Titleorigin of the superior oxygen reduction activity of zirconium nitride in alkaline media
Author:Prof. Hao Li's team (Institute for Advanced Study of Materials Science, Tohoku University, Japan, WPI-AIMR).
In hydrogen fuel cell applications, the search for non-first-class catalysts as oxygen reduction reaction (ORR) materials has been a long-standing research hotspot in the past two decades. In 2021, Professor Hao Li of Tohoku University in Japan published a theoretical research work entitled "Analysis of the limitations in the oxygen reduction activity of transition metal oxide surfaces" in Nature Catalysis as the first author.
Through theoretical modeling, it is found that there are two major bottlenecks in the ORR of oxides: 1) most oxides can only adsorb reactive oxygen at the metal apex under the ORR potential and the corresponding coverage, forming weak metal-O bonds, which makes the activation of O-O bonds difficult2) Due to the adsorption of reactive oxygen at the apex of the metal, the dipole moment caused by its adsorption is larger, which makes its ability to adsorb O and activate O-O bonds under acidic conditions weaker.
This theoretical work is highly consistent with the conclusions of most of the previous oxide-orr literaturesSuch as: Caltech John MGregoire's research group has tested the basic ORR performance of nearly 7800 groups of different oxides with high-throughput experiments, and found that there is a large gap between the activity of most oxides and ***PT.
Interestingly, unlike oxides, some metal X-compounds (e.g., nitrides) have higher activity and stability under alkaline ORR conditions. For example, in 2020, a team from the Chinese Academy of Sciences published a groundbreaking experimental work entitled "Zirconium Nitride Catalysts Surpass Platinum for Oxygen Reduction" in Nature Materials, and found that ZRN is a catalyst with extremely excellent performance on basic ORRs. This work opens up the possibility of metal nitrides on basic ORRs.
Brief introduction of the results
Recently,The team of Professor Li Hao of Tohoku University, JapanCombined with the electrocatalytic surface state analysis, DFT calculation, and microdynamic modeling under electric field-PH coupling, the catalytically active volcanic model of metal nitrides such as ZRN under alkaline ORR conditions was successfully derived, and the activity origin of ZRN under alkaline ORR conditions was explained. The core of the work is as follows: 1) the surface of Zrn is self-oxidized at the ORR potential and thus covered by Ho (Fig. 1) (this phenomenon is also consistent with some previous experimental characterizations after catalysis);2) Since the ORR reactants will be "buried" in the HO group, their adsorption capacity and dipole moment due to adsorption will be significantly reduced, which will be conducive to ORR occurrence (Fig. 2A-B).
Finally, through microdynamic modeling of pH-electric field coupling, the work found that the activity of Zrn under alkaline conditions reached a theoretical maximum (Fig. 2c). The trend of the simulated current density curve is also in good agreement with the experiment (Fig. 2d). This work illustrates the importance of analyzing the electrochemical surface state by methods such as surface Pourbaix phase diagrams prior to the analysis of activity.
The authors found that although metal nitrides and metal oxides have relatively close initial surface structures, the self-oxidation of nitrides at the ORR potential makes some nitrides a class of excellent ORR catalysts under alkaline conditions, as nitrides tend to have higher surface reactivity. This also reveals that some metal nitrides may have better ORR applications than oxides.
Figure 1The surface Pourbiax phase diagram analysis was carried out on the surface of ZRN, and the autooxidation phenomenon under the ORR potential was found.
Figure 2The electric field analysis of the self-oxidized ZRN surface and the pure ZRN surface were carried out, and the pH-related microdynamic volcano model and theoretical ORR polarization curve were derived.
Finally, based on this work, the authors designed a theoretical framework and process for exploring the ORR activity of metal X-carves (Fig. 3). Through this framework, the authors found that results were consistent with previous experiments in other metal nitride systems (e.g., TIN and HFN).
Figure 3The authors suggest a workflow for the analysis of the ORR activity of metal X.