Activating P2-NaxCoO2 for efficient water oxidation catalysis via controlled chemical oxidation

Electrocatalytic water oxidation is critically important for a wide range of emerging energy conversion devices. Co-based metal oxides are very promising candidates as high-performance oxygen evolution reaction (OER) catalysts. Here, it is shown that chemical oxidation of layered P2-Na_xCoO₂ could lead to compositionally tunable P2-Na_xCoO₂ with high OER activity. The optimal electrocatalytic activity emerges in a narrow range of sodium concentrations with Na_0·28CoO₂ exhibiting the lowest overpotential of 350 mV at 10 mA/cm² and a Tafel slope of 29 mV/dec in 0.1 M NaOH electrolyte, outperforming the benchmark RuO₂ catalyst and previous LiCoO₂-based electrocatalysts. Electrochemical measurements and X-ray spectroscopic investigations reveal that chemically oxidized P2-Na_xCoO₂ catalysts are intrinsically active toward OER, arising from the abundant oxygen vacancies, increased Co-O covalency, and enhanced conductivity after deintercalation of the Na⁺. Our findings provide new insights into the design and synthesis of cost-effective catalysts toward efficient and durable OER.     

Highly exposed NiFeOx nanoclusters supported on boron doped carbon nanotubes for electrocatalytic oxygen evolution reaction

The development of efficient and cost-effective electrocatalysts for oxygen evolution reaction (OER) is crucial for the overall water splitting. Herein, we prepared a highly exposed NiFeO_x ultra-small nanoclusters supported on boron-doped carbon nonotubes catalyst, which achieves a 10 mA/cm² anodic current density at a low overpotential of 213 mV and the Tafel slope of 52 mV/dec in 1.0 mol/L KOH, superior to the pristine NiFeO_x-CNTs and other state-of-the-art OER catalysts in alkaline media. A combination study (XPS, sXAS and XAFS) verifies that the local atomic structure of Ni and Fe atoms in the nanoclusters are similar to NiO and Fe₂O₃, respectively, and the B atoms which are doped into the crystal lattice of CNTs leads to the optimization of Ni 3d e_g orbitals. Furthermore, in-situ X-ray absorption spectroscopies reveal that the high valence state of Ni atoms are served as the real active sites. This work highlights that the precise control of highly exposed multicomponent nanocluster catalysts paves a new way for designing highly efficient catalysts at the atomic scale.     

A Bifunctional Hybrid Electrocatalyst for Oxygen Reduction and Evolution: Cobalt Oxide Nanoparticles Strongly Coupled to B,N-Decorated Graphene

The electrocatalyzed oxygen reduction and evolution reactions (ORR and OER, respectively) are the core components of many energy conversion systems, including water splitting, fuel cells, and metal–air batteries. Rational design of highly efficient non-noble materials as bifunctional ORR/OER electrocatalysts is of great importance for large-scale practical applications. A new strongly coupled hybrid material is presented, which comprises CoO_x nanoparticles rich in oxygen vacancies grown on B,N-decorated graphene (CoO_x NPs/BNG) and operates as an efficient bifunctional OER/ORR electrocatalyst. Advanced spectroscopic techniques were used to confirm formation of abundant oxygen vacancies and strong Co−N−C bridging bonds within the CoO_x NPs/BNG hybrid. Surprisingly, the CoO_x NPs/BNG hybrid electrocatalyst is highly efficient for the OER with a low overpotential and Tafel slope, and is active in the ORR with a positive half-wave potential and high limiting current density in alkaline medium.     

Controlled Assembly of Cu/Co-Oxide Beaded Nanoclusters on Thiolated Graphene Oxide Nanosheets for High-Performance Oxygen Evolution Catalysts

The use of water splitting modules is highly desired for the sustainable production of H₂ as a future energy carrier. However, the sluggish kinetics and demand of high anodic potential are the bottlenecks for half-the cell oxygen evolution reaction (OER), which severely hamper the overall conversion efficiency. Although transition metal oxides based electrocatalysts have been envisioned as cost-effective and potential contenders for this quest, nevertheless, their low conductivity, instability, and limited number of active sites are among the common impediments that need to be addressed to eventually enhance their inherent catalytic potential for enhanced OER activity. Herein, the controlled assembly of transition metal oxides, that is, Cu@CuO_x nanoclusters (NCs, ≈2 nm) and Co@CoO_x beaded nanoclusters (BNCs, ≈2 nm), on thiol-functionalized graphene oxide (G-SH) nanosheets is reported to form novel and highly efficient electrocatalysts for OER. The thiol (-SH) functionality was incorporated by selective epoxidation on the surface of graphene oxide (GO) to achieve chemically exfoliated nanosheets to enhance its conductivity and trapping ability for metal oxides in nanoscale dimensions (≈2 nm). During the electrocatalytic reaction, overpotentials of 290 mV and 310 mV are required to achieve a current density of 10 mA cm⁻² for BNCs and NCs, respectively, and the catalysts exhibit tremendous long-term stability (≈50 h) in purified alkaline medium (1 m KOH) with no dissolution in the electrolyte. Moreover, the smaller Tafel slopes (54 mV/dec for BNCs and 66 mV/dec for NCs), and a Faradic efficiency of approximately 96 % indicate not only the selectivity but also the tailored heterogeneous electrons transfer (HET) rate, which is required for fast electrode kinetics. It is anticipated that such ultrasmall metal oxide nanoclusters and their controlled assembly on a conducting surface (G-SH) may offer high electrochemical accessibility and a plethora of active sites owing to the drastic decrease in dimensions and thus can synergistically ameliorate the challenging OER process.     

The oxygen evolution reaction on cobalt: Part II. Transient measurements

Open-circuit overpotential decays on an aged cobalt electrode in the oxygen evolution range in 6 M KOH show different slopes for two overpotential regions. These slopes are lower than the Tafel slope in the same region: Tafel slopes of ～100 and ～40 mV/dec, at high and low overpotentials, respectively. compared to decay slopes of ～?60 and ～?20 to ?30 mV/dec. For a fresh cobalt electrode a decay slope of ～?40 mV/dec is found at high overpotentials. From impedance measurements during a decay it is concluded that the electrode capacitance cannot account for the decay curves obsered. By means of steady-state potentiostatic impedance measurements (with stabilization times > 24 h) it is found that the differential Tafel slope remains constant at ～40–50 mV/dec and differs considerably from the Tafel slope at high overpotentials, ～100 mV/dec. Galvanostatic pulse experiments give evidence of the presence of CoO₂ in the oxide layer.Two models which may explain the observed experimental results are analysed. Both include a potential-dependent (extra) process which is fixed by the amount of CoO₂ at the surface. In one model, CoO₂ is responsible for partial surface blockage (parallel process); in the other model, CoO₂ controls the conductivity of the top layer of the oxide layer on the electrode.     

Electrodeposited Cobalt‐Phosphorous‐Derived Films as Competent Bifunctional Catalysts for Overall Water Splitting

One of the challenges to realize large‐scale water splitting is the lack of active and low‐cost electrocatalysts for its two half reactions: H₂ and O₂ evolution reactions (HER and OER). Herein, we report that cobalt‐phosphorous‐derived films (Co‐P) can act as bifunctional catalysts for overall water splitting. The as‐prepared Co‐P films exhibited remarkable catalytic performance for both HER and OER in alkaline media, with a current density of 10 mA cm^?2 at overpotentials of ?94 mV for HER and 345 mV for OER and Tafel slopes of 42 and 47 mV/dec, respectively. They can be employed as catalysts on both anode and cathode for overall water splitting with 100 % Faradaic efficiency, rivalling the integrated performance of Pt and IrO₂. The major composition of the as‐prepared and post‐HER films are metallic cobalt and cobalt phosphide, which partially evolved to cobalt oxide during OER.     

A Bifunctional Hybrid Electrocatalyst for Oxygen Reduction and Evolution: Cobalt Oxide Nanoparticles Strongly Coupled to B,N‐Decorated Graphene

The electrocatalyzed oxygen reduction and evolution reactions (ORR and OER, respectively) are the core components of many energy conversion systems, including water splitting, fuel cells, and metal–air batteries. Rational design of highly efficient non‐noble materials as bifunctional ORR/OER electrocatalysts is of great importance for large‐scale practical applications. A new strongly coupled hybrid material is presented, which comprises CoO_x nanoparticles rich in oxygen vacancies grown on B,N‐decorated graphene (CoO_x NPs/BNG) and operates as an efficient bifunctional OER/ORR electrocatalyst. Advanced spectroscopic techniques were used to confirm formation of abundant oxygen vacancies and strong Co−N−C bridging bonds within the CoO_x NPs/BNG hybrid. Surprisingly, the CoO_x NPs/BNG hybrid electrocatalyst is highly efficient for the OER with a low overpotential and Tafel slope, and is active in the ORR with a positive half‐wave potential and high limiting current density in alkaline medium.     

Activating CoOOH Porous Nanosheet Arrays by Partial Iron Substitution for Efficient Oxygen Evolution Reaction

Iron‐substituted CoOOH porous nanosheet arrays grown on carbon fiber cloth (denoted as Fe_xCo_1?xOOH PNSAs/CFC, 0≤x≤0.33) with 3D hierarchical structures are synthesized by in situ anodic oxidation of α‐Co(OH)₂ NSAs/CFC in solution of 0.01 m (NH₄)₂Fe(SO₄)₂. X‐ray absorption fine spectra (XAFS) demonstrate that CoO₆ octahedral structure in CoOOH can be partially substituted by FeO₆ octahedrons during the transformation from α‐Co(OH)₂ to Fe_xCo_1?xOOH, and this is confirmed for the first time in this study. The content of Fe in Fe_xCo_1?xOOH, no more than 1/3 of Co, can be controlled by adjusting the in situ anodic oxidation time. Fe_0.33Co_0.67OOH PNSAs/CFC shows superior OER electrocatalytic performance, with a low overpotential of 266 mV at 10 mA cm^?2, small Tafel slope of 30 mV dec^?1, and high durability.     

Impedance measurements in different alkaline solutions on an oxygen-evolving La0.5Sr0.5CoO3 electrode

It was found that the oxygen evolution reaction on La_0.5Sr_0.5CoO₃ in strong alkaline solutions has a Tafel slope of ～ 60 mV/dec and a reaction order at constant overpotential with respect to the KOH activity of 0.6–0.8. From impedance measurements differential Tafel slopes were calculated and were found to be ～ 60 mV/dec at overpontentials > 250 mV. Effective capacitances having a broad maximum at an overpotential of about 200 mV in all alkaline solutions were calculated. The effective capacitance increased with increasing KOH concentration. Furthermore, the material decomposed at the surface when exposed to strong oxygen evolution. From the results a modified Krasil'shchikov reaction path is analysed.     

Understanding the Activity of Co-N4−xCx in Atomic Metal Catalysts for Oxygen Reduction Catalysis

Atomic metal catalysis (AMC) provides an effective way to enhance activity for the oxygen reduction reaction (ORR). Cobalt anchored on nitrogen-doped carbon materials have been extensively reported. The carbon-hosted Co-N₄ structure was widely considered as the active site; however, it is very rare to investigate the activity of Co partially coordinated with N, for example, Co-N_4−xC_x. Herein, the activity of Co-N_4−xC_x with tunable coordination environment is investigated as the active sites for ORR catalysis. The defect (di-vacancies) on carbon is essential for the formation of Co-N_4−xC_x. N species play two important roles in promoting the intrinsic activity of atomic metal catalyst: N coordinated with Co to manipulate the reactivity by modification of electronic distribution and N helped to trap more Co to increase the number of active sites.     

Enhanced electrolytic generation of oxygen gas at binary nickel oxide–cobalt oxide nanoparticle-modified electrodes

This study addresses the enhancement of the oxygen evolution reaction (OER) on glassy carbon, Au, and Pt electrodes modified with binary catalysts composed of nickel oxide nanoparticles (nano-NiO_x) and cobalt oxide nanoparticles (nano-CoO_x). Binary NiO_x/CoO_x-modified electrodes (with NiO_x initially deposited) show a high catalytic activity and a marked stability which far exceeds that obtained at the individual oxide-modified electrodes. This enhancement is demonstrated by a marked negative shift (more than ca. 600 mV) in the onset potential of the OER compared to that obtained at the unmodified electrodes. The modified electrodes show a significantly higher long-term stability, over a period of 5 h of continuous electrolysis, without any significant loss of activity towards the OER in alkaline medium. The influence of the solution pH, the loading level, and sequence of deposition of each oxide on the electrocatalytic activity of the modified electrodes is addressed with an aim to maximize the catalytic activity of the modified electrodes towards the OER. SEM imaging is used to disclose the size and morphology of the fabricated nano-NiO_x and nano-CoO_x binary catalysts at the electrode surface.     

Heterostructured Inter‐Doped Ruthenium–Cobalt Oxide Hollow Nanosheet Arrays for Highly Efficient Overall Water Splitting

The development of transition‐metal‐oxides (TMOs)‐based bifunctional catalysts toward efficient overall water splitting through delicate control of composition and structure is a challenging task. Herein, the rational design and controllable fabrication of unique heterostructured inter‐doped ruthenium–cobalt oxide [(Ru–Co)O_x] hollow nanosheet arrays on carbon cloth is reported. Benefiting from the desirable compositional and structural advantages of more exposed active sites, optimized electronic structure, and interfacial synergy effect, the (Ru–Co)O_x nanoarrays exhibited outstanding performance as a bifunctional catalyst. Particularly, the catalyst showed a remarkable hydrogen evolution reaction (HER) activity with an overpotential of 44.1 mV at 10 mA cm^?2 and a small Tafel slope of 23.5 mV dec^?1, as well as an excellent oxygen evolution reaction (OER) activity with an overpotential of 171.2 mV at 10 mA cm^?2. As a result, a very low cell voltage of 1.488 V was needed at 10 mA cm^?2 for alkaline overall water splitting.     

Sulfur‐Modified Oxygen Vacancies in Iron–Cobalt Oxide Nanosheets: Enabling Extremely High Activity of the Oxygen Evolution Reaction to Achieve the Industrial Water Splitting Benchmark

The oxygen vacancies of defective iron–cobalt oxide (FeCoO_x‐Vo) nanosheets are modified by the homogeneously distributed sulfur (S) atoms. S atoms can not only effectively stabilize oxygen vacancies (Vo), but also form the Co?S coordination with Co active site in the Vo, which can modulate the electronic structure of the active site, enabling FeCoO_x‐Vo‐S to exhibit much superior OER activity. FeCoO_x‐Vo‐S exhibits a mass activity of 2440.0 A g^?1 at 1.5 V vs. RHE in 1.0 m KOH, 25.4 times higher than that of RuO₂. The Tafel slope is as low as 21.0 mV dec^?1, indicative of its excellent charge transfer rate. When FeCoO_x‐Vo‐S (anode catalyst) is paired with the defective CoP₃/Ni₂P (cathode catalyst) for overall water splitting, current densities of as high as 249.0 mA cm^?2 and 406.0 mA cm^?2 at a cell voltage of 2.0 V and 2.3 V, respectively, can be achieved.     

Sulfur-Modified Oxygen Vacancies in Iron–Cobalt Oxide Nanosheets: Enabling Extremely High Activity of the Oxygen Evolution Reaction to Achieve the Industrial Water Splitting Benchmark

The oxygen vacancies of defective iron–cobalt oxide (FeCoO_x-Vo) nanosheets are modified by the homogeneously distributed sulfur (S) atoms. S atoms can not only effectively stabilize oxygen vacancies (Vo), but also form the Co−S coordination with Co active site in the Vo, which can modulate the electronic structure of the active site, enabling FeCoO_x-Vo-S to exhibit much superior OER activity. FeCoO_x-Vo-S exhibits a mass activity of 2440.0 A g⁻¹ at 1.5 V vs. RHE in 1.0 m KOH, 25.4 times higher than that of RuO₂. The Tafel slope is as low as 21.0 mV dec⁻¹, indicative of its excellent charge transfer rate. When FeCoO_x-Vo-S (anode catalyst) is paired with the defective CoP₃/Ni₂P (cathode catalyst) for overall water splitting, current densities of as high as 249.0 mA cm⁻² and 406.0 mA cm⁻² at a cell voltage of 2.0 V and 2.3 V, respectively, can be achieved.     

Non-Kinetic Effects Convolute Activity and Tafel Analysis for the Alkaline Oxygen Evolution Reaction on NiFeOOH Electrocatalysts

A large variety of nickel-based catalysts has been investigated for the oxygen evolution reaction (OER) in alkaline media. However, their reported activity, as well as Tafel slope values, vary greatly. To understand this variation, we studied electrodeposited Ni₈₀Fe₂₀OOH catalysts with different loadings at varying rotation rates, hydroxide concentrations, with or without sonication. We show that, at low current density (<5 mA cm⁻²), the Tafel slope value is ≈30 mV dec⁻¹ for Ni₈₀Fe₂₀OOH. At higher polarization, the Tafel slope continuously increases and is dependent on rotation rate, loading, hydroxide concentration and sonication. These Tafel slope values are convoluted by non-kinetic effects, such as bubbles, potential-dependent changes in ohmic resistance and (internal) OH⁻ gradients. As best practise, we suggest that Tafel slopes should be plotted vs. current or potential. In such a plot, it can be appreciated if there is a kinetic Tafel slope or if the observed Tafel slope is influenced by non-kinetic effects.     

Porous TiO2 Nanotubes with Spatially Separated Platinum and CoOx Cocatalysts Produced by Atomic Layer Deposition for Photocatalytic Hydrogen Production

Efficient separation of photogenerated electrons and holes, and associated surface reactions, is a crucial aspect of efficient semiconductor photocatalytic systems employed for photocatalytic hydrogen production. A new CoO_x/TiO₂/Pt photocatalyst produced by template‐assisted atomic layer deposition is reported for photocatalytic hydrogen production on Pt and CoO_x dual cocatalysts. Pt nanoclusters acting as electron collectors and active sites for the reduction reaction are deposited on the inner surface of porous TiO₂ nanotubes, while CoO_x nanoclusters acting as hole collectors and active sites for oxidation reaction are deposited on the outer surface of porous TiO₂ nanotubes. A CoO_x/TiO₂/Pt photocatalyst, comprising ultra‐low concentrations of noble Pt (0.046 wt %) and CoO_x (0.019 wt %) deposited simultaneously with one atomic layer deposition cycle, achieves remarkably high photocatalytic efficiency (275.9 μmol h⁻¹), which is nearly five times as high as that of pristine TiO₂ nanotubes (56.5 μmol h⁻¹). The highly dispersed Pt and CoO_x nanoclusters, porous structure of TiO₂ nanotubes with large specific surface area, and the synergetic effect of the spatially separated Pt and CoO_x dual cocatalysts contribute to the excellent photocatalytic activity.     

Defect‐Rich Co−CoOx‐Graphene Nanocatalysts for Efficient Hydrogen Production from Ammonia Borane

Chemical hydrogen storage ammonia borane has attracted extensive attention as a method of efficient utilization of hydrogen energy. The high‐efficiency catalysts are the main factor restricting the hydrogen production of ammonia borane. In this paper, the synergistic effect of Co and CoO_x supported on graphene (named Co?CoO_x@GO‐II) promotes the efficient hydrogen production of ammonia borane, and its catalytic hydrogen production rate can reach 5813 mL min^?1 g_Co^?1 at 298 K, the corresponding TOF is 15.33 min^?1. After five stability tests, Co?CoO_x @GO‐II maintained 65% of its original catalytic performance. The synergy of metal and metal oxide and the defects in the atomic arrangement ensure the catalytic activity, the large specific surface area of graphene ensures the dispersion and fixation. This strategy may provide a possibility to design high‐performance transition metal catalysts.     

Modulating the Electronic Structure of Porous Nanocubes Derived from Trimetallic Metal–Organic Frameworks to Boost Oxygen Evolution Reaction Performance

The preparation of noble metal‐free catalysts for water splitting is the key to low‐cost, sustainable hydrogen generation. Herein, through a pyrolysis‐oxidation process, we prepared a series of Co‐Fe‐Ni trimetallic oxidized carbon nanocubes (Co_1‐XFe_XNi‐OCNC) with a continuously changeable Co/Fe ratio (X=0, 0.1, 0.2, 0.5, 0.8, 0.9, 1). The Co_1‐XFe_XNi‐OCNC shows a volcano‐type oxygen evolution reaction (OER) activity. The optimized Co_0.1Fe_0.9Ni‐OCNC achieves a low overpotential of 268 mV at 10 mA cm^?2 with a very low Tafel slope of 48 mV dec^?1 in 1 m KOH. At the same time, the stability of the Co_0.1Fe_0.9Ni‐OCNC is also outstanding; after 1000 CV cycles, the LSV plot is almost coincident. Moreover, the potential remains almost of the same value at 10 mA cm^?2 after 12 h in comparison to the initial value. The excellent electrocatalytic properties can be attributed to the synergistic cooperation between each component. Therefore, the Co_0.1Fe_0.9Ni‐OCNC is a promising candidate instead of precious metal‐based electrocatalysts for OER.     

Preferential Co substitution on Ni sites in Ni–Fe oxide arrays enabling large-current-density alkaline oxygen evolution

Developing low-cost and high-activity transition metal oxide electrocatalysts for an efficient oxygen evolution reaction (OER) at a large current density is highly demanded for industrial application and remains a big challenge. Herein, we report vertically aligned cobalt doped Ni–Fe based oxide (Co–NiO/Fe₂O₃) arrays as a robust OER electrocatalyst via a simple method combining hydrothermal reaction with heat treatment. Density functional theory calculation and XRD Rietveld refinement reveal that Co preferentially occupies the Ni sites compared to Fe in the Ni–Fe based oxides. The electronic structures of the Co–NiO/Fe₂O₃ could be further optimized, leading to the improvement of the intrinsic electronic conductivity and d-band center energy level and the decrease in the reaction energy barrier of the rate-determining step for the OER, thus accelerating its OER electrocatalytic activity. The Co–NiO/Fe₂O₃ nanosheet arrays display state-of-the-art OER activities at a large current density for industrial demands among Fe–Co–Ni based oxide electrocatalysts, which only require an ultra-low overpotential of 230 mV at a high current density of 500 mA cm⁻², and exhibit superb durability at 500 mA cm⁻² for at least 300 h without obvious degradation. The Co–NiO/Fe₂O₃ nanosheet arrays also have a small Tafel slope of 33.9 mV dec⁻¹, demonstrating fast reaction kinetics. This work affords a simple and effective method to design and construct transition metal oxide based electrocatalysts for efficient water oxidation.

Co–NiO/Fe₂O₃ nanosheets featuring Co substitution on Ni sites can effectively regulate electronic structures and exhibit high OER activities with low overpotential (η₅₀₀ = 230 mV), small Tafel slope (33.9 mV dec⁻¹) and superb durability for 300 h.     

Role of rhodium doping into lanthanum cobalt oxide (LaCoO3) perovskite and the induced bifunctional activity of oxygen evolution and reduction reactions in alkaline medium

Transition metal oxides, especially perovskites, have been considered effective electrocatalysts for the oxygen evolution (OER) and oxygen reduction (ORR) reactions in an alkaline solution. Here, a series of lanthanum cobalt rhodium oxide perovskites with the chemical formula LaCo_1-xRh_xO₃ (LCRO, 0.1 ≤ x ≤ 0.70) were prepared through the approach of solid-phase synthesis and their bifunctional electrocatalytic activity was assessed for both the OER and ORR. The crystallinity, morphology, surface, and electrocatalytic features of the LCRO were significantly correlated with the rhodium content. The LaCo_0.7Rh_0.3O₃ electrocatalysts with x = 0.3 showed enhanced electrocatalytic bifunctional performance with a substantially lower OER/ORR onset potential of 1.38/0.73 V vs HRE, smaller Tafel slope (116/90 mV/dec), and low charge-transfer resistance, which is the most efficient catalyst among the other studied ratios and superior to the pristine lanthanum cobalt oxide benchmark electrocatalysts. The LaCo_0.7Rh_0.3O₃ electrode exhibit good bifunctional electrocatalytic behavior and long-term durability with an OER and ORR onset potential gap (ΔE = E_OER ? E_ORR) of only 0.65 V, which could be credited to the enriched oxygen vacancies, lattice expansion and the improved electrical conductivity upon the doping of larger size of Rh ions. The LaCo_1-xRh_xO₃ catalysts are obtained from abundant materials that have the potential of highly-active bifunctional OER and ORR electrocatalysts.