Fabrication and characterization of nanostructured Ni–IrO2 electrodes for water electrolysis

Nanostructured Ni–IrO₂ electrodes were fabricated by electrodeposition in a two-step procedure: first arrays of nickel nanowires (NWs) were electrodeposited within pores of polycarbonate (PC) membranes, then iridium oxide nanoparticles were deposited on the Ni metal after membrane dissolution, for improving the catalytic activity. The aim was to compare performance of these electrodes with traditional ones consisting of Ni film. Different methods of deposition of the IrO₂ electrocatalyst were investigated and the effect on electrodes stability and activity is discussed. Despite a low coverage of Ni NWs by the electrocatalyst, results indicate a faster kinetics of O₂ evolution in 1 M KOH solution and an improvement of performances for electrolysers having a nanostructured anode.     

IrO2/Nb–TiO2 electrocatalyst for oxygen evolution reaction in acidic medium

A corrosion-resistant Nb_0.05Ti_0.95O₂ material with high surface area was prepared by a sol–gel process. IrO₂ nanoparticles (about 16–33 wt%) were successfully loaded on Nb_0.05Ti_0.95O₂ powders as the electrocatalyst for oxygen evolution reaction (OER) in acidic medium. The IrO₂/Nb_0.05Ti_0.95O₂ catalyst with the IrO₂ loading of 26 wt% exhibits the best mass normalized cyclic voltammetry charge and mass normalized activity among all the IrO₂/Nb_0.05Ti_0.95O₂ catalysts because IrO₂ nanoparticles were uniformly supported on the surface of Nb_0.05Ti_0.95O₂ providing conductive channels to reduce the grain boundary resistance. Due to the anchoring effect of carrier on the catalyst, the stability of the supported IrO₂ was significantly improved as compared to the unsupported one. The IrO₂/Nb_0.05Ti_0.95O₂ catalyst with 26 wt% IrO₂ loading demonstrates the best effectiveness of the OER activity and cost.     

Electrochemical reforming of ethanol–water solutions for pure H2 production in a PEM electrolysis cell

This study reports, for the first time in literature, the electrochemical reforming of ethanol–water solutions for pure H₂ production in a PEM electrolysis cell. Hence a bimetallic 40% Pt – 20% Ru carbon based anode and a 20% Pt carbon based cathode were used. The influence of different parameters on the performance of the system for H₂ production was studied: applied potential, ethanol feeding concentration, temperature and long working operation times. In addition, the deactivation process observed on the system for long working times was evaluated and discussed in conjunction with a possible regeneration procedure in view of its further practical development. The observed deactivation was attributed to a strong chemisorption of reaction intermediates on the anodic catalyst which could be partially attenuated under the application of higher potentials, which would lead to the oxidation of the adsorbed intermediates.     

Electroactivity of RuO2–IrO2 mixed nanocatalysts toward the oxygen evolution reaction in a water electrolyzer supplied by a solar profile

Pure crystalline ruthenium–iridium oxide materials were synthesized with high surface area by co-precipitation method in ethanol medium. Cyclic and linear scan voltammetries were then undertaken to evaluate the effect of mixing iridium and ruthenium oxides on the oxygen evolution reaction (OER). The activity of these anode catalysts was evidenced through the determination of their capacitances, surface charges and Tafel slopes. It was found that the Ru_0.9Ir_0.1O₂ catalyst presented catalytic properties close to those of RuO₂, in particular for low overpotentials. The proposed synthesis method was also shown to be suitable for recovering large catalyst amount for O₂ production in a large surface proton exchange membrane water electrolyzer (PEMWE). Polarization measurements were therefore performed in a single PEMWE cell and the durability of the catalytic materials was evaluated by supplying a solar power profile. The efficiency loss at 1 A cm⁻² and 80 °C was only 90 μV h⁻¹ for 1000 h under galvanodynamic operating conditions for Ru_0.9Ir_0.1O₂ as anode, while the cell voltage varied between 1.75 and 1.85 V.     

Platinum–rare earth electrodes for hydrogen evolution in alkaline water electrolysis

In the new “Hydrogen Economy” concept, water electrolysis is considered one of the most promising technologies for hydrogen production. Novel electrocatalytic materials for the hydrogen electrode are being actively investigated to improve the energy efficiency of current electrolysers. Platinum (Pt) alloys are known to possess good catalytic activities towards the hydrogen evolution reaction (HER). However, virtually nothing is known about the effects of rare earth (RE) elements on the electrocatalytic behaviour of Pt towards the HER. In this study, the hydrogen discharge is evaluated in three different Pt–RE intermetallic alloy electrodes, namely Pt–Ce, Pt–Sm and Pt–Ho, all having equiatomic composition. The electrodes are tested in 8 M KOH aqueous electrolytes at temperatures ranging from 25 °C to 85 °C. Measurements of the HER by linear scan voltammetry allow the determination of several kinetic parameters, namely the Tafel coefficients, charge-transfer coefficients, and exchange current densities. Activation energies of 46, 59, 39, and 60 kJ mol⁻¹ are calculated for Pt, Pt–Ce, Pt–Sm and Pt–Ho electrodes, respectively. Results show that the addition of REs improves the activity of the Pt electrocatalyst. Studies are in progress to correlate the microstructure of the studied alloys with their performance towards the HER.     

CO tolerance of nano-architectured Pt–Mo anode electrocatalysts for PEM fuel cells

PEM fuel cell membrane electrode assemblies with Nafion electrolytes and commercial Pt-based cathodes were tested with Pt_0.8Mo_0.2 alloy and MoO_x@Pt core–shell anode electrocatalysts for CO tolerance and short-term stability to corroborate earlier thin-film RDE results. Polarization curves at 70 °C for the Pt_0.8Mo_0.2 alloy in H₂ with 25–1000 ppm CO showed a significant increase in CO tolerance based on peak power densities in comparison to PtRu electrocatalysts. MoO_x@Pt core–shell electrocatalysts, which showed extremely high activity for H₂ in 1000 ppm CO during RDE studies, performed relatively poorly in comparison to the Pt_0.8Mo_0.2 and PtRu alloys for the same total catalyst loading on a per area basis in MEA testing. The discrepancy is attributed to residual stabilizer from the core–shell synthesis impacting catalyst-ionomer interfaces. Nonetheless, the MoO_x@Pt electrochemical performance is superior on a per-gram-of-precious-metal basis to the Pt_0.8Mo_0.2 electrocatalyst for CO concentrations below 100 ppm. Due to cross-membrane Mo migration, the stability of the Mo-containing anode electrocatalysts remains a challenge for developing stable enhanced CO tolerance for low-temperature PEM fuel cells.     

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.     

Synthesis and evaluation of ATO as a support for Pt–IrO2 in a unitized regenerative fuel cell

An IrO₂ catalyst was prepared using a colloidal method followed by a thermal treatment. The catalyst was later mixed with Pt-Black and supported on the Sb-doped SnO₂ (ATO), synthesized through the same colloidal method. ATO was investigated as a possible catalyst support in an electrode of a regenerative fuel cell (URFC), where Pt–IrO₂ was used as the catalyst for the oxygen evolution and reduction reactions. The morphology and composition of the ATO support was investigated through transmission electron microscopy, X-ray diffraction (including Rietveld Refinement), BET analysis, and X-ray fluorescence. An ATO support was obtained with a highly homogeneous distribution and crystal sizes, measuring approximately 4–6 nm.     

The significant role of oxygen vacancy in Cu/ZrO2 catalyst for enhancing water–gas-shift performance

Three Cu/ZrO₂ catalysts were synthesized utilizing co-precipitation (CP), deposition–precipitation (DP) and deposition–hydrothermal (DH) methods, respectively. The microstructure and texture of those catalysts are characterized by means of XRD, SEM, N₂-physisorption, Raman and EPR characterizations. It is demonstrated that different morphologies and textures of ZrO₂ are formed, and the micro- and crystal structure of Cu nanoparticles as well as the concentration of oxygen vacancies of ZrO₂ are distinguish from each other. In addition, H₂-TPR technique is employed to investigate the reducibility properties of the as-synthesized Cu/ZrO₂ catalysts. It is found that the synergy interaction between Cu–ZrO₂ obtained by the DH method is the strongest, owning to the possession of the largest amount of oxygen vacancies. Furthermore, their catalytic activities with respect to the water gas shift reaction are also performed, and the Cu/ZrO₂-DH shows high catalytic activity, the reasons are the well dispersion and small crystallite size of Cu, the largest amount of oxygen vacancies, as well as the strongest interaction between Cu–ZrO₂.     

Electrocatalytic properties for the hydrogen evolution of the electrodeposited Ni–Mo/WC composites

The catalytical activity for the hydrogen evolution reaction (HER) of the electrodeposited Ni–Mo/WC composites is examined in 1 M KOH solution. The structure, surface morphology and surface composition is investigated using the scanning electron microscopy, X-ray diffraction and X-ray photoelectron spectroscopy. The electrocatalytic properties for the HER is evaluated based on the cathodic polarization, electrochemical impedance, cyclic voltammetry and chronopotentiometry methods. The obtained results prove the superior catalytic activity for the HER of Ni–Mo/WC composites to Ni–Mo alloy. The catalytic activity of Ni–Mo/WC electrodes is determined by the presence of WC nanoparticles and Mo content in the metallic matrix. The best electrocatalytic properties are identified for Ni–Mo/WC composite with the highest Mo content and the most oxidized surface among the studied coatings. The impedance results reveal that the observed improvement in the catalytic activity is the consequence of high real surface area and high intrinsic catalytic activity of the composite.     

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.     

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.     

Electrochemical evolution of hydrogen on composite La–Ni–Al/Ni–S alloy film in water electrolysis

The composite La–Ni–Al/Ni–S alloy film was obtained by molten salt electrolysis and aquatic electrodeposition in turn. The La–Ni–Al alloy film was prepared in Na₃AlF₆–La₂O₃–Al₂O₃ molten salt electrolyte by galvanostatic electrolysis at 100 mA cm^?2. The results showed that La³⁺ and Al³⁺ ions could be co-reduced on the nickel cathode and form La–Ni–Al film at c.a. ?0.5 V, which is much lower than that of the theoretical decomposition potential of lanthanum and aluminum. With high HER activity, the composite La–Ni–Al/Ni–S film (η₁₅₀ = 70 mV, 353 K) could absorb large amount of H atoms. Instead of the dissolution of the Ni–S film, the absorbed H atoms would be oxidized under intermittent electrolysis effectively and prolong the lifetime of the cathode.     

(IrOx – Pt)/Ti bifunctional electrodes for oxygen evolution and reduction

Mixed Ir–Pt electrocatalytic films on Ti metal supports were prepared via a galvanic deposition process. Two types of (Ir – Pt)/Ti electrodes were prepared with different Ir–Pt compositions (Ir/Pt atomic composition ratios of 1.74 and 0.44, based on ICP-MS measurements) and of a similar total metal loading (0.15 and 0.12 mg cm^?2). The simultaneous deposition of both metallic Ir and Pt occurred spontaneously upon immersion of a freshly etched Ti metal substrate into a composite solution of Ir(IV) and Pt(IV) complexes of variable concentration. This was followed by electrochemical anodization to convert Ir to IrO_x. Both electrodes showed homogeneous Ir and Pt dispersion on the Ti surface. The bifunctional electrocatalytic performance of (IrO_x/Ir – Pt)/Ti electrodes has been tested towards the oxygen evolution (OER) and reduction (ORR) reactions in acidic solutions. The thus prepared Ti-supported Ir–Pt film electrodes exhibited satisfactory performance towards both reactions, with mass-specific currents for OER being higher than those at a single component IrOx/Ir/Ti electrode and the ones for ORR being comparable to those at a single component Pt/Ti electrode.     

Noble fabrication of Ni–Mo cathode for alkaline water electrolysis and alkaline polymer electrolyte water electrolysis

Among the catalysts for hydrogen evolution reaction (HER) in alkaline media, Ni–Mo turns out to be the most active one. Conventional preparations of Ni–Mo electrode involve repeated spraying of dilute solutions of precursors onto the electrode substrate, which is time-consuming and usually results in cracking and brittle electrodes. Here we report a noble fabrication of Ni–Mo electrode for HER. NiMoO₄ powder was synthesized and used as the precursor. After reduction in H₂ at 500 °C, the NiMoO₄ powder layer was converted to a uniform and robust electrode containing metallic Ni and amorphous Mo(IV) oxides. The distribution of Ni and Mo components in this electrode is naturally uniform, which can maximize the interaction between Ni and Mo and benefit the electrocatalysis. The thus-obtained Ni–Mo electrode exhibits a very high catalytic activity toward the HER: the current density reaches 700 mA/cm² at 150 mV overpotential in 5 M KOH solution at 70 °C. This new fabrication method of Ni–Mo electrode is not only suitable for alkaline water electrolysis (AWE), but also applicable to the alkaline polymer electrolyte water electrolysis (APEWE), an emerging technique for efficient production of H₂.     

A study on novel combined processes for preparation of high performance Pt–Co/C electrocatalyst for oxygen reduction in PEM fuel cell

The preparation of a Pt–Co/C electrocatalyst for the oxygen reduction reaction in PEM fuel cells was achieved via a novel combined process of impregnation and seeding. The effect of seeding and non-seeding approaches on the morphologies and activities of the electrocatalyst was explored. The results indicated that the seeding or non-seeding approaches provided the similar results of Pt structure and phase composition in the Pt–Co/C electrocatalyst. However, the seeding approach provided a more uniform dispersion and smaller particle size of electrocatalyst compared with that of the non-seeding approach. Also, higher values of kinetic parameters including i₀, E₀, i_0.9V and E_10mA/cm2 were obtained in case of seeding electrocatalyst. Finally, the rotating disk electrode experimental results showed that the mechanism of oxygen reduction involved the four-electron pathway.     

Electrocatalytic performance of Nicore@Ptshell/C core–shell nanoparticle with the Pt in nanoshell

The Ni₁@Pt_0.067 core–shell nanoparticles with a thin layer of Pt shell have been prepared by colloidal template method. The structure and composition of the prepared core–shell nanoparticles have been analyzed by using transmission electron microscopy (TEM), energy dispersive spectroscopy (EDS), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS). In addition, the electrochemical performance of the prepared nanoparticles has been analyzed by potentiodynamic polarization and cyclic voltammetry (CV), by testing their activity towards oxygen reduction reaction (ORR) and methanol oxidation reaction (MOR). Experimental results indicate that the Ni₁@Pt_0.067 particles are well distributed, with an average particle size of approximately 6 nm and shell thickness of approximately 0.5 nm–2.1 nm. Compared with Pt/C, the Ni₁@Pt_0.067/C nanoparticles prepared in this study show significantly improved catalytic activity towards ORR and MOR. However, with increase in methanol concentration in the electrolyte composed of 0.5 mol L⁻¹ H₂SO₄ + x mol L⁻¹ methanol (where, x = 0, 0.2, 0.5 and 1.0), the limiting current of MOR on Ni₁@Pt_0.067/C increase remarkably, whereas the ORR activity weakens. Based on the experimental data, we analyze the mechanism underlying the impact of methanol concentration on the ORR in Ni₁@Pt_0.067/C and find that the surface of Pt has a variety of activity sites.     

Effect of RuO2–CoS2 anode nanostructured on performance of H2S electrolytic splitting system

An innovative, nanostructured composite, anode electrocatalyst, material has been developed for the electrolytic splitting of (100%) H₂S feed content gas operating at 135 kPa and 150 °C. A new class of anode electrocatalyst with general composition, RuO₂–CoS₂ has shown great stability and desired properties at typical operating conditions. This configuration showed stable electrochemical operation over the period of 24 h and also exhibited a maximum current density of (0.019 A/cm²). The kinetic behaviors of various anode-based electrocatalysts demonstrated that, exchange current density, which is a direct measure of the electrochemical reaction, increased with RuO₂–CoS₂-based anodes. Moreover, high levels of feed utilization were possible using these materials. Electrochemical performance, current density, and sulfur tolerance were enhanced compared to the other tested anode configurations. The structural, microstructural and surface behavior of RuO₂–CoS₂ anode electrocatalyst was investigated in detail.     

Efficient electrocatalyst of Pt–Fe/CNFs for oxygen reduction reaction in alkaline media

Transition metal iron-based catalysts are promising electrocatalysts for oxygen reduction reaction (ORR), and they have the potential to replace noble metal catalysts. The one-dimensional of carbon nanofibers with tubular structure can effectively promote the electrocatalytic activity, which facilitates electron transport. Herein, the Pt–Fe/CNFs were synthesized by electrospinning and subsequent calcination. Benefiting from the advantages of one-dimensional structure, Pt–Fe/CNFs-900 with fast electrochemical kinetics and excellent stability for ORR with excellent onset of 0.99 V, a low Tafel slope of 62 mV dec⁻¹ and high limiting current density of 6.00 mA cm⁻². Long-term ORR testing indicated that the durability of this catalyst was superior to that of commercial Pt/C in alkaline electrolyte. According to RRDE test, the ORR reaction process of Pt–Fe/CNFs-900 was close to four-electron transfer routes.