Relative Humidity Dependence Research Articles

On the origin of deactivation of reversal-tolerant fuel cell anodes under voltage reversal conditions

Over the past decade, extensive efforts have been carried out to increase the durability of polymer electrolyte fuel cells (PEFCs) under voltage reversal conditions primarily caused by hydrogen starvation. One common strategy is the use of reversal tolerant anodes (RTAs) that incorporate oxygen evolution reaction (OER) catalysts (e.g., IrO2) to promote harmless water electrolysis over carbon oxidation and delay severe carbon support degradation. The present study investigates the RTA catalyst deactivation mechanism under hydrogen starvation conditions and the relative humidity (RH) dependence of the OER and carbon oxidation reactions to understand the relatively quick failure of an RTA at a high RH. The comparison of the OER durability of membrane electrode assemblies with and without carbon under hydrogen starvation conditions shows that the presence of carbon deactivates the OER catalyst through carbon oxidation species poisoning. The catalytic activity of IrO2 measured by the current density response shows a linear dependence on RH, while the rate of carbon oxidation reactions increases exponentially with RH, suggesting that the decreased durability of RTAs is related to the exponentially increased rate of carbon oxidation reactions. These findings can be used to design more robust electrodes for automotive fuel cells under hydrogen starvation conditions.

A series of asymmetric zinc (II) phthalocyanines containing fluoro and alkynyl groups: Synthesis and examination of humidity sensing performance by using QCM based sensor

In this study, the three novel fluoro substituted asymmetric zinc phthalocyanines substituted with 3-hydroxy-3-methyl-1-butynyl (3), ethynyl (4), and 4-nitrophenylethynyl (5) groups were synthesized and characterized by Fourier-transform infrared spectrophotometer, ultraviolet–visible, proton nuclear magnetic resonance, and matrix-assisted laser desorption/ionization time-of-flight mass spectroscopy. The thickness dependence of relative humidity sensing performance of these compounds using quartz crystal microbalance transducers in a broad operating range of humidity was investigated. Results from this preliminary analysis indicated that responses and the response and recovery times of the 3–5 compound-based relative humidity sensors strongly depended on the thickness of the sensory layer. Experimental studies indicated that the optimized thickness as 450 nm produces a good sensitivity, and the best response and recovery times. The relative humidity sensing results demonstrated that the quartz crystal microbalance sensor coated with peripherally substituted unsymmetrical zinc(II) phthalocyanines bearing nitrophenyl group was the most useful coating for the relative humidity sensing.

Simultaneous measurements of the relative-humidity-dependent aerosol light extinction, scattering, absorption, and single-scattering albedo with a humidified cavity-enhanced albedometer

Abstract. Hygroscopic aerosols take up water and grow with increasing relative humidity (RH), giving rise to large changes in light extinction (bext), scattering (bscat), absorption (babs), and single scattering albedo (SSA, ω). The optical hygroscopic growth factors for each parameter (f(RH)ext,scat,abs,ω) are thus important for assessing aerosol effects on regional air quality, atmospheric visibility, and radiative forcing. The RH dependence of aerosol scattering and extinction has been studied in many laboratory and field studies. However, owing partly to the absence of suitable instrumentation, there are few reports of the RH dependence of aerosol absorption and ω. In this work, we report the development of a humidified cavity-enhanced albedometer (H-CEA) for simultaneous measurements of f(RH)ext,scat,abs,ω at λ=532 nm from 10 % to 88 % RH. The instrument's performance was evaluated with laboratory-generated ammonium sulfate, sodium chloride, and nigrosin aerosols. Measured hygroscopic growth factors for different parameters were in good agreement with model calculations and literature-reported values, demonstrating the accuracy of the H-CEA for measuring RH-dependent optical properties.

Hygroscopic properties of sodium and potassium salts as related to saline mineral dusts and sea salt aerosols

Mineral dust, soil, and sea salt aerosols are among the most abundant primary inorganic aerosols in the atmosphere, and their hygroscopicity affects the hydrological cycle and global climate. We investigated the hygroscopic behaviors of six Na- and K-containing salts commonly found in those primary organic aerosols. Their hygroscopic growths as a function of relative humidity (RH) agree well with thermodynamic model prediction. Temperature dependence of deliquescence RH (DRH) values for five of those salts was also investigated, which are comparable to those in literature within 1%–2% RH, most showing negative dependence on temperature. Hygroscopic growth curves of real-world soil and sea salt samples were also measured. The hygroscopic growths of two more-hydroscopic saline soil samples and of sea salt can be predicted by the thermodynamic model based on the measured water-soluble ionic composition. The substantial amounts of water-soluble ions, including Na+ and K+, in saline soil samples imply that even nascent saline soil samples are quite hygroscopic at high-RH (>80%) conditions. For three less-hygroscopic dust samples, however, measurements showed higher water uptake ability than that predicted by the thermodynamic model. The small amount of water taken up by less-hygroscopic dust samples suggests that dust particles might contain thin layers of water even to very low RH. The results of this study provide a comprehensive characterization of the hygroscopicity of Na- and K-containing salts as related to their roles in the hygroscopic behaviors of saline mineral dusts and sea salt aerosols.

Heterogeneous Ozonolysis of Squalene: Gas-Phase Products Depend on Water Vapor Concentration.

Previous work examining the condensed-phase products of squalene particle ozonolysis found that an increase in water vapor concentration led to lower concentrations of secondary ozonides, increased concentrations of carbonyls, and smaller particle diameter, suggesting that water changes the fate of the Criegee intermediate. To determine if this volume loss corresponds to an increase in gas-phase products, we measured gas-phase volatile organic compound (VOC) concentrations via proton-transfer-reaction time-of-flight mass spectrometry. Studies were conducted in a flow-tube reactor at atmospherically relevant ozone (O3) exposure levels (5-30 ppb h) with pure squalene particles. An increase in water vapor concentration led to strong enhancement of gas-phase oxidation products at all tested O3 exposures. An increase in water vapor from near zero to 70% relative humidity (RH) at high O3 exposure increased the total mass concentration of gas-phase VOCs by a factor of 3. The observed fraction of carbon in the gas-phase correlates with the fraction of particle volume lost. Experiments involving O3 oxidation of shirts soiled with skin oil confirms that the RH dependence of gas-phase reaction product generation occurs similarly on surfaces containing skin oil under realistic conditions. Similar behavior is expected for O3 reactions with other surface-bound organics containing unsaturated carbon bonds.

Roles of Relative Humidity in Aerosol Pollution Aggravation over Central China during Wintertime.

Aerosol pollution elicits considerable public concern due to the adverse influence on air quality, climate change, and human health. Outside of emissions, haze formation is closely related to meteorological conditions, especially relative humidity (RH). Partly due to insufficient investigations on the aerosol hygroscopicity, the accuracy of pollution prediction in Central China is limited. In this study, taking Wuhan as a sample city, we investigated the response of aerosol pollution to RH during wintertime based on in-situ measurements. The results show that, aerosol pollution in Wuhan is dominated by PM2.5 (aerodynamic particle size not larger than 2.5 μm) on wet days (RH ≥ 60%), with the averaged mass fraction of 0.62 for PM10. Based on the RH dependence of aerosol light scattering (f (RH)), aerosol hygroscopicity was evaluated and shows the high dependence on the particle size distribution and chemical compositions. f (RH = 80%) in Wuhan was 2.18 (±0.73), which is comparable to that measured in the Pearl River Delta and Yangtze River Delta regions for urban aerosols, and generally greater than values in Beijing. Ammonium (NH4+), sulfate (SO42−), and nitrate (NO3−) were enhanced by approximately 2.5-, 2-, and 1.5-fold respectively under wet conditions, and the ammonia-rich conditions in wintertime efficiently promoted the formation of SO42− and NO3−, especially at high RH. These secondary ions play an important role in aggravating the pollution level and aerosol light scattering. This study has important implications for understanding the roles of RH in aerosol pollution aggravation over Central China, and the fitted equation between f (RH) and RH may be helpful for pollution forecasting in this region.

Effects of Porous Carbon Morphology, Agglomerate Structure and Relative Humidity on Local Oxygen Transport Resistance

The morphology and ionomer distribution in the polymer electrolyte membrane fuel cell (PEMFC) cathode electrode (used in Toyota Mirai) are quantified with nano-scale resolution X-ray computed tomography (nano-CT). Using the nano-CT data, different shapes, sizes and compositions of agglomerates are extracted. Statistical information from multiple techniques, including transmission electron microscopy (TEM), ultra-small angle X-ray scattering (USAXS), and Brunauer-Emmett-Teller (BET) gas adsorption porosimetry are combined to reconstruct the high surface-area porous carbon (HSC) support, exterior (on the surface of carbon) and interior (inside carbon pores) catalysts, ionomer, and primary pores in the extracted agglomerates. Application of capillary condensation theory to the reconstructed agglomerate structure is shown to accurately represent the experimentally-observed relative humidity (RH) dependence of the electrochemically-active surface area (ECA). Direct numerical simulations (DNS) show that the high local O2 transport resistance () under dry conditions is mainly associated with the reduced ECA. We demonstrate that the agglomerate shape and size affect only if the primary pores are poorly accessible (i.e., capillary-condensed water filled primary pores result in lower for lower aspect ratio agglomerates). We also estimate associated with the location of catalyst (whether inside or on the surface of HSC).

(Invited) Activity and Stability of Atomically Dispersed (AD) Fe-C-N ORR Catalyst in Polymer Electrolyte Fuel Cell Environment

Activity and stability of (AD)Fe-N-C catalyst with low Fe content (1.5 at%) have been investigated in a polymer electrolyte fuel cell environment. Previous electron microscopy analyses (SEM, EDS and EELS) have revealed the presence of atomically dispersed Fe atoms coordinated with nitrogen (Fe-Nx) and a partially graphitized microporous carbon structure. In this study, 5-cm2 membrane electrode assemblies (MEAs) were prepared by hot pressing anode gas diffusion electrodes (GDEs, 0.2 mgPt/cm2) to N211 membrane, and brush painting the PGM-free cathode catalyst ink; Sigracet SG29BC GDL was used on cathode side without hot pressing. All performance tests were conducted under differential cell conditions with fixed H2, O2 and air flow rates. Polarization curves obtained in H2/O2 show double Tafel slopes which, in conjunction with the position of the redox potential observed in cyclic voltammetry traces, forms the basis for formulating a distributed ORR (oxygen reduction reaction) kinetic model with potential-dependent available sites and extracting reaction order for P(O2) and relative humidity (RH) dependence and activation energy for temperature dependence. Application of this model to polarization data collected in H2/air provides the basis for formulating the oxygen transport model and leads to 7.7±0.6 mA/cm2 modeled catalyst activity (defined as areal current density at 0.9 V IR-corrected cell voltage in H2/O2,1.5 atm, 80oC, and 100% RH), 31.5-34.3 mA/cm2 cell performance at 1.5 atm, 80oC, 100% RH, and 0.8-2 s/cm O2 transport resistance (Rcf) in the cathode catalyst layer (CCL). The derived value of Rcf for ~100-μm thick PGM-free electrode is 2-3 times higher than the measured Rcf for <10-μm thick low-Pt loaded (<0.1 mgPt/cm2) electrodes. As such, Rcf includes resistance to O2 transport in the CCL secondary pores, and the primary pores and ionomer film surrounding the Fe-Nx active sites within the microporous carbon support. Catalyst durability was characterized in accelerated stress tests (AST) with 0.6-0.95 V trapezoid potential cycles in H2/N2. Although the (AD)Fe-N-C catalyst has low initial activity, it exhibits excellent stability in ASTs considering that the measured polarization curves in H2/air show voltage losses of only 5, 15 and 30 mV after 10k, 20k and 30k cycles, respectively, for an average degradation rate of 3.5 mV/h. The voltage losses are nearly independent of the current density, suggesting that the observed performance degradation is mainly due to loss in catalyst activity and the transport resistance is nearly unaffected by AST. The kinetic model indicates a 50% loss in catalyst activity after 30k AST cycles. We observe similar degradation rate on a load cycle (LC) consisting of H2/air polarization curves under different operating pressures (1-2.5 atm), temperatures (60-95oC), O2 mole fraction (2-21%), and relative humidity (30-100%). The degradation rate is faster under a combined AST and LC, with majority of losses occurring in the load cycles. Figure 1 summarizes the modeled loss in active sites for load cycles in H2/O2 (cell 1), load cycles in H2/air (cell 2), AST cycles in H2/N2 (cell 3), and combined AST and load cycles (cell 4). The model indicates <20% increase in the O2 transport resistance in the (AD)Fe-N-C catalyst layer with >75% loss of inactive sites. We have used the coupled kinetic, O2 transport and stability model to project the improvements needed in catalyst activity and electrode structure to approach the automotive target of 1000 mW/cm2 stack power density while meeting the 1.45 kW/oC heat rejection (Q/ΔT) constraint at 2.5 atm inlet pressure, 95oC coolant exit temperature, and 1.5 cathode stoichiometry. The model indicates that we need twelve-fold higher mass activity for reducing the kinetic losses, doubling of active site density and an engineered electrode structure for 50% lower sheet resistance, as well as a 50% reduction in electrode thickness to limit the O2 transport losses. Such an automotive competitive PGM-free catalyst will exceed 22.6 mA/mg mass activity for >43.3 mA/cm2 specific activity at 1.9 mg/cm2 catalyst loading in an electrode structure that is <50-μm thick and engineered for <50 mΩ.cm2 sheet resistance. It will reach 1150 mA/cm2 current density at 656 mV for 750 mW/cm2 power density at Q/ΔT relevant operating conditions, after incurring voltage losses of 405 mV for ORR kinetics, 70 mV for combined proton transport in membrane and CCL, and 35 mV for O2 transport across the gas channel, GDL and CCL. Figure 1

(Invited) Structural Characterization of Polymer Electrolyte Fuel Cell Electrodes Using Nano-Scale Synchrotron X-Ray Computed Tomography

With improvements in the intrinsic oxygen reduction reaction activity of both Pt-based and platinum group metal-free (PGM-free) catalysts for polymer electrolyte fuel cells, in recent years more attention is being focused on utilization of the active sites in these catalysts under conditions relevant to the automotive application. Taking full advantage of the performance offered by these high activity catalysts requires comprehensive knowledge of the electrode microstructure and the spatial distribution of the catalyst, support, ionomer, and pores. Nano-scale resolution X-ray computed tomography (nano-CT) has been used as a powerful tool for non-destructive quantification of three-dimensional pore morphology and estimation of ionomer distribution in both Pt-based and PGM-free cathode catalyst layers (CCL) [1, 2]. On the other hand, due to insufficient resolution (20 nm voxel size) this technique cannot resolve the detailed structures of support, ionomer, and catalyst. We developed the “hybrid computational method” [2] combining statistical information from transmission electron microscopy (TEM), ultra-small angle X-ray scattering (USAXS), and BET with nano-CT to reconstruct the features below the nano-CT resolution. In this study, an example will be given of application of this method to a Pt alloy-based electrode. Various shape, size and composition agglomerates are reconstructed to 1 nm voxel resolution using the hybrid methodology. It is shown that ionomer film thickness is spatially non-uniform inside the agglomerates and strongly correlates with ionomer to carbon weight ratio (I/C) while it is independent of the agglomerate size. Compared to experimental observations, the application of capillary condensation theory to the reconstructed agglomerate structure is shown to accurately represent the relative humidity (RH) dependence of the electrochemically-active surface area (ECA). Furthermore, direct numerical simulations are performed to quantify localized transport losses. Effectiveness factor correlations are derived for use in macroscale models. The effects of RH, agglomerate structural properties, and pores in the carbon support on the local oxygen transport resistance are examined. It is demonstrated that low catalyst utilization is responsible for the experimentally-observed high local transport resistance under dry conditions and that agglomerate shape and size affect the local oxygen transport resistance only if the primary pores are poorly accessible.

(Invited) Performance and Durability of Automotive Fuel Cell Stacks and Systems with Low-Loaded d-PtCo/C Cathode Catalyst in Membrane Electrode Assemblies

Previous studies have shown that meeting the 30 $/kWe cost and 8,000-h durability targets for automotive fuel cell systems (FCS) requires high-activity oxygen reduction reaction (ORR) cathode catalysts that are stable under cyclic potentials at low Pt loadings (<0.1 mg-Pt/cm2). We have evaluated the performance and durability of a state-of-the-art (SOA) fuel cell stack and system under automotive relevant conditions using data obtained for a membrane electrode assembly (MEA) operated under differential cell conditions. The MEA has de-alloyed Pt3Co catalyst supported on a high surface area carbon (HSAC) in cathode, Pt catalyst supported on Vulcan carbon in anode, thin 12-μm reinforced PFSA membrane, and 200-μm thick diffusion media with microporous layers. The Pt loadings are 0.1 mg/cm2 in the cathode catalyst and 0.025 mg/cm2 in the anode catalyst. We formulated a model for ORR kinetics on d-PtCo/C catalyst by using the measured polarization data at low current densities together with the ionic conductivity of the cathode catalyst layer derived from the Galvanostatic impedance data obtained in H2/N2, and a transient solid solution model for oxide coverage as a function of potential, relative humidity and temperature. The kinetic model indicates that d-PtCo/C catalyst has 650 A/gPt mass activity, which is double the mass activity of annealed Pt catalyst that has nearly the same particle size, and easily exceeds the target of 440 A/gPt. In accelerated stress tests (AST) with 0.6-0.95 V square wave potentials in cyclic voltammetry, there are negligible changes in kinetic parameters denoting the reaction order for O2 partial pressure, activation energy and relative humidity (RH) dependence. As indicated in Fig. 1, the specific activity (µA/cmPt 2) degraded by ~33% in AST during which time the electrochemically active surface area (ECSA) decreases by 7.5% and nearly stayed constant with further decrease in ECSA. We concluded that the mass activity, which is a product of specific activity and ECSA, degrades in AST initially because of decrease in specific activity due to cobalt dissolution and subsequently because of decrease in ECSA due to coarsening of catalyst particles. We determined the oxygen transport resistance of the d-PtCo/C electrode (Rcf) by (a) using the measured polarization data at high current densities; (b) estimating mass transfer overpotentials (ηm) from the ORR kinetic model; (c) determining the limiting current densities (iL) at which ηm equals 400 mV; (d) estimating the oxygen transport resistance (ηm) from iL; and (e) extracting the pressure-dependent and pressure-independent (Rcf) parts of Rm. We concluded that after 30,000 AST cycles, the pressure-dependent part of Rm representing O2 transport resistance in gas channel and gas diffusion layer (GDL) showed small changes; however Rcf has increased by >50% at 40% RH exposure and >125% at 100% RH exposure. Lower roughness (SPt, cmPt 2/cm2) because of catalyst coarsening accounts for 8% of the increase in Rcf at 40% exposure and for 45% increase at 100% exposure. Also, 48-60% of the increase in Rcf may be associated with the reduction in O2 permeability through the ionomer film on the catalyst particles supported on HSAC. We developed an integral cell model using the differential cell data to evaluate the performance and cost of an automotive FCS with the d-PtCo/C catalyst relative to the targets of 65% peak efficiency, 8.0 kWe/g stack Pt utilization, 1000 mW/cm2 stack power density and 40 $/kW cost, all subject to 1.45 kW/K (Q/ΔT) heat rejection constraint. The analysis indicates that the SOA catalyst can achieve 9.5 ± 0.5 kWe/g stack Pt utilization and 1180 ± 55 mW/cm2 gross stack power density at 656 mV cell voltage, 2.5 atm stack inlet pressure, 95oC stack coolant outlet temperature, and 1.5 cathode stoichiometry. The projected system cost is 46.0 ± 0.7 $/kWe at high manufacturing volume (500,000 units/year). At lower manufacturing volumes, the estimated costs are 51 $/kWe at 100,000 units/year and 88 $/kWe at 10,000 units/year. We projected FCS performance degradation in terms of ECSA loss by conducting simulations at constant air flow rate (variable cathode stoichiometry) and heat rejection (Q). The simulations indicate that meeting the target of 10% derating in FCS power over lifetime requires controlling the stack operating conditions to limit the ECSA loss to 35%. At end of life, we estimate that mass activity has degraded by ~50%, specific activity by 25%, and the cell voltage at rated power by 30 mV. Increase in kinetic overpotential due to Co dissolution and coarsening of catalyst particles accounts for the majority of the projected cell voltage loss at rated power. Figure 1

Extinction coefficients of surface atmospheric aerosol above LHAASO * * Supported by National Natural Science Foundation of China (11503021, 11205126, 11847307), Key Projects of Cooperation between Provincial Colleges and Schools of Science and Technology Plan from Sichuan Province (2017JZ0034) and National Key Research and Development Program Projects (2018YFA0404201)

We investigate the extinction coefficients of the surface atmospheric aerosol over the Large High Altitude Air Shower Observatory (LHAASO), located at the Haizi Mountain, Daocheng County, China. To this end, we utilize the Longtin model, Mie scattering theory, and experimental data obtained by the Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO). Our theoretical calculations show that the total extinction coefficients of the atmospheric aerosol at the wavelength of 200–500 nm are inversely proportional to the laser wavelength, and influenced by the wind speed. From July 2015 to October 2016, the extinction coefficient of the surface atmospheric aerosols at 532 nm wavelength reached 0.04 km−1 with no wind, while it increased to 0.1 km−1 with gusts. In this period, the extinction coefficients of the surface atmospheric aerosol at 532 nm wavelength, obtained by the CALIPSO, change from 0.01 to 0.07 km−1, which is less than the values obtained the theoretical calculation and larger than the average of Tibetan Plateau in 2006−2016. These calculations and experimental evidence provide important arguments to the model of atmospheric aerosol to be applied in the calibration of LHAASO. Our results suggest that the extinction coefficients over LHAASO require further study, including research on the size distribution, shape, concentration of aerosols particles, wind dependence, relative humidity dependence, etc.

Assessing relative humidity dependent photoacoustics to retrieve mass accommodation coefficients of single optically trapped aerosol particles.

Photoacoustic spectroscopy is widely used to measure the light absorption of aerosols. However, the impact of key factors such as the effect of relative humidity and mass exchange on photoacoustic measurements are still poorly understood. We assess such measurement biases and their physical origin by analysing the photoacoustic signal of single tetraethylene glycol (TEG) particles at varying relative humidities. Our results show a decrease in the photoacoustic signal at elevated relative humidities for small particles (0.8-1.5 μm), while for larger sizes (2.2-3.2 μm) the trend is reversed. We model the photoacoustic signal to interpret the observed behaviour in terms of mass and heat flux contribution. The single particle photoacoustic signal analysis presented in this paper additionally allows for the retrieval of the mass accommodation coefficient. Fitting our experimental data to the theoretical model reveals values of αM ≈ 0.02-0.005 for water on TEG in the temperature range 295-309 K.

Fast and Reliable State-of-Health Model of a PEM Cathode Catalyst Layer

Long-term stability of polymer electrolyte membrane fuel cells under dynamic operation still has a high potential for optimization, specifically for use in the automotive industry. This stability is especially affected by the degradation processes taking place in the cathode catalyst layer and hence should be fully understood. In this work, we develop a fast and reliable state-of-health model of the cathode catalyst layer, incorporating the electrochemical degradation processes related to anodic and cathodic platinum dissolution, oxidation, Pt loss due to ion diffusion, carbon corrosion and place exchange mechanisms as well as their interaction. For the purpose of validation, the model is developed alongside a comprehensive experimental data set. A detailed parameter study taking into account temperature, relative humidity and load profile dependency was carried out. A good agreement between model and experiment was found for load ranges between 0.6 and 0.95 V. Further, good approximation of the active surface area loss for cell temperatures between 60°C and 90°C and relative humidity between 30% and 100% were achieved.

Heterogeneous OH oxidation of secondary brown carbon aerosol

Abstract. Light-absorbing organic aerosol, or brown carbon (BrC), has significant but poorly constrained effects on climate; for example, oxidation in the atmosphere may alter its optical properties, leading to absorption enhancement or bleaching. Here, we investigate for the first time the effects of heterogeneous OH oxidation on the optical properties of a laboratory surrogate of aqueous, secondary BrC in a series of photo-oxidation chamber experiments. The BrC surrogate was generated from aqueous resorcinol, or 1,3-dihydroxybenzene, and H2O2 exposed to &gt;300 nm radiation that is atomized, passed through trace gas denuders, and injected into the chamber, which was conditioned to either 15 % or 60 % relative humidity (RH). Aerosol absorption and scattering coefficients and single scattering albedo (SSA) at 405 nm were measured using a photoacoustic spectrometer. At 60 % RH, upon OH exposure, absorption first increased, and the SSA decreased sharply. Subsequently, absorption decreased faster than scattering, and SSA increased gradually. Comparisons to the modelled trend in SSA, based on Mie theory calculations, confirm that the observed trend is due to chemical evolution, rather than slight changes in particle size. The initial absorption enhancement is likely due to molecular functionalization and/or oligomerization and the bleaching to fragmentation. By contrast, at 15 % RH, slow absorption enhancement was observed without appreciable bleaching. A multi-layer kinetics model, consisting of two surface reactions in series, was constructed to provide further insights regarding the RH dependence of the optical evolution. Candidate parameters suggest that the oxidation is efficient, with uptake coefficients on the order of unity. The parameters also suggest that, as RH decreases, reactivity decreases and aerosol viscosity increases, such that particles are well-mixed at 60 % RH but not at 15 % RH. These results further the current understanding of the complex processing of BrC that may occur in the atmosphere.

Measurement of DC properties and relative humidity (RH) dependence of wideband AC current shunts

The paper describes the DC characterization of AC current shunts recently developed at the Faculty of electrical engineering and computing, University of Zagreb (UNIZG-FER). The shunts of cage type design include nine shunts that range from 1 mA up to 10 A. The following DC properties were inspected: long time drift, temperature coefficients and power coefficients. The results have shown that shunts have low temperature (<7 µΩ/Ω/°C), and power coefficients (<10 µΩ/Ω/W), and small drift which makes them suitable for precise power measurement applications. Also, relative humidity dependence of currents shunts is investigated.

Humidity dependence of electrostatic pick-and-place operation of a micro dielectric particle considering surface conductivity and capillary condensation

Relative humidity dependence on electrostatic pick-and-place operation is investigated to improve the reliability of micro-manipulation. The manipulation system consists of three elements: a conductive probe as a manipulator, a conductive plate as a substrate, and a dielectric particle as the target object for manipulation. To pick up/place the particle, a rectangular pulse voltage is applied to the probe. Capillary condensation at the particle-plate interface is theoretically considered to evaluate a detachment voltage (the voltage to detach the particle). Surface conductivity of the particle is theoretically considered to evaluate a detachment time (the time for detaching the particle). Experiments are conducted in relative humidities of 30%, 40%, 50%, and 60% by using a soda-lime glass particle with a diameter of 30 μm. It is clarified that the detachment voltage increases and the detachment time decreases by increasing relative humidity. The particle can be successfully picked up/placed by clarifying the effect of relative humidity on the detachment voltage and the detachment time. This knowledge, relative humidity dependence on electrostatic pick-and-place operation, will contribute to micro-fabrication technology by enhancing adaptability in various atmospheric conditions.

Multi-block sulfonated poly(arylene ether nitrile) polymers bearing oligomeric benzotriazole pendants with exceptionally high H2/O2 fuel cell performance

Aromatic ionomers have emerged as promising alternatives to perfluorosulfonic acid polymers to be applied as proton exchange membranes in fuel cells. However, the paradox between the ion conductivity and stability is still a challenge precluding the commercialization of aromatic ionomers. In this paper, we report the design of a novel sulfonated poly(arylene ether nitrile) multi-block structure by introducing a key component, alkyl benzotriazole (Bt) side chains, into the hydrophobic segments. The modified structure could facilitate polymer phase-separation and generate self-standing films with excellent mechanical properties, and it effectively suppresses the excessive swelling of the membrane owing to strong electrostatic interactions between the Bt chains and sulfonic acid groups. Moreover, the Bt unit could act as both a proton acceptor and proton donor, causing a dramatic increase in the ion conductivity of the membrane. The most optimal membrane possesses an ion-exchange capacity of 2.15 meq g−1 and exhibits a weaker relative humidity (RH) dependence and higher proton conductivity than the commercial Nafion 212 over the entire RH range. Remarkably, the maximum power output of the fuel cell based on the most optimal membrane reaches 1090, 856, and 451 mW cm−2 at 95%, 70%, and 30% RH, respectively, which are more than 2 times higher than those of the non-Bt analogue. Further, the current densities (I0.6) ranging up to 1500 and 1000 mA cm−2 (0.6 V) at 95% and 70% RH are both much higher than those of the Nafion. Our study provides a novel methodology for the design of aromatic ionomer structures with excellent performances for practical fuel cell application.

Humidity and temperature effects on mechanical properties and conductivity of graft-type polymer electrolyte membrane

The relative humidity (RH) and temperature dependence of the conductance and mechanical properties of poly(styrenesulfonic acid) (PSSA)-grafted poly(ethylene-co-tetrafluoroethylene) polymer electrolyte membrane (ETFE-PEM) at a grafting degree (GD) of 36%, corresponding to an ion exchange capacity (IEC) of 2.03 mmol/g with Nafion-212 as a reference, were investigated. The activation energy of ETFE-PEM was 8.2 kJ/mol, which is lower than that of Nafion-212 (13.2 kJ/mol). The tensile strength of ETFE-PEM (12 − 34 MPa) is higher than that of Nafion-212 (10 − 25 MPa) under dry, wet, or humidified conditions (at 0% and 98% RH at 80 °C). The excellent mechanical properties of ETFE-PEM under humidified conditions can be explained by partial or total compensation effects between PSSA grafts containing water and the ETFE backbone and hierarchical crystalline structures of the membrane. The pristine ETFE, grafted-ETFE, and ETFE-PEM exhibited crystallinities of 30.3%, 22.4%, and 14.6% according to X-ray diffraction results. Thus, sulfonation strongly altered the natural crystalline structures of the pristine ETFE film. Additionally, the 112 XRD reflections played an essential role in the crystalline properties, which determined the mechanical strength of the membrane. The results indicate that ETFE-PEM with an IEC of 2.03 mmol/g exhibited promising electrochemical and mechanical properties under moderately severe operating conditions for fuel cell applications.

Thermal conductivity of hygroscopic foams based on cellulose nanofibrils and a nonionic polyoxamer

Nanocellulose-based lightweight foams are promising alternatives to fossil-based insulation materials for energy-efficient buildings. The properties of cellulose-based materials are strongly influenced by moisture and there is a need to assess and better understand how the thermal conductivity of nanocellulose-based foams depends on the relative humidity and temperature. Here, we report a customized setup for measuring the thermal conductivity of hydrophilic materials under controlled temperature and relative humidity conditions. The thermal conductivity of isotropic foams based on cellulose nanofibrils and a nonionic polyoxamer, and an expanded polystyrene foam was measured over a wide range of temperatures and relative humidity. We show that a previously developed model is unable to capture the strong relative humidity dependence of the thermal conductivity of the hygroscopic, low-density nanocellulose- and nonionic polyoxamer-based foam. Analysis of the moisture uptake and moisture transport was used to develop an empirical model that takes into consideration the moisture content and the wet density of the investigated foam. The new empirical model could predict the thermal conductivity of a foam with a similar composition but almost 3 times higher density. Accurate measurements of the thermal conductivity at controlled temperature and relative humidity and availability of simple models to better predict the thermal conductivity of hygroscopic, low-density foams are necessary for the development of nanocellulose-based insulation materials.

Dry versus wet marine particle optical properties: RH dependence of depolarization ratio, backscatter, and extinction from multiwavelength lidar measurements during SALTRACE

Abstract. Triple-wavelength lidar observations of the depolarization ratio and the backscatter coefficient of marine aerosol as a function of relative humidity (RH) are presented with a 5 min time resolution. The measurements were performed at Barbados (13° N, 59° W) during the Saharan Aerosol Long-range Transport and Aerosol-Cloud interaction Experiment (SALTRACE) winter campaign in February 2014. The phase transition from spherical sea salt particles to cubic-like sea salt crystals was observed with a polarization lidar. The radiosonde and water-vapor Raman lidar observations show a drop in RH below 50 % in the marine aerosol layer simultaneously with a strong increase in particle linear depolarization ratio, which reaches values up to 0.12 ± 0.08 (at 355 nm), 0.15 ± 0.03 (at 532 nm), and 0.10 ± 0.01 (at 1064 nm). The lidar ratio (extinction-to-backscatter ratio) increased from 19 and 23 sr for spherical sea salt particles to 27 and 25 sr (at 355 and 532 nm, respectively) for cubic-like particle ensembles. Furthermore the scattering enhancement due to hygroscopic growth of the marine aerosol particles under atmospheric conditions was measured. Extinction enhancement factors from 40 to 80 % RH of 1.94 ± 0.94 at 355 nm, 3.70 ± 1.14 at 532 nm, and 5.37 ± 1.66 at 1064 nm were found. The enhanced depolarization ratios and lidar ratios were compared to modeling studies of cubic sea salt particles.