A new view of electrochemistry at highly oriented pyrolytic graphite

Major new insights on electrochemical processes at graphite electrodes are reported, following extensive investigations of two of the most studied redox couples, Fe(CN)64–/3– and Ru(NH3)63+/2+. Experiments have been carried out on five different grades of highly oriented pyrolytic graphite (HOPG) that vary in step-edge height and surface coverage. Significantly, the same electrochemical characteristic is observed on all surfaces, independent of surface quality: initial cyclic voltammetry (CV) is close to reversible on freshly cleaved surfaces (>400 measurements for Fe(CN)64–/3– and >100 for Ru(NH3)63+/2+), in marked contrast to previous studies that have found very slow electron transfer (ET) kinetics, with an interpretation that ET only occurs at step edges. Significantly, high spatial resolution electrochemical imaging with scanning electrochemical cell microscopy, on the highest quality mechanically cleaved HOPG, demonstrates definitively that the pristine basal surface supports fast ET, and that ET is not confined to step edges. However, the history of the HOPG surface strongly influences the electrochemical behavior. Thus, Fe(CN)64–/3– shows markedly diminished ET kinetics with either extended exposure of the HOPG surface to the ambient environment or repeated CV measurements. In situ atomic force microscopy (AFM) reveals that the deterioration in apparent ET kinetics is coupled with the deposition of material on the HOPG electrode, while conducting-AFM highlights that, after cleaving, the local surface conductivity of HOPG deteriorates significantly with time. These observations and new insights are not only important for graphite, but have significant implications for electrochemistry at related carbon materials such as graphene and carbon nanotubes

Residential mobility of middle-class and popular sectors: the city of Buenos Aires as an arrival destination

El artículo reflexiona acerca de los patrones de movilidad residencial de individuos y hogares de sectores populares y medios que residen en dos zonas de la Ciudad de Buenos Aires configuradas en torno a patrones disimiles de segregación. La movilidad residencial es un lente privilegiado para evidenciar de qué modo la experiencia de la clase es producida y reproducida en los modos de habitar. Mediante un abordaje cualitativo y biográfico hemos analizado las especificidades que presentan las movilidades residenciales de los diferentes sectores sociales. Se identifican patrones en tres dimensiones de la movilidad residencial: su espacialidad, los arreglos residenciales que los hogares despliegan y las motivaciones que guían sus desplazamientos. Los hallazgos presentados dan cuenta de los efectos que produce la posición en la estructura social sobre la movilidad residencial, así como su interacción con la propia estructuración del espacio.The article reconstructs residential mobility patterns of individuals and households from popular and middle-class sectors residing in two areas of the city of Buenos Aires, configured around dissimilar segregation patterns. Residential mobility is a powerful lens to show how the social class experience is produced and reproduced in modes of dwelling. Through a qualitative and biographical approach, we analyzed the specificities presented by residential mobilities of different social sectors. We identified patterns in the three dimensions of residential mobility: spatiality, the residential arrangements that households employ, and the motivations that guide their movements. The findings show the effects that the position in the social structure has on residential mobility, as well as its interaction with the spatial structure itself.publishedVersionFil: Cosacov, Andrea. Universidad Nacional de Córdoba. Facultad de Ciencias Exactas, Físicas y Naturales; Argentina.Fil: Cosacov, Andrea. Consejo Nacional de Investigaciones Científicas y Técnicas. Instituto de Investigaciones Biológicas; Argentina.Fil: Di Virgilio, María Mercedes. Consejo Nacional de Investigaciones Científicas y Técnicas. Instituto de Investigación Gino Germani; Argentina.Fil: Najman, Mercedes. Universidad de Buenos Aires. Facultad de Ciencias Sociales; Argentina.Fil: Di Virgilio, María Mercedes. Universidad de Buenos Aires. Facultad de Ciencias Sociales; Argentina.Fil: Najman, Mercedes. Consejo Nacional de Investigaciones Científicas y Técnicas. Instituto de Investigación Gino Germani; Argentina

Drivers of Change or Cut-Throat Competitors? Challenging Cultures of Innovation of Chinese and Nigerian Migrant Entrepreneurs in West Africa

L'afflux remarquable des entrepreneurs migrants chinois dans différents pays d'Afrique occidentale au cours des dernières années a été heurtée à une résistance de plus en plus farouche par des entrepreneurs locaux établis. Que le premiers ont un avantage concurrentiel sur ce dernier en raison de traits socio-culturels distinctifs, ou si l'efficacité supposée chinoise est juste une caractéristique de toutes les diasporas mercantiles, est ouvert à la question. Cette étude exploratoire des migrants entrepreneuriales chinois et nigérians au Ghana et au Bénin tente de répondre à cette question. Apparemment, les forces culturels des agents du changement migrants ne sont pas limités à des systèmes de valeurs héritées ou religions, comme une éthique protestante ou le confucianisme, mais ils sont adaptés en permanence et ont inventé de nouveau par des réseaux transnationaux de la migration dans un monde globalisé. Il n'y a aucune preuve d'une prétendue supériorité de la culture d’innovation chinois par rapport aux cultures d’innovation africains des migrants entrepreneuriales. Plutôt, il existe une capacité accrue d'innovation d'une diaspora mercantile en général vis à vis des entrepreneurs locaux, indépendamment de l'origine de la culture nationale dans lequel il est intégré. En outre, la rivalité des entrepreneurs migrants chinois et nigérians dans les marchés africains ne conduit pas nécessairement à la concurrence coupe-gorge souvent suspectée sous l'impact de la mondialisation. Souvent, les deux groupes agissent plutôt complémentaires. Cela contribue, sous certaines conditions, même à la réduction de la pauvreté dans le pays d'accueil

Tree community variation in a tropical continental island according to slope aspect and human interference

ABSTRACT Associating description of unrecorded tropical tree community structure to sampling approaches that can help determine mechanisms behind floristic variation is important to further the comprehension of how plant species coexist at tropical forests. Thus, this study had the goals of (i) evaluating tree community structure on the continental island of Marambaia (23°4’37.09”S; 43°59’2.15”W) and (ii) testing the prediction that there are local scale changes in a tropical tree community structure between slopes facing different geographic orientation and with distinct human interference history. We established 60 (0.6 ha) sampling units in three different slope sites with distinct predominant geographic orientation and human interference. We sampled all woody trees with diameter at breast height (dbh) ≥ 5 cm. We found a total of 1.170 individuals representing 220 species, 120 genera and 50 families. The overall tree community structure and structural descriptors (abundance of individuals, basal area, species richness and diversity) varied extensively between the sites. The evidence presented here supports that local scale topography variations and human interference history can be important factors contributing to the known floristic heterogeneity of the Atlantic Rainforest. Future work on the study area should focus on disentangling effects from distinct causal factors over tree community variation and species occurrence

Influence of CdTe solar cell properties on stability at high temperatures

The goal of this paper is to investigate the stability of CdTe solar cells exposed to high temperature storage. In particular, we want to investigate the behavior of cells during short-term degradation test (up to 40 h). Several cells, having different thickness of copper in the contacts and of CdTe absorber were investigated. The results, obtained through combined EQE, dark and light IV, and photovoltage decay, indicate that the Cu metal contact plays a role in the degradation of the solar cells, and provide detailed insight on the related processes

Comparison between Cu(In,Ga)Se2 solar cells with different back contacts submitted to current stress

The main goal of this paper is to compare the stability over time of two different kinds of Cu(In,Ga)Se2 solar cells: the first is designed with a molybdenum back contact, the second has a semi-transparent back contact based on ITO (Indium Tin Oxide).To achieve this goal, we subjected two different samples to a forward pulse current stress during which the main electric cell parameters (i.e. Isc, Voc, PCE, IV characteristic) were monitored. Based on the collected results, we propose a physical interpretation to explain the different behaviours shown by the samples during degradation