Tikalon Blog by Dev Gualtieri

Nanocellulose Capacitors

January 14, 2016

My father was a carpenter, so our material of choice was always wood. There were bits and pieces of wood, such as wall studs and plywood, in various neat piles in the garage and the cellar workshop. These were a great source of materials for my many science fair projects. As they say, if all you have is a hammer, everything looks like a nail, and my father even repaired a broken automobile seat with a piece of wood.

Wood is a great renewable resource, and it's no wonder that most houses are framed with wood. For the past century or more, the typical home is constructed using balloon framing. This type of construction, in which many smaller wall studs replaced fewer and heavier supporting posts at corners and principal junctions, made for easier house framing. The older construction practice required skilled joining, while the balloon houses could just be nailed together.

Wood is a combination of several chemical compounds, but its major constituent (about 42%) is cellulose. Cellulose is a crystalline polymer that derives considerable strength from the hydrogen bonding between straight, parallel chains of the cellulose polymer (see figure). I wrote about cellulose in an earlier article (Cellulosic Carbon, April 30, 2014).

Polymer chains of cellulose.

The polymer chains are straight, and the strength of wood derives from hydrogen bonding, not the coiling or branching found in other polymers.

(Illustration by Luca Laghi, via Wikimedia Commons .)

Paper is a principal wood product; and, as you affirm every time you buy another ream of paper for your printer, or haul another load of junk mail to the recycle center, paper is inexpensive. That's why a team of scientists from Linköping University (Norrköping, Sweden), the Technical University of Denmark (Roskilde, Denmark), the University of Kentucky (Lexington, Kentucky), and the KTH Royal Institute of Technology (Stockholm, Sweden) has looked at paper as an energy storage medium. They processed sheets of nanocellulose to form capacitors with a large energy density.[1-3]

A quick look at the equations describing the simplest capacitor, the parallel plate capacitor , gives us an idea of what needs to be improved to give higher energy storage ability.

C = κε_oA/d
Q = C V
I = ∂Q/∂t
W = (1/2) C V²
V = V_o e^(-t/RC)
Q = C V_o e^(-t/RC)
I = (V_o/R) e^(-t/RC)

where C is capacitance , κ is the dielectric constant , ε_o is the permittivity of free space , A is the area of the capacitor plates, d is the plate separation, Q is the charge , V is the voltage , V_o is the initial voltage, I is the current , W is the stored energy , t is time , and R is the load resistance .

A convenient unit for ε_o, the permittivity of free space, is 8.854 x 10^-12 farads per meter . If you use farads as the units of capacitance, the stored energy W will have units of joules . A watt is a joule per second. As we can see from these equations, a large plate area, small plate separation and high dielectric constant lead to high capacitance; and, thereby, high energy storage at high voltage.

The research team, led by scientists at the Laboratory of Organic Electronics of Linköping University, has developed what they call "power paper," a composite of nanocellulose and the conductive polymer, PEDOT:PSS (poly(3,4-ethylene-dioxythiophene):poly(styrene-sulfonate).[1-3] The energy storage principal is capacitance, and supercapacitors are similarly used for energy storage applications. Power paper is different, since it has a three-dimensional structure that's easy to produce in thick sheets.[3] A fifteen centimeter disk of the material, a few millimeters thick, has a capacitance of up to one farad, about the same capacitance of supercapacitors.

This flexible piece of power paper has a one farad capacitance.

(Linköping University image by Thor Balkhed.)

One other distinguishing characteristics of the material is its ability to be recharged hundreds of times, and each charge only takes a few seconds.[3] A piece of power paper has the highest charge (1 coulomb) and highest capacitance (2 farads) in an organic electronic device.[1,3] The material also has the highest measured current (1 ampere) in an organic conductor, and the highest transconductance (1 siemen) when made into a transistor.[1,3]

The nanocellulose is created using high pressure water, which breaks cellulose fibers into strands as thin as 20 nanometer in diameter.[3] The PEDOT:PSS conducting polymer is added while the nanocellulose is still in a water solution, and the polymer forms a thin coating around the fibers.[3] Says Linköping University doctoral student, Jesper Edberg, "The covered fibers are in tangles, where the liquid in the spaces between them functions as an electrolyte."[3]

A phase atomic force microscopy image of a 70 micrometer thick film of vacuum-dried power paper

(Fig. 3a of ref. 1, licensed under a Creative Commons Attribution License.)

Because of its simultaneous high ionic conductivity and electronic conductivity, power paper has an exceptional capacity for energy storage.[3] It has the further advantages that it's made from simple materials, cellulose and an inexpensive polymer. It's lightweight, contains no toxic heavy metals or other toxic chemicals, and it's waterproof.[3] The research team is looking into an industrial scale process for power paper production.[3] This research was funded by the Knut and Alice Wallenberg Foundation, and the Swedish Foundation for Strategic Research.[3]

Charge and discharge stability of a power paper capacitor.

(Fig. 5d of ref. 1, licensed under a Creative Commons Attribution License.)

Nanocellulose Capacitors

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