Contaminant Cation Effect on Oxygen Transport through the Ionomers of Polymer Electrolyte Membrane Fuel Cells

Abstract

We characterized the effects of cobalt (Co 2+ ) and other cation contaminants on the oxygen (O 2 ) transport properties of the PFSA ionomer used in polymer electrolyte membrane fuel cells (PEMFCs) and gained insight into the mechanisms by which contaminant cations inhibit O 2 transport. Such cations can be released by alloy catalysts and environmental conditions and pose a significant challenge to maintaining high current density performance with low platinum (Pt) loadings. We used a test cell capable of isolating the ionomer from the membrane electrode assembly (MEA), allowing for O 2 transport resistance (R O2 ) measurements using a limiting current technique. We contaminated ionomer membranes with Li + , Na + , Ni 2+ , Co 2+ , and Ce 3+ and found a general increase in R O2 for increased contamination levels and decreased water activity. In addition, our Co 2+ results indicated distinct concentrationdependent regimes. The other cation-form ionomers allowed us to separate the impacts of ion pair strength, multivalency, and reduced water uptake. We believe that these factors cause a more compressed hydrophilic domain and tortuous O 2 diffusion path, and a commensurate increase in R O2 . Finally, we studied the impact of Co 2+ on an operating PEMFC and found an increase in R O2 consistent with the results of our isolated membrane tests.

Resolving the nanoparticles' structure-property relationships at the atomic level: a study of Pt-based electrocatalysts

“…This is relevant for practical PEMFC applications, as several recent reports already provide evidence that M cations (e.g. Ni, Co, Cu, Fe, …) contamination in the PEMFC are decreasing the high-current density performance in MEA ( Ahluwalia et al., 2018a ; Braaten et al., 2017 , 2019 ; Papadias et al., 2018 ). Thus, it is of particularly high importance to understanding how intrinsic dissolution trends change after PCA for different Pt-M alloy systems when the surface of the nanoparticles becomes Pt rich, namely, core-shell.…”

“…In addition, the presence of a carbon shell requires additional efforts for adequate chemical activation of such electrocatalysts and exposure of active Pt-surface area such as with the use of ozone ( Baldizzone et al., 2015b ). On the bright side, however, Ni dissolution has a much less detrimental effect on proton-exchange membrane degradation as Fe or even Cu ions ( Braaten et al., 2019 ; Strlič et al., 2003 ). Furthermore, due to its low redox potential it does not block the Pt surface as Cu.…”

“…Therefore, Pt-Ni could be a viable candidate and has already well been demonstrated in the PEMFCs ( Ahluwalia et al., 2018a ; Dionigi et al., 2019 ; Han et al., 2015 ; Myers et al, 2015 ). Nevertheless, Ni ions have been shown to also have a negative effect on the O 2 transport resistance as well as influence the water uptake in the ionomer (but less detrimentally as Cu), thus affecting the three-phase-boundary and thus high current density performance in PEMFC ( Braaten et al., 2019 ). Thus, in addition to the above-stated Pt-Ni alloy synthesis challenges, significant improvements in the resilience of Pt-Ni alloy electrocatalysts against Ni leaching are still necessary for its successful deployment in end-user products.…”

“…However, although this initial dissolution is highly desired, further dissolution of M can diminish its positive ligand and/or strain effect and result in a loss of ORR performance ( Hodnik et al., 2014 ; Jovanovič et al., 2016 ; Papadias et al., 2018 ). Furthermore, the dissolved M ions can also negatively affect the overall performance, especially in the membrane-electrode assembly (MEA)( Ahluwalia et al., 2018a ; Braaten et al., 2019 ; Gasteiger et al., 2005 ; Mayrhofer et al., 2009 ). One such example is the interaction between the dissolved M ions and Pt surface that blocks the active surface and thus affects its electrochemical performance.…”

“…In this sense, the most prominent example can be shown in the case of Pt-Cu alloy where Cu ions strongly interact, namely adsorb and reduce, on the Pt surface via well-known under-potential deposition (UPD); Cu can be found on both the cathode and the anode ( Gatalo et al., 2019a , 2019b ; Jia et al., 2013 ; Yu et al., 2012 ; Zhu et al., 2020 ). The second example is the replacement of protons in the ionomer with M cations, which results in higher O 2 resistance as well as changes the water-uptake that consequently lowers the proton conductivity of the ionomer ( Braaten et al., 2017 , 2019 ). This causes ohmic losses in the cell as well as slows the ORR reaction rate, as fewer protons are available at the electrode ( Kienitz et al., 2011 ; Okada et al., 2002 ).…”