NSRRC Activity Report 2023

026 NSRRC ACTIVITY REPORT 2023 amount of Pt 2+ increased with increasing anodic potential and, interestingly, Pt 4+ suddenly formed when the applied voltage was set to cross the threshold voltage (1.75 V RHE ) of the OER. Valence-band measurements unveiled a decrease in electron density near the Fermi edge as the amount of Pt 4+ on the surface increased. This may have improved the adsorption of hydroxyl groups and water. Furthermore, depth-profiling studies were conducted to illustrate the surface oxide species evolution during the redox polarization, as shown in Fig. 2 . The results indicate that Pt δ+ was gradually converted into Pt 2+ on the surface as anodic polarization was applied before the OER (1.05–1.65 V RHE ), resulting in a mixed-valence surface of the Pt catalyst. However, the behavior of the Pt 4+ was rather different, appearing at the subsurface of the catalyst in the early stage of OER (applied voltage was set higher than 1.65 V RHE ), followed by an increment in the surface of the catalysts at a higher potential (2.65 V RHE ). The results suggest that proper valence states on the surface can further enhance the adsorption of water and hydroxyl groups and facilitate OER activity at the interface between metallic Pt and thin, porous oxide layers on the surface. Furthermore, the team collaborated with Yu-Hsu Chang (National Taipei University of Technology) to uncover the active species of the transition metal-based OER catalyst in the alkaline environment. 2 The study focused on a series of Fig. 3 : Relative Co 2p diagrams of (a) FeCo 2 O 4 /NF, (b) Zn 0.2 Fe 0.8 Co 2 O 4 /NF, (c) Zn 0.4 Fe 0.6 Co 2 O 4 /NF, and (d) Zn 0.6 Fe 0.4 Co 2 O 4 /NF. The applied potentials of OCP, 1.3, 1.5, and 2.0 V vs RHE were selected to conduct APXPS spectra. [Reproduced from Ref. 2] Zn-substituted iron cobaltite spinel (Zn x Fe 1-x Co 2 O 4 , 0 < x < 0.6 in intervals of 0.2) on nickel foam (NF), which were synthesized through a hydrothermal process. Among the catalysts with different substitution ratios, Zn 0.4 Fe 0.6 Co 2 O 4 / NF exhibited the highest OER performance, reaching an overpotential of 330 mV at a current density of 50 mA cm 2 in 1 M NaOH, while the overpotential of FeCo 2 O 4 /NF was 395 mV. The operando APXPS analysis summarized in Fig. 3 showed that the catalysts are composed of Co in spinel, Co(OH) 2 and CoO(OH) in the OCP state, whereas Zn 0.4 Fe 0.6 Co 2 O 4 /NF has the highest Co(OH) 2 compared to other samples. Meanwhile, the deconvolution results of the Co 2p spectra showed the highest formation of Co(IV)O 2 for the Zn 0.4 Fe 0.6 Co 2 O 4 /NF sample under OER conditions. Co(IV)O 2 is an important intermediate and leads to higher OER performance. The results are in agreement with the OER performance, suggesting that the activity of FeCo 2 O 4 / NF can be enhanced with well-controlled amounts of Zn substitution. In summary, PMEC cells were successfully developed and combined with APXPS for operando investigation of the chemical state changes on the catalyst surface under electrochemical reactions. Electrochemical reactions can be performed under a wide-range of pH environments by carefully choosing different ion exchange membranes. Moreover, the chemical states and formation mechanism