Display omitted •Isothermal PbO/PbCO3·2PbO energy storage system based on CO2 pressure cycling.•MnO and PbO carbonate below 50 bar of CO2 pressure between 25 and 500 °C.•Moisture is crucial for carbonation of the metal oxides.•Chemical history and origin govern performance of thermochemical storage materials. Metal carbonates are attractive materials for combining carbon capture and thermochemical energy storage. Carbonate materials feature high decomposition and formation temperatures and may be considered in applications in combination with concentrating solar power. In the present study in-situ P-XRD carbonation (1–8 bar CO2) and reactor-based experiments (1–55 bar CO2) are combined focusing on the effect of elevated CO2 pressures on carbonation of metal oxides. Carbonation of MnO and PbO at CO2 pressures between 8 and 50 bar in the presence of moisture resulted in reaction with CO2, forming the corresponding carbonates at notably lower temperatures than under dry CO2 atmosphere of 1 bar. This enables the application of metal oxide/metal carbonate reaction couples for energy storage at temperatures between 25 and 500 °C. Based on the reversible carbonation/decarbonation of PbO under varying CO2 pressures, an isothermal storage cycle between PbO/PbCO3·2PbO, triggered by changing the CO2 pressure between 2 and 8 bar, was developed.