Improving the HER Kinetics of the Nickel Phosphide Catalyst Using Electrochemical Dealloying Treatment
Authors: Jo Seunghyun , Kwon JunHwa , Cho Ki-Yeop , Eom KwangSup 
Published: December 2020

Hydrogen fuel is receiving a lot of attention owing to its high efficiency and eco-friendliness. The reforming of natural gas is conventionally used to generate hydrogen gas; however, the process generates carbon dioxide causing global warming. To solve the problem, water electrolysis, which produces hydrogen and oxygen gas from water, is actively studied in order to design a cheaper and more effective hydrogen generation system.
The major research gap of water electrolysis is the high cost due to the rarity of catalyst materials such as platinum [1,2]. As a substitute for a platinum catalyst, transition metal phosphides represented by nickel phosphide (Ni-P) has shown the comparable catalytic activity to platinum, despite the much lower cost compared to platinum. The nickel phosphide can be fabricated with a facile electroplating process which can be more suitable synthesis process of electrocatalyst, because it is not necessary to mix with a polymer such as Nafion, disturbing emission of generated gas and decreasing surface area [3]. On the other hand, the nickel phosphide has showed low stability in acid media. According to the previous researches [4], the main reason of low stability is due to the difference of corrosion potential between nickel metal and Ni-P phases. Furthermore, they suggest three strategies to increase the corrosion resistance: (i) well-alloying of nickel and phosphorus, (ii) formation of amorphous film at grain boundary, and (iii) formation of phosphate passive film on the surface [4].
In this context, we adopted the electrochemical surface treatment for the electroplated Ni-P to improve the activity and stability of Ni-P/carbon paper (CP) catalyst. The surface treatment (“dealloying surface treatement”) is to remove the ‘isolated Ni’ which has less bonding with P, which is composed of the CV cycles inducing the repetition of oxidation and reduction of Ni-P/CP catalyst. Furthermore, it is expected to form well-alloying between nickel and phosphide (recrystallization of Ni-P).
As a result, the morphology of Ni-P/CP after CV 20 cycles was similar with raw Ni-P/CP as shown in Fig. 1a, b. However, an area of P-rich portion (dark part) increased after dealloying cycles. Therefore, an atomic ratio of phosphorus increased about 186% higher after 20 cycles of CV than the Ni-P/CP which did not conduct surface treatment. In addition, the more invasion of phosphorus atom aroused a decrease of grain size of nickel metal (Fig. 1c). According to XPS data of Ni state in Fig. 1d, an intensity of peak at a binding energy of 853 eV which means a bonding of Ni2P increased after the dealloying process. That is, the bonding between Ni and P atoms increased after the treatment. An atomic ratio of P at the surface was also increased to 36.54at%, since Ni-P/CP catalyst showed 24.88at% of P. As a result, a repetition of reduction and oxidation of Ni-P during the dealloying surface treatment decreases the cystalline size of Ni and increase the P content of the Ni-P catalyst.
Due to such the changes, the Ni-P/CP showed a higher activity and stability after the dealloying surface treatment. Specifically, Ni-P/CP after CV 20 cycles showed the highest HER activity, with the overpotential of 124 mV at the current density of 10 mA cm-2, since raw Ni-P/CP had the overpotential of 179 mV (Fig. 2a). And then, the current density for Ni-P/CP after CV 20 cycles was maintained at approximately 80.4% at −0.2 VRHE for 6 hours, while raw Ni-P/CP maintained a current density of 35.6%. It is presumably because the nickel atoms can use enough electrons for the HER, which is provided from well –bonded phosphorus atoms [5].

References

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Figure 1