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The possibility to store energy efficiently and sustainably at little cost is crucial to prevent climate change and exhaustion of natural resources. In this communication we demonstrate that interconnected and porous carbon fibers easily obtained from lignin exhibit ultrafast charge-discharge and excellent energy density and cyclability performance, to be used as binderless and flexible electrodes in supercapacitors.Electrochemical capacitors, usually referred to as supercapacitors, are electrochemical devices for electrical energy storage and harvesting applications that can complement or replace batteries when high power delivery or uptake and/or long cycling stability are required. Additionally, these devices improve efficiencies in supply systems (such as internal combustion engines, renewable energy systems, batteries and fuel cells) by storing energy when in excess or not needed. Thus, they are considered as one of the most powerful technologies that will provide a more efficient and sustainable utilization of energy on a short-term and real scenario of increasing energy costs and threatening climate change. 1 In spite of such relevance, the widespread utilization of supercapacitors has not been achieved due to a high cost to performance ratio. Among different electrode candidates, the last decade has witnessed a very intense research in the capacitance and storage performance of different nanostructured carbons, like graphene, CNTs, fullerenes, CNFs, onions, templated carbons, etc. 1 Nevertheless, apart from the significant progress on fundamental aspects, the complex and expensive manufacture as well as the complicated handling and electrode processing of these carbon materials make uncertain their feasible application. Accordingly, while novel electrode materials and chemistries are being developed to improve the storage performance, further research on simpler and cheaper manufacture and processing is demanded. Activated carbons (AC) derived from biomass and polymers have been identified as the currently most viable materials for supercapacitors, from both economic and sustainability points of view. 1a To attain the specifications for widespread commercialization, various issues need to be solved, being the most important to produce AC with a high accessible surface area and a sufficiently high electrical conductivity. On the one hand, biomass and natural polymers show a heterogeneous structure and contain impurities that complicate their processing to produce adequate conductive carbon electrodes. On the other hand, the derived ACs are conventionally obtained as powder or granular materials. Both the particle-like morphology and porosity strongly increase their inter-and intra-particle electrical resistances, 1e respectively, what makes necessary their processing into electrode pastes (by using auxiliary binders and conductivity promoters). This generates many electric point contacts that decrease their mechanical and chemical stability. 2 Hence the development of binderless porous carbons by a dire...

ABSTRACT:The electro-oxidation of carbon materials enormously degrades their performance and limits their wider utilization in multiple electrochemical applications.In this work, the positive influence of phosphorus functionalities on the overall electrochemical stability of carbon materials has been demonstrated under different conditions. We show that the extent and selectivity of electroxidation in P-containing carbons are completely different to those observed in conventional carbons without P.The electro-oxidation of P-containing carbons involves the active participation of phosphorus surface groups, which are gradually transformed at high potentials from less-to more-oxidized species to slow down the introduction of oxygen groups on the carbon surface (oxidation) and the subsequent generation of (C*OOH)-like unstable promoters of electro-gasification. The highest-oxidized P groups (-C-O-P-like species) seem to distribute the gained oxygen to neighboring carbon sites, which finally suffer oxidation and/or gasification. So it is thought that P-groups could act as mediators of carbon oxidation although including various steps and intermediates compared to electroxidation in P-free materials.

Hierarchical porous carbon materials prepared by the direct carbonization of lignin/zeolite mixtures and the subsequent basic etching of the inorganic template have been electrochemically characterized in acidic media. These lignin-based templated carbons have interesting surface chemistry features, such as a variety of surface oxygen groups and also pyridone and pyridinic groups, which results in a high capacitance enhancement compared to petroleum-pitch-based carbons obtained by the same procedure. Furthermore, they are easily electro-oxidized in a sulfuric acid electrolyte under positive polarization to produce a large amount of surface oxygen groups that boosts the pseudocapacitance. The lignin-based templated carbons showed a specific capacitance as high as 250 F g(-1) at 50 mA g(-1) , with a capacitance retention of 50 % and volumetric capacitance of 75 F cm(-3) at current densities higher than 20 A g(-1) thanks to their suitable porous texture. These results indicate the potential use of inexpensive biomass byproducts, such as lignin, as carbon precursors in the production of hierarchical carbon materials for electrodes in electrochemical capacitors.