<p>Pure and (0.02, 0.04 and 0.06&#xa0;M) Cu²⁺doped NiO nanoparticles (NPs) were synthesized via hydrothermal method and characterized for supercapacitor and photocatalytic applications. XRD analysis confirmed the cubic NiO phase with slight lattice contraction and reduced crystallite size upon Cu²⁺ doping. FESEM images showed spherical, agglomerated sheet-like morphology, with decreasing particle size as doping increased, while EDAX confirmed the presence of Ni, O, and Cu without impurities. UV-Vis DRS confirmed a direct transition with the bandgap decreasing from 3.15&#xa0;eV to 2.67&#xa0;eV with increasing doping concentration. XPS confirmed Cu²⁺ incorporation in NiO lattice with mixed Ni²⁺/Cu²⁺ states, oxygen vacancies, and surface hydroxyl groups. BET analysis showed a mesoporous structure with reduced surface area after Cu²⁺ doping, while VSM confirmed weak ferromagnetism due to defect and oxygen vacancy-induced spin alignment. Electrochemical evaluations showed that 0.06&#xa0;M Cu²⁺ doped NiO NPs exhibit enhanced pseudocapacitive behavior due to additional <InlineEquation ID="IEq1"> <EquationSource Format="TEX">\(\:{Cu}^{2+}/{Cu}^{3+}\)</EquationSource> </InlineEquation> and <InlineEquation ID="IEq2"> <EquationSource Format="TEX">\(\:{Ni}^{2+}/{Ni}^{3+}\)</EquationSource> </InlineEquation> redox reactions and delivered higher specific capacitance of 421.85&#xa0;F g^− 1 (CV at 10 mV s^− 1) and 351.13&#xa0;F g^− 1 (GCD at 1&#xa0;A g^− 1), while EIS showed a reduced charge transfer resistance compared to pure NiO. The (0.06&#xa0;M) Cu²⁺ doped NiO NPs demonstrated superior photocatalytic degradation of Congo Red (CR) dye under visible sunlight, achieving 78.35% degradation within 80&#xa0;min, twice than pure NiO (41.15%). Thus, the optimized Cu²⁺ doped NiO NPs exhibit strong potential for photocatalytic remediation and high performance supercapacitor applications.</p>