Bubble size distribution and electrode coverage at porous nickel electrodes in a novel 3-electrode flow-through cell

A novel 3-electrode cell type is introduced to run parametrical studies of H₂ evolution in an alkaline electrolyte on porous electrodes. Electrochemical methods combined with a high-speed optical measurement system are applied simultaneously to characterize the electrodes and the bubble dynamics in terms of bubble size distribution and coverage of the working electrode. Three different cathodes made of expanded nickel are investigated at applied current densities of |j| = 10–200 mA cm^?2 without forced flow and at a flow rate of 5 ml min^?1. The applied current density is found to significantly influence both the size of detached bubbles and the surface coverage of the working electrode. The forced flow through the cathodes is found to strongly reduce the bubble size up to current densities of about 100 mA cm^?2, whereas the initial transient until the cathode surface is completely covered by bubbles is only marginally affected by the flow-through.     

Enhanced oxygen evolution reaction activity of NiFe layered double hydroxide on nickel foam- reduced graphene oxide interfaces

Herein, we prepared a novel nickel iron-layered double hydroxide/reduced graphene oxide/nickel foam (NiFe-LDH/RGO/NF) electrodes by two step electrodeposition processes for oxygen evolution reaction (OER). The modification of NF by RGO increased the interface conductivity and electrochemical active surface areas (ECSA) of the electrode. The NiFe-LDH/RGO/NF electrode has shown higher catalytic activity with a lower overpotential of 150 mV at the current density of 10 mA cm⁻². The NiFe-LDH/RGO/NF electrode has also shown a small Tafel slope of 35 mV per decade due to the synergy effect between the larger ECSA and the conductive RGO interface. Furthermore, the electrodes exhibits almost 10 h stability under a general current density of 10 mA cm⁻².     

Effect of the thickness of nickel electrode with aligned porous structure on hydrogen evolution reaction

Seven nickel electrodes with aligned porous structure of different thicknesses (i.e., 100, 250, 400, 500, 600, 850, and 1100 μm) were fabricated via freeze casting, and the effect of the electrode thickness on hydrogen evolution reaction (HER) was experimentally studied. The polarization curves of the porous electrodes were obtained by linear sweep voltammetry (LSV) in a 1 M KOH solution. The results show that, in the lower current density zone, the overpotential decreases with the increasing thickness of the aligned porous electrode. At higher current density, the overpotential presents a relative complex variation with the electrode thickness. For a thicker porous electrode, its electrochemically active surface area (ECSA) undoubtedly increases. Nevertheless, its bubble removal ability decreases due to deeper porous channels, which adversely affects the HER performance. It is also found that while the aligned pore orientation of the electrode is parallel to gravity direction, the electrode with a thickness of 400 μm has a trade-off between the ECSA and bubble removal ability and shows optimal performance.     

One-step electrodeposition of cauliflower-like Ni–Fe–Sn particles as a highly-efficient electrocatalyst for the hydrogen evolution reaction

Herein, a Ni–Fe–Sn coating was synthesized in-situ on Ni mesh by one-step electrodeposition at different durations. The Ni–Fe–Sn60 electrode obtained after 1 h deposition exhibits cauliflower-like morphology and the best electrocatalytic properties for the hydrogen evolution reaction (HER) compared to other electrodes. The electrode requires an overpotential of 43 mV at a current density of 10 mA cm⁻² and a small Tafel slope of 70 mV dec⁻¹ in a 1 M KOH solution. Moreover, the electrode shows outstanding stability in prolonged electrolysis and overall water splitting performance, generating a current density of 93 mA cm⁻² at 1.8 V, which is thrice that of an industry electrode. This electrocatalytic activity is ascribed to the high active surface area produced by the cauliflower-like Ni–Fe–Sn particles and the synergistic interaction of Ni, Fe and Sn. The simple synthesis method and excellent performance endow this electrode with great potential for large-scale applications.     

Electrodeposition of Ni–Fe–Mn ternary nanosheets as affordable and efficient electrocatalyst for both hydrogen and oxygen evolution reactions

The synthesis of high performance and economical electrocatalysts in the process of overall water splitting is very important for the production of hydrogen energy and has become one of the most important challenges. Here, various Ni, Ni–Fe, Ni–Mn nanosheets and Ni–Fe–Mn ternary nanosheets were created using cost-effective, versatile and binder-free electrochemical deposition methods, and the electrocatalytic activity of various electrodes for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) were investigated in an alkaline environment. Due to the high electrochemical active surface area due to the fabrication of nanosheets, the synergistic effect between different elements on the electronic structure, the high wettability due to the formation of nanosheets and the quick detachment of formed gasses from the electrode, the Ni–Fe–Mn nanosheets electrode showed excellent electrocatalytic activity. In order to deliver the 10 mA cm⁻² current density in HER and OER processes, this electrode required values of 64 mV and 230 mV overpotential, respectively. Also, the stability test showed that after 10 h of electrolysis at a current density of 100 mA cm⁻², the overpotential changes was very small (less than 4%), indicating that the electrode was excellent electrostatic stability. Also, when using as a bi-functional electrode in the full water splitting system, it only needed a cell voltage of 1528 V to deliver a current of 10 mA cm⁻². The results of this study indicate a new strategy for the synthesis of active and stable electrocatalysts.     

Enhanced electrochemical performance and stability of (La,Sr)MnO3–(Gd,Ce)O2 oxygen electrodes of solid oxide electrolysis cells by palladium infiltration

Palladium-impregnated or infiltrated La_0.8Sr_0.2MnO₃–Gd_0.2Ce_0.8O_1.9 (LSM-GDC) composites are studied as the oxygen electrodes (anodes) for the hydrogen production in solid oxide electrolysis cells (SOECs). The incorporation of small amount of Pd nanoparticles leads to a substantial increase in the electrocatalytic activity and stability of the LSM-GDC oxygen electrodes. The electrode polarization resistance (R_E) at 800 °C on a 0.2 mg cm⁻² Pd-infiltrated LSM-GDC electrode is 0.13 Ω cm², significantly smaller than 0.42 Ω cm² for the reaction on the pure LSM-GDC electrodes. The overpotential loss is also substantially reduced after the Pd infiltration; at an anodic overpotential 50 mV and 800 °C, the current increases from 0.15 A cm⁻² for the pure LSM-GDC anode to 0.47 A cm⁻² on a 0.3 mg cm⁻² Pd-infiltrated LSM-GDC. The infiltrated Pd nanoparticles enhance the stability of the LSM-GDC oxygen electrodes and are most effective in the promotion of the diffusion, exchange and combination processes of oxygen species on the surface of LSM-GDC particles, leading to the increase in the oxygen evolution reaction rate.     

CoO/NF nanowires promote hydrogen and oxygen production for overall water splitting in alkaline media

The exploration of catalysts with high activity and low cost for water splitting is still necessary. Herein, a nanowire-like morphology CoO/NF electrode is synthesized using facile hydrothermal reaction and calcination treatment. The urea can regulate its morphology during the synthetic process of CoO/NF. Electrochemical studies reveal that the as-obtained CoO/NF exhibits excellent electrocatalytic performance with overpotential of 307 mV at current density of 10 mA cm⁻² and Tafel slope of 72 mV dec⁻¹ for oxygen evolution reaction, and CoO/NF delivers current density of 10 mA cm⁻² at overpotential of 224 mV for hydrogen evolution reaction. The results of the oxygen evolution reaction stability show that the overpotential of CoO/NF electrode is only increased by 4 mV at current density of 10 mA cm⁻². The two-electrode water splitting with CoO/NF electrodes as both anode and cathode needs a cell potential of 1.76 V to reach 10 mA cm⁻². Therefore, this simple method to prepare CoO/NF electrode can enhance the properties of electrocatalysts, which makes CoO/NF a promising material to replace noble metal-based catalysts.     

Electrodeposited Ni–Co–S nanosheets on nickel foam as bioelectrochemical cathodes for efficient H2 evolution

High overpotential and soaring prices of the cathode electrode are the bottlenecks for the development of microbial electrolysis technology for hydrogen production. In this study, a novel one-step electrodeposition method has been attempted to fabricate electrodeposited cathodes in situ growth of Ni–Co–S, Ni–S, Co–S catalyst on nickel foam (NF) to reduce the overpotential of electrodes. Finally, a uniform nanosheet with a high specific surface area and more active sites is formed on the NF surface, resulting in a lower overpotential than plain NF. At 0.8 V, the Co–S/NF cathode produces a favorable 42% increase in hydrogen yield (0.68 m³·m⁻³·d⁻¹), 40% upsurge in current density (10.6 mA/cm³) and 39% rise of cathodic recovery rate (58.0 ± 3.2%) than bare NF, followed by Ni–Co–S/NF and Ni–S/NF cathode. All the electrodeposited electrodes demonstrate enhanced current density and reduced electron losses, thereby achieving efficient hydrogen production. These innovative varieties of electrodes are highly advantageous as they are relatively inexpensive and easy to manufacture with great potential in reducing costs and further real time application in large scale.     

Scaling up self-stratifying supercapacitive microbial fuel cell

Self-stratifying microbial fuel cells with three different electrodes sizes and volumes were operated in supercapacitive mode. As the electrodes size increased, the equivalent series resistance decreased, and the overall power was enhanced (small: ESR = 7.2 Ω and P_max = 13 mW; large: ESR = 4.2 Ω and P_max = 22 mW). Power density referred to cathode geometric surface area and displacement volume of the electrolyte in the reactors. With regards to the electrode wet surface area, the large size electrodes (L-MFC) displayed the lowest power density (460 μW cm⁻²) whilst the small and medium size electrodes (S-MFC, M-MFC) showed higher densities (668 μW cm⁻² and 633 μW cm⁻², respectively). With regard to the volumetric power densities the S-MFC, the M-MFC and the L-MFC had similar values (264 μW mL⁻¹, 265 μW mL⁻¹ and 249 μW cm⁻¹, respectively). Power density normalised in terms of carbon weight utilised for fabricating MFC cathodes-electrodes showed high output for smaller electrode size MFC (5811 μW g⁻¹-C- and 3270 μW g⁻¹-C- for the S-MFC and L-MFC, respectively) due to the fact that electrodes were optimised for MFC operations and not supercapacitive discharges. Apparent capacitance was high at lower current pulses suggesting high faradaic contribution. The electrostatic contribution detected at high current pulses was quite low. The results obtained give rise to important possibilities of performance improvements by optimising the device design and the electrode fabrication.     

Autologous growth of Fe-doped Ni(OH)2 nanosheets with low overpotential for oxygen evolution reaction

Highly active and durable electrocatalysts for oxygen evolution reaction (OER) play a vital role in water splitting. Despite numerous efforts, the strategies to prepare durable and effective electrocatalysts via scalable methods still remain a great challenge. In this work, we fabricated Fe-doped Ni(OH)₂ ultrathin nanosheets (Fe–Ni–OH/Ni) via autologous growing of Ni(OH)₂ from Ni foam, and in situ electrochemical-assisted doping Fe into Ni(OH)₂. Benefiting from the unique structure with large surface areas and strong coupling effects between Fe and Ni, the optimal Fe–Ni–OH electrodes exhibit remarkable catalytic performance toward OER, which requires an overpotential of 220 mV to achieve a current density of 10 mA cm⁻² with a Tafel slope of 48.3 mV dec⁻¹. The Fe–Ni–OH electrodes also possess high stability even under a high current density of 500 mA cm⁻² for 600 h with an ultralow overpotential of 290 mV. Using Ni–Fe–OH electrodes as both anode and cathode for overall water splitting, only a small overpotential of 1.57 V is required to reach a current density of 10 mA cm⁻². Moreover, the high catalytic performance and scalable preparation method can meet the emergency needs for the practical application.     

Mn–Mo–W-oxide anodes for oxygen evolution during seawater electrolysis for hydrogen production: Effect of repeated anodic deposition

MnMoW-triple oxide catalyst for chlorine-less oxygen generating electrodes from seawater electrolysis were prepared by repeated anodic deposition of their oxides on Ti/IrO₂ substrate. The durability of the electrodes were examined by electrolysis in 0.5 M NaCl solution at current density of 1000 Am⁻² and compared with their corresponding electrodes that were prepared by continuous anodic deposition. The electrodes repeatedly and continuously deposited for 90 min kept the oxygen evolution efficiency higher than 99% for 2600 h and 120 h of electrolysis, respectively. The improvement in the electrode durability by repeated deposition was attributed to the formation of single phase oxide deposits of MnMoW-oxides with optimized thickness, composition and structural water content, which enables the reduction of the electrode overpotential for the oxygen evolution reaction and increase overall stability and adhesion of the oxide deposits to Ti/IrO₂ substrate. A mechanism for oxygen evolution at the anode surface during electrolysis was proposed.     

Enhancement of hydrogen evolution reaction by Pt nanopillar-array electrode in alkaline media and the effect of nanopillar length on the electrode efficiency

Pt nanopillar-array 3D electrodes with nanopillar length of 150, 450 and 900 nm and nanopillar density of ~10⁹ cm⁻² were fabricated. Their catalytic activity for hydrogen evolution reaction (HER) was evaluated by linear sweep voltammetry and electrochemical impedance spectroscopy. In comparison with straightly electrodeposited black Pt film and forged Pt sheet electrodes, the HER current density has been significantly improved by the nanopillar-array architecture. The overpotential of HER at current density of 10 mA cm-2 at 26 °C is as low as 78 mV, lower than the black Pt film of 107 mV and the Pt sheet of 174 mV. The improvement of HER is ascribed to the low charge transfer resistance of the 3D electrode and the high desorption capability of hydrogen bubbles at the nanotips. Interestingly, the nanopillar-array 3D electrode has an optimal nanopillar length for HER. The mechanisms for the optimal nanopillar length were investigated here.     

Porous NiFe alloys synthesized via freeze casting as bifunctional electrocatalysts for oxygen and hydrogen evolution reaction

Binder-free NiFe-based electrocatalyst with aligned pore channels has been prepared by freeze casting and served as a bifunctional catalytic electrode for oxygen and hydrogen evolution reaction (OER and HER). The synergistic effects between Ni and Fe result in the high electrocatalytic performance of porous NiFe electrodes. In 1.0 M KOH, porous Ni₇Fe₃ attains 100 mA cm⁻² at an overpotential of 388 mV with a Tafel slope of 35.8 mV dec⁻¹ for OER, and porous Ni₉Fe₁ exhibits a low overpotential of 347 mV at 100 mA cm⁻² with a Tafel slope of 121.0 mV dec⁻¹ for HER. The Ni₉Fe₁//Ni₉Fe₁ requires a low cell voltage of 1.69 V to deliver 10 mA cm⁻² current density for overall water splitting. The excellent durability at a high current density of porous NiFe electrodes has been confirmed during OER, HER and overall water splitting. The fine electrocatalytic performances of the porous NiFe-based electrodes owing to the three-dimensionally well-connected scaffolds, aligned pore channels, and bimetallic synergy, offering excellent charge/ion transfer efficiency and sizeable active surface area. Freeze casting can be applied to design and synthesize various three-dimensionally porous non-precious metal-based electrocatalysts with controllable multiphase for energy conversion and storage.     

Surface engineering of CeO2–TiO2 composite electrode for enhanced electron transport characteristics and alkaline hydrogen evolution reaction

Herein, we report the fine tuning of electrocatalytic characteristics of CeO₂–TiO₂ composite by surface engineering to reduce overpotential and to improve exchange current density for enhanced alkaline hydrogen evolution reaction (HER). The enhanced electrocatalytic activity of the surface engineered CeO₂–TiO₂ composite through Ni and P decoration is attributed to the improved electron transport ability. The surface roughness characteristics and surface composition of electroactive species are tuned to generate high electronic conductivity on the surface engineered composite electrode surface. The developed hard electrode with leptokurtic surface (S_ku > 3) exhibited a high average roughness value (Sa) of 3 μm due to incorporation of the mesoporous catalyst material into it. Tuning of a compact and continuous electrode surface with critical composition of elements Ni (52 at.%), P (20 at.%), Ce (9 at.%) and Ti (8 at.%) furnishes the high conductivity (contact potential difference = 0.83 V) to the electrode. The developed electrode with surface engineered CeO₂–TiO₂ catalyst exhibited a low overpotential of −111 mV (at a high current density of 250 mA cm⁻²) and high exchange current density (1.6 × 10⁻¹ mA cm⁻²) with low charge transfer resistance (615 Ω cm²). High electrocatalytic activity and stability of the surface engineered CeO₂–TiO₂ catalyst electrode during alkaline (32 w/v.% NaOH) HER ensure its promising performance and applicability for long term HER.     

Hydrothermally/electrochemically decorated FeSe on Ni-foam electrode: An efficient bifunctional electrocatalysts for overall water splitting in an alkaline medium

A new hybrid catalyst based on Ni foam (NF) and FeSe was prepared by a facial hydrothermal method, in which Se-decorated NF was subsequently electrochemically doped by Fe. Binder-free catalyst containing electrodes were directly tested for the hydrogen and oxygen evolution reaction (HER/OER). The FeSe/NF electrode displayed an OER current density of 100 mA cm⁻² at potential of 1.42 V, and a relatively small Tafel slope of 109 mV dec⁻¹ in a 1 M KOH solution. Also, FeSe/NF electrode exhibited reasonable HER overpotential of 200 mV at 10 mAcm⁻² current density with Tafel slope of 145 mV dec⁻¹. The XRD and TEM studies revealed that the formation of heterogeneous interfaces of NiSe₂ and FeSe₂，generated more active sites that can promote better ions and electron transport in the electrode/electrolyte interfaces. Furthermore, HRTEM analysis indicates that FeSe₂ rich in Se vacancy defects can be created with suitable M − O and M − H bond for better OER and HER performance, respectively. In a-two electrode alkaline water electrolyzer, current densities of 10 mA cm⁻² and 50 mA cm⁻² were obtained at cell voltages of 1.52 V and 1.85 V, respectively, using pure FeSe–NF as both the cathode and anode.     

Electrocatalytic properties of porous Ni-Co-WC composite electrode toward hydrogen evolution reaction in acid medium

Porous Ni-Co-(WC)_x ternary composite electrodes were fabricated by means of electrodeposition on a foam Ni substrate. The surface morphology and microstructure of the electrodes were characterized by scanning electron microscopy (SEM) and X-ray diffraction (XRD). The electrocatalytic properties of porous Ni-Co-(WC)_x electrodes for hydrogen evolution reaction (HER) in 0.5 M H₂SO₄ solution at temperatures from 25 to 50 °C were conducted by means of cathodic polarization, electrochemical impedance spectroscopy (EIS), cyclic voltammetry and chronoamperometry (CA). These Ni-Co-WC electrodes are efficient electrocatalysts for HER. Compared with the porous Ni-Co electrode, the porous Ni-Co-(WC)_x electrode exhibited a lower HER overpotential, a lower electrochemical impedance, a lower apparent activation energy and a higher exchange current density. The apparent exchange current density of porous Ni-Co-(WC)_x (x = 10, 20, 30 and 40 g/l) is 2.01, 3.01, 7.8 and 19.91 times of porous Ni-Co electrode, respectively. With the increase of WC concentration and temperature, the apparent exchange current density of HER was enhanced. With the increase of WC concentration and potential, the HER resistance and the activation energy decreased. The Ni-Co-(WC_)x electrode exhibited superior corrosion resistance and stability for HER.     

Co0.85Se on three-dimensional hierarchical porous graphene-like carbon for highly effective oxygen evolution reaction

Designing an efficient, cheap and abundant catalyst for oxygen evolution reaction (OER) is crucial for the development of sustainable energy sources. A novel catalyst which could be a promising candidate for such electrocatalysts is described. Co_0.85Se supported on three-dimensional hierarchical porous graphene-like carbon (HPG) exhibits outstanding catalytic performances for OER in alkaline medium. It is found that the onset overpotential is 311 mV on the Co_0.85Se/HPG electrode, which is more 28 and 41 mV negative than that on the Co/HPG and Co₃O₄/HPG electrodes. What's more, the value of Tafel slope is 61.7 mV dec⁻¹ and the overpotential at the current density of 10 mA cm⁻² is 385 mV on this electrode. The Co_0.85Se/HPG of this work is an appealing electrocatalyst for OER in basic electrolyte.     

Cobalt-based composite films on electrochemically activated carbon cloth as high performance overall water splitting electrodes

An attractive approach to obtain effective and stable electrode for water electrolysis is to directly deposit the electrocatalyst on current collector surface. Herein, we show the influence of electrochemical activation of carbon cloth substrate on the morphology and electrocatalytic properties of bifunctional electrodes for both hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). A simple one-step electrodeposition technique was applied to directly grow mixed Co-based films on electrochemically activated carbon cloth (EACC) surface. The produced films are composed of metallic Co, and largely amorphous CoO/Co(OH)₂ phases. Variation of Co²⁺ concentration in the solution for electrodeposition enabled tuning the composition of mixed films in order to achieve the optimal HER and OER electrocatalytic performance in 0.1 M KOH. The synthesized electrodes require the overpotentials of 195 mV for HER and 340 mV for OER to deliver the current density of 10 mA/cm². The results indicate that the facile oxidation of carbon cloth prior to the electrodeposition decreases the overpotential at 10 mA/cm² by 150 and 60 mV for HER and OER respectively, thus opening the perspective of improving the activity of carbon-based self-supported composite electrocatalytic electrodes for advanced energy conversion processes.     

Modification of stainless steel fiber felt via in situ self-growth by electrochemical induction as a robust catalysis electrode for oxygen evolution reaction

The development of cost-effective, highly efficient and robust electrodes for oxygen evolution reaction (OER) is greatly significant for water-electrolysis to produce hydrogen. In this paper, we report a stainless steel fiber felt (SSF) electrode with greatly enhanced OER catalytic performance and durability. The SSF is directly treated by cyclic voltammetry (CV) method in alkaline electrolyte, which is more facile and convenient than the traditional measures. The characterization results of X-ray diffraction, X-ray photoelectron spectroscopy and scanning electron microscopy indicate that an ultra-thin layer composed of Fe/Ni/Cr hydroxides/oxides with 3D open nanoporous structure is formed on the surface of SSF after CV treatment. The electrochemical tests show that the prepared SSF electrode displays a very low overpotential of 230 mV at 10 mA cm⁻², a small Tafel slope of 44 mV dec⁻¹ and good long-term durability of 550 h in 1 M KOH. The excellent OER performance of SSF electrode is contributed to the formation of hybrid metal hydroxides/oxides on its surface via in situ self-growth by electrochemical induction. Furthermore, the electrode only requires an overpotential of 340 mV at 10 mA cm⁻² in 0.5 M Na₂CO₃/NaHCO₃ solution. It is expectable that the modified SSF will be a promising catalysis electrode for water-electrolysis in large-scale commercial production.     

Growth of ultrathin nanosheets of nickel iron layered double hydroxide for the oxygen evolution reaction

Because of low cost and abundance, nickel-iron double layered hydroxide (NiFe LDH) is seen as a viable substitute for noble-metal-based electrodes for the oxygen evolution reaction (OER). Herein, we report the growth of NiFe LDH in the form of fine nanosheets in a single step using benzyl alcohol-mediated chemistry. The electrochemical studies clearly suggest that benzyl alcohol is capable of inducing effective chemical interaction between Ni and Fe in the NiFe LDH. The overpotential to produce benchmark 10 mA cm^?2 (η₁₀) for the NiFe LDH electrode is only ～270 mV_RHE, which is much smaller than those of benchmark IrO₂ (η₁₀ = 318 mV_RHE), nickel hydroxide (η₁₀ = 370 mV_RHE) and iron hydroxide (η₁₀ = 410 mV_RHE) for the OER. The difference of the overpotential requirement increases further with increasing current density, indicating faster kinetics of the OER at the catalytic interface of the NiFe LDH. Estimation of Tafel values verifies this notion – the Tafel slopes of NiFe LDH, Ni(OH)₂, and FeOOH are calculated to be 48.6, 55.8, and 59.3 mV dec^?1, respectively. At η = 270 mV, the turnover frequency (TOF) of the NiFe LDH is 0.48 s^?1, which is ～8 and ～11 folds higher than those of Ni(OH)₂ (0.059 s^?1) and FeOOH (0.042 s^?1). In addition to Tafel and TOF, the NiFe LDH electrode has favorable electrochemically active surface area and electrochemical impedance. The electrochemical stability of the NiFe LDH electrode is assessed by conducting potentiostatic measurements at η = 270 mV_RHE (～10 mA cm^?2) and at η = 355 mV_RHE (～30 mA cm^?2) for 24 h of continuous oxygen production.