Hydrogen bubble evolution from magnetized nickel wire electrode

How to reduce the bubble coverage of the electrode is one of the key issues in water electrolysis which is related to the reduction of energy consumption. In this work, the magnetized nickel electrode with 100 μm diameter is used as working electrode in hydrogen evolution reaction (HER) during alkaline water electrolysis (1 mol/L, KOH). According to the experimental observation, both of the bubble's diameter and number have decreased at the magnetized surface compared with the unmagnetized one. The B–H loop measurement shows that the residual magnetic field intensity (Br) of the magnetized nickel electrode is up to 0.03 T. It can be found from the numerical simulation result that the residual magnetic field exactly right brings the Lorentz force and magnetohydrodynamic (MHD) convection near the electrode surface, which play the role of helping hydrogen bubble release from the electrode, even if the external magnetic field is absent in experiment. The new finding may develop a new way of utilizing magnetic field in water electrolysis for gas product elimination and energy conservation.     

Effects of magnetic field on water electrolysis using foam electrodes

Alkaline water electrolysis using foam electrodes was performed under the influence of a uniform magnetic field. The motivation of doing this work is to combine magnetic field with foam electrode to see if it can get unexpected higher water electrolysis efficiency. The result shows that the energy consumption was reduced by about 3.4% and some unique gas producing properties of foam electrodes were exhibited under the parallel-to-electrode magnetic field. The magnetohydrodynamic (MHD) convection induced by the Lorentz force can accelerate the detachment of bubbles, which are capable to be generated both on the surface and in the interior of foam electrodes simultaneously. Due to the uneven distribution of Lorentz forces, a circulating flow was formed in the electrolyzer, which is special designed to make full use of the circulating flow. The flow scoured the inner space of the foam electrodes so that the void fraction was reduced and the reaction overpotential was decreased. It should be potential to significantly improve the energy efficiency of the hydrogen manufacturing via water electrolysis and provide new ideas for the engineering design of electrolyzer.     

Water electrolysis using plate electrodes in an electrode-paralleled non-uniform magnetic field

Compared with other ways to produce hydrogen, water electrolysis is the best way to obtain ultra-pure hydrogen, but its low energy efficiency greatly limits its wide application. It was proved that external magnetic field can reduce energy consumption, thereby increase electrolysis efficiency. Most of the researchers are focused on the impact of uniform magnetic field but few on a non-uniform one. To address the industrial operation reality, in our work, water electrolysis was operated using alkaline solution and plate electrodes in a non-uniform Magnetic field. The results show that a rotational flow on the vertical plane was formed by Lorentz force within the entire cell range. Although the entrainment effect of rotating flow made the cell full of microbubbles, the cell voltage was still reduced. By measuring the voltage difference of cathode side and anode side, we think that the bubble layer in the vicinity of the electrode surface matters the most among the sources of electric resistance. And the velocity distribution near the electrode was measured by PIV, it reveals that MHD flow is the dominant effect on the flow field of the cell. The results show that non-uniform magnetic field has potential merit in industrial electrolysis process.     

Porous electrode improving energy efficiency under electrode-normal magnetic field in water electrolysis

The porous electrodes (Ni, Cu) with 110 pores per inch (PPI) are adopted in water electrolysis for hydrogen production under normal-to-electrode magnetic field. The result shows that the voltage drop between electrodes can be reduced up to 2.5% under 0.9 T field, and the electric energy efficiency is improved correspondingly. Based on the numerical simulation method, the micro-magnetohydrodynamic (micro-MHD) convection induced by Lorentz force within the porous structure is found. The results showed that although the apparent current direction is parallel to magnetic field outside the porous electrode, the electric field may be distorted within the porous structure, and the Lorentz force is involved near the rib of the micro structure where the current is not parallel to the magnetic field any more. Micro-MHD plays the role of strengthening the mass transfer and facilitating bubble to eject from the porous structure, which results in the cell voltage decreasing. The combined application of porous electrode and magnetic field should be potential to further improve energy efficiency of water electrolysis for hydrogen production.     

The “tornado” effect induced by magnetohydrodynamic convection in an electrolytic cell with a wire-electrode

High energy consumption is the key problem to be solved in water electrolysis for hydrogen production. Imposing magnetic field during electrolysis is proved to be a research-worthy method to reduce the required electrical energy, since the magnetohydrodynamic convection can be induced without additional energy input. Considering the structure of commercial electrolyzers, the magnetic field perpendicular to the electrode surface is most likely to be applied in practical engineering. But there is still a lack of research on the gas-liquid two-phase flow in the electrolytic cell under this condition. To avoid mutual blocking between a large number of bubbles and obtain clear two-phase flow images of the electrolysis process, a wire electrode is used as cathode to generate hydrogen bubbles in this work. The cell voltage is obviously reduced by external magnetic field, and an interesting “bubble tornado” is formed under the action of induced magnetohydrodynamic convection. The numerical simulation results and theoretical analysis indicate that: (1) the formation of the bubble chain is caused by low-pressure region along the vertical axis; (2) the unstable low-pressure region is the key factor leading to the formation of continuously deformed bubble chain; (3) the bubble dispersion may be related to Kelvin-Helmholtz flow instability. We anticipate our work being a starting point for the application of magnetic field in practical engineering.     

Experimental investigation on bubble growth and detachment characteristics on vertical microelectrode surface under electrode-normal magnetic field in water electrolysis

The effects of current density and electrode-normal magnetic fields on the growth and detachment characteristics of a single bubble on vertical microelectrode surface have been investigated. A high-speed camera was used to capture the bubble evolution behavior and the bubble contact characteristic parameters were extracted and analyzed with OpenCV-Python program. The results reveal that an apparent bubble coalescence behavior occurs at low current densities and can be gradually inhibited with increasing current density. With the increase of current density, the bubble growth rate, departure diameter, working electrode potential and potential fluctuations increase, while the bubble growth time first increases and then decreases continuously. The upper microelectrode surface is more easily covered than the lower microelectrode surface. The whole microelectrode can be completely covered when the current density exceeds a certain limit with and without magnetic fields. The external magnetic fields can obviously promote the bubble detachment behavior within a relatively large current density range.     

The effect of a Lorentz-force-driven rotating flow on the detachment of gas bubbles from the electrode surface

The enhanced bubble detachment in water electrolysis due to Lorentz-forces is discussed for the case of mainly parallel electric and magnetic fields. Experiments and numerical simulations were carried out to assess the velocity and pressure distribution around single rigid spheres mimicking electrolytic bubbles on a horizontal electrode in the presence of a vertical magnetic field. Astigmatism particle tracking velocimetry delivered the three-dimensional flow field and a finite volume method was used for the computations. Formerly it was assumed that the flow-induced pressure decrease at the bubble's top caused the earlier detachment under magnetic field action. However, the experimental and numerical results obtained here demonstrate that this pressure decrease is too weak as to effectively change the detachment process. Finally, an alternative explanation for the observed bubble behavior is suggested: it might result from the comparatively strong global flow generated by the additive effect of a group of bubbles.     

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。     

The effect of magnetic and optic field in water electrolysis

Hydrogen production via water electrolysis was studied under the effect of magnetic and optical field. A diode solid state laser at blue, green and red were utilized as optical field source. Magnetic bar was employed as external magnetic field. The green laser has shown a greatest effect in hydrogen production due to its non-absorbance properties in the water. Thus its intensity of electrical field is high enough to dissociation of hydronium and hydroxide ions during orientation toward polarization of water. The potential to break the autoprotolysis and generate the auto-ionization is the mechanism of optical field to reveal the hydrogen production in water electrolysis. The magnetic field effect is more dominant to enhance the hydrogen production. The diamagnetic property of water has repelled the present of magnetic in water. Consequently the water splitting occurs due to the repulsive force induced by the external magnetic field. The magnetic distributed more homogenous in the water to involve more density of water molecule. As a result hydrogen production due to magnetic field is higher in comparison to optical field. However the combination both fields have generated superior effect whereby the hydrogen yields nine times higher in comparison to conventional water electrolysis.     

Detecting and modeling oxygen bubble evolution and detachment in proton exchange membrane water electrolyzers

In this work, we discuss the effect of multiphase flow dynamics on the performance of a PEM electrolyzer. We obtained images of a flow system consisting of O₂ and water at two stages of gas production: gas evolution via bubbles and gas exiting through the channels of a flow field. We processed the obtained images of bubble evolution with a MATLAB-based bubble detection and counting algorithm, and we found that the bubble detachment sizes remained invariant within a water flow range between 0.07 and 4.65 l h⁻¹. We measured an average bubble detachment radius of 22.47 μm. We applied a bubble force balance developed by van Helden et al. [1] to model the observed effect of water flow on bubble evolution, and we found that the bubble detachment radius is a weak function of water flow when the water flow is below 60 l h⁻¹. We found that the variables that affect the bubble detachment radius the strongest were the electrode's hydrophobicity and pore size.     

Effects of magnetic field and pulse potential on hydrogen production via water electrolysis

This experiment uses nickel electrodes and adds pulse potential and magnetic force when producing hydrogen via water electrolysis and explores how related parameters are affected by magnetohydrodynamics and pulse potential. Experiments showed that the Lorentz force of the magnetic field changes the direction of convective flow of the electrolyte, which affects the flow of bubbles during electrolysis. Adding a magnetic field increases the rate of current density by roughly 15% under a normal temperature, a distance of 2 mm between electrodes and a potential of 4 V. Pulse potential instantaneously increases current and accelerates both the movement of bubbles from the electrode surface and the mass transfer rate in the electrolyte, which lowers electrochemical polarization in the diffussion layer and further increases hydrogen production efficiency. When the duty cycle is 10% and the pulse on‐time is 10 ms, almost 88% of overall power is converted, and current density increases by 680 mA/cm², which is an increase of roughly 38%. Generally, pulse potential and magnetic field effects enhance each other when added under suitable pulse potential and basic potential. Copyright © 2013 John Wiley & Sons, Ltd.     

Investigation of alkaline water electrolysis performance for different cost effective electrodes under magnetic field

Alkaline water electrolysis is the easiest methods for hydrogen production because of their simplicity. Although the simplicity is an advantage; reducing the energy consumption and maintaining the durability and the safety of these systems are the main challenges. In this paper, alkaline water electrolysis system, that uses cost effective electrode materials and magnetic field effects are presented. Cost effective electrodes such as high carbon steel, 304 stainless steel, 316L low carbon steel and graphite material are used for the hydrogen production. After the selection of the best electrode pair, effects of magnetic field to hydrogen production and change of current density are investigated for KOH electrolytes in different concentrations (5 wt%, 10 wt% and 15 wt%). According to the experimental observations the direction of the Lorentz Force affects the hydrogen production and current density. When the Lorentz Force is directed upward, it enhances the hydrogen production for 5 wt% and 15 wt% KOH solution by almost 17%. The increase in current density for 5 wt%, 10 wt% and 15 wt% concentration is 19%, 5%, 13%, respectively. Forced convection in the magnetic field enhances the separation of gas bubbles from electrode surface. Downward directed Lorentz Force decreases hydrogen production and current density values significantly. For 5 wt%, 10 wt% and 15 wt% the hydrogen production decreases by 14%, 8%, 7%, respectively. Similarly, current density for downward directed Lorentz Force decreases by 11%, 7%, 4%, respectively.     

Ultrashort-pulse laser structured titanium surfaces with sputter-coated platinum catalyst as hydrogen evolution electrodes for alkaline water electrolysis

For the first time, we report on micro- and nanostructured Ti surfaces produced by ultrashort-pulse laser processing followed by sputter deposition of Pt aiming at efficient cathode electrodes for alkaline water electrolysis. We studied the laser processing-induced surface morphology, the elemental composition of the surface, the specific surface increase, the wetting behavior as well as the activity of the hydrogen evolution reaction. It is demonstrated that ultrashort-pulse laser structuring in combination with thin layer catalyst deposition can dramatically boost the performance of cathodes for the hydrogen evolution reaction due to the enormous increase in specific surface in combination with superhydrophilic and superwetting properties leading to a rapid gas bubble detachment.     

Condensation of a bubble train in immiscible liquids

We previously developed a theoretical envelope model for single bubbles condensing in immiscible liquids, in which the convection outside the bubble is conducted through boundary layers at the front of the bubble and through the wake at the rear while the bubble accelerates, and the convection is dominated by heat transfer through the wake all over the bubble while the bubble is enveloped by its own wake at decelerating. In this paper the envelop model is extended for bubble train condensing in immiscible liquids by assuming that the envelopment occurs from start, i.e., the bubble is enveloped by the previous bubble’s wake right after detachment from the nozzle. The experimental results for freon-113 and hexane bubbles condensing in water confirm the assumption for injection frequencies higher than 12 bubbles per second.     

Recent progress in alkaline water electrolysis for hydrogen production and applications

Alkaline water electrolysis is one of the easiest methods for hydrogen production, offering the advantage of simplicity. The challenges for widespread use of water electrolysis are to reduce energy consumption, cost and maintenance and to increase reliability, durability and safety. This literature review examines the current state of knowledge and technology of hydrogen production by water electrolysis and identifies areas where R&D effort is needed in order to improve this technology. Following an overview of the fundamentals of alkaline water electrolysis, an electrical circuit analogy of resistances in the electrolysis system is introduced. The resistances are classified into three categories, namely the electrical resistances, the reaction resistances and the transport resistances. This is followed by a thorough analysis of each of the resistances, by means of thermodynamics and kinetics, to provide a scientific guidance to minimising the resistance in order to achieve a greater efficiency of alkaline water electrolysis. The thermodynamic analysis defines various electrolysis efficiencies based on theoretical energy input and cell voltage, respectively. These efficiencies are then employed to compare different electrolysis cell designs and to identify the means to overcome the key resistances for efficiency improvement. The kinetic analysis reveals the dependence of reaction resistances on the alkaline concentration, ion transfer, and reaction sites on the electrode surface, the latter is determined by the electrode materials. A quantitative relationship between the cell voltage components and current density is established, which links all the resistances and manifests the importance of reaction resistances and bubble resistances. The important effect of gas bubbles formed on the electrode surface and the need to minimise the ion transport resistance are highlighted. The historical development and continuous improvement in the alkaline water electrolysis technology are examined and different water electrolysis technologies are systematically compared using a set of the practical parameters derived from the thermodynamic and kinetic analyses. In addition to the efficiency improvements, the needs for reduction in equipment and maintenance costs, and improvement in reliability and durability are also established. The future research needs are also discussed from the aspects of electrode materials, electrolyte additives and bubble management, serving as a comprehensive guide for continuous development of the water electrolysis technology.     

Numerical Study of MHD Natural Convection of Liquid Metal with Wall Effects

Three-dimensional natural convection of liquid metal under a uniform magnetic field in a cubic enclosure is numerically simulated. The opposite sides of the cubic enclosure are isothermal, and the other four walls are adiabatic. The temperature gradient is perpendicular to the gravity vector. Three different magnetic field directions are applied in the computing model, respectively; namely, parallel to the temperature gradient, parallel to the gravity vector, and mutually perpendicular to them. Different wall electrical conductivities are also adopted in the simulation. Based on the Bousssinesq assumption, the numerical simulation of natural convection with Rayleigh number of 10⁵, 25,000, and 1000, Prandtl number (physical property of Pb-17Li under 573 K) of 0.0321, and Hartmann number of 100, 50, and 10 are performed using a projection method to simulate the incompressible liquid metal flow and a consistent and conservative scheme to calculate the electromagnetic force at a nonuniform rectangular collocated grid. The influences of magnetic field directions and wall electrical conductivities on MHD flows are also studied.     

Numerical Investigation of the Magnetic Field Effects on the Entropy Generation and Heat Transfer in a Nanofluid Filled Cavity with Natural Convection

This paper presents a numerical investigation of the entropy generation and heat transfer in a ferrofluid (water and 4% Fe₃O₄ nanoparticles) filled cavity with natural convection using a two phase mixture model and control volume technique. The effect of applying a nonuniform magnetic field on the entropy generation and heat transfer in the cavity and also the interaction of magnetic force and the buoyancy force are investigated. Based on the obtained results, applying a magnetic field will enhance the heat transfer mechanism. Furthermore, by applying the nonuniform magnetic field on the ferrofluid filled cavity with natural convection, the total entropy generation is decreased considerably at higher Rayleigh numbers. Therefore, applying a magnetic field can be considered as a suitable method for entropy generation minimization in order to have high efficiency in the system.     

Water management in membrane electrolysis and options for advanced plants

The development of polymer electrolyte membrane electrolysis (PEMEL) is driven by increasing performance to decrease the costs of electrolysis systems. One option for increasing power density is decreasing the Ohmic losses within the cell. This can be enabled by using thinner membranes, although the disadvantage of thin membranes is their lower diffusion resistivity for water, hydrogen and oxygen what influences the efficiency and the operating conditions. In this paper the water transport and the Ohmic resistance of catalyst coated membranes with different thickness are analyzed. The disadvantage of high water permeability in thin membranes can be used to change the feed configuration in stacks and systems. It is possible to feed the electrolysis only from the cathode, which simplifies the mass transport (single phase) in the anode's porous transport layer and reducing stack and system dimensions, as well as costs.     

Study on the behavior of single oxygen bubble regulated by salt concentration in photoelectrochemical water splitting

The increase of electrolyte resistance near the electrode due to bubble evolution is the main obstacle to significantly improve the efficiency of photoelectrochemical water splitting. The oxygen bubble evolution on the TiO₂ nanorod photocatalytic electrode was observed in situ by micro high-speed camera, and its corresponding current curve was measured synchronously by electrochemical workstation. The effect of single oxygen bubble evolution on current under different Na₂SO₄ electrolyte salt concentrations (0.01 M–1.0 M) was studied. When the salt concentration increases from 0.01 M to 0.8 M, the current in the bubble growth stage increases by about 2.5 times, while the bubble period and detachment diameter decrease by about 78.7% and 28.6%, respectively. When the salt concentration further increases, the current and bubble detachment diameter and period remain almost unchanged. The bubble growth coefficient reaches the optimum at 0.5 M, while the gas production rate reaches the optimum at 0.6 M. As the increase of electrolyte salt concentration, the increase rate of photocurrent density is lower than that of mass transfer coefficient, resulting in the decrease of solutal Marangoni force and the smaller bubble detachment diameter. The results show that regulating the concentration of electrolyte is an effective method to remove bubbles on the photocatalytic electrode.     

Investigation on PEM water electrolysis cell design and components for a HyCon solar hydrogen generator

Hydrogen as a secondary energy carrier promises a large potential as a long term storage for fluctuating renewable energies. In this sense a highly efficient solar hydrogen generation is of great interest especially in southern countries having high solar irradiation. The patented Hydrogen Concentrator (HyCon) concept yields high efficiencies combining multi-junction solar cells with proton exchange (PEM) membrane water electrolysis. In this work, a special PEM electrolysis cell for the HyCon concept was developed and investigated. It is shown that the purpose-made PEM cell shows a high performance using a titanium hybrid fiber sinter function both as a porous transport layer and flow field. The electrolysis cell shows a high performance with 1.83 V at 1 A/cm² and 24 °C working under natural convection with a commercially available catalyst coated membrane. A theoretical examination predicts a total efficiency for the HyCon module from sunlight to hydrogen of approximately 19.5% according to the higher heating value.