Novel alkaline water electrolysis with nickel-iron gas diffusion electrode for oxygen evolution

In the present work, a novel electrolyzer concept for alkaline water electrolysis (AEL) with a gas diffusion electrode (GDE) as anode, a conventional immersed porous cathode and a state-of-the-art Zirfon™ separator is presented and compared with a conventional electrolyzer setup. Due to the utilization of a GDE in this configuration, the electrolyte is only circulated through the cathode compartment which greatly simplifies the process. The influence of the catalyst composition and the enhanced electrode surface owing to the three-dimensional porous structure of the GDE are characterized and investigated regarding the electrode performance. Furthermore, process parameters like contact pressure and differential pressure are examined and optimized. The novel process concept with a GDE as anode reveals a similar cell potential compared to a classical electrolysis cell with a Ni/Fe-coated nickel foam anode up to 400 mA cm⁻² at 353 K and 32.5 wt% KOH and also exhibits relatively good electrochemical stability over time.     

Exploring the effect of ion concentrations on the electrode activity and stability for direct alkaline seawater electrolysis

Studying the electrode activity and stability changes caused by the increasing ion concentration during the alkaline seawater electrolysis is crucial to exploit industrial-level seawater electrolyser. Herein, the concentration of hydroxide ion (OH⁻), chlorine ion (Cl⁻), and the other ions in alkaline seawater (OIAS) is investigated to understand the activity and stability for nickel foam (NF) electrodes as both hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) electrodes. As a whole, the activity of HER electrode is mainly dropped with the increasing concentration of OH⁻, while the OER electrode is enhanced with the increasing concentration of OH⁻ and Cl⁻. However, the all (OH⁻, Cl⁻ and OIAS) increasing ion concentrations decrease the HER electrode stability, while the Cl⁻ reduces, the OH⁻ and OIAS enhances the stability of OER electrode. Moreover, the chloride evolution reaction (ClER) in 6 M NaOH with seawater can be ignored even though the concentration of salts in alkaline seawater reach to saturation.     

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.     

Electrochemically fabricated NiCu alloy catalysts for hydrogen production in alkaline water electrolysis

NiCu alloy catalysts for alkaline water electrolysis were prepared by an electrodeposition method varying the alloy composition. When the deposition potential became more positive, the bulk and surface Cu content in NiCu alloys as well as the catalyst particle size gradually increased, which were confirmed by various spectroscopic and electrochemical techniques. The surface coverage of the catalysts was found to be a function of the deposition potential, as well. The catalytic activities of the prepared NiCu alloys to hydrogen evolution reaction (HER) were investigated with cyclic voltammetry in a 6.0 M KOH electrolyte at 298 K, and the mass activities of NiCu alloys were correlated with bulk and surface Cu contents to investigate the Cu alloying effect.     

NiA and NiX zeolites as bifunctional electrocatalysts for water splitting in alkaline media

NiA and NiX zeolites were prepared and characterised using XRD, FTIR and SEM, and subsequently tested as electrodes for hydrogen (HER) and oxygen (OER) evolution reactions in alkaline media. Linear sweep voltammetry and chronoamperometry techniques showed that NiA has higher catalytic activity for these two reactions, as evidenced by higher current densities, which can be correlated with a higher weight fraction of Ni in this electrocatalyst than in the NiX and with its higher conductivity. HER and OER kinetic parameters, including Tafel slope, exchange current density and apparent activation energy were evaluated. Electrochemical impedance spectroscopy analysis yielded values of the resistance of the solution, charge transfer and mass transfer, as well as double layer capacitance and pseudo-capacitance of the working electrode, at different potentials and temperatures. Unlike the HER, during which the mass transfer resistance of the adsorbed intermediate is dominant in the case of NiA, the OER impedance response is controlled by the charge transfer process itself at the potentials of interest for these process. The overall resistance related to the HER is lower for NiA than for NiX.     

Mesoporous nickel-sulfide/nickel/N-doped carbon as HER and OER bifunctional electrocatalyst for water electrolysis

The development of non-precious metal-based highly active bi-functional electrocatalysts for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) is critical factor for making water electrolysis a viable process for large-scale industrial applications. In this study, bi-functional water splitting electrocatalysts in the form of nickel-sulfide/nickel nanoparticles integrated into a three-dimensional N-doped porous carbon matrix, are prepared using NaCl as a porous structure-forming template. Microstructures of the catalytic materials are characterized by scanning electron microscopy, transmission electron microscopy, X-ray photoelectron spectroscopy and N₂ adsorption-desorption analysis. The most active catalyst synthesized in this study exhibits a low HER overpotential of 70 mV at 10 mA cm⁻² and a low Tafel slope of 45 mV dec⁻¹. In OER, the optimized sample performs better than a state-of-the-art RuO₂ catalyst and produces an overpotential of 337 mV at 10 mA cm⁻², lower than that of RuO₂. The newly obtained materials are also used as HER/OER electrocatalysts in a specially assembled two-electrode water splitting cell. The cell demonstrates high activity and good stability in overall water splitting.     

Ultrashort pulse laser-structured nickel surfaces as hydrogen evolution electrodes for alkaline water electrolysis

In this study, we report on micro- and nanostructured Ni surfaces produced by an ultrashort pulse laser process as cathode materials for the alkaline electrolysis of water. We studied the influence of the laser-induced microstructure and surface morphology as well as a cyclic voltammetric activation process on the electrochemical activity of the hydrogen evolution reaction. Galvanostatic techniques, steady-state polarization curves to attain Tafel parameters and capacitance calculations via electrochemical impedance spectroscopy were used to analyze the electrodes. The analyses reveal that the ultrashort pulse laser process increases the specific surface on formerly flat Ni surfaces. Further, the cyclic voltammetric activation process gives rise to an increased intrinsic activity. Both effects lead to a strongly reduced overpotential value. This work demonstrates that different processes can be combined to dramatically boost the activity of Ni electrodes for the hydrogen evolution reaction.     

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.     

CoP@NC electrocatalyst promotes hydrogen and oxygen productions for overall water splitting in alkaline media

Design and synthesis of high-performance bi-functional electrocatalysts can play a crucial role for electrolytic water splitting. Herein, we develop a simple phosphating process to construct cobalt phosphide@nitrogen-doped carbon (CoP@NC) using metal organic frame (MOF) as a precursor and a template. In alkaline solution, CoP@NC-350 exhibits exceptional hydrogen and oxygen evolution reaction performances with over potentials of 75 mV and 268 mV at 10 mA cm^?2, respectively. For a symmetric CoP@NC-350 two-electrode water splitting setup, the potential can be low as 1.69 V to obtain 10 mA cm^?2. Therefore, low-temperature phosphating treatment can be a simple and promising method to produce electrocatalysts for water splitting.     

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.     

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.     

Femtosecond laser molybdenum alloyed and enlarged nickel surfaces for the hydrogen evolution reaction in alkaline water electrolysis

A simple femtosecond laser alloying process is applied under two different process gases nitrogen and air to create novel molybdenum-nickel alloyed and surface enhanced catalysts. A three-day electrochemical test protocol is applied in an alkaline half-cell at 298 K and 353 K to examine the catalytic activity and initial degradation mechanisms in the hydrogen evolution reaction. It is found, that the surface enhancement and the stability of the electrode significantly depends on the process gas. Molybdenum is degraded at the beginning of the test protocol, but it is shown that higher concentrations are not necessarily required for an increase performance. The highest catalytic activity on an electrode alloyed with molybdenum under nitrogen emerges in a steady state operation at 353 K. An overpotential of 135 mV at ?100 mA cm^?2 is measured.     

Enhanced oxygen evolution reaction kinetics through biochar-based nickel-iron phosphides nanocages in water electrolysis for hydrogen production

Highly-efficient and stable non-noble metal electrocatalysts for overcoming the sluggish kinetics of oxygen evolution reaction (OER) is urgent for water electrolysis. Biomass-derived biochar has been considered as promising carbon material because of its advantages such as low-cost, renewable, simple preparation, rich structure, and easy to obtain heteroatom by in-situ doping. Herein, Ni₂P–Fe₂P bimetallic phosphide spherical nanocages encapsulated in N/P-doped pine needles biochar is prepared via a simple two-step pyrolysis method. Benefiting from the maximum synergistic effects of bimetallic phosphide and biochar, high conductivity of biochar encapsulation, highly exposed active sites of Ni₂P–Fe₂P spherical nanocages, rapid mass transfer in porous channels with large specific surface area, and the promotion in adsorption of reaction intermediates by high-level heteroatom doping, the (Ni_0.75Fe_0.25)₂P@NP/C demonstrates excellent OER activity with an overpotential of 250 mV and a Tafel slope of 48 mV/dec at 10 mA/cm² in 1 M KOH. Also it exhibits a long-term durability in 10 h electrolysis and its activity even improves during the electrocatalytic process. The present work provides a favorable strategy for the inexpensive synthesis of biochar-based transition metal electrocatalysts toward OER, and improves the water electrolysis for hydrogen production.     

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.     

Electrochemical synthesis of Li-doped NiFeCo oxides for efficient catalysis of the oxygen evolution reaction in an alkaline environment

Oxygen evolution reaction (OER) is an essential reaction for overall electrochemical water splitting. In this present study, we adopt a facile electrochemical deposition method to synthesize the Li-doped NiFeCo oxides for OER in an alkaline medium. The scanning electron microscopy, X-ray diffraction, Brunauer-Emmet-Teller method and X-ray photo-electron spectroscopy provides the information of morphology, structure, specific surface area and electronic state of the electrocatalysts respectively. Investigates the electrochemical properties by the thin-film technique on a rotating disk electrode and in a single-cell laboratory water electrolyzer connects with electrochemical impedance spectroscopy. Among the catalysts under investigation, Ni_0·9Fe_0·1Co_1·975Li_0·025O₄ exhibits the highest activity towards oxygen evolution reaction, and explains the activity by the oxygen binding energy; such knowledge can be helped to develop better catalyst. We achieve onset over potential 220 mV and receive 10 mA cm^?2 current density at over potential 301 mV with Tafel slope 62 mV dec^?1 in 1 M KOH solution. The results are similar to recently published catalysts in the literature. In water electrolyzer, the Ni_0·9Fe_0·1Co_1·975Li_0·025O₄ modified nickel foam anode exhibits a current density of 143 mA cm^?2 at a cell voltage of 1.85 V in 10 wt% KOH and a temperature of 50 °C.     

Synthesis and characterization of electrocatalysts for the oxygen evolution in PEM water electrolysis

IrO₂, Ir_xSn_(1−x)O₂ (x = 0.7, 0.5) and Ir_xRu_(1−x)O₂ (x = 0.7, 0.5) electrocatalysts for the oxygen evolution reaction (OER) have been synthesized using the Adams fusion method. The metal oxides were characterized via X-ray diffraction, scanning electron microscopy, inductively coupled plasma-atomic emission spectrometry and nitrogen adsorption-desorption measurements to have information about their crystallographic structure, chemical composition and morphology, respectively. A controlled bulk molar fraction of Ru or Sn was introduced in the IrO₂ lattice during the synthesis with no phase separation. The electrocatalytic activity of the synthesized oxides in the OER was studied in liquid electrolyte using porous rotating-disk electrodes, in “half-cell” configuration and in a 5 cm² proton-exchange membrane water electrolysis cell. An increase of the electrical performance was observed upon Ru insertion and a severe depreciation upon Sn insertion.     

Development and performances of a 0.5 kW high-pressure alkaline water electrolyser

The purpose of this research paper is to describe the characteristics and electrochemical performances of a pressurized alkaline water electrolysis short stack (5-cells, 0.5 kW) operated at 80 °C, from atmospheric pressure up to 100 bars. Expanded grids of metallic nickel covered with specific porous catalytic structures have been used as working electrodes. A polysulfone-based diaphragm with a high ionic conductivity has been specifically designed for operation in pressurized alkaline water electrolysis cells. I–V polarization curves have been recorded at current densities up to 1000 mA/cm², at temperatures up to 80 °C and under pressures up to 100 bars. The water electrolysis efficiency of this short-stack has been determined. A specific energy consumption of ca. 4.4–4.5 kWh/Nm³ has been obtained in the high current density range. Durability tests have been performed on the short stack over 1000 h. A limited degradation rate <5 μV/h has been recorded over that period of test.     

Bubble evolution and transport in PEM water electrolysis: Mechanism,impact, and management

Proton exchange membrane water electrolysis (PEMWE), as a promising technology for hydrogen production from renewable energy sources, has great potential for industrial application. Gas bubbles are known to influence the PEMWE cell performance significantly, but a full picture of bubble behaviors and their impacts on cell performance has been lacking. In this review, we first discuss the most recent advances toward understanding the bubble evolution and transport processes as well as the mechanisms of how bubbles impact the PEMWE. Then the state-of-the-art bubble management methods to mitigate bubble-induced performance losses are summarized. Due to the similarity between PEMWE and anion exchange membrane water electrolysis (AEMWE), we also extend related discussions for AEMWE. Lastly, we present principles of bubble management, followed by an outlook of scientific questions and suggestions for future research priorities.     

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.     

The effect of Fe as constituent in Ni-base alloys on the oxygen evolution reaction in alkaline solutions at high current densities

Nanocrystalline Nickel-based alloys were investigated as catalysts for the oxygen evolution reaction (OER) at industrial operation conditions for alkaline water electrolysis. Different alloys were prepared by rapid solidification and subsequent high-energy milling. Regarding OER activity, the best efficiency was obtained for a nanocrystalline NiFe alloy in 29.9 wt.-% KOH at 298 K. However, at elevated temperature (333 K), comparable activities were determined in short-term experiments for nanocrystalline NiFe and Ni alloys as well as for polycrystalline Ni. This initially incomprehensible outcome can be explained by the incorporation of Fe, which is present as impurity in the reagent grade KOH solution, into the NiOOH anode surface layer. However, after a long-term operation, the nanocrystalline NiFe alloy shows a significantly better activity, in particular, at altering current density of up to 1 A cm⁻². As a result, the nanocrystalline NiFe alloy exhibits a very high efficiency and excellent long-term activity (375 mV overpotential at 0.3 A cm⁻²) after 95 h of operation at different loads.