Exploring Key Factors for Oxygen Evolution Reaction on La2-X
Sr
x
NiO4+Δ
by Oxygen Defect Engineering
Authors: Kobata Soma , Kimura Yuta , Amezawa Koji , Oishi Masatsugu , Fujishiro Fumito , Takahashi Ikuma , Yagi Shunsuke , Nakamura Takashi 
Published: February 2024

Introduction
Development of Oxygen Evolution Reaction (OER) catalyst is an important research issue for green hydrogen production by water splitting. Perovskite oxides have attracted attention as OER catalysts which can achieve both low cost and high catalytic activity. So far, some hypotheses have been proposed to understand key factors of catalytic activity of perovskite catalyst. Suntivich et al. reported volcano-like relationship between the OER activity and the number of the e
g-electron of the B-site transition metal and the highest catalytic activity is achieved when the number of the e
g-electron is close to 1.21. Besides, oxygen vacancies act as electrochemically active sites by incorporating reaction intermediates directly2,3. However, it is difficult to distinguish the contribution of electric state and oxygen defects on OER activity because both electronic state and oxygen defect concentration change simultaneously. For instance, the oxygen vacancy formation creates two electrons to maintain charge neutrality (2TMTM
×+OO
×↔2TMTM
'+V
O
∙∙+1/2 O2(g)).
The aim of this study is to clarify the key factors improving OER catalytic activity. For the aim, we controlled the Ni valence state and oxygen defect concentration in layered perovskite oxide La2-x
Sr
x
NiO4+δ
by tuning Sr content (x) and the amount of oxygen defect (δ) independently (Fig. 1-a), to reveal the influence of transition metal electronic state and oxygen defects on OER activity. La2-x
Sr
x
NiO4+δ
has two types of oxygen defects: oxygen vacancy (V
O
∙∙) and unoccupied interstitial site (V
i
×). Comparing OER activity of oxygen stoichiometric La2NiO4 (Ni2+), La1.8Sr0.2NiO4 (Ni2.2+) and La1.6Sr0.4NiO4 (Ni2.4+) allowed us to evaluate the influence of Ni valence state on OER activity while no oxygen vacancy and fully unoccupied interstitial sites ([V
O
∙∙]=0,[V
i
×]=2.0). Additionally, comparing OER activity of La2NiO4.1 (Ni2.2+,[V
O
∙∙]=0,[V
i
×]=1.9), La1.8Sr0.2NiO4 (Ni2.2+,[V
O
∙∙]=0,[V
i
×]=2.0) and La1.6Sr0.4NiO3.9 (Ni2.2+,[V
O
∙∙]=0.1,[V
i
×]=2.0) allowed us to evaluate the influence of oxygen defects on OER activity with negligible effect of Ni valence state.
Experimental
La2-x
Sr
x
NiO4+δ
(x=0, 0.2, 0.4) were synthesized by Pechini method. The oxygen content of La2-x
Sr
x
NiO4+δ
was controlled by annealing them under suitable oxygen partial pressure and temperature and then quenched based on oxygen nonstoichiometric data4. Obtained samples were characterized by XRD, XAS and SEM.
For the OER test, the catalyst ink was prepared by mixing the catalyst powder, acetylene black, K+-exchanged Nafion and tetrahydrofuran. 6.4 μl of the catalyst ink was sonicated and dropped onto the glassy-carbon rotating disk electrode. The electrochemical measurements were performed in O2-saturated 0.1 M KOH. The disk electrode potential was controlled between 0.3 and 0.9 V versus Hg/HgO reference electrode filled with 0.1 M KOH at a scan rate of 10 mVs-1.
Result and Discussion
Oxygen stoichiometric La2NiO4 (Ni2+), La1.8Sr0.2NiO4 (Ni2.2+) and La1.6Sr0.4NiO4 (Ni2.4+) showed similar catalytic activity regardless of Ni valence (Fig. 1-b). This implies that the electronic state of Ni has insignificant influence on catalytic activity when the sample has almost no oxygen vacancy. The observed tendency is inconsistent with the Suntivich’s result that the relationship between OER activity and e
g filling1.
La2NiO4.1 (Ni2.2+,[V
O
∙∙]=0,[V
i
×]=1.9) and La1.8Sr0.2NiO4 (Ni2.2+,[V
O
∙∙]=0,[V
i
×]=2.0) which have different amount of unoccupied interstitial sites and almost no oxygen vacancy showed similar OER activity (Fig. 1-c). In contrast, La1.6Sr0.4NiO3.9 (Ni2.2+,[V
O
∙∙]=0.1,[V
i
×]=2.0) which has oxygen vacancy and fully unoccupied interstitial sites showed much better OER activity than La1.8Sr0.2NiO4 (Ni2.2+,[V
O
∙∙]=0,[V
i
×]=2.0) which has almost no oxygen vacancy and fully unoccupied interstitial sites (Fig. 1-c). These results indicate that oxygen vacancy has high OER activity as suggested in earlier works2.3, while unoccupied interstitial site which has available space for oxygen intercalation has negligibly low catalytic activity.
The difference between oxygen vacancy and interstitial site is the presence of direct bonding with B-site cation. Reaction intermediates in oxygen vacancies can form direct bonding with Ni, while that in the interstitial site cannot. As suggested in the previous studies, a higher hybridization promotes charge transfer between the transition metal and reaction intermediate and effective charge transfer improves OER activity1,5.
Summery
In this work, to reveal the influence of transition metal electronic state and oxygen defects on OER activity, we have synthesized La2-x
Sr
x
NiO4+δ
, and controlled Ni valence state and oxygen defect concentration independently by tuning x and δ. It was revealed that oxygen vacancy boosts OER activity while influence of Ni valence state and unoccupied interstitial sites is insignificant.
References
[1] J. Suntivich, et al., Science, 334, 1383 (2011)
[2] S. Hirai, et al., J. Mater. Chem. A, 6, 15102 (2018)
[3] Y. Zhu, et al., J. Mater. Chem. A, 8, 10957 (2020)
[4] T. Nakamura, et al., Solid State Ionics. 180, 368 (2009)
[5] A. Grimaud, et al., Nat. Commun.
4, 2439 (2013)

Figure 1