Engineering Mn–Co–Cu hydroxide/oxyhydroxide electrode materials: rational composition optimization for enhanced supercapacitor performance

Abstract

Ternary mixed metal hydroxide/oxyhydroxide nanomaterials are an interesting class of pseudocapacitive electrode materials for supercapacitor applications, yet the influence of compositional variation on their electrochemical performance remains underexplored. In this work, a series of Mn–Co–Cu hydroxides/oxyhydroxides (marked as MnCoCu–x : y : z, with x : y : z = 1 : 1 : 1, 3 : 1 : 1, 5 : 1 : 1, 1 : 3 : 1, 1 : 5 : 1, 1 : 7 : 1, 1 : 9 : 1, 1 : 1 : 3, and 1 : 1 : 5, corresponding to Mn²⁺ : Co²⁺ : Cu²⁺ molar ratios) were synthesized via a simple co-precipitation method, along with the respective monometallic hydroxides or oxyhydroxides, MnO(OH), CoO(OH), and Cu(OH)₂, and selected bimetallic counterparts, MnCo–2 : 5 and CuCo–2 : 5. Examination of three-electrode supercapacitor performance of the as-prepared materials using 3 M KOH at 1 A g⁻¹ revealed the superior specific capacitance of MnCoCu–1 : 5 : 1 (1684 F g⁻¹) compared to the other materials, which followed the trend MnCoCu–1 : 1 : 5 (299 F g⁻¹) < MnCoCu–3 : 1 : 1 (404 F g⁻¹) < Cu(OH)₂ (477 F g⁻¹) < MnO(OH) (509 F g⁻¹) < MnCoCu–1 : 9 : 1 (608 F g⁻¹) < CoO(OH) (651 F g⁻¹) < MnCoCu–1 : 1 : 3 (683 F g⁻¹) < MnCoCu–5 : 1 : 1 (684 F g⁻¹) < CuCo–2 : 5 (828 F g⁻¹) < MnCoCu–1 : 1 : 1 (1084 F g⁻¹) < MnCoCu–1 : 3 : 1 (1124 F g⁻¹) < MnCoCu–1 : 7 : 1 (1204 F g⁻¹) < MnCo–2 : 5 (1321 F g⁻¹) – highlighting the significant synergistic effect of constituent metal hydroxides/oxyhydroxides in the as-prepared materials and underscoring the pivotal role of compositional tuning in enhancing the energy storage performance. Furthermore, the optimized MnCoCu–1 : 5 : 1 composite in a two-electrode asymmetric supercapacitor configuration demonstrated an appreciable energy storage performance, delivering a specific capacitance of 224 F g⁻¹, an energy density of 60.8 Wh kg⁻¹, and a power density of 280 W kg⁻¹ at 0.4 A g⁻¹. Moreover, this composition exhibited appreciable long-term cycling stability, retaining 90% of its initial capacitance and displaying a 92% coulombic efficiency up to 5000 charge–discharge cycles at 5 A g⁻¹. The superior electrochemical performance of MnCoCu–1 : 5 : 1 can be attributed to its optimal composition, offering favorable structural and electrical properties for facilitating efficient charge storage. These results highlight the significance of rational compositional engineering in ternary mixed metal hydroxide/oxyhydroxide composites for evolving next-generation high-performance supercapacitor electrodes.