Understanding the kinetics of coal electrolysis at intermediate temperatures

The kinetics of coal electrolysis was studied on a bench continuous coal electrolytic cell setup at intermediate temperatures by applying galvanostatic polarization techniques. The results showed that coal oxidation takes place during the contact of the coal particles with the electrode and it is directly related to electrode composition and the presence of Fe(III) ions in the slurry solution. Coal electro-oxidation by Fe(III) is the limiting step in the oxidation of a 0.02 g mL⁻¹ coal slurry containing 100 mM Fe(II)/Fe(III) at high currents (100 mA) at 108 °C with a 65 mL min⁻¹ flow rate. The study suggests that the films that grow at the surface of the coal particles limit the coal conversion. A mechanism is proposed to describe the coal oxidation process.     

Feasibility of hydrogen production from coal electrolysis at intermediate temperatures

Hydrogen production via coal electrolysis was evaluated at intermediate temperatures (40–108 °C). A coal electrolytic cell (CEC) was designed and constructed to carry out galvanostatic experiments with concentrated electrolyte (4 M H₂SO₄). Operating temperatures above 100 °C were found to significantly improve the kinetics of electro-oxidation of coal, coal conversion, and CO₂/coal Faradaic efficiency. CO₂/coal Faradaic efficiencies and coal conversions of up to 57.29 and 3.21%, respectively, were observed at 108 °C.     

Performance and durability of metal-supported solid oxide electrolysis cells at intermediate temperatures

Metal-supported solid oxide electrolysis cells (MS-SOECs) operating at 600–700 °C are attractive for storage of intermittent renewable electricity from solar and wind energy due to their advantages of easy sealing and fast startup. This paper reports on the fabrication of MS-SOECs consisting of dense scandium stabilized zirconia (SSZ) electrolytes, Ce_0.8Sm_0.2O_2−δ (SDC)/Ni impregnated 430L/SSZ cathodes and SmBa_0.5Sr_0.5Co₂O_5+δ (SBSCO) impregnated SSZ anodes supported on porous 430L alloys. Such cells demonstrated excellent electrolysis performance with current densities at 650 °C as high as 0.73 A⋅cm⁻² at 1.3 V in 50% H₂O-50% H₂ and 0.95 A⋅cm⁻² at 1.5 V in 90% CO₂-10% CO. Electrochemical impedance measurements indicated that the cell performance was largely limited by the ohmic losses for steam electrolysis and by the cathodic reduction reactions for CO₂ electrolysis, especially at reduced temperatures. Pronounced degradation was observed for both steam and CO₂ electrolysis over the preliminary 90-h stability measurements at 600 °C. SEM examination and EDS mapping of measured cells showed significant aggregation and coarsening of impregnated Ni particles, resulting in smaller activities for H₂O and CO₂ reduction reactions. As evidenced by the almost unaltered ohmic resistances over the measurement durations, the 430L stainless steel substrates demonstrate excellent resistances against corrosions from H₂O and CO₂ and thus show great promise for applications in reduced-temperature MS-SOECs.     

Nickel-cobalt-oxide cathodes for hydrogen production by water electrolysis in acidic and alkaline media

Mixed NiCo-oxide cathodes of various compositions were fabricated by a thermal-decomposition method and used as electrocatalysts for hydrogen production by water electrolysis in acidic and alkaline media. The oxide electrodes were found to be of a semi-crystalline structure, yielding the surface morphology characterized by a surface roughness factor going up to 25. Linear potentiodynamic and potentiostatic electrochemical measurements revealed that the Volmer reaction step controlled the kinetics of the hydrogen evolution on all the NiCo-oxide cathodes, and also on the pure metal Ni electrode (control). The Ni_0.2Co_0.8-oxide was identified as the best electrode material candidate among the investigated metal oxides, which was linked to the surface-area effect. However, its intrinsic activity was found to be lower than that of pure metallic Ni. Nevertheless, the Ni_0.2Co_0.8-oxide electrode showed a significantly higher electrocatalytic stability (fouling/deactivation tolerance) in comparison to metallic Ni.     

High efficiency water electrolysis in alkaline solution

Methods of increasing the efficiency and lowering the capital cost of state-of-the art pressure-type electrolyzers are briefly reviewed. A doubling of current density, accompanied by a halving of the internal resistance, and improved electrode activity are required. The latter may be obtained by catalysis and increases in effective surface area and temperature. An increase in the temperature to 120–130°C appears feasible without giving rise to materials problems provided asbestos separators are not used. All of the above are examined in detail, and it is shown that advanced electrodes with non-precious catalysts will be capable of operating at 1.55 V (I.R. included) at 0.4 A/cm², 120 C and 30–40 atm., which corresponds to isothermal operation.     

Improved components for advanced alkaline water electrolysis

The industrial realization of an advanced electrolysis cell for alkaline water electrolysis is gradually achieved and semitechnical cell units were constructed. The cells work with ceramic diaphragms and galvanically-deposited Raney nickel electrodes. The working temperature is 100–120°C and total pressure between 1 and 5 bar. The average energy consumption at 0.4 A cm⁻² and 100°C is 3.8 kWh m⁻³ hydrogen. The influence of cell components on total cell performance data is discussed. The variation of manufacturing parameters on the quality of single components (electrodes and diaphragm) is considered. Corrosion problems at operating conditions and their elimination by using proper construction techniques were investigated.     

Solar hydrogen production via alkaline water electrolysis

Electricity generation via direct conversion of solar energy with zero carbon dioxide emission is essential from the aspect of energy supply security as well as from the aspect of environmental protection. Therefore, this paper presents a system for hydrogen production via water electrolysis using a 960 Wp solar power plant. The results obtained from the monitoring of photovoltaic modules mounted in pairs on a fixed, a single-axis and a dual-axis solar tracker were examined to determine if there is a possibility to couple them with an electrolyzer. Energy performance of each photovoltaic system was recorded and analyzed during a period of one year, and the data were monitored on an online software service. Estimated parameters, such as monthly solar irradiance, solar electricity production, optimal angle, monthly ambient temperature, and capacity factor were compared to the observed data. In order to get energy efficiency as high as possible, a novel alkaline electrolyzer of bipolar design was constructed. Its design and operating UI characteristic are described. The operating UI characteristics of photovoltaic modules were tuned to the electrolyzer operating UI characteristic to maximize production. The calculated hydrogen rate of production was 1.138 g per hour. During the study the system produced 1.234 MWh of energy, with calculated of 1.31 MWh , which could power 122 houses, and has offset 906 kg of carbon or an equivalent of 23 trees.     

Corrosion study of nickel for alkaline water electrolysis

A study of the behaviour of nickel in the conditions of advanced electrolysis (130–180°C—40% KOH) shows the detrimental effect of oxygen in general corrosion. Corrosion can be limited by restricting sulfur content in nickel. No stress corrosion cracking is observed with nickel in KOH. Nickel appears to be more sensitive to cathodic hydrogen embrittlement than to gaseous hydrogen embrittlement.     

Optimal operating parameters for advanced alkaline water electrolysis

Advanced zero-gap alkaline electrolyzers can be operated at a significantly higher current density than traditional alkaline electrolyzers. We have investigated how their performance is influenced by diaphragm thickness, temperature and pressure. For this a semi-empirical current-voltage model has been developed based on experimental data of a 20 Nm³/h electrolyzer. The model was extrapolated to thinner diaphragm thicknesses and higher temperatures showing that a nominal current density of 1.8 A cm^?2 is possible with a 0.1 mm diaphragm at 100 °C. However, these operating parameters also lead to increased gas crossover, which limits the ability to operate at low loads. A gas crossover model has been developed, which shows that crossover is mainly driven by diffusive transport of hydrogen, caused by a high local supersaturation at the diaphragm surface. To enable a low minimum load of 10% the operating pressure should be kept below 8 bara.     

A bipolar cell for advanced alkaline water electrolysis

A new NiO diaphragm has been developed as a substitute for the conventional asbestos diaphragm used in alkaline water electrolysis. The NiO diaphragm has very promising corrosion and resistance properties and has demonstrated its good performance in long-time tests over a few thousand hours. For the production of highly active nickel electrodes, a galvanic method for the deposition of the activated Ni/Zn alloy was applied. By combining this new diaphragm and the nickel electrodes as construction elements, a bipolar cell in a sandwich arrangement was built, providing a very low cell voltage. With the described construction, further improvements of the anode should lead, in the future, to a cell which will operate at a higher heating value voltage and a current density of 500 mA cm^?2.     

Effect of pulse potential on alkaline water electrolysis performance

In this study, the effect of pulse potential on alkaline water electrolysis energy consumption is investigated. A specially designed electrical circuit is used to observe the effect of different duty cycles and frequency values on water electrolysis energy consumption in different concentration values of alkaline solution. The results show that using pulse potential enhances the mass transport of oxygen and hydrogen bubbles due to the pumping effect. This provides less contact with oxygen bubbles to improve corrosion resistance of anode electrodes. Moreover, decreasing mass transfer losses on the electrode surface resulted in a 20–25% lower energy consumption to produce 1 mol of hydrogen in the cell. The optimum frequency for 10% and 50% duty cycle and 10% and 15% concentration are investigated. For 10% duty cycle, the optimum frequency is specified around 140–200 kHz and for 50% duty cycle, it is around 380–400 kHz for all concentration values.     

The testing of electrodes for alkaline solution water electrolysis

The alkaline solution water electrolysis with the use of off-peak energy from nuclear power stations is proposed for hydrogen production in the initial stages of hydrogen energy system development. The energy consumption of this process could be reduced by the use of electrode improvements for both the anode and the cathode of bipolar electrolytic cells. These improvements comprise preparing an electrocatalytic surface layer, or the increase of an effective surface area by the use of Teflon-bonded electrodes or a Ni-layer on the asbestos diaphragm surface. The Teflon-bonded electrodes achieved the best results in the laboratory static test cell. In actual electrolytic cells difficulties with gas evolution arose. The simple and cheap electrode improvements, which were verified by the laboratory tests as well as by the test in the electrolyser module of 300 g H₂/h output were as follows: (1) Ni screen treated with a boiling solution of Na₂S in order to obtain a catalytically active surface layer of NiS_x, used as cathode; (2) Ni screen treated by thermal decomposition of Ni and Co nitrates in a molar ratio of 1:2 in order to obtain the spinel-like structure mixed oxide layer of NiO,CO₂O₃ as anode. This layer promotes resistance of the electrode surface to hot alkaline oxygen rich environments as well as the catalytic activity of the Ni screen electrode surface.     

Influence of process conditions on gas purity in alkaline water electrolysis

In this paper the influence of operating conditions on the product gas purity of a zero-gap alkaline water electrolyzer was examined. Precise knowledge of the resulting gas purity is of special importance to prevent safety shutdown when the electrolyzer is dynamically operated using a renewable energy source. The investigation in this study involves variation of temperature, electrolyte concentration and flow rate as well as different electrolyte management concepts. The experiments were carried out in a fully automated lab-scale electrolyzer with a 150 cm² zero-gap cell and approximately 31 wt% KOH at ambient and balanced cathodic and anodic pressure. The purity of the evolved gases was measured via online gas chromatography. It can be seen from the experiments that a temperature increase and flow rate decrease reduces the gas impurity when mixing catholyte and anolyte. A further reduction of gas impurity can be achieved when both cycles are being separated and a dynamic cycling strategy is applied.     

Multidimensional and transient modeling of an alkaline water electrolysis cell

A three-dimensional (3-D) transient numerical model of an alkaline water electrolysis (AWE) cell with potassium hydroxide solution is developed by rigorously accounting for the hydrogen and oxygen evolution reactions and resulting species and charge transport through various AWE components. First, the AWE model is experimentally validated against a polarization curve corresponding to a wide range of currents as high as 2.0 A·cm^?2. In general, the simulation results compare well with the measured data and further reveal the operating characteristics of AWE cells, showing key distributions of solid/electrolyte potentials and multidimensional contours of reactant and product concentrations at various current densities. In particular, the contribution of hydroxide ion (OH^?) diffusion to the ohmic losses through porous electrodes and a porous separator are quantitatively examined at low and high electrolyte flow rates. The present full 3-D AWE model provides a basic understanding of the electrochemical and transport phenomena and can be further applied to practical large-scale AWE cell and stack geometries for grid-scale hydrogen production.     

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.     

Laser structured nickel-iron electrodes for oxygen evolution in alkaline water electrolysis

In the present work, the ultra-short pulse laser ablation method is applied to create novel surface alloys on NiFe electrodes for the oxygen evolution reaction (OER) in alkaline water electrolysis. The nickel-to-iron ratio in the alloy can be controlled with the ultra-short pulse laser ablation method by varying the thickness of electrochemically deposited iron layers onto the nickel mesh substrate. Besides the application of the additional catalyst, the laser treatment enhances the surface area and a defined micro- and submicrometer structure is created in a single step. The laser structured nickel-iron electrodes show a significantly lower overpotential of 249 mV than an electrochemically deposited Ni-NiFe alloy with 292 mV at 10 mA cm⁻², 298 K and 32.5 wt% KOH for the OER, although some loss of iron over time could not be prevented.     

Electrochemical investigations on amorphous Fe-base alloys for alkaline water electrolysis

In this work different amorphous melt-spun Fe-alloys (Fe₈₂B₁₈, Fe₈₀Si₁₀B₁₀, Fe₆₀Co₂₀Si₁₀B₁₀) were investigated as cathode materials for the alkaline electrolysis of water. In particular, the influence of cobalt as well as the metalloids boron and silicon on the activity for the hydrogen evolution reaction (HER) was studied in 1 M KOH at 298 K using cyclic voltammetric, galvanostatic and polarization techniques. The electrocatalytic activity was evaluated in the view of the overpotential. It was found that cyclic voltammetric techniques can be used to activate the melt-spun Fe-alloys strongly. Different cyclic voltammetric activation procedures are discussed and the influence of the sweep rate and the potential window on the HER activity was elucidated. The experimental data indicate that the addition of metalloids and, most importantly, of cobalt improves the HER activity of the materials. Thus, the overpotential can be reduced by 200 mV compared to polycrystalline Ni.     

Structural dependence of hydrogen evolution reaction on transition metal catalysts sputtered at different temperatures in alkaline media

In the present article, we studied the catalytic activity of magnetron sputtered Mo, V, Ni, and Co thin films for hydrogen evolution reaction (HER) in the alkaline electrolyte. We find that the HER potentials (η₁₀) of the Mo and V thin film catalysts sputtered at 800 °C shift positive with respect to those of the film catalysts sputtered at 25 °C. For Mo metal the observed shift of η₁₀ was 280 mV and for V metal observed shift of η₁₀ was 390 mV. On the other hand, minimal effect of sputtering temperature on both Ni and Co thin film catalyst activity for HER was observed. Structural analysis reveals that Mo and V prepared at 800 °C have uncommon face centered cubic (fcc, 0.74 packing density) structure as opposed to room-temperature sputtered Mo and V thin films which have common body centered cubic (bcc, 0.68 packing density) structure, resulting in significant increases of the packing densities when they are prepared at 800 °C. On the other hand, the high-temperature prepared thin films of Ni and Co retained fcc structures, resulting in no density changes compared to the room-temperature prepared fcc Ni and hexagonal close packed (hcp, 0.74 packing density) Co. Impedance spectroscopy shows that fcc Mo is a better catalyst than fcc Ni, which is considered an industry standard for non-noble pure metal-based catalysts in alkaline media. Stability tests also suggest that fcc Mo thin film catalysts prepared at 800 °C are more durable than fcc Ni thin films. Our study points out that structure phases of catalysts can be a key factor governing the activities of transition metals for HER in alkaline media.     

Development of durable and efficient electrodes for large-scale alkaline water electrolysis

A new type of electrodes for alkaline water electrolysis is produced by physical vapour depositing (PVD) of aluminium onto a nickel substrate. The PVD Al/Ni is heat-treated to facilitate alloy formation followed by a selective aluminium alkaline leaching. The obtained porous Ni surface is uniform and characterized by a unique interlayer adhesion, which is critical for industrial application. IR-compensated polarisation curves prepared in a half-cell setup with 1 M KOH electrolyte at room temperature reveals that at least 400 mV less potential is needed to decompose water into hydrogen and oxygen with the developed porous PVD Al/Ni electrodes as compared to solid nickel electrodes. High-resolution scanning electron microscope (HR-SEM) micrographs reveal Ni-electrode surfaces characterized by a large surface area with pores down to a few nanometre sizes. Durability tests were carried out in a commercially produced bipolar electrolyser stack. The developed electrodes showed stable behaviour under intermittent operation for over 9000 h indicating no serious deactivation in the density of active sites.