Oxygen evolution reaction (OER) plays key roles in renewable energy technologies such as regenerative fuel cells and rechargeable metal-air batteries, but the large overpotential and slow kinetical of OER limit the performance and commercialization of such devices. Metal-organic frameworks (MOFs) as a new class of electrocatalysts exhibit the great promising to overcome the challenge of OER kinetical sluggish. However, even for these superior electrocatalysts, there are two phenomenon result in the decrease of OER performance. The electrocatalysts repel reacting water molecule, impeding the adsorption of reactant, and as-generated O2 molecule stick to the pore and surface of electrocatalysts, hindering the release of O2 molecule, further increase Ohmic drop. Therefore, it is highly desirable that develop effective OER electrocatalysts to efficiently attract reactant and disengage as-generated gas for optimal OER performance. Here, we report firstly two novel superhydrophilic and superaerophobic metal-organic frameworks, NiAlF5(H2O)(pyr)2 2(H2O) (KAUST-8, AlFFIVE-1-Ni); and its isomorph NiFeF5(H2O)(pyr)2 4(H2O) (KAUST-9, FeFFIVE-1-Ni). These materials show that the Fe3+(or Al3+) cations introduces open metal coordination sites within precisely controlled square-shaped channels with periodically arrayed fluoride anions, which composed hydrophilic MOFs and enhanced their H2O affinity. In the pillars of MOFs, the coordinated H2O molecule and three axial F atoms that point into the 1D channel are disordered over four positions, which benefit the as-generated O2 easily to release. Meanwhile, breakthrough experiments have demonstrated these types of MOFs particularly favor H2O adsorption and good O2 repulsion property. The developed MOFs exhibit specific activity at 10 milliamps per square centimeter with only 250 millivolts of overpotential, much better than 285 millivolts of state-of-the-art RuO2 electrocatalysts. Density functional theory calculations reveal the synergistic interplay between Fe and Ni (two components assume forming of active O radical and catalyzing the O-O coupling, respectively), and structural properties of superhydrophilic and superaerophobic deliver optimal electrocatalytic performance. These findings not only establish structure-function relationships bringing about the exciting prospect of further improved OER performance, but also provide fundamental understanding for the key functionality of each components of MOFs in the OER context.

Superhydrophilic and superaerophobic metal-organic frameworks for electrocatalytic oxygen evolution

2019 Fall National Meeting

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