The thermodynamic and operational boundaries to store electrical energy chemically are evaluated in this contribution. Methanol is considered as a candidate for chemical energy storage. The production of methanol from exhaust CO2 could be one way to recyle CO2 and lower the global CO2 emissions. Energetic analysis reveals that exergy losses are most severe in the parts of the system when electrical energy is converted to chemical (electrolysis) and when chemical energy is converted to electrical (power generation). In methanol production, the exergetic efficiency is 83.1%, when the chemical exergy of hydrogen and methanol, the exergy of the power input and the released heat are taken into consideration. The exergetic efficiency of the overall energy conversion-storage system including methanol as storage medium was evaluated to be between 16.2 and 20.0% depending on the applied conversion technology. Methanol is suitable not only as stationary energy storage, but it could also be used as fuel for transportation. The energy storage system with hydrogen as storage medium shows higher exergetic efficiency than the methanol route. However, the storage of hydrogen is clearly more complex and cost-intensive.

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CO2 methanation is often performed on Ni/Al2O3 catalysts, which can suffer from mass transport limitations and, therefore, decreased efficiency. Here we show the application of a hierarchically porous Ni/Al2O3 catalyst for methanation of CO2. The material has a well-defined and connected meso- and macropore structure with a total porosity of 78%. The pore structure was thoroughly studied with conventional methods, i.e., N2 sorption, Hg porosimetry, and He pycnometry, and advanced imaging techniques, i.e., electron tomography and ptychographic X-ray computed tomography. Tomography can quantify the pore system in a manner that is not possible using conventional porosimetry. Macrokinetic simulations were performed based on the measures obtained by porosity analysis. These show the potential benefit of enhanced mass-transfer properties of the hierarchical pore system compared to a pure mesoporous catalyst at industrially relevant conditions. Besides the investigation of the pore system, the catalyst was studied by Rietveld refinement, diffuse reflectance ultraviolet-visible (DRUV/vis) spectroscopy, and H2-temperature programmed reduction (TPR), showing a high reduction temperature required for activation due to structural incorporation of Ni into the transition alumina. The reduced hierarchically porous Ni/Al2O3 catalyst is highly active in CO2 methanation, showing comparable conversion and selectivity for CH4 to an industrial reference catalyst.

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