Thermal energy was shown to be efficiently converted into electrical power in a thermally regenerative ammonia-based battery (TRAB) using copper-based redox couples [Cu(NH 3 ) 4 2+ /Cu and Cu(II)/Cu].Ammonia addition to the anolyte (2 M ammonia in a copper-nitrate electrolyte) of a single TRAB cell produced a maximum power density of 115 AE 1 W m À2 (based on projected area of a single copper mesh electrode), with an energy density of 453 W h m À3 (normalized to the total electrolyte volume, under maximum power production conditions). Adding a second cell doubled both the voltage and maximum power. Increasing the anolyte ammonia concentration to 3 M further improved the maximum power density to 136 AE 3 W m À2 . Volatilization of ammonia from the spent anolyte by heating (simulating distillation), and re-addition of this ammonia to the spent catholyte chamber with subsequent operation of this chamber as the anode (to regenerate copper on the other electrode), produced a maximum power density of 60 AE 3 W m À2 , with an average discharge energy efficiency of $29%(electrical energy captured versus chemical energy in the starting solutions). Power was restored to 126 AE 5 W m À2 through acid addition to the regenerated catholyte to decrease pH and dissolve Cu(OH) 2 precipitates, suggesting that an inexpensive acid or a waste acid could be used to improve performance.These results demonstrated that TRABs using ammonia-based electrolytes and inexpensive copper electrodes can provide a practical method for efficient conversion of low-grade thermal energy into electricity. Broader contextThe utilization of waste heat for power production would enable additional electricity generation without any additional consumption of fossil fuels. Thermally regenerative batteries (TRBs) allow a carbon neutral approach for the storage and conversion of waste heat into electrical power, with potentially lower costs than solid-state devices. Here we present a highly efficient, inexpensive, and scalable ammonia-based TRB (TRAB) where electrical current is produced from the formation of copper ammonia complex. The ammonia can then be captured and concentrated by distillation of the anolyte, allowing recharge of the system. The voltage created by ammonia addition in the anolyte results in copper deposition onto the cathode, and loss of copper from the anode. However, by reversing the function of electrodes in the next cycle, there is no net loss of copper. With a 3 M anolyte ammonia, a TRAB produced the highest power density ever obtained for an aqueous-based, thermoelectrochemical system, of 136 AE 3 W m À2 . This power density was substantially higher than those produced using salinity gradient energy technologies based on generating salty and less-salty solutions using waste heat. This TRAB technology therefore represents a new and promising approach for efficient harvesting of low-grade waste heat as electrical power.

Several technologies, including pressure-retarded osmosis (PRO), reverse electrodialysis (RED), and capacitive mixing (CapMix), are being developed to recover energy from salinity gradients. Here, we present a new approach to capture salinity gradient energy based on the expansion and contraction properties of poly(acrylic acid) hydrogels. These materials swell in fresh water and shrink in salt water, and thus the expansion can be used to capture energy through mechanical processes. In tests with 0.36 g of hydrogel particles 300 to 600 μm in diameter, 124 mJ of energy was recovered in 1 h (salinity ratio of 100, external load of 210 g, water flow rate of 1 mL/min). Although these energy recovery rates were relatively lower than those typically obtained using PRO, RED, or CapMix, the costs of hydrogels are much lower than those of membranes used in PRO and RED. In addition, fouling might be more easily controlled as the particles can be easily removed from the reactor for cleaning. Further development of the technology and testing of a wider range of conditions should lead to improved energy recoveries and performance. ■ INTRODUCTIONSalinity gradients that naturally exist between seawater and river water could provide a large and renewable resource for clean energy production. The theoretical energy of mixing 1 m 3 of river water with a much larger volume of seawater is about 2.5 MJ, which is equivalent to the energy that could be captured from water flowing over a dam more than 250 m in height. 1,2 Worldwide, the potential power production from salinity gradients is estimated to be 1.4−2.6 TW, which is comparable to the current global demand for electrical power (∼2 TW). 3−5Several technologies have been developed to capture salinitygradient energy, including pressure-retarded osmosis (PRO), 6−8 reverse electrodialysis (RED), 9−11 and capacitive mixing (CapMix).12−14 In PRO, water from a low salinity solution (river water) permeates into the highly saline solution (seawater) across a semipermeable membrane, driven by the osmotic pressure difference. This water flow pressurizes the seawater, which can then be used to generate electricity using a hydroturbine.6,15 A RED process is based on using a stack of alternating cation (CEM) and anion exchange membranes (AEM). When waters with different salinities flow through channels separated by CEMs and AEMs, a voltage of ∼0.1 to 0.2 V is generated across each membrane pair due to the ion flux driven by the differences in salt concentrations. This ionic flux is then converted into electrical current through oxidation− reduction reactions at the electrodes. 9,16 The main disadvantage of PRO and RED is that they require use of large surface areas of expensive membranes that foul over time and that can be difficult to effectively clean. CapMix is a relatively new approach to capture energy from solutions with different salinities that does not necessarily require membranes. In this process, seawater and river water alternately are exposed to either plain capacitiv...

A thermally regenerative ammonia battery (TRAB) is a new approach for converting low-grade thermal energy into electricity by using an ammonia electrolyte and copper electrodes. TRAB operation at 72 °C produced a power density of 236 ± 8 Wm(-2), with a linear decrease in power to 95 ± 5 Wm(-2) at 23 °C. The improved power at higher temperatures was due to reduced electrode overpotentials and more favorable thermodynamics for the anode reaction (copper oxidation). The energy density varied with temperature and discharge rates, with a maximum of 650 Wh m(-3) at a discharge energy efficiency of 54% and a temperature of 37 °C. The energy efficiency calculated with chemical process simulation software indicated a Carnot-based efficiency of up to 13% and an overall thermal energy recovery of 0.5%. It should be possible to substantially improve these energy recoveries through optimization of electrolyte concentrations and by using improved ion-selective membranes and energy recovery systems such as heat exchangers.

Applications of microbial fuel cells (MFCs) are limited in part by low power densities mainly due to cathode performance. Successful immobilization of an Fe-N-C co-catalyst on activated carbon (Fe-N-C/AC) improved the oxygen reduction reaction to nearly a four-electron transfer, compared to a twoelectron transfer achieved using AC. With acetate as the fuel, the maximum power density was 4.7±0.2 W m(-2) , which is higher than any previous report for an air-cathode MFC. With domestic wastewater as a fuel, MFCs with the Fe-N-C/AC cathode produced up to 0.8±0.03 W m(-2) , which was twice that obtained with a Pt-catalyzed cathode. The use of this Fe-N-C/AC catalyst can therefore substantially increase power production, and enable broader applications of MFCs for renewable electricity generation using waste materials.

A review of the literature using cube-type microbial fuel cell reveals the extent in variability of power production.

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Air cathodes used in microbial fuel cells (MFCs) need to have high catalytic activity for oxygen reduction, but they must also be easy to manufacture, inexpensive, and watertight. A simple one-step, phase inversion process was used here to construct an inexpensive MFC cathode using a poly­(vinylidene fluoride) (PVDF) binder and an activated carbon catalyst. The phase inversion process enabled cathode preparation at room temperatures, without the need for additional heat treatment, and it produced for the first time a cathode that did not require a separate diffusion layer to prevent water leakage. MFCs using this new type of cathode produced a maximum power density of 1470 ± 50 mW m–2 with acetate as a substrate, and 230 ± 10 mW m–2 with domestic wastewater. These power densities were similar to those obtained using cathodes made using more expensive materials or more complex procedures, such as cathodes with a polytetrafluoroethylene (PTFE) binder and a poly­(dimethylsiloxane) (PDMS) diffusion layer, or a Pt catalyst. Even though the PVDF cathodes did not have a diffusion layer, they withstood up to 1.22 ± 0.04 m of water head (∼12 kPa) without leakage, compared to 0.18 ± 0.02 m for cathodes made using PTFE binder and PDMS diffusion layer. The cost of PVDF and activated carbon ($3 m–2) was less than that of the stainless steel mesh current collector ($12 m–2). PVDF-based AC cathodes therefore are inexpensive, have excellent performance in terms of power and water leakage, and they can be easily manufactured using a single phase inversion process at room temperature.

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