Cyclotron motion and magnetic focusing in semiconductor quantum wells with spin-orbit coupling

We investigate the ballistic motion of electrons in III-V semiconductor quantum wells with Rashba spin-orbit coupling in a perpendicular magnetic field. Taking into account the full quantum dynamics of the problem, we explore the modifications of classical cyclotron orbits due to spin-orbit interaction. As a result, for electron energies comparable with the cyclotron energy the dynamics are particularly rich and not adequately described by semiclassical approximations. Our study is complementary to previous semiclassical approaches concentrating on the regime of weaker fields.Comment: 14 pages, 8 figures included, version to appear in Phys. Rev.

Detection of spin polarized currents in quantum point contacts via transverse electron focusing

It has been predicted recently that an electron beam can be polarized when it flows adiabatically through a quantum point contact in a system with spin-orbit interaction. Here, we show that a simple transverse electron focusing setup can be used to detect such polarized current. It uses the amplitude's asymmetry of the spin-split transverse electron focusing peak to extract information about the electron's spin polarization. On the other hand, and depending on the quantum point contact geometry, including this one-body effect can be important when using the focusing setup to study many-body effects in quantum point contacts.Comment: 5 pages, 5 figure

An Experimentally Validated Steady State Polymer Electrolyte Membrane Water Electrolyser Model

A simple electrochemical model is developed to understand the overpotentials associated with a polymer electrolyte membrane water electrolyser (PEMWE) operating at room temperature (20 °C) and atmospheric pressure (1 atm). The model is validated using experimental results and fitted parameter values are reported

Two-phase flow behaviour and performance of polymer electrolyte membrane electrolysers: Electrochemical and optical characterisation

Understanding gas evolution and two-phase flow behaviour are critical for performance optimization of polymer electrolyte membrane water electrolysers (PEMWEs), particularly at high current densities. This study investigates the gas-bubble dynamics and two-phase flow behaviour in the anode flow-field of a PEMWE under different operating conditions using high-speed optical imaging and relates the results to the electrochemical performance. Two types of anode flow-field designs were investigated, the single serpentine flow-field (SSFF) and parallel flow-field (PFF). The results show that the PFF design yielded a higher cell performance than the SSFF design at identical operating conditions. Optical visualization shows a strong relationship between the flow path length and the length of gas slugs produced, which in turn influences the flow regime of operation. Longer flow path length in the SSFF results in annular flow regime at a high current density which degrades cell performance. The annular flow regime was absent in the PFF design. It was found the effect of flow rate on performance depends strongly on operating temperature in both flow patterns. Results of this study indicate that long channel length promotes gas accumulation and channel-blocking which degrades performance in PEMWEs

Graphitic Carbon Nitride Materials for Energy Applications

Polymeric layered carbon nitrides were investigated for use as catalyst support materials for proton exchange membrane fuel cells (PEMFCs) and water electrolyzers (PEMWEs). Three different carbon nitride materials were prepared: a heptazine-based graphitic carbon nitride material (gCNM), poly (triazine) imide carbon nitride intercalated with LiCl component (PTI-Li+Cl-) and boron-doped graphitic carbon nitride (B-gCNM). Following accelerated corrosion testing, all graphitic carbon nitride materials were found to be more electrochemically stable compared to conventional carbon black (Vulcan XC-72R) with B-gCNM support showing the best stability. For the supported Pt, Pt/PTI-Li+Cl- exhibited the best durability with only 19% electrochemical surface area (ECSA) loss versus 36% for Pt/Vulcan. Superior methanol oxidation activity was observed for all gCNM supported Pt catalysts on the basis of the catalyst ECSA. Preliminary results on IrO2 supported on gCNM using a PEMWE cell revealed an enhancement in the charge-transfer resistance as the current density increases when compared to unsupported IrO2. This may be attributed to a higher active surface area of the catalyst nanoparticles on the gCNM support

Carbon Nitride Materials as Efficient Catalyst Supports for Proton Exchange Membrane Water Electrolyzers

This work was supported by the UK Engineering and Physical Sciences Research Council. PFM acknowledges EPSRC grant EP/L017091/1 and the EU Graphene Flagship under Horizon 2020 research and innovation program grant agreement No. 696656—Graphene Core 1 for support. A. Belen Jorge acknowledges the UK Engineering Physical Research Council (EPSRC) First Grant EP/P031323/1 scheme. T. Miller acknowledges an EPSRC Postdoctoral Fellowship (EP/P023851/1

Current density mapping and optical flow visualisation of a polymer electrolyte membrane water electrolyser

A polymer electrolyte membrane water electrolyser (PEMWE) employing a segmented current collector made from a printed circuit board (PCB) with optical access to the channel has been demonstrated for the first time. The cell allows the local current density, flow regime and bubble formation dynamics to be studied in real time. Transition is observed from a flow of discrete bubbles at the start of the channel to long bullet shaped bubbles towards the end of the channel associated with a significant increase in local current density

Correlative study of microstructure and performance for porous transport layers in polymer electrolyte membrane water electrolysers by X-ray computed tomography and electrochemical characterization

The porous transport layer (PTL) in polymer electrolyte membrane water electrolysers (PEMWEs) has the multiple roles of delivering water to the electro-catalyst, removal of product gas, and acts as a conduit for electronic and thermal transport. They are, thus, a critical component for optimized performance, especially at high current density operation. This study examines the relationship between the microstructure and corresponding electrochemical performance of commonly used titanium sinter PTLs. Four PTLs, with mean pore diameter (MPD) ranging from 16 μm to 90 μm, were characterized ex-situ using scanning electron microscopy and X-ray computed micro-tomography to determine key structural properties. The performance of these PTLs was studied operando using polarization and electrochemical impedance spectroscopy. Results showed that an increase in mean pore size of the PTLs correlates to an increase in the spread and multimodality of the pore size distribution and a reduction in homogeneity of porosity distribution. Electrochemical measurements reveal a strong correlation of mean pore size of the PTLs with performance. Smaller pore PTLs showed lower Ohmic resistance but higher mass transport resistance at high current density of 3.0 A cm−2. A non-monotonic trend of mass transport resistance was observed for different PTLs, which suggests an optimal pore size beyond which the advantageous influence of macroporosity for mass transport is diminished. The results indicate that maximizing contact points between the PTL and the catalyst layer is the overriding factor in determining the overall performance. These results guide PTL design and fabrication of PEMWEs

Two-dimensional model of low-pressure PEM electrolyser: Two-phase flow regime, electrochemical modelling and experimental validation

International audienceBased on proton conduction of polymeric electrolyte membrane (PEM) technology, the PolymerElectrolyte Membrane Water Electrolyser (PEMWE) offers an interesting solution forefficient hydrogen production. During the electrolysis of water in PEMWE, water is split intooxygen, protons and electrons at the anode and a water-gas two-phase flow results. The aimof this study is to investigate the link between the two-phase flow at the anode side and cellperformance under low-pressure conditions. We have developed a two-dimensional stationaryPEMWE model that takes into account electrochemical reaction, heat transfer, masstransfer (bubble flow) and charge balance through theMembrane Electrodes Assembly (MEA).In order to take into account the changing electrical behaviour, our model combines twoscales of descriptions: at microscale within anodic active layer and MEA scale. The watermanagement at both scales is strongly linked to the Not Coalesced Bubble regime (NCBregime) or the Coalesced Bubble regime (CB regime). Therefore, water content close to activesurface areas depends on two-phase flow regimes. Our simulation results demonstrate thatthe coalesced phenomenon is associated with improvement of mass transfer, a decrease inohmic resistance and an enhancement of the PEMWE efficiency. At low and medium currentdensity values, themodel has been validated using two separate experiment electrolysis cells

Operando flow regime diagnosis using acoustic emission in a polymer electrolyte membrane water electrolyser

Polymer electrolyte membrane water electrolysers (PEMWE) are a key technology for producing clean (‘green’) hydrogen for decarbonisation of the transport sector and grid stabilisation utilising increasing levels of renewable energy. In this work, acoustic emission analysis is used as a non-destructive, operando diagnostic tool to provide information about the relative number and size of gas bubbles generated locally within a PEMWE, providing effective characterisation of the local flow conditions. An optically transparent single-channel PEMWE is used to investigate the relationship between the acoustic signals obtained and the two-phase flow conditions inside the cell. The number of acoustic hits, their frequency, and average peak amplitude is reported for several flow rates and current densities. Using high-speed imaging, the average bubble number and size in the flow cannels is compared to the acoustic signal. Results show good correlation between the number of acoustic ‘hits’ and the number of bubbles passing through the flow channel. The size of bubbles is also shown to affect the average frequency of the hits. Consequently, the transition between bubbly and slug flow regime can be identified by acoustic emission analysis, paving the way for a simple, low-cost, non-destructive means of mapping flow inside commercial-scale PEMWEs