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Portobello Power

October 26, 2015

While materials scientists pride themselves on their development of many technologically useful materials, they're often outclassed by nature. At the time I was doing research in superconductivity, our laboratory was using so much liquid helium that it was economical to have our own liquefaction plant. The helium liquefier that we used had gaskets made from natural leather, since it was an inexpensive material capable of maintaining flexibility at low temperatures.

Integrated circuit manufacturers have used apricot pits as an abrasive for deflashing plastic IC packages. Apricot pits, as well as walnut shells, are ideal media for this application, since they have the proper mechanical properties, they're non-toxic, and they don't produce problematic mineral dust.

A raft of apricots (USDA)A raft of apricot, looking like atoms on the surface of a cubic crystal.

Turkey is the foremost apricot producing country in the world. Its annual production of about 800,000 metric tons far outstrips the US annual production of about 55,000 metric tons.

(United States Department of Agriculture, Agricultural Research Service, photo by Scott Bauer, via Wikimedia Commons.)

There's another reason to love apricots. Along with mulberries and other natural products, they're mentioned as an aphrodisiac in Shakespeare's, A Midsummer Night's Dream.[1]
Be kind and courteous to this gentleman;
Hop in his walks and gambol in his eyes;
Feed him with apricocks and dewberries,
With purple grapes, green figs, and mulberries;
The honey-bags steal from the humble-bees,
And for night-tapers crop their waxen thighs
And light them at the fiery glow-worm's eyes,
To have my love to bed and to arise;

Mushrooms are a common and inexpensive natural product. Although my colleague corporate scientists and I used to joke about the mushroom management of our companies, we didn't see anything technologically useful about mushrooms as a material source. Now, materials scientists from the University of California, Riverside, have found that pyrolyzed carbon derived from Portobello mushrooms (Agaricus bisporus) acts as an excellent anode material for lithium-ion batteries.[2-3]

A Portobello mushroom (Lisa Redfern)Underside of the cap (pileus) of a Portobello mushroom (Agaricus bisporus).

(Wikimedia Commons photo by Lisa Redfern.)

First, a word about nomenclature. In my reading of the references for this article, I found that people use alternative spellings for "Portobello mushroom." As shown in the figure, a Google Search finds 927,000 websites in which the mushroom is spelled as "Portobello," about half as many using "Portabella," and a considerable number using two other spellings. This must be how written languages without vowels developed. The preferred spelling is "Portobello."

How to spell 'Portobello mushroom'Mushroom spelling by popular vote.

(Google Search results of the number of websites using various spellings, graphed using Gnumeric.)

Carbon is a typical anode material for lithium-ion batteries, and its usual source is synthetic graphite produced by pyrolysis of sucrose.[2] The porosity of the graphite, which results in a high surface area, is important to the anode application, so a subsequent treatment is required to "activate" the carbon. The usual activation process involves exposing the material to concentrated potassium hydroxide (KOH), and then heating to create mesopores and/or micropores, and this allows a battery capacity of more Than 1100 mA-h/g.[2]

Treatment with a strong base like KOH is not environmentally friendly, and that has encouraged scientists to look for a biomass replacement.[3] Engineers at the University of California, Riverside, decided to look at mushroom biomass, since mushrooms have naturally high porosity. For mushrooms, this porosity is important to allow the absorption of water and air. Furthermore, mushrooms have a high concentration of potassium salts, which would allow pore growth during use.[3]

Portobello mushroom carbon process
Diagram showing how carbon battery material is created from Portobello mushrooms. The ribbon-like mushroom material is heated at 500°C, where it dehydrates and mostly converts to carbon. A final heating at 1100°C results in full pyrolysis, creating hierarchically porous carbon nanoribbons. (UC Riverside illustration.)

The process for creation of carbon anode material from Portobello mushrooms, a two-stage pyrolysis, is shown in the above figure. Oxygen-rich organic compounds and potassium compounds in the mushroom skins create inner void spaces in carbon nanoribbons, and the resulting material is analogous to activated carbon.[2] The pyrolysis at 900°C produces a hierarchically porous material with pore size ranging from sub-nanometer to tens of nanometers.[3]

Portobello mushroom carbon porous hierarchy
A transmission electron micrograph of hierarchically porous nanoribbons produced from Portobello mushrooms. Shown are macropores (b), mesopores (d) and worm-like micropores (f). (Portion of fig. 4 of ref. 2, licensed under a Creative Commons Attribution-NonCommercial-NoDerivs 4.0 International License.)[2]

While conventional activated carbon anodes allow the lithium ions to fill most of the material during the first few cycles, the battery capacity degrades after that time since electrode damage occurs. The Portobello mushroom material behaves differently. Says Brennan Campbell, a graduate student in Materials Science and Engineering at UC Riverside, and a co-author of a paper about this research,
"With battery materials like this, future cell phones may see an increase in run time after many uses, rather than a decrease, due to apparent activation of blind pores within the carbon architectures as the cell charges and discharges over time."[3]

The Portobello mushroom anode material was shown to have a significant capacity increase for charge/discharge cycles beyond the number for conventional anodes. The anodes had a capacity of more than 260 mAh/g after 700 cycles.[2]

Portobello mushroom carbon anode charge-discharge curve
The charge/discharge plot of Portobello mushroom anodes. (Portion of Fig. 6 of ref. 2, licensed under a Creative Commons Attribution-NonCommercial-NoDerivs 4.0 International License.)[2]

The six million electric vehicles forecast to be built by 2020 would require nearly 900,000 tons of graphite for battery anodes.Clearly, a "green" manufacturing process is required.[3] The UC Riverside research team is developing prototype pouch batteries based on this novel type of anode. Patents have been filed.[3]

References:

  1. William Shakespeare, "A Midsummer Night's Dream," via Project Gutenberg.
  2. Brennan Campbell, Robert Ionescu, Zachary Favors, Cengiz S. Ozkan, and Mihrimah Ozkan, "Bio-Derived, Binderless, Hierarchically Porous Carbon Anodes for Li-ion Batteries," Scientific Reports, vol. 5, article no. 14575 (September 29, 2015), doi:10.1038/srep14575, This is an open access paper with a PDF file available here.
  3. Sean Nealon, "Making Batteries with Portabella Mushrooms," University of California, Riverside, Press Release, September 29, 2015.

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