KEY CONCEPTS
• Hydrogen production at neutral pH is currently not as effective as at acidic pH due to a lower proton concentration, need for a second reaction step and lower effectiveness of platinum catalysts.
• A multifunctional catalyst has now been developed that more effectively dissociates water by using catalysts that strongly bind hydrogen and hydroxyl groups.
• The catalyst achieved the lowest overpotential ever seen when splitting water at neutral pH.

Pathways to manufacture hydrogen in an efficient, environmentally friendly manner continue to be pursued because of the value placed on using this material as an intermediate in a wide variety of applications. One prominent use of hydrogen is in polymer electrolyte membrane fuel cells that are under evaluation by the automotive industry as a sustainable alternative to the internal combustion engine. 

One clean process to produce hydrogen is the splitting of water. In a previous TLT article, researchers reported the development of a porous, pyrochlore-type oxide based on yttrium and ruthenium that is effective in splitting water (1). This catalyst exhibited better reactivity in the oxygen evolution reaction, which is the slow step in the water splitting process.

But this work was done under the traditional acidic conditions where the main catalyst currently used, platinum, is most effective. Dr. Cao-Thang Dinh, post-doctoral researcher in the Edward S. Rogers Sr. department of electrical and computer engineering at the University of Toronto in Toronto, Ontario, Canada, says, “Acidic medium is very favorable because of the high concentration of protons that are present on the surface of the catalyst, enabling the reduction reaction to occur to form hydrogen molecules.”

Water splitting also can be conducted at neutral pH but is more difficult to achieve. Dinh says, “At a neutral pH of 7, the proton concentration is several orders of magnitude lower than what is detected under acidic pH. Hydrogen formation also involves a second step in neutral pH. Initially the water molecule must undergo a dissociation reaction that yields a hydrogen cation and a hydroxyl anion. The formation of the hydrogen molecule takes place in the second step.”

One other concern is the favored platinum catalysts are not as effective in neutral pH by two to three orders of magnitude. 

Dinh and his colleague, Dr. F. Pelayo García de Arquer, post-doctoral researcher at the University of Toronto, believe that running the water splitting reaction at neutral pH opens the door to new environmentally favorable processes. García de Arquer says, “Hydrogen formation could be incorporated in this way into a biocompatible environment where enzymes could, for example, upgrade carbon dioxide in a sustainable manner. Such processing is not possible under acidic conditions because these enzymes would not be compatible.”

The cost effectiveness of the hydrogen evolution reaction also would improve under neutral pH. Dinh and García de Arquer say, “The hydrogen evolution reaction could be scaled up if conducted in a more cost-effective manner using readily available salt water. Under acidic conditions, additional processing steps (such as desalination) must be done to enable salt water to be used, which adds cost to hydrogen formation. More cost-effective and widely available transition metal catalysts such as copper that would not operate efficiently in acidic medium also can be utilized. This would reduce the reliance on platinum and other costly and scarce precious metals.”

Previous approaches to generate hydrogen at neutral pH were done by modifying the surface texturing of the catalyst or incorporating metal hydroxides in the catalyst to facilitate dissociation of water.

A new catalyst with multiple components has now been developed that exploits functionalities effective at binding hydrogen and hydroxyl species.

Anisotropic doping strategy
Dinh, García de Arquer, Edward Sargent, university professor in the Edward S. Rogers Sr. department of electrical and computer engineering at the University of Toronto, and their colleagues used an anisotropic surface doping strategy to maximize the dissociation of water. Dinh says, “We evaluated the catalytic activity of a number of metal oxide clusters on the surface of a polished copper foil. The best candidate was chromium oxide, which was found to strongly bind hydroxyl groups. To improve hydrogen binding, we evaluated a nickel catalyst on the copper surface because this metal has been found to bind hydrogen stronger than copper and platinum. Copper is included in the catalyst to promote hydride coupling.”

An image of the catalyst is shown in Figure 1.


Figure 1. The multifunctional catalyst shown here was found to be effective at producing hydrogen at neutral pH. (Figure courtesy of the University of Toronto.)

The researchers evaluated the multifunctional catalyst for hydrogen evolution in a three-electrode cell that contained a one molar potassium phosphate buffer solution maintained at a neutral pH of 7. Dinh says, “We analyzed the reaction rate of the multicomponent catalyst by determining the current density generated versus a reversible hydrogen electrode.”

Modeling studies were done to better understand the high-catalytic activity of the multicomponent catalyst. Based on this analysis, the researchers determined that the order in which the components in the catalyst were added is important. A more porous catalyst was prepared using copper oxide nanowires on a copper foam. This was followed by depositing nickel and then chromium oxide on the three-dimensional copper oxide nanowire scaffold. 

García de Arquer says, “We evaluated this catalyst and found that it achieved an overpotential (amount of electric energy required to isolate hydrogen from water) of 48 millivolts at a current density of 10 milliamps per square centimeter. This figure represents the lowest overpotential ever seen at neutral pH.”

Future work will concern determining the mechanism of the multicomponent catalyst. Dinh adds, “We also would like to improve the reactivity of the catalyst through fine tuning and do further durability testing beyond 24 hours.”

Additional information on this research can be found in a recent article (2) or by contacting Sargent at ted.sargent@utoronto.ca. 

REFERENCES
1. Canter, N. (2018), “New porous water splitting catalyst,” TLT, 74 (12), pp. 16-17.
2. Dinh, C., Jain, A., García De Arquer, F., De Luna, P., Li, J., Wang, N., Zheng, X., Cai, J., Gregory, B., Voznyy, O., Zhang, B., Liu, M., Sinton, D., Crumlin, E. and Sargent, E. (2018), “Multi-site eletrocatalysts for hydrogen evolution in neutral media by destabilization of water molecules,” Nature Energy, available here.