• A non-precious metal catalyst, cobalt phosphide, has been found to effectively split water in acidic pH without the need for platinum.
• The catalyst can be readily scaled up through a new process and demonstrated good, consistent performance.
• A power failure encountered during long-term stability testing showed that the catalyst has strong robustness and may be suitable for applications involving renewable sources of power.

Hydrogen has significant uses in industrial applications such as in the refining of fuels and the manufacture of basic chemicals such as ammonia and methanol. Interest in using hydrogen stems from the opportunity to manufacture it using renewable energy, along with the potential growth in its market as an energy source in fuel cells or as an alternative to the internal combustion engine. 

A number of approaches have been tried to manufacture water electrolyzers with the favored strategy being that of using polymer electrolyte membranes (PEMs) in an acidic environment. In a previous TLT article (1), researchers reported the development of a multifunctional catalyst based on oxides of copper, chromium and nickel that is effective in splitting water at neutral pH. The objective of operating at neutral pH is to develop a more environmentally favorable process that may be more cost effective in being able to use readily available salt water without further processing. The catalyst demonstrated the lowest potential seen to date for splitting water at neutral pH.

Dr. Thomas Jaramillo, director of the SUNCAT Center for Interfacial Science and Catalysis, a joint partnership between Stanford University in Palo Alto, Calif., and SLAC National Accelerator Laboratory in Menlo Park, Calif., says, “There are two commercial approaches for splitting (or electrolysis) of water with the early one emerging nearly a century ago involving operating under alkaline conditions. In more recent years, the PEM technology has been commercialized and has proven to be very effective with great promise in the long-run.”

The problem with the PEM technology is that it employs platinum catalysts, which are very effective but also very expensive. The result is that researchers have been looking for more cost-effective options that involve the use of non-precious metals. Jaramillo says, “Many non-precious metal catalyst options exist for electrolysis of water under alkaline conditions. Unfortunately, acidic media is not compatible with non-precious metals because most options do not survive under the severe conditions. 

One approach for identifying a non-precious metal catalyst was found in the discovery that natural enzymes (hydrogenases and nitrogenases) were found to produce hydrogen under neutral pH conditions. Jaramillo says, “One non-precious metal catalyst inspired by these enzymes was molybdenum disulfide.” 

This is, of course, the same material that is widely used as a solid lubricant. Jaramillo continues, “The performance benefits seen with molybdenum disulfide led us and others to evaluate a series of other ionic compounds to determine their efficacy as catalysts.”

Jaramillo and coworkers, including doctorate student McKenzie Hubert and research associate Dr. Laurie King, scaled up a phosphide-based catalyst for use in commercial water electrolyzers. Together with collaborators at Nel Proton Onsite in Wallingford, Conn., the team found that the non-precious metal catalyst can split water effectively in acidic pH without the need for platinum.

Cobalt phosphide
Jaramillo and his colleagues scaled up the synthesis of cobalt phosphide as a non-precious metal catalyst and found that it is stable in the corrosive acidic environment of a PEM electrolyzer operating under true commercial conditions. Jaramillo says, “This catalyst is not as effective as platinum but represents a step in the direction of seeking a non-precious metal catalyst with comparable performance.”

The researchers developed a synthesis of cobalt phosphide that can readily be scaled up. Cobalt phosphide was prepared by impregnating cobalt nitrate onto Vulcan carbon followed by vapor-phase phosphidation. The process used enabled the catalyst to be mechanically adhered to the substrate used in the electrolysis process. 

Jaramillo says, “The process for synthesizing cobalt phosphide enabled us to produce sufficient material for use in a commercial scale 86 cm² PEM electrolyzer.” The image of the electrolyzer used to produce hydrogen from water is shown in Figure 3.


Figure 3. Researchers scaled up production of the non-precious metal catalyst, cobalt phosphide to prepare sufficient material that can be used in the 86 cm² PEM electrolyzer shown. (Figure courtesy of Stanford University and the SLAC National Accelerator Laboratory.)

The morphology of the catalyst showed that cobalt phosphide nanoparticles with an average diameter of five nanometers were well dispersed on the carbon support. Jaramillo says, “The catalyst we prepared scaled up well and demonstrated good, consistent performance during our evaluation.”

The cobalt phosphide catalyst was studied versus a commercial platinum catalyst in both lab-scale cyclic voltammetry testing and in a commercial scale electrolyzer using gas diffusion electrodes. Operating conditions were 400 psi and 50 C. Jaramillo says, “The gas diffusion electrodes are contained on the membrane electrode assembly. This is a three-phase set up with liquid water passing through the solid catalyst leading to the generation of gaseous hydrogen and oxygen.”

The reaction occurring at the anode produces oxygen and hydrogen ions that move across the membrane to meet up with electrons flowing through an external circuit at the cathode, allowing for the production of hydrogen.

The researchers found that the cobalt phosphide catalyst produced hydrogen at a turnover frequency of approximately 0.87 molecules per second to achieve the operating current density of 1.8 ampere per square centimeter. Negligible changes to catalyst efficiency were observed over 14,700+ hours of continuous hydrogen production.

Jaramillo noted that a power failure did occur during the long-term stability test, but the catalyst displayed strong robustness in resuming splitting of water once power was restored. He says, “While we did not plan to interrupt the hydrogen generation process, the resiliency demonstrated by the catalyst after the power failure suggests that this type of catalyst might be suitable for applications involving renewable sources of power (such as wind or solar) that can suddenly stop producing electricity.”

The researchers will be continuing to evaluate other types of non-precious metal catalysts to find one that comes closer in performance to platinum.

Additional information on this research can be found in a recent article (2) or by contacting Jaramillo at jaramillo@stanford.edu. 

REFERENCES
1. Canter, N. (2019), “Hydrogen production at neutral pH,” TLT, 75 (3), pp. 12-13.
2. King, L., Hubert, M., Capuano, C., Manco, J., Danilovic, N., Valle, E., Hellstern, T., Ayers, K. and Jaramillo, T. (2019), “A non-precious metal hydrogen catalyst in a commercial polymer electrolyte membrane electrolyzer,” Nature Nanotechnology, 14 (11), pp. 1071-1074.