Characterisation of the adsorption sites of hydrogen on Pt/C fuel cell catalysts

A key component of a hydrogen fuel cell is a catalyst to dissociate dihydrogen to hydrogen atoms. In the present study, the adsorption of hydrogen on Pt/C fuel cell catalysts has been investigated by inelastic neutron scattering spectroscopy.

Monitoring a clean Pt(50%)/C catalyst with low energy neutron spectroscopy, after exposure to dihydrogen at 20 K, as it was heated to room temperature, showed three distinct temperature regimes: (i) a decrease in intensity from 10 to 60 K, (ii) a rise to a maximum between 60 and 120 K and then (iii) a slow fall-off towards room temperature. We assign the three regions as: (i) desorption of physisorbed dihydrogen, (ii) dissociation of dihydrogen to give an adsorbed layer and (iii) damping of the response by an increasing Debye–Waller factor.

The vibrational INS spectra of a series of Pt/C catalysts prepared under varying conditions were similar indicating that the same types of site are common to all the catalysts, although the relative proportions of each site are sample dependent. Features at 520, 950 and part of the intensity at 1300 cm⁻¹ are assigned to hydrogen on (1 1 1) faces, in good agreement with single crystal data. The mode at 640 cm⁻¹ is assigned as the doubly degenerate asymmetric stretch of Pt(1 0 0) faces with the symmetric stretch near 550 cm⁻¹.

We assign the bending mode of the on-top site to the feature at 470 cm⁻¹. The Pt–H stretch mode was observed at 2079 cm⁻¹. This is a significant result: this is the first time that hydrogen on the on-top sites has been observed on nanosized platinum particles supported on high surface area carbon black. The width of the INS peak is surprisingly large and may give additional information on the type and relative proportions of the crystallographic faces present on the catalyst particles.     

The electrochemical reduction of VO2 in acidic solution at high overpotentials

An investigation has been carried out to attempt to understand the unusually low apparent symmetry factor observed during the reduction of V(5) at higher overpotentials at carbon electrodes (typically <0.13, or >460 mV decade⁻¹). This reaction is of interest because it occurs in vanadium redox-flow batteries (VRBs) during discharge. Polarisation curves were measured using a rotating disk electrode (RDE). The reaction was not solution mass transport controlled, was pH independent (ca from 0 to 1), and the observed Tafel slope was unaffected by V(5) concentration over a range from 0.031 to 280 mM. Electrode double layer capacitance measurements were also carried out in sulphuric acid with and without vanadium. These tests showed that the presence of V(5) caused a suppression of the normal carbon surface quinone pseudocapacitance, as well as the appearance of two new pseudocapacitance peaks, one around 0.175-0.2 V and the other around 0.675-0.725 V versus SCE. The observed results do not appear consistent with a precipitated film causing diffusion limitations or causing IR drop. A model is developed to try to explain the data, which involves electron transfer through an adsorbed layer of vanadium.     

Biomic river restoration: A new focus for river management

River management based solely on physical science has proven to be unsustainable and unsuccessful, evidenced by the fact that the problems this approach intended to solve (e.g., flood hazards, water scarcity, and channel instability) have not been solved and long‐term deterioration in river environments has reduced the capacity of rivers to continue meeting the needs of society. In response, there has been a paradigm shift in management over the past few decades, towards river restoration. But the ecological, morphological, and societal benefits of river restoration have, on the whole, been disappointing. We believe that this stems from the fact that restoration overrelies on the same physical analyses and approaches, with flowing water still regarded as the universally predominant driver of channel form and structural intervention seen as essential to influencing fluvial processes. We argue that if river restoration is to reverse long‐standing declines in river functions, it is necessary to recognize the influence of biology on river forms and processes and re‐envisage what it means to restore a river. This entails shifting the focus of river restoration from designing and constructing stable channels that mimic natural forms to reconnecting streams within balanced and healthy biomes, and so levering the power of biology to influence river processes. We define this new approach as biomic river restoration.     

Consumer‐diet discrimination of δ13C and δ15N: Source‐ and feeding‐oriented patterns based on gut content analysis in a large subtropical river of China

Understanding the trophic discrimination (?¹³C and ?¹⁵N) between consumers and diets in fluvial systems remains difficult because of the variable food sources and complex predator–prey interactions from headwaters to the estuaries. Here, stable carbon (δ¹³C) and nitrogen (δ¹⁵N) isotopes in fish and invertebrates from a large subtropical river in southern China were determined to explore trophic discrimination in conjunction with a gut content analysis. The ?¹³C values showed significant differences (p < .05) among functional feeding groups, with fish, shrimp, and insect scrapers presenting higher ?¹³C values (1.20 ± 0.23‰ to 1.51 ± 0.31‰) than other groups. The ?¹⁵N values varied significantly between invertebrates (0.64 ± 0.17‰ of insect collector‐gatherers to 1.63 ± 0.36‰ of shrimp predators) and fish (1.98 ± 0.19‰ of detritivores to 2.71 ± 0.43‰ of crustaceavores) and exhibited an increasing tendency from primary to secondary consumers. A linear regression analysis revealed that the longitudinal changes in ?¹³C and ?¹⁵N were closely associated with the δ¹³C of periphyton, the δ¹⁵N of particulate organic matter (POM) in water, and the relative contribution (%) of periphyton and organic detritus to the diet composition of consumers. These results indicated that discrimination factors might not only be influenced by the isotope signatures of basal food sources but also downstream shifts in dominant food items utilized by consumers. In particular, trophic discrimination between periphyton– and detritus–based food chains, such as “epilithic diatoms–shrimp scrapers–crustaceavorous fish” and “POM–bivalves–molluscivorous fish,” displayed regionally specific patterns. When back‐calculating to the diet assimilation and trophic position in subtropical streams and rivers, we suggest using the basin‐scale ?¹³C value of 0.96 ± 0.26‰ for all consumers and ?¹⁵N values of 1.07 ± 0.32‰ for invertebrates and 2.38 ± 0.37‰ for fish.     

New clues for understanding the magnetic behavior of UPt3

We present results of a fairly complete study on which dopants M result in single-phase U_1−xM_xPt₃ samples. The resulting new samples. M = Y, Lu, Sc, Hf, Zr, have been characterized by low temperature magnetic susceptibility and specific heat for x 0.07. Based on a previously proposed model, the c/a ratios of the present alloys would be expected to result in magnetic behavior as observed in U_1−xTh_xPt₃ and U(Pt_1−xPd_x)₃. On the contrary, no magnetism is found; the Sc-doped samples actually show a suppression of spin fluctuations. A new proposal is made to explain which properties are critical for the magnetic behavior of UPt₃, as well as its doped derivatives.     

Thin‐film transistor mobility limits considerations

The mobility limiting scattering mechanisms for amorphous semiconductors and polar polycrystalline semiconductors are studied in the context of developing new high‐performance thin‐film transistor (TFT) channel layer materials for large‐area electronics. A physics‐based model for carrier transport in an amorphous semiconductor is developed to estimate the mobility limits of amorphous semiconductor TFTs. The model involves band tail state trapping of a diffusive mobility. Simulation reveals a strong dependence on the band tail density of states. This consideration makes it difficult to realize a high‐performance p‐type oxide TFT. A polar crystalline semiconductor may offer a higher mobility but is fundamentally limited by polar optical phonon scattering. Any crystalline TFT channel layer for practical large‐area applications will not be a single crystal but polycrystalline, and therefore, grain size and grain boundary‐dependent scattering will further degrade the transport properties.