Electrochemical Deposition of Organic-inorganic Composites for Supercapacitors

Ming-Hua Bai, Xiaoxia Liu*

Department of Chemistry, Northeastern University , Shenyang, 110819, China

Electrochemical capacitors (supercapacitors) are widely recognized as an important class of energy storage devices.Development of high performance supercapacitors is highly desirable to meet the increasing demand for energy storage devices. Conducting polymers, including polyaniline (PANI) and polypyrrole (PPy) have promising applications in a variety of technologic fields, including supercapacitor. One-dimensional (1D) growth control of conducting polymer, directing to polymer nanofibers, has aroused great interest because an ordered arrangement of the polymer chains favours higher conductivity and better performance in charge storage. The growth of nanofibers is known to be intrinsic to PANI, however heterogeneous nucleation on the initially-formed PANI nanofibers would result in irregularly-shaped PANI particles. The suppression of this overgrowth on the surface active sites of initially-formed PANI nanofibers has been achieved by some chemical polymerization methods, including aqueous/organic interfacial polymerization, rapidly-mixed reactions, which led to the formation of nanofibrous PANI. However, only nonfibrous, granular powder PPy can be yielded by these methods since fibrillar structure is not intrinsic for PPy and so it is very hard for PPy to grow one dimensionally. Electrochemical deposition is very attractive due to the ability to anchor the product onto substrate materials in the desired quantity, shape and size in one single step, enabling the final application to be performed easier.

In this work, we will present the one-dimensional growth of conducting polymer through electrochemical co-deposition with inorganic oxide. Pseudocapacitive properties of the obtained composite films are studied as well. The local environment at the electrode surface for polymerization was tried to be controlled by the electrodeposition of inorganic oxide from their precursors like VO²⁺, in which process proton may be released and some of the anodic charges may be consumed. Composites with improved electrochemical performance were obtained through 1D growth control of the conducting polymer, leading to increased surface area and organic-inorganic synergistic effect.

Acknowledgements

We gratefully acknowledge financial supports from National Natural Science Foundation of China (project number: 21273029) and Research Foundation for Doctoral Program of Higher Education of China (project number: 20120042110024).

Electrochemical capacitors, called supercapacitors, are promising energy storage system because of high power density, high charge/discharge rate and excellent electrochemical stability. However, compared with batteries such as Li-ion battery, the specific capacitance and energy density of the supercapacitor, especially electro double layer cpapacitor (EDLC), is relatively low. Graphene is intensively researched as a promising candidate for electrode of supercapacitors in that it has high surface area and conductivity that can increase the capacitance of supercapacitors. But still, graphene based supercapacitors could not achieve the expected performance because of the nature of the two dimensional structure of graphene that can be stacked and get reduced the active surface area. In order to solve this problem, there are many research have been performed to change graphene structure three dimensional and control the pore size and surface area of graphene. On the other hand, there have been many research that try to complement graphene by introducing metal oxide that have high capacitance but also high resistance, due to the pseudocapacitance, expecting to take the advantages of both materials.

In this study, we synthesized the hollow reduced graphene oxide(rGO)/metal oxide composite for the supercapacitors electrode. The micro-sized polystyrene sphere was synthesized by polymerizing the styrene and coated with the positively/negatively charged reduced graphene oxide sheets layer by layer in the aqueous solution. After removing polystyrene templates by organic solvent, the metal oxide nano-particles are grown on the surface of the hollow rGO structures. The hollow rGO/metal oxide composite was successfully synthesized and confirmed by scanning electron microscope and transmission electron microscope. The structure of hollow rGO/metal oxide composite could be controlled by template's size and layer of positively/negatively charged rGO. The hollow rGO/metal oxide composite showed excellent specific capacitance, energy density and power density.

In order to meet strong demands for electronic structure analysis of green devices, namely 1) power generation devices such as polymer electrolyte fuel cells (PEFC), 2) power efficient devices such as graphene FET and Resistive RAM, and 3) energy storage devices such as Li ion battery (LIB), we have developed two soft X-ray nano-spectroscopy systems at the SPring-8 University-of-Tokyo (UT) outstation. One is operando soft-X-ray emission spectroscopy (XES)¹⁾ forPEFC cathode catalysts and Li ion battery, and the other is scanning photoelectron microscopy with 70 nm spatial resolution²⁾, which has been used to analyze graphene FET and organic FET in operando.

First, we analyzed electronic structures of carbon-related catalysts alternative to Pt for PEFC in order to elucidate the oxygen reduction reaction (ORR) mechanism. We prepared metal phthalocyanine-based carbon catalysts with 1-2% nitrogen and less than 1 % of Fe for PEFC. Photoelectron spectroscopy and first principles calculation revealed that zigzag edge carbons with neighboring graphite-like nitrogen are ORR active sites. B ased on these analyses, we fabricated fuel cell stack for PEFC which showed comparable performance to Pt catalysts. Furthermore, we have taken operando XES spectra of Fe 2p-3d transition during power generation, revealing that Fe impurity may act as an ORR catalyst³⁾.

Next, we analyzed the change of Fe 3d states accompanied with the Li intercalation/ deintercalation process by resonant photoemission spectroscopy. Since the battery voltage should reflect the energy difference between Li 2s and Fe 3d down-spin state, we measured the change in Fe 3d down-spin states for LiFePO₄ (3.4V) and Li₂FeP₂O₇ (3.6V) and found that 0.2 eV shift from PO₄ to P₂O₇ poly-anions directly reflects battery voltage. Furthermore, the operando XES method was applied to cathode materials LiMn₂O₄ in Li ion battery to reveal the electronic structure change of Mn with changing OCV (open circuit voltage). It was demonstrated that the Mn³⁺ and Mn⁴⁺ states are successfully distinguished using high-energy-resolution resonant XES⁴⁾. Multiplet calculations⁵⁾ have been performed to determine the electronic structures in comparison with operando XES spectra for both Fe and Mn chemical states in FC and LIB, respectively.

This work has been done in collaboration mainly with Y. Harada, H. Niwa,T. Aoki, Y. Nabae, Y. Nanba and D. Asakura.

References

1) Y. Harada et al., Rev. Sci. Instrum. 83, 013116 (2012). 2) K. Horiba et al., Rev. Sci. Instrum. 82,113701(2011). 3) H. Niwa et al., Electrochemistry Com. 35, 57 (2013). 4) D. Asakura et al., SPring-8 BL07LSU Activity Report 2012. 5) Y. Nanba et al., PCCP 14, 7031 (2014).

Understanding the role of water in of di-oxygen electrochemistry at atomic and molecular levels is the key to driving technological innovations that are urgently needed to deliver reliable, affordable and environmentally friendly energy [1-4]. Surprisingly, all previous studies have treated the water molecule as reactants needed to satisfy the stoichiometry of the reaction, rather than as vital hydrogen-donor molecules that can promote the rates of transformation of oxygen intermediates to final products. It is the impact of water on di-oxygen electrochemistry that constitutes the focus of our paper. First, we introduce a universal model that is capable of rationalizing, and ultimately understanding, electrocatalysis of the oxygen reaction in aqueous media, as well as in Li-O₂ electrochemistry in organic environments. The model is based on the formation of HO_ad···H₂O (alkaline) and LiO₂···H₂O (organic solvents) complexes that place water in a configurationally favorable position for proton transfer to O₂^- and HO₂^- intermediates that are formed on neighboring active sites. We propose that monometallic electrodes modified by omnipresent oxygenated spectators such as OH_ad and LiO₂ are, in fact, bifunctional catalysts capable of facilitating different parts of the overall multi-electron process: providing adsorption sites for the formation of complexes as well as bare metal sites to facilitate the electron transfer to O₂, O₂^- and HO₂^-.

Moreover, we demonstrate that water plays a dual role in Li-O₂ electrochemistry, acting simultaneously as a promoter in the production of Li₂O₂ and also as a catalyst, regenerating itself through a sequence of steps that include the recombination of H⁺ and OH^- back to water. Water acting as a catalyst has not, to the best of our knowledge, previously been reported for any electrochemical reaction.

References:

1. N. M. Markovic; “Interfacing Electrochemistry”; Nature Materials; 12(2013)101-102

2. R. Subbaraman, D. Tripkovic, K-C. Chang, D. Strmcnik, A. P. Paulikas, H.P. Hurinsit, M. Chan, J. Greeley, V. Stamenkovic and N. M. Markovic; “Trends in Activity for the Water Electrolyzer Reactions on 3d-M(Ni,Co,Fe,Mn)-Hydr(oxy)oxide Catalysts”; Nature Materials, 11 (2012) 550-557.

3. R. Subbaraman, D. Tripkovic, D. Strmcnik, K-C. Chang, M. Uchimura, A. P. Paulikas, V. Stamenkovic and N. M. Markovic; “Enhancing Hydrogen Evolution Activity in Water Splitting by Tailoring L⁺-Ni(OH)₂-Pt Interfaces”; Science, 334 (2011) 1256-1260

4. H. Gasteiger and N.M. Markovic; Fuel Cells - " Just a Dream-or Future Reality ", Science, 324 (2009) 48-49.

Graphene, which has a one-atom thickness with two-dimensional honeycomb carbon nanostructure, has been significantly explored as an electrode material for electrochemical energy storage devices, owing to the high surface area (~ 2630 m²/g), sufficient porosity, superior conductivity, and excellent electrochemical stability of graphe n e . In contrast to conventional porous-carbon materials, the ultrahigh surface area of graphene does not depend on the pore structure and distribution in the solid state but comes from open channels between the 2- d imen s ional nanosheets. The chemical properties of graphene are dependent on the nature and abundance of edge sites since the concentration of edge sites at the periphery of graphene sheets is quite high in comparison to the basal plane sites.[1] One obvious challenge would be to utilize the 2D carbon nanostructure with regard to its large specific surface area and edge sites for the potential application in energy storage devices.[2]

At the same time, metal oxide/graphene nanocomposites are also of considerable interest for electrochemical energy storage applications owing to their outstanding electrical, chemical, and electrochemical properties. These excellent properties of metal oxide/graphene nanocomposites are generated from synergistic combination of graphene with metal oxide on the nanometer scale. Since the pseudocapacitive reaction of metal oxide is known to be a surface reaction, only the surface or a very thin surface layer of the oxide can participate in the pseudocapacitive reaction. [3-7] Therefore metal oxide/graphene nanocomposites should be synthesized in a way that metal oxide forms only on the surface of graphene on the nanometer scale with a control over particle size, particle size distribution, coating layer thickness, thickness uniformity and loading amount of metal oxide on graphenes to improve both high power and high energy properties for electrochemical energy storage applications.

In this study, we report on the synthesis and electrochemical characterization of graphene and metal oxide/graphene nanocomposites for electrochemical energy storage applications.

References

[ 1 ] S.H. Park, S.M. Bak and K.B. Kim, J. Mater. Chem. 21 ( 2 011 ) 680

[ 2 ] C.W. Lee and K.B. Kim, Nanoscale, 5 (2013) 9604

[ 3 ] S.M . Ba k, and K.B. Kim, J. Mater. Chem. 21 ( 2011) 17309

[ 4 ] H.K. Kim, S.M. Bak, and K.B. Kim, Electrochem. Commun. 12 ( 2010 ) 1768

[ 5 ] K.B. Kim, Nanoscale, 5 (2013) 6804

[ 6 ] S.H. Park and K.B. Kim, Scientific Reports DOI: 10.1038/srep06118 (Accepted, 2014)

[ 7 ] H.K. Kim and K.B. Kim, Chemistry of Materials DOI: 10.1021/cm5020898 (Accepted, 2014)

The desire to increase the utilization of sustainable energy sources such as solar and wind is hampered by their intermittent nature. Large scale energy storage capacity is needed to maximize utilization of these sources, specifically to avoid large amounts of renewable energy being wasted when their supply exceeds demand.

Over the last years we have studied the electrochemical reduction of CO₂ to various value-added chemicals such as carbon monoxide (CO), formic acid, and methane. When coupled to renewable energy sources such as wind and solar, this process can produce carbon-neutral fuels or commodity chemicals, possibly providing a method for storage of otherwise wasted excess energy from intermittent renewable sources [1].

For this process to become economically feasible, more active and stable catalysts as well as better electrodes are necessary such that CO₂ electrolyzers can be operated at sufficient conversion (current density >250 mA/cm²), reasonable energetic efficiency (>60%), and sufficient product selectivity (Faradaic efficiency >90%). For CO production, a key reactant in the Fischer-Tropsch process, the best performance reported to date is current densities on the order of 90 mA/cm² and energy efficiencies up to 45%, when operating at ambient conditions [2]. This presentation will focus on new catalysts systems for efficient conversion of CO₂ to CO: (i) Ag nanoparticles supported on TiO₂ [3]; (ii) Au nanoparticles supported on multiwall nanotubes; and (iii) metal-free N-doped carbons. These catalysts have been characterized in a 3-electrode cell and in an electrolyzer. Current densities of between 100 and 250 mA/cm² as well as energy efficiencies of up to 70% were obtained. The electrodes in all these cases are prepared using automated airbrushing [2], which reduced catalyst loadings to 0.75 mg/cm² for Ag and 0.17 mg/cm² for Au. These performance levels, together with the lower cost due to low precious metal loading (due to the use of catalyst supports), or even the elimination of precious metals altogether (N-doped carbons), brings electrochemical reduction of CO₂ to CO closer to economic feasibility.

We also performed an economic / life-cycle analysis of this process, to determine whether this technology can become, economically viable for large scale application in the storage of energy from renewable sources, and/or in the reduction of greenhouse gas emissions.

References

[1] H.R. Jhong, S. Ma, P.J.A. Kenis, Current Opinion in Chemical Engineering 2 (2013) 191.

[2] H.R. Jhong, F.R. Brushett, P.J.A. Kenis, Advanced Energy Materials 3 (2013) 589.

[3] S. Ma, Y. Lan, G.M.J. Perez, S. Moniri, P.J.A. Kenis, ChemSusChem 7 (2014) 866.