A new robust and highly active bifunctional catalyst developed by Rice University and the University of Houston splits water into hydrogen and oxygen without the need for expensive metals such as platinum. The work, the team suggests, provides a facile strategy for fabricating highly efficient electrocatalysts from earth-abundant materials for overall water splitting.
The electrolytic film produced at Rice and tested at Houston is a three-layer structure of nickel, graphene and a ternary metal phosphide (FeMnP, iron, manganese and phosphorus). The foamy nickel gives the film a large surface, the conductive graphene protects the nickel from degrading and the metal phosphide carries out the reaction. A paper on the work is published in the journal Nano Energy.
FeMnP exhibits high electrocatalytic activity toward both the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER). Utilizing FeMnP/GNF as both the anode and the cathode for overall water splitting, the team achieved a current density of 10 mA cm−2 at a cell voltage of as low as 1.55 V with excellent stability. Complementary density functional theory (DFT) calculations suggested that facets exposing both Fe and Mn sites are necessary to achieve high HER activity.
Rice chemist Kenton Whitmire and Houston electrical and computer engineer Jiming Bao and their labs developed the film to overcome barriers that usually make a catalyst good for producing either oxygen (OER) or hydrogen (HER), but not both simultaneously.
Regular metals sometimes oxidize during catalysis. Normally, a hydrogen evolution reaction is done in acid and an oxygen evolution reaction is done in base. We have one material that is stable whether it’s in an acidic or basic solution.—Kenton Whitmire
The discovery builds upon the researchers’ creation of a simple oxygen-evolution catalyst revealed earlier this year. In that work, the team grew a catalyst directly on a semiconducting nanorod array that turned sunlight into energy for solar water splitting.
The new catalyst also requires less energy, Whitmire said.
If you want to make hydrogen and oxygen, you have to put in energy, and the more you put in, the less commercially viable it is. You want to do it at the minimum amount of energy possible. That’s a benefit of our material: The overpotential (the amount of energy required to trigger electrocatalysis) is small, and quite competitive with other materials. The lower you can get it, the closer you come to making it as efficient as possible for water splitting.—Kenton Whitmire
Graphene, the atom-thick form of carbon, is key to protecting the underlying nickel. One to three layers of graphene are formed on the nickel foam in a chemical vapor deposition (CVD) furnace, and the iron, manganese and phosphorus are added on top of that, also via CVD and from a single precursor.
Tests by Bao’s lab compared nickel foam and the phosphide both with and without graphene in the middle and found the conductive graphene lowered charge-transfer resistance for both hydrogen and oxygen reactions.
Whitmire said the material is scalable and should find use in industries that produce hydrogen and oxygen or by solar- and wind-powered facilities that can use electrocatalysis to store off-peak energy.
Zhenhuan Zhao of the University of Houston and the University of Electronic Science and Technology of China, Chengdu, is co-lead author of the paper. Co-authors are Andrew Leitner, Jing-Han Chen and Zhiming Wang of Rice and Hari Thirumalai, Lixin Xie, Fan Qin, Kamrul Alam, Lars Grabow, Shuo Chen, Dezhi Wang and Zhifeng Ren of the University of Houston. Whitmire is a professor of chemistry and associate dean of the Wiess School of Natural Sciences at Rice. Bao is an associate professor of electrical and computer engineering at the University of Houston and an adjunct professor at the University of Electronic Science and Technology of China.
Supporting the research were Rice University, the National Science Foundation (NSF) and the Robert A. Welch Foundation. Computing resources were provided by the University of Houston uHPC cluster, the NSF-supported Extreme Science and Engineering Discovery Environment and the Department of Energy Office of Science National Energy Research Scientific Computing Center.
Zhenhuan Zhao, Desmond E. Schipper, Andrew P. Leitner, Hari Thirumalai, Jing-Han Chen, Lixin Xie, Fan Qin, Md Kamrul Alam, Lars C. Grabow, Shuo Chen, Dezhi Wang, Zhifeng Ren, Zhiming Wang, Kenton H. Whitmire, Jiming Bao (2017) “Bifunctional metal phosphide FeMnP films from single source metal organic chemical vapor deposition for efficient overall water splitting,” Nano Energy, Volume 39, Pages 444-453, doi: 10.1016/j.nanoen.2017.07.027