A custom-made electrochemical cell was developed for real-time in situ observation of chemical and structural changes in Ni and NiFe LDHs by using Raman spectroscopy, well known for its sensitivity to vibrations of metal oxygen bonds. First of all, we confirmed that lattice oxygens of Ni LDH participate in the oxygen evolution reaction process, while no participation of lattice oxygens was observed for NiFe LDH. Moreover, we found that the NiFe lattice gets more disordered as Fe is more added and consequently there is a relationship between the lattice disorder and OER activity. In other words, we proposed optimizing lattice order as a strategy to improve the OER performance of NiFe LDH catalyst. These results have been presented at an international conference and also published through a high impact peer-reviewed journal.
According to previous reports, it turned out that if Fe is too much added to Ni-based oxides, FeOOH begins to form in the bulk layer, resulting in decrease of the OER activity. However, we revealed that FeOOH nanoclusters are much more active for the OER than NiFe LDH, if the FeOOH is formed and distributed on the surface of NiOOH (oxidized form of Ni oxides) but not inside of the catalyst layer. By using Raman spectroscopy, the FeOOH-NiOOH catalyst was monitored in real time during the OER process and finally the relevant mechanism was experimentally proved for the first time. In this mechanism, Fe was thought of as an oxygen evolving site and helped to proceed by the neighboring Ni site. The result was published in a world-leading peer-reviewed journal.
In addition to such Fe-containing Ni based oxides, Co oxide, one of the most promising candidates for alkaline OER, was also studied during the project. By using operando surface enhanced Raman spectroscopy, we found for the first time that a superoxide intermediate exists in the catalytic cycle of OER on the Co oxide and it is likely originated from both hydroxide anions of electrolyte and lattice oxygens. Based on the experimental finding, we proposed a mechanism where lattice oxygens participate directly in the OER and the rate-determining step (RDS) is liberation of the oxygen molecule rather than O-O bond formation that has been generally known as RDS. This work was presented in an international conference and then published through a world-leading peer-reviewed journal.
High entropy fluoride, a new class of materials recently developed, was studied in terms of synthetic method and its application. A simple mechanochemical way of synthesizing high entropy fluorides containing possibly 4 to 7 transition metals (Co, Cu, Mg, Ni, Zn, Mn, and Fe) in equiatomic ratios was successfully developed and the newly synthesized samples were used to see a potential application to oxygen evolution reaction. The OER performance was more active over a state of the art noble metal catalyst, IrO2. The result suggests the mechanochemical synthesis of high entropy fluorides would be promising for electrocatalyst development. The result was published through a high impact peer-reviewed journal as open access article.
