Debye relaxation mechanisms of microwave heating of near-critical polar gases

In this work, the average time of slow dielectric relaxation in sub- and supercritical polar gases is considered in terms of a combination of two Debye relaxation mechanisms. Under the assumption of thermal equilibrium, the corresponding relaxation times are theoretically predicted to be proportional to the mean binary collision time and mean translational deceleration time of free molecules with the proportionality factor close to unity. The microwave heating rate for pressurized pure gases and gas mixtures in constant-pressure and constant-density conditions is found from the Poynting formula. Temperature-dependent frequency spectra of the isobaric and isochoric heating rates for sub- and supercritical water are obtained with an allowance for non-Debye high-frequency relaxations. Perceptible temperature effects in the supercritical water heating rates at relaxation frequency are predicted when pressures/densities are high enough and considerable reduction of the heating time for higher pressures/densities is expected.     

High-performance and durable water electrolysis using a highly conductive and stable anion-exchange membrane

The commercialization of anion-exchange membrane water electrolysis (AEMWE) is crucial to produce low-cost and high-purity hydrogen. However, water electrolysis with an anion-exchange membrane (AEM) has limitations such as its low ionic conductivity and stability, leading to low performance and durability of AEMWE. In this study, we developed high-performance and stable AEMWE employing an AEM without aryl-ether backbone structure. To achieve high-performance and durable AEMWE, the effect of various parameters that is suitable for the adapted AEM was investigated. As a result, the AEM adapted in this work performed much better and was more durable than the conventional AEM (FAA-3). Moreover, it exhibited high efficiency under pure water feeding conditions. These results were attributed to high-efficient and durable AEM caused by its absence of aryl-ether backbone. This work suggests the potential use of polyphenylene structure as aryl-ether free backbone of AEM on AEMWE operated using alkaline solution and/or pure water.     

Acid mine drainage wastewater photoelectrolysis for hydrogen fuel generation: Preliminary results

Acid mine drainage (AMD) occurs when sulfide composite materials are exposed to oxygen gas, water, and microorganisms present in the environment. An alternative for the treatment of this residual water is the generation of hydrogen gas by electrolysis using a photovoltaic system. In this work, an electrolytic cell with 304 stainless steel electrodes was to form hydrogen gas. After 390 minutes of hydrogen generation, it was observed that the accumulated amount of H₂ was 254 mL when PV panels were used as the current source. When using AMD, hydrogen generation was 5.5 times higher compared to that using the sulfuric acid solution under the same experimental conditions. The electrolyte AMD conductivity diminished from 3850 μS cm⁻¹ to 2960 μS cm⁻¹. The decrease in conductivity may be related to removal of metal ions from the solution by the formation of insoluble compounds. After 390 minutes of testing, electrolysis using a PV panel resulted in a 9-fold increase in the total solid content. The reduction of iron and manganese ions in AMD samples was approximately 60% and 10%, respectively. No decrease in sulfate concentration and low pH variation were observed.     

Dense 1,2,4,5-tetramethylimidazolium-functionlized anion exchange membranes based on poly(aryl ether sulfone)s with high alkaline stability for water electrolysis

Anion exchange membranes with enough alkaline stability and ionic conductivity are essential for water electrolysis. In this work, a class of anion exchange membranes (PAES-TMI-x) with dense 1,2,4,5-tetramethylimidazolium side chains based on poly(aryl ether sulfone)s are prepared by aromatic nucleophilic polycondensation, radical substitution and Menshutkin reaction. Their chemical structure and hydrophilic/hydrophobic phase morphology are characterized by hydrogen nuclear magnetic resonance (¹H NMR) and atomic force microscope (AFM), respectively. The water uptake, swelling ratio and ionic conductivity for PAES-TMI-x are in the range of 23.8%–48.3%, 8.3%–14.3% and 18.22–96.31 mS/cm, respectively. These AEMs exhibit high alkaline stability, and the ionic conductivity for PAES-TMI-0.25 remains 86.8% after soaking in 2 M NaOH solution at 80 °C for 480 h. The current density of 1205 mA/cm² is obtained for the water electrolyzer equipped with PAES-TMI-0.25 in 2 M NaOH solution at 2.0 V and 80 °C, and the electrolyzer also has good operation stability at current density of 500 mA/cm². This work is expected to provide a valuable reference for the selection and design of cations in high-performance AEMs for water electrolysis.     

A highly efficient hydrogen generation electrolysis system using alkaline zinc hydroxide solution

Alkaline water electrolysis is a well-established conventional technique for hydrogen production. However, due to its relatively high energy consumption, the cost of hydrogen produced by this technique is still high. Here in this work, we report for the first time the application of alkaline zinc hydroxide solution (composed of sodium zincate and potassium zincate in NaOH and KOH solutions, respectively) as an efficient, simple and recursive electrolyte for producing clean hydrogen through a continuous dual-step electrolysis process. The ionic conductivity, electrodes current density, and hydrogen evolution rate were measured in a wide range of the electrolyte concentrations (0.1–0.59 M). Also, the cell efficiency was studied at different ranges of current density (0.09–0.25 A/cm2) and applied potential (1.8–2.2 V). Results indicated that the application of alkaline zinc hydroxide solution at the optimum electrolyte concentration can enhance the hydrogen evolution rate minimally by a factor of 2.74 (using sodium zincate) and 1.47 (using potassium zincate) compared to the conventional alkaline water electrolysers. The results of this study could be helpful to better understand the electrochemical behaviour of the alkaline water electrolysers when sodium zincate and potassium zincate are used as ionic activators for enhancing hydrogen evolution.     

Critical aspects in the development of anodes for use in seawater electrolysis

The global water crisis limits the full implementation of purified water electrolysis across the world. Thus, seawater electrolysis has been identified as a powerful option to meet the requirements for sustainable production of green hydrogen without the constraint of using pure water. As in pure water electrolysis, seawater electrolysis research has been oriented to produce a durable, electrocatalytic, and selective anode.Even though seawater electrolysis was proposed for the first time in 1980, the research found on this topic grew exponentially in the last few years. Nevertheless, researchers do not give a clear insight about the impact that important variables have in the process of seawater electrolysis and, in particular, the anodic process. In this work, an in-depth literature review on articles reporting the development of various anode materials and the conditions in which they have been tested was carried out. The conclusions reveal a need to standardize some parameters for testing the anodes, such as simulated seawater composition, pH of the solution, the method through which parasitic reactions are measured and the choice of secondary reactions to be considered in the process. A standardization of these parameters will allow researchers to compare results, which in turn will allow collaborative work towards the goal of finding a feasible process for seawater electrolysis.     

预磁极化条件下PEM水电解制氢特性与效率研究

为提高质子交换膜（proton exchange membrane,PEM）水电解制氢速率、降低电解所需能耗,针对磁场预极化条件下蒸馏水的分子极性和应力特性进行研究,通过构建磁场环境下氢质子的能级跃迁微观物理模型与磁化矢量——极化氢质子浓度对应的宏观数学模型,对不同磁场强度下电解液的离子电导率、电流密度和制氢速率进行定性和定量分析,并利用自主搭建的可调节预磁极化PEM水电解制氢试验平台对所提出方法的有效性进行重复试验。试验结果表明,经过预磁极化处理的蒸馏水电导率提高了2~3倍,且随着磁场强度的增加,PEM电解电流密度不断增大,极间电圧不断减小,制氢速率明显提升。     

Assessment of the FAA3-50 polymer electrolyte in combination with a NiMn2O4 anode catalyst for anion exchange membrane water electrolysis

A commercial FAA3-50 membrane was investigated as a solid polymer electrolyte in an alkaline water electrolysis. An improved chemical treatment based on alkaline KOH solution was carried out. A limited degradation of the functional groups was observed allowing to maintain a good anion conductivity approaching 55 mS cm-1 at 100 °C. Thermal stability up to 200 °C was assessed by thermal analysis.A specific membrane-electrodes assembly based on FAA3-50 anionic membrane and NiMn₂O₄ anode catalyst was developed and investigated in a single cell for water electrolysis at a moderate temperature (50 °C).Performance stability was assessed by a potential cycling-based durability test for 1000 h by varying the cell potential between 1 and 1.8 V for the FAA3-50 and NiMn₂O₄ based-MEA.According to this evaluation, both the FAA3-50 membrane and the NiMn₂O₄ catalyst appear sufficiently stable for electrolysis operation under mild operating temperatures.     

Optimal operating conditions evaluation of an anion-exchange-membrane electrolyzer based on FUMASEP® FAA3-50 membrane

Hydrogen production via water electrolysis is considered the “greenest” way because it does not produce any direct carbon emissions when powered by renewable sources. Among the different technologies of electrolyzers, increasing interest is registered by that one based on anion-exchange membranes (AEMs). In this work, a FAA3-50 anion-exchange membrane (from FuMa-Tech) is used, after the KOH solution (1 M) exchange, as electrolyte/separator in an electrolysis cell of 5 cm² geometrical area. Commercial IrO₂ and 40% Pt/C catalysts are used at the anode and cathode, respectively, to evaluate the membrane under the most convenient conditions. The influence of cell temperature, membrane-electrode assembly (MEA) procedure (catalyst-coated membrane or catalyst coated electrode), and pure water or KOH solution on electrolyzer performance are analyzed. It appears that the catalyst-coated membrane approach, using the FAA3-50 membrane, allows higher temperature operation. However, diluted KOH solution is necessary to increase the membrane conductivity and the cell performance.     

Application of green hydrogen with theoretical and empirical approaches of alkaline water electrolysis: Life cycle-based techno economic and environmental assessments of renewable urea synthesis

Alkaline water electrolysis which is the most commercialized and mature technology of water electrolysis was researched to improve performance by the Korea Institute of Energy Research (KIER). In line with the trend of energy shift, renewable urea production through hydrogen production from alkaline water electrolysis was proposed in this work. To validate the process modeling of renewable urea production and hydrogen performance analysis with I–V curves was assessed. Economic and life cycle assessments were conducted to provide quantitative guidelines for renewable urea production. Absolutely, the influential factor of unit urea production cost was hydrogen from alkaline water electrolysis and environmental assessment results as well. Moreover, the guidelines for renewable urea production were provided through cost estimation and life cycle assessment. In summary, hydrogen production from alkaline water electrolysis had a significant impact on urea production and for this reason, research on alkaline water electrolysis should continue for further development.     

Efficiency of hydrogen production systems using alternative nuclear energy technologies

Nuclear energy can be used as the primary energy source in centralized hydrogen production through high-temperature thermochemical processes, water electrolysis, or high-temperature steam electrolysis. Energy efficiency is important in providing hydrogen economically and in a climate friendly manner. High operating temperatures are needed for more efficient thermochemical and electrochemical hydrogen production using nuclear energy. Therefore, high-temperature reactors, such as the gas-cooled, molten-salt-cooled and liquid-metal-cooled reactor technologies, are the candidates for use in hydrogen production. Several candidate technologies that span the range from well developed to conceptual are compared in our analysis. Among these alternatives, high-temperature steam electrolysis (HTSE) coupled to an advanced gas reactor cooled by supercritical CO₂ (S-CO₂) and equipped with a supercritical CO₂ power conversion cycle has the potential to provide higher energy efficiency at a lower temperature range than the other alternatives.     

Recent development in electrocatalysts for hydrogen production through water electrolysis

Hydrogen is a carbon-free alternative energy source for use in future energy frameworks with the advantages of environment-friendliness and high energy density. Among the numerous hydrogen production techniques, sustainable and high purity of hydrogen can be achieved by water electrolysis. Therefore, developing electrocatalysts for water electrolysis is an emerging field with great importance to the scientific community. On one hand, precious metals are typically used to study the two-half cell reactions, i.e., hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). However, precious metals (i.e., Pt, Au, Ru, Ag, etc.) as electrocatalysts are expensive and with low availability, which inhibits their practical application. Non-precious metal-based electrocatalysts on the other hand are abundant with low-cost and eco-friendliness and exhibit high electrical conductivity and electrocatalytic performance equivalent to those for noble metals. Thus, these electrocatalysts can replace precious materials in the water electrolysis process. However, considerable research effort must be devoted to the development of these cost-effective and efficient non-precious electrocatalysts. In this review article, we provide key fundamental knowledge of water electrolysis, progress, and challenges of the development of most-studied electrocatalysts in the most desirable electrolytic solutions: alkaline water electrolysis (AWE), solid-oxide electrolysis (SOE), and proton exchange membrane electrolysis (PEME). Lastly, we discuss remaining grand challenges, prospect, and future work with key recommendations that must be done prior to the full commercialization of water electrolysis systems.     

Polymer anion-selective membrane for electrolytic water splitting: The impact of a liquid electrolyte composition on the process parameters and long-term stability

In the present study the impact of KOH replacement by a NaHCO₃ or Na₂CO₃ solution on the performance of alkaline water electrolysis and the life-time of an anion-selective polymer electrolyte was assessed. In the first instance, the impact of the electrolyte composition on the kinetics of the electrode reactions was studied. Subsequently the ionic conductivity of the membrane, in the form of individual anions, and the efficiency of the alkaline water electrolysis process were evaluated by means of electrochemical impedance spectroscopy and a laboratory single-cell alkaline water electrolyzer. The anion used was observed to make a significant impact both on the ionic conductivity as well as on the kinetics of the anode reaction, resulting in reduced electrolysis efficiency. A stability test revealed kinetics of chemical degradation of the polymer anion-selective membrane in a Na₂CO₃ solution similar to those in a KOH environment at 70 °C. An alternative approach of decreasing the temperature to 50 °C prolonged the chemical stability and made less impact on the process efficiency.     

Supercritical water gasification of glucose in fluidized bed reactor: A numerical study

Supercritical water fluidized bed (SCWFB) is a new reactor concept for gasification of biomass and coal in supercritical water. In this paper, physical fields, residence time and gas yield in a SCWFB reactor were investigated numerically based on the Eulerian two-fluid method with the kinetic theory of granular flow. A three-step reaction model including steam reforming, water-gas shift and methanation was used to describe the supercritical water gasification of glucose. Distributions of velocity and temperature were obtained, and the results show that the mixing of preheated water and cold glucose solution at the bottom in the bed leads to a region with low solid volume fraction and local swirl flow. In the freeboard, most of reactants flow near the wall and with a velocity much higher than the superficial velocity. The reaction rates and conversion ratio of glucose at different regions in the reactor were also obtained. Distribution of residence time was found to be non-uniform, and its effect on glucose gasification was analyzed. In addition, the effects of operation condition and reactor structure on gas yield and residence time were studied to explore best operation rules for increasing gas yield. The results from this work may be of interest to operators attempting to obtain more information in the reactor and provide instruction for the design of SCWFB reactor.     

Separators for alkaline water electrolysis prepared by plasma-initiated grafting of acrylic acid on microporous polypropylene membranes

Highly hydrophilic separators for alkaline water electrolysis were prepared by plasma-initiated grafting of acrylic acid on porous polypropylene (PP) membranes. The membranes were activated in a low-pressure radio-frequency discharge in oxygen and subsequently graft polymerization of acrylic acid was performed in aqueous solution. The membranes were characterized by gravimetric grafting degree (GD), SEM, FTIR, critical wetting surface tension (CWST) test, mechanical strength, and electrolytic conductivity. Moreover, the membranes were applied as separators in alkaline electrolysis cell, and content of hydrogen in the produced oxygen was measured to determine membrane permeability to hydrogen dissolved in the electrolyte. It was observed that increasing GD improves performance of membranes as separators in alkaline electrolysis, although the particular effects on the electrolytic conductivity and hydrogen permeability strongly depend on structure the of initial PP substrate. Ageing test conducted in 30 wt% KOH at 60 °C revealed that although considerable degrafting took place at beginning of the test, the remaining polyacrylic acid provided highly hydrophilic character to membrane for 7000 h of the test.     

Linear sweep voltammetry measurements and factorial design model of hydrogen production by HCl/CuCl electrolysis

CuCl electrolysis is considered a key process in the Cu–Cl cycle of hydrogen production where H₂ gas is produced by oxidation of CuCl particles dissolved in concentrated HCl solution. A lower electrochemical cell voltage than water electrolysis is a significant advantage of CuCl electrolysis and makes this process attractive for hydrogen production. In order to achieve integration of the electrolysis process with hydrolysis and decomposition reactions of the Cu–Cl cycle, experimental development of this process and analysis of operational factors are presented in this paper. A lab-scale CuCl electrolysis unit is fabricated and the influence of operating parameters including HCl and CuCl concentrations, applied current density, temperature, and solution flow rates on the cell potential and hydrogen production rate are experimentally investigated and analyzed. The present test bench and measurement techniques are described for characterization of the experiment. A fractional factorial design is performed, based on design of experiment methods, to find a correlation between cell voltage and operation factors. The present model predicts the effects of various operating variables on the cell voltage to provide new insight into integration of the electrolysis process. Close agreement between measured and theoretical hydrogen production rate, with the current efficiency of about 93%, indicates high accuracy of the experimental tests.     

Hydrogen production by methanol aqueous electrolysis using photovoltaic energy: Algerian potential

Hydrogen is considered as the most promising energy carrier for providing a clean, reliable and sustainable energy system. It can be produced from a diverse array of potential feed stocks including water, fossil fuels and organic matter. Electrolysis is the best option for producing hydrogen very quickly and conveniently. Water electrolysis as a source of hydrogen production has recently gained much attention since it can produce high purity hydrogen and can be compatible with renewable energies. Besides the water electrolysis, aqueous methanol electrolysis has been reported in several studies. The aqueous methanol electrolysis proceeds at much lower voltage than that with the water electrolysis. As a result of the substantially lower operating voltage, the energy efficiency for methanol electrolysis can be higher than that for water electrolysis. In this paper, we are interesting to methanol electrolysis in order to produce hydrogen. The relation linking hydrogen production rate to the power needed to electrolyse a unit volume of aqueous methanol solution has been determined. Using this relation, the potential of hydrogen from aqueous methanol solution using a PV solar as the energy system has been evaluated for different locations in Algeria.     

Spatially decoupled hydrogen evolution in alkaline conditions with a redox targeting-based flow battery

Conventional water electrolyzer simultaneously generates H₂ and O₂ in neighboring electrode compartments which raises various issues by gas mixing. Thus, decoupled water electrolysis is desired to avoid the problems. Motivated by the above, spatially decoupled hydrogen evolution reaction (HER) is demonstrated in this work, leveraging a redox targeting-based flow battery employing iron triethanolamine (Fe(III)TEA)-K₂MnO₄ redox couple. This system spatially separates HER from the electrode compartment. Sustained hydrogen is achieved by reaction between regenerated Fe(II)TEA and water in the negative electrolyte storage tank. The Faradaic efficiency of the decoupled HER is 70% and keeps stable upon prolonged operation. The onset overpotential of HER is 24 mV which is lower than previous studies. We anticipate the approach presented in this work would offer an alternative solution to water electrolysis for scalable and safe hydrogen production with improved gas purity.     

Thermodynamic analysis of models used in hydrogen production by geothermal energy

Four models are developed for the use of geothermal energy for hydrogen production. These include using geothermal work output as the work input for an electrolysis process (Case 1); using part of geothermal heat to produce work for electrolysis process and part of geothermal heat in an electrolysis process to preheat the water (Case 2), using geothermal heat to preheat water in a high-temperature electrolysis process (Case 3), and using part of geothermal work for electrolysis and the remaining part for liquefaction (Case 4). These models are studied thermodynamically, and both reversible and actual (irreversible) operation of the models are considered. The effect of geothermal water temperature on the amount of hydrogen production per unit mass of geothermal water is investigated for all four models, and the results are compared. The results show that as the temperature of geothermal water increases the amount of hydrogen production increases. Also, 1.34 g of hydrogen may be produced by one kg of geothermal water at 200 °C in the reversible operation for Case 1. The corresponding values are 1.42, 1.91, and 1.22 in Case 2, Case 3, and Case 4, respectively. Greater amounts of hydrogen may be produced in Case 3 compared to other cases. Case 2 performs better than Case 1 because of the enhanced use of geothermal resource in the process. Case 4 allows both hydrogen production and liquefaction using the same geothermal resource, and provides a good solution for the remote geothermal resources. A comparison of hydrogen production values in the reversible and irreversible conditions reveal that the second-law efficiencies of the models are 28.5%, 29.9%, 37.2%, and 16.1% in Case 1, Case 2, Case 3, and Case 4, respectively.