Effects of electrode thickness and crystal water on pseudocapacitive performance of layered birnessite MnO₂

Manganese dioxide (MnO₂) nanomaterials with two-dimensional (2D) layered birnessite structures are promising pseudocapacitive electrode materials. However, the effects of structural factors on their electrochemical performance is not fully understood. We synthesize alkali-free crystal water containing 2D layered birnessite MnO₂ electrodes with controllable mass loading from 0.1 to 19.3 mg cm^-2 to investigate the effects of electrode thickness and crystal water functions on crystal structure and pseudocapacitive behavior, to promote its industrialization. We find that the crystal water enlarges the interlayer space of birnessite MnO₂ with electrolyte ions transported much more easily, resulting in higher specific capacitance of 702 F g^-1 (70.2 mF cm^-2) and excellent cycling stability of 20 000 charge-discharge cycles even at a mass loading of up to 10.8 mg cm^-2. Such gains in specific capacitance are weakened significantly with raised mass loading. Thus, compared to a carbon cloth substrate, a carbon nanotube film with enhanced electron space transport capability presents better performance, indicating an effective strategy for higher mass loading cases. The present work sheds light on an efficient method for achieving high capacitance and mass loading together, for practical applications.