Ni plates were conditioned in purified 1 M KOH to obtain a Ni(OH)2 thin film (Figure ). The effectiveness of such an electrochemical conditioning to form Ni(OH)2 from Ni metal was demonstrated in a recent publication. Two sets of samples were prepared. One was used for XPS and UPS measurements, the other one for FTIR, Raman, and an extended activity measurement. The experimental sequence is outlined in Figure . Figure shows the OER overpotential at 1 mA/cm 2 , referenced to 1.23 V. The dashed bars show the overpotential of the created Ni(OH)2 thin films measured in KOH at the end of the conditioning sequence showing similar values for each sample. The samples were subsequently activated at 1.6 V vs. RHE in 1 M LiOH, NaOH, KOH, and CsOH to form the OER active NiOOH phase. The electrochemical performance was evaluated with a chronopotentiometry measurement at 1 mA/cm 2 (Figure ), chronoamperometry at 1.6 V vs. RHE (Figure ), and a CV up to 1.8 V vs. RHE (Figure ) in the presence of the respective alkali hydroxide solutions. The activity follows the order LiOH < NaOH ≤ KOH < CsOH. This is in line with trends reported in literature. The first set of activated samples was taken out of the electrochemical cell at 1.6 V vs. RHE and transferred quickly into ultra-high vacuum (UHV) to stabilize the phase (left bar in Figure ). By then, some NiOOH can have decomposed into Ni(OH)2, but the majority of the sample is still in its NiOOH phase as formed during the electrochemical activation. This approach and the stability of the NiOOH phase was studied in a previous paper. The NiOOH phase shows the characteristic shape of the Ni 2p core level (Figure ). The valence band and core levels are shifted to lower binding energies compared to Ni(OH)2, indicating a p-type electronic structure of the NiOOH phase. In our previous paper, we could assign the peak at 528.9 eV in the O 1s spectrum to the unprotonated oxygen in the NiOOH structure. In the results presented in Figure , the intensity of this peak depends on the type of electrolyte used during activation. Namely, it is largest after activation in CsOH, followed by KOH, and then NaOH and LiOH. To understand this spectral evolution, a second set of samples was activated (right bar in Figure ) and transferred as quickly as possible into the FTIR sample compartment, which was evacuated for the FTIR reflection-absorption spectroscopy measurement. Immediately afterwards, a Raman spectrum was collected ex-situ. Figure compares the FTIR spectra of the Ni(OH) 2 phase before activation, the NiOOH phase after activation, and the difference spectra with Ni(OH)2 as the reference spectrum. The spectra show that the phase composition before activation was similar for all samples, namely β-Ni(OH)2, with a strong lattice OH stretching vibration at 3646 cm -1 and a smaller bending vibration at around 520 cm -1 , and some α-Ni(OH)2 with a lattice mode at 607-634 cm -1 and an OH bending vibration at 1380-1410 cm -1 . Intercalated H2O contributes intensity at 3590 cm -1 and 1630 cm -1 . To conclude, applying the conditioning procedure to polished Ni metal creates mostly β-Ni(OH)2 with some disorder appearing in the form of turbostratic α-Ni(OH)2. This combination could signify some α/βinterstratification. After activation, the sharp peak at 3646 cm -1 is lost, which means the loss of non-hydrogen-bonded OH groups. This is partly caused by a general decrease of OH groups in NiOOH compared to Ni(OH)2, while the remaining OH groups are now bonded to hydrated H2O. The interfacial water appears in the spectra as a broad feature from 2670-3630 cm -1 . An increased hydration of the material after activation in LiOH to CsOH is hardly visible in the difference spectra NiOOH / Ni(OH)2. However, this could be caused by difficulties in correctly determining the gradient of the baseline in these spectra. In the FTIR spectra, no trend can be derived that correlates with the variation of the electrolyte. The situation is different for Raman spectroscopy. Here, Figure displays the Eg and Ag lattice vibrations of NiOOH at about 477cm -1 and 555 cm -1 , respectively, and some intensity from 900-1160 cm -1 , which is assigned to NiOO -. The sharp peak at 1068 cm -1 is attributed to carbonate in the electrolyte. The ratio of the Eg and Ag lattice vibrations was found to be sensitive to the structure of NiOOH, whereby a higher peak ratio was found for γ-NiOOH. In Figure , a correlation of the peak ratio with the kind of electrolyte becomes apparent. The spectra were normalized to visualize the relative increase of the 555 cm -1 band after activation in LiOH compared to after activation in CsOH, signifying an increasing amount of γ-NiOOH with increasing cation size.