Synthesis of hydronium-potassium jarosite; Effect of pH and aging time on their structural, morphological and electrical properties

: Structural and morphological properties of the hydronium-potassium jarosite microstructures were investigated in this work, and their electrical properties were evaluated. All microstructures were synthesized at a reasonable temperature of 343 K with a reduced reaction time of 3 hours. Increase in the pH from 0.8 to 2.1 decreased the particle sized from 3 µm to 200 nm and increasing the aging time from 0, 3 to 7 days resulted in semispherical, spherical and euhedreal jarosite structures, respectively. A Rietveld analysis also was done, finding that increasing pH, the amount of hydronium substitution by potassium in the cationic site also increases, having a 77.72 % of hydronium jarosite (JH) plus 22.29 % potassium jarosite (JK) at pH 0.8; 82.44 % (JH) and 17.56 % (JK) at pH 1.1, and 89.98 % (JH) plus 10.02 % (JK) at pH 2.1. The results obtained in this work show that the obtained hydronium potassium jarosite microstructures with reduced particle size and euhedreal morphology can be used as anode materials for improving the life time of lithium ion batteries, due that during the analysis of the voltage obtained using electrodes made with this particles and graphite, this ranged from 0.89 to 1.36 V.


Introduction
Jarosite, and echo-rich mineral pertains to trigonal crystal system with rhombohedral structure and can be acquired from sites highly acidic surroundings like in the Spanish southeast, where jarosite was found for the first time in the Jaroso Ravine in the Almagrera Sierra from where its name came [1]. Jarosite type-compounds have been of great interest mainly for mineralogists and metallurgists devoted to extractive metallurgy, because the jarosite contain valuable metals such as silver, gold, etc., in their crystalline structure which can be recovered by alkaline decomposition processes [2][3][4][5]. During the elimination of iron in the leaching process, where the residues had to be filtered and discarded jarosite [6], was predominantly used where the particle size and morphology structure in the potential range of 1.5 -4 V vs. Li/Li +, via reduction/oxidation between Fe 3+ and Fe 2+ , which corresponds to a theoretical capacity of 166 mA h g -1 , which makes it a promising cathode material for LIBs [16,[26][27][28].
All the above studies and others, related to the jarosite type-compounds have managed to find interesting advances, not only in the innovation of obtaining these materials, which by modifying parameters and synthesis methods have found appropriate morphologies and particle sizes to be able to evaluate new properties that place them as a material with possible applications in new fields such as that of energy storage. Currently the field of portable electronics continues to be monopolized by first generation LiCoO2 layered cathodes in Lithium-Ion Batteries (LIBs) and recent research has turned in favor of removing Co due to its toxicity and method of obtaining. For this reason, they are looking for new materials that are low cost with great capacity to behave either as anode or cathode [29][30][31].
Therefore, this work demonstrates the improvements such as low operation temperature (343 K), low synthesis time (3 h), changes in pH and aging time applied to the traditional method of synthesis for the jarosite type-compounds to obtain a new modified method making the synthesis both economic and environmental friendly. Changes in pH (0.8, 1.2 and 2.1) and the aging time (0, 2 and 7 days) after the synthesis, have contributed significantly in the reduction of the particle size and morphologies spherical and euhedreal. Some authors have pointed that the high crystalline structure of jarosite type compound, its particle size and morphology, have great impact in the cycle-life and experimental achievable capacity for the galvanostatic charge-discharge in the LIBs [32]. Finally, the so obtained synthetic jarosite structures could be utilized as a novel material in Li ion batteries according with the results obtained in the Daniell Cell for the electrodes made of jarosite and graphite.

Synthesis of hydronium-potassium jarosite
Synthesis of potassium jarosite employed in this work was similar to the method reported by Dutrizac and Kaiman [33], and Salinas et al. [19] with considerable modifications. And were prepared modifying the method used as well as all chemicals, Iron (III) sulfate, Fe2(SO4)3 and potassium sulfate, K2SO4, as source of potassium jarosite; sodium hydroxide, NaOH and sulfuric acid, H2SO4, were utilized for adjusting pH in this work and were purchased from Sigma Aldrich with high purity (> 99 %). In the present work, 0.15 M of Fe2(SO4)3 and K2SO4, aqueous solutions were separately prepared. In a 0.5 L three-neck flask containing the mixture of Fe2(SO4)3 and K2SO4 in 0.3L deionized water equipped with pH measurements system, whose temperature was maintained at 343 K. Fig. 1 shows the experimental setup for obtaining the jarosite powders. The above mixed solution has a pH 1.1 and it was left for 3 h with a stirring rate ̴ 200 s -1 , which results in brown precipitates. The procedure was replicated for pH 0.8 and 2.1 samples just by adding H2SO4 and NaOH, respectively, drop wise through the left neck of the flask by monitoring the pH with a potentiometer placed in the right neck of the flask. The reaction time ̴ 3 h, synthesis temperature ̴ 343 K and stirring rate 200 s -1 were kept constant for all the samples. Finally, precipitates in each case were collected separately, filtered and dried at room temperature. Additionally, two more samples with pH 2.1 were synthesized with an additional aging time of 3 and 7 days before the filtration of precipitates. All the samples obtained are shown in table 1.

Material Characterization
All the obtained jarosite powders synthesized were characterized by Low Vacuum Scanning Electron Microscopy (LV-SEM) in a JEOL JSM5900-LV machine equipped with an Oxford EDS and operated at 20 kV to observe surface morphology, chemical composition and the effect of pH and sintering temperature on the particle size. The X-ray diffraction analysis was carried out by the powder technique in a Bruker D8 Discover diffractometer, with a CuKα = 1.5406 Å radiation source, operating at 40 kV and 40 mA. Diffraction patterns were collected in a 2θ range from 10° to 70° with an increment step size of 0.03°to identify the phase compound and the crystalline structure of jarosite. Finally, the X ray diffraction patters were subjected to a Rietveld analysis using a Topas 2 software; R 3e factor and χ 2 were parameters used to indicate the accuracy of this refinement of XRD patters.

Preparation of electrodes and evaluation of electrical conductivity
To evaluate the electrical properties of the powders of hydronium-potassium jarosite, electrodes of a mixture of jarosite and graphite were prepared. The procedure to elaborate the electrodes was the following; 0.1 g of graphite was weighed for each electrode, and from each jarosite sample 0.1 and 0.5 g were weighed respectively to obtain two electrodes from each sample. Then 15 to 20 of silicone oil were added with a syringe to form the paste that was subsequently deposited on a piece of transparent plastic acetate that served as a mere support for the paste, as is shown in Figure 2.  For the electrical tests, a supersaturated solution (60 ml) of calcium chloride (CaCl2) in injectable water was prepared to be used as electrolyte, a Cu coin was used as cathode, and the different electrodes prepared with paste of carbon + Jarosite synthesized at different pH´s.

XRD Analysis
The X-ray diffraction patterns of synthesized potassium jarosite powders obtained for all samples are shown in Fig. 4. A glance on patterns shows the formation of synthetic potassium jarosite (H3O-K)Fe3(SO4)2(OH)6) with preferential orientation in (113) plane. It is evident from Fig. 4 that all the powders exhibit the rhombohedral structure of potassium jarosite matching with International Centre for Diffraction Data Powder Diffraction Files (ICDD PDF 22-0827). All samples with different pH values and aging times synthesized showed a well-defined crystallinity, and no other peaks related to different jarosite forms, or products related to precursor residues can be identified; therefore, a solid solution of hydronium -potassium jarosite powders were synthesized. Presence of precipitates during the aging makes the structures to react with the released ions like H3O + , K + , Fe 3+ , and OH -, which restricts the growth in desired planes or vanishes some planes.
We believe that the presence of adequate OH-ions for the pH value 2.1 resulted in lower crystallite size and deviated lattice parameters.
On the other hand, both increase in pH and aging time there is shift in the principle (113) plane for all jarosite structures, which is replotted and shown in Fig. 5. In order to ascertain the effect of pH and aging time the line broadening analysis of (113) peak using Debye Scherrer equation was employed and found the crystallite size and lattice parameters for all samples and tabulated in Table   2.  Because NaOH was used to control pH values, a Rietveld analysis was done (like was described in section 2.2) to determine if sodium jarosite was formed during synthesis. Table 3 shows the Rietveld results, where can be observed that only the potassium jarosite was formed with important substitution of hydronium, given so a solid solution of hydronium-potassium jarosite. In conclusion, it was found that increasing pH, the amount of hydronium substitution by potassium in the alkaline site of jarosite, also increases.   Table. 3.

Scanning Electron Microscopy (SEM) and Energy Dispersive Spectrometry (EDS) analysis
Jarosite powders show similar agglomerates ~ 2 μm, for samples prepared with pH values 0.8 and 1.1 (refer Figs. 6a-6d). As prepared powders with pH 1.1 show a morphology a dense morphology with very broad distribution of grain size ranging from 500 nm to 3 μm (Figs. 6c-6d).
Decrease of pH to 0.8 by adding sulphuric acid, size of the agglomerates and density of the particles decreased (Figs. 6a-6b) due to the relative reduction of OHion concentration. In both samples with pH values 0.8 and 1.1 no specific morphology was observed, only agglomerates with random size and density were noticed. As the pH value increased to 2.1, exceptionally, an elliptical or semi-spherical morphology with particle size around 1 μm of the jarosite particles were observed (Figs. 6e, and 6f). Powders with similar pH 2.1 with increased aging time of 3 and 7 days the morphology changed drastically to spherical and euhedreal structures, respectively (Figs. 6g-6j).
Particle sizes around 500 and 200 nm were obtained for powders aged for 3 and 7 days respectively.
Particle size decreased with increase in aging time from 2 μm to 200 nm. The particles sizes obtained in SEM are in contrast with XRD crystallite sizes. Therefore we believe that the particles observed in SEM are constituted with smaller crystallites. As prepared jarosite solution is acidic with pH 1.1 and as the pH of solution is increased by addition of strong base, NH4OH, mainly K + and OHions are liberated which makes the solution more soluble and these OHions are situated on the facets of the formed nuclei [26]. Later the growth and agglomeration are restricted by OHions resulting some elliptical particles (Figs. 6e and 6f). When the precipitates are left in the supernatant for 3 days, increased H3O starts incorporating into the structure and allowing the growth in a direction which resulted in the spherical like morphology (Figs. 6g and 6h). Furthermore, increase in the aging time to 7 days, besides incorporation of the H3O, majority of the planes starts decomposing and forms particles with different facets resulting in euhedral potassium jarosite structures (Figs. 6i and 6j). Therefore, pH is a very important parameter to obtain the desired surface morphology. The decomposition of the planes is clear from the XRD analysis (Figs. 4 and 5) where the planes (012) and (113) reduces to only (113) confirming the decomposition of planes and subsequently resulting euhedral structures (Fig. 6j). For contemplating the liberation and incorporation of cations and anions in each sample EDS analysis was performed and the obtained results were tabulated in Table. 4.  Table 4 we compare the obtained elemental composition values with the theoretical and synthetic jarosite composition values reported by the traditional synthesis method employed firstly by Dutrizac and Kaiman 33. According to the obtained EDS results, it can be observed that the values of K and Fe closer to the values reported in the traditional synthesis method with increase in pH from 0.8 to 2.1 which we believe is due to the utilization of very less molar concentration (0.15 M) compared to reported work. Whereas, the contents of SO4 and (H3O + OH) were relatively higher which is conventional because as the quantity of anions liberated increases, the solution pH also increases, pH utilized in the theoretical and synthetic jarosite was 1.2. Therefore, by comparing XRD, SEM and EDS analysis it can be concluded that the aging time makes the jarosite to form euhedreal structures and increased pH makes the size of the particle more micrometric. The results obtained in this work are the preliminary work, where in future detailed analysis of aging time and pH is studied to obtain the nanometric jarosite structures.

Electrical analysis
According the obtained results during electrical tests, it can be seen in Table 5 that the electrochemical reaction executed by the electrodes prepared with hydronium-potassium jarosite and graphite gave good results. Table 5 shows that the best results found were for the jarosite synthesized at pH 2.1 with 3 and 7 days of aging, where morphology is spherical and euhedreal, respectively. According to these preliminary results, it can be seen that the jarosite synthesized at pH 2.1 and with a resting time of 3 and 7 days, shows the best results, obtaining an average potential of 1.35 V, which is close to the potential generated by a standard battery that is 1.5 V and a little above the voltage generated by the rechargeable battery that is 1.2 V.
These results also indicate that particle size and morphology play an important role, since these materials have average particle sizes ranging from 1 to 0.2 microns, and spherical and euhedreal morphologies (Figs. 6g-h and 6i-h), which coincide with some researchers who pointed that the particle size and morphology of crystalline jarosite play an important role in the ability of these compounds to serve as cathode in LIB´s [6,32].

Conclusions
Wet chemical synthesis of potassium jarosite powders was successfully performed and synthesis performed at reduced synthesis temperature at 70 C and for only 3 h of reaction time.
The results obtained in this work directs that the morphological and micro-structural properties of the hydronium-potassium jarosite obtained have a strong dependency on synthesis parameters, show the results obtained in this work in comparison with other done by some researches along the time, according the morphology, particle size and electrical properties of these kind of solid solutions. K-rGO nanosheets Dissolution of KNO3-FeSO47H2O-GO.
Particle size over 4 m. 545 mAh g -1 at the end of 1000 cycles at 500 mAh