Role of Reaction Intermediate Diffusion on the Performance of Platinum Electrodes in Solid Acid Fuel Cells

Understanding the reaction pathways for the hydrogen oxidation reaction (HOR) and the oxygen reduction reaction (ORR) is the key to design electrodes for solid acid fuel cells (SAFCs). In general, electrochemical reactions of a fuel cell are considered to occur at the triple-phase boundary where an electrocatalyst, electrolyte and gas phase are in contact. In this concept, diffusion processes of reaction intermediates from the catalyst to the electrolyte remain unconsidered. Here, we unravel the reaction pathways for open-structured Pt electrodes with various electrode thicknesses from 15 to 240 nm. These electrodes are characterized by a triple-phase boundary length and a thickness-depending double-phase boundary area. We reveal that the double-phase boundary is the active catalytic interface for the HOR. For Pt layers ≤ 60 nm, the HOR rate is rate-limited by the processes at the gas/catalyst and/or the catalyst/electrolyte interface while the hydrogen surface diffusion step is fast. For thicker layers (>60 nm), the diffusion of reaction intermediates on the surface of Pt becomes the limiting process. For the ORR, the predominant reaction pathway is via the triple-phase boundary. The double-phase boundary contributes additionally with a diffusion length of a few nanometers. Based on our results, we propose that the molecular reaction mechanism at the electrode interfaces based upon the triple-phase boundary concept may need to be extended to an effective area near the triple-phase boundary length to include all catalytically relevant diffusion processes of the reaction intermediates.

The effect of melt water content and isothermal annealing time on the formation and evolution of clinopyroxene-titanomagnetite clusters

<p>Crystal clustering influences the formation of crystal mushes and the rheology and differentiation of magmas. Heterogeneous nucleation is known to be an important cluster-forming mechanism, but there has been little systematic experimental study of cluster formation and evolution.</p><p>In this study, we analysed dynamic crystallization experiments from Pontesilli et al. (2019), focusing on clusters of clinopyroxene (cpx) and titanomagnetite (tmt). These experiments aimed to reproduce the crystallisation behaviour of dry (nominally 0 wt.% H<sub>2</sub>O) and hydrous (2 wt.% H<sub>2</sub>O added) Etnean trachybasalt at mid-crustal storage conditions (400 MPa, 1100°C, NNO+1 oxygen buffer, corresponding to undercooling of 120°C and 80°C respectively). After superheating at 1300°C for 30 minutes, samples were cooled at 80°C/min to 1100°C and annealed for dwell times ranging from 0.5h to 8h.</p><p>Electron backscatter diffraction (EBSD) maps and image analysis were used to quantify clustering parameters such as tmt number density, “shared perimeter fraction” (“SPF”, the fraction of total tmt boundary length shared between cpx and tmt), “fraction of touching tmt” (“FTT”, the fraction of all tmt grains that are touching cpx), and the crystallographic orientation relationships (CORs) between cpx and tmt. Dry samples generally show a higher number density of tmt crystals than wet samples. SPF and FTT are highest (≥ 0.40 and ≥ 0.93 respectively) in the 0.5h duration dry experiments. Both parameters fall to ≤ 0.25 and ≤ 0.75 respectively after 4h of annealing. In wet experiments, SPF and FFT are lower (≤ 0.33 and ≤ 0.79 respectively) at 0.5h annealing time and do not decrease strongly with annealing.</p><p>EBSD maps reveal that > 70 % of tmt grains are in contact with cpx in all analysed samples. Tmt exhibits two closely related CORs to cpx. More than 60% of total tmt-cpx boundary length in all samples follows COR 1 ([-110]<sub>tmt</sub>[010]<sub>cpx</sub>, [111]<sub>tmt</sub>(100)*<sub>cpx</sub>, [-1-12]<sub>tmt</sub>[001]<sub>cpx</sub>) or  COR 2 ([-110]<sub>tmt</sub>[010]<sub>cpx</sub>, [-1-11]<sub>tmt</sub>(-101)*<sub>cpx</sub>, [112]<sub>tmt</sub>[101]<sub>cpx</sub>). COR frequencies suggest a strong influence of water content and annealing time on their formation. In the 0.5h duration dry experiment, tmt-cpx boundaries following COR 1 are twice as frequent by length as those following COR 2, whereas in the 0.5h duration wet experiment, COR 2 boundaries are 5 times more frequent by length than COR 1 boundaries. In both wet and dry experiments the length ratio of COR 1 : COR 2 boundaries approaches 1 with longer annealing times.</p><p>The degree of undercooling (as imposed by the different water contents) is the most important influence on the microstructural clustering parameters, leading to lower overall number densities of tmt as well as affecting the SPF and FTT values at short durations and the subsequent evolution of these parameters with increasing annealing time. The high frequency of tmt-cpx CORs is consistent with heterogeneous nucleation. However, the mechanisms controlling which CORs develop are unclear. Annealing does not fully erase CORs or microstructural signatures of clustering, suggesting that crystal clusters erupted in volcanic products could still preserve signs of their formation.  </p><p>Pontesilli et al. (2019), Chem Geol 510:113-129. 10.1016/j.chemgeo.2019.02.015</p><p>Funded by the Austrian Science Fund (FWF): P 33227-N</p>

Histomorphometric case-control study of subarticular osteophytes in patients with osteoarthritis of the hip

Objective The objective of this cross-sectional case-control study was to determine the prevalence and size of marginal and subarticular osteophytes in patients with osteoarthritis (OA), and to compare these to that of a control group. Design We investigated femoral heads from 25 patients with OA following hip replacement surgery, and 25 femoral heads from a control group obtained post-mortem. The area and boundary length of the femoral head, marginal osteophytes, and subarticular osteophytes were determined with histomorphometry. Marginal osteophytes were defined histologically as bony projections at the peripheral margin of the femoral head, while subarticular osteophytes were defined as areas of bone that expanded from the normal curvature of the femoral head into the articular cartilage. Results The prevalence of OA patients with marginal- and subarticular osteophytes were 100 and 84%, respectively. Whereas the prevalence of the participants in the control group with marginal- and subarticular osteophytes were 56 and 28%, respectively. The area and boundary length of marginal osteophytes was (median (Interquartile range)) 165.3mm2 (121.4–254.0) mm2 and 75.1 mm (50.8–99.3) mm for patients with OA compared to 0 mm2 (0–0.5) mm2 and 0 mm (0–0.5) mm for the control group (P < 0.001). For the subarticular osteophytes, the area and boundary length was 1.0 mm2 (0–4.4) mm2 and 1.4 mm (0–6.5) mm for patients with OA compared to 0 mm2 (0–0.5) mm2 and 0 mm (0–0.5) mm for the control group (P < 0.001). Conclusion As expected, both marginal- and subarticular osteophytes at the femoral head, were more frequent and larger in patients with OA than in the control group. However, in the control group, subarticular osteophytes were more prevalent than expected from the minor osteophytic changes at the femoral head margin, which may suggest that subarticular osteophytes are an early degenerative phenomenon that ultimately might develop into clinical osteoarthritis.

Subarticular Osteophytes in Patients with Osteoarthritis of the Hip. A Histomorphometric Cross-sectional Case-control Study.

Objective The objective of this cross-sectional case-control study was to determine the prevalence and size of marginal and subarticular osteophytes in patients with osteoarthritis (OA), and to compare these to that of a control group. Design We investigated femoral heads from 25 patients with OA following hip replacement surgery, and 25 femoral heads from a control group obtained post-mortem . The area and boundary length of the femoral head, marginal osteophytes, and subarticular osteophytes were determined with histomorphometry. Marginal osteophytes were defined histologically as bony projections at the peripheral margin of the femoral head, while subarticular osteophytes were defined as areas of bone that expanded from the normal curvature of the femoral head into the articular cartilage. Results The prevalence of OA patients with marginal- and subarticular osteophytes were 100% and 84%, respectively. Whereas the prevalence of the participants in the control group with marginal- and subarticular osteophytes were 56% and 28%, respectively. The area and boundary length of marginal osteophytes was (median (Interquartile range)) 165.3mm 2 (121.4 – 254.0) mm 2 and 75.1 mm (50.8 – 99.3) mm for patients with OA compared to 0 mm 2 (0 – 0.5) mm 2 and 0 mm (0 – 0.5) mm for the control group (P < 0.001). For the subarticular osteophytes, the area and boundary length was 1.0 mm 2 (0 – 4.4) mm 2 and 1.4 mm (0 – 6.5) mm for patients with OA compared to 0 mm 2 (0 – 0.5) mm 2 and 0 mm (0 – 0.5) mm for the control group (P < 0.001). Conclusion As expected, both marginal- and subarticular osteophytes at the femoral head, were more frequent and larger in patients with OA than in the control group. However, in the control group, subarticular osteophytes were more prevalent than expected from the minor osteophytic changes at the femoral head margin, which may suggest that subarticular osteophytes are an early degenerative phenomenon that ultimately might develop into clinical osteoarthritis.

Influence of Boundary Conditions on Cracking of Sanxingdui Moon Bay City Wall

The Sanxingdui Moon Bay City Wall, in China, is built of pale-yellow sand and yellow-brown clay, and the yellow-brown layers are suffering severely. In order to discuss the desiccation cracks in a section of this wall, this paper conducts evaporation tests in laboratory and observes the variations in the characteristics of the boundary conditions of the bottom surface of the sample. The results show that the boundary conditions of the bottom surface of the sample consist of two free boundaries, two sliding boundaries, and one fixed boundary from the outside to the inside. During the drying process, the free boundary extends, but the fixed boundary shrinks; the sliding boundary also remains basically stable. We obtain the surface tensile stress distribution under different boundary conditions through stress analysis and deduce that the tensile stress distribution has a trapezoidal pattern. Moreover, we calculate the lower limit on the crack spacing of the sample using the energy method, and the double of the sum of the maximum sliding boundary length and the free boundary length is the upper limit to the crack spacing. Maintaining the stability of the soil moisture content of the site is the main factor in the protection of the earthen archaeological site under the humid conditions of the museum above the site. In order to reduce the water evaporation rate of the soil, the surface of the site should be coated by a protectant, the main determining features of which are permeability, tensile strength, and compatibility with the earthen site.

Contracting thin disks

We answer a question of Liokumovich–Nabutovsky–Rotman showing that if [Formula: see text] is a Riemannian 2-disk with boundary length [Formula: see text], diameter [Formula: see text] and area [Formula: see text] then [Formula: see text] can be filled by a homotopy [Formula: see text] with [Formula: see text] bounded by [Formula: see text].

Stability and Optimization of a Pressure-Loaded Trapezoidal Vault with Semi-Rigid Joints

The critical pressure is determined for a trapezoidal vault with rigid members and semi-rigid joints. For maximal volume enclosed per boundary length, it is found that the critical pressure is highest when the vault symmetrical, with top three pieces 39.64% of the base length. The upper two joints should also be heavily strengthened.