Researchers at the University of Houston have developed a catalyst—composed of easily available, low-cost materials and operating far more efficiently than previous catalyst—that can split water into hydrogen and oxygen.
The robust oxygen-evolving electrocatalyst consists of ferrous metaphosphate on self-supported conductive nickel foam that is commercially available in large scale. The catalyst yields current densities of 10 mA/cm2 at an overpotential of 177 mV, 500 mA/cm2 at only 265 mV, and 1,705 mA/cm2 at 300 mV, with high durability in alkaline electrolyte of 1 M KOH even after 10,000 cycles. This represents an activity enhancement by a factor of 49 in boosting water oxidation at 300 mV relative to the state-of-the-art IrO2 catalyst. A paper on their work is published in Proceedings of the National Academy of Sciences (PNAS).
To realize the overall water electrolysis for H2 production, oxygen evolution reaction (OER), also named water oxidation, plays another key role. OER is also an important oxidative reaction in obtaining carbon fuels from CO2 reduction and achieving rechargeable metal–air batteries, and it has been meticulously studied for more than half a century. However, owing to the sluggish four-proton-coupled electron transfer and rigid oxygen–oxygen bonding, this key process remains a major bottleneck in the water-splitting process.
State-of-the-art OER catalysts, such as iridium dioxide (IrO2) and ruthenium dioxide (RuO2), still require overpotentials of around 300 mV to achieve current densities on the order of 10 mA/cm2, not to mention their scarcity and high costs, which severely hinder the substantial market penetration of this technique. Thus, it is highly desirable and imperative to develop robust and stable oxygen-evolving electrocatalysts from earth-abundant and cost-effective elements.
Commercial water electrolyzers require a competent electrocatalyst that can efficiently deliver an oxidative current density above 500 mA/cm2 with long-term stability at overpotentials < 300 mV. Although various earth-abundant materials have been proven to be efficient catalysts for oxygen evolution, such as transition metal oxides, hydroxides, oxyhydroxides, phosphates, phosphides, metal–organic frameworks, and carbon nanomaterials, many of them cannot meet the aforementioned commercial criteria for water–alkali electrolyzers, and, most importantly, they may not survive long in high-current operation. To this end, we report a highly efficient electrocatalyst for oxygen evolution reaction yielding current densities of 10 and 500 mA/cm2 at overpotentials of 177 and 265 mV, respectively, both of which are the lowest overpotential values for the corresponding current densities ever reported, and showing excellent long-term stability.—Zhou et al.
Although it is simple in theory, splitting water into hydrogen and oxygen is a complex process, requiring two separate reactions—a hydrogen evolution reaction and an oxygen evolution reaction, each requiring a separate electrode. While hydrogen is the more valuable component, it can’t be produced without also producing oxygen. And while efficient hydrogen catalysts are available, the lack of an inexpensive and efficient oxygen catalyst has created a bottleneck in the field.
Co-author Shuo Chen said oxygen evolution reactions often depend upon an electrocatalyst using a noble metal—iridium, platinum or ruthenium. But those are expensive and not readily available.
In this work, we discovered a highly active and stable electrocatalyst based on earth-abundant elements, which even outperforms the noble metal based ones. Our discovery may lead to a more economic approach for hydrogen production from water electrolysis.—Shuo Chen
The fabrication process for such an exceptional electrocatalyst is compatible with industrial standards and is economically viable for large-scale production. We believe our finding is a giant step toward practical and economic production of hydrogen by water splitting, which will significantly contribute to the effort to reduce the consumption of fossil fuels.—Zhou et al.
Haiqing Zhou, Fang Yu, Jingying Sun, Ran He, Shuo Chen, Ching-Wu Chu, and Zhifeng Ren (2017) “Highly active catalyst derived from a 3D foam of Fe(PO3)2/Ni2P for extremely efficient water oxidation” PNAS doi: 10.1073/pnas.1701562114