MATERIALS FOR SUPERCAPACITORS ELECTRODES: PREFORMANCE AND NEW TRENDS Bologna, May 23 th 2017 M. Federica De Riccardis SSPT-PROMAS-MATAS OUTLINE Basic concepts EDLC and PC Porosity Electrode materials Carbon Metal oxide Conducting polymers Structures Electrode (0D, 1D, 2D, 3D) Device (symmetric, asymmetric, battery-like) 2 1
Basic concepts Charge storage mechanisms Electric double layer Electrostatic process Stored charge proportionally to surface area Specific areal capacitance = 0.1/0.2 F/m 2 Pseudocapacitive Faradaic redox reaction Stored charge into surface area or bulk Specific areal capacitance = 1/5 F/m 2 Current collector Separator 3 Structural characteristics of a SC electrode Current collector Electrolyte filled pore Active material Separator The performance of supercapacitors is mainly determined by the electrochemical activity and kinetic feature of the electrodes. The pore structure of electrode materials is closely related to ion and electron transport processes. Electrodes with proper pore structure are highly desirable. Porous electrodes should meet the following requirements: Sufficient pore volume Suitable ion channel Plentiful chemically active sites 4 2
Electrode Materials Double layer capacitors Activated carbon Carbon nanostructures Supercapacitors Pseudocapacitors Metal oxides Conducting polymers Composites (Symmetric) Hybrid supercapacitors EDL/PS (Asymmetric) Battery-like 5 Electrode Materials CARBON materials Activated carbons (ACs), CNTs, CNFs, graphene, carbon aerogels, ordered mesoporous carbons (OMCs), hierarchical porous carbons (HPCs) are widely utilized due to their: easy accessibility, good processing ability, large surface area/porosity, low electrical resistivity, robust surface chemical environment, physicochemical stability, low cost. The typical capacitance of carbon materials is 50 200 F/g in aqueous electrolytes, 30 100 F/g in organic electrolytes 20 70 F/g in ionic liquids. 6 3
Electrode Materials METAL OXIDES Noble metal oxides and cheap metal oxides are more attractive than carbon materials thanks to their much larger capacitance, due to multi-electron transfer during fast Faradaic reactions. RuO2: high specific capacitance and very long cycling life, but high cost. Cheaper metal oxides (e.g. MnO2 and NiO): poor electrical conductivity and poor cycling stability. Also NITRIDES (tungsten nitride, titanium nitride, etc.) have very good pseudocapacitance. 7 Electrode Materials CONDUCTING POLYMERS They store charge in the bulk and the energy density is considerably higher than the surface redox materials. As a drawback, the power density is affected by slow ion diffusion in the bulk material but the conductivity is higher than the metal oxides. The dopant level is an essential parameter in the electrochemical charge storage. The p-doped polymers have a better performance than n-doped polymers, requiring a large negative potential and having a high resistance. The p-doping occurs with removal of -electrones from the conjugation, leading to net positive charge. 8 4
Structures of electrode materials Solid nanoparticles 0 D Hollow nanoparticles Core-shell nanoparticles Nanostructured electrode materials 1 D 2 D Homostructures (nanorods, nanowires, nanotubes) Heterostructures (structural/electrical core/ active material shell) Homostructures (graphene) Heterostructures (graphene and metal oxides) 3 D 3D carbon based materials Metal foams 9 0D nanomaterials 0D materials based on carbon can be solid or hollow or can have a core-shell structure. Activated carbons have high surface area (up to 3000 m 2 /g) and a wide range of pore size distribution including micropores (<2 nm), mesopores (2 50 nm), and macropores (>50 nm). 0D onion like carbon Specific areal capacitance < 10 F/cm 2 D. Pech, M. Brunet, H. Durou, P. Huang, V. Mochalin, Y. Gogotsi, P.-L. Taberna and P. Simon, Nat. Nanotechnol. (2010) The combination of faradaic and non-faradaic materials into core shell 0D nanostructures offers considerable advantages: enhanced electrical conductivity, less agglomeration, robust chemical and mechanical stability. Specific capacitance: HCS-PANI ~ 520 F/g HCS ~ 270 F/g (a) FESEM and (b) TEM images of hollow carbon spheres (HCS) coated by PANI. 10 Z. Lei, Z. Chen and X. Zhao, J. Phys. Chem. C, (2010) 5
1D nanomaterials B. Kim, H. Chung and W. Kim, J. Phys. Chem. C, (2010) Carbon nanotubes (CNTs) have several applications in energy storage field. Vertically aligned CNTs directly grown on conductive substrates, have moderate to high surface area (120 500 m 2 /g), porous structure, superior electronic conductivity, and excellent mechanical and thermal stability. Specific capacitance ~200 F/g @ 20 A/g 1D nanostructured current collector core can provide a support for active sites, forming a core-shell 1D material. Specific capacitance ~ 1400 F/g @ 5mV/s Z. Yu and J. Thomas, Adv. Mater. (2014) 11 1D nanomaterials SEM image PANI+CNTs TEM image PANI/CNTs combines the large pseudocapacitance of the conducting polymers with the fast charging/discharging double layer capacitance and excellent mechanical and electrical properties of the carbon nanotubes. CNTs Specific capacitance ~110 F/g @ 50 mv/s PANI/CNTs Specific capacitance ~260 F/g @ 50mV/s M. F. De Riccardis, et al., Functional Characterisations Of Hybrid Nanocomposite Films Based On Polyaniline And Carbon Nanotubes, Advances in Science and Technology Vol. 79 (2013) pp 81-86 12 6
1D nanomaterials Ternary composite electrode structure formed by CNT + M.O.+ C.P. Specific energy ~ 73 Wh/kg and a cyclic retention of 81% at 1000 cycles Ye Hou, Yingwen Cheng, Tyler Hobson, Jie Liu, Nano Lett., (2010) 13 2D nanomaterials GRAPHENE with RuO 2, MnO 2, WO, NiO, VO.. RuO 2 graphene (30 wt% graphene sheets) hybrid showed a specific capacitance of 370 F/g @ 2mV/s 280 F/g @ 40 mv/s in 1M KOH solution Specific capacitance of MnO 2 graphene 310 F/g @ 2mV/s 228 F/g @ 500 mv/s Pure graphene 104 F/g @ 2mV/s 103 F/g @ 500 mv/s H. Wang, Y. Liang, T. Mirfakhrai, Z. Chen, H.S. Casalongue, and H. Dai, Nano Res.(2011) 95% of capacitance retention after 15 000 cycles 14 7
3D nanomaterials CARBON AEROGELS, MESOPOROUS CARBONS, CARBON CLOTHS 3D flower-like -Ni(OH) 2 /GO/CNTs composite prepared via a phase transformation method. Specific capacitance 1815 F/g (@ 2 A/g cycling performance of 97% capacitance retention after 2000 cycles at 10 A/g X. Ma, J. Liu,C. Liang, X. Gong and R. Che, J. Mater. Chem. A (2014) 15 3D nanomaterials Carbon or Metal foam A particular combination of -lightweight substrate -porous nanostructure design -conductivity modification. (a) Porous Ni films as the substrate (b) 3D GF by CVD growth from the Ni film substrate (c) Co 3 O 4 nanowires by hydrothermal growth on the GF (d) PEDOT MnO 2 composite shell by co-electrodeposition X. Xia, D. Chao, Z. Fan, C. Guan, X. Cao, H. Zhang, H.J. Fan, Nanoletters (2014) 16 8
3D nanomaterials Electrospun porous carbon nanofibers (PCNFs) As 3D structures, porous electrospun fibers obtained by pyrolysis of polymer mats allow to have high specific surface area and high pore volume. Different approaches are used for porous CNFs: Self-activation Physical/chemical activation Incorporation of sacrificial components 17 3D nanomaterials Self-activation Lignin (with a high oxygen content) can be self-activated during an heating process, by realising the groups containing oxygen and producing many pores. CNF = 100 nm SSA ~ 580 m 2 /g Specific capacitance = 50 F/g @2 A/g Capacitance delay = 10% after 6000 cycles C. Lai, Z. Zhou, L. Zhang, X. Wang, Q. Z, Y. Zhao, et al.j. Power Sources (2014) Physical/chemical activation H 2 O steam activation is used to generate pores. PAN fibres activated at different temperatures Pores = 0.5 nm SSA ~ 500 m 2 /g Specific capacitance = 175 F/g @ 1 A/g C- Kim, J. Choi, W- Lee, K. Yang,.Electrochinìmica Acta (2004) 18 9
3D nanomaterials Incorporation of sacrificial components Blend with sacrificial components can be selectrively removed by treating in solvent or direct annealing, generating porous structures. PAN+Nafion CNF ~ 200 nm SSA ~ 1600 m 2 /g Specific capacitance = 210 F/g @ 100mV/s C. Lai, Z. Zhou, L. Zhang, X. Wang, Q. Z, Y. Zhao, et al.j. Power Sources (2014) Some inorganic materials can be sacrified to form ultra-pores. Pores in follow fibres are generated by etching of SiO 2 nanoparticles (bamboolike) PAN+SiO 2 Pores ~ 0.6/100 nm nm SSA ~ 1900 m 2 /g Specific capacitance = 230 F/g @ 5 A/g Y. Sun, R.B. Sills, X. Hu, Z. Wei Seh, X. Xiao, H. Xu, W. Luo, et al. Nanoletters (2015) 19 3D nanomaterials Hybrid electrode based on ECNFs and MO or CP (two components) CNFs have been shown to be an efficient matrix for MO A shell of MnO2 nanosheets onto ECNFs was obtained by in situ redox deposition Specific capacitance = 560 F/g @ 1 A/g retaining 94% of its initial capacitance after 1500 cycles J.G. Wang, Y. Yang, Z. H. Huang, F. Kang, Electrochimica Acta (2011) ECNF-CNTs web was prepared by electrospinning. PPy coated the electrospun ACNF/CNT (PPy/ACNF/CNT) by in situ chemical polymerization SSA = 1170 m 2 /g Specific capacitance = 330 F/g Y. W. Ju, G.-R. Choi, H. R. Jung,W. J. Lee, Electrochimica Acta (2008) 20 10
3D nanomaterials Hybrid electrode based on ECNFs and MO and CP (three components) J.-G. Wang, Y. Yang, Z.-H. Huang, F. Kang, J. Mater. Chem. (2012) ECNFs was coated by MnO 2 /PPy (~ 20 nm) Specific capacitance = 700 F/g Areal capacitance = 1.4 F/cm 2 21 3D nanomaterials Advantages of electrodes produced by E.S. 1) The size of active materials can be reduced to nanoscale, proving sufficient contact between electrolyte and active materials for shortening the ion diffusion pathway. 2) The carbon matrix enables a high electrical conductivity of the composite electrode and can be directly used as active material. 3) The introduction of a porous structure is of a great importance in reducing the volume change of active materials, so acquiring long term cycling stability. 4) Selective doping of these nanomaterials can increase the conductivity and surface property, and therefore also the electrochemical activity. 5) A continuous fibre network offers a good mechanical and electrical interconnection of the hybrids, achieving freestanding electrodes. 22 11
Hybrid Supercapacitors Type Electrode Mechanism EDLC Carbon and carbon derivates Electrochemical Double Layer capacitance Pseudocapacitor Metal oxides, conducting polymers Reversible Faradaic reactions Symmetric Asymmetric Battery-like Same electrodes, also with composite materials Different electrodes with different materials Anode: battery electrode Cathode: carbon Combination of reversible Faradaic reaction and Ellectrochemical double layer capacitance Combination of reversible Faradaic reaction and Ellectrochemical double layer capacitance Lithium intercalation and de-intercalation mechanism like in battery 23 Future challanges Design the high surface area electrode to properly utilize the total area. The nanostructure morphology should be able to extend the life time of the device. A support structure should maintain the structure of active material. The internal resistance has to be minimum. Since, it is influenced by the thickness of active material (especially in MO) and by interfacial resistance between the current collector and the electrode, it should work upon manufacturing of binder free electrode, without affecting its integrity. 24 12
Thank you for your attention M. Federica De Riccardis federica.dericcardis@enea.it 13