Tailoring platinum-based catalysts is of great research interest in the fields of electrochemical energy conversion and storage, as well as other applications. Now, an approach has been developed to boost the activity of platinum catalysts at the atomic scale.
Catalysts, especially platinum-based catalysts, play a crucial role in electrochemical energy conversion systems, such as water electrolysers or fuel cells, which involve water splitting or the hydrogen oxidation and the oxygen reduction reactions1. When pursuing efficient and stable platinum-based catalysts for diverse electrocatalytic reactions, one of the main challenges is achieving high activity with a minimum amount of the expensive and scarce platinum — that is, achieving high platinum mass activity2.
The intrinsic activity and the exposure of active sites in catalysts are related to the specific activity (SA) and the electrochemically active surface area (ECSA), respectively. These factors are of great importance in the application of catalysts, as they are in direct proportion to catalytic performance. Despite decades of efforts by scientists in this field, traditional synthesis protocols frequently lead to sacrificing either SA or ECSA. To address the key challenge of platinum-based catalysts, new synthesis protocols and concepts need to be explored to achieve high SA and ECSA at the same time.
Now, writing in Nature Catalysis, Xiangfeng Duan, Yu Huang, Philippe Sautet and colleagues report the development of an atomic tailoring approach to boost the electrocatalytic activity of platinum catalysts with minimum loss of active sites3. By partial electrochemical dealloying of platinum–nickel alloy nanowires, the researchers were able to create platinum nanowires modified with single nickel atoms (SANi-PtNWs). Figure 1a provides a schematic diagram of the SANi-PtNWs. With optimized SA and retained ECSA, the single-atom tailored platinum nanowires demonstrated significantly enhanced electrochemical performance for diverse electrochemical reactions, namely, the hydrogen evolution reaction (HER), the methanol oxidation reaction and the ethanol oxidation reaction.
Previous studies have also focused on doping or hybridizing platinum with other hetero-element species, as well as nanostructure engineering. Both are considered effective approaches to improve the performance of platinum-based catalysts. For example, nickel hydroxide has been decorated on the surface of platinum to facilitate the dissociation of adsorbed H2O and H2, leading to significantly enhanced intrinsic activities towards hydrogen oxidation and evolution reactions4,5,6. Several interesting studies have achieved excellent HER performance in acidic and alkaline media by improving intrinsic activities through alloying platinum with other transition metals (nickel, silver, palladium and so on)7,8. Platinum has also been fabricated with pine-shaped nanostructures, achieving increased ECSA and high electrocatalytic performance9. However, hetero-metal alloying and nanostructure engineering approaches usually suffer from either ECSA loss or low intrinsic activity. It remains a great challenge to discover an effective approach to simultaneously increase the SA and ECSA of platinum-based catalysts. Its achievement would be an important step towards the design of efficient and stable catalysts for practical applications.
In this study, the researchers decorate the surface of platinum with single nickel atoms, instead of nickel or nickel hydroxide nanoparticles, using a two-step alloying and partial electrochemical dealloying process. This effective synthesis approach also allowed the researchers to tailor the loading amount of nickel species by controlling the dealloying cyclic voltammetry cycles. Remarkably, with an increasing number of cycles, the ECSA gradually increased until the nickel species were completely removed. The result suggests that the electrochemical dealloying process plays a pivotal role in leaching the nickel species into single atoms. This leads to maximum exposure of platinum active sites that neighbour nickel species. Element mapping using electron energy loss spectroscopy, combined with high-angle annular dark-field scanning transmission electron microscopy, revealed atomically distributed nickel atoms on the platinum nanowires (Fig. 1b–d). The single atomic nature of the nickel species was further confirmed using synchrotron radiation-based X-ray absorption fine structure analysis, which has high structural sensitivity with no ambiguity (Fig. 1e,f).
The electrocatalytic HER performance of the SANi-PtNWs was first evaluated in alkaline media. Compared with previously reported nickel hydroxide decorated platinum (20–60 m2 gPt–1), the SANi-PtNWs exhibited a much higher ECSA (106.2 ± 4.5 m2 gPt–1). More importantly, with a low platinum loading of 2.0 μg cm–2, the specific activity of the SANi-PtNWs was as high as 10.72 ± 0.41 mA cm–2 at –70 mV, versus a reversible hydrogen electrode.
The researchers also conducted density functional theory calculations to study the high intrinsic activity of the SANi-PtNWs for HER. The results indicated that the HER activity of the superficial platinum atoms was electronically promoted by the neighbouring single nickel atoms. The considerably improved HER performance of the SANi-PtNWs can be attributed to the cooperative work of the single nickel atom-induced high intrinsic activity and the retained high ECSA, by means of the single-atom tailoring approach. Following the same strategy, greatly enhanced electrocatalytic activities towards methanol and ethanol oxidation reactions were also achieved by the SANi-PtNWs.
Taken together, the researchers provide an efficient single-atom tailoring strategy to simultaneously gain high active site exposure and intrinsic activity for diverse reactions using an electrochemical dealloying pathway. Despite this achievement, further works should explore more robust single-atom tailoring strategies to boost the development and mass production of efficient and stable catalysts, not only for scientific research, but also for potential applications. In addition, the catalyst systems should also be extended to a wide variety of low-cost and earth-abundant element systems, which are essential for many practical applications. If these can be effectively realized, a tremendous boom in the fields of electrochemical energy conversion, storage and other utilizations can be anticipated.
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Li, F., Baek, JB. Atomic tailoring of platinum catalysts. Nat Catal 2, 477–478 (2019). https://doi.org/10.1038/s41929-019-0302-y
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