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High activity and durable oxygen evolution single atoms supported by tungsten carbide
(Nanowerk Spotlight) Hydrogen has been referred as the next-generation clean energy source for replacing fossil fuels, while its sustainable and scalable production is considered as the bottleneck for the development of proton exchange membrane fuel cells and hydrogen-fuel-cell vehicles.
One option is the production of hydrogen fuel by water electrolysis. However, to overcome the high overpotentials of water electrolysis, an efficient oxygen evolution catalyst is required.
Indeed, exploring new types of non-noble metal-based oxygen evolution and hydrogen evolution catalysts is one of the key tasks for the future development of a successful hydrogen economy.
In the past decades, although a series of oxygen evolution catalysts has been reported to show low overpotentials, the turnover frequency (TOF) values are still far away from satisfied.
For instance, the commercial noble-metal-based catalyst IrO2 usually shows a low TOF around 0.01 s-1, the widely investigated promising no-noble-metal catalyst FeNi-LDH only shows a TOF value around 0.05 s-1.
Therefore, there is an urgent need to further improve OER catalytic performances in terms of atomic activities and utilization rates to maximize the TOFs.
The most promising approach to maximizing the atomic utilization rates and synergistic effects between active sites is to disperse the catalytically active metal compounds down to the atomic level, that is, to prepare single-atom catalysts.
"However, the currently reported single-atom catalysts only show improvements in electrocatalytic reduction reactions, such as O2/CO2/N2 reduction and H2 evolution reactions. When applied in the OER, the single atom catalysts usually show insufficient activity and durability," says Dr. Shuang Li, an electrocatalyst researcher and hybrid nanomaterials specialist at Technische Universität Berlin (Germany). "Hence, the development of highly active and stable single atom OER catalysts which show high TOF values is of crucial importance for the future application of water electrolysis."
One major challenge in designing the high active single atom OER catalysts is that most of the current reported single atom catalysts are based on a strong heteroatom coordination environment to fix the single atom center.
However, this strong heteroatom coordination environment changes the electronic environment (d-band center) of the metal atoms through ligand effects, which are highly correlated with adsorbate binding energy and thus unfavorably influence the catalytic activity.
“Therefore, we are aiming at designing of highly durable and conductive support materials with well-defined structures that can stabilize catalytic metal atoms without the aid of strong heteroatom coordination, which is indeed a critical challenge.” says Dr. Li.
Taking this critical challenge, recently, the team from Prof. Chong Cheng at Sichuan University, Dr. Shuang Li and Prof. Arne Thomas at Technische Universität Berlin, and Dr. Yi Wang at Max Planck Institute for Solid State Research used metal carbides as the carrier to support the transition metal Fe and Ni atoms to engineer single-atom oxygen evolution catalysts for the first time.
The related research results, first-authored by Dr. Li, have been published in Nature Materials ("Oxygen-evolving catalytic atoms on metal carbides").
Schematic diagram of carbide material structure
Figure 1. a. Schematic diagram of material structure; b. Carbide crystal particles under spherical aberration high-resolution electron microscope; c. Atom arrangement and crystal plane orientation in tungsten carbide crystal; d, e. FeNi single atom on the surface of carbide; f. Atomic-level element distribution; atomic-level EDX spectrum corresponding to atoms at positions 1-6. (Image provided by the researchers) (click on image to enlarge)
Distinct from previous studies, this novel WCx support shows obvious advantages for supporting different single atoms, especially its non-strong bonding features, which resulted in high mobility of the supported atoms, which might be the key point for realizing excellent OER activities.
The researchers also realized a single atom resolution HAADF-STEM mapping and EDX analysis of the materials. This study reveals that single-atom catalysts can be stably supported on the surface of tungsten carbide. Due to the unique structure of tungsten carbide, the atomically dispersed FeNi catalytic sites have a moderate binding force with the W and C atoms on the surface, thus realizing the construction of a highly efficient oxygen evolution catalyst.
The catalyst has an overpotential of 237 mV at 10 mA cm−2. What is more noteworthy is that it achieves ultra-high mass activity (33.5 A mg-1 FeNi at η = 300 mV overpotential) and catalytic conversion rate (4.96 s-1 at ŋ = 300 mV overpotential), which is more than 100 times that of traditional catalysts, and also exhibits stability of over 1000 hours.
Electrochemical OER performance results of various materials
Figure 2. Electrochemical OER performance results of materials. (Image provided by the researchers) (click on image to enlarge)
Furthermore, the synthetic method of the catalysts is based on a simple metal-organic hybrid precursor. This novel synthetic strategy shows obvious advantages for large-scale production, especially its fast and eco-friendly processing in water.
Dr. Li, who has devoted herself to this research area for 10 years, has published several papers on different metal-organic hybrid precursors to synthesize different electrochemical catalysts, and now she has become a principal investigator in the chemistry department of Technische Universität Berlin, supported by DFG.
The researchers believe that the application of metal carbides as single-atom carriers proposed in this work opens up a promising path for the further development of single-atoms-based electrochemical catalytic reactions.
Source: Provided as a Nanowerk exclusive by Prof. Dr. Chong Cheng, Department of Chemistry, Freie Universität Berlin

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