Regulation of Metal Bond Strength Enables Large-Scale Synthesis of Intermetallic Nanocrystals for Practical Fuel Cells

Energy Chemistry Chemistry Engineering Energy Engineering Thermodynamics Materials Chemistry Electromagnetism Physics
fuel cells intermetallic nanocrystals metal bond strength low-temperature synthesis optimized electrocatalysts

In recent years, fuel cells, as a clean and renewable energy technology, have garnered widespread attention. However, the extensive application of fuel cells faces the challenge of the stability of oxygen reduction reaction (ORR) electrocatalysts. L10-structured intermetallic nanocrystals (INCs) with chemically ordered structures, due to their lower formation energies (e.g., the atomic formation energy for ordered L10-PTFE is approximately -0.232 eV) and higher cohesive energies, demonstrate greater stability compared to disordered A1-PTM, making them one of the most promising electrocatalysts in the field of fuel cells. However, the high-temperature annealing treatment required to achieve this ordered structure (typically >600°C) often leads to severe particle sintering, morphological changes, and reduced order, making it difficult to mass-produce this type of electrocatalyst and limiting its practical application in fuel cells.

Research Background and Motivation

To address the abovementioned issues, our research team proposed a novel strategy of low melting point metal (M’ = Sn, Ga, In) induced bond strength weakening to lower the activation energy (Ea), promote the ordering process of PTM (M = Ni, Co, Fe, Cu, and Zn) catalysts, and thereby prepare high PT content (≥40 wt%) L10-PT-M-M’ intermetallic nanocrystals at lower temperatures (≤450°C), achieving production at scales up to ten grams. This research not only involves experimental verification but also elucidates the fundamental mechanism of low-temperature ordering through X-ray spectroscopy studies, in situ electron microscopy, and theoretical calculations, tightly coupling spectral analysis, material synthesis, and practical application testing.

Research Team and Publication Information

This research was completed in collaboration between teams from Peking University and Huazhong University of Science and Technology. The authors include Jiashun Liang, Yangyang Wan, Houfu Lu, Xuan Liu, Fan Lv, Shenzhou Li, Jia Xu, Zhi Deng, Junyi Liu, Siyang Zhang, Yingjun Sun, Mingchuan Luo, Gang Lu, and Jiantao Han. The research article was published in the April 2024 issue of Nature Materials, with DOI https://doi.org/10.1038/s41563-024-01901-4.

Experimental Design and Methods

Experimental Processes

  1. Material Preparation and Characterization: Carbon-supported ternary PT50NI50-XM’X alloy nanocrystals (X=7, 10, 15) were prepared using a wet chemical method. Structural characterizations were conducted using transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), and scanning transmission electron microscopy (STEM). X-ray diffraction (XRD) and differential scanning calorimetry (DSC) tests were used to determine their crystalline structures and degrees of ordering.
  2. Low-Temperature Ordering Treatment: The prepared PT50N35SN15 was annealed at 450°C in an H2/Ar gas atmosphere to obtain ordered L10-PT50NI35SN15/C alloy nanocrystals.
  3. Electrochemical Performance Testing: The electrochemical performance of the nanocrystals was measured using a rotating disk electrode (RDE), and the catalyst’s performance in a practical hydrogen-air fuel cell was evaluated through membrane electrode assembly (MEA) tests.

Research Results

  1. Structural Characterization and Ordering Mechanism: TEM, HRTEM, and STEM observations revealed atomic-scale images showing the ordered arrangement of the L10 structure. XRD and DSC test results indicated that after introducing Sn, the ordering temperature of PT50NI35SN15 significantly decreased to 410°C, and the activation energy also greatly reduced. Kissinger equation calculations showed an activation energy of 181.2 kJ mol-1, significantly lower than the 267.7 kJ mol-1 of pure PT50NI50.
  2. Electrochemical Performance: L10-PT50NI35GA15/C exhibited high current density at 1.67 A cm-2 under hydrogen-air conditions at 0.7 V and retained 80% of its current density after 90,000 cycles, surpassing the performance indicators set by the U.S. Department of Energy (DOE), making it one of the best cathode electrocatalysts in practical proton exchange membrane fuel cells (PEMFC).

Other Important Findings

The study also demonstrated the general applicability of the LMIBSW strategy to other PTM alloys, such as PT50FE45SN5 and PT50CU45SN5, which exhibited similar low-temperature ordering phenomena, further emphasizing the generality and value of this strategy. Density functional theory (DFT) calculations revealed that bond strength weakening caused by M’ doping is the fundamental reason for reduced Ea and promoted low-temperature ordering. The determination of this mechanism provides theoretical guidance for the design of a wide range of alloy systems.

Research Significance

  1. Scientific Value: Revealing the mechanism by which Sn promotes low-temperature ordering deepens the understanding of bond strength regulation’s impact on ordering.
  2. Practical Application Value: Provides high-performance, low-cost, and scalable electrocatalysts for PEMFC applications in heavy-duty vehicles (HDV), promoting the application of fuel cells under practical conditions.

Research Highlights

  1. Innovative Approach: Proposes a low melting point metal induced bond strength weakening strategy, effectively solving the problem of grain growth and morphological changes caused by high-temperature annealing, achieving high PT content alloy materials.
  2. Significant Performance Improvement: The developed L10-PT-NI-M’/C catalyst demonstrates extremely high current density and good stability under practical fuel cell conditions, far exceeding existing commercial products.
  3. Universality: This strategy is applicable not only to the PT-NI-SN system but also shows effectiveness in various other alloy systems, highlighting the strategy’s universality and great application potential.

Through this research, a revolutionary catalyst synthesis strategy is provided, possessing high practical value and laying a theoretical and technical foundation for the design of high-performance fuel cell electrocatalysts in the future.