Oxygen Evolution Reaction In Alkaline Solution Research Articles

Construction of iron-doped nickel cobalt phosphide nanoparticles via solvothermal phosphidization and their application in alkaline oxygen evolution

Multi-metallic phosphides offer the possibility to combine the strategies of surface reconstruction, electronic interaction and mechanistic pathway tuning to achieve high electrocatalytic oxygen evolution activity. Here, iron-doped nickel cobalt phosphide nanoparticles (FexCoyNi2-x-yP) with the crystalline NiCoP phase are for the first time synthesized by the solvothermal phosphidization method via the reaction between metal–organic frameworks and white phosphorus. When used to electrochemically catalyze oxygen evolution reaction (OER), the Fe0.4Co0.8Ni0.8P supported by nickel foam requires only 248 mV overpotential to achieve 10 mA cm−2 current densities, and is robust towards the long-term OER in 1 M KOH. The higher number of electrochemically active sites can account for the good OER activity, along with the improved intrinsic activity which is caused by the electron interaction that optimizes the adsorption energy of hydroxyl intermediates, and that increases the acidity of high-valent metal centers. The OER mechanistic pathway involves both adsorbate and lattice oxygen. Surface conversion is observed after OER in alkaline solution, and metal phosphide layer transforms to metal oxides and (oxy)hydroxides.

Improved electrocatalytic activity of selenide nanoparticle for oxygen evolution reaction

Oxygen Evolution Reaction (OER) plays a vital role in water electrolysis, but its slow kinetics and expensive catalysts are obstacles to widespread use. To improve the OER activity, it is crucial to create a well-structured design for electrocatalysts based on transition metals, which may be applied to a conducting polymer substrate. This research reports a facile hydrothermal synthesis and remarkable catalytic behavior of NbSe2/g-CN as an OER catalyst in an alkaline condition. The fabricated electrocatalysts were examined utilizing numerous analytical techniques to assess their crystallinity, oxidation states and morphology. NbSe2/g-CN shows enhanced electro-catalytic behavior to OER in alkaline solution (1.0 KOH) with minor overpotential (196 mV) and Tafel slop (38 mV/dec) than pristine NbSe2 electrocatalyst was associated with the exceptional conductivity of NF and its highly porous structure, along with a greater SSA. In addition, the durability of the electrocatalyst was examined via cyclic stability and chronoamperometry study over 40 h. This study demonstrates that TMSe with specific morphology can improve electrocatalytic activity when combined with g-CN, demonstrating its potential for reliable and eco-friendly energy generation

Co/Co3O4 Heterojunctions Encased in Porous N-Doped Carbon Nanocapsules for High-Performance Cathode of Rechargeable Zinc-Air Batteries.

A long-term goal of rechargeable zinc-air batteries (ZABs) has always been to design bifunctional electrocatalysts that are robust, effective, and affordable for the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). It has become a feasible method to construct metal/metal oxide interfaces to achieve superior electrocatalytic performance for ORR and OER by enhanced charge transfer. In this study, Co/Co3O4 heterojunctions were successfully prepared and encased in porous N-doped mesoporous carbon (Co/Co3O4@NC) via a simple condensation-carbonization-etching method. The extensive specific surface area of Co/Co3O4@NC facilitates effective interaction between the electrolyte and the catalyst, thereby enabling sufficient exposure of active sites for the ORR and the OER, consequently enhancing the rate of transport of active species. The well-designed Co/Co3O4@NC delivers superior ORR catalytic activity with a half-wave potential of 0.82 V (vs RHE) and a low overpotential of 347 mV at 10 mA cm-2 for OER in alkaline solution. The power density of Co/Co3O4@NC-based alkaline aqueous ZAB (156.5 mW cm-2) is superior to the commercial Pt/C + IrO2-based alkaline aqueous ZAB, and the cycling stability of ZAB is up to 220 h. In addition, Co/Co3O4@NC-based ZAB shows a high power density (50.1 mW cm-2). The construction of metal/metal oxide heterojunction encased in N-doped mesoporous carbon provides a novel route for the design of bifunctional electrocatalysts for high-performance ZABs.

Tracking Oxygen Evolution Activities of Perovskite Oxide Catalysts By 3D Electrochemical Impedance Spectroscopy

IntroductionPromoting the oxygen evolution reaction (OER) is important for more efficient electrochemical water splitting because its sluggish reaction kinetics degenerate the overall efficiency. Perovskite oxides (ABO3) are known as promising electrocatalysts for the OER, which have a flexibility of composition and structure. Among these electrocatalysts, Ba0.5Sr0.5Co0.8Fe0.2O3–δ (BSCF) exhibits good OER activities in an alkaline solution1. One of its specific properties is the change of the surface structure and composition during the OER in alkaline solutions, leading the improvement of the OER activity2. In general, the OER activities of metal oxides significantly depend on electrochemical surface area (ECSA) and activity per surface area (specific activity). However, it is not easy to decouple these two factors when the surface structure and composition change continuously during the OER.Electrochemical impedance spectroscopy (EIS) has been used as a useful tool, which provides information related to electrode/electrolyte interface3. Here, in order to reveal the origin of the OER activities behavior, we used 3D EIS including time axis and tracked the ECSA and specific activity of perovskite oxides during the OER in alkaline solutions.Experimental methodsPerovskite oxides used in this study (BSCF, La0.5Sr0.5Co0.8Fe0.2O3–δ (LSCF) and LaCoO3–δ (LCO)) were synthesized by the sol–gel method.The OER activities of obtained perovskite oxides were evaluated using a three-electrode electrochemical cell with a rotating disk electrode (RDE) in 1.0 mol dm–3 KOH solution. A working electrode was prepared by drop-casting an ink suspension with perovskite oxides (0.25 mg cm–2), acetylene black (0.05 mg cm–2), and Nafion (0.05 mg cm–2) on glassy carbon. Pt wire and a reversible hydrogen electrode (RHE) were used as the counter and reference electrodes, respectively. 3D EIS was obtained in the following steps: First, cyclic voltammetry (CV) was performed at certain cycles (1, 5, 10 ,30, and 50 cycles). Second, the electrode was left at open circuit potential for 5 min. Finally, potentiostatic EIS (PEIS) was recorded 5 times (approximately 10 min), and then Nyquist plots at 5 min from the beginning of the first PEIS were compared.Results and discussionFrom CV measurements of perovskite oxides, while the OER activities of BSCF increased, those of LSCF and LCO slightly decreased with increasing CV cycles. Figure shows Nyquist plots of perovskite oxides after certain CV cycles. The impedance spectra show a semicircle in the high- and mid-frequency regions, respectively. While a semicircle in the high-frequency region was independent on the direct current (DC) potential, the mid-frequency region semicircle was dependent on DC potential. The contrasting behavior indicated that the semicircle in the mid-frequency region corresponds to the charge transfer resistance, which contains the information of double-layer capacitance for the OER. Therefore, we tracked the semicircle in the mid-frequency region during the OER.A semicircle in the mid-frequency region of BSCF gradually decreased with CV cycles, indicating that the BSCF surface changed to a preferred structure during the OER cycles. As a result of the equivalent circuit fitting, the charge transfer resistance decreased and the double-layer capacitance increased with CV cycles. On the other hand, a semicircle in the mid-frequency region of LSCF and LCO remained unchanged or slightly increased.More detailed evaluation of their ECSA and specific activities during OER will be discussed at the conference.Reference[1] J. Suntivich, K. J. May, H. A. Gasteiger, J. B. Goodenough and Y. Shao−Horn, Science, 334, 1383 (2011).[2] K. J. May, C. E. Carlton, K. A. Stoerzinger, M. Risch, J. Suntivich, Y.-L. Lee, A. Grimaud and Y. Shao-Horn, J. Phys. Chem. Lett., 3, 3264 (2012)[3] V. A. Alves, L. A. da Silva, and J. F. C. Boodts, Electrochim. Acta, 44, 1525 (1998). Figure 1

D‐Orbital Manipulated Ru Nanoclusters for High‐Efficiency Overall Water Splitting at Industrial‐Level Current Densities

AbstractOwing to the Pt‐like electrocatalytic capability and moderate price, Ru‐based catalysts are considered as the Pt alternatives for electrochemical water splitting. However, they demonstrate limited catalytic performance under industrial‐level current densities. Herein, a novel electrocatalyst with an extremely low amount (0.85 wt.%) of Ru nanoclusters anchored on Cr‐doped Fe‐metal–organic frameworks (Ru@Cr─FeMOF) through a robust Cr─O─Ru bond is presented. The study unveils that such an architecture facilitates fast electron transfer and manipulates the highest occupied d orbital and d‐band centers of Ru sites, favoring both oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) catalysis. The as‐prepared catalyst performs excellent reaction activity of 21 mV@10 mA cm−2 for HER and 230 mV@50 mA cm−2 for OER in alkaline solution, and realizes excellent water‐splitting performance at industrial‐level current densities (1.72 V@1000 mA cm−2), surpassing the state‐of‐the‐art literatures.

Anodized Stainless Steel 316 and 304 Alloys As Bifunctional Water Splitting Electrocatalytic Materials in Alkaline Media

The world is facing a continued increase in population that is accompanied by a high demand for energy. The dependence on fossil fuels caused demand and environmental crises. Therefore, it is necessary to switch to sustainable, clean, and renewable sources of energy immediately. Using hydrogen as a fuel is considered the most cleanest and promising alternative. Producing hydrogen using water electrolysis is the most environmentally friendly and sustainable method, especially when the electricity is generated from renewable sources. Employing electrolysis for hydrogen production is currently limited due to its high cost and low efficiency. For improving the efficiency of the electrolyzers and significantly decreasing the cost, it is necessary to increase the rate of hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) by designing more active, stable, easy-to-fabricate electrocatalysts that are bifunctional and could be commonly fabricated for both the HER and OER.The different stainless steel (SS) alloys are common and inexpensive Fe-Ni-based alloys that recently attracted attention not only as conductive substrates for HER and OER catalysts but as electrocatalysts themselves.1 Untreated SS is poorly active towards HER or OER and needs further modification, such as electrochemical anodization followed by high-temperature annealing in different gases, to reach an acceptable activity.2,3 Anodized SS annealed under O2 is shown to be catalytically active towards OER while annealing under H2 is required for HER activity.2,3 Herein, the effect of anodizing the SS316 and SS304 alloys at different potentials and bath compositions (H2O, NH4F, H2SO4, and H3PO4 contents) on the electrode morphology and surface composition will be discussed. The anodized SS electrodes are annealed in different thermal annealing atmospheres (O2, and H2) and annealing temperatures (400-800 Cº). The physical/chemical structure of as received SS (R-SS), the annealed SS (A-SS), the anodized SS (AN-SS), and the anodized-annealed SS (A-AN-SS) electrodes were evaluated using XRD, FESEM, EDS, and XPS. The electrocatalytic behavior of the SS electrodes towards HER and OER in Alkaline solutions is evaluated using cyclic voltammetry (CV), linear sweep voltammetry (LSV), chronopotentiometry (CP), and electrochemical impedance spectroscopy (EIS). The results will be discussed thoroughly to correlate the electrochemical behavior to the electrode morphology and surface composition.

Regulating the electronic structure of Ni2P by one-step Co, N dual-doping for boosting electrocatalytic performance toward oxygen evolution reaction and urea oxidation reaction

The development of efficient bifunctional electrocatalysts for oxygen evolution reaction (OER) and urea oxidation reaction (UOR) is critical for hydrogen production and wastewater purification. In this work, we propose a facile synthetic method for Co and N dual-doped Ni2P directly grown on Ni foam (Co-Ni2P-N/NF) using hydrothermal and annealing process. Simultaneous Co and N dual-doping into Ni2P not only modifies the surface electronic structure, but also generates a multitude of active sites with high valence states, which are beneficial for improving electrocatalytic kinetics for both OER and UOR. As a result, the Co-Ni2P-N/NF catalyst exhibits a low overpotential of 329 mV to deliver a current density of 100 mA cm−2 for OER in alkaline solution, which is much lower than that of the state-of-the-art RuO2 electrocatalyst. In addition, the urea-assisted water oxidation process exhibits a significant reduction of approximately 163 mV in the required potential at 100 mA cm−2 compared to that of the OER, which highlights the remarkable potential of the prepared Co-Ni2P-N/NF electrocatalyst in facilitating the purification of wastewater and hydrogen production with significantly lower energy consumption.

Rapid synthesis of nickel–iron-oxide (NiFeOx) solid-solution nano-rods thin films on nickel foam as advanced electrocatalyst for sustained water oxidation

The development of earth-rich, noble-metal-free, and highly electroactive catalysts to accelerate the oxygen evolution reaction (OER) is a formidable challenge for the establishment of water-splitting technologies. Here, a solid-solution nickel-iron oxide (NiFeOx) electrocatalyst is readily grown on a Ni-foam (NF) substrate by a rapid and economical aerosol-assisted chemical vapor deposition process. In particular, the NiFeOx thin film fabricated in 40 min acquires a distinctive nanorod structure and proves to be stable and efficient for OER in alkaline solutions. It is shown that the catalyst needs a low over-potential of 226 mV to reach the typical current density of 10 mA/cm2, and it can approach a remarkable current density level of 1000 mA/cm2 by taking a slightly higher over-potential of 139 mV. The small Tafel slope of 64 mv.dec−1 and splendid electrochemical stability of 40 h at high current densities outperform many known FeNi-based anodes as well as commercial IrO2 and RuO2. The unique structure of the thin film offers many electroactive sites and a high surface area in combination with an improved electrical conductivity of NF, which is believed to play an imperative role in the excellent activity of the catalyst. The cost-effective and simple strategy to fabricate NiFeOx nano-fibrous is very attractive for the development of electrocatalysts for water splitting.

Fabrication of amorphous FeCoNiCuMnPx high-entropy phosphide/carbon composites with a heterostructured fusiform morphology for efficient oxygen evolution reaction

Amorphous high-entropy materials with abundant defects, coordinatively unsaturated sites, and loosely bonded atoms could exhibit excellent electrocatalytic performance. However, how to fabricate such materials with nanostructure as well as amorphous structure is still full of challenges. In this work, high-entropy metal organic framework (HE-MOF) is employed as the self-sacrificial template to fabricate FeCoNiCuMnPx high-entropy phosphide/carbon (HEP/C) composites. The obtained composite shows a heterostructured fusiform morphology, in which the HEP is encapsulated by a carbon layer, revealing high electron conductivity as well as rich catalytic active sites for oxygen evolution reaction (OER). Beside, it is found that there is a short-range ordered crystal structure in the amorphous phase, which is beneficial for revealing high OER catalytic activity as well as good stability. As a result, the optimum HEP/C composite shows an overpotential 239 mV@10 mA cm−2 with a small Tafel slope of 72.5 mV dec−1 for catalyzing OER in alkaline solution.

Solution Plasma‐Assisted Multivariate Metal Nanoalloys Encapsulated with Carbon Dots for Efficient Oxygen Evolution Reaction

AbstractTo accelerate the reaction kinetics of electrocatalytic oxygen evolution reaction (OER), a multicomponent CDs@(PdFeNiCo)Nbx nanohybrid comprising multiple metal nanoalloys and carbon quantum dots (CDs) was in situ generated by treating the mixture of Nb2C quantum dots (Nb2C QDs) and a variety of metal ions (M2+=Pd2+, Fe2+, Co2+, Ni2+) through the solution plasma (SP) modification technique. The CDs@(PdFeNiCo)Nbx catalyst was further explored as an electrocatalyst for OER in alkaline solution. Under nitrogen plasma irradiation with high input power (200 W), diverse bimetallic nanoalloy particles, including NbPd3, FeNb, NbNi3, and CoNbO4 nanoalloys, were generated, accompanied with the decomposition of Nb2C QDs into CDs. In view of the synergic effect between multivariate metal active sites and highly conjugated CD network, the developed CDs@(PdFeNiCo)Nbx electrocatalyst showed a small overpotential of 240 mV for achieving the current density of 10 mA cm−2, together with a very low Tafel slope of 62 mV dec−1, which substantially outperformed those of the state‐of‐the‐art metal electrocatalysts and was competitive with most metal oxides. The CDs@(PdFeNiCo)Nbx electrocatalyst also showed a negligible overpotential change during 30 h chronopotentiometry measurement. The present work can provide a feasible alternative strategy for the development of an excellent OER electrocatalyst using the SP modification method.

Hierarchical porous tri-metallic NiCoFe-Se/CFP derived from Ni-Co-Fe Prussian blue analogues as efficient electrocatalyst for oxygen evolution reaction

The progress of inexpensive, high-efficiency, and steady oxygen evolution reaction (OER) electrocatalysts is of great importance to promoting water splitting for green hydrogen production. Herein, tri-metallic NiCoFe selenide catalyst backed up by carbon fiber paper (CFP) was synthesized by a facile selenization of NiCoFe Prussian blue analogues (PBAs) for OER in alkaline solutions. The NiCoFe-Se/CFP inherited the porous nanostructure of the metal-organic frameworks (MOFs) precursors prepared by rapid cyclic voltammetry electrodeposition. Benefiting from the 3D hierarchical porous structure, optimized electronic structure of NiCoFe selenides and high conductivity, the synthesized electrocatalyst exhibits outstanding catalytic activity to the corresponding mono-metallic or bi-metallic selenides. Specifically, the NiCoFe-Se/CFP electrode demands an overpotential of 221 mV to attain the 10 mA cm−2 current density in 1.0 M KOH solution and a low Tafel slope of 38.6 mV dec-1. The prepared catalyst also displays good stability and durability. These findings prove a feasible strategy to further improve the catalytic activities of non-precious metal based OER electrocatalysts by the cooperation of structure design and chemical component modification.

Lithium Electrochemical Tuning Engineering in an Aqueous System of LiCoO2 for Enhanced Oxygen Evolution Activity

LiCoO2 is a promising catalyst for oxygen evolution reaction (OER), yet its OER activity is still unsatisfactory. Here, the delithiated LiCoO2 nanoparticles (NLCO-x cycle) are prepared via a new lithium electrochemical tuning in an aqueous two-electrode system for promoting OER in alkaline solutions. As a result, quantitative deintercalation of Li+ can be achieved through an aqueous two-electrode system and the OER performance of NLCO-5 cycle is significantly improved over that of pristine LiCoO2, with a low overpotential of 365 mV at 10 mA cm−2, a small Tafel slope of 55 mV dec−1, and low charge transfer resistance. Performance improvement results from electrochemical delithiation tuning the electronic structure of LiCoO2. The Li+ deintercalation process is accompanied by the partial oxidation of Co3+ to Co4+ and the increase of oxygen vacancies, which is associated with the enhanced intrinsic activity of the catalyst. This work paves a simple avenue for reasonable design of effective oxygen electrocatalysts by electrochemical tuning in aqueous systems.

Core-shell heterojunction engineering of Ni0.85Se-O/CN electrocatalyst for efficient OER

Efficient electrocatalysts are of great consideration for lowering the potential barrier for the oxygen evolution reaction (OER). The Ni0.85Se-O/CN multicore core–shell coated structured microspherical electrocatalyst was prepared by a simple hydrothermal method for promoting OER in alkaline solution. With highly conductive Ni0.85Se-O as the core and g-C3N4 with abundant active sites as the shell, the electrocatalyst exhibits excellent catalytic OER performance with a Tafel slope of 82.5 mV dec−1 and an overpotential of 240 mV @ 10 mA cm−2. What’s more, the catalyst exhibits excellent long-term stability, which is facilitated by the protection of the core (Ni0.85Se-O) from corrosion and oxidation by the shell (g-C3N4). In addition, in-depth mechanistic analysis on catalytic activity of materials were performed by density functional theory (DFT). This work highlights the importance of core–shell structural engineering to improve the catalytic activity of materials and provides new possibilities for the progress of Ni-based catalysts in the field of oxygen evolution reaction.

Ruddlesden–Popper Oxides LaSrM11−xM2xO4±δ (M1, M2—Fe, Co, Ni) Synthesized by the Spray-Pyrolysis Method as Promising Electrocatalysts for Oxygen Evolution Reaction

Ruddlesden–Popper (RP) transition-metal oxide phases with the general formula An+1BnO3n+1 are versatile functional materials that can accommodate a large variety of compositions without compromising structural stability. Substitutions at the A and B sites allow for the precise control of functional properties of these materials. This opens wide possibilities for rational design. In particular, some of these materials were demonstrated to be efficient and stable catalysts for electrochemical oxygen evolution reaction (OER)—one of the key processes in fuel cells and water electrolyzers. In this work, RP phases LaSrM11−xM2xO4±δ (M1, M2—Fe, Co, Ni) with unreported stoichiometry are prepared from aqueous solutions of metal nitrates using the ultrasonic spray-pyrolysis (USP) technique. We found that the phase purity of samples synthesized by USP is higher as compared to samples prepared by solid-state synthesis or by precipitation from aqueous solutions followed by calcination, used in previous studies of RP oxides. LaSrFe0.5Ni0.5O4–δ (LSNF) oxides are found to be very active in OER in alkaline solutions, with overpotential 0.27 V at j = 0.1 A cm–2 of visible electrode surface in a 5 M solution of KOH. This overpotential is on par with the noble-metal-based OER electrocatalysts. Moreover, the catalytic performance of LSNF in OER is found to be stable over the electrolysis time even in the strongly alkaline solution. These two factors let us conduct the water splitting process in more concentrated electrolytes decreasing the energy cost of hydrogen production by water electrolysis.

Nest-like Ag-doped NiMoO4/NF with rich oxygen vacancies as robust catalysts for highly efficient oxygen evolution

The development of low-cost and highly efficient electrocatalysts for oxygen evolution reaction (OER) with good conductivity, high specific surface area and abundant oxygen vacancies is of great significance for the future renewable energy conversion and storage system. Herein, Ag doped NiMoO4 materials on the Ni foam (NiMoO4/Ag/NF) with rich oxygen vacancies were successfully prepared by hydrothermal strategy and layer by layer assembly method. The Ag ions are doped in-situ to promote electron transfer and compensate for the poor conductivity of metal oxides. The suitable air annealing treatment is used to create rich oxygen vacancies. The high specific surface area and one-dimensional characteristics of the cross mesoporous nanorod skeleton are responsible for the increased exposure of active centers and the rapid charge transfer behavior. The NiMoO4/Ag/NF exhibits high activity for OER in alkaline solution (an overpotential of 275 mV at the current density of 20 mA cm−2) and low Tafel slope (53.67 mV dec−1), and a satisfying durability in 1.0 M KOH. This study provides an effective method for the preparation of Ag doped one-dimensional nanomaterials with excellent OER catalytic performance and electrochemical stability.

Manganese, iron co-doped Ni2P nanoflowers as a powerful electrocatalyst for oxygen evolution reaction

MnFe-Ni 2 P/NF is synthesized by a simple hydrothermal and subsequent phosphorylation reaction. The prepared material has excellent OER activity with an overpotential of 220 mV at a current density of 10 mA cm −2 . • Trimetallic transition metal phosphide nanoflowers are in-situ synthesized on Ni foam. • The unique flower-like structure provides more active sites for the OER reaction. • MnFe-Ni 2 P/NF shows high catalytic stability for OER in alkaline media. • MnFe-Ni 2 P/NF shows higher catalytic activity than that of bimetallic phosphides. The development of abundant, high-performance electrocatalysts for oxygen evolution reactions (OER) is an ongoing challenge to renewable energy. Herein, we rationally synthesized a manganese (Mn) and iron (Fe) doped Ni 2 P nanosheet (MnFe-Ni 2 P) as a highly efficient electrocatalyst for OER in alkaline solution by a simple hydrothermal method and phosphorylation. The bimetal-doped phosphide nanosheets are directly grown on the conductive nickel foam (NF) to form a 3D structure which facilitates to expose active sites as much as possible. Due to the electron interaction between the Mn, Fe and Ni, which contributes to ion diffusion and gas release, trimetallic MnFe-Ni 2 P/NF delivers an extremely low overpotential of 220 mV at 10 mA cm −2 and a small Tafel slope of 53 mV dec −1 in alkaline electrolyte towards OER. Furthermore, the excellent stability of MnFe-Ni 2 P/NF lasts for at least 24 h at a high current density of 50 mA cm −2 without much deviation. We demonstrate that the trimetallic MnFe-Ni 2 P/NF benefits from the doping of Mn, Fe elements and the unique flower-like structure, exhibits more effective electrocatalytic activity compare to conventional noble metal catalysts and Ni 2 P series materials.

Construction of hierarchical Co2P/Ni2P heterostructures on Ni foam as efficient bifunctional electrocatalyst for overall water splitting

High-performance bifunctional electrocatalysts for both HER (hydrogen evolution reaction) and OER (oxygen evolution reaction) are very pivotal for large-scale production of H 2 by water splitting. Herein, a series of novel hierarchical Co 2 P- x /Ni 2 P- y @ NF samples have been fabricated via a simple synthesizing method and employed as bifunctional catalytic electrodes for overall water splitting. Owing to the hierarchical heterostructure and interaction between the Co 2 P nanowires and Ni 2 P nanosheets , the optimal Co 2 P-1/Ni 2 P-1 @NF sample exhibited remarkable catalytic ability for both HER and OER in alkaline solution (79 mV and 310 mV at 10 mA cm -2 , respectively.). As a result, the water electrolysis device based on Co 2 P-1/Ni 2 P-1 @NF || Co 2 P-1/Ni 2 P-1 @NF system only needs 1.63 V to offer 10 mA cm -2 and keeps excellent durability for 40 h, demonstrating its great practical application value for overall water splitting. • Co 2 P-x/Ni 2 P-y @ NF was fabricated and used as bifunctional catalytic electrodes for water splitting. • The Co 2 P-1/Ni 2 P-1 @ NF exhibited admirable catalytic ability for both HER and OER in alkaline solution. • The water electrolysis device based on Co 2 P-1/Ni 2 P-1 @ NF || Co 2 P-1/Ni 2 P-1 @ NF only needs 1.63 V to offer 10 mA cm-2.

CoNi nanoalloy-Co-N4 composite active sites embedded in hierarchical porous carbon as bi-functional catalysts for flexible Zn-air battery

Exploiting bifunctional electrocatalysts with excellent activity and stability towards oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) is of great importance for the practical application of rechargeable metal–air batteries. Herein, we report a novel bimetallic-ion co-exchange combined pyrolysis strategy using bio-MOF-1 as the self-template to synthesize NiCo nanoalloy-Co-N4 embedded hierarchical porous carbon (CoNi-CoN4-HPC-900), which endows good electrical conductivity, large surface area and rich highly-dispersed CoNi-CoN4 active sites identified from X-ray absorption spectroscopy, resulting in outstanding electrocatalytic activity for both ORR and OER in alkaline solution. In situ Raman spectroscopy testifys that the electrocatalytic processes occur via the redox of Co(Ni)(II and Ⅲ) species. Density functional theory calculations are performed to study the ORR and OER activity on the CoNi-CoN4 and CoN4 active sites. The CoNi-CoN4-HPC-900 endows a flexible rechargeable Zn-air battery with an open circuit voltage of 1.50 V and a peak power density of 116 mW cm−2, and excellent flexibility and cycling stability as a cathode. This bimetallic-ion-coexchange strategy of MOFs gives an effective methodology involving bimetallic-ion-coexchange strategy to design and prepare bifunctional electroctalysts in metal-air batteries but also brings an intensive insight into ORR and OER fundamentals.

Comparison of hydrothermal and electrodeposition methods for the synthesis of CoSe2/CeO2 nanocomposites as electrocatalysts toward oxygen evolution reaction

Promoting efficacious and low-cost catalysts for the oxygen evolution reaction (OER), as the sluggish half-reaction of the water splitting, is inevitable to make sustainable energy technologies more promising. In this work, we report a series of novel nanocomposites comprising CeO2 nanorods decorated with CoSe2 nanoparticles. The nanocomposites were prepared via a conventional hydrothermal synthesis or a rapid electrodeposition process, and their structure, morphology, and electrochemical performance toward OER in alkaline solution were compared. To tune the electrocatalytic activity, the mass ratio of CoSe2 to CeO2 was systematically varied. Compared with the hydrothermal synthesis, the much faster electrodeposition method yielded a nanocomposite with a similar or slightly better performance in OER. This nanocomposite exhibited an overpotential of 290 mV (at 10 mA cm−2 current density), a Tafel slope of 53 mV dec−1, and excellent electrochemical stability for 15 h. Overall, these findings demonstrate the great potential of CoSe2/CeO2 nanocomposites as effective OER electrocatalysts for future applications.

Surface oxidation of cobalt carbonate and oxide nanowires by electrocatalytic oxygen evolution reaction in alkaline solution

The electrocatalytic water electrolysis is the most eco-friendly technique for hydrogen generation, which is governed by the electrode reaction kinetics involving cathodic hydrogen evolution reaction (HER) and anodic oxygen evolution reaction (OER) in common alkaline electrolytes. Cobalt oxide (Co3O4) and related compounds are the most efficient OER catalysts, replacing the noble metals. In this work, the surface oxidations of the cobalt carbonate (Co(CO3)0.5OH·0.11H2O) and Co3O4 nanowires during the OER are carefully investigated by contrasting the polarization curves, Tafel plots, and x-ray photoelectron spectroscopy (XPS) spectra, before and after the 1000th cyclic voltammetry (CV) cycling in 1 M KOH alkaline solution. The overpotentials required to reach a current density (j) of 20 mA cm−2 (η 20) are estimated to be 313 mV for the 300 °C-calcined Co3O4, 350 mV for the 400 °C-calcined Co3O4, 365 mV for the 500 °C-calcined Co3O4, and 373 mV for the cobalt carbonate (Co(CO3)0.5OH·0.11H2O). The Tafel slope of cobalt carbonate (Co(CO3)0.5OH·0.11H2O) nanowires is the highest at 93 mV dec−1, while it is measured to be 57 mV dec−1 for the 300 °C-calcined Co3O4, 47 mV dec−1 for the 400 °C-calcined Co3O4, and 79 mV dec−1 for the 500 °C-calcined Co3O4. As a result, the oxidation from Co2+ to Co3+ within Co3O4 during the OER is detected, which improves the OER activity. On the other hand, the formation of cobalt hydoxide is found on the surface of the Co3O4 nanowires during the OER in alkaline solution, which decreases the OER activity. For the surface oxidation of the cobalt carbonate (Co(CO3)0.5OH·0.11H2O) nanowires, the increase in the amounts of Co3+ and oxygen vacancy and the formation of O-C-O and carbonates are found, which highly enhance the OER activity. These findings indicate that the surface redox kinetics during the electrocatalytic reactions should be considered important in order to enhance the electrocatalytic activity, and furthermore can provide insight into future challenges in designing advanced electrocatalysts.