Hemp Fibers Make Better Supercapacitors Than Graphene

biodata (1981610) writes "BBC News is reporting findings published in the journal ACS Nano by Dr David Mitlin from Clarkson University. Dr. Mitlin's team took waste hemp stems and recycled the material into supercapacitors with performance as good, or better, than those built from graphene, at a fraction of the raw materials cost. "We're making graphene-like materials for a thousandth of the price - and we're doing it with waste. The hemp we use is perfectly legal to grow. It has no THC in it at all - so there's no overlap with any recreational activities," Mitlin says.

Comments

suitable for home use?

[russejl]

This is potentially exciting... no pun intended :-)

The article abstract says:
The nanosheets are ideally suited for low (down to 0 C) through high (100 C) temperature ionic-liquid-based supercapacitor applications: At 0 C and a current density of 10 A g–1, the electrode maintains a remarkable capacitance of 106 F g–1. At 20, 60, and 100 C and an extreme current density of 100 A g–1, there is excellent capacitance retention (72–92%) with the specific capacitances being 113, 144, and 142 F g–1, respectively. These characteristics favorably place the materials on a Ragone chart providing among the best power–energy characteristics (on an active mass normalized basis) ever reported for an electrochemical capacitor: At a very high power density of 20 kW kg–1 and 20, 60, and 100 C, the energy densities are 19, 34, and 40 Wh kg–1, respectively. "

Which possibly suggests that the materials are suitable for indoor use (but not in cars unless you happen to operate in a non-freezing climate) which could have some very practical applications. Solar panels are becoming attractive and I'd like a storage bank but would like to avoid batteries because of the slow charge, expense, and maintenance. A super capacitor, of course, is attractive. Off the top of my head, I don't know what the power density of this type of capacitor is relative to lead acid deep cycle batteries. Still, I smile though :)

Re:suitable for home use?

[cbhacking]

The power density is really nowhere close to a battery. Supercaps make sense for things where you actually need really massive charge and/or discharge spikes, over very short times. Think railguns, or a camera flash that can fire multiple times without needing to recharge between shots (if it charged enough to begin with), or possibly a smoothing system for charging batteries from a very spiky power source (hypothetically, this could scale to things like harvesting lightning, though at present that's not at all practical). They aren't practical for long-term storage, either due to energy density or due to their tendency to lose power over time pretty quickly.

A sufficiently large battery bank will have no problem with the charge speed of a photovoltaic array (which is actually rather slow). A small bank might reach saturation voltage - where the batteries are still charging but can't charge any *faster* or they'll take damage from overvolting - fairly quickly if fed by a large array, but that's not the real problem with a small bank; the real problem is not having enough storage capacity.

Expense is considerable, especially if you go with the low-maintenance options like gel-cells. However, supercaps are, at this time, not something you can buy a huge bank of at any price (certainly not the hemp-based ones). If you could get a meaningful capacity of the graphene ones it would probably cost many times as much. Maybe the hemp ones will change that, but don't hold your breath.

Maintenance is much less than it sounds. Wet-cells (typical lead-acid batteries) need topping up with water periodically, and occasionally may need equalization charges; the first can be done by a reminder to go do so every month, and the latter doesn't even need to be that often. Pretty much every other aspect of maintenance should be handled by a good enclosure for the batteries and a good charge controller. The controller costs a bit but you want one of the good ones anyhow; they perform DC-DC voltage conversion to take the output of the solar cells (which can easily be at least 25% higher voltage than the batteries will charge at) and down-convert it, extracting some extra current in the process (some energy is lost in this process, but it's typically a 10%-20% net positive for the 12V gel-cells my family uses). Speaking of gel-cells, those will save you on maintenance (at a cost of more money up front and a more severe voltage sensitivity that limits charge rate a bit harder). Such batteries are basically install-and-forget, but you'd need to be tremendously lazy for them to be worthwhile for a home installation; they are typically for marine usage (as my parents do) where never needing to open the cells (to add water) is a significant plus.

Re:suitable for home use?

[mlts]

I'd love these for a solar array where energy stored for unit volume is important, but not critical (like it is in a car or RV) for a number of reasons:

1: Hemp is a lot less nasty for the environment than graphene.

2: This could be used as a buffer for the chemical batteries, since you don't have to worry about limiting amps coming in. Come "shoulder hours", the supercaps can be charging the batteries at exactly the amperage rate they need even after the sun is down, greatly improving the system's efficiency.

3: The lifespan of a capacitor is a lot longer than a battery because electricity is stored physically, not chemically. So, if space is less of an issue, large supercaps can be used without worrying about replacement every 5-10 years (or 2-3 years with Li-ion) batteries.

So, for an off-grid circuit (one that never goes near mains power and pretty much acts as a UPS), having this technology would go far.

Gives new meaning...

[ilsaloving]

This will give new meaning to the term 'magic smoke'.

( http://en.wikipedia.org/wiki/M... for those that don't know)

No overlap with recreational activities?

[kruach aum]

What about basket weavers, you insensitive clod!

Re:Legal... sort of

[GrahamCox]

Which is nuts, actually. Hemp is a brilliant raw material with hundreds of practical uses which *should*, if people had any sense of balance, far outweigh the small issue of the cannabinoids. It could probably even be selectively bred to eliminate that aspect, but no, concern about a few potheads sends legislators into a tailspin. This is why we can't have nice things.