Activity and Durability of Mn Binary Oxide-Based Electrocatalyst for an Alternative Anode
Authors: Kohara Yuma , Ishihara Akimitsu , Matsuzawa Koichi 
Published: February 2024

Compared with alkaline water electrolysis (AWE), proton exchange membrane water electrolysis (PEMWE) has several advantages such as high current density, compact system and high purity of hydrogen gas. However, a conventional anode of PEMWE utilizes precious metal oxide such as iridium oxide (IrO2) with high cost and low resources. Recently, it has been reported that the performance of low-loading IrO2 anodes deteriorates due to the potential fluctuations simulated variable renewable energies [1-2]. Thus, it is required to develop a low-cost catalyst with high durability against the potential fluctuations. We have focused on the tantalum oxide-based electrocatalyst because of its low cost and high chemical stability in acid. In this study, we have investigated on the catalytic activity and durability of Mn added Ta oxide-based thin film (Mn-TaOx) for the alternative anode of PEMWE. For the purpose of improvement of durability, Mn-TaOx coated with ZrO2 fabricated by the Atomic Layer Deposition (ALD) was also prepared to evaluate the durability of potential cycling.

A Ti rod was used as the substrate, and Mn-TaOx was prepared by RF magnetron sputtering deposition. Mn disk was used as the sputtering target, and the Ta plate was placed on the Mn target to prepare Mn-TaOx. Mn addition was fixed at 50 at%, and it was controlled by the area ratio considering sputtering rate of Mn and Ta metal. Partial pressure of Ar and O2 gas were kept constant at 0.23 Pa for 20 min during the sputtering. The substrate heating temperature was constant at 673 K during the sputtering. The prepared Mn-TaOx electrode was coated with ZrO2 fabricated by ALD (ZrO2 coating Mn-TaOx). The thickness of coating ZrO2 was varied in the range of 0.5 to 2.0 nm. All electrochemical measurements such as oxygen evolution reaction (OER) was used by conventional three electrode cell in 1 M H2SO4 at 303 K. In addition, electrochemical impedance spectroscopy (EIS) was also performed to estimate several factors such as charge transfer resistance (R
ct), film resistance (R
film) and so on. The protocol of potential cycling as a durability test was set the potential in the range of 1.5 to 2.0 V with 50 mV s-1 for 10000 cycles, and the slow scan voltammetry was conducted every 2000 cycle in the range of 1.2 to 2.0 V with 5 mV s-1 to compare the OER activity before and after test.

The OER current of ZrO2 coating Mn-TaOx was smaller than that of Mn-TaOx without coating (Mn50-TaOx (0 nm)). In particular, the OER current of Mn-TaOx coated with 2 nm of ZrO2 (“2 nm”) was greatly decreased because the film thickness of ZrO2 is too thick so that it has low electric conductivity. Furthermore, the APD-ZrO2 covered on the surface, and it is also hard to react with reactant at active site. From results of EIS, the R
ct and R
film of “2 nm” increased. On the other hand, the OER current of Mn-TaOx coated with 1 nm of ZrO2 (“1 nm”) did not drastically decrease compared to Mn-TaOx and it was suggested that “1 nm” relatively maintained its activity.

Figure 1 shows the geometric current density (i
geo) at 2.0 V of ALD-ZrO2 coated on Mn-TaOx after potential cycling. The i
geo at 2.0 V of Mn-TaOx without coating also shows in Fig. 1. The i
geo of Mn50-TaOx (0 nm) was decreased about half value after 2000 cycles of durability test, but that of Mn-TaOx coated with ZrO2 did not decrease significantly even after 2000 cycles of potential cycling. In particular, the i
geo of “1 nm” was the highest current after the 2000 cycles of potential cycling in this study, and the degradation didn’t observe in this cycle range. According to the results from XPS spectrum, a small amount of ZrO2 was detected on the surface of “1 nm” after the potential cycling. The redox peak of Mn(Ⅲ)/(Ⅳ) [3] observed at the ALD-ZrO2 coated on Mn-TaOx was also detected after the potential cycling, so that the Mn species as an active site of Mn-TaOx for the OER was still remained. Therefore, the reason why the “1 nm” was relatively stable is that the amounts of both active and protect site have maintained during the durability test.

Acknowledgement

This work is partially supported by Suzuki Foundation.

References

(1) Ayers, Curr. Opin. Electrochem., 18, 9 (2019).

(2) M. Alia, and G. C. Anderson, J. Electrochem. Soc., 166, F282 (2019).

(3) Morita, C. Iwakura, and H. Tamura, Electrochim. Acta, 24, 357 (1979).

Figure 1