Cleaner production of ammonium paratungstate by membrane electrolysis-precipitation of sodium tungstate solution

The production of ammonium paratungstate(APT) is riddled with the generation of wastewater,which causes environmental problems.To solve the problem of wastewater generation at source,a membrane electrolysis-NH3·H2O precipitation method,which prevents wastewater generation and recycles the reagents used in the process,was proposed and investigated in this study.The electrolysis process was investigated based on parameters such as initial cathodic and anodic NaOH concentrations,and current density.The results showed that an increase in current density and initial cathodic NaOH concentration and a decrease in the initial anodic NaOH concentration would enhance the separation of tungsten and sodium.The optimum condition was found at a current density of 666 A·m^(-2),initial anodic and cathodic NaOH concentrations of 69 g·L^(-1) and 40 g·L^(-1),with a current efficiency of 75.40%,and energy consumption for producing 1 ton of NaOH was 2184 kW·h.The precipitation process was investigated based on the acidic high W/Na molar ratio solution obtained by the electrolysis process with NH3·H2O as the precipitant.Parameters such as excessive coefficient,temperature,and W/Na molar ratio were studied.The result showed that the variation of excessive coefficient and solution temperature had an opposite effect on the purity of the APT,while an increase in the W/Na molar ratio would increase the product purity.The precipitation product obtained had a purity of 99.6% and was characterized using X-ray diffraction,inductively coupled plasma,and scanning electron microscopy.The methods proposed in this study could provide fundamental information for the design of a cleaner APT production process.

Numerical investigation of water and temperature distributions in a proton exchange membrane electrolysis cell

A three-dimensional, non-isothermal, two-phase model for a PEM water electrolysis cell(PEMEC) is established in this study.An effective connection between two-phase transport and performance in the PEMECs is built through coupling the liquid water saturation and temperature in the charge conservation equation. The distributions of liquid water and temperature with different operating(voltage, temperature, inlet velocity) and physical(contact angle, and porosity of anode gas diffusion layer) parameters are examined and discussed in detail. The results show that the water and temperature distributions, which are affected by the operating and physical parameters, have a combined effect on the cell performance. The effects of various parameters on the PEMEC are of interaction and restricted mutually. As the voltage increases, the priority factor caused by the change of inlet water velocity changes from the liquid water saturation increase to the temperature drop in the anode catalyst layer. While the priority influence factor caused by the contact angle and porosity of anode gas diffusion layer is the liquid water saturation. Decreasing the contact angle or/and increasing the porosity can improve the PEMEC performance especially at the high voltage. The results can provide a better understanding of the effect of heat and mass transfer and the foundation for optimization design.

Three-dimensional multiphase modeling of a proton exchange membrane electrolysis cell with a new interdigitated-jet hole flow field

The flow field structure on the bipolar plate significantly affects the performance of the proton exchange membrane electrolysis cell(PEMEC).This paper proposes a new interdigitated-jet hole flow field(JHFF)design to improve the uniformities of liquid saturation,temperature,and current density distributions.The common single-path serpentine flow field(SSFF)and interdigitated flow field(IFF)are used as comparative references to constitute three PEMEC cases.An advanced numerical model has been established to simulate the performance of the PEMEC using CFD software.The results show that,due to the perpendicular mainstream and the pressure difference,the JHFF enhances the mass and heat transfer inside the porous electrode by introducing strong forced convection,which promotes gas removal underneath the ribs and cooling.Compared with the comparative flow fields,the uniformities of liquid saturation,temperature,and current density distributions by using the JHFF at the anode side are increased by 19.1%,53.2%,and 40.4%,respectively.Further,mainly owing to the largest conductive area,the PEMEC with the JHFF has superior polarization performance,which is 8.05%higher than the PEMEC with the SSFF.

A new method for direct synthesis of Li2CO3 powders by membrane electrolysis

A new method for the direct synthesis of Li2CO3 powders by membrane electrolysis from LiC1 solution is demonstrated in this paper, where a novel electrolysis system combining ventilation, agitation and loop filtration functions was reported. The aim of this work is to explore the effect of the starting concentration of LiC1 on the phase and micromorphology of Li2CO3 crystals and thereafter to explore the mechanism of crystallization and grain growth law. Scanning electron microscopy （SEM） images indicate that the particles become irregular polycrystalline from well-defined flower-like and the micro-crystals change from lamellar to needle-like and subsequently to smaller globular granules, and the surface of the crystals becomes smooth with LiC1 concentration increasing from 50 to 400 g.L^-3. The crystalline phases of the different samples were characterized using powder X-ray diffraction （XRD） and the results prove that pure LiaCO3 crystals can be obtained in a single step by the electrolysis method. The particle size distributions show that both volume mean crystal sizes and the full width at half maximum （FWHM） decrease when the starting LiC1 concentration increases from 50 to 300 g.L 3 and also decreases from 400 to 300 g-L^-3.

Ternary layered double hydroxide oxygen evolution reaction electrocatalyst for anion exchange membrane alkaline seawater electrolysis

Anion exchange membrane(AEM)water electrolyzers are promising energy devices for the production of clean hydrogen from seawater.However,the lack of active and robust electrocatalysts for the oxygen evolution reaction(OER)severely impedes the development of this technology.In this study,a ternary layered double hydroxide(LDH)OER electrocatalyst(NiFeCo-LDH)is developed for high-performance AEM alkaline seawater electrolyzers.The AEM alkaline seawater electrolyzer catalyzed by the NiFeCo LDH shows high seawater electrolysis performance(0.84 A/cm^(2)at 1.7 Vcell)and high hydrogen production efficiency(77.6%at 0.5 A/cm^(2)),thus outperforming an electrolyzer catalyzed by a benchmark IrO_(2)electrocatalyst.The NiFeCo-LDH electrocatalyst greatly improves the kinetics of the AEM alkaline seawater electrolyzer,consequently reducing its activation loss and leading to high performance.Based on the results,this NiFeCo-LDH-catalyzed AEM alkaline seawater electrolyzer can likely surpass the energy conversion targets of the US Department of Energy.

Internal Polarization Field Induced Hydroxyl Spillover Effect for Industrial Water Splitting Electrolyzers

The formation of multiple oxygen intermediates supporting efficient oxygen evolution reaction(OER)are affinitive with hydroxyl adsorption.However,ability of the catalyst to capture hydroxyl and maintain the continuous supply at active sits remains a tremendous challenge.Herein,an affordable Ni2P/FeP2 heterostructure is presented to form the internal polarization field(IPF),arising hydroxyl spillover(HOSo)during OER.Facilitated by IPF,the oriented HOSo from FeP2 to Ni2P can activate the Ni site with a new hydroxyl transmission channel and build the optimized reaction path of oxygen intermediates for lower adsorption energy,boosting the OER activity(242 mV vs.RHE at 100 mA cm-2)for least 100 h.More interestingly,for the anion exchange membrane water electrolyzer(AEMWE)with low concentration electrolyte,the advantage of HOSo effect is significantly amplified,delivering 1 A cm^(-2)at a low cell voltage of 1.88 V with excellent stability for over 50 h.

Enhancement of current density using effective membranes electrode assemblies for water electrolyser system

The goal of this study was to develop and design a composite proton exchange membrane（PEM） and membrane electrode assembly（MEA） that are suitable for the PEM based water electrolysis system. In particular,it focuses on the development of sulphonated polyether ether ketone（SPEEK） based membranes and caesium salt of silico-tungstic acid（Cs Si WA） matrix compared with one of the transition metal oxides such as titanium dioxide（TiO2）, silicon dioxide（SiO2） and zirconium dioxide（ZrO2）. The resultant membranes have been characterized by Fourier transform infrared spectroscopy, scanning electron microscopy, ion exchange capacity（IEC）, water uptake and atomic force microscopy. Comparative studies on the performance of MEAs were also conducted utilizing impregnation-reduction and conventional brush coating methods. The PEM electrolysis performance of SPEEK-Cs Si WA-ZrO2 composite membrane was more superior than that of other membranes involved in this study. Electrochemical characterization shows that a maximum current density of 1.4 A/cm^2 was achieved at 60 °C, explained by an increased concentration of protonic sites available at the interface.

Interfacial engineering of atomic platinum-doped molybdenum carbide quantum dots for high-rate and stable hydrogen evolution reaction in proton exchange membrane water electrolysis

Platinum(Pt)-based electrocatalysts remain the only practical cathode catalysts for proton exchange membrane water electrolysis(PEMWE),due to their excellent catalytic activity for acidic hydrogen evolution reaction(HER),but are greatly limited by their low reserves and high cost.Here,we report an interfacial engineering strategy to obtain a promising low-Pt loading catalyst with atomically Pt-doped molybdenum carbide quantum dots decorated on conductive porous carbon(Pt-MoCx@C)for high-rate and stable HER in PEMWE.Benefiting from the strong interfacial interaction between Pt atoms and the ultra-small MoCx quantum dots substrate,the Pt-MoCx catalyst exhibits a high mass activity of 8.00 A·mgPt−1,5.6 times higher than that of commercial 20 wt.%Pt/C catalyst.Moreover,the strong interfacial coupling of Pt and MoCx substrate greatly improves the HER stability of the Pt-MoCx catalyst.Density functional theory studies further confirm the strong metal-support interaction on Pt-MoCx,the critical role of MoCx substrate in the stabilization of surface Pt atoms,as well as activation of MoCx substrate by Pt atoms for improving HER durability and activity.The optimized Pt-MoCx@C catalyst demonstrates>2000 h stability under a water-splitting current of 1000 mA·cm^(−2)when applied to the cathode of a PEM water electrolyzer,suggesting the potential for practical applications.

Modulating metal-organic frameworks for catalyzing acidic oxygen evolution for proton exchangemembrane water electrolysis

Proton exchangemembrane(PEM)water electrolysis represents one of the most promising technologies to achieve green hydrogen production,but currently its practical viability is largely affected by the slow reaction kinetics of the anodic oxygen evolution reaction(OER)in an acidic environment.While noble metal-based catalysts containing iridium or ruthenium are excellent catalysts for the acidic OER,their practical use in PEM electrolyzers is hindered due to their low abundance and high cost.Most recently,metal-organic frameworks(MOFs)have been demonstrated as a perfect platform to facilitate the design of acidic OER catalysts with both high efficiency and cost-effectiveness.Here,we pro-vide a timely and comprehensive overview of the recent progress on MOF-based acidic OER catalysts.The fundamental mechanisms of the acidic OER are first introduced,followed by a summary of the development of pristine MOFs and MOF derivatives as acidic OER catalysts.Importantly,a number of catalyst design strategies are discussed aiming at improving the acidic OER catalytic per-formance of MOF-based candidates.The integration of MOF-based catalysts into real PEM water electrolyzers is also included.Finally,future research directions are provided to achieve better MOF-based catalysts operational in acidic envi-ronments and PEM devices.

Improving the water electrolysis performance by manipulating the generated nano/micro-bubbles using surfactants

The impeded mass transfer rate by on-site-generated gas bubbles at both cathode and anode dramatically reduces the energy conversion efficiency of the proton exchange membrane water electrolyzer(PEMWE).Herein,we report a surfactant-assistant method to accelerate the nano/micro-bubble detachment and the mass transfer rate by reducing the surface tension,resulting in an increase in overall efficiency.Four kinds of surfactants are studied in this work.Only potassium perfluorobutyl sulfonate(PPFBS),which has the structural similarity to Nafion,shows a significant promotion of activity and stability for both hygrogen evolution reaction(HER)and oxygen evolution reaction(OER)in the acidic medium at the high current density region.The HER overpotential at 0.1 A·cm−2 decreased 22%,and the current density at−0.4 V increased 31%by adding PPFBS.The promotion of overall efficiency by PPFBS on a homemade PEMWE was also proven.The reduced surface tension and electrostatic repulsion were the probable origins of the accelerated bubble detachment.

Boosting the oxygen evolution reaction performance of wrinkled Mn(OH)_(2) via conductive activation with a carbon binder

Electrochemical water splitting is one of the most reliable approaches for environmental-friendly hydrogen production.Because of their stability and abundance,Mn-based materials have been studied as electrocatalysts for the oxygen evolution reaction(OER),which is a more sluggish reaction in the water splitting system.To increase the OER activity of Mn,it is imperative to facilitate the structural change of Mn oxide to the active phase with Mn_(3)+species,known as the active site.Here,we present the relationship between the electronic conductivity in the catalyst layer and the formation of the Mn active phase,δ-MnO_(2),from wrinkled Mn(OH)_(2).Mn(OH)_(2) has poor conductivity,and it disrupts the oxidation reaction toward MnOOH orδ-MnO_(2).Adjacent conductive carbon to Mn(OH)_(2) enabled Mn(OH)_(2) to be oxidized toδ-MnO_(2).Furthermore,after repetitive cyclic voltammetry activation,the more conductive environment resulted in a higher density ofδ-MnO_(2) through the irreversible phase transition,and thus it contributes to the improvement of the OER activity.

Stabilizing high-efficiency iridium single atoms via lattice confinement for acidic oxygen evolution

Stable and efficient single atom catalysts(SACs)are highly desirable yet challenging in catalyzing acidic oxygen evolution reaction(OER).Herein,we report a novel iridium single atom catalyst structure,with atomic Ir doped in tetragonal PdO matrix(IrSAs-PdO)via a lattice-confined strategy.The optimized IrSAs-PdO-0.10 exhibited remarkable OER activity with an overpotential of 277 mV at 10 mA·cm^(-2) and long-term stability of 1000 h in 0.5 M H_(2)SO_(4).Furthermore,the turnover frequency attains 1.6 s^(-1) at an overpotential of 300 mV with a 24-fold increase in the intrinsic activity.The high activity originates from isolated iridium sites with low valence states and decreased Ir–O bonding covalency,and the excellent stability is a result of the effective confinement of iridium sites by Ir–O–Pd motifs.Moreover,we demonstrated for the first time that SACs have great potential in realizing ultralow loading of iridium(as low as microgram per square center meter level)in a practical water electrolyzer.

Corrosion Investigation by Scanning Electrochemical Microscopy of AISI 446 and Ti-Coated AISI 446 Ferritic Stainless Steel as Potential Material for Bipolar Plate in PEMWE

The components of proton exchange membrane water electrolysers frequently experience corrosion issues, especially at high anodic polarization, that restrict the use of more affordable alternatives to titanium. Here, we investigate localized corrosion processes of bare and Ti-coated AISI 446 ferritic stainless steel under anodic polarization by scanning electrochemical microscopy (SECM) in sodium sulphate and potassium chloride solutions. SECM approach curves and area scans measured at open-circuit potential (OCP) of the samples in the feedback mode using a redox mediator evidence a negative feedback effect caused by the surface passive film. For the anodic polarization of the sample, the substrate generation-tip collection mode enables to observe local generation of iron (II) ions, as well as formation of molecular oxygen. For the uncoated AISI 446 sample, localized corrosion is detected in sodium sulphate solution simultaneously with oxygen formation at anodic potentials of 1.0 V vs. Ag/AgCl, whereas significant pitting corrosion is observed even at 0.2 V vs. Ag/AgCl in potassium chloride solution. The Ti-coated AISI 446 sample reveals enhanced corrosion resistance in both test solutions, without any evidence of iron (II) ions generation at anodic potentials of 1.2 V vs. Ag/AgCl, where only oxygen formation is observed.

Supporting IrO_(x)nanosheets on hollow TiO_(2)for highly efficient acidic water splitting

The efficiency of proton exchange membrane water electrolysis(PEM-WE)for hydrogen production is heavily dependent on the noble metal iridium-based catalysts.However,the scarcity of iridium limits the large-scale application of PEM-WE.To address this issue,it is promising to select an appropriate support because it not only enhances the utilization efficiency of noble metals but also improves mass transport under high current.Herein,we supported amorphous IrO_(x)nanosheets onto the hollow TiO_(2)sphere(denoted as IrO_(x)),which demonstrated excellent performance in acidic electrolytic water splitting.Specifically,the annealed IrO_(x)catalyst at 150℃in air exhibited a mass activity of 1347.5 A·gIr^(−1),which is much higher than that of commercial IrO_(2)of 12.33 A·gIr^(−1)at the overpotential of 300 mV for oxygen evolution reaction(OER).Meanwhile,the annealed IrO_(x)exhibited good stability for 600 h operating at 10 mA·cm^(−2).Moreover,when using IrO_(x)and annealed IrO_(x)catalysts for water splitting,a cell voltage as low as 1.485 V can be achieved at 10 mA·cm^(−2).The cell can continuously operate for 200 h with negligible degradation of performance.

Supported Cu/Ni Bimetallic Cluster Electrocatalysts Boost CO_(2) Reduction

Supported metal clusters with the integrated advantages of single-atom catalysts and conventional nanoparticles held great promise in the electrocatalytic carbon dioxide reduction(ECO_(2)R)operated at low overpotential and high current density.However,its precise synthesis and the understanding of synergisti-cally catalytic effects remain challenging.Herein,we report a facile method to synthesize the bimetallic Cu and Ni clusters anchored on porous carbon(Cu/Ni-NC)and achieve an enhanced ECO_(2)R.The aberration-corrected high-angle annular dark-field scanning transmission electron microscopy and synchrotron X-ray absorption spectroscopy were employed to verify the metal dispersion and the coordination of Cu/Ni clusters on NC.As a result of this route,the target Cu/Ni-NC exhibits excellent electrocatalytic performance including a stable 30 h electrolysis at 200 mA cm^(-2) with carbon monoxide Faradaic efficiency of∼95.1%using a membrane electrode assembly electrolysis cell.Combined with the in situ analysis of the surface-enhanced Fourier transform infrared spectroelectrochemistry,we propose that the synergistic effects between Ni and Cu can effectively promote the H_(2)O dissociation,thereby accelerate the hydrogenation of CO_(2)to*COOH and the overall reaction process.

Materials Engineering toward Durable Ru-Based Electrocatalysts for Acidic Oxygen Evolution Reaction

Proton exchange membrane water electrolysis(PEMWE)is considered one of the most promising pathways for producing green hydrogen(H2).However,the sluggish kinetic of the anodic oxygen evolution reaction(OER)hinders the overall efficiency of PEMWE.In the past few decades,ruthenium(Ru)-based materials have been developed as highly active and cost-effective OER catalysts while faced with significant durability challenges.To this end,addressing the durability issues of Ru catalysts is imperative for their practical employment in PEMWE.In this review,state-of-the-art advances in understanding the degradation mechanisms of Ru catalysts in acidic conditions are comprehensively discussed.Then,materials engineering strategies to mitigate degradation through the rational design of stable Ru-catalysts are highlighted.Finally,some prospects are provided in terms of exploring the long-term stability of Ru-based catalysts.This review is anticipated to foster a better understanding of Ru-based catalysts in acidic OER and work on novel strategies for the design of stable Ru-based materials.