Carbon materials are generally preferred as anodes in supercapacitors; however, their low capacitance limits the attained energy density of supercapacitor devices with aqueous electrolytes. Here, we report a low-crystalline iron oxide hydroxide nanoparticle anode with comprehensive electrochemical performance at a wide potential window. The iron oxide hydroxide nanoparticles present capacitances of 1,066 and 716 F g⁻¹ at mass loadings of 1.6 and 9.1 mg cm⁻², respectively, a rate capability with 74.6% of capacitance retention at 30 A g⁻¹, and cycling stability retaining 91% of capacitance after 10,000 cycles. The performance is attributed to a dominant capacitive charge-storage mechanism. An aqueous hybrid supercapacitor based on the iron oxide hydroxide anode shows stability during float voltage test for 450 h and an energy density of 104 Wh kg⁻¹ at a power density of 1.27 kW kg⁻¹. A packaged device delivers gravimetric and volumetric energy densities of 33.14 Wh kg⁻¹ and 17.24 Wh l⁻¹, respectively.

Atomically dispersed metal catalysts for the oxygen reduction reaction, including their synthesis, characterization, reaction mechanisms and electrochemical energy application, are reviewed.