Biochar-based composites as electrode active materials in hybrid supercapacitors with particular focus on surface topography and morphology
Introduction
Considering the electrochemical characteristics (specific capacity, cycling stability, etc.), cost, and environmental impacts of all supercapacitors (SCs), improving the quality of the active materials that have the highest importance in the storage mechanism has gained noticeable attention [1,2]. Carbohydrate and biopolymer materials seem like the ideal candidates for a supercapacitor as carbon is the second most abundant element in the biosphere. Table 1 shows the main differences in terms of precursors, operational conditions and features among conventional carbon-based materials (particularly activated carbon and graphene) and biochar. The main disadvantages of the activated carbon and graphene are their dependence on fossil fuel components, and their energy-intensive synthesis conditions [3,4]. These issues are both costly and not eco-friendly, specifically when using synthesis processes that require toxic reagents. Recently, in order to address these concerns, biochar based electrodes have been introduced. Biochar is obtained from heat treating biomass and industrial waste and can be used as a substitute for the current electrode materials due to their adjustable and modifiable physical and chemical surface features [3,5]. In addition, compared with the non-renewable activated carbon and graphene, biochar is considered to have a sustainable cycle in which less carbon returns to the atmosphere due to the adsorption of CO2 onto the carbon matrix of biochar. The biochar cycle is displayed in Fig. 1. This process leads to the removal of carbon dioxide from the carbon cycle and thus mitigates climate change. It is estimated that 0.1–0.3 billion tons of CO2 can be adsorbed from the atmosphere through carbon storage in biochar [6].
To apply the biochar-based electrodes in practical supercapacitors, the following challenges need to be met:
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Developing a simple, inexpensive, scalable, and environmentally friendly method to produce biochar with high specific surface areas, controllable pore size, pore geometry, and pore connection.
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Developing supercapacitors with high cycling performance and power density as well as an energy density close to that of current supercapacitor batteries [13,14].
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Enhancing the energy density of biochar-based supercapacitors through introducing pseudocapacitive materials, such as heteroatoms, metal oxides, and conductive polymers [7], [8], [9], [10], [11], [12].
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Investigating the charge storage mechanisms and selecting suitable electrolyte materials, systems, and characterization methods.
This review focuses on the more recent studies of hybrid supercapacitors (HSCs) arising from BBCs. In particular, this review paper provides an overview of the effects of advanced modification techniques on surface topography and morphology and electrochemical behavior. It is worth reminding that critical factors such as electrical conductivity, improvement of active sites, and charge storage mechanism should be discussed in deep details regarding BBCs.
Section snippets
Conventional modification methods of biochar
Many different hybrid materials combining biochar with Transition Metal Oxides (TMOs) have been developed as electrode active materials in energy storage systems. Most state-of-the-art hybrid structures with excellent capacitive performance have been reviewed comprehensively by Thomas et al. Simply put, modified biochar can be synthesized via chemical oxidation, chemical reduction, or metal loading [15].
Two-dimensional (2D) biochar architectures
Surface topography and morphology, BET surface area, and graphitization level of biochar also have a great influence on the performance of the SCs [1,2]. Non-activated biochar usually possesses low surface area, density, and conductivity hindering their application for fabrication of the working electrodes. Thus, further activation and modification are needed to improve their electrochemical behavior. Some non-activated biochars have a unique porous structure making them an interesting
Application of biochar composite in supercapacitor electrodes
There are two types of SCs based on the charge storage mechanism, namely EDLCs and pseudocapacitors. EDLCs stores electrical energy near the electrode-electrolyte interface using a non-faradic double-layer capacitive process; while pseudocapacitors employ faradic redox reactions of electroactive materials (AM) [64]. As shown in Fig. 6, pseudocapacitors are mainly made out of transition metal oxides/hydroxides (i.e. MnO2, Fe3O4, NiCo2O4, Co3O4, Ni(OH)2, Co(OH)2, etc.). In contrast, AM in EDLCs
Conclusion and future perspectives
In summary, this extended overview presents a wide range of synthesis methods for biochar-based composites and their positive effect on improving hybrid supercapacitors by combining the EDLC of biochar and pseudocapacitance of nano-materials. Combined activation method including the integration of physical and chemical activations and heteroatom doping has become an important practice for reaching a high-performance supercapacitor. This study provided insight into choosing suitable biomass with
Declaration of Competing Interest
The authors whose names are listed immediately below certify that they have NO affiliations with or involvement in any organization or entity with any financial interestand they all confirm the submission of the manuscript to the Journal of Energy Storage.
Acknowledgement
This research is financially supported by Ontario Ministry of Agriculture, Food and Rural Affairs (OMAFRA). The authors would like to acknowledge Mr. Anthony Broeders for proofreading the manuscript.
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