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 (≥500 μm), an intermediate vertically-aligned MnCo2O4 core-nanoflake array (50–100 nm), topmost ultra-thin MnO2 nanosheets (∼10 nm) and short-range ordered mesopores (∼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.