The Effect of Carbon Support Surface Functionalization on PEM Fuel Cell Performance, Durability, and Ionomer Coverage in the Catalyst Layer

Abstract

Surface functionalization of nitrogen-containing species on carbon support has been shown to affect electrocatalytic activity and carbon-ionomer interaction in the fuel cell electrodes, but studies of the functionalization on mass transport are limited. We reported two schemes for grafting positively (nitrogen groups) charged species and a scheme for negatively (sulfonates) charged species on the surfaces of three carbon materials. The functionalization was characterized with BET, XPS, contact angle measurements, and zeta potentials. In full-cell tests, improvements over high current densities were observed in samples reacted with para-phenylenediamine or ammonia, whereas the performance decreased after functionalization with sulfonate groups. The improvement at high current densities exceeded the mass-activity improvement and was attributed to reduced mass-transfer polarizations. Furthermore, a statistical approach was developed to examine the changes in ionomer surface coverage using STEM images with fluorine EDS maps. Durability studies followed the DOE’s protocol of potential cycling between 1.0 V and 1.5 V. Faster rates of carbon loss were found to occur after all three surface functionalization schemes due to preferential oxidation of the weaker covalently bonded functional groups, but despite the increased rates of loss, after amination the most durable carbon support showed end-of-life performance comparable to the initial performance.

Carbon black functionalized by grafting of Azo‐generated‐radicals as electrocatalyst support for the oxygen reduction reaction

“…[13][14][15][16][17] Indeed, the local O 2 mass transport resistance can be expected to lower using N-modified carbon material, thanks to two distinct effects: (i) the blocking of the support micropores after functionalization, preventing the Pt NPs deposition too far inside these pores, and (ii) a more uniform spread of the ionomer onto the electrocatalyst due to attractive interactions between the nitrogen functional groups and the ionomer. [13,[18][19][20][21] Additional beneficial features of N-modified carbon support were also put forward, such as a stronger anchoring of Pt NPs onto the carbon surface, or better intrinsic activity of the Pt NPs for the ORR, due to the electronic influence of the support nitrogen functional groups with Pt (effect commonly referred to as strong metal-support interaction, SMSI). [22][23][24][25][26] The carbon supports are commonly functionalized by an oxidation step (e.g., refluxing of the carbon material with highly concentrated HNO 3 ) followed by an amination step (reaction of the oxidized carbon with ammonia gas at various elevated temperature).…”

“…[27] Another extensively studied functionalization strategy is the grafting onto the carbon material of aryl radicals generated in situ by the reduction of diazonium cations. [19,22,23,[28][29][30][31] The generation of the aryl radicals generally takes place in highly acidic aqueous media with nitrite and constitutes a simple and versatile method for functionalization. Our initial goal was to investigate another grafting synthesis route, replacing the aforementioned aryl radicals by alkyl radicals generated from the thermal decomposition of azo compounds.…”

Scalable Sacrificial Templating to Increase Porosity and Platinum Utilisation in Graphene-Based Polymer Electrolyte Fuel Cell Electrodes

“…These moieties have been reported as having aided in the 'binding' of the thiophilic Pt nanoparticles [55][56][57]. The functionalisation of carbon with sulfur-containing organic species has been reported as having improved catalyst durability; however, they have also been reported as having poisoned the Pt catalyst's surface in some cases [26,58]. The aqueous reduction of chloroplatinic acid was used to deposit ca.…”

“…High surface area carbons (HSAC) typically possess a high microporosity and are less graphitic than low surface area carbons (LSAC). The reduced graphitisation and the presence of surface oxygen functional groups in HSAC make them highly susceptible to oxidative damage, limiting fuel cell durability [25,26]. Carbon corrosion of the support is one of the main degradation mechanisms for an active fuel cell, and occurs faster under fuel starvation or high humidity operations that are common in real-world applications [25,27].…”

Correlation between the surface characteristics of carbon supports and their electrochemical stability and performance in fuel cell cathodes

“…The surface functionalisation of carbon supports has been demonstrated to affect the ionomer coverage in the catalyst layer as well as its performance and durability in the fuel cell. 24 Therefore, an increasing interest and need is emerging to correlate surface and structure properties of carbon supports with the performance and stability to corrosion of the corresponding PEMFC cathodes. 6,8 In this study, carbons with a similar surface area but different surface chemistries were used as supports for the deposition of Pt nanoparticles synthesised by the microwaveassisted polyol-mediated method.…”