The proton exchange membrane fuel cells (PEMFCs) are expected to be one of the most suitable power sources for electric vehicles to replace the traditional energy,¹ but its commercialization is restricted by some problems, such as high cost, low durability and inadequate facilities and so on.² Among these problems, the high cost, unsatisfactory activity and durability of Pt catalyst as the key part in fuel cells become particularly important.³ It is one of promising solutions to design and prepare low Pt loading catalysts⁴ with high efficiency and low cost. Recently, it is found that the (111) facets possess the highest oxygen reduction reaction (ORR) activity in the three low index facets of Pt alloys,⁵ and thus Pt-based octahedral catalysts with maximal (111) surface area have been paid more attention. However, in methods commonly employed, the capping agents difficult to remove not only block the active sites but also hinder the electron transfer between catalyst and support. Therefore, surfactant-free method is employed, and the growth of specific facet is synergistically regulated by suitable organic reducing agent and organic ligands of precursors to replace capping agents.⁶

The reaction conditions have important influence on the crystal growth process, crystal size, bulk and surface atom arrangement. In this work, impacts of reaction temperature, reaction time and ratio of metal elements on the formation of PtNi octahedral nanocrystals were investigated to optimize preparation conditions. The results demonstrate that reaction temperatures can affect crystallization rates, and lower temperature leads to excessively slow reduction and crystallization rate, which will result in small particles and incomplete (111) facet growth, whereas a higher temperature will cause a faster crystallization rate and larger particles, which will result in loss of active sites and decreased ORR performance. The influence of reaction time on electrochemical performance mainly depends on the synergistic effect between crystal growth and metal atom migration caused by concentration gradients. For one thing, crystal sizes are increased with the reaction time prolonged, which will result in the decrease of active area; for another, the atom concentration gradients resulted from different electrochemical deposition potentials impel Pt atom to migrate towards the surface, which is beneficial to the increase of active sites. The mechanism is more complex for impact of different atom ratios. We suggest that the ORR activities of PtNi octahedral nanocrystals with different Pt/Ni atom ratios are determined by the delicate balance between the thickness of Pt surface skin, the Ni content near the surface and the remaining (111) facets. Accordingly, reaction temperature of 140 °C, reaction time of 42 h and Pt/Ni atom ratio of 1: 1 were selected to prepare the PtNi/C octahedral nanocrystal catalyst in order to ensure optimal ORR performance.

Under the optimized reaction conditions, PtNi/C octahedral nanocrystal catalyst with highly active (111) facets as surface was successfully prepared by a facile one-pot surfactant-free solvothermal method using acetylacetonate salts as precursors and dimethylfomamide (DMF) as reducing agent, and it exhibits well-defined structure and enhanced ORR performance compared with commercial Pt/C catalyst. Its mass activity and specific activity are 224.5 mA mg_Pt^-1 and 618.5 μA cm_Pt^-2, which are about 2.6 times and 3.8 times of Pt/C (Johnson Matthey, JM), respectively. The electrochemically active surface areas (ECSAs) of Pt/C (JM) decrease from 53.2 m² g_Pt^-1 to 32.5 m² g_Pt^-1 after 2000 cycles accelerated durability testing (ADT), indicating a loss of 38.9%, whereas the attenuation rate in ECSA of PtNi/C octahedral nanocrystal catalyst is only 23.1%. The mass activity of Pt/C (JM) at 0.9 V vs. RHE is reduced from 86.4 mA mg_Pt^-1 to 30.6 mA mg_Pt^-1 after durability test, and the attenuation rate is up to 64.6%; nevertheless, the mass activity of PtNi/C octahedral nanocrystal catalyst is dropped by 46.1%, and its mass activity after 2000 cycles is yet higher than the initial value of Pt/C (JM).

Our work provides some valuable insights in the preparation condition optimization of PtNi octahedral catalyst, and has a certain guiding significance for further research on the preparation of shape-selective catalysts.

References

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6. C. H. Cui, L. Gan, M. Heggen, S. Rudi, P. Strasser, Nat. Mater., 12, 765 (2013).