A n+ ] = 1 M concentration, we expand our computational analysis to investigate the stability of MnO 2 polymorphs across a broad range of solvent pH and voltage. Pourbaix diagrams, which plot the most stable phases at each value of pH and applied voltage, indicate how MnO 2 electrodes are likely to evolve under various conditions. However, the original Pourbaix diagram for the Mn oxide system published in the Pourbaix Atlas considered only select phases with Mn-O-H composition. It therefore remains unknown whether A x MnO 2 compounds are thermodynamically stable under relevant electrochemical conditions. It is also unclear how these compounds evolve with changing pH and voltage.