Ultraviolet/ozone treatment for boosting OER activity of MOF nanoneedle arrays

In this study, we developed a hydrothermal method to grow Fe-based MOF nanoneedle arrays on a nickel foam as well as a combined ultraviolet/ozone (UV/O3) treatment strategy to improve the OER activities of the nonprecious metal-organic frameworks (MOFs) to the level even higher than that of the commercially-available precious metal catalysts (e.g., IrO2). It was found that Fe atoms existing in the Fe-MOF nanoneedles through coordination bonds were partially transformed into active FeOx species in-situ by the synergistic effect of UV and O3, because UV treatment can dissociate some coordination bonds to form free Fe ions, which are simultaneously oxidized to FeOx species by ozone. As a result, the combined UV/O3 treatment greatly reduced the resistance, increased the specific surface area and the number of active sites, and hence the increased catalytic activity and stability. Specifically, UV/O3 treated Fe-MOF nanoneedle arrays supported on nickel foam exhibited an excellent OER performance with a low overpotential (218 mV at 10 mA cm-2) and low Tafel slope (38.8 mV dec-1), outperformed the commercial IrO2 catalyst. This simple methodology developed in this study should have broad implications for the development of other new low-cost, but highly efficient, catalysts for OER and beyond.