ZnO–InN nanostructures with tailored photocatalytic properties for overall water-splitting

ZnO-based electrodes for one-step photocatalytic water splitting are designed by incorporating InN. The electronic and optical properties of (ZnO)_1−x(InN)_x alloys and ZnO with InN-like cluster formations ZnO:(InN)_x are analyzed by means of first-principles approaches. We calculate the energy gaps E_g, the band-edge energies relative to the vacuum level, and the optical absorption, employing the GW₀ method to describe single-particle excitations and the Bethe–Salpeter equation to model the two-particle exciton interactions. For ZnO and InN, the valence-band maximum (VBM) is E_VBM ≈ −7.3 and −5.7 eV, and the energy gap is E_g ≈ 3.3 and 0.7 eV, respectively. Incorporating InN into ZnO, the random (ZnO)_1−x(InN)_x alloys up-shifts the VBM and down-shifts the conduction-band minimum (CBM). In addition, the presence of InN-like clusters enhances this effect and significantly narrows the band gap. For instance, the VBM and the energy gap for 12.5% InN are E_VBM ≈ −6.5 and −6.1 eV, and E_g ≈ 2.2 and 1.9 eV for the alloy and the cluster structure, respectively. This impact on the electronic structure favors thus visible light absorption. With proper nanoclusters, the band edges straddle the redox potential levels of H⁺/H₂ and O₂/H₂O, suggesting that ZnO–InN nanostructures can enhance the photocatalytic activity for overall solar-driven water splitting.     

Microstructure and hydrogenation thermokinetics of ZrTi0.2V1.8 alloy

The microstructures and phase composition of the pseudobinary ZrTi_0.2V_1.8 alloy were examined by scan electron microscope (SEM) and X-ray diffraction (XRD). Before hydrogenation, the hypoeutectic structure accompanied with ZrV₂ + (ZrV₂ + Zr) spherical-like texture has been observed in ZrTi_0.2V_1.8 and the dominant phase could be ascribed to the C15 Laves phase. Hydrogen absorption pressure–composition isotherms (P–C isotherms) and hydriding kinetics of ZrTi_0.2V_1.8 were investigated by pressure reduction method using Sievert apparatus from 673 to 823 K. At hydrogen concentration 0.65 (H/A), the relative partial molar enthalpy and entropy calculated by Van’t Hoff equation are −60 ± 1 kJ mol⁻¹ and −119 ± 1 J mol⁻¹ K⁻¹, respectively. In addition, two stages in the hydrogen absorption reaction between 673 and 823 K could be attributed to the different hydrogen absorption mechanisms including redistribution of the hydrogen atoms in the hydride phase and the diffusion of hydrogen in the β-phase. The activation energy E_a of the alloy is ∼3.6 kJ mol⁻¹ for the first absorption stage and ∼61.9 kJ mol⁻¹ for the second one.     

Photocatalytic hydrogen production from aqueous methanol solutions under visible light over Na(BixTa1−x)O3 solid-solution

A series of equivalent substitution solid-solution Na(Bi_xTa_1−x)O₃ (x = 0–0.10) was prepared by a simple hydrothermal method using Ta₂O₅ and NaBiO₃ as precursors. The Na(Bi_0.08Ta_0.92)O₃ photocatalyst exhibited the highest performance of H₂ evolution (59.48 μmol h⁻¹ g⁻¹) under visible-light irradiation (λ > 400 nm) without co-catalyst, whereas no H₂ evolution is observed for NaTaO₃ under the same conditions. The UV-Vis spectra indicate that the Na(Bi_0.08Ta_0.92)O₃ powders can absorb not only ultraviolet light like pure NaTaO₃ powder but also the visible-light spectrum. The absorption edge corresponds to a band gap of 2.88 eV. The results of density functional theory calculation illuminate that the visible-light absorption bands in the Na(Bi_xTa_1−x)O₃ catalysts are attributed to the band transition from the O2p to the Bi2s + 2p + Ta5d hybrid orbital.     

An experimental investigation of electro-osmotic drag coefficients in a polymer electrolyte membrane fuel cell

Through the use of a water balance experiment, the electro-osmotic drag coefficients of Nafion 115 were obtained under several conditions (as a function of water content and thermodynamics conditions). For the cases when the anode was fully hydrated (corresponding to water content λ ≈ 14 in the adjacent membrane) and the cathode suffered from drying when dry air was supplied (λ ≈ 2), the electro-osmotic drag coefficients varied from 0.82 (±0.06) to 0.50 (±0.03) H₂O/H⁺ when the current density varied from 0.4 to 1.0 A cm⁻² (95% confidence level). When the current density increased, the electro-osmotic drag coefficient decreased. When the water content at the anode increased from λ ≈ 5 to λ ≈ 14, the cathode was supplied with dry air (λ ≈ 2), and the fuel cell discharged constant current density at 0.6 A cm⁻², the electro-osmotic drag coefficient increased from 0.44 (±0.06) to 0.68 (±0.06) H₂O/H⁺ (95% confidence level). Higher relative humidity gas leads to a higher electro-osmotic drag coefficient at constant current density.     

Doping levels, trap density of states and the performance of co-doped CdTe(As,Cl) photovoltaic devices

Doping, compensation and photovoltaic performance have been investigated in all-metal-organic vapour-phase deposition (MOCVD) grown CdTe/CdS solar cells that were co-doped with arsenic and chlorine.Although arsenic chemical concentration is in the range of 10¹⁷-1.5×10¹⁹ cm⁻³, the maximum net acceptor concentration is only in the order of 10¹⁴ cm⁻³, as determined by capacitance-voltage characteristics. Admittance spectroscopy revealed shallow traps at 0.055 eV which were attributed to As_Te; its compensation by Cd_i is discussed. Formation of the alloy CdS_xTe_1−x is linked to deep levels at E_V+∼0.55 eV and E_V+∼0.65 eV. Limits to the diffusion of photo-generated carriers were considered to be important in determining photovoltaic performance rather than carrier lifetime. Prospects for optimizing the performance of such co-doped MOCVD-grown devices are discussed.     

Metabolic shift and electron discharge pattern of anaerobic consortia as a function of pretreatment method applied during fermentative hydrogen production

We have made an attempt to evaluate the variation in the electron discharge (ED) pattern of anaerobic consortia as a function of pretreatment viz., chemical, heat-shock, acid and oxygen-shock in comparison with untreated mixed consortia during fermentative hydrogen (H₂) production. Experiments were performed with dairy wastewater as substrate using anaerobic mixed consortia as biocatalyst (pretreated individually and in combination). Cyclic voltammetry (CV) elucidated significant variation in the ED pattern of mixed consortia along with H₂ production and substrate degradation (SD) as a function of pretreatment method applied. Higher ED was observed with all pretreated consortia which can be attributed to the stable proton (H⁺) shuttling due to the suppression of methanogenic activity. Oxygen-shock method and untreated consortia showed lower H₂ production and higher SD among the variations studied, while, combined pretreated consortia resulted higher H₂ production and lower SD. Lower ED observed with untreated consortia suggests the H⁺ reduction during methanogenesis rather than the inter-conversion of metabolites, which is presumed to be necessary for H₂ production. ED observed with combined pretreated consortia corroborated well with the observed H₂ production. Redox pairs were visualized on the voltammograms with almost all the experimental variations studied except untreated consortia. The potentials (E₀) of redox pairs observed were corresponding to intracellular electron carriers viz., NAD⁺/NADH (E₀ −0.32 V) and FAD⁺/FADH₂ (E₀ −0.24 V).     

Synthesis and characterization of 2,1,3-benzothiadiazole-thieno[3,2-b]thiophene-based charge transferred-type polymers for photovoltaic application

New low-band-gap copolymers, including thieno[3,2-b]thiophene and 2,1,3-benzothiadiazole, were synthesized as photovoltaic materials. Thiophene was introduced to provide extended π-conjugation length and charge transfer properties. A band gap (E_g^op=1.62 eV, E_g^ec=1.51 eV) of this polymer was investigated through UV-vis spectroscopy and cyclic voltammetry. A bulk heterojunction structure of glass/indium tin oxide (ITO)/PEDOT:PSS/polymer-PCBM(1:3)/LiF/Al was fabricated for investigating photovoltaic properties. PC₇₁BM was used as an acceptor material, due to its increased absorption in the visible region, in comparison with PC₆₁BM. In this polymer, incident photon-to-current conversion efficiency (IPCE) was as high as 50%. Moreover, maximum power conversion efficiency (PCE) of up to 1.72% was achieved under AM 1.5 G conditions. It demonstrated relatively high V_OC (0.67 V) and J_SC (6.86 mA/cm²), while a low fill factor value (0.37) was obtained.     

Hydrogen storage properties of Ti1.4V0.6Ni + x Mg (x = 1–3, wt.%) alloys

We prepared Ti_1.4V_0.6Ni ribbons by arc-melting and subsequent melt-spinning techniques. Ti_1.4V_0.6Ni + x Mg (x = 1, 1.5, 2, 2.5 and 3, wt.%) composite alloys were obtained by the mechanical ball-milling method. The structures and hydrogen storage properties of alloys were investigated. Ti_1.4V_0.6Ni + x Mg composite alloys contained icosahedral quasicrystalline phase, Ti₂Ni-type phase, β-Ti solid-solution phase and metallic Mg. The electrochemical and gaseous hydrogen storage properties of alloys were improved with Mg addition. Ti_1.4V_0.6Ni + 2 Mg alloy showed maximum electrochemical discharge capacity of 282.5 mAh g⁻¹ as well as copacetic high-rate discharge ability of 82.3% at the discharge current density of 240 mA g⁻¹ compared with that of 30 mA g⁻¹, and the cycling life achieved above 200 mAh g⁻¹ after 50 consecutive cycles of charging and discharging. The hydrogen absorption/desorption properties of Ti_1.4V_0.6Ni + x Mg (x = 1, 2 and 3, wt.%) alloys were better than Ti_1.4V_0.6Ni. Ti_1.4V_0.6Ni + 3 Mg alloy also exhibited a favorable hydrogen absorption capacity of 1.53 wt.%. The improvement in the hydrogen storage characteristics caused by adding Mg may be ascribed to better hydrogen diffusion and anti-corrosion ability.     

Well-defined core/shell pDA@Ni-MOF heterostructures with photostable polydopamine as electron-transfer-template for efficient photoelectrochemical H2 evolution

Here, novel core/shell polydopamine@Ni-MOF (pDA@Ni-MOF) heterogeneous nanostructures are synthesized via a simple one-pot nucleation-growth technique. This rational core/shell design method provide a uniform Ni-MOF shell thickness (shell: ～ 10 nm) as well as homogeneous wrapping of pDA templates with quite narrow size distributions. The obtained band properties of bare pDA (E_CB = ?0.35 eV and E_VB = 2.95 eV vs normal hydrogen electrode (NHE)) and bare Ni-MOF (E_CB = ?0.49 eV and E_VB = 2.85 eV vs NHE) clearly revealed charge separation is occurred on pDA by absorbing light due to π-π1 transition, and photogenerated electrons on conduction band (CB) of pDA was migrated to CB of Ni-MOF. Specifically, the photoelectrochemical (PEC) water performance of pDA@Ni-MOF photoanodes with highest current density is recorded as 8.61 mA/cm² at 0.77 V vs. RHE under visible LED irradiation, which is significantly higher than bare pDA (0.008 V vs. RHE) and bare Ni-MOF (0.011 V vs. RHE) at the same conditions. Note that, the higher photon absorption properties of pDA in core together with high interaction valence bond between two semiconductors could generate electron rich state giving rise to faster electron transfer kinetics as next generation of MOF based hybrid materials with regular morphologies.     

Gallium (III) sulfide as an active material in lithium secondary batteries

In an attempt to identify an active material for use in lithium secondary batteries with high energy density, we investigated the electrochemical properties of gallium (III) sulfide (Ga₂S₃) at 30 °C. Ga₂S₃ shows two sloping plateaus in the potential range between 0.01 V and 2.0 V vs. (Li/Li⁺). The specific capacity of the Ga₂S₃ electrode in the first delithiation is ca. 920 mAh g⁻¹, which corresponds to 81% of the theoretical capacity (assuming a 10-electron reaction). The capacity in the 10th cycle is 63% of the initial capacity. Ex situ X-ray diffraction and X-ray absorption fine structure analyses revealed that the reaction of the Ga₂S₃ electrode proceeds in two steps: Ga₂S₃ + 6Li⁺ + 6e⁻ ? 2Ga + 3Li₂S and Ga + xLi⁺ + xe⁻ ? Li_xGa.     

In situ quantification of the in-plane water content in the Nafion membrane of an operating polymer-electrolyte membrane fuel cell using H micro-magnetic resonance imaging experiments

Spatial, quantitative, and temporal information regarding the water content distribution in the transverse-plane between the catalyst layers of an operating polymer-electrolyte membrane fuel cell (PEMFC) is essential to develop a fundamental understanding of water dynamics in these systems. We report ¹H micro-magnetic resonance imaging (MRI) experiments that measure the number of water molecules per SO₃H group, λ, within a Nafion^®-117 membrane between the catalyst stamps of a membrane-electrode assembly, MEA. The measurements were made both ex situ, and inside a PEMFC operating on hydrogen and oxygen. The observed ¹H MRI T₂ relaxation time of water in the PEM was measured for several known values of λ. The signal intensity of the images was then corrected for T₂ weighting to yield proton density-weighted images, thereby establishing a calibration curve that correlates the ¹H MRI density-weighted signal with λ. Subsequently, the calibration curve was used with proton density weighted (i.e., T₂-corrected) signal intensities of transverse-plane ¹H MRI images of water in the PEM between the catalyst stamps of an operating PEMFC to determine λ under various operational conditions. For example, the steady state, transverse-plane λ was 9 ± 1 for a PEMFC operating at ∼26.4 mW cm⁻² (∼20.0 mA, ∼0.661 V, 20 °C, flow rates of the dry H₂(g) and O₂(g) were 5.0 and 2.5 mL min⁻¹, respectively).     

Nanocarbon boosts energy-efficient hydrogen production in carbon-assisted water electrolysis

To improve upon our previously reported slow hydrogen evolution rate R_H at the energy-efficient lower voltages in CAWE (carbon-assisted water electrolysis) at room temperature, new results using different carbons and catalysts to improve R_H are reported here. Compared to earlier results with carbon GX203, about a ten-fold increase in R_H is reported using high surface area carbon BP2000 at the operating voltage E^o = 1.12 V. With added FeSO₄ catalyst, E^o is lowered to 0.72 V without lowering R_H, representing about 30% decrease in the energy barrier of the process. For comparison, in water electrolysis without carbon, measurable R_H is observed only for E^o ≥ 2 V. This large improvement in R_H at the energy efficient E^o = 0.72 V is suggested to result from nanoscale particle size of carbon BP2000 as well as from electrons provided by the catalyst through the reaction Fe²⁺ ? Fe³⁺ + e⁻. By measuring the amounts of H₂ evolved at the cathode and CO₂ evolved at the anode using gas chromatography, the mechanism for CAWE is established to be the reaction: C (s) + 2H₂O (?) → CO₂ (g) + 2H₂ (g). The reaction slows down with time as carbon is depleted by oxidation.     

Electron transfer reaction between Au25 nanocluster and phenothiazine-tetrachloro-p-benzoquinone complex

Single-electron transfer (SET) between donor-acceptor complex PTZ–TCBQ (phenothiazine：tetrachloro-p-benzoquinone = 1/1) and thiol-stabilized gold nanoclusters Au₂₅(SC₂H₄Ph)₁₈⁻TOA⁺ (abbreviated as Au₂₅⁻) is firstly researched here. Cyclic voltammetry (CV), UV–vis spectroscopy, electron spin resonance (ESR) and nuclear magnetic resonance (NMR) are utilized to study their electron transfer reaction. The characteristic results show only one-electron transfer from Au₂₅⁻ to the complex, and the excess amount of PTZ–TCBQ complex cannot oxidate Au₂₅⁰ further as ¹HNMR observed. So those results indicate that this well defined gold nanoclusters with a molecular-like redox behavior as an electron donor can be applied as a solar photocatalytic nanomaterial, SET catalyst and so on.     

Analysis of the carbon anode in carbon conversion cells

We examine further the electrochemical oxidation of carbon in molten carbonate, based on analysis of published research. Ascending and descending branches of voltage hysteresis found in current sweeps of atomically-ordered graphite and of disordered carbon (coal char) are separated by about 0.20–0.25 V and by 0.10–0.15 V for ordered and disordered forms, respectively, over a wide band of current density, 0.03–0.10 A/cm². The higher voltage of the descending branch is in rough agreement with prediction of the Y. Li model for the carbon/carbonate electrode in the same current range, for ordered graphite (L_a = 70–100 nm) and for disordered structures (L_a = 3–5 nm), respectively. We suggest that the amplitude of the hysteresis represents the difference between the overvoltage requirements for 2- and 4 electron net transfer processes, respectively. The 2 e− reaction (C + CO₃²⁻ = CO + CO₂ + 2e⁻) dominates the low current segment (LCS) of our previous analysis, and the more hindered 4e− transfer reaction (C + 2CO₃²⁻ = 3CO₂ + 4e⁻) dominates the high current segment (HCS). The voltage increase separating LCS from HCS is effected by accumulation of CO₂ within small, melt-filled pores to form highly supersaturated solutions of CO₂, which enhance anode voltage by a concentration overpotential of 0.10–0.25 V. Overpotential increases with reaction extent until (1) overall polarization inhibits the interior reaction and shifts CO₂ production to the more accessible exterior surface, or, (2) at a critical concentration (dependent on surface tension and pore diameter) bubbles nucleate and block current flow in the pores. Further support for this picture comes from the often-reported deviation of the gas composition from the CO/CO₂ ratio of the Boudouard equilibrium at atmospheric pressure, as open circuit conditions are approached in an electrochemical cell. Our interpretation accounts for the mole fraction of CO₂ at open circuit being greater than predicted from the Boudouard equilibrium.     

Hydrogen storage on uncharged and positively charged Mg-decorated graphene

The storage of H₂ molecules was studied theoretically on charged and uncharged Mg decorated graphene surfaces using density-functional theory and by incorporating the van der Waals (vdW) interactions. We found that an increase in the number of Mg atoms and H₂ molecule increases the net interaction of the hydrogen molecule with the surface. The Mg-Gr⁺ has the hydrogen storage capacity of up to nine H₂ molecules, with the average adsorption energy of −0.134 eV/H₂. Also, we found that hydrogen molecules play an important role in the interaction between the graphene surface and the Mg atom. The charge density difference analysis showed that electron transfer occurs from H₂ molecules to Mg atom in uncharged system. However, the Bader charge analysis showed that the positive charges in the Mg-Gr⁺ and nH₂-Mg-Gr⁺ systems are concentrated on the Mg atom. When the number of H₂ molecules reaches more than 4, the charge transfer instead occurs from the Mg atom to H₂ molecules as well as to the graphene surface. This results in better interaction between the Mg atom and the Gr⁺ surface.     

2LiH + M (M = Mg, Ti): New concept of negative electrode for rechargeable lithium-ion batteries

xLiH + M composites, where M = Mg or Ti, are suggested as new candidates for negative electrode for Li-ion batteries. For this purpose, the xLiH + M electrode is prepared using the mechanochemical reaction: MH_x + xLi → xLiH + M or by simply grinding a xLiH + M mixture. The most promising electrochemical behaviour is obtained with the (2LiH + Mg) composite prepared via a mechanochemical reaction between MgH₂ and metallic Li leading to a very divided composite in which Mg crystallites of 20 nm size are embedded in a LiH matrix. Reversible capacities of 1064 mAh g⁻¹ (three times as much as the one of graphite) and 600 mAh g⁻¹ are reached for these phase mixtures after 1 and 28 h of grinding in vertical and planetary mill, respectively. The (2LiH + Ti) mixture prepared via the mechanochemical reaction between TiH₂ and Li exhibits a reversible capacity of 428 mAh g⁻¹. From X-ray diffraction measurements, the performances of the electrodes are attributed to the electrochemical conversion reaction: M + xLiH ↔ MH_x + xLi⁺ + xe⁻ (M = Mg, Ti) followed for M = Mg by an alloying process where M reacts with lithium ions to form Mg_1−xLi_x alloys.     

Hydrogen trapped at intermetallic particles in aluminum alloy 6061-T6 exposed to high-pressure hydrogen gas and the reason for high resistance against hydrogen embrittlement

Hydrogen (¹H) trapped at intermetallic particles (IPs) in an aluminum alloy, 6061-T6, was visualized with secondary ion mass spectrometry (SIMS) by precisely excluding the false signal which is caused by background hydrogen (H_BG). The interference of the H_BG was avoided by a unique continuous pre-sputtering (pre-digging) by a primary ion beam of SIMS into a sample in combination with silicon sputtering prior to the SIMS measurement of the sample and we succeeded in visualizing the exact signal of ¹H trapped by IPs at subsurface layer of the sample charged in high-pressure hydrogen gas. The thermal desorption analysis clarified that the desorption energy (E_d) of the IPs was 200 kJ/mol or higher, which was extremely higher than E_d for lattice interstice, dislocations, and vacancies. High density hydrogen was concentratedly trapped at IPs in the subsurface layer in contact with the hydrogen gas. This nature causes an extremely low effective hydrogen diffusivity of 6061-T6 of the order of 10^?14 m²/s even at 200 °C and may eventually give a high HE resistance to 6061-T6.     

Properties of the lithium and graphite-lithium anodes in N-methyl-N-propylpyrrolidinium bis(trifluoromethanesulfonyl)imide

The ternary [Li⁺]_0.09[MePrPyr⁺]_0.41[NTf₂⁻]_0.50 room temperature ionic liquid was obtained by dissolution of solid lithium bis(trifluoromethanesulfonyl)imide (LiNTf₂) in liquid N-methyl-N-propylpyrrolidinium bis(trifluoromethanesulfonyl)imide ([MePrPyr⁺][NTf₂⁻]), and studied as an electrolyte for lithium-ion batteries. The graphite-lithium (C₆Li) anode, working together with vinylene carbonate as an additive showed ca. 90% of its initial discharge capacity after 50 cycles. The addition of vinylene carbonate to the neat ionic liquid results in the formation of the protective coating (SEI) on both the lithium and graphite anodes. The SEI formation increases the rate of the charge transfer reaction as well as protects the anode from chemical passivation (corrosion). The graphite-lithium (C₆Li) anode shows good cyclability and Coulombic efficiency in the presence of 10 wt.% of vinylene carbonate as an additive to the ionic liquid.     

Lithium electrolytes based on modified imidazolium ionic liquids

In this study, new electrolytes for Li-ion batteries in the form of lithium salt solutions in room temperature imidazolium ionic liquids (RTIL) are reported. The ionic liquids applied, for higher reduction potential stability, were substituted at position C2 with oligooxyethylene groups of various length ([Im nEO]⁺X⁻; where: n = 0, 3, 7, 20 and X = Cl, BF₄, N(CF₃SO₂)₂). It was found that they are good solvents for lithium salts (LiBF₄, LiN(CF₃SO₂)₂, {[CH₃(OCH₂CH₂)₃O]₃BC₄H₉}Li) forming liquid solutions of low glass transition temperature (T_g in the −70 to −40 °C range). Ionic conductivity depends on the length of oxyethylene substituent in Im nEO and on the concentration of the salt applied, for 10 mol%, σ_RT is of the order of 10⁻⁴ S cm⁻¹. On the basis of polarization measurements by the variable-current method, the proportion of lithium cations in electric charge transfer (t₊) has been determined. The values obtained (typical for ionic liquids) are low and depend on n and lithium salt concentration but do not exceed a dozen or so percent.     

Degradation in phosphoric acid doped polymer fuel cells: A 6000 h parametric investigation

This paper reports an experimental study of the degradation of single PBI-based high temperature MEAs doped with phosphoric acid. The study is carried out by operating the single MEAs for long periods in steady state, the degradation is quantified considering the voltage decay rate. Besides the most common operating condition suggested by the MEAs producer (T = 160 °C, i = 0.2 A cm⁻², λH₂=1.2${\lambda }_{{\text{H}}_2}=1.2$, λ_air = 2), the study also investigates higher operating temperature (T = 180 °C), higher current density (i = 0.4 A cm⁻²) and double air flow rate (λ_air = 4). A temperature of 180 °C accelerates the degradation of the MEA which increases from around 8 μV h⁻¹ up to around 19 μV h⁻¹. On the opposite side, operating the MEA at i = 0.4 A cm⁻² reduces the voltage degradation rate down to 4 μV h⁻¹ and increases the power output making this condition particularly interesting. EIS, CV and LSV are used to clarify the causes of degradation. A consistent increase in the charge transfer resistance is observed and is related to the loss of catalyst active area due to catalyst agglomeration, carbon corrosion and possible acid leaching. Concerning the electrolyte membrane, a slight decrease in the proton conductivity is measured, a major effect on degradation is played by the increasing gas crossover rate and by the short circuit current.