III. SOLID STATE SYNTHESIS AND WET CHEMICAL PROCESSING OF
3.2 S YNTHESIS & P ROCESSING
3.2.1 Can+1MnnO3n+1 (n= 1, 2, 3)
Each of Ca4Mn3O10, Ca3Mn2O7, and Ca2MnO4 parent powders, along with (Ca0.85Ag0.15)3Mn2O6.775, were synthesized via solid state sintering. Stoichiometric amounts of MnO (Alfa Aesar, 98%) and CaCO (Alfa Aesar, 98%) were mixed in
isopropyl alcohol in a micronizing mill (McCrone Group, USA) with alumina milling media for 30 min and dried on a hot plate. Ca4Mn3O10 and Ca3Mn2O7 were calcined at 900
°C for 48 h. After calcination, the powders were pelletized and heated in a furnace (Carbolite, UK). Ca4Mn3O10 was sintered at 1350 °C for 24 h with a 50 K/min ramp rate and air quenched to room temperature. Ca3Mn2O7 was sintered at 1250 °C for 24 h with a 25 K/min ramp rate and air quenched to room temperature. Ca2MnO4 was sintered at 1200
°C for 12 h with a 10 K/min ramp and cool rate. All samples were re-heated multiple times until the desired phase purity was achieved. (Ca0.85Ag0.15)3Mn2O6.775 was synthesized by adding stoichiometric amounts of Ag2O (Johnson Matthey Catalog Company, 99+%) to Ca3Mn2O7. Ag2O and Ca3Mn2O7 was then mixed for 30 min in a micronizing mill in isopropyl alcohol and dried on a hot plate. The dried powders were pelletized and placed in a powder bed in an alumina crucible. The pellets were sintered at 1250 °C for 24 h with a 25 K/min ramp and cool rate with an additional 50 mL/min flowing O2.
Proton exchange powders of each Ca4Mn3O10, Ca3Mn2O7, and (Ca0.85Ag0.15)3Mn2O6.775 were obtained by ultrasonicating 0.5 g of powder in 45 mL of 0.5 N HNO3 (Fisher Scientific) in a 40kHz ultrasonic bath for 4 h with intermittent shaking every 20 min to redisperse powders from the bottom of the 50 mL centrifuge tube. After the 4 h proton exchange was complete, powders were washed three times in DI water (12,000 rpm for 10 mins, and decant off the rinse water) and dried on a hot plate at 55°C.
XRD was used to check for an increase in the interlayer spacing of proton exchanged powders. The ion exchange process was completed two additional times, three times in total. Proton exchanged Ca2MnO4 was processed using 0.1 N HNO3 under the same conditions. The ion exchange process for Ca2MnO4 was completed a total of two times.
Ca2MnO4 was prone to dissolution in higher molarity acids.
powder and TBAOH mixture were ultrasonicated in a 40kHz ultrasonic bath for 4 h with intermittent shaking every 20 min. After the 4 h exfoliation was complete, the sample was centrifuged at 10,000 rpm for 10 min. The resulting dark brown nanosheet suspension was decanted. The centrifuge container was refilled with DI water and shaken to remove built up powder on the centrifuge tube wall. Samples were centrifuged again at 10,000 rpm for 10 min and decanted. This process was repeated several (5-6) times or until the suspension no longer contained a reasonable amount of nanosheets.
( 𝑚𝑎𝑠𝑠 𝑜𝑓 𝐻+ 𝑒𝑥𝑐ℎ𝑎𝑛𝑔𝑒𝑑 𝑝𝑜𝑤𝑑𝑒𝑟 [𝑔]
1 ) (1 𝑚𝑜𝑙 𝑜𝑓 𝐻+𝑒𝑥𝑐ℎ𝑎𝑛𝑔𝑒𝑑 𝑝𝑜𝑤𝑑𝑒𝑟 [𝑚𝑜𝑙]
𝑀𝑀 𝑜𝑓 𝐻+𝑒𝑥𝑐ℎ𝑎𝑛𝑔𝑒𝑑 𝑝𝑜𝑤𝑑𝑒𝑟 [𝑔] )
(𝑖𝑛𝑡𝑒𝑟𝑙𝑎𝑦𝑒𝑟 𝑖𝑜𝑛𝑠 𝑖𝑛 𝐻+𝑒𝑥𝑐ℎ𝑎𝑛𝑔𝑒𝑑 𝑝𝑜𝑤𝑑𝑒𝑟 [𝑚𝑜𝑙]
𝑓𝑜𝑟𝑚𝑢𝑙𝑎 𝑢𝑛𝑖𝑡 𝐻+𝑒𝑥𝑐ℎ𝑎𝑛𝑔𝑒𝑑 𝑝𝑜𝑤𝑑𝑒𝑟 [𝑚𝑜𝑙] ) (20)
( 1 𝑚𝑜𝑙 𝑇𝐵𝐴𝑂𝐻 [𝑚𝑜𝑙]
1 𝑚𝑜𝑙 𝑖𝑛𝑡𝑒𝑟𝑙𝑎𝑦𝑒𝑟 𝑖𝑜𝑛𝑠 𝑖𝑛 𝐻+𝑒𝑥𝑐ℎ𝑎𝑛𝑔𝑒𝑑 𝑝𝑜𝑤𝑑𝑒𝑟 [𝑚𝑜𝑙]) ( 𝐿 𝑇𝐵𝐴𝑂𝐻 [𝐿]
𝑚𝑜𝑙𝑠 𝑇𝐵𝐴𝑂𝐻 [𝑚𝑜𝑙])
Nanosheet floccules were obtained by adding 1N HNO3 to the nanosheet suspension dropwise at a rate of 1 mL/min (20 drops/min) until a pH of 1.7 was achieved.
At this point the suspension was flocculated, and 1.0 N NaOH (Fisher Scientific) was added to bring the flocculated suspension up to the desired pH. Floccules were stirred at 700 rpm for 24 h. After 24 h the stir bar was removed, and the supernatant was decanted after most of the floccules settled to the bottom of the beaker. Typically, 15-25 mL of the remaining floccules and liquid were separated into 50 mL centrifuge containers and diluted up to 45 mL. Floccules were centrifuged at 12,000 rpm for 10 min and washed several times until there were no TBAOH bubbles after shaking the centrifuge tubes. Floccules were dried at 60 °C overnight (12-15 h). Freeze-dried samples usually took 6-7 days to fully dry. The flocculated suspension equilibrated at this pH for 24 h and was washed until there was no visible TBAOH in the liquid. Note: when shaken, TBAOH produces several small bubbles at the surface of the liquid. Floccules were dried on a glass plate in a Heratherm drying oven (ThermoFisher Scientific, USA) at 60 °C overnight ~15 h.
3.2.2 KCa2A3O10 (A = Ta, Nb)
Dion-Jacobson oxide layered perovskites were prepared by milling 50% excess K2CO3 (Alfa Aesar, 99.0% min) to account for vaporization of the alkali, stoichiometric
amounts of CaCO3, and either Nb2O5 (Alfa Aesar, 99.9% metals basis) or Ta2O5 (Alfa Aesar, 99% metals basis) in isopropyl alcohol in a micronizing mill for 30 min. Excess K2CO3 was also added at 10 mol%, but this produced a 3D pyrochlore phase. The 50 mol%
excess K2CO3 was used to mitigate pyrochlore formation. The milled powders were then dried on a hot plate until the isopropyl alcohol was evaporated. KCa2Nb3O10 powder was sintered at 1200 °C for 12 h with a 10 K/min ramp rate. KCa2Ta3O10 was pelletized and sintered in a powder bed at 1350 °C for 24 h with a 10 K/min ramp and cool rate. An additional 3D pyrochlore phase was present in each batch of KCa2Ta3O10. The XRD confirmed KCa2Nb3O10 powder was completely pure after one sintering step. These results were consistent with the literature for synthesis of both materials.96-99 While the extra phase in the parent form, KCa2Ta3O10, was not desirable, it was not present in the reassembled nanosheets. The 3D pyrochlore phase did not delaminate, so it was contained in the solids during exfoliation, rather than the colloidal suspension. Stoichiometric amounts, typically 15 mol%, of Ag2O was added to the sintered Dion-Jacobson powder and milled for an additional 30 min in IPA and sintered in air using the same parameters as the un-doped powder.
Proton exchange powders were obtained by ultrasonicating 0.5 g of powder in 45 mL of 1.0 N HNO3 or 1.0 N HCl (Fisher Scientific) for 4 h in a 40 kHz ultrasonic bath with intermittent shaking every 20 min to redisperse powders from the bottom of the centrifuge tube. Proton exchanging in HNO3 and HCl did not dissolve powders. Gravimetric capacitance measurements were performed on powders exchanged with HCl. After each proton exchange was complete, the powders were washed three times (centrifuged at 12,000 rpm). This process was repeated 3 times. After the third 4 h proton exchange, powders were washed three times in DI water and dried on a hot plate. XRD was used to
rpm for 10min. The resulting white nanosheet suspension was decanted. The centrifuge container was refilled with DI water and shaken to remove built up powder on the centrifuge tube wall. Samples were centrifuged again at 10,000 rpm for 10 min and decanted. This process was repeated several (5-6) times or until the suspension no longer contained a noticeable amount of nanosheets.
The nanosheet suspension was reassembled by adding 6.0 N HCl at a rate of 1mL/min until the pH was 1.7. 1.0 N NaOH was then added to bring the suspension up to the desired pH and stirred for 24 h at 700 rpm. After 24 h the stir bar was removed, and the supernatant was decanted after most of the floccules settled to the bottom of the beaker.
Typically, 15-25 mL of the remaining floccules and liquid were separated into 50 mL centrifuge containers and diluted up to 45 mL. Floccules were centrifuged at 12,000 rpm for 10 min and washed several times until there were no TBAOH bubbles after shaking the centrifuge tubes. TBAOH bubbles are small soap like bubbles that disappear several seconds after shaking. Water bubbles are larger and disappear almost immediately after shaking. Floccules were dried at 60 °C overnight (12-15 h). Freeze-dried samples usually took 6-7 days to fully dry.
Reduction of H(Ca0.85Ag0.15)2Ta3O10 occurred in 4% H2 atmosphere in a Bruker D8 advance diffractometer (Bruker, USA) equipped with an Anton Paar HTK1200 furnace (Anton Paar, Austria) and alumina sample holder. Samples were heated to 300 °C for 30 min. XRD peaks characteristic of Ag metal confirmed reduction.
3.2.3 K(Ca0.85Ag0.15)2Nb3O9.85 and KCa2(Nb0.85B0.15)3O~10 (B = V, Cr, Mn, Co) B-site doped powders were batched using K2CO3, CaCO3, Nb2O5, and dopant oxide powders: V2O5 (Acros Organics, 99.6+%), Cr2O3 (J.T. Baker, 99%), MnO2, and Co3O4
with a molar ratio of K : Ca : Nb : dopant of 1.1 : 2 : 2.55 : 0.45 were mixed in isopropyl alcohol in a micronizing mill for 30 minutes and dried on a hot plate. A-site doped powder was batched using K2CO3, CaCO3, Nb2O5, Ag2O raw materials with a molar ratio of K : Ca : Ag : Nb of 1.1 : 1.7 : 0.3 : 3. These powders were mixed in isopropyl alcohol in a micronizing mill for 30 min and dried on a hot plate. Mixed powders were sintered in an Al2O3 crucible in a Carbolite furnace at 1200 °C for 12 h, 1200 °C for 20 h, and then sintered at 1300 °C for 20 h with a 10 K/min ramp and cool rate in a powder bed. The
powders were not pelletized or calcined prior to sintering. XRD revealed only K(Ca0.85Ag0.15)2Nb3O9.85 powders were phase pure.
3.2.4 K(Ti0.85Mn0.15)NbO5 and K3(Ti0.85Mn0.15)5NbO14
K(Ti0.85Mn0.15)NbO5 powder was batched using K2CO3, Nb2O5, TiO2, MnO2 with a molar ratio of K : Nb : Ti : Mn, 1.1 : 1 : 0.85 : 0.15 in IPA in a micronizing mill for 30 minutes and dried on a hot plate. K3(Ti0.85Mn0.15)5NbO14 powder was batched using the same process but with a molar ratio of K :Nb : Ti : Mn, 1.1 : 4.25 : 0.75 : 1. XRD revealed impurity phases in both powders.
3.2.5 Ba4Mn3O10
Ba4Mn3O10 parent powder was synthesized using solid state methods.
Stoichiometric amounts of BaCO3 (Alfa Aesar, 99.0 – 101.0%), and MnO2 were mixed for 30 minutes in a micronizing mill in isopropyl alcohol and dried on a hot plate. The resulting powder was pelletized and sintered for 72 h at 1350 °C with a 5 K/min ramp rate and air quenched to room temperature.100 XRD confirmed phase purity.
Proton exchanged powder was obtained by ultrasonicating 0.5 g of parent powder with 45 mL of 0.25 N H2SO4 (Fisher Scientific). A BaSO4 second phase was identified along with proton exchanged powder, presumably through dissolution of Ba and precipitation of BaSO4. Exfoliation with TBAOH was unsuccessful.
3.2.6 Na0.7CoO2
Na0.7CoO2 parent powder was synthesized by mixing stoichiometric amounts of Na2CO3 (J.T. Baker, 95%) and Co3O4 (Alfa Aesar, 99.7%) with 5 wt% excess Na to account for alkali volatility. The raw materials were mixed for 30 minutes in IPA in a
is known to produce a pink colored solution.101 An additional proton exchange was completed to ensure complete replacement of Na.
Exfoliation was performed by mixing 0.35 g H0.7CoO2 powder with 2.5 mL of 1.5 M TBAOH and 30 mL of DI water and ultrasonicating for 4 h in a 40kHz ultrasonic bath with intermittent shaking every 20 min. Exfoliated nanosheets were centrifuged at 10,000 rpm for 10 min. Additional nanosheets were obtained by refilling the centrifuge tube with DI water and repeating the centrifuge process.
Floccules were synthesized by adding 1.0 N H2SO4 to the nanosheet suspension at a rate of 1 mL/min (20 drops/min) until a pH of 1.7 was achieved. NaOH was then added to bring the suspension up to the desired pH.
3.2.7 Na0.7(Co0.85Mn0.15)O2
Na0.7(Co0.85Mn0.15)O2 parent powder was synthesized by mixing stoichiometric amounts of Na2CO3, Co3O4, and Mn3O4 (Sigma Aldrich, 97%) with 5 wt% excess Na to account for alkali volatility. The raw materials were mixed for 30 minutes in IPA in a micronizing mill, transferred to an alumina crucible, and sintered at 850 °C for 18 h at a 10 K/min heating and cooling rate.
3.2.8 Carbon electrodes
Carbon binder electrodes on Ni foil substrates were prepared for electrochemical measurements. An 80 : 15 : 5 mass ratio of active material (30 mg) to carbon (Alfa Aesar, 99.9+%) (5.6 mg) to poly(vinylidene fluoride) (1.9 mg) was mixed in a smooth agate crucible for 15 min. The mixture was transferred to a small vial on a stir plate. 3-5 drops of 1-Methyl-2pyrrolidinone (NMP) (Sigma-Aldrich, anhydrous, 99.5%) were added and the mixture was stirred for at least 6 h at 700 rpm. Typically the NMP, carbon, binder, and active material mixture was stirred overnight (~15 h).
Nickel foil (Alfa Aesar, 99+% metals basis) was prepared by cutting a 1.5 cm x 3 cm area and ultrasonicated in a 40kHz ultrasonic bath in 1N HNO3 and DI water for 20 min each to remove NiO. A maximum of four Ni foils were cleaned in each 50 mL centrifuge tube. Ni foils were then dried in a drying oven at 100 °C for 5 min before the initial mass was recorded. The electrode slurry was deposited onto the Ni foil substrate pre-masked with a 1 cm x 1 cm area to deposit some of the mixture atop of the area. A
razor blade was used to create a smooth electrode surface by doctor blading the slurry onto the Ni substrate