Study of Co–W crystalline alloys as hydrogen electrodes in alkaline water electrolysis

Binary Co–W crystalline alloys, Co₉₅W₅, Co₉₀W₁₀, Co₈₅W₁₅, Co₈₀W₂₀ and Co₇₀W₃₀ (atomic %) were investigated in view of their possible applications as electrocatalytic materials for hydrogen evolution reaction (HER). The electrocatalytic efficiency of the electrodes was studied on the basis of electrochemical data obtained from steady-state polarization and electrochemical impedance spectroscopy (EIS) techniques in oxygen-free 1 M NaOH solution at 298 K. The results were compared with those obtained on polycrystalline Co. Moreover, literature data concerning the electrocatalytic activity of polycrystalline Ni and Ni–Mo alloys, which are considered good electrocatalyst materials for the hydrogen evolution reaction in alkaline solutions, were also reported for comparison. The values of Tafel slope, b, exchange current density, j₀, and overpotential at the current density of 250 mA cm⁻², η₂₅₀, indicated outstandingly high electrocatalytic activity of Co–W electrodes. The best performance towards the HER demonstrates the Co₉₀W₁₀ alloy in accordance with the prediction based on the electronic structure calculations and the enhanced density of states at the Fermi level of the 3d Co band.     

Evaluation of the Zirfon® separator for use in alkaline water electrolysis and Ni-H2 batteries

Zirfon^® is a porous composite separator material composed of a polysulfone matrix and ZrO₂ which is present as a powder. The manufacturing is based on the film-casting technique. The separator is very stable in concentrated KOH solutions at elevated temperatures. Even with a high loading of ZrO₂ it is possible to produce very flexible separators with attractive mechanical properties. The main challenge for this Zirfon^® material is to replace the asbestos diaphragms which are presently used in industrial alkaline water electrolysis. Different companies are already testing this separator. Excellent results have been obtained. Another field of interest is related to the use of such a separator in Ni-H₂ batteries both for earth and space applications.     

Cobalt-chrome activation of the nickel electrodes for the HER in alkaline water electrolysis – Part II

Catalyst based on cobalt and chrome was investigated as cathode material for hydrogen production process via water electrolysis. Electrocatalytic efficiency of proposed system was studied using quasi-potentiostatic, galvanostatic and impedance spectroscopy techniques of the catalyst obtained by in situ electrodeposition in an alkaline, 6 M KOH, electrolyser. In accordance to our previous studies, synergetic effect of cobalt complex and chrome salt is observed, with its maximum at high temperatures and for high current densities (industrial conditions). The Tafel slopes were found to be around 120 mV and exchange current densities in the range of 10⁻³ mA cm⁻² up to 10⁻² mA cm⁻². Results are presented to show the Tafel slopes, the exchange current densities, the apparent energy of activation and the apparent electrochemical surface of in situ formed Co–Cr catalyst. This study shows that catalytic performance of Co–Cr was achieved not only from the increase of the real surface area of electrodes, but also from the true catalytic effect.     

Zinc–nickel alloy electrodeposits for water electrolysis

Electrodeposited zinc–nickel alloys of various compositions were prepared. A suitable electrolyte and conditions to produce alloys of various compositions were identified. Alloys produced on electroformed nickel foils were etched in caustic to leach out zinc and to produce the Raney type, porous electro catalytic surface for hydrogen evolution. The electrodes were examined by polarization measurements, to evaluate their Tafel parameters, cyclic voltammetry, to test the change in surface properties on repeated cycling, scanning electron microscopy to identify their microstructure and X-ray diffraction. The catalytic activity as well as the life of the electrode produced from 50% zinc alloy was found to be better than others.     

Electrochemical preparation and characterization of C/Ni–NiIr composite electrodes as novel cathode materials for alkaline water electrolysis

The binary NiIr coatings as novel and effective catalysts were electrochemically prepared on a Ni-modified carbon felt electrode (C/Ni–NiIr) in view of their possible application as cathode materials for the alkaline water electrolysis. The surface morphology and chemical composition of the electrodes were investigated by scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDX) techniques. Their hydrogen evolution activity was assessed by electrochemical techniques. It was found that, the preparation of NiIr co-deposits on the Ni-modified C substrate enhances the hydrogen evolution activity. The electrodes have wide space, which is an advantage for diffusion of ions and hydrogen bubbles through inner zones and reduction of diffusion resistance. The high hydrogen evolution activity of the C/Ni–NiIr electrode was mainly attributed to the finer surface structure, high surface area and the higher numbers of the catalytically active centers.     

VOF modelling of gas–liquid flow in PEM water electrolysis cell micro-channels

In this study, the gas–liquid flow through an interdigitated anode flow field of a PEM water electrolysis cell (PEMEC) is analysed using a three-dimensional, transient, computational fluid dynamics (CFD) model. To account for two-phase flow, the volume of fluid (VOF) method in ANSYS Fluent 17.2 is used. The modelled geometry consists of the anode channels and the anode transport layer (ATL). To reduce the complexity of the phenomena governing PEMEC operation, the dependence upon electro-chemistry is disregarded. Instead, a fixed source of the gas is applied at the interface between the ATL and the catalyst layer. An important phenomenon that the model is able to capture is the gas–liquid contact angle on both the channel wall and ATL-channel interface. Particularly, the latter interface is crucial in capturing bubble entrainment into the channel. To validate the numerical simulation, photos taken of the gas–liquid flow in a transparent micro-channel, are qualitative compared against the simulation results. The experimental observations confirm the models prediction of long Taylor bubbles with small bubbles in between. From the simulation results, further intriguing details of the flow are revealed. From the bottom to the top of the outgoing channel, the film thickness gradually increases from zero to 200 μm. This increase in the film thickness is due to the particular superficial velocity field that develops in an interdigitated flow. Here both the superficial velocities change along the length of the channel. The model is capable of revealing effect of different bubble shapes/lengths in the outgoing channel. Shape and the sequence of the bubbles affect the water flow distribution in the ATL. The model presented in this work is the first step in the development of a comprehensive CFD model that comprises multiphase flow in porous media and micro-channel, electro-chemistry in catalyst layers, ion transport in membrane, hydrogen evolution, etc. The model can aid in the study of gas–liquid flow and its impact on the performance of a PEMEC.     

Ni–Zn nanosheet anchored on rGO as bifunctional electrocatalyst for efficient alkaline water-to-hydrogen conversion via hydrazine electrolysis

Bifunctional Ni-15 at.% Zn/rGO catalyst was fabricated by a two-step electrodeposition to be used for efficient alkaline water-to-hydrogen conversion via hydrazine electrolysis. Experiments show that the as-deposited Ni-15 at.% Zn/rGO nanosheet arrays with porous structure possesses excellent catalytic activity and stability towards both hydrogen evolution reaction (HER) and hydrazine oxidation reaction (HzOR). A small overpotential of 49 mV at 10 mA cm⁻² with a low Tafel slope of 26.3 mV dec⁻¹, and a retention rate of 91.4% after 12 h at 10 mA cm⁻² are observed for Ni-15 at.% Zn/rGO towards HER. Moreover, Ni-15 at.% Zn/rGO also shows an extra-high current density of 1097 mA cm⁻² at 0.6 V vs RHE with a low Tafel slope of 33.5 mV dec⁻¹, and a high durability of 90.5% after 5000 s towards HzOR. Moreover, two-electrode cell was constructed using Ni-15 at.% Zn/rGO as both cathode and anode for HER and HzOR, achieving 100 mA cm⁻² at an ultralow cell voltage of 0.418 V. The above outstanding bifunctional catalytic performance should be attributed to its large ECSA, high electrical conductivity and most importantly, its superaerophobic surface induced by the porous structure with nanosheet arrays.     

Decomposition of hydriodic acid by electrolysis in the thermochemical water sulfur–iodine splitting cycle

Although several technologies, such as reactive distillation and catalytic membrane reactor, have been proposed to improve HI conversion efficiency, they still experience several challenges for the application in HI section. In this study, an electrochemical cell was employed for hydriodic acid decomposition under the presence of iodine. Several commercial proton-exchange membranes (PEMs), namely, Nafion 117 and Nafion 115, were used as separators for the electrochemical cell. Anodization of iodide anion occurred at the graphite electrode in the anode compartment. Hydrogen was generated by the reduction reaction of hydrogen cations, which migrated from anolyte to catholyte. In electrolysis experiments, PEM showed good performance in terms of high transport number of proton and low iodine permeation. Several parameters, such as operating temperature, HI molarity, and I₂ molarity in anolyte, which affected current efficiency, iodine permeance, and electric resistance of test cell, were investigated. High operating temperature and I₂ molarity in anolyte enhanced the permeability of iodine, which had several negative influences on electrochemical cell performance. Although current efficiency was negatively affected by increasing temperature and I₂ molarity, it still remained above 0.85 in the range of 30 °C–75 °C. Ohmic resistance, which is a component of cell resistance, offered by PEM was investigated with Nafion 117 and 115. Apart from graphite plates, activated carbon papers were adopted as electrodes to reduce the overpotentials due to their high specific surface characteristic.     

An alkaline water electrolyzer with nickel electrodes enables efficient high current density operation

Low-temperature industrial water electrolysis is typically conducted using either liquid alkaline electrolytes or acidic polymer electrolyte membranes (PEMs). The latter approach is considered to be more efficient but also more expensive as it requires Pt and Ir based catalysts. This study reports on an alkaline water electrolyzer with Ni electrodes that operates at a current density of 2 A/cm² with a cell voltage of 1.85 V, which provides a comparable voltage-current characteristic to the state-of-the-art PEM water electrolyzers. Thin Ni mesh electrodes with surface areas that are thousand times higher than the geometric area were manufactured by an easily scalable and cheap process, i.e. metallurgical hot dip galvanization with subsequent de-alloying. With a thin porous polymer of approximately $140\text{μm}$ as the diaphragm a low cell resistance of 0.11 Ω cm^?2 was obtained.     

Production of hydrogen by the electrochemical reforming of glycerol–water solutions in a PEM electrolysis cell

An alternative method for producing hydrogen from renewable resources is proposed. Electrochemical reforming of glycerol solution in a proton exchange membrane (PEM) electrolysis cell is reported. The anode catalyst was composed of Pt on Ru–Ir oxide with a catalyst loading of 3 mg cm⁻² on Nafion. Part of the energy carried by the produced hydrogen is supplied by the glycerol (82%) and the remaining part of the energy originates from the electrical energy (18%) with an energy efficiency of conversion of glycerol to hydrogen of around 44%. The electrical energy consumption of this process is 1.1 kW h m⁻³ H₂. Compared to water electrolysis in the same cell, this is an electrical energy saving of 2.1 kW h N m⁻³ H₂ (a 66% reduction). Production rates are high compared with comparable sized microbial cells but low compared with conventional PEM water electrolysis cells.     

Towards integration of hydrolysis,decomposition and electrolysis processes of the Cu–Cl thermochemical water splitting cycle

The thermochemical copper–chlorine (Cu–Cl) cycle is a promising technology that can utilize various energy sources such as nuclear and solar energy to produce hydrogen with minimal or no emissions of greenhouse gases. Past investigations have primarily focused on the design and testing of individual unit operations of the Cu–Cl cycle. This paper investigates the chemical streams flowing through each individual process from the aspect of system integration. The interactions between each of the two immediate upstream and downstream processes are examined. Considering the integration of electrolytic hydrogen production and cupric chloride hydrolysis steps, it is evident that an intermediate step to concentrate CuCl₂ and reduce HCl composition is required. Spray drying and crystallization, serving as the intermediate steps, are examined from the aspects of energy requirements and viability of engineering. Regarding the integration of the hydrolysis and oxygen production steps, thermodynamic and XRD analysis results are presented to study the mutual impacts of these two steps on each other. Within the hydrolysis reactor, high conversion of CuCl₂ to Cu₂OCl₂ is preferable for the integration because it reduces the release of chlorine gas during the oxygen production. Considering the integration of the oxygen production step and electrolysis of CuCl, pulverization is needed for the solidified CuCl. The recovery of CuCl vapour entrained in oxygen gas requires further research. Residual CuCl₂ introduced from the hydrolysis step into the oxygen production step may be further entrained by CuCl into the electrolytic hydrogen production cell. Additionally, thermal energy integration patterns are briefly discussed while integrating the various chemical streams of the Cu–Cl cycle. Steam generated from the heat recovery of cuprous chloride can be introduced into the hydrolysis reactor to serve as a reactant.     

Semi-empirical model and experimental validation for the performance evaluation of a 15 kW alkaline water electrolyzer

This paper presents a semi-empirical mathematical model for predicting the electrochemical behavior of an alkaline water electrolysis system, based on the polarization curve and Faraday efficiency as a function of the current density under different operating conditions, such as, temperature and pressure. Also, the gas impurities of hydrogen in oxygen have been modeled for safety reasons due to its importance when the electrolyzer is dynamically operated using renewable energy sources. The different parameters defined in the model have been calculated by MATLAB, using a non-linear regression, on the basis of experimental data obtained in a 15 kW alkaline test bench. The simulated and measured values have been compared to ensure the accuracy and validity of the proposed model. In this sense, the error has been evaluated for the voltage with an average result of 5.67 mV per cell and for the Faraday efficiency and the gas impurities of hydrogen in oxygen with a value lower than 1%. These results show an excellent correlation between experimental and modeled data, so the model is a useful design and optimization tool for alkaline electrolyzers. Also, a sensitivity analysis has been used to determine the most influential operating variables in the performance of the electrolyzer.     

Electrocatalytic properties of Ti/Pt–IrO2 anode for oxygen evolution in PEM water electrolysis

A novel Pt–IrO₂ electrocatalyst was prepared using the dip-coating/calcinations method on titanium substrates. Titanium electrodes coated with oxides were investigated for oxygen evolution. Experimental results showed that Ti/Pt–IrO₂ electrode containing 30 mol% Pt in the coating exhibited significantly higher electrocatalytic activity for oxygen evolution compared to Ti/IrO₂ prepared by the same method, which is also supported by the electrochemical impedance data. Stability tests demonstrated Pt–IrO₂ electrocatalyst had a service cycle of 10,000 times in 0.1 M H₂SO₄ solution. And the anode surface had hardly discovered cracks and had compact structures, which contributed to stable nature of the electrode together with good conductivity and specific interaction between Pt and IrO₂ formed during the calcination. Furthermore, the enhanced catalytic activity for O₂ evolution at Ti/Pt–IrO₂ electrode is preliminarily discussed using the Mott–Schottky analysis.     

Development of Ni–Fe based ternary metal hydroxides as highly efficient oxygen evolution catalysts in AEM water electrolysis for hydrogen production

A number of mixed metal hydroxide oxygen evolution reaction (OER) catalysts i.e. Ni–Fe, Ni–Co, Ni–Cr, Ni–Mo, Ni–Fe–Co, Ni–Fe–Mo and Ni–Fe–Cr were prepared by cathodic electrodeposition and characterised by SEM, TEM, EDS, XPS and micro X-CT. The compositions of selected catalysts were optimised to give lower OER overpotentials in alkaline media. Further optimisation of Ni–Fe based ternary metal hydroxide catalysts such as Ni–Fe–Co and Ni–Fe–Mo was carried out, showing improved performance at high current densities up to 1 A cm⁻² in 1 M NaOH, 333 K. The influence of electrodeposition parameters such as current density, pH, electrodeposition time and temperature on the electrocatalytic performance of ternary Ni–Fe–Co metal hydroxide was further investigated and optimised. The durability of the optimised catalyst was tested at a current density of 0.5 A cm⁻² in an anion exchange membrane (AEM) water electrolyser cell at 4 M NaOH, 333 K, demonstrating stable performance over 3.5 h.     

One-step synthesis of amorphous Ni–Fe–P alloy as bifunctional electrocatalyst for overall water splitting in alkaline medium

Design of inexpensive and highly efficient bifunctional electrocatalyst is paramount for overall water splitting. In this study, amorphous Ni–Fe–P alloy was successfully synthesized by one-step direct-current electrodeposition method. The performance of Ni–Fe–P alloy as a bifunctional electrocatalyst toward both hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER) was evaluated in 30 wt% KOH solution. It was found that Ni–Fe–P alloy exhibits excellent HER and OER performances, which delivers a current density of 10 mA cm^?2 at overpotential of ～335 mV for HER and ～309 mV for OER with Tafel slopes of 63.7 and 79.4 mV dec^?1, respectively. Moreover, the electrolyzer only needs a cell voltage of ～1.62 V to achieve 10 mA cm^?2 for overall water splitting. The excellent electrocatalytic performance of Ni–Fe–P alloy is attributed to its electrochemically active constituents, amorphous structure, and the conductive Cu Foil.     

A water–water preheater and a steam–water heater network: Temperature dynamics

The dynamic simulation of an integrated, double pipe heat exchanger network was validated through experimentation. A steam–water, concentric tube, heat exchanger was coupled to a water–water preheater. When the preheater was configured for cocurrent flow with equal fluid velocities in its annulus and core, Lagrangian-based derivations yielded analytical solutions that accurately predicted observed temperature dynamics. When the preheater was configured for countercurrent flow with distinct fluid velocities in its annulus and core, analytical solutions for the heater and connecting tubing were coupled with Eulerian based numerical solutions for the preheater. Programming used Mathcad. Nonlinear regression analysis of steady state data was used to determine system parameters. The significance of time delays through the integration of unit operations is illustrated.     

Influence of SiW12O404− on the electrocatalytic behaviour of Pt–Co alloy supported on carbon for water electrolysis in 3 M KOH aqueous solution

Pt–Co alloy supported on carbon (Pt–Co) was electroactivated with or without SiW₁₂O₄₀⁴⁻ and used as cathodes for the hydrogen evolution reaction (HER) in 3 M KOH at 70°C. These electrodes were characterised through neutron activation (NA) and electrochemical impedance spectroscopy (EIS). The best HER performances were observed on Pt–Co electroactivated with STA. The enhanced HER electrocatalytic activity observed on Pt–Co electroactivated with STA was attributed to an increase in its active surface area and to its surface chemical composition.