Phosphoric Acid Concentration Research Articles

Hydrometallurgy two stage process for preparation of (Nd, La, Ce)2O3 from end-of-life NiMH batteries

The present work aims to investigate the recovery of light rare earth elements (LREEs) oxides from end-of-life NiMH batteries using a hydrometallurgical process followed by effective precipitation. The operational leaching parameters such as phosphoric acid concentration, temperature, and the solid–liquid ratio were first optimized by Box-Behnken design. The results reveal that under optimum conditions ([H3PO4] = 2 mol/L, T = 80 °C, and S/L = 1: 10 g/mL) the leaching efficiencies of Ni, Co reach 98.1% and 99.3%. While La, Ce, and Nd elements remain in the leaching residue as (La, Ce, Nd)PO4 with yields of 98.2%, 98.6%, and 99.6% for La, Ce, and Nd, respectively. Afterward, the (La, Ce, Nd)PO4 is leached with HCl acid, then the rare earth oxalate was precipitated using oxalic acid at a pH of 1.8 and then the product was calcined at 800 °C for 2 h in order to synthesize the (Nd, La, Ce)2O3. The analysis using scanning electron microscopy (SEM) coupled with energy dispersive X-ray spectroscopy (EDX) confirms the homogeneity of (Nd, La, Ce)2O3 particles that have two morphologies, i.e., flower and sticks with a particle size between 3 and 6 μm. The unit cell parameters of (Nd, La, Ce)2O3 were calculated after Rietveld refinement of the XRD patterns, in the space group of Fm-3m are a = b = c = 0.57921 nm and the volume equal to 0.194322 nm3.

Corrosion behavior and passive film properties of nickel-based alloy in phosphoric acid

Corrosion behavior and passive film properties of nickel-based alloy (Inconel 625) in phosphoric acid were investigated, by means of a variety of electrochemical methods combined with surface analysis techniques. Results showed that Inconel 625 exhibited strong passive behavior in phosphoric acid but their corrosion resistance was deteriorated with increasing concentration of phosphoric acid. The increasing concentration of phosphoric acid also accelerated the corrosion rate. The passive film of Inconel 625 was composed of an inner layer rich in Cr2O3 and an outer layer rich in FeOOH. The initial corrosion of Inconel 625 in phosphoric acid originated from the complex inclusions composed of N, Nb and Ti with a corn rich in Mg and O. At concentrations lower than 1 mol/L, inner layer of the passive film remained intact and only the outer layer was damaged. At concentrations of phosphoric acid higher than 1 mol/L, both the inner and outer layers were destroyed. The insoluble corrosion products Fe phosphate compounds precipitated on the surface of Inconel 625, which accumulated with increasing concentrations of phosphoric acid and became a loose layer covering the surface of Inconel 625 at concentrations of phosphoric acid higher than 1 mol/L.

Phosphoric acid pretreatment of poplar to optimize fermentable sugars production based on orthogonal experimental design.

The recalcitrant structure of raw poplar limited the production of fermentable sugars when applied as the material in the pretreatment of biochemical conversions. Phosphoric acid pretreatment is an efficient method to destroy the compact lignocellulose matrix presence in the poplar. In this study, phosphoric acid pretreatment of poplar was optimised by an orthogonal experimental design [L9(33)] to improve enzymatic digestibility through investigating the effects of reaction temperature, time duration, and phosphoric acid concentration. The optimal conditions were selected based on the variance of chemical compositions, hemicellulose removal ratio, and delignification of the woody material after pretreatment. The optimum enzymatic hydrolysis yield of up to 73.44% was obtained when the phosphoric acid pretreatment performed at 190°C for 150min under 1.5% of v/v phosphoric acid concentration.

Dephosphorization of nitric acid solutions from the leaching of monazite ferrous ores under hydrothermal conditions by selective precipitation of iron(III) hydroxyphosphate (giniite)

The precipitation of phosphoric acid as a hydroxyphosphate from nitric acid solutions in the presence of iron(III) oxides and hydroxyoxides under hydrothermal conditions was studied. It was shown that the dephosphorization process proceeds with the formation of iron(III) hydroxyphosphate (giniite) Fe5(PO4)4(OH)3∙2H2O. The most efficient and speedy process is implemented at temperatures of 180–200 °C and above. It was found that the concentration of phosphoric acid in the solution as a result can be reduced to 0.001 mol/L and below. Lanthanide, uranium, and thorium nitrates are stable under conditions of hydrothermal dephosphorization of solutions, in contrast to iron(III) nitrates decomposing to hematite and nitric acid. The features of the studied hydrothermal process were used to increase the efficiency of direct nitric acid leaching of monazite iron-bearing ores of the Chuktukon rare metal deposit.

Effects of Spodumene Flotation Tailings on Mechanical Properties of Acid-Based Geopolymer Mortar

This study focuses on using spodumene flotation tailings (SFT) to prepare phosphoric acid-activated metakaolin geopolymer, in which the replacement of metakaolin (MK) by a high percentage (up to 75 wt.%) of tailings was achieved. The compressive strength of geopolymer mortar was significantly improved with SFT as aggregates. In addition, the mechanical properties could also be enhanced by an increased concentration of phosphoric acid (H3PO4) solution or a decreased aggregate particle size. The optimized geopolymer mortar composite was SFT:MK = 3:1, which was activated by H3PO4 solution with a concentration of 51 vol%, followed by curing at 55 °C for 24 h. On the other hand, properties of the geopolymer mortar could also be affected by the morphology of the aggregates. For example, SFT as aggregates could produce more interconnected pores compared to standard sand. The major chemical structural units of geopolymer mortar were -P-O-Al- and AlPO4, which could be spontaneously generated according to the thermodynamic calculation results. Finally, many aluminum ions and a small amount of silicon ions could be leached from the tailings under acidic conditions.

Zirconium Phosphate Assisted Phosphoric Acid Co-Catalyzed Hydrolysis of Lignocellulose for Enhanced Extraction of Nanocellulose.

The high mechanical strength, large specific surface area, favorable biocompatibility, and degradability of nanocellulose (CNC) enable it to be a potential alternative to petroleum-based materials. However, the traditional preparation of CNCs requires a large amount of strong acid, which poses a serious challenge to equipment maintenance, waste liquid recycling, and economics. In this study, a solid and easily recoverable zirconium phosphate (ZrP) was used to assist in the phosphoric acid co-catalyzed hydrolysis of lignocellulose for extracting CNCs. Due to the presence of acidic phosphate groups, ZrP has a strong active center with a high catalytic activity. With the assistance of ZrP, the amount of phosphoric acid used in the reaction is significantly reduced, improving the equipment's durability and economic efficiency. The effects of the process conditions investigated were the phosphate acid concentration, reaction temperature, and reaction time on the yield of CNCs. The Box-Behnken design (BBD) method from the response surface methodology (RSM) was applied to investigate and optimize the preparation conditions. The optimized pre-treatment conditions were 49.27% phosphoric acid concentration, 65.38 °C reaction temperature, and 5 h reaction time with a maximal cellulose yield (48.33%). The obtained CNCs show a granular shape with a length of 40~50 nm and a diameter of 20~30 nm, while its high zeta potential (-24.5 mV) make CNCs present a stable dispersion in aqueous media. Moreover, CNCs have a high crystallinity of 78.70% within the crystal type of cellulose Ⅰ. As such, this study may pioneer the horizon for developing a green method for the efficient preparation of CNC, and it is of great significance for CNCs practical production process.

MOF-assisted synthesis of mesoporous zirconium phosphate for enhanced cadmium removal from water

It is unprecedently found that a zirconium-based metal-organic framework (MOF-808) can be a source of zirconium for the synthesis of zirconium phosphate (ZrP) with discrete disc nanocrystals (4.8 to 5.2 nm in size), normally obtained at high temperature or high phosphoric acid concentration. In contrast to the routinely obtained ZrP phase, the one derived from zirconium MOF under similar conditions offers dispersed particles with more active and accessible adsorption sites for efficient removal of cadmium. Indeed, the cadmium removal performance was found to be 88.27 mg/g compared to the conventional ZrP (67.48 mg/g) under the same conditions, bringing thus clear evidence of the virtues of using Zr-MOF as a template for the design of novel nanostructured materials with enhanced removal efficiency.

Leaching of rare earths from Abu Tartur (Egypt) phosphate rock with phosphoric acid

The leaching of rare earth elements (REEs) from Egyptian Abu Tartur phosphate rock using phosphoric acid has been examined and was subsequently optimized to better understand if such an approach could be industrially feasible. Preliminary experiments were performed to properly define the design of experiments. Afterward, 24 full factorial design was implemented to optimize the leaching process. Optimum REEs leaching efficiency (96.7 ± 0.9%) was reached with the following conditions: phosphoric acid concentration of 30 wt.-% P2O5, liquid/solid ratio, mL/g, of 5:1, at 20 °C, and 120 min of leaching time. The apparent activation energy of the dissolution of REEs from phosphate rock using the phosphoric acid solution was -19.6 kJ/mol. D2EHPA was subsequently applied as an organic solvent for REEs separation from the acquired leach liquor. REEs stripping and precipitation were conducted, and finally, rare earth oxides with a purity of 88.4% were obtained. The leach liquor was further treated with concentrated sulfuric acid to recover the used phosphoric acid and produce gypsum with a purity of >95% at the same time. A flow diagram for this innovative cleaner production process was developed, and larger-scale experiments are proposed to further understand this promising approach to comprehensive phosphate rock processing.

Analysis and estimation of cross-flow heat exchanger fouling in phosphoric acid concentration plant using response surface methodology (RSM) and artificial neural network (ANN)

The production of phosphoric acid by dehydrated process leads to the precipitation of unwanted insoluble salts promoting thus the crystallization fouling build-up on heat transfer surfaces of the exchangers. During the acid concentration operation, the presence of fouling in heat exchangers results in reducing the performance of this equipment, in terms of heat transfer, while increasing energy losses and damaging the apparatus. To mitigate these adverse effects of fouling, it is necessary to forecast the thermal resistance of fouling to schedule and perform exchanger cleaning. In this context, artificial neural network and response surface methodology were used to estimate thermal resistance of fouling in a cross-flow heat exchanger by using the operating data of the concentration loop. The absolute average relative deviations, mean squared errors, root mean squared errors and correlation coefficients were used as indicators error between the experimental and estimated values for both methods. The best fitted model derived from response surface methodology method was second order polynomial while the best architecture topology, for the artificial neural network method, consists of three layers: input layer with six input variables, hidden layer with six hidden neurons and an output layer with single output variable. The interactive influences of operating parameters which have significant effects on the fouling resistance were illustrated in detail. The value of correlation coefficient for the output parameter from the response surface methodology is 0.9976, indicating that the response surface methodology as an assessment methodology in estimating fouling resistance is more feasible compared with the artificial neural network approach.

Synthesis of Hydrosiapatite from Muscle Shell Waste Using The Precipitation Method

Hydroxyapatite is a calcium phosphate compound that is the main inorganic component of bones andteeth. Hydroxyapatite has a main role in the medical world because of its identical chemical propertiesand structure to human bone. Kupang shells have a high CaCO3 content of 95-99% by weight. So thisconcurrence, the mussel shells are to be used as good as an ingredient for making Hydroxyapatite. Thevariables used in this research are Temperature of the Furnace and Phosphoric Acid Concentration,which were 700ºC, 750 ºC, 800 ºC, 850 ºC, 900 ºC and Phosphate Acid Concentrations 0.4M, 0.6M,0.8M, 1M, and 1.2M. The product results were tested with XRD analysis to determine the crystalstructure contained in the product and the level of Hydroxyapatite in the product. The best researchresults in this study at a temperature of 900ºC at a concentration of 1.2M phosphoric acid with aHydroxyapatite content of 100% with lattice parameters an (Aº)=b (Aº) = 9..422, c (Aº)=6.8835, andgamma = 120º.

Growth, Yield, and Grain Quality of Barley (Hordeum vulgare L.) Grown across South Korean Farmlands with Different Temperature Distributions

Climate change has disrupted several aspects of food systems, but perhaps one of the most alarming effects on global nutrition is the decrease in grain production as well as the reduction in the protein content and quality of the grain. Over the last several decades, due to climate change, suitable areas in Korea for barley cultivation have been moving northward compared to the past. Thus, the objective of this study was to determine how different climatic factors such as temperature impact barley growth at different stages (December, February, and April) and the yield at harvest in four group areas (G1, G2, G3, and G4) with different climates. Differences in the temperatures between areas during the growing season and the variability in growth and yields were noted. Additionally, the chemical composition of the soils and the mineral content of the leaves at the heading stage as well as the main constituents and amino acid composition of the barley seeds grown in different areas were considered. On average, the tiller number/m2, plant height, and dry aboveground plant parts/m2 in G1 areas were lower than in other group areas when measured before overwintering (December), after overwintering (February), and at the heading stage (April). However, there was no difference in these parameters between G2 and G3 areas. In 2020, the order of yield levels was G4 > G2 = G3 > G1. The yield in G1 areas was 37% less than in G4 areas. In 2021, yield levels were similar with the order of yield being G4 = G3 > G1 = G2. Also similar to the prior year, yield in G1 areas was 39% less than in G4 areas. The minimum and daily average temperatures during the growing season (October to June) were in the order of G4 > G3 > G2 > G1. Growth parameters in the colder G1 areas were lower than in other group areas, which suggests that the minimum and daily average temperatures in December, February, and April may be responsible for the lower crop growth and yield. Crude protein, lipid, and ash contents in the G1 and G2 areas were higher than in G3 and G4 areas. There was no variation in most kinds of amino acids between the group areas. Organic matter, available phosphoric acid, potassium (K), calcium (Ca), and zinc (Zn) contents in the soil of G1 areas were higher than in soils elsewhere. In addition, there was no consistency among most other mineral contents in the plants between the group areas. Overall, the growth and yield in G1 areas were lower than in other areas. Thus, it was concluded that these areas were still not suitable for barley cultivation regardless of climate change.

Development and characterization of novel activated carbons based on reed canary grass

Activated carbons were prepared from a novel precursor, reed canary grass, through microwave-induced activation in nitrogen, carbon dioxide, and air environments using phosphoric acid chemical activating agent. BET surface area of activated carbons was modeled as a function of phosphoric acid concentration and radiation time. The optimal preparation conditions for the activated carbon prepared in presence of nitrogen was 35 wt% phosphoric acid concentration and 6.9 min radiation time with BET surface area of 521 ± 6 m2/g and yield of 44.7 ± 1.2 wt%. The optimal preparation condition in the carbon dioxide environment was phosphoric acid concentration of 65 wt% and radiation time of 4 min, giving a BET surface area of 699 ± 7 m2/g and yield of 44.8 ± 1.2 wt%. In case of air as the activation gas, the yield was 15.6 ± 0.6 wt% and below which was much lower than the yield values when the activated carbons were prepared in nitrogen and carbon dioxide environments.

Properties and Bifunctional Catalytic Activity of Niobium-Doped Silica-Titania: Effect of Phosphoric Acid Treatment

The effect of phosphoric acid treatment on the physical-chemical properties and catalytic activity of the niobium-doped silica-titania bifunctional catalyst was investigated. As part of the synthesis procedure to produce xPO4−/Nb/TiO2-SiO2, different concentrations of phosphoric acid (H3PO4) were used (x= 0, 0.05, 0.10, 0.15, 0.20, 0.25 M). As shown by XRD analysis, the samples synthesized using 0–0.20 M H3PO4 were in amorphous form, as featureless diffractograms were obtained, indicating the PO4− groups were dispersed homogeneously on the surface of Nb doped SiO2-TiO2. Due to the increased concentration of acid, other compounds were formed in the samples by reactions between PO4− and Nb and/or Ti. Additionally, UV-Vis DRS results indicated that the presence of the PO4− group accelerated the transformation of hydrated tetrahedral Ti species into isolated tetrahedral Ti species. An experimental investigation of the catalytic performance of the catalyst was conducted using 1,2-epoxyoctane as an oxidant for the epoxidation of 1-octene to 1,2-octanediol. It has been demonstrated that H3PO4 treatment was essential for oxidative and acidity active site formation. The current research findings strongly suggested that Nb-doped TiO2-SiO2 treated with 0.2 M H3PO4 was the most effective bifunctional catalyst in generating 1,2-octanediol.

Effective removal of fluorine ions in phosphoric acid by silicate molecular sieve synthesized by hexafluorosilicic acid

Silicone zeolite prepared from hexafluorosilicic acid is a good adsorbent for fluoride removal. The morphology and structure of the prepared composites were characterized by XRD, FT-IR and BET. XRD images showed that fluorine was adsorbed on the surface of zeolite and the FT-IR spectra showed that there are different kinds of silicon and aluminum in silica-based zeolite. Therefore, through the studied of regeneration-adsorption cycle of zeolite, fluoride desorption and regeneration mechanism of zeolite defluorination was explained, and the adsorption performance and regeneration adsorption performance of zeolite defluorination were evaluated. The effects of phosphoric acid concentration, operating condition and specific surface area of zeolite and Si/Al ratio of zeolite on adsorption performance were investigated. Silicate molecule sieve shows strong adsorption capacity for fluoride (F-), and the adsorption capacity reached 352.46 mg/g under the optimal conditions. The Freundlich and Langmuir isotherm models for aluminosilicate MCM-41 and NaA zeolite which experimental data of F- adsorption on in phosphoric acid were fitted to investigate the adsorption mechanism. In the mechanism of anion adsorption, F- or SiF62- is bound by electrostatic adsorption of hydroxyl on the surface of adsorbent with anion adsorption. The adsorbed SiF62− ions can be regenerated when the environmental pH increases to value higher than that of zero charge point of zeolite. There is a competitive adsorption between OH– and F- with negative charge, which reduces the adsorption efficiency of adsorbent for F-. The NaA zeolite regenerated with NH3·H2O showed better fluorine re-adsorption performance.

Elastic properties of porous silicon nitride fabricated via a low-temperature processing route

Porous silicon nitride is a promising material for many advanced applications due to its many outstanding properties. In this work, porous silicon nitride was fabricated at 1200 °C using a mixture of silicon nitride powder and phosphoric acid. The dual role of phosphoric acid as inorganic binder and pore former has been critically analyzed through a detailed structural characterization. Elastic constants of the porous silicon nitride samples were determined using non-destructive ultrasound phase spectroscopy, and the influence of the phosphoric acid content on the elastic properties was studied at depth. Both longitudinal and shear elastic constants were determined and Young's modulus and Poisson's ratio were calculated as a function of porosity considering the actual elastic symmetry of the fabricated samples. The experimentally determined elastic constants were compared with the predictions from different analytical models to understand the influence of porosity and pore morphology on the stiffness of the fabricated samples.

Enhancing Glucose Recovery from Hibiscus cannabinus L. through Phosphoric Acid Pretreatment

Non-food lignocellulosic biomass is an attractive source owing to its abundance as a renewable resource and cost-effectiveness. Hibiscus cannabinus L., commonly known as kenaf, is a fiber-producing plant with high cellulose yield and non-food biomass. This study aimed to enhance the glucose recovery (GR) of kenaf biomass (KB). The bark and core fibers of KB are rich in glucan content and low in lignin content. Based on its glucan and lignin contents, KB has considerable potential as a feedstock for synthesizing monomer sugars, which can produce biofuel and high-value compounds. Therefore, the bark and core fibers were treated at a moderate temperature with various concentrations of phosphoric acid, followed by enzymatic hydrolysis. After pretreatment, the chemical composition of both feedstocks was changed. Phosphoric acid substantially affected the elimination of partial lignin and hemicellulose, which led to enhanced enzymatic hydrolysis. The maximum hydrolysis efficiency (HE) and GR of bark and core fibers were achieved when both feedstocks were treated with 75% phosphoric acid. Compared with untreated feedstocks, HE increased by approximately 5.6 times for bark and 4.7 times for core fibers. However, GR was enhanced approximately 4.9-fold for bark and 4.3-fold for core fibers.

Water resistance of fly ash phosphoric acid-based geopolymer

Phosphoric acid (PA)-based geopolymers are different from alkali-activated materials. The PA-based geopolymers have good mechanical properties, but the related research is still less, especially the research concerning the durability of geopolymers has received less attention, which has brought some trouble to the engineering application. In this article, to further study the water resistance of PA-based geopolymer prepared using fly ash, the unconfined compressive strength tests were conducted to evaluate the water resistance. Meantime, the Scanning Electron Microscopy (SEM), SEM-EDS, SEM mapping, Mercury Intrusion Porosimetry, X-ray Diffraction, and Infrared Spectroscopy tests were used to reveal the mechanism of strength weakening of the geopolymers after soaking. The results show that the water resistance of the geopolymer first increased and then decreased with the increasing PA concentration. When the PA concentration was 50%, the geopolymer with the optimal PA concentration gave the best water resistance. The compressive strength was decreased significantly after soaking, and the strength first decreased rapidly and then reduced slowly with the soaking time increasing. At 7 days of soaking, the strength decreased rapidly with a decrease of 30–50%. After 7 days of soaking, the strength decreased slightly. After soaking, the at 180 days of soaking, the geopolymer specimens cracked except for the geopolymer with the optimal PA concentration. If the water stability of the acid-based geopolymer is not improved, it is difficult to meet the requirement of engineering application. The microscopic analysis shows that quartz and mullite did not participate in the polymerization, and the crystalline compounds formed in the geopolymer matrix, such as CaPO3(OH)·2H2O, CaHPO4, AlPO4, and CaPO3·OH, basically did not change after soaking, but some calcium-containing crystalline compounds leached after recrystallization after soaking in water, and the Si–O–P unit in the amorphous gel in the geopolymer hydrolyzed after soaking, also leading to the decrease the geopolymer strength.

Melamine Adsorbed Catalyst for Mitigating Phosphoric Acid Poisoning in High Temperature Polymer Electrolyte Membrane Fuel Cell

High-temperature polymer electrolyte membrane fuel cell (HT-PEMFC) is operate at over 100 °C, at which the water in the fuel cell cannot exist in the liquid phase. That is why the PBI-based membrane with phosphoric acid is used in HT-PEMFC and phosphoric acid as an electrolyte to carry hydrogen ions in the form of phosphate ions. However, phosphate ions are attached to the surface of the platinum catalyst and inhibit the catalytic activity [1]. In particular, phosphate ions attached to the cathode prevent oxygen from reacting to platinum and become a major resistance factor [2,3]In this study, melamine was introduced to the Pt/C under 7 different concentrations in a water/ethanol-based solution at ambient temperature and pressure conditions. In the prepared catalyst, the melamine layer formed on the Pt nanoparticles protects phosphate ions while allowing oxygen to pass through.When the catalyst surface was analyzed using XPS, a clear N 1s peak was confirmed in common in samples containing 10 mM or more melamine. In addition, when the surface element ratio was analyzed, samples of 20 mM or more showed a ratio of harmful nitrogen atoms from melamine of 4 wt% or more in common.Elemental analysis using EDS mapping and shape change through transmission electron microscopy (TEM) as a result showed no significant difference depending on the melamine concentration, but showed a nitrogen content of about 7 wt% in common.In a 0.1 M HClO4 electrolyte, the binding of melamine can be confirmed through the shape change of the hydrogen oxidation and oxygen reduction peaks on cyclic voltammetry. Also, the 5 mM melamine sample showed the greatest half-wave potential in the ORR polarization curve at 1600 rpm. When the CV was measured while increasing the phosphoric acid concentration, the commercial platinum under the high phosphoric acid concentration had a very reduced ECSA, but the melamine sample showed an increased ECSA. In addition, the electrochemical performance of commercial Pt/C through the ORR polarization curve decreased significantly as the phosphoric acid concentration increased. On the other hand, it was confirmed that the reduction amount was greatly reduced in the sample in which 10 mM or more of melamine was synthesized. As a result, the performance under 1 M phosphoric acid solution was the best in the 20 mM melamine sample, in which the voltage at 0.2 mA/cm2 is 0.817 V at melamine 10 mM versus 0.715 V at Pt/C.

(Invited) Phosphoric Acid Redistribution in HT-Pemfcs: Scratching the Surface

There is a growing consensus that electrochemical energy conversion of hydrogen and high-energy-density fuels will be essential in the global transition towards a sustainable energy future. High-temperature proton exchange membrane fuel cells (HT-PEMFCs) that use doped phosphoric acid can operate above 100 °C under anhydrous conditions and provide an ideal solution for heat rejection in automotive fuel cell applications. An imperative but poorly understood research area of HT-PEMFCs is phosphoric acid redistribution in the membrane electrode assembly (MEA). While the phosphoric acid redistribution during pre-conditioning in MEAs improves fuel cell performance, further acid redistribution can be detrimental to the lifetime of the fuel cells.Here, we present our recent discovery regarding phosphoric acid redistribution in phosphoric acid doped membrane-based HT-PEMFCs. We first show the phosphoric acid concentration change in polybenzimidazole-based MEAs under various operating conditions. This result explains more than 10,000 hours of stable operation for HT-PEMFCs under certain operating conditions. Next, we investigate the phosphoric acid redistribution behavior in the MEA by calculating the energetics of phosphoric acid clusters in the membrane and electrodes, emphasizing the importance of phosphoric acid retention in the membrane. Lastly, we explain how acid redistribution can be controlled by introducing ion-pair coordinated membranes, providing future directions for obtaining more durable HT-PEMFCs for various applications.

(Invited) NMR Investigation of Proton Transport in Concentrated Aqueous and Polymer Electrolytes Based on Polyphosphoric Acid

Concentrated aqueous electrolytes have received much attention in recent years due to their high ionic conductivity and electrochemical window that usually exceeds that of the water electrolysis limitation. In collaboration with Ziyue Li and Fei Wang (Fudan University) we have investigated water/polyphosphoric acid solutions ranging in concentration from 1.25 to 16.6 molar being considered for proton-based batteries. Self-diffusion coefficients measured by pulsed field gradient NMR as well as spin-lattice time relaxation measurements were determined for 31P and 1H. The results shed light on the observed maximum in conductivity at the 5.9 molar concentration.For polymer electrolyte membrane fuel cells (PEMFC), polybenzimidazole (PBI) membranes with high phosphoric acid content have been developed previously by collaborator Brian Benicewicz (University of South Carolina) using the so-called PPA process. Recently, a carefully controlled drying method has been discovered by Benicewicz and co-worker Laura Murdock in which a PBI membrane prepared by the PPA process is transformed into a dense PBI film. Though originally developed for flow battery applications, it was found that this dense film could be re-acidified to yield a mechanically robust PEM. Furthermore this method provides a synthetic route to PBI films without the use of any organic solvents. The re-doped PBI films display high ionic conductivity at elevated temperatures, similar to the starting PBI gel membranes prepared by the PPA process, while containing nearly a factor of two less acid than the latter and also exhibiting enhanced mechanical properties. NMR spectroscopy was used to characterize the structure and dynamics of the modified and the original PBI/PPA membranes. In addition to pulsed field gradient diffusion studies, we have performed solid state one-dimensional 31P, 13C and two-dimensional 31P{1H} and 13C{1H} HETCOR NMR measurements, which yield additional information on structural differences between the membranes and how these differences may affect transport properties.