Surface pH modulation of amorphous manganese oxide for enhanced supercapacitor performance

Abstract

Manganese oxide shows great promise as an electrode material for supercapacitors due to its high theoretical specific capacitance, cost-effectiveness, abundant oxygen vacancies, and adjustable surface characteristics. Current research extensively focuses on incorporating external dopant ions to create defects, aiming to enhance the inherent low efficiency of electron/ion transport in manganese oxide. Nevertheless, this strategy also modifies the surface charge of manganese oxide, a parameter often overlooked. This study effectively regulated the surface pH of oxidized manganese through swift redox reactions in an acidic or alkaline environment. The acidic environment enhances the deprotonation of the manganese oxide surface, reducing the surface pH, whereas alkaline conditions elevate the surface pH. Through the lower surface pH, manganese oxide exhibits a remarkable enhancement in its pseudocapacitive performance. When tested at a scanning rate of 5 mV s⁻¹, K–AMO displays a specific capacitance of 258.88 F g⁻¹ and sustains 104% of its capacitance over 10 000 cycles. The symmetric supercapacitor developed by K–AMO achieves a peak energy density of 56.11 Wh kg⁻¹ at a power density of 1000 W kg⁻¹, showcasing outstanding cycling durability and offering significant potential for practical application. The influence of surface pH on the electronic structure and ion transport kinetics of manganese oxide is elucidated using ex situ XPS analysis and in situ ATR-SEIRAS. Decreasing the surface pH activates additional electrochemically active sites, improves H⁺ and cation insertion/extraction reactions, and boosts pseudocapacitance contribution. This study introduces a novel interface design for amorphous manganese oxide-based supercapacitors by employing surface pH modulation strategies. It investigates the combined influences of counter cations and surface chemistry, laying the groundwork for improving the performance of energy storage materials.