Time-Scale-Resolved Capacitance and Relaxation-Time Analysis of Glucose Electrooxidation on NiO Electrodes

Electrochemical impedance spectroscopy (EIS) is one of the essential tools for probing interfacial charge transfer in electrochemical systems. Transforming impedance data into alternative domains can reveal hidden processes that are masked in the frequency domain. In the present study, we investigate glucose oxidation on NiO by combining electrochemical capacitance spectroscopy (ECS) and the distribution of relaxation times (DRT). NiO was prepared via thermal decomposition and characterized structurally. Then, we show how capacitance analysis separates double layer and pseudocapacitive contributions, while DRT provides time-resolved insights into the individual electrochemical steps. Using ECS, we observe that increasing glucose concentration (0 to 5 mM) enhances pseudocapacitance and reduces charge transfer resistance. ECS also reveals that the low frequency capacitance increases linearly with glucose concentration, whereas the double layer capacitance remains constant. On the other hand, DRT spectra exhibit two peaks, which we attribute to two processes: a fast process associated with OH^- adsorption and a slower process attributed to direct glucose oxidation. The dominant peak shifts to shorter relaxation times and decreases in amplitude with increasing glucose, consistent with accelerated kinetics and lower resistance. DRT signatures of ascorbic acid, dopamine and uric acid differ from that of glucose, demonstrating selectivity. These findings establish, for the first time, the DRT as a powerful time scale domain transduction method that critically complements ECS, and offers mechanistic insights for the advanced design of non-enzymatic glucose sensors for diabetes monitoring.