Influence of the cellulose substrate on the electrochemical properties of paper-based polypyrrole electrode materials
Henrik Olsson
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Daniel O. Carlsson
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Gustav Nystrom
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Martin Sjodin
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Leif Nyholm
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Maria Strmme
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L. Nyholm (&) Department of Materials Chemistry
, The A
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H. Olsson D. O. Carlsson G. Nystrom M. Strmme (&) Nanotechnology and Functional Materials, The A ngstrom Laboratory, Uppsala University
, P. O. Box 534, 751 21 Uppsala,
Sweden
The influence of the cellulose substrate on the electrochemical performance of supercapacitor electrode materials made of polypyrrole (PPy) and cellulose is investigated. Composites were synthesized by chemical polymerization of pyrrole on dispersed fibers of cellulose from Cladophora algae and dispersed wood cellulosebased commercial filter papers, respectively, as well as on Cladophora cellulose and filter paper sheets. The resulting composites, which were characterized using scanning electron microscopy, cyclic voltammetry, and elemental analysis, were found to exhibit specific charge capacities proportional to the PPy content of the composites. The highest specific capacity (i.e., 171 C/g composite or 274 C/g PPy) was obtained for composites made from dispersed Cladophora cellulose fibers. The higher specific capacities for the Cladophora cellulose composites can be explained by the fact that the Cladophora cellulose fibers were significantly thinner than the wood cellulose fibers. While the PPy was mainly situated on the surface of the Cladophora cellulose fibers, a significant part of the PPy was found to be present within the wood fibers of the filter paper-based M. Sjodin composites. The latter can be ascribed to a higher accessibility of the aqueous pyrrole solution to the wood-based fibers as compared to the highly crystalline algae based cellulose fibers. The present results clearly show that the choice of the cellulose substrate is important when designing electrode materials for inexpensive, flexible and environmentally friendly paper-based energy storage devices.
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There is currently a high demand for lightweight,
inexpensive, flexible, and environmentally friendly energy
storage devices for a number of applications including new
types of portable electronics, propulsion of all-electric or
hybrid vehicles as well as for the management of
intermittent renewable energy sources [16].
As a result of these needs, research is currently carried
out to develop both thin flexible lithium-ion batteries [6]
and novel current collectors for such devices [7]. Another
approach to flexible energy storage involves the utilization
of conducting polymers, which offer promising aspects in
terms of easy manufacturing, inexpensive raw materials
and environmental friendliness [8, 9]. Since the discovery
of the conductive properties of doped polyacetylene [10], a
wide variety of polymers with conductive properties have
been presented [11, 12]. One of the most promising
candidates is polypyrrole [11] (PPy) which can be polymerized
in aqueous solutions, and offers good stability as well as a
theoretical charge storage capacity of 418 C/g assuming a
Cl- doping level of 33 % [13]. A commonly used approach
for making PPy electrodes is to polymerize the monomer,
pyrrole, onto a substrate. This can be done by electro
polymerization using electrically conducting substrates
[14], but also by chemical polymerization onto both
electrically conducting [15] and non-conducting substrates
[1618].
Two main problems with conducting polymer based
batteries and supercapacitors are their poor cycling
stabilities [11, 19], and rate restrictions due to mass transport
limitations within thick polymer layers [20, 21]. To
overcome the latter problem, we have previously developed a
material based on a chemically deposited thin (*50 nm)
layer of PPy coated on a large surface area substrate of
dispersed nanofibrous cellulose from the Cladophora algae
[22], which could be used in both paper-based
supercapacitors [23, 24] and in electrochemically controlled anion
exchange procedures [2527] including DNA extraction
[28] and hemodialysis [29, 30]. We have also demonstrated
that it is possible to produce a material with similar
electrochemical properties using dispersed microfibrillated
cellulose from wood as substrate for the chemical
polymerization of PPy [31]. In addition, it was recently shown
that the PPy-Cladophora cellulose composites exhibited
excellent cycling stability, with only 0.7 % loss in
capacitance over 4 000 cycles, when used as the electrodes in an
aqueous symmetric supercapacitor device [32]. It was
further shown that no significant loss in capacitance
occurred when charging the device to 1.8 V due to an
intrinsic self-protective mechanism which prevented
degradation of the PPy [32].
In situ polymerization of conducting polymers on
cellulose can be carried out based on three main approaches
involving (i) mixing or soaking the cellulose in a monomer
solution followed by addition of the oxidant [18], (ii)
soaking the cellulose in a solution of the oxidant followed
by addition of mon (...truncated)