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Electrochemical CO2 capture approaches, where electrochemical reactions control the sorbent’s CO2 affinity to drive subsequent CO2 absorption/desorption, have gained substantial attention due to their low energy demands compared to temperature-swing approaches. Typically, the process uses separate electrochemical and mass-transfer steps, producing a 4-stage (cathodic/anodic, absorption/desorption) process, but recent work proposed that these energy demands can be further reduced by combining the electrochemical and CO2 mass-transfer reactor units. Here, we used computational models to examine the practical benefit of combining electrochemical sorbent reactivation with CO2 absorption due to this combination’s implicit assumptions about the process rate and therefore, the reactor size and cost. Comparing the minimum energy demand and process time of this combined reactor to those of the separated configuration, we found that the combined absorber can reduce the energy demand by up to 67% but doing so can also increase the process time by several orders of magnitude. In contrast, optimizing the solution chemistry could benefit both the energy demand and process time simultaneously. 

Prussian blue analogs (PBAs) are used as electrode materials in energy storage and water deionization cells due to their reversible cation intercalation capability. Despite extensive research on their performance and intercalation mechanisms, little attention has been given to their behavior under open-circuit conditions. Recent studies using symmetrical PBA electrodes in two electrode deionization cells reported that after constant current cycling in dilute NaCl (<0.2 M), the cell voltage dropped under open-circuit conditions, which substantially increased the amount of energy consumed for deionization. However, it remains unclear which electrode (anode/cathode) experienced potential drift and if it was influenced by the low salinity of the electrolyte. Here, we performed a series of electrochemical experiments under different charging and discharging regimes and electrolyte compositions to determine the processes that contributed most significantly to open-circuit potential drift. The data indicated that charge redistribution within the electrode was the main contributor to open circuit potential drift, with electrode dissolution and parasitic reactions playing negligible roles. A one-dimensional finite element model was constructed to simulate charge redistribution by accounting for cation diffusion under open-circuit conditions. The open-circuit potential profiles generated by the model were validated against experimental trends, confirming the occurrence of charge redistribution. A Monte Carlo analysis of the model was conducted to determine the relationship of potential drift to key factors such as applied current, electrode thickness, diffusion coefficient of intercalating ions, and intercalation capacity. Subsequently, a dimensionless number (Da) was developed based on the Dahmköhler number to relate the extent of potential drift resulting from combinations of these factors. The analyses revealed a strong positive correlation between simulated potential drift andDa. Among the key factors studied here, the diffusion coefficient and applied current had the largest impact onDaand, consequently, on potential drift. 

Several capacitive deionization (CDI) cell architectures employ ion-exchange membranes to control the chemistry of the electrolyte contacting the electrodes. Here, we experimentally examined how exposing carbon electrodes to either a saline electrolyte or an electrolyte containing a soluble redox-active compound influenced deionization energy demands and long-term stability over ∼50 hours. We specifically compared the energy demands (W h L −1 ) required to deionize 20 mM NaCl to 15 mM with a 50% water recovery as a function of productivity (L m −2 h −1 ). Relative to a conventional membrane capacitive deionization (MCDI) cell, flowing saline electrolyte over the electrodes did not affect energy demands but increased electrode salt adsorption capacities and capacity retention over repeated cycles. Exposing the electrodes to an electrolyte containing a redox-active compound, which made the cell behave similarly to an electrodialysis system, dramatically reduced energy demands and showed remarkable stability over 50 hours of operation. These experimental results indicate that using a recirculated soluble redox-active compound in the electrolyte contacting the electrodes to balance charge leads to far more energy efficient brackish water deionization than when charge is balanced by the electrodes undergoing capacitive charging/discharging reactions. 

The decreasing cost of electricity produced using solar and wind and the need to avoid CO 2 emissions from fossil fuels has heightened interest in hydrogen gas production by water electrolysis. Offshore and coastal hydrogen gas production using seawater and renewable electricity is of particular interest, but it is currently economically infeasible due to the high costs of ion exchange membranes and the need to desalinate seawater in existing electrolyzer designs. A new approach is described here that uses relatively inexpensive commercially available membranes developed for reverse osmosis (RO) to selectively transport favorable ions. In an applied electric field, RO membranes have a substantial capacity for proton and hydroxide transport through the active layer while excluding salt anions and cations. A perchlorate salt was used to provide an inert and contained anolyte, with charge balanced by proton and hydroxide ion flow across the RO membrane. Synthetic seawater (NaCl) was used as the catholyte, where it provided continuous hydrogen gas evolution. The RO membrane resistance was 21.7 ± 3.5 Ω cm 2 in 1 M NaCl and the voltages needed to split water in a model electrolysis cell at current densities of 10–40 mA cm −2 were comparable to those found when using two commonly used, more expensive ion exchange membranes.