MnO2/MnCo2O4/Ni heterostructure with quadruple hierarchy: a bifunctional electrode architecture for overall urea oxidation

A three-dimensional MnO2/MnCo2O4/Ni core-shell heterostructured electrode has been fabricated through a facile method. This electrode architecture consists of four levels of interconnected hierarchy: a primary macroporous Ni foam scaffold ( greater than or equal to 500 mu m), an intermediate vertically-aligned MnCo2O4 core-nanoflake array (50-100 nm), topmost ultra-thin MnO2 nanosheets ( similar to 10 nm) and short-range ordered mesopores ( similar to 5 nm) on the MnO2 nanosheets. This freestanding, hierarchical porous electrode has advantages in enhancing electroactive surface area, enabling efficient mass transport through the porous structure. The heterostructured electrode exhibits a low onset potential (1.33 V vs. RHE), a high anodic peak current density (1000 mA cm-2 g-1 at 1.7 V vs. RHE) and long-term catalytic stability for urea oxidation, which surpasses previous reported electrode materials for urea electrolysis. Remarkably, the MnO2/MnCo2O4/Ni electrode possesses bifunctional catalytic activity for both urea oxidation and hydrogen evolution. A urea electrolytic cell with both anode and cathode using the heterostructured electrodes has been fabricated and a current density of 10 mA cm-2 has been achieved at a cell voltage of 1.55 V. This noble metal-free quadruple hierarchy electrode shows potential as a new platform for multi-purpose applications.