Boosting hydrogen evolution activity and durability of Pd–Ni–P nanocatalyst via crystalline degree and surface chemical state modulations
Graphical abstract
Introduction
With the growing energy crisis related to fossil fuels, a large number of studies have been done on the evolution of renewable energies [1]. Hydrogen generation from water electrolysis has advantages for clean energy production as it excludes carbon emissions [[2], [3], [4], [5], [6]]. Platinum-based electro-catalysts show fast kinetics in the hydrogen evolution reaction (HER) [[7], [8], [9], [10], [11]]; however, they are expensive and incur severe degradation during long-term electrolysis. It is still challenging to develop efficient, durable, and economical electro-catalysts for advancing large-scale commercialization of the HER.
Palladium and Pt belong to the same family in the Periodic Table and show very similar catalytic properties in many cases. Palladium has become the best substitute for Pt in many fields, including fuel-cell catalysts [12]. However, it is well known that Pd-based nanomaterials are one type of the most versatile hydrogenation catalysts in the chemical industry [[13], [14], [15]]. It has also been reported that hydro-treating catalysts could be potentially used for the HER [16]. Moreover, the Gibbs free energy of hydrogen adsorption (△GH*) on Pd is close to that on Pt at high exchange current density [[16], [17], [18]]. Therefore, Pd-based materials could be one class of promising electro-catalysts for the HER [4,19].
Ternary metal phosphides, such as Ni–Co–P [20,21], Fe–Ni–P [22,23], and Co–Fe–P [24], involving bimetallic synergistic effects and metal-P covalent bonds are very active as water-spitting electro-catalysts [2,25,26]. Although these materials have been extensively reported on, they still cannot reach the level of Pt for the HER. Previously, we fabricated Pd-based ternary metal phosphides (Pd–Ni–P) as electro-catalysts for methanol oxidation [27,28]. Hu et al. reported that the addition of Ni could significantly improve the stability of bulk metallic glass electro-catalyst in the HER [29]. Moreover, in many cases Ni itself could serve as the main active site for the HER [26,[30], [31], [32], [33], [34], [35], [36]]. The doping of Ni and P in Pd lattices not only increases the active sites of catalysts, but leads to the formation of positively charged Pd atoms in the form of Pd–P covalent bonds [37,38]. As a result, the adsorption energy of hydrogen (Hads) to Pd would be weakened [38] to release gaseous H2 through the Heyrovsky reaction (Pd-Hads + H+ + e− → Pd + H2) or Tafel reaction (2Pd-Hads → 2Pd + H2) [4].
Although Pd-based ternary metal phosphides (Pd-M-P) could be potentially used as a type of efficient catalyst for the HER, scant research has been done relative to this field [29]. To reduce the cost of catalysts, the applications of nanoscale materials as the cathodes are necessary. However, a study of Pd-M-P nanocatalysts in the HER, to our best knowledge, is still lacking. To this end, we present herein for what we believe is the first time a study of the catalytic performance of Pd–Ni–P nanoparticles (NPs) in the HER, in which the crystalline degree and surface-chemical-state- dependent electro-catalysis of the ternary metal phosphides are mainly investigated. As a result, the face-centered-cubic (fcc) Pd–Ni–P NPs annealed at 400 °C show the best HER activity with a very small overpotential of 32 mV to realize a current density of 10 mA cm−2, a Tafel slope of 62 mV decade−1, and a high mass activity of 1.23 mA μg−1Pd, superior to Pd NPs, Pd–P NPs, Pd–Ni NPs, and Pd–Ni–P NPs annealed under different temperatures. Furthermore, this catalyst is also highly stable and the current density barely decreases after 20 h of electrolysis.
Section snippets
Fabrication of Pd1.5Ni1P/C, Pd1.5Ni1P/C-400, and Pd1.5Ni1P/C-500 catalysts
The Pd–Ni–P nanocatalysts are fabricated with a solvothermal synthesis/deposition procedure according to our previous method [27]. Palladium(II) acetylacetonate (0.24 mmol) and nickel (II) acetylacetonate (0.16 mmol) as Pd and Ni precusors, and excess amount of triphenylphosphine (TPP, 0.88 mmol) as P precursor are used. Three precursors mixed with tetrabutylammonium bromide (1.0 mmol), trioctylphosphine oxide (3 mmol), and oleylamine (6.5 mL) were deaerated with N2 for 20 min. After that, the
Results and discussion
Transmission electron microscopy (TEM) of Pd1.5Ni1P/C (Fig. 1a), Pd1.5Ni1P/C-400 (Fig. 1b), and Pd1.5Ni1P/C-500 (Fig. 1c) are illustrated. The NP sizes increase gradually from 3.2, to 3.7, to 4.1 nm with temperatures due to Ostwald ripening (Fig. 1d and S1). Energy-dispersive X-ray spectroscopy (EDX) and laser-ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) show a result in close agreement regarding the real composition of Pd1.5Ni1P/C, namely approximately Pd43Ni27P30, which
Conclusions
In summary, the correlations among HER catalytic properties with crystalline degree and chemical states of a class of ternary metal phosphide (Pd–Ni–P) were investigated. The fcc structural Pd–Ni–P alloy annealed at 400 °C was improved to be the most active and stable catalyst. Through several contrast experiments, the catalytic properties of the ternary Pd-based metal phosphide was proved to be superior in terms of overpotential, Tafel slope, mass activity, and stability. This suggests that
Acknowledgement
This work was supported by the Fundamental Research Funds for the Central Universities (2018XKQYMS19). We thank LetPub (www.letpub.com) for its linguistic assistance during the preparation of this manuscript.
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