Sodium can serve as an alternative to lithium in rechargeable batteries as the reversible storage mechanisms for sodium ions are very similar (e.g., earlier post). Similarly, sodium has recently attracted attention as a replacement for lithium in alkali-metal-air batteries. These batteries are promising systems that provide very high theoretical energy densities; however, the use of pure alkali metals (both Li and Na) as anodes create safety and cost issues associated with their reactivity and the expense of the required dry-assembly process, the developers of this new hybrid fuel cell noted.

Thus, we have designed a novel energy conversion and storage system using seawater, or more precisely, the NaCl dissolved in seawater, as a sodium source. The use of naturally-abundant seawater as a sodium source renders unnecessary any additional processing and allows for a substantial reduction of the manufacturing cost for energy storage and conversion devices. The herein-reported device is an intermediate system between batteries and fuel cells and is thus referred to as a hybrid fuel cell.

Differing from conventional batteries, which comprise alkali-metal-containing intercalation or insertion materials as electrodes in a closed system, this novel concept gains its active material from seawater, which is circulated in the open cathode. Such an abundant supply of active material (sodium dissolved in seawater) enables the use of various alloying-anode materials, such as Si, Sn or Ge, overcoming the limitation introduced by the irreversibility of the first and, to a lesser extent, subsequent alloying processes.

… circulating seawater in an open-system electrode corresponds to a continuous supply of sodium ions, which gives this system superior cycling stability and allows the application of various anodes by compensating for irreversible charge losses. The negative electrode of this novel hybrid battery/fuel cell system is, instead, closed and separated from the open-seawater positive electrode by a NASICON solid electrolyte. The negative electrode might be composed of sodium metal, in metal-seawater configuration, or a sodium-ion host (e.g., an alloying material), in full sodium-ion configuration.

—Kim et al.

For the electrochemical studies in the paper, the fabricated the negative electrode from an 80:10:10 (wt.%) mixture of hard carbon or Sn-C, SuperP carbon black as a conductive additive, and poly(vinylidene fluoride) as a binder. Seawater containing NaCl was used as the positive electrode.

Charge–discharge curves of seawater-hybrid fuel cells with (a) hard carbon and (b) Sn/C anodes; (c) cycling performance recorded with negative electrodes in a hybrid-seawater fuel cell system (0.05 mA cm−2). Kim et al. Click to enlarge.

The non-aqueous liquid electrolyte on the negative-electrode side was a 1 M solution of NaClO₄ in ethylene carbonate/diethyl carbonate (DEC; 1/1 vol.%). As the solid electrolyte, a NASICON (Na₃Zr₂Si₂PO₁₂) ceramic electrolyte with a diameter of 20 mm and a thickness of 2 mm was used. The conductivity was determined to be σ_Na=9 × 10⁻⁴ S cm⁻1 at room temperature by dielectric spectrometry. A carbon paper with a thickness of 280 μm was used as the current collector in the positive-electrode compartment containing seawater. The Sn-C anode was kept in liquid electrolyte (1 M NaClO₄ in ethylene carbonate/DEC) for 1 day and washed by DEC for the stored Sn-C anode.

The redox processes at the cathode are:

Charge: 4NaCL → 4Na⁺ + 2Cl₂ + 4e^-

Discharge: 4Na⁺ + 2H₂O + O₂ + 4e^- → 4NaOH

During charging, the Na⁺ ions present at the cathode diffuse through the NASICON electrolyte and transfer to the negative electrode, with the release of gaseous Cl₂ is released. Upon discharge, the oxygen dissolved in seawater is reduced, resulting in the formation of NaOH in the presence of water and sodium ions. The participation of oxygen in the reduction reaction boosts the theoretical discharge potential to 3.11 V from 1.88 V in de-aerated water.

• With hard carbon as the sodium-ion negative-electrode, the discharge capacity (sodium uptake) was 114.4 mAh g⁻¹, whereas the irreversible capacity amounted to 60 mAh g⁻¹. This latter value decreased with increasing cycle number, although the reversible capacity slightly decreased. Such an electrochemical performance is typical of hard carbons, the authors said, indicating that seawater can serve as the source of sodium ions as well as conventional cathode materials in sodium-ion batteries.

• Anodes based on the high-capacity Sn-C nanocomposite showed a first-cycle irreversible capacity of ~200 mAh g⁻¹, and the reversible capacity was ~300 mAh g⁻¹. The team attributed the “rather high irreversible capacity” is to electrolyte decomposition at the particle surface, resulting in the formation of a solid-electrolyte interphase, as well as structural rearrangement occurring upon the first sodiation within the micron-sized composite particles. For subsequent cycles, the reversible capacity increased to >300 mAh g⁻¹ at the 5^th cycle, accompanied by a continuously decreasing irreversible capacity (~90 mAh g⁻¹ at the 5^th cycle).

Generally, the performance of both the anode materials (hard carbon and Sn-C nanocomposite) in combination with the seawater cathode is very stable upon continuous cycling, showing a remarkably low capacity fading of only 0.02% and 0% after 30 cycles for the hard carbon and Sn-C anode, respectively). These results again highlight the great advantage of an almost infinite supply of sodium ions by employing the open-system seawater cathode.

… hybrid fuel cells using seawater as the positive electrode show great promise as next-generation energy conversion and storage systems that allow both high energy density and low environmental impact at a low cost. In addition, this system can be easily scaled up. It appears noteworthy that the gaseous Cl₂ released upon charge might be trapped somehow and later utilized for other applications. Indeed, the production of gaseous Cl₂ might provide another great advantage of this technology, adding some value to this new device.

—Kim et al.

Resources

• Jae-Kwang Kim, Franziska Mueller, Hyojin Kim, Dominic Bresser, Jeong-Sun Park, Du-Hyun Lim, Guk-Tae Kim, Stefano Passerini and Youngsik Kim (2014) “Rechargeable-hybrid-seawater fuel cell” NPG Asia Materials 6, e144 doi: 10.1038/am.2014.106