| 11. |
- Nielsen, Ida, et al.
(författare)
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Water driven phase transitions in Prussian white cathode materials
- 2022
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Ingår i: Journal of Physics. - : Institute of Physics (IOP). - 2515-7655. ; 4:4
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Tidskriftsartikel (refereegranskat)abstract
- Prussian white (PW, Na2Fe [Fe(CN)(6)] center dot zH(2)O) is a promising cathode material for use in sodium-ion batteries for large-scale energy storage applications, which demand long cycling life-times. However, for non-aqueous battery applications PW must not contain any water, and yet dehydration induces a large volume change destabilizing the structure and reducing the cycling life. The material undergoes multiple phase transitions as a function of both the sodium and water content, however, the mechanism behind is poorly understood. Here, we use neutron diffraction to explore the influence of water on the structure of PW. For the first time, two structures for a single composition of PW were observed near room temperature independent of the synthesis method. These structures differ in the FeN6 and FeC6 octahedral tilting configurations, which is connected to the ordering of water in the framework. The removal of water modulates the magnitude of pre-existing structural distortions, if it is itself disordered within the structure, rather than modifying the nature of the distortions. These results provide a robust fundamental understanding of the chemical driving force impacting the nature and magnitude of structural distortions in Prussian blue analogues. The insights provide guidance for designing tilt-engineering ultimately enabling new materials with enhanced long-term electrochemical performance in battery applications.
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| 12. |
- Ojwang, Dickson O., 1985-, et al.
(författare)
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Guest water hinders sodium-ion diffusion in low-defect Berlin green cathode material
- 2022
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Ingår i: Dalton Transactions. - : Royal Society of Chemistry (RSC). - 1477-9226 .- 1477-9234. ; 51:38, s. 14712-14720
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Tidskriftsartikel (refereegranskat)abstract
- Among Prussian blue analogues (PBAs), NaxFe[Fe(CN)(6)](1-y)center dot nH(2)O is a highly attractive cathode material for sodium-ion batteries due to its high theoretical capacity of similar to 170 mA h g(-1) and inexpensive raw materials. However, concerns remain over its long-term electrochemical performance and structural factors which impact sources of resistance in the material and subsequently rate performance. Refined control of the [Fe(CN)(6)] vacancies and water content could help in realizing its market potential. In this context, we have studied a low-defect Berlin green (BG) Na0.30(5)Fe[Fe(CN)(6)](0.94(2))center dot nH(2)O with varied water content corresponding to 10, 8, 6, and 2 wt%. The impact of water on the electrochemical properties of BG was systematically investigated. The electrodes were cycled within a narrow voltage window of 3.15-3.8 V vs. Na/Na+ to avoid undesired phase transitions and side reactions while preserving the cubic structure. We demonstrate that thermal dehydration leads to a significantly improved cycling stability of over 300 cycles at 15 mA g(-1) with coulombic efficiency of >99.9%. In particular, the electrode with the lowest water content exhibited the fastest Na+-ion insertion/extraction as evidenced by the larger CV peak currents during successive scans compared to hydrated samples. The results provide fundamental insight for designing PBAs as electrode materials with enhanced electrochemical performance in energy storage applications.
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| 13. |
- Ojwang, Dickson O., et al.
(författare)
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Influence of sodium content on the thermal behavior of low vacancy Prussian white cathode material
- 2020
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Ingår i: Dalton Transactions. - : ROYAL SOC CHEMISTRY. - 1477-9226 .- 1477-9234. ; 49:11, s. 3570-3579
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Tidskriftsartikel (refereegranskat)abstract
- Rechargeable sodium-ion batteries are the most attractive substitutes for lithium-ion batteries in large-scale energy storage devices due to wide spread reserves and low-cost of sodium resources and the similarities between sodium and lithium chemistry. However, finding a suitable cathode material is still a hurdle to be overcome. To date, Prussian white (PW), NaxFe[Fe(CN)(6)](y)center dot nH(2)O has stood out as one of the most promising Na-host materials due to its low cost, facile synthesis and competitive electrochemical capacity. Despite this, there are concerns that this material will thermally decompose at relatively low temperatures to form cyanogen gas, which is a safety hazard. Thus, low vacancy NaxFe[Fe(CN)(6)](y)center dot nH(2)O (x = 1.5, 1, 0.5 and 0) has been synthesized, and the influence of x on its thermal behavior systematically investigated. It is demonstrated that the thermal decomposition temperature, water content and moisture sensitivity of the samples strongly depend on the sodium content. The sample with x = 1.5 is found to be the most thermally stable and has the highest water content under the same experimental conditions. In addition, the sodium-rich samples (x = 1.5, 1 and 0.5) have higher surface water than the sodium-deficient one (x = 0). The local structure for this sample is also very different to the sodium-rich ones. Our findings offer new insights into the profound implications of proper material handling and safer operating conditions for practical Na-ion batteries and may be extended to analogous systems.
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| 14. |
- Ojwang, Dickson O., et al.
(författare)
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Moisture-Driven Degradation Pathways in Prussian White Cathode Material for Sodium-Ion Batteries
- 2021
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Ingår i: ACS Applied Materials and Interfaces. - : American Chemical Society (ACS). - 1944-8244 .- 1944-8252. ; 13:8, s. 10054-10063
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Tidskriftsartikel (refereegranskat)abstract
- The high-theoretical-capacity (∼170 mAh/g) Prussian white (PW), NaxFe[Fe(CN)6]y·nH2O, is one of the most promising candidates for Na-ion batteries on the cusp of commercialization. However, it has limitations such as high variability of reported stable practical capacity and cycling stability. A key factor that has been identified to affect the performance of PW is water content in the structure. However, the impact of airborne moisture exposure on the electrochemical performance of PW and the chemical mechanisms leading to performance decay have not yet been explored. Herein, we for the first time systematically studied the influence of humidity on the structural and electrochemical properties of monoclinic hydrated (M-PW) and rhombohedral dehydrated (R-PW) Prussian white. It is identified that moisture-driven capacity fading proceeds via two steps, first by sodium from the bulk material reacting with moisture at the surface to form sodium hydroxide and partial oxidation of Fe2+ to Fe3+. The sodium hydroxide creates a basic environment at the surface of the PW particles, leading to decomposition to Na4[Fe(CN)6] and iron oxides. Although the first process leads to loss of capacity, which can be reversed, the second stage of degradation is irreversible. Over time, both processes lead to the formation of a passivating surface layer, which prevents both reversible and irreversible capacity losses. This study thus presents a significant step toward understanding the large performance variations presented in the literature for PW. From this study, strategies aimed at limiting moisture-driven degradation can be designed and their efficacy assessed.
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| 15. |
- Ojwang, Dickson Odhiambo, 1985-
(författare)
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Prussian blue analogue copper hexacyanoferrate : Synthesis, structure characterization and its applications as battery electrode and CO2 adsorbent
- 2017
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Prussian blue (PB) and Prussian blue analogues (PBAs) are compounds with potential applications in a large variety of fields such as gas storage, poison antidotes, electrochromism, electrochemistry and molecular magnets. The compounds are easy to synthesize, cheap, environmentally friendly and have been pursued for both fundamental research and industrial purposes. Despite the multifunctionality of PB and PBAs, they have complicated compositions, which are largely dependent on the synthesis methods and storage conditions. Thus, performing investigations on such compounds with defined composition, stoichiometry and crystal structure is essential.This thesis has focused on synthesis and detailed structure characterization of copper hexacyanoferrate (CuHCF) via X-ray powder diffraction (XRPD), neutron powder diffraction (NPD), thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), inductively coupled plasma-optical emission spectroscopy (ICP-OES), scanning electron microscopy-energy dispersive spectroscopy (SEM-EDS), Mössbauer spectroscopy, extended X-ray absorption fine structure (EXAFS), infrared (IR) and Raman techniques. In addition, kinetics of thermal dehydration process, CO2 adsorption and CO2 adsorption kinetics were investigated. Moreover, in operando synchrotron X-ray diffraction experiments were performed to gain insight into the structure-electrochemistry relationships in an aqueous CuHCF/Zn battery during operation.
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| 16. |
- Ojwang, Dickson O., et al.
(författare)
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Structure Characterization and Properties of K-Containing Copper Hexacyanoferrate
- 2016
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Ingår i: Inorganic Chemistry. - : American Chemical Society (ACS). - 0020-1669 .- 1520-510X. ; 55:12, s. 5924-5934
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Tidskriftsartikel (refereegranskat)abstract
- Copper hexacyanoferrate, Cu-II[Fe-III(CN)(6)](2/3)center dot nH(2)O, was synthesized, and varied amounts of IC ions were inserted via reduction by K2S2O3 (aq). Ideally, the reaction can be written as Cu-II[Fe-III(CN)(6)](2/3)-nH(2)O + 2x/3K(+) + 2x/3e(-)K(+) <-> K-2x/3 Cu-II[Fe-x(II).Fe-1-x(II),(CN)(6)](2/3)-nH(2)O. Infrared, Raman, and Mossbauer spectroscopy studies show that Fe-II is continuously reduced to Fell with increasing x, accompanied by a decrease of the a-axis of the cubic Fn (3) over barm unit cell. Elemental analysis of K by inductively coupled plasma shows that the insertion only begins when a significant fraction similar to 10% of the Fe-III, has already been reduced. Thermogravimetric analysis shows a fast exchange of water with ambient atmosphere and a total weight loss of similar to 26 wt % upon heating to 180 degrees C, above which the structure starts to decompose. The crystal structures of Cu-III[Fe-III(CN)(6)](2/3)center dot nH(2)O and K2/3Cu[Fe(CN)(6)](2/3)center dot nH(2)O were refined using synchrotron X-ray powder diffraction data. In both, one-third of the Fe(CN)(6) groups are vacant, and the octahedron around Cull is completed by water molecules. In the two structures, difference Fourier maps reveal three additional zeolitic water sites (8c, 32f, and 48g) in the center of the cavities formed by the-Cu-N-C-Fe- framework. The K-containing compound shows an increased electron density at two of these sites (32f and 48g), indicating them to be the preferred positions for the K+ ions.
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| 17. |
- Ojwang, Dickson O., et al.
(författare)
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The adsorption kinetics of CO2 on copper hexacyanoferrate studied by thermogravimetric analysis
- 2018
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Ingår i: Microporous and Mesoporous Materials. - : Elsevier BV. - 1387-1811 .- 1873-3093. ; 272, s. 70-78
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Tidskriftsartikel (refereegranskat)abstract
- The CO2 adsorption and CO2 adsorption kinetics were evaluated by thermogravimetry on two Prussian blue analogues, K2x/3CuII [Fe-x(II) Fe-1-x(III) (CN)(6)](2/3), with nominally K-free x = 0.0 and K-rich x = 1.0. Differential isosteric heats of adsorption were determined from adsorption isotherms using the Clausius-Clapeyron equation and integral values by differential scanning calorimetry. The average differential heats of CO2 adsorption are 28 kJ/mol for x = 0.0 and 33 kJ/mol for x = 1.0. Both compositions show small maxima in differential heat at similar to 1 mmol/g. The integral adsorption heats were determined to be 26 kJ/mol for both x = 0.0 and x = 1.0. The kinetic CO(2 )adsorption/desorption curves can be modeled by a double exponential function describing two parallel processes with different rate constants. The activation energies for CO2 adsorption on x = 0.0 were 6 (1) kJ/mol for the faster component and 16 (1) kJ/mol for the slower one, while the corresponding values for x = 1.0 were 9 (1) kJ/mol and 7 (1) kJ/mol, respectively. The maximum CO2 uptake for both compositions was found to be similar to 4.5 mmol/g, 19.8 wt%, at 1 bar and 273 K. The materials exhibited fast adsorption kinetics and stable cyclic performance at room temperature. The kinetics were slower for the samples with x = 1.0 than for x = 0.0 which may be attributed to interactions between CO2 molecules and K+ ions.
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| 18. |
- Ojwang, Dickson, et al.
(författare)
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The adsorption kinetics of CO2 on copper hexacyanoferrate studied by thermogravimetric analysis
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Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- The CO2 adsorption and CO2 adsorption kinetics were evaluated by thermogravimetry on two Prussian blue analogues, K2x/3CuII[FeIIx FeIII1-x(CN)6]2/3, with nominally K-free x = 0.0 and K-rich x = 1.0. Differential isosteric heats of adsorption were determined from adsorption isotherms using the Clausius-Clapeyron equation and integral values by differential scanning calorimetry. The average differential heats of CO2 adsorption are 28 kJ/mol for x = 0.0 and 33 kJ/mol for x = 1.0. Both compositions show small maxima in differential heat at ~1 mmol/g. The integral adsorption heats were determined to be 26 kJ/mol for both x = 0.0 and x = 1.0. The kinetic CO2 adsorption/desorption curves can be modeled by a double exponential function describing two parallel processes with different rate constants. The activation energies for CO2 adsorption on x = 0.0 were 6(1) kJ/mol for the faster component and 16(1) kJ/mol for the slower one, while the corresponding values for x = 1.0 were 9(1) kJ/mol and 7(1) kJ/mol, respectively. The maximum CO2 uptake for both compositions was found to be ~4.5 mmol/g, 19.8 wt %, at 1 bar and 273 K. The materials exhibited fast adsorption kinetics and stable cyclic performance at room temperature. The kinetics were slower for the samples with x = 1.0 than for x = 0.0 which may be attributed to interactions between CO2 molecules and K+ ions.
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| 19. |
- Otieno, Austine O., et al.
(författare)
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Pineapple peel biochar and lateritic soil as adsorbents for recovery of ammonium nitrogen from human urine
- 2021
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Ingår i: Journal of Environmental Management. - : Elsevier. - 0301-4797 .- 1095-8630. ; 293
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Tidskriftsartikel (refereegranskat)abstract
- Human urine is a rich source of nitrogen which can be captured to supplement the existing sources of nitrogen fertilizers thus contributing to enhanced crop production. However, urine is the major contributor of macronutrients in municipal wastewater flows resulting into eutrophication of the receiving water bodies. Herein, pineapple peel biochar (PPB), and lateritic soil (LS) adsorbents were prepared for the safe removal of ammonium nitrogen (NH4+-N) from human urine solutions. Physicochemical properties of PPB, and LS were characterized by scanning electron microscopy-energy dispersive spectroscopy (SEM-EDS), X-ray powder diffraction (XRD), Fourier transform infrared spectroscopy (FTIR) and thermogravimetric analysis (TGA) to investigate the relationship of their properties with NH4+-N adsorption. Langmuir, Freundlich, and Dubinin-Radushkevich (D-R) isotherm models were employed to correlate the experimental equilibrium adsorption data. The effect of contact time and initial concentration of NH4+-N adsorption was also evaluated. The D-R isotherm model best described the behaviour of NH4+-N adsorption on both PPB and LS based on the coefficient of correlation values. This model showed that the adsorption of NH4+-N on both samples was a physical process with PPB and LS having mean surface adsorption energies of 1.826 × 10−2, and 1.622 × 10−2 kJ/mol, respectively. The PPB exhibited a slightly higher adsorption capacity for NH4+-N (13.40 mg/g) than LS (10.73 mg/g) with the difference attributed to its higher surface area and porosity. These values are good indicators for assessing the effectiveness of the materials for adsorption of NH4+-N from human urine.
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| 20. |
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