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Highlights• Development of highly stable and active cathode catalyst supports is reported.• Pt nanoparticles deposited on the stable supports show enhanced catalytic activity.• Potential holding at 1.2V showed enhanced support stability in H 2 -air polarization.• Rated power densities of 0.18-0.23 g Pt kW −1 are achieved with the novel catalysts. Abstract Novel procedures are developed for the synthesis of highly stable carbon composite catalyst supports (CCCS-800 °C and CCCS-1100 °C) and an activated carbon composite catalyst support (A-CCCS). These supports are synthesized through: (i) surface modification with acids and inclusion of oxygen groups, (ii) metal-catalyzed pyrolysis, and (iii) chemical leaching to remove excess metal used to dope the support. The procedure results in increasing carbon graphitization and inclusion of non-metallic active sites on the support surface. Catalytic activity of CCCS indicates an onset potential of 0.86 V for the oxygen reduction reaction (ORR) with well-defined kinetic and mass-transfer regions and ~2.5% H 2 O 2 production in rotating ring disk electrode (RRDE) studies. Support stability studies at 1.2 V constant potential holding for 400 h indicate high stability for the 30% Pt/A-CCCS catalyst with a cell potential loss of 27 mV at 800 mA cm −2 under H 2 -air, 32% mass activity loss, and 30% ECSA loss. Performance evaluation in polymer electrolyte membrane (PEM) fuel cell shows power densities (rated) of 0.18 and 0.23 g Pt kW −1 for the 30% Pt/A-CCCS and 30% Pt/CCCS-800 °C catalysts, respectively. The stabilities of various supports developed in this study are compared with those of a commercial Pt/C catalyst.

Carbon composite catalyst (CCC) and activated carbon composite catalyst (A-CCC), both containing active catalytic sites for oxygen reduction reaction (ORR), were synthesized and used as supports to develop hybrid cathode catalysts (HCCs). HCCs are a combination of CCC or A-CCC supports and Pt or Co-doped Pt catalysts. Uniform Pt deposition on these supports was accomplished through surface modification and modified polyol processes. The Co-doped Pt was synthesized at 800°C in the presence of polyaniline protective coating. The HCCs, namely Pt/A-CCC, Co-doped Pt/CCC, and L-Co-doped Pt/CCC catalysts showed peak power densities of 944, 857, and 1050 mW cm−2, respectively, which are much higher than the commercial Pt/C (746 mW cm−2). Furthermore, the Pt/C showed very high power density loss (63%) when compared to HCCs (16–26% loss) after 30,000 potential cycles (0.6–1.0 V). The Pt/A-CCC catalyst showed excellent support stability when subjected to potential holding at 1.2 V for 400 h (27 mV loss at 800 mA cm−2) and 5,000 potential cycles between 1.0 and 1.5 V (25 mV loss at 1500 mA cm−2) which are less than that of 2017 US DOE targets (≤30 mV loss). The durability studies indicated that the HCCs are promising cathode catalysts for transportation applications.

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A B S T R A C TCathode catalyst based on Co-doped Pt deposited on carbon composite catalyst (CCC) support with high measured activity and stability under potential cycling conditions for polymer electrolyte membrane (PEM) fuel cells was developed in this study. The catalyst was synthesized through platinum deposition on Co-doped CCC support containing pyridinic-nitrogen active sites followed by controlled heattreatment. High resolution transmission electron microscopy (HRTEM) and X-ray diffraction (XRD) studies confirmed uniform Pt deposition (Pt/CCC catalyst, d Pt = 2 nm) and formation of Co-doped Pt/CCC catalyst (d Pt = 5.4 nm) respectively. X-ray energy dispersive spectrometry (XEDS) line-scan studies showed the formation of Co-core Pt-shell type catalyst with a Pt-shell thickness of $0.75 nm. At 0.9 V iR-free , the Co-doped Pt/CCC catalyst showed initial mass activity of 0.44 A mg Pt À1 and 0.25 A mg Pt À1 after 30,000 potential cycles between 0.6 and 1.0 V corresponding to an overall measured activity loss of 42.8%. The commercial Pt-Co/C showed initial mass activity of 0.38 A mg Pt À1 and $70% loss of activity after 30,000 cycles. The enhanced catalytic activity at high potentials and stability of mass activity for the Co-doped Pt/CCC catalyst are attributed to the formation of compressive Pt lattice catalyst due to Co doping. The Co-doped Pt/CCC showed stable open circuit potential close to 1.0 V under H 2 -air with an initial power density of 857 mW cm À2 and only 16% loss after 30,000 cycles. Catalyst durability studies performed between 0.6 and 1.0 V indicated that Co doping increased the onset potential for PtO 2 formation close to 1.0 V vs. reversible hydrogen electrode (RHE). The enhanced catalytic activity and stability of Co-doped Pt/CCC catalyst are attributed to (i) higher onset potential for PtO 2 formation resulting in less PtO 2 formation during potential cycling which alleviates Pt dissolution in the reverse scan (ii) higher stability of CCC used as a support compared with commercially used supports, and (iii) optimized electrochemical properties of the catalyst and the support which result in synergistic effect between pyridinic nitrogen catalytic sites from the Co-doped CCC support and compressive Pt-lattice catalyst.

Combined experimental and simulation techniques were applied to evaluate the validity of the commonly applied Tafel-approximation for the calculation of the mass and specific activities. A one dimensional agglomerate-based macro-homogeneous model was developed and solved analytically and numerically to understand the effects of exchange current density, porosity, agglomerate size, Nafion-film thickness and Pt loading on the mass activity. Porosity higher than 60% or agglomerates bigger than 100 nm at higher loadings may cause the loss of the validity of the Tafel-approximation and consequently the mass and specific activities decrease. Since these effect do not fully interpret the measured data, a new structural parameter was introduced. The real pore structure was estimated by using fractal theory. The volumetric catalyst density was corrected by the fractal dimension measured by Hg-porosimetry, which finally gave a good agremment with the experimental values.

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The mass and specific activities are intensively used as the characterization parameters for the effectiveness of the catalyst in a fuel cell. These quantities were found to be independent of catalyst loading, but the data presented in this work shows that the mass and specific activity are not independent of the loading at ultra-low Pt loadings. In this work the dependence of mass activity on platinum loading was studied. A one dimensional simplified agglomerate-based macro-homogeneous model was developed and solved analytically and numerically to understand the effects of porosity, agglomerate size, and Pt loading on the mass activity.