Materials Horizons View Article Online REVIEW View Journal | View Issue Biopolymer hybrid electrodes for scalable electricity storage Cite this: Mater. Horiz., 2016, 3,174 ab a a a S. Admassie, F. N. Ajjan, A. Elfwing and O. Ingana¨s* Powering the future, while maintaining a cleaner environment and a strong socioeconomic growth, is going to be one of the biggest challenges faced by mankind in the 21st century. The first step in overcoming the challenge for a sustainable future is to use energy more efficiently so that the demand for fossil fuels can be reduced drastically. The second step is a transition from the use of fossil fuels to renewable energy sources. In this sense, organic electrode materials are becoming increasingly attractive compared to inorganic electrode materials which have reached a plateau regarding performance and have severe drawbacks in terms of cost, safety and environmental friendliness. Using organic composites based on conducting polymers, such as polypyrrole, and abundant, cheap and naturally occurring Received 3rd November 2015, biopolymers rich in quinones, such as lignin, has recently emerged as an interesting alternative. These Accepted 14th January 2016 Creative Commons Attribution 3.0 Unported Licence. materials, which exhibit electronic and ionic conductivity, provide challenging opportunities in the DOI: 10.1039/c5mh00261c development of new charge storage materials. This review presents an overview of recent developments in organic biopolymer composite electrodes as renewable electroactive materials towards sustainable, www.rsc.li/materials-horizons cheap and scalable energy storage devices. Introduction green plants. Biopolymers contribute to the conversion of solar energy to biochemical energy. The chemical energy found in Biopolymers are the most abundant polymers on Earth, in the organic solids or liquid fuels for use in combustion is a plant This article is licensed under a sequence of cellulose, lignin, proteins and DNA. They form the product of recent photosynthesis as well as photosynthesis over skeleton and have active functions in living systems and in millions of years that has been converted to fossil fuel deposits. The flow of carbon compounds in this greater scheme of things hinges on the combined electron and proton transfer affected Open Access Article. Published on 09 February 2016. Downloaded 13/02/2017 11:28:44. a Biomolecular and Organic Electronics, IFM, Linko¨ping University, S-581 83, by quinone molecules. Derivatives of quinones are found in the Linko¨ping, Sweden. E-mail: [email protected], [email protected] b Department of Chemistry, Addis Ababa University, PO Box 1176, Addis Ababa, biomembranes of bacteria, plants and animals and play a decisive Ethiopia role in photosynthetic energy conversion and in respiration. Dr Shimelis Admassie is currently Fatima Nadia Ajjan was born in an Associate Professor of Physical Madrid, Spain in 1986. She Chemistry at Addis Ababa Univer- received her Bachelor of Chemistry sity, Addis Ababa, ETHIOPIA. His degree from the Autonoma research interest includes renewable University of Madrid in 2010 and energy conversion, electrochemical her MSc degree in Chemical science energy storages and sensors. and nanotechnology from the Complutense University of Madrid in 2012, Spain. She is currently a PhD student under the supervision of Prof. Olle Ingana¨s at Linko¨ping University, Sweden. Her research S. Admassie F. N. Ajjan interests mainly focus on the field of energy conversion and storage and their applications in various energy storage devices including supercapacitor devices. 174 | Mater. Horiz., 2016, 3, 174--185 This journal is © The Royal Society of Chemistry 2016 View Article Online Review Materials Horizons The quinones couple the electronic charges from the active from wind turbines and solar photovoltaics. In this review, we will photosynthetic reaction center in photosynthetic bacteria and summarizesomeofourrecentresults using quinone containing plants to drive proton diffusion over the biomembrane via biopolymer hybrids obtained from lignins and related studies the reduction of a diffusing quinone. The flow of electrons that lay the foundation for developing cheap, scalable and eventually creates a proton gradient, which is the basic element environmentally friendly energy storage materials. of bioenergetics. The pH difference between the two sides of the membrane is used to form ATP from ADP, the intermediate energy form in bioenergetics. Therefore, quinones are found all Lignin from plants and its uses over the surface of the Earth in all biological systems. The simplest redox couple based on a quinone/hydroquinone couple Plants build cellulose, hemicelluloses and lignins in their growth will exchange two electrons and two protons. For a quinone, the processes, and E1011 tons of cellulose are produced annually. charge density of 2 moles of electrons per 110 g or 487 mA h gÀ1 Lignin forms some 20–30% of the solid weight of plants, whereas is obtained. A solid electrode composed of quinone, and capable the remaining 70–80% is dominantly cellulose and hemicellulose. of both electronic and ionic conduction, would thus have an Softwood plants carry more lignin (closer to 30%) than hardwood impressive charge density. Since quinones are already fully (closer to 20%). Lignin is a complex three-dimensional polymer integrated in the biological material flow they do not cause concerns based on the combination of three monolignols, phenylpropane for environmental accumulation and disruption. Hence, they compounds, in different proportions, and with different com- are very attractive compared to inorganic electrode materials. positions between plant species and plant elements3 (Fig. 1(a) Present day electrochemical energy storage systems are not and (b)). The role of lignin in these plants is that of a structural fully scalable and are expensive.1 Hence, breakthroughs in the polymer, which together with cellulose and hemicellulose forms storage of electricity would enable an accelerated use of renew- the cell walls of the plant cell (Fig. 1(c)). able electricity in electrical power grids, vehicles and in mobile Aromatic lignin is the cause for the yellow color of news- Creative Commons Attribution 3.0 Unported Licence. electronics when far away from the grid system. It can also have paper grade paper and is due to the photochemical reaction a major impact in places that are far away from the electrical driven by absorption of UV light in the lignin polymer. If the grids by dramatically changing the lifestyles of a large part of yellow color is not required, lignin is removed to make white the global population who still lack access to electricity.2 paper that does not bleach over time. The aromatic phenylpropane Hence, cheap and scalable materials are desired materials for monomers lead to a more hydrophobic polymer than cellulose. electrical energy storage that can be scaled-up to match the Therefore, lignins play a role in the transport of water in plants need coming from the growing use of intermittent electricity and blocks evaporation of water. The source of phenylpropane is This article is licensed under a Anders Elfwing was born in Olle Ingana¨s is a professor Stockholm, Sweden in 1975. He of biomolecular and organic Open Access Article. Published on 09 February 2016. Downloaded 13/02/2017 11:28:44. received his M. Eng. degree in electronics, IFM, Linko¨pings 2002 from Linko¨ping University, Universitet, Sweden. He received Sweden. After positions at Uppsala his MSc degree in engineering University and Novozymes physics from Chalmers University Biopharma in Lund he returned to of Technology (1977), BSc degree in academia in 2009. Currently, he is philosophy and economics from working on his Applied Physics PhD Go¨teborg University (1978), and program under the supervision of PhD degree in applied physics at Prof. Olle Ingana¨s at Linko¨ping Linko¨ping University in 1984. He University, Sweden. His research was appointed as a professor in A. Elfwing interest focuses on process O. Ingana¨s 1999. Ingana¨sreceivedtheGo¨ran development for energy generation Gustafsson prize in physics in 1997, and storage. and was a Wallenberg Scholar in 2010. He was elected as a member of the Royal Swedish Academy of Sciences, class of physics, in 2006, and is the chairman of the Nobel committee for the prize in physics. Ingana¨s has focused on studies of the class of conjugated polymers throughout the areas of polymer physics, electrochemistry, electronics and optics. He has contributed to a number of start-up companies in the field of electronic polymers. His current interest includes energy conversion and energy storage with organic photovoltaic devices and organic supercabatteries, as well as the use of biopolymers as organisers of electronic polymers. This journal is © The Royal Society of Chemistry 2016 Mater. Horiz., 2016, 3, 174--185 | 175 View Article Online Materials Horizons Review Fig. 1 (a) Lignin polymeric structure, (b) lignin monomeric building blocks, and (c) spatial arrangement of cellulose, hemicellulose and lignin in the cell walls of lignocellulosic biomass. the amino acid phenylalanine, which is glycosylated to form Electrochemistry of lignin monolignols. These are water soluble, and can be transported across the plant membrane to form the lignin polymer, outside The insulating properties of lignin (and cellulose) are well this membrane. Polymerisation mechanisms are debated, but known and introducing electronic conductivity