Capacitive mixing (CapMix) is the general name for several different electrode-based technologies being developed to produce electrical energy from salinity gradients. Electricity generation using a CapMix process is based on a cycle of charging and discharging electrodes. The electrodes in a CapMix device are sequentially exposed to two solutions that have large differences in salinity, such as naturally occurring freshwater and seawater, or solutions with salinity differences created using thermolytic salts and waste heat (for example, from industrial processes) in conventional technologies such as distillation.

CapMix was inspired by conventional energy storage devices, such as supercapacitors and batteries. In contrast to batteries, which require an external electrical power source to charge, CapMix cells are charged with using renewable sources of energy in the form of salinity gradients. CapMix processes were only recently invented in the past six years, and thus they are in an early stage of development. The power densities of CapMix processes have only reached ~0.2 Wm–2 based on the electrode projected area, which is a bit lower than other salinity gradient energy (SGE) technologies such as pressure-retarded osmosis (PRO, ~10 Wm–2) and reverse electrodialysis (RED, ~1 Wm–2) (based on total membrane area).

There have been three approaches to creating a CapMix device, which are differentiated based on how the electrodes charge: (1) capacitive double layer expansion (CDLE), (2) capacitive energy extraction based on the Donnan potential (CDP), (3) and mixing entropy battery (MEB). These three technologies are summarized in Table 1, along with the key original citations for these different concepts. For CDLE, a pair of bare porous carbon electrodes are charged due to the electrical potential produced from changes in the thickness of the electrical double layer (i.e., the layer of charge near the electrode surface that is different from the solution), which changes depending on the salt concentration in the solution. For the CDP process, an ion-exchange polymer coating (or ion exchange membrane) is placed on top of the electrodes, which produces a potential (i.e., the Donnan potential), which can charge the membranes in a high concentration solution and discharge in a low concentration solution. Both of these processes rely on the use of materials that build up charge on either the electrode (CDLE) or membrane (CDE) surface, but in both cases there are no chemical reactions. In contrast, the MEB process makes use of oxidation/reduction reactions, and thus it uses battery electrodes to drive salt concentration-dependent electrode potentials.

Table 1. Three different concepts of CapMix.

Electrical energy in a CapMix process is usually captured using a four-step cycle, in which the cell is alternatively charged and discharged in different salinity solutions (Figure 1a). For the CDLE and CDP processes, a high concentration salt solution is introduced to the cell, which decreases the cell voltage, and thus the charge increases on the electrodes at this lower cell voltage (step 1 & 2). Then, the solution inside is replaced by a low concentration salt solution, which increases the cell voltage, and current is discharged (step 3 & 4). Due to the voltage rise and drop upon changing the solution in a cycle, more energy can be collected in the low concentration solution than that used during charging in the high concentration solutions. Thus, there is net energy captured due to the differences between the voltages, which can be seen as a “voltage window” in Figure 1b. For either CDP or CDLE, energy can be harvested without electrical charging. However, more net energy can be captured in CDP by further charging the electrodes using a power source (‘Forced CDP’ in Figure 1b) as this increases the size of the voltage window. The same four-step cycle is also used in the MEB, but the direction of the voltage charging is the opposite of that for the capacitive systems, as the voltage is higher during discharge in the high salt solution, and lower in the low salt solution.

Figure 1. (a) Schematic diagram of the four-step cycle for harvesting energy and (b) voltage profiles during the cycle. (Source: Hatzell et al., Energy Environ. Sci. 7 (2014) 1159)

The main goal of the research being conducted at Penn State is to increase the power densities produced by the different CapMix processes. We have looked at how altering the properties of carbon-based electrodes affects the natural rise and fall potentials in high and low salt concentrations (CDLE- surface chemistry). We have also investigated CapMix energy production using flow electrodes (carbon particles that are suspended in water) that move between four different reactors (CDP- flow electrodes). Another way to increase power densities is combining CapMix with other processes. Previous work on combining another SGE technology (RED) with microbial fuel cells (MFCs), inspired us to examine using CapMix electrodes in a bioelectrochemical system (BES). The CapMix carbon electrodes coated with ion-exchange membranes were placed in a separate chamber located between the BES anode and cathode chambers (Figure 2a). When the CDP was operated while the BES produced energy, the power density of CDP was significantly enhanced to ~500 mWm-2 with NaCl solutions (versus from ~10 mW-2 without BES; see work on CapMix in BES).

We are currently looking at using thermolytic salts, such as ammonium bicarbonate (AmB), in CapMix processes. The advantage of AmB is that waste heat can be used to volatilize the two species in the form of ammonia and carbon dioxide gases (NH4HCO3(aq) à NH3(g) + CO2(g) + H2O(l)), so that they could then be condensed into a concentrated AmB solution, leaving behind a relatively dilute AmB solution. These engineered salinity differences in the two resulting solutions allows us to capture waste heat energy as electricity. Using AmB (rather than NaCl) in the CDP combined with the BES has increased the performance of the CapMix electrodes to ~900 mWm-2 (Figure 2b; CapMix in BES). We are trying to develop other CapMix approaches to make use of AmB for waste heat conversion to electricity.
Figure 2 Capmix

Figure 2. (a) Schematic diagram of CDP-BES system and (b) energy extracted as a function of BES current with NaCl and AmB. (Source: Hatzell et al., Energy Environ. Sci. 7 (2014) 1159)

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