Surface-defect engineering of a nickel hexacyanoferrate material for high-performance printed flexible supercapacitors

Abstract

Flexible supercapacitors (FSCs) are considered to be a promising electrochemical energy source for wearable electronic products. However, the absence of high-performance electrode materials and effective manufacturing techniques of devices hinder the volume production of FSCs. Here, a facile surface-etching method is investigated to prepare high-performance nickel hexacyanoferrate (NiHCF) electrode materials, and further, a screen-printing strategy is employed to realize high-volume production of FSCs. Compared with conventional NiHCF electrodes, the Na₂Ni[Fe(CN)₆]-based electrode with surface defect offers a specific capacity of 184 C g⁻¹ (100 C g⁻¹ for conventional NiHCF), and its capacity retention is 91% after 10 000 cycles. According to density functional theory (DFT) calculations, the excellent cycling stability can be attributed to the negligible structural distortion during the Na⁺ insertion/extraction process (∼6% volume change). Subsequently, symmetric FSCs are constructed by printing the NiHCF electrode materials on flexible PET substrates, displaying excellent areal capacitance of 7.3 mF cm⁻² (0.1 mA cm⁻²) and energy density and power density of 0.65 μW h cm⁻² and 0.4 mW cm⁻², respectively. The surface-defect engineering provides a low-cost strategy to construct nickel hexacyanoferrate materials with outstanding electrochemical properties. The printed electronic techniques we have demonstrated for large-scale and low-cost printing FSCs applications are inspiring.