Multivalent ruthenium immobilized by self-supported NiFe–organic frameworks for efficient electrocatalytic overall water splitting

Abstract

Doping transition metal electrocatalysts with Ru is an effective strategy to achieve the balance between high-efficiency performance and low preparation cost. Herein, we prepared a series of Ru-doped and self-supported Ru_x-NiFe-MOF/NFF electrodes under solvothermal conditions. In the synthesis process, some Ru³⁺ ions were reduced to metallic Ru nanoparticles and further coated by NiFe-MOF sheets, while the residual Ru³⁺ ions were diffused into the lattice of NiFe-MOFs. The optimal Ru_9.1-NiFe-MOF/NFF showed high electrocatalytic activity for the hydrogen evolution reaction (HER) with an overpotential of 17 mV @ 10 mA cm⁻², which was much better than that of many reported catalysts. For the oxygen evolution reaction (OER), a low overpotential of 202 mV @ 10 mA cm⁻² was required. When used as both an anodic and a cathodic electrocatalyst, the Ru_9.1-NiFe-MOF/NFF couple demonstrated outstanding overall water splitting performance with a cell voltage of 1.47 V @ 10 mA cm⁻². Moreover, the electrocatalytic activity of the bifunctional Ru_9.1-NiFe-MOF/NFF electrode could be maintained over 90 h. Based on analysis using various characterization techniques, the electrocatalytic performance of Ru_9.1-NiFe-MOF/NFF could be attributed to the intrinsic activity of Ru sites, the synergistic effect of various metal species, and the in situ transformation of Ni/Fe sites. This work provides insight into the design of a bifunctional Ru-doped electrocatalyst for overall water splitting.