What is limiting the potential window in aqueous sodium‐ion batteries? Online study of the hydrogen‐, oxygen‐ and CO₂‐evolution reactions at NaTi₂(PO₄)₃ and Na_0.44MnO₂ electrodes

Abstract NaTi 2 (PO 4 ) 3 (NTP) and Na 0.44 MnO (NMO), their derivatives, have emerged as the most promising materials for aqueous Na‐ion batteries. For both, NTP NMO, avoiding evolution of hydrogen oxygen is found to be mandatory in order mitigate material dissolution. Intriguingly, however, no direct determination reactions (HER OER) has yet been carried out. Using differential electrochemical mass spectrometry (DEMS) we directly identify onset potentials HER OER. Surprisingly, potential window significantly smaller than suggested by commonly employed cyclic voltammetry measurements. CO evolution, upon decomposition carbon black, observed at an 1.61 V RHE , which 0.25 more cathodic OER NMO electrode. Our results show that state‐of‐the‐art additive plays a crucial role stability positive electrode ion battery.