Effect of Polybenzimidazole Coating on Acetylene Black for Durability of Polymer Electrolyte Membrane Fuel Cell

Abstract

One of the difficulties to retain Pt interfacial structure during PEMFC operation is arose from the high activity and large surface energy of the Pt nanoparticle, which facilitate i) Ostwald ripening, where the Pt particle size increases by a dissolution-redeposition process upon redox cycling and ii) agglomeration of Pt nanoparticles by sintering. These phenomena lead to the loss of Pt interfacial area and deteriorate the PEMFC performance. In addition, stability of carbon supports used as an electron conductive Pt supporting material is also essential to improve Pt stability since carbon corrosion catalyzed by Pt also takes place at a high potential (mainly over 1.0 V), which triggers the detachment of Pt and subsequent agglomeration. Thus, the use of durable carbon supports against the oxidation is crucial to increase the PEMFC durability. Since it is known that sp2 carbons are more thermodynamically stable than sp3 carbons upon oxidation, the use of highly graphitized carbons such as carbon nanotubes (CNTs) have been investigated to employ and it is proved that the use of such carbons mitigates the Pt agglomeration and thus enhance the PEMFC durability. However, an absence of the anchoring sites for Pt deposition on such graphitized carbon surface such as -OH and -COOH connected on sp3 carbons make the uniform deposition of Pt nanoparticles rather difficult. And it is pointed out that the weak interaction between the graphitized carbon surface and Pt nanoparticles often induces the sintering of Pt particles and, therefore the improvement of the Pt‒carbon interfacial strength will further improve the Pt stability. Although, general strategy to introduce anchoring sites for the highly graphitized carbons is to oxidize the surface by harsh chemical or physical treatments, such oxidation always involved the introduction of the sp3 sites and resulted in the decrease of the durability of carbon supports. We have developed smart method to introduce anchoring sites without creating sp3 carbons on the highly graphitized carbon surface, where the carbons are coated by polymer having anchoring sites for metal ions. In this method, polymer such as polybenzimidazole (PBI) is coated on the pristine (non-oxidized) carbons and can act as the anchoring site for the Pt deposition. Therefore, Pt can be deposited onto the surface of highly graphitized carbon such as CNTs without oxidizing the CNTs. Indeed, we reported that PEMFC having PBI-coated CNT as a carbon support showed excellent durability upon fuel cell accelerated durability test (ADT) between 1.0 ‒ 1.5 V. In that research, oxidized-CNT was used as a comparison since direct Pt deposition onto the pristine CNT was rather difficult and, in addition, we used the ADT protocol designed to degrade the carbon (1.0 – 1.5 V). Thus, we successfully demonstrated the advantage of the PBI coating to preserve inherent carbon durability but the effect of the PBI coating for the Pt stability was not investigated. We also demonstrated that the PBI-coating on Vulcan could enhance the durability of Vulcan-based PEMFC. However, in that research, PBI coating improved Vulcan stability mainly because the coverage of micropore of Vulcan decelerated the oxidation of the carbon surface during the ADT between 1.0 ‒ 1.5 V. Therefore, the effect of PBI coating for the stability of Pt particles was not observed. In this study, to clarify the effect of PBI coating for Pt stability and PEMFC durability, ADT between 0.6 ‒ 1.0 V designed to study Pt stability was applied for the PEMFC having PBI-coated carbons as a carbon support. Here, Acetylene Black (AB) was chosen as the carbon support since 1) AB shows high electrochemical stability due to their high crystallinity and Pt sintering triggered by the carbon corrosion during the ADT can be avoided and 2) direct Pt deposition on AB was possible and no oxidation pretreatment is necessary for the reference samples. As the result, we found that MEA employing PBI-coated AB shows only 8% decrease in the maximum power density, while MEA containing non-coated AB shows 34% decrease after the ADT. Structural analysis of the electrocatalysts after the ADT reveals that the PBI anchors both Pt nanoparticle and ionomer upon ADT and maintains their interfacial structure. We conclude that the polymer-coating method is promising both for the improvement of the durability and the activity of the PEMFC.