Enhanced pseudocapacitive energy storage of oxides grown on nanoporous alloys by solid solution

Abstract

Development of transition-metal-oxides (TMO) based high-energy density aqueous pseudocapacitors requires simultaneously high charge storage capacity, high electronic conductivity and large redox potential window. Engineering the interfacial electronic structure among electrolyte, oxides and current collectors and thus modifying the corresponding multi-redox reactions displays a great potential to address the above challenges. Herein, a series of nanoporous alloy@oxide composite electrodes are prepared by self-oxidation of the nanoporous NiMMn alloys, where various dopants of 3d transition metal elements (M = Ti, V, Cr, Fe, Co, Cu) are introduced to tune the electronic structures prior to oxidation of the alloys. Within the alloy-core and oxide-shell structure, it is found that all the dopants effectively suppress the hydrogen production to ensure the high operation voltage. Additionally, Co dopant displays the largest energy density (145 mWh cm−3) comparing to others. The hydrogen suppression stems from the tuning of the work function of NiMn via doping different 3d elements so that the out-layer oxide electronic structures downshift accordingly. Further DFT calculation on the doped active surfaces (001) and (111) illustrates that Co dopant results in the large amounts of density of states within the external scan voltage −0.9 V ~ 0.6 V (vs. Ag/AgCl electrode), yielding the high capacitance to accommodate the electrons. This work provides insights into the design of the promising capacitive materials with a high energy density via interface engineering of the core alloy and the out-layer oxides.