Co3O4 Nanorods—Efficient Non-noble Metal Electrocatalyst for Oxygen Evolution at Neutral pH

BaRuO3 coated Ti plate as an efficient and stable electro-catalyst for water splitting reaction in alkaline medium

Water splitting using an electrochemical device to produce hydrogen fuel is a green and economic approach to solve the energy and environmental crisis. The realistic design of durable electro-catalysts and their synthesis using a simplistic technique is a great challenge to produce hydrogen by water electrolysis. Herein, we report a stable highly active barium ruthenium oxide (BRO) electro-catalysts over Ti plate using a soft chemical method at low temperature. The synthesized material shows facile hydrogen evolution reaction (HER) as well as oxygen evolution reaction (OER) in alkaline medium with over-potentials of 195 mV and 300 mV, respectively at a current density of 10 mA cm-2. The excellent stability lasts for at least 24 h without any degradation for both the HER and OER at the current density of 10 mA cm-2, inferring the practical applications of the material toward production of green hydrogen energy. Certainly, the synthesized catalyst is capable adequately for the overall water splitting at a cell voltage of 1.60 V at a current density of 10 mA cm-2 with an impressive stability for at least 24 h, showing a minimum loss of potential. Thus the present work contributes to the rational design of stable and efficient electro-catalysts for overall water splitting reaction in alkaline media.

In Situ Reconstructed Mo-doped Amorphous FeOOH Boosts the Oxygen Evolution Reaction

Developing a fast and highly active oxygen evolution reaction (OER) catalyst to change energy kinetics technology is essential for making clean energy. Herein, we prepare three-dimensional (3D) hollow Mo-doped amorphous FeOOH (Mo-FeOOH) based on the precatalyst MoS₂ /FeC₂ O₄ via in situ reconstruction strategy. Mo-FeOOH exhibits promising OER performance. Specifically, it has an overpotential of 285 mV and a durability of 15 h at 10 mA cm^-2 . Characterizations indicate that Mo was included inside the FeOOH lattice, and it not only modifies the electronic energy levels of FeOOH but also effectively raises the inherent activity of FeOOH for OER. Additionally, in situ Raman analysis indicates that FeC₂ O₄ gradually transforms into the FeOOH active site throughout the OER process. This study provides ideas for designing in situ reconstruction strategies to prepare heteroatom doping catalysts for high electrochemical activity.

Recent Progress on Bimetallic-Based Spinels as Electrocatalysts for the Oxygen Evolution Reaction

Electrocatalytic water splitting is a promising and viable technology to produce clean, sustainable, and storable hydrogen as an energy carrier. However, to meet the ever-increasing global energy demand, it is imperative to develop high-performance non-precious metal-based electrocatalysts for the oxygen evolution reaction (OER), as the OER is considered the bottleneck for electrocatalytic water splitting. Spinels, in particular, are considered promising OER electrocatalysts due to their unique properties, precise structures, and compositions. Herein, the recent progress on the application of bimetallic-based spinels (AFe₂ O₄ , ACo₂ O₄ , and AMn₂ O₄ ; where A = Ni, Co, Cu, Mn, and Zn) as electrocatalysts for the OER is presented. The fundamental concepts of the OER are highlighted after which the family of spinels, their general formula, and classifications are introduced. This is followed by an overview of the various classifications of bimetallic-based spinels and their recent developments and applications as OER electrocatalysts, with special emphasis on enhancing strategies that have been formulated to improve the OER performance of these spinels. In conclusion, this review summarizes all studies mentioned therein and provides the challenges and future perspectives for bimetallic-based spinel OER electrocatalysts.

Mn-doped Co3O4 for acid, neutral and alkaline electrocatalytic oxygen evolution reaction

This work reports the application of Mn-doped Co3O4 oxides in the electrocatalytic oxygen evolution reaction (OER). The materials were characterized by structural, morphological, and electrochemical techniques. The oxides with higher Co : Mn molar ratio presented a lower electron transfer resistance, and consequently the most promising OER activities. Pure Co3O4 shows an overpotential at j = 10 mA cm-2 of 761, 490, and 240 mV, at pH 1, 7, and 14, respectively, and a high TOF of 1.01 × 10-1 s-1 at pH 14. Tafel slopes around 120 mV dec-1 at acidic pH and around 60 mV dec-1 at alkaline pH indicate different OER mechanisms. High stability for Co3O4 was achieved for up to 15 h in all pHs, and no change in the structure and morphology after the electrocatalysis was observed. The reported excellent OER activity of the Mn-Co oxides in a wide pH range is important to broaden the practical applicability in different electrolyte solutions.

Electrochemical Characteristics of Nanosized Cu, Ni, and Zn Cobaltite Spinel Materials

For a long time, transition metal oxide systems have been considered well explored materials in heterogeneous catalysis. Amongst, the spinel-type oxides, materials such as cobaltites (Co3O4) received significant attention, owing to their use in many industrial applications. In the present study, nanosized Cu, Ni, and Zn cobaltite spinel oxides were synthesized by a simple hydrothermal method. Physicochemical characterization of the synthesized materials was performed utilizing XRD, HRTEM, CO2-TPD, and XPS techniques. The textural characteristics (BET-surface area, pore size, etc.) of samples were determined from N2 physisorption measurements at −196 °C. The CO2-electrocatalytic reduction was selected as a model reaction to evaluate the electrochemical performance of the synthesized spinel cobaltites. For Ni, Cu, and Zn spinel materials, hydrogen was produced as the main product at the whole potential, along with other products, such as CO and HCOOH. Despite the advantages, the catalytic electrochemical CO2 reduction performance of spinel cobaltite catalysts is still far from adequate, which is principally ascribed to the low number of active sites combined with poor electrical conductivity.

Concurrent occurrence of electrochemical dissolution/deposition of cobalt-calcium phosphate composite

Amorphous cobalt-calcium phosphate composite (CCPC) films are electrochemically prepared on various electrodes by utilizing the solid phase of hydroxyapatite as a phosphate source. The CCPC film formation is surface process in which the dissolution of hydroxyapatite and the deposition of CCPC film concurrently occur on the electrode surface without the mass transfer of phosphate ions into the bulk solution. Elemental, crystallographic, and morphological analyses (EDX, ICP-AES, XPS, and XRD) indicate that the CCPC is composed of amorphous cobalt oxide with calcium and phosphate. The film exhibits durable oxygen evolution reaction (OER) catalytic properties under neutral and basic aqueous condition. Compared to using solution phase of phosphate source, our preparation method utilizing solid hydroxyapatite has advantage of preventing unnecessary chemical reaction between phosphate and other chemical species in bulk solution.

A Ni-MOF nanosheet array for efficient oxygen evolution electrocatalysis in alkaline media

Ni-MOF(bpdc) nanosheet array on nickel foam (Ni-MOF/NF) is a superior OER catalyst with a requirement of an overpotential of 350 mV to attain 20 mA cm⁻² in 1.0 M KOH.

In situ construction and post-electrolysis structural study of porous Ni2P@C nanosheet arrays for efficient water splitting

Carbon-confined Ni₂P porous nanosheet arrays functioned as a superior water-splitting catalyst. Impressively, this porous-hybrid catalyst just required an OER overpotential of 243 mV to deliver 15 mA cm⁻² owing to the formation of γ-NiOOH layer.

Recent Advances in the Development of Water Oxidation Electrocatalysts at Mild pH

Developing anodic oxygen evolution reaction (OER) electrocatalysts with high catalytic activities is of great importance for effective water splitting. Compared with the water-oxidation electrocatalysts that are commonly utilized in alkaline conditions, the ones operating efficiently under neutral or near neutral conditions are more environmentally friendly with less corrosion issues. This review starts with a brief introduction of OER, the importance of OER in mild-pH media, as well as the fundamentals and performance parameters of OER electrocatalysts. Then, recent progress of the rational design of electrocatalysts for OER in mild-pH conditions is discussed. The chemical structures or components, synthetic approaches, and catalytic performances of the OER catalysts will be reviewed. Some interesting insights into the catalytic mechanism are also included and discussed. It concludes with a brief outlook on the possible remaining challenges and future trends of neutral or near-neutral OER electrocatalysts. It hopefully provides the readers with a distinct perspective of the history, present, and future of OER electrocatalysts at mild conditions.

Spin state engineered ZnxCo3-xO4 as an efficient oxygen evolution electrocatalyst

Oxygen evolution is the key step in the oxidation of water in electrolyzers and photoelectrochemical cells for the production of hydrogen. Developing a non-precious metal oxide catalyst with good electrocatalytic activity for the oxygen evolution reaction (OER) is very challenging. In this work, nanostructured ZnxCo3-xO4 has been shown as an efficient catalyst with a low overpotential for the OER in 0.1 M KOH solution. Substitution of Co2+ in the spinel oxide Co3O4 with Zn2+ creates a higher number of high-spin Co3+, which is found to be directly correlated with the OER activity of ZnxCo3-xO4. Zn0.8Co2.2O4 (x = 0.8) with the optimum amount of Co2+/Co3+ and high-spin Co3+ content showed a very low overpotential of ∼250 mV, at 10 mA cm-2, with a turnover frequency of ∼3 × 10-3 s-1 for the OER. The high Faradaic efficiency along with the stability of Zn0.8Co2.2O4 and electrocatalytic activity comparable with that of precious metal oxides indicate that this composition is a promising catalyst for water oxidation.

Hierarchical Nickel-Cobalt Dichalcogenide Nanostructure as an Efficient Electrocatalyst for Oxygen Evolution Reaction and a Zn-Air Battery

A unique three-dimensional (3D) structure consisting of a hierarchical nickel-cobalt dichalcogenide spinel nanostructure is investigated for its electrocatalytic properties at benign neutral and alkaline pH and applied as an air cathode for practical zinc-air batteries. The results show a high oxygen evolution reaction catalytic activity of nickel-cobalt sulfide nanosheet arrays grown on carbon cloth (NiCo₂S₄ NS/CC) over the commercial benchmarking catalyst under both pH conditions. In particular, the NiCo₂S₄ NS/CC air cathode shows high discharge capacity, a narrow potential gap between discharge and charge, and superior cycle durability with reversibility, which exceeds that of commercial precious metal-based electrodes. The excellent performance of NiCo₂S₄ NS/CC in water electrolyzers and zinc-air batteries is mainly due to highly exposed electroactive sites with a rough surface, morphology-based advantages of nanosheet arrays, good adhesion between NiCo₂S₄ and the conducting carbon cloth, and the active layer formed of nickel-cobalt (oxy)hydroxides during water splitting. These results suggest that NiCo₂S₄ NS/CC could be a promising candidate as an efficient electrode for high-performance water electrolyzers and rechargeable zinc-air batteries.

Co(OH)2 Nanoparticle-Encapsulating Conductive Nanowires Array: Room-Temperature Electrochemical Preparation for High-Performance Water Oxidation Electrocatalysis

It is highly desired but still remains challenging to design and develop a Co-based nanoparticle-encapsulated conductive nanoarray at room temperature for high-performance water oxidation electrocatalysis. Here, it is reported that room-temperature anodization of a Co(TCNQ)₂ (TCNQ = tetracyanoquinodimethane) nanowire array on copper foam at alkaline pH leads to in situ electrochemcial oxidation of TCNQ^- into water-insoluable TCNQ nanoarray embedding Co(OH)₂ nanoparticles. Such Co(OH)₂ -TCNQ/CF shows superior catalytic activity for water oxidation and demands only a low overpotential of 276 mV to drive a geometrical current density of 25 mA cm^-2 in 1.0 m KOH. Notably, it also demonstrates strong long-term electrochemical durability with its activity being retrained for at least 25 h, a high turnover frequency of 0.97 s^-1 at an overpotential of 450 mV and 100% Faradic efficiency. This study provides an exciting new method for the rational design and development of a conductive TCNQ-based nanoarray as an interesting 3D material for advanced electrochemical applications.

A Co-MOF nanosheet array as a high-performance electrocatalyst for the oxygen evolution reaction in alkaline electrolytes

A Co-MOF nanosheet array on Ni foam (Co-MOF/NF) acts as a superior electrocatalyst for the oxygen evolution reaction, needing an overpotential of only 311 mV to drive a geometrical catalytic current density of 50 mA cm⁻² in 1.0 M KOH.

An amorphous Co-carbonate-hydroxide nanowire array for efficient and durable oxygen evolution reaction in carbonate electrolytes

The development of earth-abundant catalysts toward high-efficient and durable oxygen evolution reaction (OER) electrocatalysis in the carbonate electrolyte is in great demand but remains a huge challenge. In this communication, we describe the development of a Co-carbonate-hydroxide nanowire array on nickel foam (CoCH/NF) via in situ electrochemical conversion of the Co(CO₃)_0.5(OH)·0.11H₂O nanowire array. When utilized as a 3D catalyst electrode for the OER in 1.0 M KHCO₃ (pH: 8.3), as-formed CoCH/NF demands overpotential of only 332 mV to drive a geometrical catalytic current density of 10 mA cm^-2, with its catalytic activity being maintained for at least 130 h. Impressively, it also demonstrates a high turnover frequency value of 0.22 mol O₂ s^-1 at an overpotential of 500 mV.

Homologous Catalysts Based on Fe-Doped CoP Nanoarrays for High-Performance Full Water Splitting under Benign Conditions

The design and development of earth-abundant electrocatalysts for efficient full water splitting under mild conditions are highly desired, yet remain a challenging task. A homologous Fe-doped Co-based nanoarray incorporating complementary catalysts is shown to effect high-performance and durable water splitting in near-neutral media. Iron-doped cobalt phosphate borate nanoarray on carbon cloth (Fe-Co-Pi-Bi/CC) derived from iron-doped cobalt phosphide on CC (Fe-CoP/CC) through oxidative polarization behaves as a highly active bimetallic electrocatalyst for water oxidation with an overpotential of 382 mV to afford a catalytic current density of 10 mA cm^-2 in 0.1 m potassium borate (K-Bi, pH 9.2). Fe-CoP/CC is also highly active for the hydrogen evolution reaction, capable of driving 10 mA cm^-2 at an overpotential of only 175 mV in 0.1 m K-Bi. A two-electrode water electrolyzer incorporating Fe-Co-Pi-Bi/CC as anode and Fe-CoP/CC as cathode achieves 10 mA cm^-2 water-splitting current at a cell voltage of 1.95 V with strong long-term electrochemical durability.

Facilitating Active Species Generation by Amorphous NiFe-Bi Layer Formation on NiFe-LDH Nanoarray for Efficient Electrocatalytic Oxygen Evolution at Alkaline pH

Searching for a simple and fast strategy to effectively enhance the oxygen evolution reaction (OER) performance of non-noble-metal electrocatalysts in alkaline media remains a significant challenge. Herein, the OER activity of NiFe-LDH nanoarray on carbon cloth (NiFe-LDH/CC) in alkaline media is shown to be greatly boosted by an amorphous NiFe-Borate (NiFe-B_i ) layer formation on NiFe-layered double hydroxide (NiFe-LDH) surface. Such a NiFe-LDH@NiFe-B_i /CC catalyst electrode only needs an overpotential of 294 mV to drive 50 mA cm^-2 in 1.0 m KOH; 116 mV less than that needed by NiFe-LDH/CC. Notably, this electrode also demonstrates strong long-term electrochemical durability. The superior activity is ascribed to the pre-formed amorphous NiFe-B_i layer effectively promoting active species generation on the NiFe-LDH surface. This work opens up exciting new avenues for developing high-performance water-oxidation catalyst materials for application.

Bimetallic Nickel-Substituted Cobalt-Borate Nanowire Array: An Earth-Abundant Water Oxidation Electrocatalyst with Superior Activity and Durability at Near Neutral pH

There is an urgent demand to develop earth-abundant electrocatalysts for efficient and durable water oxidation under mild conditions. A nickel-substituted cobalt-borate nanowire array is developed on carbon cloth (Ni-Co-Bi/CC) via oxidative polarization of NiCo₂ S₄ nanoarray in potassium borate (K-Bi). As a bimetallic electrocatalyst for water oxidation, such Ni-Co-Bi/CC is superior in catalytic activity and durability in 0.1 m K-Bi (pH: 9.2), with a turnover frequency of 0.33 mol O₂ s^-1 at the overpotential of 500 mV and nearly 100% Faradaic efficiency. To drive a geometrical catalytic current density of 10 mA cm^-2 , it only needs overpotential of 388 mV, 34 mV less than that for Co-Bi/CC, outperforming reported non-noble-metal catalysts operating under benign conditions. Notably, its activity is maintained over 80 000 s. Density functional theory calculations suggest that the O* to OOH* conversion is the rate-determining step and Ni substitution decreases the free energy on Co-Bi from 2.092 to 1.986 eV.

Interconnected Network of Core-Shell CoP@CoBiPi for Efficient Water Oxidation Electrocatalysis under Near Neutral Conditions

Developing earth-abundant electrocatalysts for efficient and stable water oxidation under near neutral conditions is of great importance but still remains a key challenge. Herein, we demonstrate the development of an interconnected network of core-shell CoP@CoBiPi through anodic polarization of a CoP nanoarray in potassium borate aqueous electrolyte (KBi). This 3 D CoP@CoBiPi exhibits high catalytic activity for water oxidation at pH 9.2 and needs an overpotential (η) of only 410 mV to drive a geometrical catalytic current density of 10 mA cm^-2 , with a high turnover frequency of 819 h^-1 at an overpotential of 610 mV. Remarkably, this catalyst also demonstrates high long-term electrochemical stability with its activity being maintained for at least 27 h in KBi. This study provides us an attractive earth-abundant 3 D catalyst electrode for water-splitting devices toward efficient and stable water oxidation under benign conditions.

A nickel–borate–phosphate nanoarray for efficient and durable water oxidation under benign conditions

A nickel–borate–phosphate nanoarray (Ni–Bi–Pi/CC) topotactically converted from a nickel phosphide nanoarray (Ni₂P/CC) acts as a durable catalyst electrode for water oxidation needing an overpotential of 440 mV to drive 10 mA cm⁻² in 0.1 M K–Bi (pH: 9.2).