A ¹H NMR investigation of a birnessite-like MnOx electrode

Manganese oxides (MnOx) represent an important and versatile class of electrode materials for electrochemical energy-storage technologies, particularly those based on aqueous electrolytes ranging from strongly alkaline to mildly acidic pH. The charge-storing functionality of such oxides can be further enhanced by expressing them in nanoscale forms incorporated into porous carbon substrates. Because the resulting MnOx@carbon hybrid materials are themselves structurally and compositionally complex, and charge-storage mechanisms in aqueous media may involve supporting electrolyte cations (e.g., Li⁺, Na⁺) or inevitably protons (from H₂O), a wide range of characterization tools is required to understand and optimize electrochemical function. In this study, we use ¹H NMR spectroscopy to probe the structural environments available to protons in such hybrids as synthesized by electroless deposition of birnessite-like MnOx onto carbon nanofoam papers (MnOx@CNFP). Two features are observed in the ¹H spectrum of this material, attributed to protons located at different distances away from the MnO₆ octahedral layers coating the carbon walls. We show that chemical exchange occurs between protons in these two environments in a two-step process. We also use ¹H chemical shift imaging of the MnOx@CNFP electrode in a device-like configuration and verify that this technique can isolate the signal of this electrode architecture in a device with multiple components (separator, electrolyte, counter electrode). These results showcase the ability of NMR to interrogate the structural environments of protons in an architected MnOx@CNFP electrode and serve as the foundation for future in situ/operando investigations of this system.