Aamir Razaq

Ultrafast All-Polymer Paper-Based Batteries

Conducting polymers for battery applications have been subject to numerous investigations during the last two decades. However, the functional charging rates and the cycling stabilities have so far been found to be insufficient for practical applications. These shortcomings can, at least partially, be explained by the fact that thick layers of the conducting polymers have been used to obtain sufficient capacities of the batteries. In the present letter, we introduce a novel nanostructured high-surface area electrode material for energy storage applications composed of cellulose fibers of algal origin individually coated with a 50 nm thin layer of polypyrrole. Our results show the hitherto highest reported charge capacities and charging rates for an all polymer paper-based battery. The composite conductive paper material is shown to have a specific surface area of 80 m2 g−1 and batteries based on this material can be charged with currents as high as 600 mA cm−2 with only 6% loss in capacity over 100 subsequent charge and discharge cycles. The aqueous-based batteries, which are entirely based on cellulose and polypyrrole and exhibit charge capacities between 25 and 33 mAh g−1 or 38−50 mAh g−1 per weight of the active material, open up new possibilities for the production of environmentally friendly, cost efficient, up-scalable and lightweight energy storage systems.

A Nanocellulose Polypyrrole Composite Based on Microfibrillated Cellulose from Wood

It is demonstrated that it is possible to coat the individual fibers of wood-based nanocellulose with polypyrrole using in situ chemical polymerization to obtain an electrically conducting continuous high-surface-area composite. The experimental results indicate that the high surface area of the water dispersed material, to a large extent, is maintained upon normal drying without the use of any solvent exchange. Thus, the employed chemical polymerization of polypyrrole on the microfibrillated cellulose (MFC) nanofibers in the hydrogel gives rise to a composite, the structure of which—unlike that of uncoated MFC paper—does not collapse upon drying. The dry composite has a surface area of ∼90 m2/g and a conductivity of ∼1.5 S/cm, is electrochemically active, and exhibits an ion-exchange capacity for chloride ions of 289 C/g corresponding to a specific capacity of 80 mAh/g. The straightforwardness of the fabrication of the present nanocellulose composites should significantly facilitate industrial manufacturing of highly porous, electroactive conductive paper materials for applications including ion-exchange and paper-based energy storage devices.

Influence of the type of oxidant on anion exchange properties of fibrous Cladophora cellulose/polypyrrole composites

The electrochemically controlled anion absorption properties of a novel large surface area composite paper material composed of polypyrrole (PPy) and cellulose derived from Cladophora sp. algae, synthesized with two oxidizing agents, iron(III) chloride and phosphomolybdic acid (PMo), were analyzed in four different electrolytes containing anions (i.e., chloride, aspartate, glutamate, and p-toluenesulfonate) of varying size.The composites were characterized with scanning and transmission electron microscopy, N2 gas adsorption,and conductivity measurements. The potential-controlled ion exchange properties of the materials were studied by cyclic voltammetry and chronoamperometry at varying potentials. The surface area and conductivity of the iron(III) chloride synthesized sample were 58.8 m2/g and 0.65 S/cm, respectively, while the corresponding values for the PMo synthesized sample were 31.3 m2/g and 0.12 S/cm. The number of absorbed ions per sample mass was found to be larger for the iron(III) chloride synthesized sample than for the PMo synthesized one in all four electrolytes. Although the largest extraction yields were obtained in the presence of the smallest anion (i.e., chloride) for both samples, the relative degree of extraction for the largest ions (i.e., glutamate and p-toluenesulfonate) was higher for the PMo sample. This clearly shows that it is possible to increase the extraction yield of large anions by carrying out the PPy polymerization in the presence of large anions. The results likewise show that high ion exchange capacities, as well as extraction and desorption rates, can be obtained for large anions with high surface area composites coated with relatively thin layers of PPy.

High-Capacity Conductive Nanocellulose Paper Sheets for Electrochemically Controlled Extraction of DNA Oligomers

Highly porous polypyrrole (PPy)-nanocellulose paper sheets have been evaluated as inexpensive and disposable electrochemically controlled three-dimensional solid phase extraction materials. The composites, which had a total anion exchange capacity of about 1.1 mol kg−1, were used for extraction and subsequent release of negatively charged fluorophore tagged DNA oligomers via galvanostatic oxidation and reduction of a 30–50 nm conformal PPy layer on the cellulose substrate. The ion exchange capacity, which was, at least, two orders of magnitude higher than those previously reached in electrochemically controlled extraction, originated from the high surface area (i.e. 80 m2 g−1) of the porous composites and the thin PPy layer which ensured excellent access to the ion exchange material. This enabled the extractions to be carried out faster and with better control of the PPy charge than with previously employed approaches. Experiments in equimolar mixtures of (dT)6, (dT)20, and (dT)40 DNA oligomers showed that all oligomers could be extracted, and that the smallest oligomer was preferentially released with an efficiency of up to 40% during the reduction of the PPy layer. These results indicate that the present material is very promising for the development of inexpensive and efficient electrochemically controlled ion-exchange membranes for batch-wise extraction of biomolecules.

Ionic motion in polypyrrole-cellulose composites: trap release mechanism during potentiostatic reduction.

This work investigates the movement of anions during potentiostatic controlled reduction of novel composite materials consisting of high surface area cellulose substrates, extracted from the Cladophora sp. algae, coated with thin ( approximately 50 nm) layers of the intrinsically conducting polymer (ICP) polypyrrole. The coating was achieved by chemical polymerization of pyrrole on the cellulose fibers with iron(III) chloride and phosphomolybdic acid, respectively. The composites are in the form of paper sheets and can be directly immersed into an electrolyte solution for ion absorption/desorption. The motion of glutamate and aspartate anions during cathodic polarization was investigated as a function of preceding anodic polarization at various potentials. The composite was found to exhibit memory effect as the response to a cathodic polarization of constant magnitude produced different responses depending on the magnitude of the preceding anodic potential. After the application of a cathodic potential to the composite, the reduction current curvesgenerated by anions leaving the compositewere found to initially increase in magnitude followed by a monotonic decay. A similar response has not been described and analyzed for electrochemical reduction of anion containing ICP materials earlier. A theoretical model was developed to aid the analysis of the experimental data. The model accounts for both freely mobile anions and anions that may be temporarily trapped in a contracting PPy network during cathodic polarization. By fitting the recorded reduction current curves to this model, detailed information about the ionic movement in the composite could be obtained, which may be used to further optimize the materials properties of conducting polymer systems aimed for specific electrochemical ion exchange processes.

Influence of the nanocellulose raw material characteristics on the electrochemical and mechanical properties of conductive paper electrodes

Paper-based conductive electrode materials of polypyrrole (PPy) and nanocellulose (NC) have received much attention lately for applications in non-metal-based energy storage devices, ion exchange, etc. The aim of this study was to study how the primary characteristics of NC raw materials impact and electrochemical properties of conductive NC–PPy composite sheets. Three NC raw materials were used: Cladophora cellulose (NCUU) produced at Uppsala University, Cladophora cellulose (NCFMC) produced at FMC Biopolymer, and microfibrillated cellulose (NCINN) produced at Innventia AB. Composite paper sheets of PPy coated on the substrate NC material were produced. The NC raw materials and the composites were characterized with a battery of techniques to derive their degree of crystallinity, degree of polymerization, specific surface area, pore size distribution, porosity, electron conductivity, charge capacity and tensile properties. It was found that the pore size distribution and overall porosity increase upon coating of NC fibres for all the samples. The charge capacity of the composites was found to decrease with the porosity of the samples. It was further found that the mechanical strength of the pristine NC sheets was largely dependent on the overall porosity, with NCINN having the highest mechanical strength and lowest porosity in the series. The mechanical properties of the composite NC–PPy sheets were significantly diminished as compared with pristine NC sheets because of the impaired H-bonding between fibres and PPy-coated nanofibres. It was concluded that to improve the mechanical properties of PPy–NC sheets, a fraction of additive bare NC fibres is beneficial. Future study may include the effect of both soluble and insoluble additives to improve the mechanical strength of PPy–NC sheets.

Spatial Mapping of Elemental Distributions in Polypyrrole-Cellulose Nanofibers using Energy-Filtered Transmission Electron Microscopy

The energy-filtered transmission electron microscopy (EFTEM) technique has been used to study ion-exchange processes in conductive polymer composite nanofibers. The elemental distributions of carbon, nitrogen, oxygen, chlorine, boron, phosphorus, molybdenum, and sulfur within polypyrrole-cellulose nanofibers, used as potential controlled electrochemical solid phase extraction media, have been studied by EFTEM. The distribution of ions within the polypyrrole-cellulose nanofibers and the penetration depth of ions into the material as a function of the size and charge of the latter were investigated. Further, the spatial distribution of single stranded DNA hexamers inside polypyrrole-cellulose nanofibers was mapped subsequent to the electrochemically controlled extraction of DNA from a borate buffer solution. The results show that the EFTEM mapping technique provides unpreceded possibilities for studies of the distribution of ions inside conductive polymer composites.

Electrochemically Controlled Separation of DNA Oligomers with High Surface Area Conducting Paper Electrode

Conductive paper materials consisting of conductive polymers and cellulose are promising for high-tech applications (energy storage and biosciences) due to outstanding aspects of environmental friendliness, mechanical flexibility, electrical conductivity and efficient electroactive behavior. Recently, a conductive composite paper material was developed by covering the individual nanofibers of cellulose from the green algae Cladophora with a polypyrrole (PPy) layer. The PPy-Cladophora cellulose composite paper is featured with high surface area (80 m2 g-1), electronic conductivity (~2 S cm-1), thin conductive layer (~50 nm) and easily up-scalable manufacturing process. This doctoral thesis reports the development of the PPy-Cladophora composite as an electrode material in electrochemically controlled solid phase ion-exchange of biomolecules and all-polymer based energy storage devices. First, electrochemical ion-exchange properties of the PPy-Cladophora cellulose composite were investigated in electrolytes containing three different types of anions, and it was found that smaller anions (nitrate and chloride) are more readily extracted by the composite than lager anions (p-toluene sulfonate). The influence of differently sized oxidants used during polymerization on the anion extraction capacity of the composite was also studied. The composites synthesized with two different oxidizing agents, i.e. iron (III) chloride and phosphomolybdic acid (PMo), were investigated for their ability to extract anions of different sizes. It was established that the number of absorbed ions was larger for the iron (III) chloride-synthesized sample than for the PMo-synthesized sample for all four electrolytes studied. Further, PPy-Cladophora cellulose composites have shown remarkable electrochemically controlled ion extraction capacities when investigated as a solid phase extraction material for batch-wise extraction and release of DNA oligomers. In addition, composite paper was also investigated as an electrode material in the symmetric non-metal based energy storage devices. The salt and paper based energy storage devices exhibited charge capacities (38−50 mAh g−1) with reasonable cycling stability, thereby opening new possibilities for the production of environmentally friendly, cost efficient, up-scalable and lightweight energy storage systems. Finally, micron-sized chopped carbon fibers (CCFs) were incorporated as additives to improve the charge-discharge rates of paper-based energy storage devices and to enhance the DNA release efficiency. The results showed the independent cell capacitances of ~60-70 F g-1 (upto current densities of 99 mA cm2) and also improved the efficiency of DNA release from 25 to 45%.