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Conductive wrapping greatly improves supercapacitor performance of graphene/MnO2 electrodes; applicable to a range of advanced battery electrode materials

Wrap
(Left) Schematic illustration showing conductive wrapping of graphene/MnO2 (GM) to introduce an additional electron transport path (in a discharge cycle). (Right) Summary plot of specific capacitance values for three different electrode systems: GM-, GMC-, and GMP-based textiles at various current densities. Credit: ACS, Yu et al. Click to enlarge.

Researchers at Stanford University led by Professors Yi Cui and Zhenan Bao have developed a “conductive wrapping” method that greatly improves the supercapacitor performance of hybrid graphene/MnO2 (GM)-based nanostructured electrodes. This approach, they conclude, is also applicable to a wide range of insulating energy storage electrode materials such as sulfur, LiMnPO4, and silicon in lithium-ion batteries.

In a paper published in the ACS journal Nano Letters, they report that by three-dimensional (3D) conductive wrapping of graphene/MnO2 nanostructures with carbon nanotubes or conducting polymer, specific capacitance of the electrodes (considering total mass of active materials) has substantially increased by 20% and 45%, respectively, with values as high as 380 F/g achieved. The ternary composite electrodes also exhibited excellent cycling performance with >95% capacitance retention over 3,000 cycles.

Supercapacitors offer a number of attractive attributes, including high power capability, long cycle lifetime, and fast charge and discharge rates. However, the energy storage density of existing supercapacitors is limited—generally an order of magnitude lower than that of batteries, according to the team. Thus, improving the energy density while maintaining the high power density and cycling stability for supercapacitor devices remains a primary research focus in the field.

Pseudocapacitive transition-metal oxides such as RuO2, NiO, and MnO2, have been studied extensively as active electrode materials for supercapacitors owing to their high energy density and large charge- transfer-reaction pseudocapacitance which is based on fast and reversible redox reactions at the electrode surface, resulting in much higher specific capacitance exceeding that of carbon-based materials using electric double layer charge storage.

Although MnO2 is considered to be the most attractive oxide material owing to high abundance of Mn, low cost, and environmental friendliness, the poor conductivity of MnO2 (10-5–10-6 S/cm) remains a major challenge and limits the rate capabilities for high power performance, thus hindering its wide applications in energy storage systems.

...To realize many practical applications that require large capacitance and high energy storage, the high mass loading of active MnO2 materials usually leads to the increased electrode resistance and the decreased specific capacitance, because MnO2 becomes densely packed with limited electrochemically active surface area, resulting in only a very thin top layer (up to a few hundreds of nanometers) of oxide nanomaterials participating in the charge storage process.

To solve these critical problems, we developed a “three- dimensional (3D) conductive wrapping” method to rationally design ternary systems based on graphene/MnO2/CNT (GMC) and graphene/MnO2/poly(3,4-ethylenedioxythiophene) poly- (styrenesulfonate) (PEDOT:PSS) (GMP) composites for high-performance electrochemical electrodes...An ultra-thin layer of single-walled CNTs (SWNTs) or conducting polymer that wraps around graphene/MnO2 three-dimensionally not only provides an additional electron transport path besides the graphene layer underneath MnO2 nanomaterials but actively participates in the charge storage process as both can contribute to the energy storage of the whole film via electric double layer capacitance or pseudocapacitance.

Such [a] 3D conductive wrapping approach would provide a promising design direction for optimizing the electrochemical performance of insulating metal-oxide based electrode materials and could be generally applicable to many promising but challenging energy storage electrode materials in which the electron transport limits the device performance.

—Yu et al.

The researchers prepared GM textile electrodes using a two-step solution-based coating process they had recently developed; conductive wrapping to form a GMC system involved simply dipping the electrodes into a SWNT (single wall nanotube) ink solution and subsequently drying in a vacuum oven at 100 °C for 10 minutes.

Similarly, they prepared GMP-based electrodes by coating GM materials with a commercially available PEDOT:PSS solution via the “dip and dry” method.

Such a 3D conductive wrapping approach represents an effective and convenient technique to improve the specific capacitance and rate capability of oxide-based supercapacitors and can be applicable to a wide range of insulating energy storage electrode materials such as sulfur, LiMnPO4, and silicon in lithium-ion batteries. For example, conductive polymer can be used for wrapping sulfur cathode materials to enhance electrode conductivity and contain polysulfide intermediates, therefore minimizing polysulfide dissolution and improving the performance of Li-S batteries.

—Yu et al.

Resources

  • Guihua Yu, Liangbing Hu, Nian Liu, Huiliang Wang, Michael Vosgueritchian, Yuan Yang, Yi Cui, and Zhenan Bao (2011) Enhancing the Supercapacitor Performance of Graphene/MnO2 Nanostructured Electrodes by Conductive Wrapping. Nano Letters DOI: 10.1021/nl2026635

Comments

HarveyD

One more important step to further reduce the cost and increase the energy density of storage devices. Another dozen such steps and viable-affordable extended range BEVs will become a reality.

Can it be achieved before 2020?

Treehugger

Harvey

What is the magic of 2020 date that you put everywhere as the dead line for every new innovations ? will the world stop to turn after 2020 is there no new super cap or battery ? or did you program your last day in 2020?

HarveyD

Post 2020 years will see major changes in the efficiency and cost of energy storage devices, solar panels, electrified vehicles and wireless charging systems (both fixed and on-the move). The current decade is the spring board to it. Meanwhile, you could buy one of the high quality HEV, such as the Prius family, or a post-2015 improved PHEV.

The purchase of practical, extended range, mass produced, affordable BEVS will have to wait till 2020.

Treehugger

Harvey

I don't think the diversion from fossil energy will happen as you describe it, it will be very incremental not hockey stick. The trouble for new green energy as a name "natural gaz" cheap, clean, plentiful. If I was a vendor of EV cars or solar panel I would be worried of the historically low price of Nat Gaz, and also of the historical de-correlation of nat gaz price and oil price that happened in the 2 past years

Bob Wallace

Tree - look at installation graphs for wind and solar. They are not showing an incremental, straight line rate but the accelerating curve of a hockey stick.

Bring an affordable, adequately ranged EV to the market and the transition away from liquid fuels will likely take the same form.

Get the price of the Leaf down about $6k and the range up to 200 miles and I would expect sales to zoom and continue accelerating until the gas guzzler is pushed to the curb.

Look at all the other technological changes we've made in the last few decades. Computers replacing typewriters, adding machines and ledger books. Cell phones replacing land lines. CDs replacing vinyl and MP3s replacing CDs. Digital replacing film. All transitions slow at the onset which became landslides.

Go back and look how autos replaced horses. Henry Ford introduced the Model T in 1908 and by the 1930s horses were largely absent on city streets.

You reach a point at which it is generally recognized that the new technology is superior to the old technology and the old technology collapses.

(The futures market expects NG prices to rise over the coming years. Right now an expectation of >60% in the next eight years.)

HarveyD

I tend to agree with Bob. BEVs take off point may be close to 2020 or shortly thereafter. Meanwhile (for the next 10 - 15 years or so) :

1. sales of ICE will progressively decrease, specially after 2015.
2. sales of HEVs will start to peak between 2015 & 2020.
3. sales of PHEVs will rise for 10 to 15 years.
4. sales of BEVs will progressively rise and take off after 2020.
5. sales of FCs vehicles and FCs PHEVs will rise between 2020 & 2025

Sometime between 2020 and 2030 we may see an interesting competition between BEVs and FCs while ICEs are fading away like horses did.

NG/SG may be more appropriate for certain home uses and cleaner power generation stations.

Roy_H

I find this article somewhat confusing. They talk about capacitance, but describe a battery. I think the end product is an improved battery using this capacitive film. The improvement is not in energy density, but in power density.

DaveD

This is very exciting...potentially. But it's so hard to keep it all straight with these recent announcements and even harder to tell which, if any, will ever come to fruition.

Good luck to all of them and I hope it pans out for some of them.

David Freeman

@DaveD - I couldn't tell either :-) but I hope it's for batteries. Even A123 batteries (with high power ratings) have trouble in cold weather. If this approach lets batteries approach super-capacitor power rating, than we can use smaller batteries for hybrids, create a wider range of performance cars off of a single battery pack and even push LiIon batteries down into regular cars as the starter battery.

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