Nickel phosphide nanoparticle shown to be efficient non-noble metal electrocatalyst for hydrogen production
In the electrochemical reduction of water to molecular hydrogen, the hydrogen evolution reaction (HER) is facilitated by noble metal catalysts such as platinum (Pt), which generate large cathodic current densities for this reaction at low overpotentials.
A research team led by Raymond Schaak, a professor of chemistry at Penn State University, now reports that nanoparticles of nickel phosphide (Ni2P)—the two component elements of which are inexpensive and earth-abundant—have demonstrated among the highest HER activity of any non-noble metal electrocatalyst reported to date. A paper on the work is published in the Journal of the American Chemical Society.
Schaak’s team came to investigate Ni2P as an HER catalyst by first recognizing commonalities between HER and hydrodesulfurization (HDS) catalysts; Ni2P is a well-known HDS catalyst.
One non-precious-metal alternative to Pt is MoS2, which has high HER activity and exhibits good stability in acidic solutions. MoB and Mo2C have also been identified as active HER catalysts in both acidic and alkaline solutions.
The first-row metal nickel, which is significantly more abundant than Mo, is often used as an electrocatalyst for the HER, with active electrocatalysts produced by use of alloys such as Ni−Mo, Ni−Mo−Zn, Ni−Fe,9 and Ni−P. These Ni-based catalysts are not, however, stable in acidic solutions, in which proton exchange membrane-based electrolysis is feasible and operational. Addition of nitrogen to Ni−Mo, to form Ni−Mo−N composites, has been shown to significantly improve the acid stability of such alloys, but such systems still show significantly lower HER activity and/or stability in acidic solutions than noble metals such as Pt.
MoS2, an active earth-abundant HER electrocatalyst, is also highly active for hydrodesulfurization (HDS). Both HDS and the HER rely on the catalyst to reversibly bind H2, with H2 dissociating to produce H2S in HDS and with protons bound to the catalyst to promote the formation of H2 in the HER.
...These commonalities between the mechanisms and putative active sites of MoS2 for both HDS and HER catalysis suggest that other materials that are known HDS catalysts may also provide active electrocatalysts for the HER. Nickel phosphide (Ni2P)...is a well-known HDS catalyst, and Ni2P also produces H(g) via the water−gas shift reaction.—Popczun et al.
To create the nickel phosphide nanoparticles, team members began with metal salts that are commercially available. They then dissolved these salts in solvents, added other chemical ingredients, and heated the solution to allow the nanoparticles to form. The researchers were able create a nanoparticle that was quasi-spherical—not a perfect sphere, but spherical with many flat, exposed edges.
The small size of the nanoparticles creates a high surface area, and the exposed edges means that a large number of active sites are available, Schaak explained.
Team members at the California Institute of Technology test the nanoparticles’ performance in catalyzing the necessary chemical reactions. Led by Nathan S. Lewis, the George L. Argyros Professor of Chemistry at the California Institute of Technology, the researchers placed the nanoparticles onto a sheet of titanium foil and immersed that sheet in a solution of sulfuric acid. Next, the researchers applied a voltage and measured the current produced. They found that not only were the chemical reactions happening as they had hoped, they also were happening with a high degree of efficacy.
They tested more than 20 individual Ni2P electrodes and found that the HER activities were highly consistent. Their overpotentials compared favorably to the behavior of other non-Pt HER electrocatalysts in acidic aqueous solutions with similar mass loadings, including bulk Mo2C and MoB, Mo2C nanoparticles deposited on carbon nanotube supports, MoS2 nanoparticles anchored on reduced graphene oxide, and unsupported Ni−Mo−N nanosheets.
Faradaic yields for hydrogen evolution of the Ni2P nanoparticles and of Pt nanoparticles were estimated by maintaining catalyst-loaded Ti foil working electrodes at a cathodic current of 10 mA for 50 min, resulting in passage of 30 C of charge. Over 50 minutes, Ni2P and Pt produced identical amounts of H2, and the amount of H2 evolved agreed closely with the theoretical value based on Faraday’s law, implying a quantitative faradaic yield.
In summary, nanostructured Ni2P, with a high accessible surface area and a high density of exposed (001) facets that have been predicted to be active for catalyzing the HER, is indeed an active HER electrocatalyst. Because Ni2P is also a well-known HDS catalyst (as is MoS2), these observations suggest that other known HDS catalysts are also interesting candidates for identifying new highly active, earth-abundant HER electrocatalysts.
Furthermore, chemical substitution and additional nanostructuring efforts, both of which have been demonstrated to improve catalytic HDS performance, are promising routes to possibly obtaining further improvement in the HER activity of Ni2P, as well understanding in detail the relationship between the HER activity and the quantity and characteristics of different exposed facets of Ni2P in such systems.—Popczun et al.
In addition to Schaak and Lewis, other researchers who contributed to this study include Eric J. Popczun, Carlos G. Read, Adam J. Biacchi, and Alex M. Wiltrout from Penn State; and James R. McKone from the California Institute of Technology.
The research was funded by the US National Science Foundation and the US Department of Energy. The team has filed a patent application.
Eric J. Popczun, James R. McKone, Carlos G. Read, Adam J. Biacchi, Alex M. Wiltrout, Nathan S. Lewis, and Raymond E. Schaak (2013) Nanostructured Nickel Phosphide as an Electrocatalyst for the Hydrogen Evolution Reaction. Journal of the American Chemical Society doi: 10.1021/ja403440e