Abstract
In order to help the introduction on the automotive market of polymer electrolyte fuel cells (PEFCs), it is mandatory to develop highly performing and stable catalysts. The main objective of this work is to investigate PtNi/C catalysts in a PEFC under low relative humidity and pressure conditions, more representative of automotive applications. Carbon supported PtNi nanoparticles were prepared by reduction of metal precursors with formic acid and successive thermal and leaching treatments. The effect of the chemical composition, structure and surface characteristics of the synthesized samples on their electrochemical behavior was investigated. The catalyst characterized by a larger Pt content (Pt3Ni2/C) presented the highest catalytic activity (lower potential losses in the activation region) among the synthesized bimetallic PtNi catalysts and the commercial Pt/C, used as the reference material, after testing at high temperature (95 °C) and low humidification (50%) conditions for automotive applications, showing a cell potential (ohmic drop-free) of 0.82 V at 500 mA·cm−2. In order to assess the electro-catalysts stability, accelerated degradation tests were carried out by cycling the cell potential between 0.6 V and 1.2 V. By comparing the electrochemical and physico-chemical parameters at the beginning of life (BoL) and end of life (EoL), it was demonstrated that the Pt1Ni1/C catalyst was the most stable among the catalyst series, with only a 2% loss of voltage at 200 mA·cm−2 and 12.5% at 950 mA·cm−2. However, further improvements are needed to produce durable catalysts.
Highlights
Fuel cell technology is very close to the market application; a further optimization and a decrease of costs are still necessary [1,2]
The occurrence of the primitive cubic structure in the sample treated at high temperature is evident from the presence of the superlattice reflections, i.e., (001), (110) and (210) and from a better matching with the Joint
PtNi electrocatalysts were prepared by the formic acid method and successive thermal and leaching treatments in order to assess the best composition in terms of catalytic activity, fuel cell performance and stability
Summary
Fuel cell technology is very close to the market application; a further optimization and a decrease of costs are still necessary [1,2]. Several mechanisms have been proposed to explain the high ORR activity of Pt–Ni structures [25,26], the best composition is still not clear, in particular under practical fuel cell conditions These catalysts show insufficient durability, and suffer from a loss of structural integrity by metal segregation and de-alloying [16]. Some preparation procedures can effectively produce stable catalysts with an enrichment of Pt in the outermost layers of alloyed bimetallic catalysts [7,27,28] These regard an induced surface segregation of Pt by high-temperature annealing and a removal of the less noble transition metal from the alloy surface by pre-leaching in an appropriate acid. A leaching procedure in 0.5 M HClO4 was carried out for the synthesized PtNi catalysts after the thermal treatment to remove the non-alloyed
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