Elsevier

Electrochimica Acta

Volume 292, 1 December 2018, Pages 292-298
Electrochimica Acta

Impedance spectroscopy assessment of catalyst coated Nafion assemblies for proton exchange membrane fuel cells

https://doi.org/10.1016/j.electacta.2018.09.163Get rights and content

Highlights

  • High performance catalyst coated membranes (CCM) made of ketjenblack based TKK Pt catalysts.

  • Systematic impedance spectroscopy study of ketjenblack based TKK Pt catalysts for PEMFC.

  • Combined contribution of intrinsic charge transfer and ionic properties in predicting fuel cell performances.

Abstract

We report on the employment of the electrochemical impedance spectroscopy (EIS) to characterize catalyst coated membrane (CCM) electrode assemblies (MEA) as a function of ionomer percentage versus relative humidity, in proton exchange membrane fuel cell (PEMFC). The catalyst layer was fabricated by spraying TKK catalyst containing Nafion ionomer directly onto Nafion membranes, sandwiched between two identical mesoporous layer coated gas diffusion layers (GDLMPL). Relative humidity was varied for the MEA's containing various Nafion contents (MEAM; M: Nafion wt. percentage per mass of Pt). The MEA20 had low charge transfer (RCT) and high ionic (Rion) resistances, while MEA60 showed high RCT and low Rion. Both generated inferior performances to MEA30, MEA40, and MEA50. These observations simply ruled out RCT or Rion to independently explain the fuel cell performance. However, the MEA30, MEA40, and MEA50 had a combination of low RCT and Rion, all producing power densities larger than 1.1 W cm−2. This indicates that both RCT and Rion would effectively contribute to the performance of the MEA's, where the maximum power density sharply dropped when sum of the normalized RCT and Rion (i.e. RT, total resistance) was ∼0.5 Ω or higher. It appears that the RT may be a reliable while simple parameter for assessing the ketjenblack-based TKK with Nafion as ionomer for PEMFC. Such diagnostic tool can be further evaluated and extended to other catalysts with different supports and ionomer compositions.

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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