(Invited) MOF-Derived Nanosheet Arrays As Advanced Electrocatalysts for Water Splitting

Abstract

Developing efficient, nanostructured electrocatalysts with desired compositions and structures is of great significance for improving the efficiency of water splitting toward hydrogen production. In this regard, metal organic framework (MOF) derived nanoarrays have attracted great attention as promising electrocatalysts because of their diverse compositions and adjustable structures. This presentation summarizes our recent work on the design and fabrication of MOF-derived nanosheet arrays toward enhanced catalytic activity for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) as well as overall water splitting.First, heterostructured inter-doped ruthenium-cobalt oxide ((Ru-Co)Ox) hollow nanosheet arrays were prepared on carbon cloth for efficient overall water splitting. Benefiting from the desirable compositional and structural advantages of more exposed active sites, optimized electronic structure, and interfacial synergy effect, the (Ru-Co)Ox nanoarrays exhibited outstanding performance as a bifunctional catalyst. Particularly, they showed a remarkable hydrogen evolution reaction (HER) activity with an overpotential of 44.1 mV at 10 mA cm–2 and a small Tafel slope of 23.5 mV dec–1, as well as an excellent oxygen evolution reaction (OER) activity with an overpotential of 171.2 mV at 10 mA cm−2. As a result, a very low cell voltage of 1.488 V was needed at 10 mA cm–2 for alkaline overall water splitting.Second, Mo-doped ruthenium–cobalt oxide (Mo-RuCoOx) nanosheet arrays were produced for high-efficiency water splitting through combining electronic and vacancy engineering. The unique Mo-RuCoOx nanosheet arrays were able to act as a high-performance bifunctional electrocatalyst toward both HER and OER. Theoretical calculations and experimental results reveal that the incorporation of Ru and Mo can effectively tune the electronic structure, and the controllable Mo dissolution coupling with the oxygen vacancy generation during surface reconstruction is able to optimize the adsorption energy of hydrogen/oxygen intermediates, thus greatly accelerating the kinetics for both HER and OER. As a result, the Mo-RuCoOx nanoarrays exhibit remarkably low overpotentials of 41 mV and 156 mV at 10 mA cm–2 for HER and OER in 1 M KOH, respectively. Furthermore, the two-electrode electrolyzer assembled by the Mo-RuCoOx nanoarrays requires a cell voltage as low as 1.457 V to achieve 10 mA cm–2 for alkaline overall water splitting.Third, hollow nanosheet arrays assembled by ultrafine ruthenium-cobalt phosphide nanocrystals were fabricated toward exceptional pH-universal hydrogen evolution. The development of high-efficiency electrocatalysts for pH-universal HER is promising for constructing feasible water splitting systems at all pH values, but it remains challenging. A facile approach toward hollow nanosheet arrays assembled by ultrafine ruthenium-cobalt phosphide (Ru-CoxP) nanocrystals was developed through conversion from the MOF template. The synergic effects of optimized electronic structure, increased active sites, and rapid charge/mass transfer endowed the Ru-CoxP nanoarrays with outstanding electrocatalytic performance toward pH-universal HER. While exhibiting remarkably low overpotentials of 34.6 mV and 22.7 mV at 10 mA cm–2 in 1 M KOH and 0.5 M H2SO4, respectively, the Ru-CoxP nanoarrays showed an extremely low overpotential of 21.6 mV at 10 mA cm–2 in 1 M phosphate buffer solution (PBS). Furthermore, they were able to stably drive a Pt-free neutral electrolyzer for overall water splitting at 10 mA cm−2 with a cell voltage as low as 1.557 V.