Tuning methanol oxidation activity: impact of Cu content on sputtered NiO thin film electrocatalysts

Electrochemical methanol oxidation (MOR) is a pivotal process for sustainable energy conversion in direct methanol fuel cells (DMFCs). This study reports the precision fabrication of Cu-doped NiO thin films on FTO substrates via a co-sputtering deposition technique, with Cu concentrations optimized at 2.5, 4, and 7%. Comprehensive characterization (XRD, SEM, EDS, and XPS) confirmed high crystallinity and a systematic increase in film thickness from 53 to 180 nm with higher Cu- loading. Optical analysis revealed a significant band gap narrowing from 3.64 eV (pure NiO) to 3.24 eV (7% Cu-doped NiO), indicating enhanced electronic conductivity. Electrochemical evaluations (CV, EIS, and CA) demonstrated that the 7% Cu-doped NiO catalyst exhibits superior electrocatalytic activity in 1.0 M NaOH, achieving the lowest onset potential of 0.37 V (vs. Ag/AgCl). Kinetic insights derived from Tafel analysis further support this trend, where the Tafel slope decreased from 178 mV dec −1 for pristine NiO to 120 mV dec −1 for the 7% Cu-doped NiO (linear sweep voltammetry at 5 mV s −1 ), corresponding to a reduction of approximately 32.5%. This reduction in the Tafel slope, coupled with a significantly diminished charge-transfer resistance, indicates faster interfacial electron transfer kinetics. The 7% Cu-doped NiO electrode not only optimized the Ni(II)/Ni(III) redox cycle but also demonstrated exceptional stability across various methanol concentrations (0.25–2.0 M). These findings position Cu-doped NiO as a high-performance, cost-effective electrocatalyst, offering a robust strategy for advancing alkaline DMFC technologies .