Beijing University of Chemical Technology reported the stabilization strategy of hydrogen ruthenium-based anode from PEM electrolysis of water

On March 15, 2023, Beijing time, the team of Professor Sun Xiaoming of Beijing University of Chemical Technology published a research result entitled “Eliminating over-oxidation of ruthenium oxides by niobium for highly stable electrocatalytic oxygen evolution in acidic media” in the journal Joule.

This achievement reports the role of monodisperse metal niobium site in regulating the electronic structure of ruthenium oxide to improve the oxygen evolution stability of ruthenium oxide at high current density. The introduction of niobium weakens the covalence of ruthenium-oxygen bonds, strengthens the electron transfer of local structure, reduces the valence state of ruthenium sites and stabilizes lattice oxygen, thereby inhibiting the excessive oxidation of ruthenium oxide, realizing the stability of ruthenium oxide at a current density of 300 mA cm-2 in proton exchange membrane electrolyzed water (PEMWE) hydrogen production device, and providing new insights for the design of Ru-based OER (Oxygen evolution reaction) catalysts with high stability in acidic media.

The corresponding authors of the paper are Professor Sun Xiaoming and Professor Kwong Yun; The first author is Liu Hai, a 2020 doctoral student at Beijing University of Chemical Technology.

The high current density and high dynamic responsiveness of hydrogen production by proton exchange membrane electrolysis of water (PEMWE) can be coupled with renewable energy to achieve efficient hydrogen production, accelerating the realization of China’s “carbon peak” and “carbon neutrality” goals. At present, anode catalytic materials for hydrogen production by electrolysis of water in proton exchange membranes rely heavily on the precious metal iridium. However, the high price and low catalytic activity of iridium and China’s low iridium reserves limit the development of China’s proton exchange membrane electrolysis water hydrogen production industry. Compared with iridium (1105 yuan/g), ruthenium (117 yuan/g) has high oxygen evolution catalytic activity, but the excessive oxidation of ruthenium oxide in the oxygen evolution process leads to poor stability, which restricts its application in proton exchange membrane water electrolysis to hydrogen production. In order to inhibit the excessive oxidation of ruthenium oxide, a large number of research work on transition metal doped ruthenium oxide has been carried out at home and abroad. Although these works can increase the activity of ruthenium oxide to a certain extent, the dissolution of the transition metals used in the oxygen evolution process and the participation of lattice oxygen limit its stability at high current densities.

In this work, Professor Sun Xiaoming’s team based on the hydrotalcite valence ion repulsion and metal oxide solid solution theory (Chem. Soc. Rev. 2021, 50(15), 8790), proposed a strategy of monodisperse high-valence poorly soluble metal niobium doped with ruthenium oxide, taking advantage of the difference in the interaction between niobium and ruthenium and oxygen, thereby enhancing the charge transfer inside ruthenium oxide and weakening the covalence of ruthenium oxide. This aspect can regulate the adsorption of oxygen evolution reaction intermediates at the ruthenium site and improve their oxygen evolution activity. More importantly, this can inhibit the participation of lattice oxygen in oxygen evolution and excessive oxidation of ruthenium sites, and ultimately improve the stability of ruthenium oxide at high current densities. Finally, the developed Nb0.1Ru0.9O2 catalyst has a voltage attenuation rate of only 1/100 of the literature at a current density (200 mA cm-2) that is 20 times larger than the current density (10 mA cm-2), which is expected to replace the use of iridium-based oxygen evolution catalysts.

Figure 1: Structural characterization of niobium-doped ruthenium oxide

The research group prepared ruthenium oxide nanoparticles with different niobium content (5%-30%) and uniform doping by sol-gel method, and the doping of niobium brought obvious lattice expansion effect. In particular, the niobium in Nb0.1Ru0.9O2 is in a monodisperse state in the crystal lattice, and the bulk phase and surface distribution are very consistent.

Figure 2: Oxygen evolution performance of niobium-doped ruthenium oxide

The oxygen evolution performance test showed that the prepared Nb0.1Ru0.9O2 had the best oxygen evolution activity, its oxygen evolution overpotential at 10 mA cm-2 was only 204 mV, and it had a low Tafel slope of 47.9 mV dec-1, which was better than most of the currently reported ruthenium-based oxygen evolution catalysts. At the same time, the attenuation rate of Nb0.1Ru0.9O2 during the stability test at 200 mA cm-2 current density is only 25 μV h-1, which is only 1/100 of the attenuation rate of the catalyst reported in the literature at 10 mA cm-2 current density test. In addition, the PEM device with Nb0.1Ru0.9O2 as the anode can operate stably for 100 hours at a current density of 300 mA cm-2.

Figure 3: Characterization of electronic structure before and after oxygen evolution stability of Nb0.1Ru0.9O2

X-ray absorption near-edge spectroscopy and X-ray photoelectron spectroscopy before and after long-term stability test show that the monodisperse niobium site doping introduces a strongly interacting Nb-O bond, which not only weakens the covalence of Ru-O bonds, improves the electron density of ruthenium sites, but also inhibits the participation of lattice oxygen in oxygen precipitation and eliminates excessive oxidation at high current densities at ruthenium sites. In addition, the oxidation of niobium during the OER process further reduces the valence state of ruthenium, ensuring the stability of the catalyst at high current density.

Figure 4: Study on oxygen evolution activity and stability mechanism of Nb0.1Ru0.9O2

DFT (Density Functional Theory) theoretical calculation confirmed that the doping of niobium reduced the energy barrier of Nb0.1Ru0.9O2 oxygen evolution reaction speed control step (*O+H2O→*OOH+H++e-). In situ infrared spectroscopy observed that the adsorption peak position (1132 cm-1) of the intermediate *OO/*OOH in the process of Nb0.1Ru0.9O2 oxygen evolution was redshifted compared with the position of *OO/*OOH (1180 cm-1) of the pure ruthenium oxide characteristic intermediate, which confirmed that the adsorption of *OOH intermediate by niobium doping enhanced ruthenium site was the reason for the increase of OER activity. CV analysis with wide voltage range showed that niobium doping effectively reduced the peak area of Ru4+/Ru6+ redox peaks in ruthenium oxide, and the disappearance of Ru6+/Ru8+ redox peaks on the CV curve of niobium-doped samples confirmed that niobium doping inhibited excessive oxidation of ruthenium sites, thereby improving its stability at high current density.

This study not only puts forward the design idea of monodisperse poorly soluble metal doped ruthenium oxide, but also reveals its positive role in improving the stability of ruthenium oxide oxygen evolution, and will also guide the design and optimization of a series of highly stable metal doped ruthenium oxide oxygen evolution catalysts. The research was supported by the National Natural Science Foundation of China (Grant No.: 21935001), and the relevant research has successfully applied for invention patents. (Source: Science Network)

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