A Study of MnO₂ with Different Crystalline Forms for Pseudocapacitive Desalination

Recent research on materials for capacitive deionization (CDI) has shown that intercalation materials have salt removal capacities (>40 mg g^-1) much higher than those of carbon materials (∼15 mg g^-1). However, little work has been done to elucidate the relationship between the microstructure of an intercalation material and its desalination performance. Herein, we report the desalination performances of various crystalline forms of MnO₂ in a hybrid CDI setup without the use of ion-exchange membranes. MnO₂ materials used in our experiments were either poorly crystalline or crystalline forms of 1D hollandite α-MnO₂, 2D birnessite δ-MnO₂, and 3D spinel λ-MnO₂. X-ray photoelectron spectroscopy performed on electrochemically cycled MnO₂ showed redox changes associated with intercalation processes in crystalline MnO₂, whereas poorly crystalline MnO₂ showed no such changes. It was further shown that surface adsorption is dominant in poorly crystalline MnO₂ and that poorly crystalline forms of α-MnO₂ and δ-MnO₂ exhibited the highest salt removal capacities of 0.17 and 0.16 mmol g^-1 (9.93 and 9.35 mg g^-1), respectively. These performances are comparable to state-of-the-art carbon materials and are remarkable considering the low surface areas (<400 m² g^-1) of MnO₂ materials.