The recent development of anion exchange membranes (AEM) has renewed interest in alkaline water electrolysis for large-scale production of hydrogen gas. These systems can achieve current densities comparable to PEM-based systems while enabling the use of non-PGMs as catalyst materials. New electrocatalysts are required to interface with such AEMs in advanced alkaline water electrolyser designs. Amorphous alloys have previously been proposed as electrocatalysts due to their unique atomic structure, which results in electrochemical properties particularly suitable for catalysis. These properties include a high density of co-ordinatively unsaturated sites, a much greater solute solubility allowing for highly tunable homogeneous compositions, and a high stability against surface segregation of solute atoms [1-3]. To date, amorphous alloys have been underutilized as catalysts due to the lack of suitable methods to increase their electrochemically active surface area. Mechanical alloying and subsequent dealloying can be used to overcome this issue to yield 3D mesoporous and nanoporous amorphous structures. The present work assesses the role of structure and chemistry on the catalytic activity of Ni-Nb-Y amorphous alloys towards the hydrogen evolution reaction. Ni-based alloys exhibit some of the highest activity among the non-PGM catalysts while yttrium and niobium are excellent glass formers and offer interesting catalytic performance, particularly yttrium which has shown high catalytic activity when alloyed with nickel [4, 5].
Experimental Methods:
Cryogenic ball milling of elemental powders was used to synthesize a novel high surface area amorphous Ni79.2Nb12.5Y8.3 alloy. Composite amorphous alloys with minor secondary crystalline phases (Ni5Y and Y2O3) were also synthesized and evaluated. X-ray diffraction and electron microscopy were used to analyze the alloy structures and particle morphology. Working electrodes were prepared by synthesizing catalyst suspensions consisting of the alloyed powder along with Nafion® 117 solution and iso-propanol and drop casting said suspensions on glassy carbon electrodes. Electrochemical characterization was assessed through steady state polarization (Tafel) curves, cyclic voltammetry, and AC impedance spectroscopy.
Results and Discussion:
The Ni79.2Nb12.5Y8.3 amorphous alloy exhibited improved intrinsic catalytic performance compared to nickel, and the addition of minor secondary crystalline phases, namely Ni5Y, resulted in even greater intrinsic performance. Electrochemical testing revealed this trend to be consistent even when testing amorphous composite materials containing higher (Ni81.3Nb6.3Y12.5) and lower (Ni77.1Nb18.8Y4.2) Y content. Tafel measurements for Ni-Nb-Y alloys can be seen in figure 1 with corresponding data summarized in table 1. These alloys highlight the improved catalytic performance to be a result of the secondary Ni5Y phase as opposed to the total alloying content. These results warrant further electrochemical characterization of multiphase amorphous nickel alloys with finely dispersed crystallinity for catalyst development.
Conclusions and Future Work:
The mechanical alloying process can be used to produce highly tunable chemical compositions where then the addition of finely dispersed secondary crystalline phases result in improved catalytic activity. The preliminary electrochemical results illustrate that mechanically alloyed Ni-based amorphous materials are promising catalyst precursors for clean electrochemical hydrogen production.
References:
[1] A. Molnar, G. Smith, and M. Bartok, Adv. Catal., 36, 329-383 (1989).
[2] J. Deng, H. Li, and W. Wang, Catal. Today, 51, 113-125 (1999).
[3] M. Carmo, R. Sekol, S. Ding, G. Kumar, J. Schroers and A. Taylor. ACS Nano, 5 (4) 2979-2983 (2011).
[4] F. Rosalbino, S. Delsante, G. Borzone, and E. Angelini. Int. J. Hydrogen Energy, 33, 6696-6703 (2008).
[5] F. Rosalbino, S. Delsante, G. Borzone and E. Angelini. J. Alloys Compd., 429, 270-275, (2007)