Surface charge density-dependent performance of Ni–Al layered double hydroxide-based flexible self-powered generators driven by natural water evaporation

Abstract

Natural water evaporation (NWE) can be used as a driving force to generate electricity. NWE-driven generators (NWEGs) require materials with charged surfaces. However, the quantitative adjustment of the surface charge density (dc) remains difficult and the relationship between dc and NWEG performance has not been clarified. Herein, Ni–Al layered double hydroxide (LDH) films with tailored structural dc in the range of 2.52–4.59 e/nm2 were synthesized by precisely adjusting the molar ratio of Al3+ to Ni2+. The Ni–Al LDH films were assembled into NWEGs by the spraying method and then their electrical performance was investigated. Strong positive correlations were observed between dc, open-circuit voltage (Voc), and short-circuit current (Isc); that is, the larger dc, the higher Voc and Isc. The maximum Voc of ~0.6 V, Isc of ~0.3 μA, and output power density of ~15 μW/cm3 were achieved simultaneously for a 4 × 1.5 cm NWEG with a maximum dc of 4.59 e/nm2. A reasonable increase of dc resulted in improved device performance, which was attributed to the positive correlation between the densities of both hydroxide ions in the diffusion layer and positive charge on the Ni–Al LDH film surface. In addition, a relatively stable total Voc of ~2.5 V and Isc of ~1.5 μA were obtained using eight NWEGs with a modified individual generator structure. The eight assembled NWEGs successfully powered a simple electronic watch. The spraying method used to fabricate the NWEGs not only greatly shortened the device molding time, but also markedly improved device stability. This study reveals a new avenue to guide the design of materials and to improve the performance of flexible self-powered NWEGs.