The doping factor approach is a simple, sometimes a “back-of-the-envelope” method, to predict concentrations of native point defects upon doping with aliovalent dopants. The doping factor is the ratio between the concentration of quasi-free electrons in the doped state and their concentration in a reference state, typically the undoped state. In binary compounds, as long as the Boltzmann statistics hold, the neutrality equation becomes a polynomial equation for the doping factor. Once the doping factor is found, all the concentrations of the native point defects are readily determined. While other computational methods allow us to find those concentrations, it is healthy for the researcher to have such a simple tool for calculating the resulting new concentrations. Furthermore, this method might be the only one available in cases with small deviations from stoichiometry. The method is explained and demonstrated using a simple case of magnesium oxide.

The increasing global energy demand necessitates the development of sustainable electrochemical energy technologies. Anion-exchange membrane fuel cells (AEMFCs) present a promising alternative to proton-exchange membrane fuel cells (PEMFCs) due to their ability to utilize platinum-group metal (PGM)-free catalysts, which significantly reduce costs and resource dependency. In this work, we prepare various PGM-free catalysts for the oxygen reduction reaction (ORR) via an ionothermal synthesis using cyclodextrin and magnesium nitrate. The synthesis conditions were optimized and the electrocatalysts were investigated with both physical and electrochemical characterization techniques. The catalysts’ ORR performance was assessed using rotating disk electrode (RDE) measurements and single-cell AEMFC tests. The optimized FeNC-CD3 catalyst exhibited a half-wave potential of 0.90 V vs RHE in 0.1 M KOH in the RDE test and a peak power density of 599 mW cm⁻² in an AEMFC, showcasing its potential as a viable alternative to conventional Pt-based cathode catalysts. This work highlights the critical role of hierarchical porosity in enhancing the ORR activity and paves the way for the development of cost-effective and efficient AEMFC technology.

Metal–organic frameworks (MOFs) are promising precursors for creating metal–nitrogen–carbon (M–N–C) electrocatalysts with high performance, though maintaining their structure during pyrolysis is challenging. This study examines the transformation of a Zn-based MOF into an M–N–C electrocatalyst, focusing on the preservation of the carbon framework and the prevention of Zn aggregation during pyrolysis. A highly porous Zn–N–C electrocatalyst derived from Zn-TAL MOF (where TAL stands for the TalTech-UniTartu Alliance Laboratory) was synthesized via optimized pyrolysis, yielding notable electrocatalytic activity toward oxygen reduction reaction (ORR). Scanning electron microscopy (SEM) and X-ray diffraction spectroscopy (XRD) analyses confirmed that the carbon framework preserved its integrity and remained free of Zn metal aggregates, even at elevated temperatures. Rotating disc electrode (RDE) tests in an alkaline solution showed that the optimized Zn–N–C electrocatalyst demonstrated ORR activity on par with commercial Pt/C electrocatalysts. In an anion-exchange membrane fuel cell (AEMFC), the Zn–N–C material pyrolyzed at 1000 °C exhibited a peak power density of 553 mW cm^–2 at 60 °C. This work demonstrates that Zn-TAL MOF is an excellent precursor for forming hollow Zn–N–C structures, making it a promising high-performance Pt-free electrocatalyst for fuel cells.

We measure the dynamic electrical properties of a spontaneously charged glass surface in an electrolyte solution by using a MHz-level surface acoustic wave (SAW) actuator to introduce a same frequency mechanical wave in the glass substrate. The mechanical wave vibrates ions in the nanometer-thick electrical double layer (EDL) to appear at the glass/electrolyte interface.The out-of-equilibrium EDL leaks electrical field, which is modulated by ion vibration frequency, revealing the presence of ions and their dynamic motion. A previous study excited EDLs on the piezoelectric lithium niobate substrate of a SAW actuator in contact with an electrolyte solution; it remained unclear whether the mechanical or electrical components of the SAW in the piezoelectric substrate dominated the EDL excitation. We isolate the SAW mechanical component in glass and show that it introduces a similar ion electrokinetic vibration in the excited EDL at the glass/electrolyte interface using Sodium Nitrate and Sodium Chloride solutions as electrolytes:The measured electrical field leakage spectra are of similar magnitude to the ones measured in the previous study and exhibit similar non-monotonic behaviors, taking local maxima where the SAW period (inverse of its frequency) is synchronized with the ion relaxation times in the EDL. At these frequencies, the synchronization maximizes ion vibration displacement, thereby amplifying the electrical field leakage. Our findings may be used to study electrokinetic properties of solid surfaces and ion dynamics in EDLs. Moreover, SAW-actuated fluidic platforms may support out-of-equilibrium EDLs of consequence to, e.g., applications involving ion selective membranes and the film stability of electrolyte solutions.

The rapid advancement of wearable electrophysiological monitoring has driven a growing demand for soft, conductive materials, particularly hydrogels, due to their exceptional mechanical properties, high conductivity, and biocompatibility. However, hydrogel materials are prone to freezing at subzero temperatures, which leads to material failure. Additionally, prefabricated hydrogel electrodes often lack sufficient conformability and long-term stability, thereby limiting their practical applications. We report a skin-printable hydrogel-based epidermal bioelectrode designed for continuous, high-precision wireless electrophysiological monitoring. The hydrogel, synthesized via an effective one-pot process, comprises gelatin, glycerol, ammonium chloride, and water. Its reversible thermal phase transition enables precise printing onto glabrous and hairy human skin. The in-situ formed hydrogel electrode exhibits excellent adhesion at both the hydrogel/skin and hydrogel/electrode interfaces, with adhesion force reaching 0.9 N/cm and 2.6 N/cm, respectively. The incorporation of glycerol and ammonium chloride imparts remarkable anti-freezing properties, allowing the hydrogel to maintain outstanding mechanical flexibility across a broad temperature range, from ultra-low temperatures of −80 °C to the phase transition temperature of 43.9 °C. Remarkably, the hydrogel retains its optical transparency even when immersed in liquid nitrogen. Additionally, the hydrogel demonstrates superior self-healing, high water retention, and recyclable properties. We successfully validated the hydrogel’s potential for personalized healthcare through high-fidelity electrocardiogram recordings over 80 min during various daily activities, demonstrating its practical applicability. This work highlights the versatility and functionality of skin-printable hydrogel as a promising material for next-generation epidermal electronics.

This work presents a Monte Carlo (MC) based microscopic model for simulating the extraction of oil from oil-in-water emulsions under the influence of surface acoustic waves (SAWs). The proposed model is a two-dimensional Ising-lattice gas model that employs Kawasaki dynamics to mimic the interactions between oil, water, and air, as well as external forces such as gravity and acoustic stress. By incorporating both acoustic streaming and acoustic radiation pressure, the model captures key experimental observations, including selective oil extraction and droplet motion under SAW excitation. The results highlight the critical role of acoustic radiation pressure in enabling oil film formation and detachment, governed by the balance between capillary and acoustic stresses. The study provides qualitative agreement with experimental findings and offers insights into the essential mechanisms driving acoustowetting-induced phase separation, demonstrating the utility of discrete modeling for complex fluid dynamics problems.

Rotating magnetic field is an efficient method of actuation of synthetic colloids in liquids. In this Letter we theoretically study the effect of the thermal noise on torque-driven steering of magnetic nanohelices. Using a combination of numerical and analytical methods, we demonstrate that surprisingly a weak thermal noise can substantially disrupt the orientation and rotation of the nanohelix, severely impeding its propulsion. The results of Langevin simulations are in excellent agreement with the numerical solution of the Fokker-Planck equation and the analytical effective field approximation.

The growth of ultrathin layers of oxides by atomic layer deposition (ALD) is well documented for oxide substrates such as SiO₂, Bi₂O₃, Al₂O₃, in which oxygen is the only negatively charged atom. In contrast, the knowledge regarding ALD growth on substrates containing other negatively charged atoms, such as halogens, is quite limited. The commonly used bismuth oxyhalide (BiOX) family of materials are characterised by a low density of surface hydroxyls, required for the initiation of thermal ALD growth of oxides, thus hampering the ability to grow ultrathin layers of oxides on their surface. This restriction becomes even more severe if the process has to be performed at low temperatures. In this work, we show that high quality Al₂O₃ can be grown on bismuth oxyhalide materials by low temperature ALD, upon performing UV-ozone surface pretreatment. The effect of pretreatment on the BiOX photocatalysts was studied by wettability measurements and FTIR. The coating conformality was monitored by both XPS and via the ability of the ultrathin layers to suppress the photocatalytic activity of the substrates. The capability to form dense, conformal aluminium oxide layers on BiOX substrates opens a door for low-temperature preparation of organic–inorganic hybrid devices on such and similar compounds.

Desalination via reverse osmosis (RO) membrane technology is a preferred solution to the ongoing global challenges of freshwater scarcity. The active separation layer of RO membranes is a polyamide thin film (<200 nm), whose morphology critically influences membrane performance. However, conflicting descriptions of trends between morphology and performance abound in the literature due to the lack of a rigorous morphological description of these membranes. Notably, comprehensive three-dimensional (3D) morphological characterization of these membranes has so far been conducted exclusively under dry conditions, which contrasts with the operational, hydrated state of these membranes. Here, we present, for the first time, characterization of the hydrated 3D nanoscale morphology of polyamide films from commercial brackish water (BW) and seawater (SW) membranes using cryo-transmission electron microscopy (cryo-TEM) tomography. Our findings reveal significant morphological differences between hydrated and dry membranes, resulting in variations in key structural parameters that impact performance. Both SW and BW membranes swell and increase in total volume and thickness upon hydration, with BW membranes exhibiting more pronounced swelling (32% vs 7% in volume and 35% vs 11% in effective thickness), primarily due to the lower degree of cross-linking of BW membranes. Additionally, while the surface area decreases upon hydration for both SW and BW membranes, indicating a smoothing of surface nodules and cavities, surface roughness remains unchanged, suggesting that current roughness measurement methods such as atomic force microscopy do not capture intrinsic morphological features. Overall, this study demonstrates the feasibility of employing cryo-TEM tomography techniques to characterize RO membrane morphology under operation relevant conditions, thus enabling a better linkage between membrane morphology and performance.

This paper describes the first implementation of a flipped, one-week workshop on heat integration that was taught in Spring 2024 to the 3rd Year cohort of 138 students in Chemical Engineering at Imperial College, London. The “flipped” workshop consisted of three online lessons that cover the core materials on pinch design of heat exchanger networks, which the students were required to complete ahead of each of the corresponding three face-to-face class meetings, which focused on problem-solving exercises largely carried out by the students themselves. The paper describes the teaching methodology applied, presents and analyses the results of a survey conducted to assess the students’ perceptions and degree of satisfaction with the workshop. Learning outcomes relevant to the workshop topic, that is, the ability to design and optimize heat exchanger networks in realistic plant-wide settings, are also presented and compared to those of previous years. The main conclusion is that the short workshop format can successfully achieve the learning objectives, even for relatively large class sizes. Evidently, this workshop can be taught effectively in this concentrated form provided that the workshop participants are given access to the online lessons in advance of the class exercises.