Scientists at Ulsan National Institute of Science and Technology (UNIST) in Korea and Karlsruhe Institute of Technology in Germany have developed a new energy conversion and storage system using seawater for the cathode in a fuel cell.

We’ve seen that sodium can serve as an alternative to lithium in rechargeable batteries. That’s because the reversible storage mechanisms for sodium ions are very similar. Sodium is also attracting attention as a replacement for lithium in alkali-metal-air batteries.

Seawater Sodium Chloride Fuel Cell Layout.  Schematic illustration of the designed hybrid-seawater fuel cell and a schematic diagram at the charged–discharged state. Click image for the largest viw.

Seawater Sodium Chloride Fuel Cell Layout. Schematic illustration of the designed hybrid-seawater fuel cell and a schematic diagram at the charged–discharged state. Click image for the largest view.

The Korean-German team’s paper has been published in the journal NPG Asia Materials as open access, available to everyone. The new system is described as an intermediate between a battery and a fuel cell, and is accordingly referred to as a hybrid fuel cell.

The circulating seawater in the open-cathode system results in a continuous supply of sodium ions. The circumstances allow the system to operate with superior cycling stability allowing the application of various alternative anodes to sodium metal by compensating for irreversible charge losses.

Hard carbon and tin-carbon nanocomposite electrodes were successfully applied as anode materials, yielding highly stable cycling performance and reversible capacities exceeding an initially impressive 110 mAh g−1 with hard carbon and 300 mAh g−1 with tin-carbon electrodes.

Both lithium and sodium batteries are promising systems that provide very high theoretical energy densities. But the use of pure alkali metals such as lithium and sodium for the anodes creates safety and cost issues associated with their reactivity and the expense of the required dry-assembly process.

The authors of the paper about the new hybrid fuel cell note:

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 silicon, tin or germanium, 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.

For the experiments discussed in the paper the team fabricated the negative electrode from an 80:10:10 (wt.%) mixture of hard carbon or tin-carbon, SuperP carbon black as a conductive additive, and poly(vinylidene fluoride) as a binder. Seawater containing sodium chloride was used as the positive electrode.

During charging, the sodium+ ions present at the cathode diffuse through s NASICON electrolyte and transfer to the negative electrode, where the release of gaseous chlorine2 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 5th cycle, accompanied by a continuously decreasing irreversible capacity (~90 mAh g−1 at the 5th cycle).

In the study the authors explain, “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,” they said.

This is a pretty big idea that actually works from the first lab bench prototype. There is a lot of potential here. For now the impressive thing is the intuitiveness of the research team. These resources have been constantly at our disposal and only now have been noticed and experiments begun to make use of them.


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