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Chapter 3. Sustainable Hydrocarbon Fuels by Recycling CO2 with Renewable/Nuclear Energy 58 estimate based on hydrogen production using the latest promising thermochemical cycles versus using commercial alkaline electrolysis cells concluded that, when driven by the same concentrated solar thermal power source, the technologies have comparable costs [89]. The achievable electrolysis cost for a given electricity cost may be lower than estimated in that study because high temperature electrolysis has the potential to be significantly cheaper than today’s commercial alkaline electrolysis [32]. 3.2.2.3. Electrolysis An electrolysis cell dissociates H2O or CO2 using electricity. Attractive in its simplicity, electrolysis performs the dissociation in a single step without any need for moving parts, and the products are released separately in the anode and cathode compartments of the cell. For solar conversion, a PV cell could be coupled to an electrolysis cell, which would comprise a full system without moving parts. Thermolytic and thermochemical reactors, on the other hand, need a mechanical sun-tracking system to concentrate the direct incident sunlight to collect high temperature heat. Electrolysis cells, like photovoltaic cells, are ideal for mass production and automated maintenance. An especially interesting property of an electrolysis cell is the ability to choose the efficiency at which it operates. For example, the enthalpy of H2O dissociation corresponds to a cell voltage of 1.48 V whereas the free energy corresponds to a H2O electrolysis reversible potential of 1.23 V at 25 °C and 0.96 V at 850 °C (Figure 3-4). Assume for a moment that the internal resistance is a constant 1 Ω at any applied voltage for both a low-temperature cell (25 °C) and a high-temperature (850 °C) cell, and that the open-circuit voltages (OCV) are at the reversible potentialsv. Then, isothermal operation of the cells, by applying the thermoneutral voltage of 1.48 Vvi, yields currents of 0.25 A [= (1.48 V – 1.23 V) / 1 Ω] and 0.52 A [= (1.48 V – 0.96 V) / 1 Ω] respectively. Both cells are being run at 100% efficiency in electricity-to- chemical energy conversion. From the higher rate of hydrogen production it is clear the high temperature cell has an advantage – even for the same internal resistance, more hydrogen is being produced because part of the dissociation energy is supplied as heat (the difference in ΔH and ΔG shown in Figure 3-4), which is being supplied as the inevitable Joule heat produced by the internal electrical resistance of the cell (alternatively, high- temperature heat from an v This assumes that the activities or partial pressures of the reactants and products (H2, O2, and H2O) are such that the Nernst potential is the same as the reversible potential—that the equilibrium constant in the Nernst equation is 1, e.g. for a high temperature cell, pH2/pH2O = 1 and pO2 = 1 are assumed. In reality, for practical implementation it would be desirable to supply a much higher H2O content, e.g. pH2/pH2O should approach zero, resulting in an OCV lower than 0.96 V, which would increase the current to greater than 0.52 A in the calculation that follows. vi Note that the high temperature cell is electrolyzing steam. The thermoneutral voltage for the steam electrolysis reaction at 850 °C is 1.29 V, but the thermoneutral voltage for the cell is still 1.48 V if one includes the heat exchanger that is needed to generate steam for the cell if liquid water is the input.PDF Image | Electrolysis of CO2 and H2O
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