High-performance imidazole-containing polymers for applications in high temperature polymer electrolyte membrane fuel cells

This work focuses on the development of high temperature polymer electrolyte membranes(HT-PEMs)as key materials for HT-PEM fuel cells(HT-PEMFCs).Recognizing the challenges associated with the phosphoric acid(PA) doped polybenzimidazole(PBI) membranes,including the use of carcinogenic monomers and complex synthesis procedures,this study aims to develop more cost-effective,readily synthesized,and high-performance alternatives.A series of superacid-catalyzed polyhydroxyalkylation reactions have been carefully designed between p-terphenyl and aldehydes bearing imidazole moieties,resulting in a new class of HT-PEMs.It is found that the chemical structure of aldehyde-substituted N-heterocycles significantly impacts the polymerization reaction.Specifically,the use of 1-methyl-2-imidazole-formaldehyde and 1 H-imidazole-4-formaldehyde monomers leads to the formation of high-viscosity,rigid,and ether-free polymers,denoted as PTIm-a and PTIm-b.Membranes fabricated from these polymers,due to their pendent imidazole groups,exhibit an exceptional capacity for PA absorption.Notably,PTIm-a,carrying methylimidazole moieties,demonstrates a superior chemical stability by maintaining morphology and structural stability during 350 h of Fenton testing.After being immersed in 75 wt% PA at 40℃,the PTIm-a membrane reaches a PA content of 152%,maintains a good tensile strength of 13.6 MPa,and exhibits a moderate conductivity of 50.2 mS cm^(-1) at 180℃.Under H_(2)/O_(2) operational conditions,a single cell based on the PTIm-a membrane attains a peak power density of 732 mW cm^(-2) at 180℃ without backpressure.Furthermore,the membrane demonstrates stable cycle stability over 173 h within 18 days at a current density of 200 mA cm^(-2),indicating its potential for practical application in HT-PEMFCs.This work highlights innovative strategies for the synthesis of advanced HT-PEMs,offering significant improvements in membrane properties and fuel cell performance,thus expanding the horizons of HT-PEMFC technology.

Effect of electrolyte temperature on the nano-carbonitride layer fabricated by surface nanocrystallization and plasma treatment on a γ-TiAl alloy

The distribution of nano-carbonitrides produced by the treatments of surface nanocrystallization and plasma electrolytic carbonitriding on a γ-TiAl was investigated by means of figure analysis. The skewness and kurtosis of Gaussian shape distribution curves were studied and the effect of electrolyte temperature was determined. The usage of lower temperatures of the electrolyte is more suitable for achieving lower sizes of complex nano-carbonitrides. The surface roughness of treated samples was measured and it was observed that there is an optimum level of electrolyte temperature for surface roughness increase （difference between two measured data）.

Alumina Solubility in Na_3AlF_6-K_3AlF_6-AlF_3 Molten Salt System Prospective for Aluminum Electrolysis at Lower Temperature

The alumina solubility in the title system within the composition range of KR{m（K3AlF6）/[m（K3AlF6）＋ m（Na3AlF6）]} 10%―50%, a ternary Na3AlF6-K3AlF6-AlF3 molten system with 23%―29%（mass fraction） AlF3 was investigated by measuring the mass loss of a rotating sintered corundum disc. And the following empirical equation was derived when superheat degree was no more than 60 °C： w（Al2O3）sat=A×（T/1000）B, where A= –1.85774＋ 26.754234w（AlF3）–0.3683–0.00783KR2.363＋0.010266KR2.3048＋0.7902w（AlF3）0.00652, B=112.4625–53.2567w（AlF3）0.4236＋ 5.1079w（AlF3）0.9241＋0.01542w（AlF3）1.3540. Considering both higher alumina solubility and not too high superheat de gree are required, alumina solubility of different compositions at not the same temperature but the same superheat degree was studied, which will be more industrial helpful for selecting prospective compositions. The results show that the composition deserved to be further tested in lower temperature cells is 10%―30% KR and 23%―26%（mass fraction） AlF3.

Water-induced electrode poisoning and the mitigation strategy for high temperature polymer electrolyte membrane fuel cells

Engineering failure of membrane electrode assembly caused by increasingly fuel poisoning in the high temperature polymer electrolyte membrane fuel cells fed with humidified reformate gases is firstly demonstrated herein this work. Based on the results of the in-situ environmental scanning electron microscope, electrochemical analyses, and limiting current method, a water-induced phosphoric acid invasion model is constructed in the porous electrode to elucidate the failure causations of the hindered hydrogen mass transport and the enhanced carbon monoxide poisoning. To optimize the phosphoric acid distribution under the inevitably humidified circumstance, a facile and effective strategy of constructing acid-proofed electrode is proposed and demonstrates outstanding stability with highly humidified reformate gases as anode fuel. This work discusses a potential defect that was rarely studied previously under practical working circumstance for high temperature polymer electrolyte membrane fuel cells, providing an alternative opinion of electrode design based on the fundamental aspects towards the engineering problems.

Discharge Characteristic of Micro-Arc Oxidation on Aluminum Alloy under the Changing Electrolyte Temperature

The electrolyte temperature has a great influence on the performance of the coating prepared by micro-arc oxidation (MAO). The behavior of MAO discharge in the changing electrolyte temperature has been investigated. Compared to constant electrolyte temperature in conventional MAO process, the process has different discharge characteristics under the changing electrolyte temperature. The amplitude of pulse voltage was detected to study the change of discharge characteristic under the constant-current control of MAO power supply. Three successive discharge stages were differentiated by the variable the pulse voltage versus process time. Since there were significant changes in the sound, the sound signals were acquired and the audio analysis was used to describe the changing of the MAO discharge at different stages. Optical emission spectroscopy (OES) was employed in situ to unveil how the micro-discharge changed with the temperature increasing. Scanning electron microscopy (SEM) was used to characterize the morphology of the coatings on 6N01 aluminum alloy prepared by normal process with the constant-temperature control of the MAO electrolyte and by the process under the changing electrolyte temperature. A mode of film growth and micro-discharge was given to describe the effects of the changing electrolyte temperature in the whole MAO process.

Properties and Application of Double Alkali Earth Doped Ce_ (0.9)Ca_(0.1-x) Sr_xO_( 1.90)

A series of solid solution material based on ceria, Ce 0.9 Ca 0.1- x Sr x O 1.90 ( x =0, 0 04, 0 05, 0 06), were synthesized by a solid reaction method. Ceria doped with two alkali earth elements has cubic fluorite structure. The ionic conductivity of these materials at high temperature was studied by impedance spectra, and an increasing effect of ionic conductivity was found. A maximum conductivity is achieved when the effective ionic radius is near to the critical radius (0 1106 nm). Some fuel cells were made using these materials as electrolyte. The output power and current of the SOFC with the electrolyte of double doped ceria are better than those of YSZ and single doped ceria, and the open circuit voltage of the double doped ceria is also higher than that of the single doped ones.

Phosphorus induced activity-enhancement of Fe-N-C catalysts for high temperature polymer electrolyte membrane fuel cells

Fe-N-C materials with atomically dispersed Fe–N_(4) sites could tolerate the poisoning of phosphate,is regarded as the most promising alternative to costly Pt-based catalysts for the oxygen reduction in high temperature polymer electrolyte membrane fuel cells(HT-PEMFCs).However,they still face the critical issue of insufficient activity in phosphoric acid.Herein,we demonstrate a P-doping strategy to increase the activity of Fe-N-C catalyst via a feasible one-pot method.X-ray absorption spectroscopy and electron microscopy with atomic resolution indicated that the P atom is bonded with the N in Fe–N_(4) site through C atoms.The as prepared Fe-NCP catalyst shows a half-wave potential of 0.75 V(vs.reversible hydrogen electrode(RHE),0.1 M H_(3)PO_(4)),which is 60 and 40 mV higher than that of Fe-NC and commercial Pt/C catalysts,respectively.More importantly,the Fe-NCP catalyst could deliver a peak power density of 357 mW·cm^(−2)in a high temperature fuel cell(160℃),exceeding the non-noble-metal catalysts ever reported.The enhancement of activity is attributed to the increasing charge density and poisoning tolerance of Fe–N_(4) caused by neighboring P.This work not only promotes the practical application of Fe-N-C materials in HT-PEMFCs,but also provides a feasible P-doping method for regulating the structure of single atom site.

Effects of ionic strength and temperature on the aggregation and deposition of multi-walled carbon nanotubes

The aggregation and deposition of carbon nanotubes（CNTs） determines their transport and fate in natural waters.Therefore,the aggregation kinetics of humic-acid treated multi-walled carbon nanotubes（HA-MWCNTs） was investigated by time-resolved dynamic light scattering in NaCl and CaCl_2 electrolyte solutions.Increased ionic strength induced HA-MWCNT aggregation due to the less negative zeta potential and the reduced electrostatic repulsion.The critical coagulation concentration（CCC） values of HA-MWCNTs were 80 mmol/L in NaCl and 1.3 mmol/L in CaCl_2 electrolyte,showing that Ca^（2＋） causes more serious aggregation than Na~＋.The aggregation behavior of HA-MWCNTs was consistent with Derjaguin-Landau-Verwey-Overbeek theory.The deposition kinetics of HA-MWCNTs was measured by the optical absorbance at 800 ran.The critical deposition concentrations for HA-MWCNT in NaCl and CaCl_2 solutions were close to the CCC values,therefore the rate of deposition cannot be increased by changing the ionic strength in the diffusion-limited aggregation regime.The deposition process was correlated to the aggregation since larger aggregates increased gravitational deposition and decreased random Brownian diffusion.HA-MWCNTs hydrodynamic diameters were evaluated at 5,15 and 25℃.Higher temperature caused faster aggregation due to the reduced electrostatic repulsion and increased random Brownian motion and collision frequency.HA-MWCNTs aggregate faster at higher temperature in either NaCl or CaCl_2electrolyte due to the decreased electrostatic repulsion and increased random Brownian motion.Our results suggest that CNT aggregation and deposition are two correlated processes governed by the electrolyte,and CNT transport is favored at low ionic strength and low temperature.

Temperature stability of symmetric activated carbon supercapacitors assembled with in situ electrodeposited poly (vinyl alcohol) potassium borate hydrogel electrolyte

The temperature stability of supercapacitor（SC） is largely determined by the properties of the electrolyte.Hydrogel electrolytes（HGE）, due to their hydrophilic polymer skeleton, show different temperature stability to that of liquid aqueous electrolytes. In this study, symmetric activated carbon（AC） SCs had been assembled with in situ electrodeposited poly（vinyl alcohol） potassium borate（PVAPB） HGE. The electrochemical performance of the SCs was systematically studied at different temperatures. Results show that the conductivity of PVAPB HGE is comparable with that of liquid aqueous electrolytes at different temperatures. The operating temperature range of PVAPB HGE SCs is -5–60°C, while those of the 1 mol/L Na2SO4SCs and the 0.9 mol/L KClSCs are 20–80°C and 20–40°C, respectively. The specific capacitance of PVAPB HGE SC is higher than those of SCs using liquid aqueous electrolytes at any temperature. The excellent temperature stability of PVAPB HGE makes it possible to build stable aqueous SCs in the wider temperature range.