Hydrous and Amorphous Cobalt Phosphate Thin-Film Electrodes Synthesized by the SILAR Method for High-Performing Flexible Hybrid Energy Storage Devices

Recently, excellent electrochemical performance and good conductivity of transition metal phosphates (TMPs) have been obtained, assuring their potential as a cathode in hybrid supercapacitors. Also, amorphous, hydrous materials are supposed to be exemplary active materials for high-performing supercapacitors because of their unique porous structure, structural flexibility, and rich defects. Therefore, the present investigation describes a simple synthesis method for hydrous and amorphous cobalt phosphate thin-film preparation by the simple successive ionic layer adsorption and reaction (SILAR) method on flexible stainless steel (SS) substrate for supercapacitor application. The structural and morphological analyses reveal mesoporous, agglomerated nanoparticle-like, hydrous, and amorphous cobalt phosphate over the SS substrate. The mesoporous nanoparticles of cobalt phosphate material possess the uppermost specific capacitance of 1147 F g^-1and 630.7 C g^-1specific capacity at a 1 mA cm^-2current density. To demonstrate practical relevance, hybrid supercapacitor devices were assembled with cobalt phosphate and rGO (reduced graphene oxide) as cathode and anode electrodes, respectively. Furthermore, the assembled hybrid aqueous supercapacitor device (S-CP4//KOH//rGO) delivers 44.8 Wh kg^-1specific energy (SE) at a specific power (SP) of 4.8 kW kg^-1with 126 F g^-1specific capacitance. The accumulated all-solid-state hybrid supercapacitor device (S-CP4//PVA-KOH//rGO) achieved the uppermost 77 F g^-1specific capacitance and SE of 27.37 Wh kg^-1at SP of 1.5 kW kg^-1with an outstanding 94% capacitive retention over 5000 cycles. Such remarkable supercapacitive performance results demonstrate that the SILAR method is an easy synthesis process for the binder-free preparation of cathode based on hydrous, amorphous cobalt phosphate thin films for hybrid supercapacitor devices.