Low-cost and environmentally friendly electrochemical energy storage systems are crucial to address the increasing global energy demand. Nanomaterials can play a pivotal role in catalysing charge storage and/or exchange, still the underlying mechanism often remains poorly investigated, as for ZnO/ZnS nanostructures onto Ni foam. In this work, we investigate hydrothermally grown ZnO/ZnS nanostructures decorating Ni foam for energy storage application. Morphology, structure and composition are evaluated via electron microscopy-based methodologies. The electrochemical energy storage performance is evaluated by cyclic voltammetry (CV) measurements with the aim to highlight the energy storage mechanism. When nickel foam (NF) is used as substrate, the system shows a predominant pseudocapacitive behaviour. By contrast, a modest and capacitive performance is measured on graphene paper (GP). Mott-Schottky (M-S) and open circuit potential (OCP) measurements suggests a key role of hole reservoir in ZnS decoration which boosts NF performances.