Unraveling the Pivotal Role of the W ⁵⁺ / W ⁶⁺ Ratio in Substoichiometric Tungsten Oxide Nanorods in Their Performance as a Redox-Capacitive Electrode and as an Efficient Electrocatalyst for the Oxygen Evolution Reaction

Herein, we have elucidated the influence of surface oxygen vacancy, aspect ratio, and crystalline water of nanostructured substoichiometric tungsten oxide (WO 3– x; 0 < x < 1), which behaves as a redox-capacitive electrode in alkaline medium in a three-electrode configuration. The samples have been synthesized by simply varying the hydrothermal reaction time, say 6 h (W1), 12 h (W2), and 18 h (W3), in a reducing environment. A comparative performance evaluation reveals that a nanorod aspect ratio of 11.8 and a higher W 5+ / W 6+ ratio (∼1.96 for W 4f 7/2 and 1.44 for W 4f 5/2 ) for the W2 sample facilitate enhanced electrochemical charge storage and significantly affect the oxygen evolution reaction (OER) kinetics. This is mediated by the enhanced availability of electrochemically active sites for faradic charge transfer at the electrode–electrolyte interface. With W 5+ -rich tungsten oxide (W2)-coated nickel foam as a cathode in KOH electrolyte, the three-electrode cell configuration shows a capacity of 310 C/g at a current density of 1 A/g along with a cycling stability of over 15,000 cycles. The device performance in a hybrid asymmetric supercapacitor (ASC) configuration shows pseudocapacitive behavior. Having a comparatively higher electrochemical active surface area (ECSA), the W2 sample also performs as a better electrocatalyst with a Tafel slope of 58 mV/dec and an overpotential of 400 mV vs RHE to reach a current density of 100 mA/cm 2, along with a stability of up to 50 h. The nanorod morphology and crystal framework of the W2 electrode remain largely intact after prolonged OER testing.