Proton-exchanged Layer Research Articles

Materials descriptors for advanced water dissociation catalysts in bipolar membranes.

The voltage penalty driving water dissociation (WD) at high current density is a major obstacle in the commercialization of bipolar membrane (BPM) technology for energy devices. Here we show that three materials descriptors, that is, electrical conductivity, microscopic surface area and (nominal) surface-hydroxyl coverage, effectively control the kinetics of WD in BPMs. Using these descriptors and optimizing mass loading, we design new earth-abundant WD catalysts based on nanoparticle SnO2 synthesized at low temperature with high conductivity and hydroxyl coverage. These catalysts exhibit exceptional performance in a BPM electrolyser with low WD overvoltage (ηwd) of 100 ± 20 mV at 1.0 A cm-2. The new catalyst works equivalently well with hydrocarbon proton-exchange layers as it does with fluorocarbon-based Nafion, thus providing pathways to commercializing advanced BPMs for a broad array of electrolysis, fuel-cell and electrodialysis applications.

Surface-modified nanoclay incorporated anion exchange membrane facilitating performance in self-humidifying bipolar membrane fuel cell

Bipolar membrane fuel cell (BPMFC) undergoes self-humidification due to formation of water at the interface of proton exchange layer (PEL) and anion exchange layer (AEL). Ion conductivity and dimensional stability of individual layers (PEL/AEL) play crucial role in the ohmic resistance/interface resistance of the BPM. A series of polysulfone based composite anion exchange membranes (AEMs) were synthesized by varying 2, 4, 6, and 8 % w/w of surface modified montmorillonite (MMT) nano clay (SM-MMT), and their electrochemical/physiochemical properties were optimized and compared with commercial AEM fumasep®, FAS-50. The AEM with 6 wt% SM-MMT (AEM-SM-MMT-06) showed improved OH− conductivity (103.34 mS cm−1, 80 °C) compared to both AEM-pristine (72.68 mS cm−1, 80 °C) and commercial FAS-50 (86.61 mS cm−1, 80 °C), while dimensional stability (i.e. swelling ratio, %) of AEM-SM-MMT-06 (16.43 %) much higher compared to pristine AEM (47.37 %), but closer to fumasep® (FAS-50, at 80 °C, 15.78 %). Finally, BPM fuel cell test carried out without external humidification showed peak power density of 331.2 mW cm−2 with nafion-212 (PEL)/AEM-SM-MMT-06 (AEL), followed by other two BPMs (nafion-212/FAS-50, 270 mW cm−2; nafion-212/AEM-pristine, 246 mW cm−2).

Electroosmotic flow steers neutral products and enables concentrated ethanol electroproduction from CO2

<h2>Summary</h2> Electrochemical reduction of carbon dioxide (CO₂RR) converts intermittent renewable energy into high energy density fuels, such as ethanol. Membrane electrode assembly (MEA) electrolyzers are particularly well-suited for CO₂-to-ethanol conversion in view of their low ohmic resistance and high stability. However, over 75% of the ethanol produced at the cathode migrates through the membrane where it is diluted by the anolyte and may be oxidized. The ethanol concentration that results is two orders of magnitude below the 10 wt % standard set by the incumbent industrial process, fermentation. Here, we reverse the direction of ion and electroosmotic transport by means of a porous proton exchange layer, thereby blocking both the convective and diffusive routes of ethanol loss. With this strategy, we eliminate ethanol crossover to the anode (< 1%) and achieve an ethanol concentration of 13.1 wt % directly from the cathode outlet.

Accelerating Water Dissociation in Bipolar Membranes and Electrocatalysis

Catalyzing water dissociation (WD), H2O -> H+ + OH-, is practically important for fabricating bipolar membranes (BPMs) that couple different pH environments into a single electrochemical device and for accelerating (electro)catalytic reactions that consume water. We use a BPM electrolyzer for quantitative measurement of WD kinetics in the absence of soluble electrolyte. We discover that the rate of WD on metal-oxide nanoparticle surfaces is affected by the local pH, consistent with a proton-transfer mechanism. We combine WD catalysts efficient near the acidic proton-exchange layer with those efficient near the alkaline hydroxide-exchange layer and yield WD overpotentials < 10 mV across the BPM at 20 mA cm-2. We demonstrate pure-water BPM electrolyzers that maintain a gradient of ~ 14 pH units between the basic anode and acidic cathode that operate continuously at 500 mA cm-2 with a total electrolysis voltage of ~ 2.2 V. We further investigate the WD activity of metal electrocatalysts in the BPM junction and find a direct correlation with their electrocatalytic activity for the hydrogen-evolution reaction (HER) in alkaline media. This result is direct evidence of the rate-limiting role of WD in alkaline HER and guides advanced (electro)catalyst design for (electrochemical) reactions where water is a reactant.

Accelerating water dissociation in bipolar membranes and for electrocatalysis.

Catalyzing water dissociation (WD) into protons and hydroxide ions is important both for fabricating bipolar membranes (BPMs) that can couple different pH environments into a single electrochemical device and for accelerating electrocatalytic reactions that consume protons in neutral to alkaline media. We designed a BPM electrolyzer to quantitatively measure WD kinetics and show that, for metal nanoparticles, WD activity correlates with alkaline hydrogen evolution reaction activity. By combining metal-oxide WD catalysts that are efficient near the acidic proton-exchange layer with those efficient near the alkaline hydroxide-exchange layer, we demonstrate a BPM driving WD with overpotentials of <10 mV at 20 mA·cm-2 and pure water BPM electrolyzers that operate with an alkaline anode and acidic cathode at 500 mA·cm-2 with a total electrolysis voltage of ~2.2 V.

Influence of surface treatment on the structure and properties of proton-exchanged waveguides in lithium niobate

The influence of reverse proton exchange (RPE) and some types of surface pretreatment (exposure to Piranha or RCA-1 solutions or to HNO3+HF mixture) on the structure and properties of proton-exchanged layers has been studied in X-cut lithium niobate (LN). It has been shown that the rate of phase transitions in PE layers during RPE is greater than that during annealing at the same temperature. Pretreatment of LN surface before PE by Piranha or RCA-1 solutions slightly increases refractive index change Δne in the resulting waveguide, whereas pretreatment by HNO3+HF mixture decreases it.

Research of selective etching in LiNbO3 using proton-exchanged wet etching technique

Lithium niobate material (LN) has shown great application potentials in the fabrication of integrated optical devices due to its excellent physical properties, especially with the occurrence of lithium niobate-on-insulator (LNOI) substrate. However, the greatest challenge of micro/nano optical devices based on LN material lies in the precise etching process and thus limits its applications. In this paper, we comprehensively analyze the etching results treated by the proposed proton-exchanged wet-etching method (PEWE) combining with theoretical simulations and experiments. It is found that the proton-exchanged layer in the LN material can be easily etched after using a mixture acid of HF/HNO3, leading to the improvement of etching rate and surface morphology. The lowest roughness of the optical waveguide is measured to be 0.81 nm, which is beneficial for the performance improvement of LN-based optical devices. Ultimately, a quasi-vertical sidewall of the upper part of optical waveguide with improved surface morphology is successfully realized by utilizing the PEWE. Moreover, this method could also be extended to improve the performance of LNOI-based optical devices and pave the way for ultra-compact photonic integrated circuits based on LNOI.

Моделирование кривых дифракционного отражения рентгеновских лучей от протонообменных слоев монокристалла ниобата лития

In this work, within the framework of the dynamic theory of the X-ray scattering, a model is constructed that describes the diffraction reflection curves obtained from the proton-exchange layers of a lithium niobate monocrystal subjected to the post-exchange annealing. Planar proton-exchange waveguides based on the X-cut of the lithium niobate monocrystal are obtained experimentally. It was shown that the proton implantation leads to the formation of the new crystalline phases with a larger lattice parameter. The structure changes (by the method of the diffraction structural analysis) and the optical properties (by the method of the mode spectroscopy) of the obtained waveguides are investigated in the various temperature regimes. The microstrain of the crystal lattice caused by еру proton implantation was estimated by the analysis of X-ray diffraction line broadening. It is shown the thickness of the waveguide layer and the number of new crystalline phases depend not only on the temperature regime of the proton exchange, but also on the duration of post-exchange annealing. The simulation of the experimentally obtained curves of the diffraction reflection is carried out within the framework of the described model. As a result of the modeling, the depth of the waveguide layer was determined, which is consistent with the data obtained by the method of the mode spectroscopy. The models of the assumed profiles of the crystal lattice deformation caused by proton implantation are presented. The average values of the lattice microstrain and the phase composition of proton-exchange lithium niobate layers are determined.

Structural phase transitions during annealing of proton-exchanged layers in X-cut and Z-cut lithium niobate

Phase transitions during annealing of proton-exchanged (PE) layers in X-cut and Z-cut lithium niobate (LN) crystals were studied. The following sequence of phase transitions is proposed: β1, β2-phases → κ2-phase → κ1-phase → α-phase. Main feature of κ2-phase → κ1-phase transition in X-cut LN is the formation of particles of κ1-phase, which create a modulated structure. In case of Z-cut LN, the particles of κ1-phase do not arise and PE-waveguide consists of coherent layers until they completely transform into α-phase.

ОСОБЕННОСТИ СТРУКТУРНО-ФАЗОВЫХ ПРЕВРАЩЕНИЙ ПРИ ОТЖИГЕ ПРОТОНООБМЕННЫХ СЛОЕВ НА Z-СРЕЗЕ КРИСТАЛЛА НИОБАТА ЛИТИЯ

С использованием методов оптической микроскопии в поляризованном свете, модовой спектроскопии, ИК-спектроскопии и рентгеноструктурного анализа подтверждена последовательность фазовых превращений, происходящих при отжиге протонообменных слоев на Z-срезе кристалла ниобата лития: b2-, b1-фазы ® k2-фаза ® k1-фаза ® α-фаза. Установлено, что в процессе отжига в ходе фазовых превращений когерентность слоев, образованных из различных фаз, не нарушается, что обусловлено меньшей величиной деформаций (напряжений) формирующихся протонообменных фаз по сравнению с деформациями (напряжениями) на X-срезе кристалла. &#x0D; Работа выполнена при финансовой поддержке гранта РФФИ (проект № 17-43-590309 р_а).

ВЛИЯНИЕ ОБРАТНОГО ПРОТОННОГО ОБМЕНА НА СТРУКТУРУ И СВОЙСТВА ПРОТОНООБМЕННЫХ СЛОЕВ НА X-СРЕЗЕ КРИСТАЛЛА НИОБАТА ЛИТИЯ

The changes in the structure and optical properties of proton-exchange layers during reverse proton exchange were investigated on X cut lithium niobate. It was shown that the phase transformations in proton-exchange layers at reverse proton exchange proceed faster than during annealing at the same temperature;  1 phase particles are not formed at reverse proton exchange.

Structural phase transformations of proton-exchanged layers of lithium niobate during annealing

ABSTRACTKinetics of phase transitions in the surface layer of proton-exchanged planar waveguides on X-cut of lithium niobate crystal during annealing at 330, 350, and 370°C was investigated by optical methods (dark field microscopy, crossed-polarizers bright field microscopy, and mode spectroscopy). Nucleation and growth stages of phase transformations were identified. Based on mode spectroscopy data and analysis of previous works, we assume that transformation of κ2-phase into κ1-phase and further transformation into α-phase were observed during experiment.

Hybrid microfiber–lithium-niobate nanowaveguide structures as high-purity heralded single-photon sources

We propose a compact, fiber-integrated architecture for photon-pair generation by parametric downconversion with unprecedented flexibility in the properties of the photons produced. Our approach is based on a thin-film lithium niobate nanowaveguide, evanescently coupled to a tapered silica microfiber. We demonstrate how controllable mode hybridization between the fiber and waveguide yields control over the joint spectrum of the photon pairs. We also investigate how independent engineering of the linear and nonlinear properties of the structure can be achieved through the addition of a tapered, proton-exchanged layer to the waveguide. This allows further refinement of the joint spectrum through custom profiling of the effective nonlinearity, drastically improving the purity of the heralded photons. We give details of a source design capable of generating heralded single photons in the telecom wavelength range with purity of at least 0.95, and we provide a feasible fabrication methodology.

Electric-optical property of the proton exchanged phase modulator in single-crystal lithium niobate thin film.

The electric-optical property of the proton exchanged phase modulator in an x-cut single-crystal lithium niobate thin film was studied. Proton exchanged waveguides generally suffered from a deteriorated electric-optical coefficient. By introducing a shallow proton exchange layer (thickness = 0.165 μm), most energy of the optical mode was allowed to guide in the untouched single-crystal lithium niobate film, making contribution to the effective electric-optical coefficient as high as 29.5 pm/V, which was very close to that of the bulk lithium niobate (r33 = 31 pm/V). A 12 V voltage applied to the electrodes located on the two sides of the waveguide induced a 0.097 nm shift of the Fabry-Perot resonant peak. Considering the wavelength difference of the neighboring resonant peaks (0.228 nm) and the length of the electrodes (2.3 mm), the voltage-length product was as low as 6.5 V·cm, indicating the efficient electric-optical modulation.

Interface and thickness dependent domain switching and stability in Mg doped lithium niobate

Controlling ferroelectric switching in Mg doped lithium niobate (Mg:LN) is of fundamental importance for optical device and domain wall electronics applications that require precise domain patterns. Stable ferroelectric switching has been previously observed in undoped LN layers above proton exchanged (PE) phases that exhibit reduced polarization, whereas PE layers have been found to inhibit lateral domain growth. Here, Mg doping, which is known to significantly alter ferroelectric switching properties including coercive field and switching currents, is shown to inhibit domain nucleation and stability in Mg:LN above buried PE phases that allow for precise ferroelectric patterning via domain growth control. Furthermore, piezoresponse force microscopy (PFM) and switching spectroscopy PFM reveal that the voltage at which polarization switches from the “up” to the “down” state increases with increasing thickness in pure Mg:LN, whereas the voltage required for stable back switching to the original “up” state does not exhibit this thickness dependence. This behavior is consistent with the presence of an internal frozen defect field. The inhibition of domain nucleation above PE interfaces, observed in this study, is a phenomenon that occurs in Mg:LN but not in undoped samples and is mainly ascribed to a remaining frozen polarization in the PE phase that opposes polarization reversal. This reduced frozen depolarization field in the PE phase also influences the depolarization field of the Mg:LN layer above due to the presence of uncompensated polarization charge at the PE-Mg:LN boundary. These alterations in internal electric fields within the sample cause long-range lattice distortions in Mg:LN via electromechanical coupling, which were corroborated with complimentary Raman measurements.

Interference Lithography Patterned Nanogratings in LiNbO&lt;sub&gt;3&lt;/sub&gt; Fabricated by Dry Etching

Channel waveguides have been fabricated in x-cut lithium niobate (LiNbO3) by proton exchange (PE) method and optically measured. The thickness and the optical constants of the thin PE layer were characterized using a prism coupling technique. The PE area was plasma etched and a 2.775-μm total etching depth was achieved. The measured average etching rate is 92.5 nm/min. One-and two-dimensional dense arrays of LiNbO3 nanostructures have also been fabricated by using interference lithography (IL) and inductively coupled plasma reactive ion etching (ICP-RIE) techniques.Intorduction

Phase composition and stress in LiTaO3 proton-exchanged optical waveguides

The phase composition of the top layer of Li1−xHxTaO3 waveguide layers produced at different modifications of the proton exchange (PE) technology has been analyzed based on their IR reflection spectra. These spectra contain new bands within the range , each phase having its own reflection spectrum. Since the top layer is actually the strongest proton-exchanged one of all sublayers building the waveguide layer, the recognition of the top sublayer's phase in many cases could be used to make conclusions about the phases building the rest of the entire PE layer. The intrinsic stress caused by crystal lattice deformation due to the PE was calculated by an optical integral method. An attempt to explain the level of stress is made based on the phase composition of the studied samples.

Low Temperature Polymer Electrolyte Fuel Cell Performance Degradation

The goal of this research was to quantify the degradation experienced by a polymer electrolyte fuel cell after storage at subzero temperatures ranging from 0 to -40 ° C. The performance loss was determined by comparing the polarization and other applicable power curves before and after the subzero storage cycle. It was found that there are two distinct types of degradation experienced by the membrane. The first type was identified as a variance of the actual voltage-current relationship of the cell. This mode of degradation only pertained to the initial freeze/thaw cycle and was not observed after any number of subsequent cycles. The second type of degradation was only observed during the subsequent cycles, and mainly affected the high power regions of the operating range. A 5% reduction in current density and power output was observed as a result of further freeze/thaw cycles. Mass transport limitations may have been caused by the destruction of the meso-porous gas diffusion and micro-porous layers. The pore size, pore volume, and membrane surface area were quantified using an Autosorb physisorption tool. The results showed that the pore diameter of the catalyst and proton exchange layer did not increase significantly. However, the pore volume increased by a factor of ten and was confirmed by the surface area measurements of the membrane.

Structure and Properties of Proton Exchanged Layers in +Z Cut and –Z Cut Lithium Niobate

The phase structure of planar and channel, un-annealed and annealed proton exchanged waveguides made in benzoic acid and benzoic acid + lithium benzoate melts on Z cut of congruent lithium niobate crystal was investigated using mode spectroscopy, IR spectroscopy, XRD, SEM, AFM, microindentation, and wet etching techniques. In un-annealed waveguides, three layers (probably β2, β1, and κ2 phases) can be distinguished. At least two different structural layers are present in annealed channel waveguides. Proton exchange (225oC, 28 h) in benzoic acid + 3 mol.% of lithium benzoate leads to formation of κ2 phase and a thin layer of α phase.

Proton-exchanged optical waveguides in LiTaO3: phase composition and stress

Planar optical waveguide layers were obtained in Z-cut LiTaO3 crystal substrates via proton exchange. Two different media were used as proton sources: benzoic acid melt and lithium hydrogensulphate vapors, controlling the thickness of the waveguides by duration and temperature of the proton exchange process and also by post-exchange annealing. The intrinsic stress caused by the penetration of the hydrogen ions into the crystal lattice was estimated by the optical integral method. The phase composition of proton-exchanged layers was analyzed based on the mode and IR-absorption spectra. An attempt to relate the level of stress to the level of proton doping has been made.