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

NaTi2(PO4)(3) (NTP) and Na0.44MnO2 (NMO), and their derivatives, have emerged as the most promising materials for aqueous Na-ion batteries. For both, NTP and NMO, avoiding the evolution of hydrogen and oxygen is found to be mandatory in order to mitigate material dissolution. Intriguingly, however, no direct determination of the hydrogen and oxygen evolution reactions (HER and OER) has yet been carried out. Using differential electrochemical mass spectrometry (DEMS) we directly identify the onset potentials for the HER and OER. Surprisingly, the potential window is found to be significantly smaller than suggested by commonly employed cyclic voltammetry measurements. CO2 evolution, upon decomposition of carbon black, is observed at an onset potential of 1.61 V-RHE, which is 0.25 V more cathodic than the OER for the NMO electrode. Our results show that the state-of-the-art carbon additive plays a crucial role in the stability of the positive NMO electrode in the ion battery.