17-3 The Effects of Potassium Ferrocyanide/Potassium Ferricyanide and their Derivatives on the Performance of Solid-State Supercapacitor Xiangyang Zhou, Xiaoyao Qiao, Chen Zhang, and Yuchen Wang Department of Mechanical and Aerospace Engineering, University of Miami 1251 Memorial Drive, Coral Gables, Florida, USA, FL33124 [email protected] / 1-305-284-3287 Azzam N. Mansour, Gordon H. Waller, and Curtis A. Martin NSWCCD, 9500 MacArthur Boulevard, West Bethesda, Maryland, USA MD20817 [email protected] / 301-227-4451 Abstract: In previous studies, we have demonstrated that function of mediators, including increasing capacitance of the addition of metal cyanide mediators or redox species supercapacitor and promoting the ionic conductivity of the into polymer electrolytes can effectively increase the electrolyte, have been observed and analyzed. specific energy and power of a mediator containing As a mediator, Prussian blue derivatives or analogues supercapacitor relative to a supercapacitor without a (PBAs) have fast reversible redox kinetics and high specific mediator. The benefits of combining mediators into the capacitance. However, compared to other redox mediators, composite electrode, which contains carbon powder and PBAs have unique advantages in practical applications: polymer electrolyte, are the introduction of pseudo- capacitance, promotion of ionic and electronic 1. PBAs are insoluble in most solvents. For other redox conductivities, and serving as additional ion sources for mediators, such as K3Fe(CN)6/K4Fe(CN)6 and NaI/I2, electric double layer capacitance. In order to further which are soluble in an aqueous electrolyte, an expensive enhance the performance of the mediator supercapacitor selective membrane must be employed to prevent the and to elucidate the charge/discharge mechanisms, a mediator-crossover issue for non- solid-state SC. systematic study was conducted. In this report, we will 2. PBAs containing multi-valent transition metals, such as present the research progress in three aspects: 1) synthesis K2MnFe(CN)6, could further improve the capacitance of of mediators in the form of KMFe(CN)6 (M=transition K3Fe(CN)6 and K4Fe(CN)6, because Mn can have oxidation metal); 2) structural characterization of the synthesized states of 2+, 3+, and 4+ under normal potential range. compounds using X-ray Photoelectron Spectroscopy (XPS) 3. For EDLCs and pseudocapacitors, the ions are consumed and X-ray diffraction (XRD); 3) experimental validation of to form the EDL or complete redox reactions. However, time dependent charge and discharge process. The full cell PBAs can provide extra ions to the electrolyte to maintain the measurements revealed that the specific energy of concentration, as shown in the following redox reaction: supercapacitors with mediators where M is Mn, Fe, Co, 2+ 2+ 3+ 3+ - + increased by more than 40% relative to that of K2Mn Fe (CN)6 → Mn Fe (CN)6 + 2e + 2K supercapacitors containing K3Fe(CN)6/K4Fe(CN)6. The objectives of the present research work are as follows. 1. Develop reliable and repeatable chemical synthesis Keywords: supercapacitor; potassium ferrocyanide, procedure to produce series of transition metal based PBAs. potassium ferricyanide; mediator; solid-state; XPS; XRD. 2. Characterize the synthesized PBAs using XPS and XRD Introduction to verify the structures and compositions of the PBAs. A mediator is a redox material that undergoes fast and 3. Fabricate mediator supercapacitors using the PBAs to reversible reactions. Examples of mediator pairs are NaI/I2, evaluate the performance of the mediator supercapacitors K4Fe(CN)6/K3Fe(CN)6. Our research group proposed a new as a function of mediators. concept of mediator supercapacitor (SC), which is 4. Study the effect of the mediators as ion providers during composed of large amount of activated carbon and small charge/discharge. amount of mediator [ 1 , 2 ]. This type of SC can be considered as an EDLC combined with a pseudocapacitor, Experimental Approach or a hybrid SC. The Faradaic reactions can increase the Sample Preparation: The K Fe(CN) and K Fe(CN) capacitance of the electrodes, replenish the number of free 4 6 3 6 mediators were obtained from Sigma-Aldrich. An attempt ions in the electrolyte, and minimize reduction of ionic was made to prepare a number of K Fe(CN) and conductivity during the charge/discharge processes. 3 6 K4Fe(CN)6 analogues using the following procedures: In previous work by Zhou’s group, a couple of (1) Dissolve 0.01 mole of transition metal (Mn2+, Co2+, mediators/redox species were reported to be effective in Ni2+, Cu2+, Zn2+) nitrate in 100 mL of deionized water with enhancing the performance of supercapacitors. The magnetic stirring at room temperature to get Solution 1. 292 (2) Dissolve 0.01 mole K3Fe(CN)6 or K4Fe(CN)6 in 100 mL the stoichiometry of the cyanide group. Considering the of deionized water with magnetic stirring at room semi-quantitative nature of XPS composition analysis, the temperature to get Solution 2. potassium concentrations of 11.0 and 11.4 for (3) Add these two solutions dropwise into 100 mL of K2MnFe(CN)6 and K2CoFe(CN)6, respectively, are in deionized water with magnetic stirring at room general agreement with its nominal value of 12.5. temperature. Precipitate forms immediately but stirring However, the potassium concentrations of 6.3 and 5.5 for continues for another 2 hours. K2CuFe(CN)6 and K2ZnFe(CN)6 are well below nominal value indicating that these two mediators will not form (4) The precipitate is centrifuged at 6500 rpm, washed with pure phases with the indicated nominal stoichiometry deionized water for three times, and dried in nitrogen-filled using the synthesis procedure described here. glovebox at 50°C for 1 day. (5) The raw product is grounded by hands with a mortar Table 1. Summary of XPS composition of various mediators. and pestle and kept in a nitrogen-filled glovebox. Sample C N O K Fe M % The general chemical reactions are shown as follow: K4Fe(CN)6 40.7 22.8 9.8 22.5 4.2 N/A K3Fe(CN)6 + M(NO3)2→ KMFe(CN)6 + 2KNO3 K3Fe(CN)6 43.8 26.5 6.6 18.5 4.7 N/A K Fe(CN) + M(NO ) → K MFe(CN) + 2KNO K2MnFe(CN)6 39.0 30.7 5.9 11.0 5.8 7.6 4 6 3 2 2 6 3 a b K2CoFe(CN)6 39.1 31.7 4.1 11.4 5.5 8.3 K4Fe(CN)6 + Fe(NO3)3→ KFeFe(CN)6 + 3KNO3 K2CuFe(CN)6 43.3 33.4 5.6 6.3 6.10 5.3 K ZnFe(CN) 44.2 29.7 6.4 5.5 4.88 9.4 XPS and XRD Measurements: XPS spectra were collected 2 6 using Physical Electronics VersaProbe II Scanning XPS KMnFe(CN)6 44.0 29.4 7.9 5.6 5.2 8.0 KFeFe(CN)6 46.1 27.2 13.3 0.5 12.9 N/A Microprobe using monochromatic Al Kα (1486 eV) X-ray a b source with a focused beam size of 200 microns rastered KCoFe(CN)6 42.3 34.2 6.3 0.6 4.9 11.7 over and area of 1000 x 200 microns. The structure and KCuFe(CN)6 46.8 31.2 9.0 0.4 6.1 6.5 phase purity of mediators were investigated by powder X- KZnFe(CN)6 51.7 28.5 7.0 0.4 4.0 8.4 ray diffraction using the Bruker D8 Advance a The Fe 2p3/2 concentration is distorted due to interference Diffractometer with Cu Kα X-rays (λ = 1.5404 Å). b with the Co Auger lines. The Co 2p3/2 concentration is Supercapacitor Fabrication: The electrodes were prepared distorted due to interference with the Fe Auger line. using the following steps: 1) adding 0.05 g polyvinylidene The potassium concentration of 5.6 is roughly close to the fluoride (PVDF) into 3 mL N-methyl-2-pyrrolidone solvent nominal value of 6.7 for KMFe(CN)6 where M is Mn but (NMP) and stirring at 80°C until PVDF is totally dissolved is reduced significantly below 1 when M is Fe, Co, Cu, or (this process may take 3 hours), 2) adding 0.1 g of Zn. Hence, the samples with potassium concentrations mediator, 0.9 g of activated carbon (AC), and 0.05 g of below 1 do not support the formation of significant conductive carbon to the previous solution and stirring for quantities of KMFe(CN)6. The concentrations of Fe are in 2 hours to form consistent slurry, 3) coating the composite qualitative agreement with nominal value for all slurry onto a 50 cm2 carbon paper with a drawdown tool, mediators. The Mn, Cu and Zn concentrations are and 4) The coated electrode is then dried in a glovebox at somewhat close to the nominal value. Note that the Fe 80°C for 24 hours. The same procedure was used to concentration for KFeFe(CN)6 is close to the nominal prepare an electrode without mediators and used as a value of 13.3. To be noted, oxygen was found to be reference for comparison purposes. Two aluminum foils present in all mediators with concentrations in the range were used as current collectors, a PVDF with 1 M of of 4.2-8.6 % except in the case of KFeFe(CN)6 where the tetraethylammonium tetrafluoroborate based polymer concentration is highest at 13 %. The presence of oxygen electrolyte membrane similar to those described in Refs. 1 was also confirmed on the basis of XRD indicating that and 2 were used as the separator. The SC cell was oxygen was not only present as surface contaminant but assembled in a glove box with nitrogen using a vacuum also in the bulk of the sample most likely in the form of sealing machine.