Investigation of the non-Arrhenius behavior of fast ion conducting glasses

Traditional glassy ion conductors exhibit Arrhenius temperature dependence of the d.c. conductivity. Recently, Kinc and Martin1 reported the discovery of a Fast Ion Conducting (FIC) glass with ionic conductivities as high as 10-2 (O-cm)-1. Surprisingly, while this is a very high conductivity for a glassy material, it is still several orders of magnitude lower than that predicted by the low temperature Arrhenius behavior. While Kinc and Martin did a through investigation of these materials at low temperatures, they did not explore the room temperature and above behavior. They proposed a simple model to explain their observed non-Arrhenius ionic conductivity, but the full study of the behavior was not been made.;Several researchers have since attempted to explain the cause of the behavior observed by Kinc and Martin; however, no conclusive evidence has been given for the true origin of this behavior. Most of the models have been purely mathematical fits, with no basis in the physical world. Other researchers simply write off the observed behavior as a fluke of crystallized or phase separated samples. The present investigation looks at the high temperature behavior of the same glass compositions that Kinc and Martin looked at to determine if there is ionic conductivity saturation or perhaps even an ionic conductivity maximum. This work goes on to develop a theory that explains the observed results in a physical manner that is based on current knowledge ionic conductors and glass structure.;1Kincs, J., Martin, S. W., Phys. Rev. Let., 76, 70--7

Investigation of the non-Arrhenius behavior of fast ion conducting glasses

Traditional glassy ion conductors exhibit Arrhenius temperature dependence of the d.c. conductivity. Recently, Kinc and Martin1 reported the discovery of a Fast Ion Conducting (FIC) glass with ionic conductivities as high as 10-2 (O-cm)-1. Surprisingly, while this is a very high conductivity for a glassy material, it is still several orders of magnitude lower than that predicted by the low temperature Arrhenius behavior. While Kinc and Martin did a through investigation of these materials at low temperatures, they did not explore the room temperature and above behavior. They proposed a simple model to explain their observed non-Arrhenius ionic conductivity, but the full study of the behavior was not been made.;Several researchers have since attempted to explain the cause of the behavior observed by Kinc and Martin; however, no conclusive evidence has been given for the true origin of this behavior. Most of the models have been purely mathematical fits, with no basis in the physical world. Other researchers simply write off the observed behavior as a fluke of crystallized or phase separated samples. The present investigation looks at the high temperature behavior of the same glass compositions that Kinc and Martin looked at to determine if there is ionic conductivity saturation or perhaps even an ionic conductivity maximum. This work goes on to develop a theory that explains the observed results in a physical manner that is based on current knowledge ionic conductors and glass structure.;1Kincs, J., Martin, S. W., Phys. Rev. Let., 76, 70--73</p

PEM fuel cell cathode contamination in the presence of cobalt ion (Co2+)

This paper reports the effects of Co\ub2\u207a contamination on PEM fuel cell performance as a function of Co\ub2\u207a concentration and operating temperature. A significant drop in fuel cell voltage occurred when Co\ub2\u207a was injected into the cathode air stream, and Co\ub2\u207a contamination became more severe with decreasing temperature. To investigate in detail the mechanism of Co\ub2\u207a poisoning, AC impedance was monitored before and during Co\ub2\u207a injection, revealing that both charge transfer and mass transport related processes deteriorated significantly in the presence of Co\ub2\u207a, whereas membrane conductivity decreased to a lesser extent. Surface cyclic voltammetry and contact angle measurements further revealed changes in physical properties, such as active Pt surface area and hydrophilicity, furthering our understanding of the contamination process.Cet article signale les effets de la contamination par le Co2+ sur le rendement de piles MEP en fonction de la concentration et de la temp\ue9rature du Co2+. Une baisse importante de la tension d\u2019une pile \ue0 combustible s\u2019est produite lorsque du Co2+ a \ue9t\ue9 inject\ue9 dans le flux d\u2019air de la cathode et la contamination par le Co2+ s\u2019est aggrav\ue9e lorsqu\u2019on a abaiss\ue9 la temp\ue9rature. Afin d\u2019\ue9tudier en d\ue9tail le m\ue9canisme d\u2019empoisonnement au Co2+, on a suivi l\u2019imp\ue9dance c.a. avant et pendant l\u2019injection de Co2+, ce qui a r\ue9v\ue9l\ue9 que les m\ue9canismes li\ue9s au transfert des charges et au transport de masse se d\ue9t\ue9rioraient consid\ue9rablement en pr\ue9sence de Co2+, alors que la conductivit\ue9 de la membrane subissait une moindre diminution. Des mesures de voltamp\ue9rom\ue9trie cyclique de surface et des angles de contact ont en outre r\ue9v\ue9l\ue9 des modifications des propri\ue9t\ue9s physiques, comme la taille de la surface active du Pt et le caract\ue8re hydrophile, ce qui am\ue9liore notre connaissance du processus de contamination.Peer reviewed: YesNRC publication: Ye

Effect of Co2+ on oxygen reduction reaction catalyzed by Pt catalyst, and its implications for fuel cell contamination

The oxygen reduction reaction (ORR) catalyzed by Pt was studied in the presence of Co2+ using cyclic voltammetry (CV), rotating disk electrode (RDE), and rotating ring-disk electrode (RRDE) techniques in an effort to understand fuel cell cathode contamination caused by Co2+. Findings indicated that Co2+ could weakly adsorb on the Pt surface, resulting in a slight change in ORR exchange current densities. However, this weak adsorption had no significant effect on the nature of the ORR rate determining steps. The results from both RDE and RRDE indicated that the overall electron transfer number of the ORR in the presence of Co2+ was reduced, with ~9% more H2O2 being produced. We speculate that the weakly adsorbed Co2+ on Pt could react with the H2O2 intermediate and form a Co2+\u2013H2O2 intermediate, inhibiting the further reduction of H2O2 and thus resulting in more H2O2 production. The fuel cell performance drop observed in the presence of Co2+ could be attributed to the reduction in overall electron transfer number and the increase in H2O2 production. Higher production could intensify the attack by H2O2 and its radicals on membrane electrode assembly components, including the ionomer, carbon support, Pt particles, and membrane, leading to fuel cell degradation.Peer reviewed: YesNRC publication: Ye

Durability of PEM fuel cell cathode in the presence of Fe3+ and Al3+

The contamination effects of Fe\ub3\u207a and Al\ub3\u207a on the performance of polymer electrolyte membrane fuel cells were investigated by continuously injecting Fe\ub3\u207a or Al\ub3\u207a salt solution into the air stream of an operating fuel cell. Both metal ions individually caused significant cell performance degradation at a level of only 5ppmmol in air. In addition, elevated temperature accelerated fuel cell performance degradation in the presence of Fe\ub3\u207a. Moreover, the presence of Fe\ub3\u207a in an operating fuel cell resulted in the cell\u2019s sudden death, due to the formation of membrane pinholes that may have been promoted by the enhanced production of peroxy radicals catalyzed by Fe species. Half-cell tests in liquid electrolyte revealed that the presence of Al\ub3\u207a in the electrolyte changed the kinetics and mechanisms of the oxygen reduction reaction by reducing the kinetic current densities and the electron transfer number.Peer reviewed: YesNRC publication: Ye

Cerebral microbleeds and stroke risk after ischaemic stroke or transient ischaemic attack:a pooled analysis of individual patient data from cohort studies

BACKGROUND Cerebral microbleeds are a neuroimaging biomarker of stroke risk. A crucial clinical question is whether cerebral microbleeds indicate patients with recent ischaemic stroke or transient ischaemic attack in whom the rate of future intracranial haemorrhage is likely to exceed that of recurrent ischaemic stroke when treated with antithrombotic drugs. We therefore aimed to establish whether a large burden of cerebral microbleeds or particular anatomical patterns of cerebral microbleeds can identify ischaemic stroke or transient ischaemic attack patients at higher absolute risk of intracranial haemorrhage than ischaemic stroke. METHODS We did a pooled analysis of individual patient data from cohort studies in adults with recent ischaemic stroke or transient ischaemic attack. Cohorts were eligible for inclusion if they prospectively recruited adult participants with ischaemic stroke or transient ischaemic attack; included at least 50 participants; collected data on stroke events over at least 3 months follow-up; used an appropriate MRI sequence that is sensitive to magnetic susceptibility; and documented the number and anatomical distribution of cerebral microbleeds reliably using consensus criteria and validated scales. Our prespecified primary outcomes were a composite of any symptomatic intracranial haemorrhage or ischaemic stroke, symptomatic intracranial haemorrhage, and symptomatic ischaemic stroke. We registered this study with the PROSPERO international prospective register of systematic reviews, number CRD42016036602. FINDINGS Between Jan 1, 1996, and Dec 1, 2018, we identified 344 studies. After exclusions for ineligibility or declined requests for inclusion, 20 322 patients from 38 cohorts (over 35 225 patient-years of follow-up; median 1·34 years [IQR 0·19-2·44]) were included in our analyses. The adjusted hazard ratio [aHR] comparing patients with cerebral microbleeds to those without was 1·35 (95% CI 1·20-1·50) for the composite outcome of intracranial haemorrhage and ischaemic stroke; 2·45 (1·82-3·29) for intracranial haemorrhage and 1·23 (1·08-1·40) for ischaemic stroke. The aHR increased with increasing cerebral microbleed burden for intracranial haemorrhage but this effect was less marked for ischaemic stroke (for five or more cerebral microbleeds, aHR 4·55 [95% CI 3·08-6·72] for intracranial haemorrhage vs 1·47 [1·19-1·80] for ischaemic stroke; for ten or more cerebral microbleeds, aHR 5·52 [3·36-9·05] vs 1·43 [1·07-1·91]; and for ≥20 cerebral microbleeds, aHR 8·61 [4·69-15·81] vs 1·86 [1·23-1·82]). However, irrespective of cerebral microbleed anatomical distribution or burden, the rate of ischaemic stroke exceeded that of intracranial haemorrhage (for ten or more cerebral microbleeds, 64 ischaemic strokes [95% CI 48-84] per 1000 patient-years vs 27 intracranial haemorrhages [17-41] per 1000 patient-years; and for ≥20 cerebral microbleeds, 73 ischaemic strokes [46-108] per 1000 patient-years vs 39 intracranial haemorrhages [21-67] per 1000 patient-years). INTERPRETATION In patients with recent ischaemic stroke or transient ischaemic attack, cerebral microbleeds are associated with a greater relative hazard (aHR) for subsequent intracranial haemorrhage than for ischaemic stroke, but the absolute risk of ischaemic stroke is higher than that of intracranial haemorrhage, regardless of cerebral microbleed presence, antomical distribution, or burden. FUNDING British Heart Foundation and UK Stroke Association