New Rutgers non-noble metal catalyst for hydrogen evolution performs as well as Pt in both acid and base
Researchers at Rutgers University have developed a new noble metal-free catalyst—Ni5P4 (nickel-5 phosphide-4)—performing on par with platinum for the hydrogen evolution reaction (HER) in both strong acid and base. The development, the team concludes in a paper published in the RSC journal Energy & Environmental Science, can offer a key step towards industrially relevant electrolyzers competing with conventional H2 sources.
Currently, renewable hydrogen may be produced from water by electrolysis with either low efficiency alkaline electrolyzers that suffer 50–65% losses, or by more efficient acidic electrolyzers using expensive rare platinum group metal catalysts (Pt). Consequently, the authors noted, research has focused on developing alternative, cheap, and robust catalysts made from earth-abundant elements.
Here, we show that crystalline Ni5P4 evolves H2 with geometric electrical to chemical conversion efficiency on par with Pt in strong acid (33 mV dec-1Tafel slope and −62 mV overpotential at −100 mA cm−2 in 1 M H2SO4). The conductivity of Ni5P4 microparticles is sufficient to allow fabrication of electrodes without conducting binders by pressing pellets. Significantly, no catalyst degradation is seen in short term studies at current densities of −10 mA cm−2, equivalent to ~10% solar photoelectrical conversion efficiency.—Laursen et al.
|Left: schematic of Ni5P4 surface showing water adsorption and conversion to H atoms and to H2 product. Right: current output versus voltage input for Ni5P4. Click to enlarge.|
Ni5P4 has the potential to replace platinum in two types of electrochemical cells: electrolyzers that make hydrogen by splitting water through hydrogen evolution reaction (HER) powered by electrical energy, and fuel cells that make electricity from combining hydrogen and oxygen, said Rutgers Chemistry Professor Charles Dismukes, a co-corresponding author of the paper.
Platinum is the benchmark material for both devices as it has the best conversion efficiency. However, while platinum may be acceptable for making jewelry and low volume specialty applications, it is too expensive for large-scale applications such as energy storage and conversion. Our new HER catalyst, Ni5P4, has the strong potential to overcome this challenge.—Charles Dismukes
Scientists have been working for years to develop low-cost replacements for platinum and other noble metals used in these devices. Ni5P4 is the most promising new material presently available that combines both the energy conversion efficiency of noble metals, yet is much more affordable based on the high natural abundance of its elements—over a million times greater than platinum.—Professor Martha Greenblatt, co-corresponding author
The researchers believe that Ni5P4 should lower the material costs of both electrolyzers and fuel cells, while maintaining the efficiencies of these technologies for electrical conversion.
The next step for the research is to test the operating stability and efficiency of the compound over extended time periods in commercial electrolyzers and fuel cells. As these devices have different requirements for operation, independent tests for both will be needed. Rutgers has partnered with Proton OnSite of Wallingford, Conn., a commercial manufacturer of electrolyzers, to test Ni5P4 as an appropriate HER catalyst.
To achieve the overall water splitting process, the HER catalyst cathode will be combined with an oxygen-evolving (OER) catalyst anode. The Rutgers team has previously developed a noble-metal-free OER catalyst—LiCoO2 (lithium cobalt oxide)—that has shown promising performance in preliminary tests at Proton OnSite.
If used together, these catalysts could eliminate the need for expensive noble metal based electrode materials, Greenblatt said.
A patent is pending on the technology.
Funding for the Ni5P4 research was provided by the Air Force Office of Scientific Research, NATCO Pharma Ltd. and Rutgers, while the OER research is now being funding by the Department of Energy Office of Energy Efficiency and Renewable Energy.
In related work, scientists from Rutgers and Proton OnSite are partnering with members of the Solar Fuels Institute (SOFI) of Telluride, Colo. on a demonstration project that seeks to build a solar-powered mobile electrolyzer for making public demonstrations showing the production of a renewable liquid fuel using only sunlight, water and carbon dioxide as inputs.
A. B. Laursen, K. R. Patraju, M. J. Whitaker, M. Retuerto, T. Sarkar, N. Yao, K. V. Ramanujachary, M. Greenblatt and G. C. Dismukes (2015) “Nanocrystalline Ni5P4: a hydrogen evolution electrocatalyst of exceptional efficiency in both alkaline and acidic media” Energy & Environmental Science 8, 1027-1034 doi: 10.1039/C4EE02940B