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| Structure of the rechargeable Li-air battery based on GNSs as an air electrode. Credit: ACS, Yoo and Zhou. Click to enlarge. |
Researchers at Japan’s National Institute of Advanced Industrial Science and Technology (AIST) have demonstrated that metal-free graphene nanosheets (GNSs) show good performance as a catalyst for reducing oxygen in the air in Li-air batteries with a
hybrid electrolyte, although with poor cycling performance. However, heat-treated GNSs not only provided a similar catalytic activity in reducing oxygen in the air, but also showed much more stable cycling performance in a Li-air battery.
Li-air batteries are the subject of increased research focus (earlier post) for use as high energy batteries, given their capacity of 5-10 times that of Li-ion batteries. Li-air batteries basically use a catalytic air cathode that converts oxygen to lithium peroxide; an electrolyte; and a lithium anode.
However, one of serious challenges to Li-air batteries is that the solid reaction
product (Li2O or Li2 O2) is insoluble in
organic electrolytes, and so clogs on the air electrode (cathode) in the discharge process. If the air electrode is fully clogged, O2 from the atmosphere cannot be reduced.
AIST earlier developed a new type of Li-air battery with a hybrid
electrolyte designed to overcome that problem. (Earlier post.) In
that new battery, an organic electrolyte is used on the anode (metallic lithium) side,
and an aqueous electrolyte is used on the cathode (air) side. These two electrolytes are separated by a solid-state electrolyte [a lithium super-ion-conductor
glass film (LISICON)] so that they do not intermix.
Only lithium ions pass through the solid electrolyte, and the battery reactions proceed smoothly. The discharge reaction product is not a solid substance such as lithium oxide (Li2O), but lithium hydroxide (LiOH), which dissolves in the aqueous electrolyte; clogging of the pores does not occur at the carbon cathode.
Challenges remain, including the corrosion of the air electrode catalysts.
Corrosion of the air electrode catalysts is
considered to result from oxidation of carbon to form Li2CO3. It is therefore important to choose suitable electrode materials to improve the performance of the
battery.
In general, the air electrode for a Li-air
battery is prepared from a combination of
Pt-Au or a metal oxide such as Mn3O4
supported on a carbon material. In almost
all cases, mesoporous carbon has been used as the support for the metal
nanoparticles. Such mesoporous carbon-supported electrocatalysts have shown
quite moderate performance in Li-air batteries, and several major obstacles
arising from the carbonaceous air cathode, such as carbon’s oxidation in both charge
and discharge processes, remain to be overcome if the cycling efficiency and
cycle life of Li-air batteries are to be improved.
…GNSs were expected to be useful novel catalysts for Li-air batteries.
However, until now, although GNSs have been widely investigated as support hosts
for catalysts, metal-free GNSs have not been directly used as catalysts in air electrodes for either fuel cells or Li-air batteries.—Yoo and Zhou
In a paper published in the journal ACS Nano, Eunjoo Yoo and Haoshen Zhou report prepared both GNSs and heat-treated GNSs by a chemical method, and studied their electrochemical
properties as air-electrode catalysts for the new type of Li-air battery.
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| Discharge curves of 20 wt % Pt/CB, GNSs, and AB at a current density of 0.5 mA&iddot;cm-2 for 24 h (a), the correlation between cycle number and capacity of GNSs (b). Charge-discharge curves of GNSs (c) and heat-treated GNSs (d) at a current density of 0.5 mA&iddot;cm-2 from 1 to 50 cycles. Credit: ACS, Yoo and Zhou. Click to enlarge. |
At 0.5 mA cm-1, the GNSs showed a high discharge voltage that was near that of the 20 wt % Pt/carbon black. Although the detailed mechanism for the oxygen reduction reaction on GNSs without metal is unclear, concluded Yoo and Zhou in their paper. they expect GNSs to be good catalysts for cathode electrode of Li-air batteries.
Resources
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Eunjoo Yoo, Haoshen Zhou (2011)> Li-Air Rechargeable Battery Based on Metal-free Graphene Nanosheet Catalysts. ACS Nano Article ASAP doi: 10.1021/nn200084u