Effects of Oxygen Defects in Zr Oxide-Based Thin Films on Oxygen Reduction Reaction in Acidic Media
Authors: Watanabe Yuri , Matsuzawa Koichi , Monden Ryuji , Nagai Takaaki , Ishihara Akimitsu 
Published: February 2024

The use of carbon-supported platinum as cathode catalysts of polymer electrolyte fuel cells has some problems such as high cost and low amount of resources of platinum. Therefore, we have attempted to develop substitute of platinum based on group 4 metal oxides. In the case of Zr oxide-based powder catalysts, we found that zirconium oxide-based compounds with oxygen defects and deposited carbon derived from organometallic precursors had definite activity of oxygen reduction reaction (ORR) [1][2]. However, the ORR activities of these catalysts still remain low. In order to increase the ORR activity drastically, it is necessary to clarify the catalyst design. This requires an understanding of the active sites and the electron conduction path separately, which were not separated in powder catalysts. On the other hand, Sugino et al. investigated the effect of oxygen vacancies and nitrogen doping on the active site based on first-principles calculations [3]. Defects by oxygen vacancies and nitrogen dopants were introduced into the tetragonal ZrO2(101) slab. It was found that the energy barrier of the rate-determining step, or the removal of adsorbed OH, was comparable to (or only slightly lower than) that of the pristine surfaces, indicating that defects play only a minor role in enhancing the ORR activity. In other words, it was suggested that oxygen vacancies and nitrogen dopants contributed to the emergence of ORR activity by enhancing the electrical conductivity of ZrO2 rather than by forming high quality active sites. Simple models are suitable for experimentally confirming the results of theoretical calculations and for understanding the active sites and the electron conduction paths separately. Therefore, we attempted to prepare nitrogen doped ZrOx nanofilms by RF magnetron sputtering method with different targets and atmospheres and to investigate their potential as model electrodes.
ZrOx thin films of ca. 3-5 nm thickness were prepared by RF magnetron sputtering method using glassy carbon (GC) plates as substrates and ZrN or Zr metal plates as targets under Ar, Ar+O2 and Ar+N2. The sputtering power, the deposition time, and the substrate heating temperature were 100 W, 10 min, and 300 ℃, respectively. The flow rates of Ar, O2, and N2 gas were controlled to keep the total chamber pressure at approximately 0.6 Pa. When the gases were mixed, the partial pressure ratio was set to 1:1. Electrode prepared with a ZrN target in Ar was denoted as "ZrN_Ar", and electrodes prepared with a Zr target in Ar:O2 and Ar:N2 were denoted as "Zr_Ar+N2" and "Zr_Ar+O2", respectively. A conventional three-electrode cell was used for electrochemical measurements with Reversible Hydrogen Electrode (RHE) as a reference and a GC plate as a counter electrode in 0.5 M sulfuric acid at 30 ℃. The oxygen reduction current density i
ORR (geometric area standard) was calculated by subtracting the current in N2 from the current in O2.
Figure 1 shows the effect of sputtering condition on potential-ORR current curves of the ZrOx thin film electrodes. For comparison, the result of the GC substrate only is also plotted as "GC only." The ORR activity of the ZrN_Ar was lower than that of the GC only. In addition, the current of the steady-state cyclic voltammogram of the ZrN_Ar was smaller than that of the GC only. This may be due to the electrochemical oxidation of ZrN surface and the formation of an insulating layer without active sites, presumably Zr hydroxide. On the other hand, the onset potential of the Zr_Ar+N2 and the Zr_Ar+O2 for the ORR were higher than that of the GC only and the Zr_Ar+N2 showed highest ORR activity. The steady-state cyclic voltammograms of the Zr_Ar+N2 and the Zr_Ar+O2 were larger than that of GC only, revealing that these electrodes might have high conductivities. In addition, XPS measurements suggest that the Zr_Ar+N2 contains nitrogen and the Zr_Ar+N2 and the Zr_Ar+O2 have oxygen vacancies. These results suggested that their high ORR activities were caused by nitrogen doping and oxygen defects. Therefore, the electrodes prepared by RF magnetron sputtering method may be able to separate the active sites from the electron conduction by changing the sputtering conditions such as film thickness.
Acknowledgement
The authors thank the financial support of the New Energy and Industrial Technology Development Organization (NEDO).
References
(1) A. Ishihara et al., J. Phys. Chem. C, 123, 18150 (2019).
(2) M. Chisaka et al., ACS Omega, 2, 678 (2017).
(3) S. Muhammady et al., J. Phys. Chem. C, 126, 15662 (2022).

Figure 1