Locking the Atomic Ni Leaching on Ordered PtNi Nanostructure for Long-Term Hydrogen Production
Authors: Jin Song , Park Minseon , Jeong Jae-Yeop , Park Junyoung , Woo Seung-min , Kim Chiho , Lee Jooyoung , Kim Won Bae , Seo Min Ho , Choi Sung Mook 
Published: December 2024

To advance the practical performance and durability of alkaline electrolyte membrane water electrolyzers (AEMWE), addressing the sluggish kinetics compared to acidic conditions is imperative in developing highly active and durable electrocatalysts. For mitigating the issues of recession leading to lower catalytic activity, short-term durability, and reduced cost-effectiveness associated with conventional Pt-based electrocatalysts, incorporating transition metals is an effective strategy by modifying the metal-metal (M-M) lattice and electronic structure. Among the transition metals for alloying with Pt, especially, Ni is a suitable secondary atom because it is well-known to promote water dissociation and has a proper ∆GH* value close to Pt. However, the Ni atom undergoes a thermodynamic transformation into the Ni(OH)2, leading to dissolution or leaching, and is randomly redistributed during the hydrogen evolution reaction (HER) in alkaline conditions. To address these drawbacks associated with conventional alloy catalysts, there is growing interest in ordered intermetallic nanostructures, which exhibit unique and locally specific geometrical properties. The intermetallic nanostructure could be an effective remedy due to its high chemical and structural stability, attributed to a strong d-d orbital hybridization with an atomically ordered state.

In our study, intermetallic PtNi nanostructures are introduced to intrinsically expedite catalytic activity and prolonged durability through the combinational approaches of both computation and experiments. Specifically, the degradation mechanism of electrocatalysts for hydrogen evolution reaction (HER) in alkaline conditions is still questionable despite numerous related research works to enhance catalytic activity and durability. To unlock the degradation mechanism and demonstrate the robust durability of intermetallic-nanostructured HER electrocatalyst, the dissolution potential on the surfaces is theoretically predicted by density functional theory (DFT) calculations. Intermetallic PtNi/C shows higher dissolution potential on overall surfaces such as (111), (110), and (100), indicating more energy requirement for leaching Ni atoms compared to randomly alloyed PtNi/C. Furthermore, theoretical catalytic activity is analyzed using Gibbs free energy diagrams and electronic structure calculations based on density of states (DOS) and Crystal Orbital Hamiltonian Population (COHP) from DFT calculations. These results provide meaningful insights that Ni alloying significantly improves electrocatalysis for the HER compared to conventional Pt catalysts.

To experimentally validate these findings, intermetallic PtNi/C are synthesized via the NaBH4 reduction method at a low temperature followed by heat treatment in the N2 atmosphere. Physicochemical analyses including X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and scanning transmission electron microscopy (STEM) with energy-dispersive spectrometry (EDS) confirm atomically controlled structures with lattice strain and electronic structure modifications. To investigate the catalytic performance and the synergist effect of Ni alloying and intermetallic structure for hydrogen production, the electrochemical tests were evaluated in alkaline conditions from half-cell (active area: 0.196 cm2) to practical-scale AEMWE system (active area: 64 cm2). The HER performance was enhanced with lower overpotential at 10 mA cm-2 by alloying Ni atoms into Pt catalysts. According to the results of the accelerated degradation test (ADT), intermetallic PtNi/C well-maintained stability with a lower degradation rate compared to randomly disordered PtNi/C and commercial Pt/C. Notably, a large 3-cell stack cell evaluated the activity and durability for practically industrial feasibility that it showed a negligible degradation rate over a 2000 h operation period.

In summary, we successfully unlock the degradation mechanism and synergistic effect of Ni alloying with ordering engineering effect in theory, furthermore, experimentally validate from intrinsic property to practical-scale feasibility. We believe that controlling nanoparticle structure at the atomic scale is an attractive strategy to achieve highly efficient and stable low-PGM electrocatalysts for hydrogen evolution in AEMWE.

This work has been supported by the National Research Foundation of Korea (NRF) grant funded by the Ministry of Science and ICT in the Republic of Korea (MSIT) (NRF-2022R1A2C2093090) and was supported by the Ministry of Trade, Industry & Energy (MOTIE, Korea) under the Technology Innovation Program (20019175) supervised by the Korea Evaluation Institute of Industrial Technology(KEIT).

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