport characteristics using impedance spectroscopy. This is considering the importance of the support material in the performance of the resulting MEA [21], where information for a given, well studied support such as Vulcan-based carbon may not be valid to be simply extended to catalysts that contain ketjenblack (KB) supports (e.g. TKK). As such, one important question would be how such properties may correlate with fuel cell performances when KB is incorporated as catalyst coated membrane (CCM) support. On this basis, this paper is aimed at using EIS over MEA's made of KB-based catalysts with various Nafion contents as ionomer, coated directly onto Nafion 212. These will be subjected to various relative humidity (RH) conditions. We will also show that our MEA's constructed of the symmetric CCM's generate interesting performances that are dependent upon Nafion content and RH. Then, by means of EIS we will discuss how the two factors influence charge/ion transport properties. It will also be discussed that the MEA with combined optimal charge/ion transport properties performs more superiorly.
Section snippets
Chemicals
2-propanol (IPA; Fisher, assay >99.5%), Nafion solution (Ion Power Inc.; 5% wt.), commercial Pt/C (TKK 46.6%), microporous layer coated gas diffusion layer (GDLMPL; BC29), hydrogen peroxide (EMD; 30%), and sulfuric acid (ACP; 98.08) were used as received. Nafion NRE212 membranes (Ion Power; designated as 212) were activated at 80 °C in H2O, H2O2, H2SO4, and finally in H2O, then washed with deionized water several times and stored at room temperature in deionized water.
Preparation of catalyst coated membranes (CCM)
In a typical preparation,
Results and discussion
Fig. 1A shows potentiodynamic response at 100 mV s−1 of an MEA containing the CL with 30% Nafion (relative to Pt), MEA30, as a function of the relative humidity (RH). The voltammograms display the characteristic electrochemical response of Pt in acid [22], where the H electrochemistry features can be observed below 0.3–0.4 V, with H adsorption (HAds) and the H desorption (HDes) on the cathodic and anodic sweeps, respectively [23]. Above 0.6 V, oxide formation (anodic) stripping (cathodic)
Conclusions
Impedance study was conducted over the MEA's containing various Nafion contents (20–60%) under 20–100% relative humidity (RH). It was found that ionic resistance (Rion, at 425 and 850 mV) decreased with increasing Nafion, while charge transfer resistance (RCT, 100 mV; hydrogen region) most significantly increased when the Nafion quantity was larger than 40%. At 20% Nafion (i.e. MEA20), large Rion values were obtained, whereas it resulted in relatively low RCT values. On the other hand, the
Acknowledgement
Financial support of this work was provided by the University of Ontario Institute of Technology and Natural Science and Engineering Research Council of Canada through a Strategic Grant in partnership with Ballard Systems.
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