A Li-doped Co3O4 oxygen evolution catalyst for non-precious metal alkaline anion exchange membrane water electrolysers

Li-doped Co₃O₄ (Li_xCo_3−xO₄, x = 0, 0.07, 0.21, 0.35, 0.49) spinel powders were prepared with a thermal decomposition method and characterized by XRD, SEM, TEM, and XPS. The Li_xCo_3−xO₄ samples were formed as tetragonal powders with a simple spinel structure and with particle sizes about 30–40 nm. All Li_xCo_3−xO₄ samples exhibited a 50 mV more negative onset potential for oxygen evolution reaction (OER) than Co₃O₄. The influence of Li-doping is discussed regarding cation distribution, electronic conductivity and oxygen binding energy. Li_0.21Co_2.79O₄ exhibited the highest OER activity amongst the five samples. A single cell, non-precious metal alkaline anion exchange membrane water electrolysers (AAEMWE) with Li_0.21Co_2.79O₄ anode exhibited a current density of 300 mA cm⁻² at a voltage 2.2 to 2.05 V at temperatures of 20–45 °C and the stability was examined with a continuous operation for 10 h at 300 mA cm⁻² and at 30 °C.     

Zeolite-templated IrxRu1−xO2 electrocatalysts for oxygen evolution reaction in solid polymer electrolyte water electrolyzers

Ir_xRu_1−xO₂(1 ≥ x ≥ 0) with nanorod structure were successfully synthesized by employed pre-filling the Ir and/or Ru guest species into the peripheral-pore of NH₂-modified as-synthesized SBA-15 and explored as electrocatalyst for oxygen evolution reaction (OER) in water electrolyzers. Various physicochemical parameters for zeolite template and/or Ir_xRu_1−xO₂ were obtained by SEM, TEM, XRD, EDX and N₂ gas absorption/desorption measurements. The morphology for prepared Ir_xRu_1−xO₂ samples with individual and/or cluster nanorods was changed with the component difference. Cyclic voltammetry, linear sweep voltammetry, electrochemical impedance spectroscopy, steady state polarization curves and stability tests were performed to investigate the catalytic activity and stability of these electrocatalysts for OER. The cell with catalyst RuO₂ showed best catalytic performance with the lowest onset potential (1.374 V at 10 mA cm⁻²), which may be ascribed to regular nanoclusters and larger outer active surface area. Meanwhile, the cell stability tests suggested that the addition of IrO₂ in Ir_xRu_1−xO₂ improved the stability of the RuO₂ catalyst.     

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.     

Effect of V substitution at B-site on the physicochemical and electrocatalytic properties of spinel-type NiFe2O4 towards O2 evolution in alkaline solutions

Spinel-type ternary transition metal oxides of nickel, iron and vanadium with composition NiFe_2−xV_xO₄ (0 ≤ x ≤ 1) have been synthesized by a hydroxide precipitation method and investigated for their physicochemical and electrochemical catalytic properties towards the oxygen evolution reaction (OER) in alkaline solutions at 25 °C. The OER study indicates that substitution of V from 0.25 to 1.0 mol for Fe in the spinel matrix increases the electrocatalytic activity of the oxide greatly; the activity being the greatest with x = 0.5 mol. At low overpotentials, the Tafel slope and the order for the OER with respect to OH⁻ concentration were found to be ∼40 mV and ∼2, respectively.     

Performance of a PEM electrolyzer using RuIrCoOx electrocatalysts for the oxygen evolution electrode

The Proton Exchange Membrane Water Electrolyzer (PEMWE) can be coupled to renewable energy sources (solar radiation and wave energy), which produce the necessary electricity for splitting the water. In this work the performance of a PEMWE using RuIrCoO_x as anodic electrocatalyst had been examined. The oxide powder was synthesized using a chemical reduction method, followed by thermal oxidation. The electrochemical properties of the electrocatalysts were examined by cyclical and lineal voltammetry in 0.5 M H₂SO₄. It was found that RuIrCoO_x oxide electrodes present a stable performance for OER. The PEMWE was designed and in-home built. Chrono-potentiometric experiments were recorded in the current range of 0.25 mA cm⁻² to 75 mA cm⁻² at 300 s. The current pulses length is chosen to be sufficiently long so that the voltage remains constant. Their intrinsic electrocatalytic activity in combination with their large surface area and stability are quite promising for the development of economically feasible electrocatalysts for (PEMWE).     

Electronic and structural engineering of NiCo2O4/Ti electrocatalysts for efficient oxygen evolution reaction

The oxygen evolution reaction (OER) involves four electron transfer processes and is of great significance in water electrolysis. The development of efficient and robust non-precious OER electrocatalysts remains a critical challenge for the production, storage and conversion of renewable energy. Herein, vertically NiCo₂O₄ nanosheets are grown on Ti mesh via a facile solvothermal method which is followed by low-temperature calcination. The NiCo₂O₄/Ti catalyst exhibits outstanding OER performance with a low overpotential of 353 mV to drive the current density of 10 mA cm^?2 and a Tafel slope of 61 mV dec^?1 in alkaline solution. Moreover, the stable electrocatalyst undergoes negligible degradation in alkaline media at least 20 h. The acceleration of the electrochemical OER likely stems from the facile electron transfer promoted by the NiCo₂O₄/Ti interface as revealed by X-ray photoelectron spectroscopy. This work introduces a novel strategy for the establishment low-cost electrocatalysts for electrochemical water splitting.     

Novel FexCr2−x(MoO4)3 electrocatalysts for oxygen evolution reaction

Transition metal mixed oxides of Fe, Cr and Mo with nominal compositional formula, Fe_xCr_2−x(MoO₄)₃ (x = 0, 0.25, 0.50 and 0.75) have been obtained by a co-precipitation method and investigated for their structural and electrocatalytic properties by XRD, TEM, XPS, BET, electrochemical impedance spectroscopy and anodic Tafel polarization. Results show that introduction of Fe for Cr from 0.25 to 0.75 mol into the Cr₂(MoO₄)₃ matrix improved the electrocatalytic activity toward the O₂ evolution reaction (OER) in 1 M KOH considerably; the magnitude of improvement being maximum with 0.5 mol Fe. Values of the Tafel slope were close to 35 mV at low and 2.303RT/F at high overpotentials on Fe-substituted oxides. The OER follows nearly second order kinetics in OH⁻ concentration at low overpotentials.     

Ultrathin MoS2 nanosheets in situ grown on rich defective Ni0.96S as heterojunction bifunctional electrocatalysts for alkaline water electrolysis

Developing earth-abundant and highly active bifunctional electrocatalysts are critical to advance sustainable hydrogen production via alkaline water electrolysis but still challenging. Herein, heterojunction hybrid of ultrathin molybdenum disulfide (MoS₂) nanosheets and non-stoichiometric nickel sulfide (Ni_0.96S) is in situ prepared via a facile one-step hydrothermal strategy, followed by annealing at 400 °C for 1 h. Microstructural analysis shows that the hybrid is composed of intimate heterojunction interfaces between Ni_0.96S and MoS₂ with exposed active edges provided by ultrathin MoS₂ nanosheets and rich defects provided by non-stoichiometric Ni_0.96S nanocrystals. As expected, it is evaluated as bifunctional electrocatalysts to produce both hydrogen and oxygen via water electrolysis with a hydrogen evolution reaction (HER) overpotential of 104 mV at 10 mA cm⁻² and an oxygen evolution reaction (OER) overpotential of 266 mV at 20 mA cm⁻² under alkaline conditions, outperforming most current noble-metal-free electrocatalysts. This work provides a simple strategy toward the rational design of novel heterojunction electrocatalysts which would be a promising candidate for electrochemical overall water splitting.     

Ni3S2-MoSx nanorods grown on Ni foam as high-efficient electrocatalysts for overall water splitting

In order to solve the problem of large overpotential in water electrolysis for hydrogen production, transition metal sulfides are promising bifunctional electrocatalysts for hydrogen evolution reaction/oxygen evolution reaction that can significantly reduce overpotential. In this work, Ni₃S₂ and amorphous MoS_x nanorods directly grown on Ni foam (Ni₃S₂-MoS_x/NF) were prepared via one-step solvothermal process, which were used as a high-efficient electrocatalyst for overall water splitting. The Ni₃S₂-MoS_x/NF composite exhibits very low overpotentials of 65 and 312 mV to reach 10 mA cm⁻² and 50 mA cm⁻² in 1.0 M KOH for HER and OER, respectively. Besides, it exhibits a low Tafel slope (81 mV dec⁻¹ for HER, 103 mV dec⁻¹ for OER), high exchange current density (1.51 mA cm⁻² for HER, 0.26 mA cm⁻² for OER), and remarkable long-term cycle stability. This work provides new perspective for further the development of highly effective non-noble-metal materials in the energy field.     

MoS2/NiFeS2 heterostructure as a highly efficient electrocatalyst for overall water splitting at high current densities

Searching for efficient, stable and low-cost nonprecious catalysts for oxygen and hydrogen evolution reactions (OER and HER) is highly desired in overall water splitting (OWS). Herein, presented is a nickel foam (NF)-supported MoS₂/NiFeS₂ heterostructure, as an efficient electrocatalyst for OER, HER and OWS. The MoS₂/NiFeS₂/NF catalyst achieves a 500 mA cm⁻² current density at a small overpotential of 303 mV for OER, and 228 mV for HER. Assembled as an electrolyzer for OWS, such a MoS₂/NiFeS₂/NF heterostructure catalyst shows a quite low cell voltage (≈1.79 V) at 500 mA cm⁻², which is among the best values of current non-noble metal electrocatalysts. Even at the extremely large current density of 1000 mA cm⁻², the MoS₂/NiFeS₂/NF catalyst presents low overpotentials of 314 and 253 mV for OER and HER, respectively. Furthermore, MoS₂/NiFeS₂/NF shows a ceaseless durability over 25 h with almost no change in the cell voltage. The superior catalytic activity and stability at large current densities (>500 mA cm⁻²) far exceed the benchmark RuO₂ and Pt/C catalysts. This work sheds a new light on the development of highly active and stable nonprecious electrocatalysts for industrial water electrolysis.     

Electrochemical performance of La2Cu1−xCoxO4 cathode materials for intermediate-temperature SOFCs

K₂NiF₄-type structure oxides La₂Cu_1−xCo_xO₄ (x = 0.1, 0.2, 0.3) are synthesized and evaluated as cathode materials for intermediate temperature solid oxide fuel cells (IT-SOFCs). The materials are characterized by XRD, SEM and electrochemical impedance spectrum (EIS), respectively. The results show that no reaction occurs between La₂Cu_1−xCo_xO₄ electrode and Ce_0.9Gd_0.1O_1.95 (CGO) electrolyte at 1000 °C. The electrode forms good contact with the electrolyte after sintering at 800 °C for 4 h in air. The electrode properties of La₂Cu_1−xCo_xO₄ are studied under various temperatures and oxygen partial pressures. The optimum composition of La₂Cu_0.8Co_0.2O₄ results in 0.51 Ω cm² polarization resistance (R_p) at 700 °C in air. The rate limiting step for oxygen reduction reaction (ORR) is the charge transfer process. La₂Cu_0.8Co_0.2O₄ cathode exhibits the lowest overpotential of about 50 mV at a current density of 48 mA cm⁻² at 700 °C in air.     

Electrode properties of Co-doped Ca2Fe2O5 as new cathode materials for intermediate-temperature SOFCs

Brownmillerite oxide Ca₂Fe_2−xCo_xO₅ (x = 0.2, 0.4, 0.6) was characterized by XRD, SEM and electrochemical impedance spectrum (EIS), respectively. Ca₂Fe_2−xCo_xO₅ has no reaction with Sm_0.2Ce_0.8O_1.9 (SDC) electrolyte at 1100 °C for 10 h in air. The thermal expansion coefficient (TEC) of Ca₂Fe_2−xCo_xO₅ increased with increasing Co content, and the TEC value was compatible with SDC. The electrode properties of Ca₂Fe_2−xCo_xO₅ were studied under various temperatures and oxygen partial pressures. The polarization resistance (R_p) of Ca₂Fe_2−xCo_xO₅ with x = 0.2, 0.4 and 0.6 are 0.23, 0.48 and 1.05 Ω cm² at 700 °C in air, respectively. The rate-limiting step for oxygen reduction reaction was the charge transfer process. Ca₂Fe_1.8Co_0.2O₅ cathode exhibits the lowest overpotential of about 50 mV at a current density of 70 mA cm⁻² at 700 °C in air.     

Syngas production at intermediate temperature through H2O and CO2 electrolysis with a Cu-based solid oxide electrolyzer cell

Solid Oxide Electrolyzer Cells (SOECs) are promising energy devices for the production of syngas (H₂/CO) by H₂O and/or CO₂ electrolysis. Here we developed a Cu–Ce_0.9Gd_0.1O_2−δ/Ce_0.8Gd_0.2O_2−δ/Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3−δ-Ce_0.8Gd_0.2O_2−δ cell and performed H₂O and CO₂ electrolysis experiments in the intermediate temperature range (600°C–700 °C). As a baseline, the cell was first tested in fuel cell operation mode; the sample shows a maximum power density peak of 104 mW cm⁻² at 700 °C under pure hydrogen and air. H₂O electrolysis testing revealed a steady production of hydrogen with a Faraday's efficiency of 32% at 700 °C at an imposed current density of −78 mA cm⁻². CO production was observed during CO₂ electrolysis but higher cell voltages were required. A lower efficiency of about 4% was obtained at 700 °C at an imposed current density of −660 mA cm⁻². These results confirm that syngas production is feasible by water and carbon dioxide electrolysis but further improvements from both the manufacturing and the electrocatalytic aspects are needed to reach higher yields and efficiencies.     

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.     

Heterostructural Co/CeO2/Co2P/CoP@NC dodecahedrons derived from CeO2-inserted zeolitic imidazolate framework-67 as efficient bifunctional electrocatalysts for overall water splitting

The design and fabrication of highly active, robust and cost-efficient electrocatalysts for both oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) is of great significance towards overall water splitting, but remains challenging as well. Herein, we report, for the first time, heterostructural Co/CeO₂/Co₂P/CoP@NC dodecahedrons as bifunctional electrocatalyst, in which abundant interfaces are formed between different components. Typical ZIF-67 (ZIF = zeolitic imidazolate framework) dodecahedrons with pre-inserted CeO₂ nanowires were selected as precursors to synthesize Co/CeO₂/Co₂P/CoP@NC via a direct carbonization process followed by phosphidation, simultaneously generating the strong coupled heterojunction interfaces through interactions between CeO₂ and Co_xP species. Abundant porous structure leads to the exposure of more active sites and the carbon encapsulation of nanodomains sustains the high robustness and conductivity and the synergistic effect between the multi-components heterostructure. Benefiting from the above collective advantages, the Co/CeO₂/Co₂P/CoP@NC electrocatalysts exhibit small overpotentials of 307 and 195 mV to derive 10 mA cm⁻² for OER and HER, respectively. Furthermore, an alkaline electrolyzer assembled by using Co/CeO₂/Co₂P/CoP@NC as both cathode and anode can achieve a current density of 10 mA cm⁻² at a low voltage of 1.76 V and work continuously for over 15 h. This work would provide a rational protocol for fabrication multi-phase interface enriched electrocatalysts toward highly efficient energy conversion.     

La0.75Sr0.25Cr0.5Mn0.5O3 as hydrogen electrode for solid oxide electrolysis cells

La_0.75Sr_0.25Cr_0.5Mn_0.5O₃ (LSCM) has been applied as hydrogen electrode (cathode) material in solid oxide electrolysis cells operating with different steam concentrations (20, 40, 60 and 80 vol.% absolute humidity (AH)) using 40 sccm H₂ carrier gas at 800, 850 and 900 °C, respectively. Impedance spectra and voltage-current curves were measured as a function of cell electrolysis current density and steam concentration to characterize the cell performance. The cell resistance decreased with the increase in electrolysis current density while increased with the increase in steam concentration under the same electrolysis current density. At 1.6 V applied electrolysis voltage, the maximum consumed current density increased from 431 mA cm⁻² for 20 vol.% AH to 593 mA cm⁻² for 80 vol.% AH at 850 °C. Polarization and impedance spectra experiments revealed that LSCM-YSZ hydrogen electrode played a major role in the electrolysis reaction.     

Structure-design and synthesis of nickel-cobalt oxide/sulfide/phosphide composite nanowire arrays for efficient overall water splitting

The construction of cost-effective bifunctional electrocatalysts with the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) is significant for efficient overall water splitting. Herein, this work demonstrates a novel strategy for the synthesis of nickel-cobalt oxides/sulfides/phosphides composite (denoted as NiCoO–2P/S) nanoarrays on Ni foam. In this method, Ni–Co bimetallic oxide nanowires on Ni foam were partially phosphorized and sulfurized simultaneously in situ to yield Ni–Co oxide/sulfide/phosphide composite. The NiCoO–2P/S arrays have good interfacial effects and display many holes in the nanowires, giving it the advantage of large accessible surfaces on the nanowires and a beneficial for the release of gas bubbles, resulting in an excellent OER performance with a low overpotential (η) of 254 mV at 100 mA cm^?2 and good HER activity (η₁₀ = 143 mV at 10 mA cm^?2). The electrocatalytic test results demonstrate small Tafel slopes (82 mV dec^?1 for HER, 88 mV dec^?1 for OER) and the satisfying durability in an alkaline electrolyte, indicating that the HER and OER activity was enhanced by the introduction of the Ni/Co sulfides and phosphides into Ni–Co oxides composite nanowires. Furthermore, the as-prepared NiCoO–2P/S catalyst can be used as both the anode and the cathode simultaneously to realize overall water splitting in the two-electrode electrolyzer. This system can be driven at low cell voltages of 1.50 and 1.68 V to achieve current densities of 10 and 100 mA cm^?2, respectively. This work provides an alternative strategy to prepare high-performance bifunctional electrochemical materials and demonstrates the advantages of Ni–Co oxide/sulfide/phosphide composites for water splitting.     

Hydrogen production via highly efficient electrocatalyst based on 3D NixCo1-x(OH)2/NiFe-AM induce overall water splitting

The main factors limiting water splitting producing hydrogen production are overpotential, activity and persistence of electrocatalysts. Herein, a novel Ni_xCo_1-x(OH)₂ coupled with NiFe amorphous compound array growing on nickel foam substrate (expressed as Ni_xCo_1-x(OH)₂/NiFe-AM) was developed by facile hydrothermal and electrodeposition methods. Significantly, Ni_xCo_1-x(OH)₂/NiFe-AM with this unique structural exhibits superior activity and stability in the two half reactions of water electrolysis. In addition, when tested in an alkaline electrolyte with a current density of 10 mA cm⁻², the overpotentials of HER and OER was 157 mV and 196 mV (60 mA cm⁻²), respectively. The stability can up to 60 h. These test results show through constructing hierarchical nano-thron architecture enhanced electrocatalytic activity to produce hydrogen and oxygen.     

Efficient overall water splitting using nickel boride-based electrocatalysts

Currently there is tremendous interest in the discovery of low cost and efficient electrocatalysts for the oxygen evolution reaction (OER) and the hydrogen evolution reaction (HER). In this work, iron-doped nickel boride (Fe_xNi_1-xB) and nickel boride (NiB) were successfully grown on 3D self-supporting graphene (SSG) electrodes via a one-step reduction approach. The Fe_0.2Ni_0.8B/SSG electrode required a very low overpotential of only 263 mV for OER (the best OER activity achieved to date for a metal boride). NiB/SSG showed modest OER performance but excellent HER activity. A water electrolyzer comprising Fe_0.2Ni_0.8B/SSG and NiB/SSG delivered a current density of 10 mA cm⁻² at a voltage of only 1.62 V. Further, the Fe_0.2Ni_0.8B/SSG and NiB/SSG catalysts showed excellent stability with no deactivation observed over 14 h of testing. Results demonstrate that nickel boride-based electrocatalysts are promising lost cost alternatives to precious metal-based electrocatalysts for OER, HER and overall water splitting.     

Mesoporous spinel NiFe oxide cubes as advanced electrocatalysts for oxygen evolution

Oxygen evolution reaction (OER) is the rate-controlling step of the electrochemical water splitting. The slow kinetics hinders large-scale H₂ production. Herein, the spinel NiFe oxides were prepared by directly pyrolyzing nickel hexacynoferrate precursors in air. The NiFe oxides were presented as mesoporous nanocubes with a specific surface area of 125 m² g⁻¹. The mesoporous spinel NiFe oxide nanocubes can afford a geometric current of 10 mA cm⁻² at a low overpotential of a 0.24 V and a small Tafel slope of 41 mV dec⁻¹ in alkaline solution. The specific activity can reach up to 0.37 mA cm⁻² with a turnover frequency of 0.93 s⁻¹. The superior OER activity of the NiFe oxide nanocubes (NiFeO NCs) can outperform those of the state-of-the-art IrO₂ catalysts, and compare favorably with other spinel transition metal oxides reported recently under identical condition. NiFeO NCs also show a long-term durability without significant loss of the OER activity. Our works provide a new strategy to develop efficient, robust and earth-abundant spinel NiFe oxides as advanced OER electrocatalysts to replace the expensive commercial IrO₂ catalysts for water splitting in the industrial scale.