The Gibbs free energy released during mixing solutions of different salinities, widely known as salinity gradient energy (SGE), can be harvested for useful work.(1?4) The global potential of SGE is estimated to be approximately 2 TW on the basis of mixing fresh water (e.g., major rivers) with saline ocean water.(1,2) SGE can be harvested by reverse electrodialysis (RED) process, in which cations and anions selectively move through ion exchange membranes (IEMs) under their respective concentration gradients to generate electricity.(3,5?8) RED can be highly attractive due to low operation pressure,(2,9) abundant feedwater resources,(10?12) and low energy conversion loss.(1,13?15) Existing literature has addressed various aspects of RED ranging from the synthesis of ion exchange materials/membranes,(16?19) development of RED modules,(14,20?25) and design and optimization of RED processes.(5,12,26?29)

Major developments have occurred in recent years, extending RED applications far beyond the simple mixing of freshwater with seawater. For example, Logan and co-workers formulated the concept of RED osmotic heat engine.(30?33) In this novel approach, a low-grade heat is used to generate extremely high salinity gradients (e.g., with a salinity ratio of ?800 using ammonia bicarbonate solutions), which can be converted into electricity in a subsequent RED step.(31?34) Compared to conventional RED based on freshwater/seawater mixing, the high salinity gradients engineered in such osmotic heat engines translate into significantly enhanced power generation.(30,31) Similarly, the hybridization of RED and bioelectrochemical process can be used to oxidize organic matter in wastewater, which produces more electricity at improved efficiency due to the simultaneous recovery of SGE and bioenergy of biomass and the reduction of overpotential at the electrodes.(30,31,35,36) Many additional promising alternatives have been reported in recent years, such as concentration flow cells(37?39) and reverse osmosis-RED hybridization.(40?44)

An interesting opportunity exists for coupling RED with controlled chemical reactions for enhanced energy production. For example, industrial wastewaters often contain large quantities of waste acid/base, and their controlled mixing in an RED process has the potential to greatly enhance the power generation. Conceptually, one can arrange the acid solution and the base solution in an alternative manner, separated by a compartment for neutralization (Figure 1). The neutralization reaction of H+ and OH– greatly reduces their concentration in this compartment (), leading to additional salinity gradient caused by these ions. The total voltage is then contributed by the H+ and OH– gradients, in addition to the Na+ and Cl– gradients due to Na+ from NaOH and Cl– from HCl, respectively (eq 1):



where N represents the number of the repeating units, ? indicates the permselectivity of IEMs, R is gas constant (8.314 J/mol·K), T indicates the absolute temperature (e.g., 298 K in this study), z refers to the charge of the salt ions (e.g., z = 1 for Na+ and Cl–), F is Faraday constant (96 485 C/mol), a is the activity of a solution with the subscript AS, BS, and NS indicating acid, base, and neutral solutions, respectively. According to eq 1, the total energy production in an RED neutralization cell originates from two driving forces: the contribution from salt ions such as Na+ and Cl– in a similar fashion to a conventional RED process, and that from salinity gradients of H+ and OH–.

In the current study, we demonstrate the feasibility of reaction-enhanced RED process using controlled neutralization of HCl and NaOH. We systematically investigated the critical factors governing the performance of the RED chemical cell. The mechanistic insights of the effects of salt ions uphill transport on power generation were gained in this study. These findings have important implications to the incorporation of chemical reaction with RED for enhanced power production.

Figure 1. (a) The schematic diagram of a repeating unit of the RED chemical cell, comprising alternatively profiled acid and base solutions separated by electrolyte solution for neutralization, the salinity ratios of H+/OH– over IEMs is greatly enhanced due to their neutralization reaction in neutral compartment, compared to the salinity ratios of Na+/Cl– across IEMs in a conventional RED process, (b) the theoretical polarization curves of RED neutralization cell (REDn) using 0.1 M HCl/NaOH with a 0.01 M NaCl as the neutral solution. For comparison, a conventional RED (REDc) is also presented where the acid and base are both replaced by 0.1 M NaCl.

Figure 2. Voltage output (a) and power production (b) of REDc and REDn. The empty symbols represent the REDc using 0.1 and 0.01 M NaCl solutions as the salinity source. The solid symbols represent the REDn using 0.1 M HCl and 0.1 M NaOH as acid and base solutions, respectively, and 0.01 M NaCl solution as neutral solution. For REDc (empty symbols), the acid and base compartments were both filled by 0.1 M NaCl, and 0.01 M NaCl was used in the neutral compartment. The error bar represents the standard deviation of three parallel experiments.

Figure 3. Open circuit voltage of the REDn cell with and without air sparging in neutralization solution compartment.

Figure 4. Voltage output (a) and power production (b) of REDn operating with pure acid/base solutions (AS/BS) and mixture of AS/BS and NaCl solutions. The solid symbols represent the stack using pure AS/BS of 0.03, 0.1, or 0.3 M, and the empty symbols represent the stack using mixture of AS/BS and NaCl solutions (?: ionic strength of 0.3 M (0.1 M AS/BS + 0.2 M NaCl); ?: ionic strength of 0.1 M (0.03 M AS/BS + 0.07 M NaCl)). The feed streams of neutral compartments were 0.01 M NaCl for all cases. The error bar represents the standard deviation of three parallel experiments.

Figure 5. Open circuit voltage (a), power production (b), and internal resistance (c) of the REDc stack (empty symbol) and REDn stack (solid symbol) as a function of the neutral solution (NS) concentrations (0.001–0.02 M NaCl). Either 0.1 M NaCl (in the case of REDc) or 0.1 M HCl/NaOH (in the case of REDn) was as the feed streams of acid/base compartments. The dashed lines in Figure 4 has a constant slope of 0.42 V per log increase in the neutral solution concentration, as predicted by the Nernst equation. The error bar represents the standard deviation of three parallel experiments.

Figure 6. Increasing molarity of Cl– (in the base solution) and Na+ (in the acid solution) in an REDn due to their respective uphill transport from the neutral solution. The experiments were performed at a constant current density of 4.8 A/m2. 0.1 M HCl/NaOH was as the feed streams of acid/base compartments. The neutral solution was 0.001, 0.002, 0.005, 0.01, or 0.02 M NaCl. The error bar represents the standard deviation of three parallel experiments.

This study reports a novel REDn cell for enhanced power production by taking advantage of the additional salinity gradient derived from acid/base neutralization. This technology can be potentially used to recover energy while treating waste acid/base. Potential environmental applications include the treatment of the highly acidic solutions of acid mine drainage(57,58) and landfill leachate(59,60) (e.g., neutralization with lime or carbonate salts(61?63)), provided that the mass balance of these streams can be justified. In addition, acidic and alkaline chemical wastes from a wide range of industries (food processing,(64?66) iron and steel industry,(67?70) etc.) can be potentially used. Nevertheless, future studies are needed to further address the challenge of membrane fouling(71?73) and alkali resistance(74?77) in such harsh environments. Whereas ion exchange membranes can withstand strongly acidic conditions, they tend to have weaker resistance to bases. Luckily, there have been promising progresses on the fabrication of base-resistant ion exchange membranes.(74,75) Though the current study has focused on strong acid and base solutions of identical concentrations and equal volumes, it would be interesting to further explore the use of solutions with different concentrations and volumes as well as weak acids/bases (e.g., lime instead of NaOH). One may also take advantage of the different transport rates of H+ and OH– through CEM and AEM to further optimize the power performance.(78) In addition to the hybridization of the RED technology with acid/base neutralization, the concept of RED chemical cell can be further extended to other types of reactions, such as metal–ligand coordination chemistry.(69) Future studies need to systematically assess the technical and economic feasibility of such opportunities.