Functionalisation of Carbon Supports for PtNi Nanoparticles as Fuel Cell Catalysts

Abstract

To enhance efficiency, catalysts are employed in fuel cells to facilitate the anode and cathode reactions. The most common catalyst is composed of platinum-based nanoparticles, distributed on a carbon black (CB) support. This composite catalyst degrades during usage, gradually lowering the fuel cell performance. Hence, the objective of this work was to decrease catalyst degradation by increasing the particle-support interaction through modification of the carbon black surface with functional groups. The functionalisation on the carbon black Vulcan XC72R was carried out in two steps: a chemical oxidation, followed by a condensation reaction of the produced COOH groups with diamines (ethylenediamine (EDA), triethylenetetramine (TETA), phenylenediamine (PDA)) or a dithiol (butanedithiol (BDT)). The chemical oxidation was conducted with a three-to-one mixture of concentrated sulphuric and nitric acid for varying durations. The successful oxidation was indicated by an increase of the O and H content with roughly a third of the O content originating from COOH groups and the remainder attributed to a variety of oxide groups. The degree of oxidation increased with longer oxidation times, accompanied with a reduction of the BET surface area. Short oxidation times (1-4 hours) provided a sufficient amount of COOH groups while maintaining a large surface area. The amine and thiol groups were introduced by either direct treatment of an oxidised carbon with a diamine, or via an acyl chloride intermediate step, followed by diamine/dithiol treatment. The latter synthesis route demonstrated higher conversion rates, but entailed increased sulphur impurities. The functionalisations with EDA and TETA were the most successful with EDA mainly displaying monofunctional and TETA displaying bridged functionalisations. To obtain the nanoparticle-support catalysts, pre-synthesised colloidal PtNi or Pt nanoparticles were deposited on the modified carbon blacks via a precipitation method, adjusted according to the changing polarity of the substrates and nanoparticles. The catalytic activity was investigated using electrochemical half-cell measurements of a working electrode containing a catalyst thin-film. The thin-film morphology, produced by drying a catalyst suspension (ink) on the working electrode, is influenced by the drying conditions. Stationary drying resulted in the formation of a coffee-ring. Flow-restricted drying in a container with a small hole was able to produce more uniform thin-films. However, the influence of the thin-film morphology was negligible towards the electrochemical active surface area (ECSA). The ECSA was employed to quantify the catalytic activity of the synthesised catalysts before and after an accelerated stress test (AST) to assess the degradation rate. The investigated samples comprised either PtNi or Pt nanoparticles, supported one of the following CB supports: unfunctionalised Vulcan XC72R, oxidised CB, EDA-functionalised, TETA-functionalised. As a reference, the synthesised catalysts were compared with the commercial catalyst (Pt on CB). Of the prepared catalysts, PtNi catalysts demonstrated higher catalytic activity than Pt catalysts. For both particle types, the Vulcan XC72R-supported catalysts displayed the highest ECSAs, and the highest degree of degradation. The lowest degradation was achieved for PtNi with the EDA-functionalised support and for Pt with the TETA-functionalised support, which also displayed initial ECSAs, comparable to Vulcan XC72R. This may be attributed the preferred interaction of the PtNi with primary amine groups (EDA) and of the Pt particles with secondary amine groups (bridged binding of TETA). Furthermore, the reduced relative degradation rate may be attributed to a removal of the carbon fine structures in the oxidation step. Compared to all prepared catalysts, the commercial catalyst HiSPEC 3000 displayed a higher initial ECSA, but also a higher degree of degradation. To conclude, the amine-functionalisation of the carbon supports for PtNi and Pt nanoparticles successfully improved the electrochemical stability of the catalysts while maintaining ECSA values comparable to the unfunctionalised carbon support.