Tuning Hydrogen-Bond Network within Stacked 2D Nanolayer for Enhanced Oxygen Evolution Reaction

Introduction Water splitting is a key to producing hydrogen with renewable electricity. The reaction consists of two reactions: hydrogen evolution and oxygen evolution reaction (OER), and the sluggish reaction kinetics of OER limits the overall energy efficiency 1) . Although the OER activity was continuously improved, mainly by optimizing the binding ability between reactants and the catalysts, the current strategy is reaching its limits. Recently, it has been reported that the hydrogen bond network of water molecules around the catalyst surface influences OER activity, and Lewis acidic metal cations in electrolytes can be used to tune its structures 2) . However, the contribution of cations to the OER activity is not noticeably manifested since the cation existence ratio is orders of magnitude lower than that of water molecules. Here, we focused on layered manganese oxide (MnO 2 ) 3) , which can trap metal complexes, water molecules, and cations within its nanolayer, as an OER catalyst. We synthesized the MnO 2 catalysts having Ni 2+ complex (for OER active site) and non-catalytic Li + , K + , and Cs + (for tuning the hydrogen bond network of confined water) co-inserted into the nanolayer. The effect of the hydrogen bond network on the OER activity was clarified by utilizing the nanosized interlayer space of layered MnO 2 as a tunable electrochemical reaction field. Methods The layered MnO 2 was electrodeposited on an FTO substrate using a solution containing manganese sulfate (MnSO 4 ) and a guest cation (TBA + Cl - ). Subsequently, the electrodeposited samples were immersed in various ion exchange solutions to synthesize target catalysts (Ni, Ni-Li, Ni-K, Ni-Cs/MnO 2 ). X-ray diffraction (XRD) was used to estimate the interlayer distances between the crystal phases of samples. The electronic states of Ni active sites were evaluated by X-ray photoelectron spectroscopy (XPS). Linear sweep voltammetry (LSV) was performed using three-electrode cells in 0.1 M TBAOH at a scan rate of 10 mV s – 1 to evaluate the OER activity. Results and discussion The XRD patterns showed the equally spaced peaks for each sample, confirming the layered structure for all synthesized catalysts (Fig.1a). Furthermore, the calculated interlayer distances matched with the hydration radius of expected interlayer cations, suggesting the successful insertion of the target cations. Ni2p peaks from XPS spectra showed the same peak positions throughout the samples, indicating a negligible change in the electronic states of Ni (Fig. 1b). The OER activity changed markedly depending on the non-catalytic alkaline cations, and the OER activity was in the order of Cs + < K + < Li + (Fig.1c). The observed trend was opposite to the activity of Ni disk electrodes in alkaline-cation electrolytes (LiOH < KOH < CsOH) (Fig.1d). The results suggest that alkaline cations differently modulate the hydrogen-bond network in the bulk electrolyte and inside the nanosized interlayer space, thus leading to different OER activity. We will also discuss the correlation between the hydrogen-bond network structure around the Ni 2+ complex and OER activity by probing the behavior of confined water within the interlayer with operando surface-enhanced infrared spectroscopy (SEIRAS). Reference 1) Liu, X. Chem. Commun ., 52, 5546–5549 (2016). 2)M. Görlin, et al. Nat. Commun., 11, 6181 (2020). 3)K. Fujimoto, et al . J. Phys. Chem ., 122, 8406–8413 (2018). Figure 1