Quantitative Evaluation of the Activity of Nickel Ion Site of LiNi0.5Mn1.5O4 Spinel for Oxygen Evolution Reaction

Suntivich et al. found a volcano relationship between the eg occupancy in perovskite oxide and OER activity. eg filling of ~ 1.2 is the optimum value1. Nevertheless, some materials do not follow this trend. A typical example is oxides containing a high valence Ni(4+) state. To elucidate the effect of high-valence Ni on OER activity, it is necessary to evaluate the OER activity of materials with stable Ni in a high oxidation state (4+). However, in case of the perovskite family, the formation of Ni4+ requires calcination under high oxygen partial pressures (60-200 bar). In contrast, LiNi0.5Mn1.5O4 (LNMO) with a spinel structure is known that lithium ions are desorbed by oxidation to produce tetravalent nickel ions quantitatively. Therefore, by using this material as an oxygen-evolving electrode, it is possible to quantitatively evaluate the activity of tetravalent nickel ions. Herein, we report the OER activity of nickel-manganese spinel, in which Ni4+ was stable up to the bulk. LiNi0.5Mn1.5O4 powder was synthesized as follows. Stoichiometric amounts of LiNO3, Ni(NO3)2.6H2O, and Mn(NO3)2.6H2O were dissolved in ultrapure water. A four-to-five-fold molar excess of citric acid was added while stirring. This solution was heated at 400 °C to obtain the precursor. The obtained powder was calcined at 800 °C for 12 h and at 700 °C for 48 h under pure O2. Various levels of chemical delithiation were achieved by reacting LiNi0.5Mn1.5O4 with nitronium tetrafluoroborate in acetonitrile solution for 48 h at 25 ºC. The degree of delithiation was controlled by changing the ratio of LiNi0.5Mn1.5O4 to NO2BF4. The suspension of LiNi0.5Mn1.5O4 and NO2BF4 was stirred for 48 h to obtain a uniform particle composition. Subsequently, the suspension was filtered, thoroughly washed with acetonitrile at least three times, and dried overnight in a vacuum oven. Then, chemical composition was determined using inductively coupled plasma optical emission spectrometry, X-ray diffraction (XRD) patterns were collected in BL02B2 at SPring-8, Japan. XAS was conducted for Mn and Ni K-edges, XAS for Mn, Ni L-edges, and O K-edge was also conducted. This study focused on the electrocatalytic activity of LixNi0.5Mn1.5O4 spinels with different lithium contents. The lithium (x) content in LixNi0.49Mn1.51O4 was controlled by topochemical delithiation using NO2BF4 in an acetonitrile solution. Upon applying this approach, nickel changed from the divalent to tetravalent state, while the tetravalent state of manganese was maintained. The OER activity was found to increase with decreasing x. x = 0.00 showed the highest OER activity. The Tafel slopes indicated that the RDS changed from OH− adsorption to proton transfer from OH*; thus, the OER kinetics was enhanced. The XAS spectra of Ni and O indicated that the presence of Ni4+ caused a downshift in the unoccupied Ni 3d state and decreased the energy barrier of charge transfer from the oxygen intermediate species to the metal active site; hence, the OER activity was enhanced. This study revealed the mechanism underpinning the high OER activity of high-valence Ni(4+) and also provided a approach for the development of highly active catalyst materials, namely, topochemical delithiation. Acknowledgement This work is based on results obtained from a project (JPNP14021) commissioned by the New Energy and Industrial Technology Development Organization (NEDO) of Japan. References Suntivich, J.; May, K. J.; Gasteiger, H. A.; Goodenough, J. B.; Shao-Horn, Y., Science 2011, 334, 1383-1385. Ren Y. et al., ACS Appl. Energy Mater. 2021 4 (10), 10731-10738