Graphene-Co₃O₄ nanocomposite as electrocatalyst with high performance for oxygen evolution reaction

Dual Applications of Cobalt-Oxide-Grafted Carbon Quantum Dot Nanocomposite for Two Electrode Asymmetric Supercapacitors and Photocatalytic Behavior

Carbon materials, such as graphene, carbon nanotubes, and quantum-dot-doped metal oxides, are highly attractive for energy storage and environmental applications. This is due to their large surface area and efficient optical and electrochemical activity. In this particular study, a composite material of cobalt oxide and carbon quantum dots (Co3O4-CQD) was prepared using cobalt nitrate and ascorbic acid (carbon source) through a simple one-pot hydrothermal method. The properties of the composite material, including the functional groups, composition, surface area, and surface morphology, were evaluated by using various methods such as ultraviolet, Fourier transform infrared, X-ray diffraction, Raman, X-ray photoelectron spectroscopy, Brunauer-Emmett-Teller, scanning electron microscopy, and transmission electron microscopy analysis. The electrochemical performance of the Co3O4-CQD composite has been studied using a three-electrode system. The results show that at 1 A g-1, the composite delivers a higher capacitance of 1209 F g-1. The asymmetric supercapacitor (Co3O4-CQD//AC) provided 13.88 W h kg-1 energy and 684.65 W kg-1 power density with a 96% capacitance retention. The Co3O4-CQD composite also demonstrated excellent photocatalytic activity (90% in 60 min) for the degradation of methylene blue dye under UV irradiation, which is higher than that of pristine Co3O4 and CQD. This demonstrates that the Co3O4-CQD composite is a promising material for commercial energy storage and environmental applications.

Boron-Doped Activated Carbon Supports for Cobalt-Catalyzed Oxygen Evolution in Alkaline Electrolyte

Activated carbons (ACs) are the most widely used and attractive support materials for electrocatalytic applications because of their significant surface areas, high electrical conductivities, and moderate affinities toward supported metal catalysts. However, the corrosive behavior of ACs at oxidative potentials causes an inevitable reduction in the active surface area of supported catalysts, resulting in the continuous deterioration of their electrocatalytic performance. Therefore, the introduction of corrosion-resistant durable catalyst supports is essential for sustainable and efficient electrocatalysis. Here, we modified ACs to obtain different boron (B)-doped structures via doping-temperature controls to investigate the corrosion resistance of B-doped ACs. With increasing doping temperature, the B-doped ACs exhibited a decreased defect density and enhanced crystallinity owing to the accelerating dopant-induced graphitization. We found that the substitution of B atoms into the carbon lattice improved the structural integrity of the carbon structure, and cyclic voltammetry (CV) tests suggested that the highly B-substituted structures caused electrochemical surface passivation against carbon corrosion. Moreover, B-doped ACs significantly contributed to the increase in loading mass of cobalt (Co)-based catalyst on them and the electrochemical durability toward the oxygen evolution reaction as catalyst-support hybrid. The B22 (B-doped AC obtained at a 2200 °C B-doping temperature)-supported Co catalyst with the lowest oxidation current exhibited a voltage change of 32 mV at a current density of 10 mA/cm² (ΔE_j=10) after 10,000 cycles, which was a factor of ∼7 higher cycle durability and stability than that of the conventional IrO₂ catalyst (ΔE_j=10 = 205 mV). Here, we propose that surface engineering by B-doping to improve the structural integrity of ACs is an attractive method for designing durable electrocatalytic support materials.

Square-Facet Nanobar MOF-Derived Co3O4@Co/N-doped CNT Core-Shell-based Nanocomposites as Cathode Materials for High-Performance Supercapacitor Studies

The binary as well as ternary nanocomposites of the square-facet nanobar Co-MOF-derived Co₃O₄@Co/N-CNTs (N-CNTs: nitrogen-doped carbon nanotubes) with Ag NPs and rGO have been synthesized via an easy wet chemical route, and their supercapacitor behavior was then studied. At a controlled pH of the precursor solution, square-facet nanobars of Co-MOF were first synthesized by the solvothermal method and then pyrolyzed under a controlled nitrogen atmosphere to get a core-shell system of Co₃O₄@Co/N-CNTs. In the second step, different compositions of Co₃O₄@Co/N-CNT core-shell structures were formed by an ex-situ method with Ag NPs and rGO moieties. Among several bare, binary, and ternary compositions tested in 6 M aqueous KOH electrolyte, a ternary nanocomposite having a 7.0:1.5:1.5 stoichiometric ratio of Co₃O₄@Co/N-CNT, Ag NPs, and rGO, respectively, reported the highest specific capacitance (3393.8 F g^-1 at 5 mV s^-1). The optimized nanocomposite showed the energy density, power density, and Coulombic efficiency of 74.1 W h.kg^-1, 443.7 W.kg^-1, and 101.3%, respectively, with excellent electrochemical stability. After testing an asymmetrical supercapacitor with a Co₃O₄@Co/N-CNT/Ag NPs/rGO/nickel foam cathode and an activated carbon/nickel foam anode, it showed 4.9 W h.kg^-1 of energy density and 5000.0 W.kg^-1 of power density.

The pH Influence on the Water-Splitting Electrocatalytic Activity of Graphite Electrodes Modified with Symmetrically Substituted Metalloporphyrins

Hydrogen, considered to be an alternative fuel to traditional fossil fuels, can be generated by splitting water molecules into hydrogen and oxygen via the use of electrical energy, in a process whose efficiency depends directly on the employed catalytic material. The current study takes part in the relentless search for suitable and low-cost catalysts relevant to the water-splitting field by investigating the electrocatalytic properties of the O₂ and H₂ evolution reactions (OER and HER) of two metalloporphyrins: Zn(II) 5,10,15,20-tetrakis(4-pyridyl)-porphyrin and Co(II) 5,10,15,20-tetrakis(3-hydroxyphenyl)-porphyrin. The TEM/STEM characterisation of the porphyrin samples obtained using different organic solvents revealed several types of self-assembled aggregates. The HER and OER experiments performed on porphyrin-modified graphite electrodes in media with different pH values revealed the most electrocatalytically active specimens. For the OER, this specimen was the electrode manufactured with one layer of Co-porphyrin applied from dimethylsulfoxide, exhibiting an overpotential of 0.51 V at i = 10 mA/cm² and a Tafel slope of 0.27 V/dec. For the HER, it was the sample obtained by drop casting one layer of Zn-porphyrin from N,N-dimethylformamide that displayed a HER overpotential of 0.52 V at i = -10 mA/cm² and a Tafel slope of 0.15 V/dec.

Innovative Cost-Effective Nano-NiCo2O4 Cathode Catalysts for Oxygen Reduction in Air-Cathode Microbial Electrochemical Systems

Microbial electrochemical systems (MESs) can harvest bioelectricity from varieties of organic matter in wastewater through electroactive microorganisms. Oxygen reduction reaction (ORR) in a cathode plays an important role in guaranteeing high power generation, which can be enhanced by cathode catalysts. Herein, the tiny crystalline grain nanocrystal NiCo₂O₄ is prepared via the economic method and utilized as an effective catalyst in air-cathode MESs. The linear sweep voltammetry results indicate that the current density of 2% nano-NiCo₂O₄/AC cathode (5.05 A/m²) at 0 V increases by 20% compared to the control (4.21 A/m²). The cyclic voltammetries (CVs) and the electrochemical impedance spectroscopy (EIS) showed that the addition of nano-NiCo₂O₄ (2%) is efficient in boosting the redox activity. The polarization curves showed that the MESs with 2% nano-NiCo₂O₄/AC achieved the highest maximum power density (1661 ± 28 mW/m²), which was 1.11 and 1.22 times as much as that of AC and 5% nano-NiCo₂O₄. Moreover, the adulteration of nano-NiCo₂O₄ with a content of 2% can not only enable the electrical activity of the electrode to be more stable, but also reduce the cost for the same power generation in MESs. The synthetic nano-NiCo₂O₄ undoubtedly has great benefits for large-scale MESs in wastewater treatment.

Orbital Dependence in Single-Atom Electrocatalytic Reactions

Transition metal single-atom catalysts supported on N-doped graphene (TM-N-C) could serve as an ideal model for studying orbital dependence in electrocatalytic reactions because the atom on the catalytic active site has discrete single-atom-like orbitals. In this work, the catalytic efficiency of Fe-N-C for the oxygen evolution reaction (OER) under a small structural perturbation has been comprehensively investigated with density functional theory calculations. The results suggest that the subtle local environment of a single atom can significantly modulate the catalytic reactivity. Further analysis demonstrates that the energy level of the TM d_z² orbital center, rather than the d-band center, is responsible for the OER catalytic efficiency as the d_z² orbital participates mainly in the reactions. Essentially, the d-band theory can be extended to the sub-d orbital level, and a small perturbation of the crystal field, induced by lattice strain or z-direction displacement of the TM atom, can prominently change the sub-d orbital associated with the reaction and in turn affect the catalytic activity.

Water electrolysis: from textbook knowledge to the latest scientific strategies and industrial developments

Replacing fossil fuels with energy sources and carriers that are sustainable, environmentally benign, and affordable is amongst the most pressing challenges for future socio-economic development. To that goal, hydrogen is presumed to be the most promising energy carrier. Electrocatalytic water splitting, if driven by green electricity, would provide hydrogen with minimal CO2 footprint. The viability of water electrolysis still hinges on the availability of durable earth-abundant electrocatalyst materials and the overall process efficiency. This review spans from the fundamentals of electrocatalytically initiated water splitting to the very latest scientific findings from university and institutional research, also covering specifications and special features of the current industrial processes and those processes currently being tested in large-scale applications. Recently developed strategies are described for the optimisation and discovery of active and durable materials for electrodes that ever-increasingly harness first-principles calculations and machine learning. In addition, a technoeconomic analysis of water electrolysis is included that allows an assessment of the extent to which a large-scale implementation of water splitting can help to combat climate change. This review article is intended to cross-pollinate and strengthen efforts from fundamental understanding to technical implementation and to improve the 'junctions' between the field's physical chemists, materials scientists and engineers, as well as stimulate much-needed exchange among these groups on challenges encountered in the different domains.

Catalytic Properties of Free-Base Porphyrin Modified Graphite Electrodes for Electrochemical Water Splitting in Alkaline Medium

Hydrogen generation via electrochemical water splitting is considered an eco-friendly pathway for obtaining this desired alternative energy source, and it has triggered an intensive search for low cost and efficient catalysts. Within this context, four free-base porphyrins were studied as heterogeneous catalysts for the oxygen and hydrogen evolution reactions (OER and HER) in alkaline aqueous solutions. TEM and STEM analyses of samples obtained by drop-casting the porphyrins from different organic solvents on TEM grids revealed a rich variety of aggregates due to the self-assembling property of the porphyrin molecules. Modified electrodes were manufactured by applying the four tetrapyrrolic macrocycles from various solvents on the surface of graphite supports, in one or more layers. Experiments performed in 0.1 M and 1 M KOH electrolyte solutions allowed the identification of the most electrocatalytically active electrodes for the OER and HER, respectively. In the first case, the electrode was manufactured by applying three layers of 5-(4-pyridyl)-10,15,20-tris(4-phenoxyphenyl)porphyrin on the graphite substrate from N,N-dimethylformamide solution was identified as overall catalytically superior. In the second case, the electrode obtained by applying one layer of 5,10,15,20-tetrakis(4-allyloxyphenyl)-porphyrin from benzonitrile solution displayed an HER overpotential value of 500 mV at i = −10 mA/cm2 and a Tafel slope of 190 mV/dec.

Microwave-Assisted Coprecipitation Synthesis and Local Structural Investigation on NiO, β-Ni(OH)2/Co3O4 Nanosheets, and Co3O4 Nanorods Using X-ray Absorption Spectroscopy at Co-Ni K-edge and Synchrotron X-ray Diffraction

Developing the most straightforward, cheapest, and eco-friendly approaches for synthesizing nanostructures with well-defined morphology having the highest possible surface area to volume ratio is challenging for design and process. In the present work, nanosheets of NiO and β-Ni(OH)₂/Co₃O₄, and nanorods of Co₃O₄ have been synthesized at a large scale via the microwave-assisted chemical coprecipitation method under low temperature and atmospheric pressure. X-ray absorption spectroscopy (XAS) measurements, which comprises both X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) techniques, have been carried out at Co and Ni K-edges to probe the electronic structure of the samples. Also, the local atomic structural, chemical bonding, morphological, and optical properties of the sample were systematically investigated using XAS, synchrotron X-ray diffraction (SXRD), Raman spectroscopy, FTIR, transmission electron microscopy (TEM), and UV-visible spectroscopy. The normalized XANES spectra of the β-Ni(OH)₂/Co₃O₄ nanosheets show the presence of Ni²⁺ and a mixed oxidation state of Co. The disorder factor decreases from β-Ni(OH)₂/Co₃O₄ to Co₃O₄ with increasing Co-O bond length. The SXRD pattern analyzed using Rietveld refinement reveals that NiO has a face-centered cubic phase, Co₃O₄ has the standard spinal structure, and β-Ni(OH)₂/Co₃O₄ has a mixed phase of hexagonal and cubic structures. TEM images revealed the formation of nanosheets for NiO and β-Ni(OH)₂/Co₃O₄ samples and nanorods for Co₃O₄ samples. FTIR and Raman spectra show the formation of β-Ni(OH)₂/Co₃O₄, which reveals the fingerprints of Ni-O and Co-O.

Fabrication and Integration of Functionalized N-rGO-Ni/Ag and N-rGO-Ni/Co Nanocomposites as Synergistic Oxygen Electrocatalysts in Fuel Cells

Fabrication of composites by developing simple techniques can be an efficient way to modify the desire properties of the materials. This paper presents a detailed study on synthesis of low cost and efficient nitrogen doped reduced graphene oxide nickle-silver (N-rGO-Ni/Ag) and nickel-cobalt (N-rGO-Ni/Co) nanocomposites as electrocatalysts in fuel cell using one-pot blended reflux condensation route. An admirable correlation in the structures and properties of the synthesized nanocomposites was observed. The Oxygen Reduction Reaction (ORR) values for N-rGO-Ni/Ag and N-rGO-Ni/Co calculated from the onset potential, using Linear Sweep Voltammetry (LSV), were found to be 1.096 and 1.146. While the half wave potential were determined to be 1.046 and 1.106, respectively, N-rGO-Ni/Ag and N-rGO-Ni/Co. The Tafel and bi-functional (ORR/OER) values were calculated as 76 and 35 mV/decade and 1.23 and 1.12 V, respectively, for N-rGO-Ni/Ag and N-rGO-Ni/Co. The lower onset and half wave potential, low charge transfer resistance (Rct = 1.20 Ω/cm2) and internal solution resistance (Rs = 8.84 × 10-1 Ω/cm2), lower Tafel values (35 mV), satisfactory LSV measurements and mass activity (24.5 at 1.056 V for ORR and 54.9 at 1.056 for OER) demonstrate the remarkable electrocatalytic activity of N-rGO-Ni/Co for both ORR and OER. The chronamperometric stability for synthesized nanocomposites was found satisfactory up to 10 h.

Anchoring Sites Engineering in Single-Atom Catalysts for Highly Efficient Electrochemical Energy Conversion Reactions

Single-atom catalysts (SACs) have been at the frontier of research field in catalysis owing to the maximized atomic utilization, unique structures and properties. The atomically dispersed and catalytically active metal atoms are necessarily anchored by surrounding atoms. As such, the structure and composition of anchoring sites significantly influence the catalytic performance of SACs even with the same metal element. Significant progress has been made to understand structure-activity relationships at an atomic level, but in-depth understanding in precisely designing highly efficient SACs for the targeted reactions is still required. In this review, various anchoring sites in SACs are summarized and classified into five different types (doped heteroatoms, defect sites, surface atoms, metal sites, and cavity sites). Then, their impacts on catalytic performance are elucidated for electrochemical reactions based on their distance from the metal center (first coordination shell and beyond). Further, SACs anchored on two typical types of hosts, carbon- and metal-based materials, are highlighted, and the effects of anchoring points on achieving the desirable atomic structure, catalytic performance, and reaction pathways are elaborated. At last, insights and outlook to the SAC field based on current achievements and challenges are presented.

Reduced Graphene Oxide Supported Cobalt-Calcium Phosphate Composite for Electrochemical Water Oxidation

We report the oxygen evolution reaction (OER) catalyst composed of cobalt–calcium phosphate on reduced graphene oxide (CoCaP/rGO). Our catalyst is prepared by the anodic electrolysis of calcium phosphate/rGO mixture loaded on indium-tin-oxide (ITO) in Co2+ aqueous solution. TEM, XPS and XRD experiments confirm that the crystal phase of calcium phosphate (CaP) is transferred into an amorphous phase of calcium oxide with phosphate (5.06 at%) after anodic electrolysis. Additionally, the main cation component of calcium is replaced by cobalt ion. The current–voltage characteristics of CoCaP/rGO showed a shoulder peak at 1.10 V vs. NHE, which originated from Co2+ to higher oxidation states (Co3+ or Co4+) and a strong wave from water oxidation higher +1.16 V vs. NHE at neutral condition (pH 7). CoCaP and CoCaP/rGO showed 4.8 and 10 mA/cm2 at 0.47 V of overpotential, respectively. The enhanced OER catalytic activity of CoCaP/rGO arises from the synergetic interaction between the amorphous phase of CoCaP and electric conducting graphene sheets.

Electrocatalytic Oxygen Evolution by Hierarchically Structured Cobalt-Iron Composites

The development of scalable routes to highly active and efficient oxygen evolution reaction (OER) electrocatalysts based on earth-abundant materials is crucial for post-fossil fuel energy schemes. Here, we demonstrate how commercial copper foam electrodes can be functionalized for water oxidation using a facile electrodeposition process. The resulting composite electrode features hierarchically structured cobalt-iron-based catalyst particles, which offer channel-like structures for the transport of electrolyte and release of oxygen gas bubbles. We report high electrocatalytic OER performance as demonstrated by high current densities at low overpotentials (293 mV at j = 50 mA cm^-2) and long-term stability under technologically relevant alkaline conditions (>24 h in 1.0 M aqueous KOH).

Strategies to Develop Earth-Abundant Heterogeneous Oxygen Evolution Reaction Catalysts for pH-Neutral or pH-Near-Neutral Electrolytes

The anodic oxygen evolution reaction (OER) is the bottleneck of water splitting to produce hydrogen due to its sluggish kinetics. In order to lower the energy cost, highly active and cost-efficient OER catalysts need to be used to overcome the OER reaction barrier, especially in neutral pH. Compared to alkaline or acidic electrolytes, pH-neutral or pH-near-neutral electrolytes are considered to be cheaper and safer, and water from rivers and the sea could be used directly under such conditions. However, OER under neutral pH is challenging compared to the OER catalysts for alkaline conditions. Therefore, OER catalysts for neutral or near-neutral pH have not been pursued significantly and, hence, there are limited advances in this area. Here, the progress made in the research and development of earth-abundant heterogeneous catalysts for OER in three pH-neutral or pH-near-neutral systems, namely, the phosphate system, the carbonate system, and the borate system, are systematically reviewed and summarized for the first time. Strategies to develop high-performance OER catalysts for neutral pH are reviewed and summarized. In addition, future challenges and opportunities in this field are discussed, which may shed some light on the future developments of earth-abundant heterogeneous catalysts for OER in neutral or near-neutral pH.

CdS quantum dots encapsulated within the mesopores of MCM-41 and interlayers of montmorillonite as photocatalysts for rhodamine-B degradation in aqueous solution

Capping agent-free CdS quantum dots (CdS-QDs) were synthesized within the mesopores of MCM-41 and interlayers of montmorillonite (MMT), using a safe manner by a facile ion exchange-precipitation protocol. The mesopores of MCM-41 and interlayers of MMT controlled the growth of CdS-QDs. The obtained CdS-QDs@MCM-41 and CdS-QDs/MMT were characterized by X-ray diffraction (XRD) analysis, energy-dispersive X-ray (EDX), diffuse reflectance UV-Vis, and photoluminescence spectroscopies. Photodegradation of rhodamine-B (RhB) over these embedded CdS-QDs was investigated under UV-Vis light irradiation. The influences of some parameters on the photodegradation of RhB such as pH, temperature, and UV-Vis irradiation time were investigated. The results showed that the CdS-QDs/MMT and CdS-QDs@MCM-41 have high efficiencies for RhB photodegradation under UV-Vis illumination.

Facile doping of nickel into Co3O4 nanostructures to make them efficient for catalyzing the oxygen evolution reaction

Designing a facile and low-cost methodology to fabricate earth-abundant catalysts is very much needed for a wide range of applications. Herein, a simple and straightforward approach was developed to tune the electronic properties of cobalt oxide nanostructures by doping them with nickel and then using them to catalyze the oxygen evolution reaction (OER) in an aqueous solution of 1.0 M KOH. The addition of a nickel impurity improved the conductivity of the cobalt oxide, and further increased its activity towards the OER. Analytical techniques such as scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS) and powder X-ray diffraction (XRD) were used to investigate, respectively, the morphology, composition and crystalline structure of the materials used. The nickel-doped cobalt oxide material showed randomly oriented nanowires and a high density of nanoparticles, exhibited the cubic phase, and contained cobalt, nickel and oxygen as its main elements. The nickel-doped cobalt oxide also yielded a Tafel slope of 82 mV dec⁻¹ and required an overpotential of 300 mV to reach a current density of 10 mA cm⁻². As an OER catalyst, it was shown to be durable for 40 h. Electrochemical impedance spectroscopy (EIS) analysis showed a low charge-transfer resistance of 177.5 ohms for the nickel-doped cobalt oxide, which provided a further example of its excellent OER performance. These results taken together indicated that nickel doping of cobalt oxide can be accomplished via a facile approach and that the product of this doping can be used for energy and environmental applications.

Designing a facile and low-cost methodology to fabricate earth-abundant catalysts is very much needed for a wide range of applications.

Recent advances in two-dimensional materials and their nanocomposites in sustainable energy conversion applications

Two-dimensional (2D) materials have a wide platform in research and expanding nano- and atomic-level applications. This study is motivated by the well-established 2D catalysts, which demonstrate high efficiency, selectivity and sustainability exceeding that of classical noble metal catalysts for the oxygen reduction reaction (ORR), oxygen evolution reaction (OER), and/or hydrogen evolution reaction (HER). Nowadays, the hydrogen evolution reaction (HER) in water electrolysis is crucial for the cost-efficient production of a pure hydrogen fuel. We will also discuss another important point related to electrochemical carbon dioxide and nitrogen reduction (ECR and N2RR) in detail. In this review, we mainly focused on the recent progress in the fuel cell technology based on 2D materials, including graphene, transition metal dichalcogenides, black phosphorus, MXenes, metal-organic frameworks, and metal oxide nanosheets. First, the basic attributes of the 2D materials were described, and their fuel cell mechanisms were also summarized. Finally, some effective methods for enhancing the performance of the fuel cells based on 2D materials were also discussed, and the opportunities and challenges of 2D material-based fuel cells at the commercial level were also provided. This review can provide new avenues for 2D materials with properties suitable for fuel cell technology development and related fields.

Carbon Derived from Soft Pyrolysis of a Covalent Organic Framework as a Support for Small-Sized RuO2 Showing Exceptionally Low Overpotential for Oxygen Evolution Reaction

Electrochemical water splitting is the most energy-efficient technique for producing hydrogen and oxygen, the two valuable gases. However, it is limited by the slow kinetics of the anodic oxygen evolution reaction (OER), which can be improved using catalysts. Covalent organic framework (COF)-derived porous carbon can serve as an excellent catalyst support. Here, we report high electrocatalytic activity of two composites, formed by supporting RuO2 on carbon derived from two COFs with closely related structures. These composites catalyze oxygen evolution from alkaline media with overpotentials as low as 210 and 217 mV at 10 mA/cm2, respectively. The Tafel slopes of these catalysts (65 and 67 mV/dec) indicate fast kinetics compared to commercial RuO2. The observed activity is the highest among all RuO2-based heterogeneous OER catalysts-a touted benchmark OER catalyst. The high catalytic activity arises from the extremely small-sized (∼3-4 nm) RuO2 nanoparticles homogeneously dispersed in a micro-mesoporous (BET = 517 m2/g) COF-derived carbon. The porous graphenic carbon favors mass transfer, while its N-rich framework anchors the catalytic nanoparticles, making it highly stable and recyclable. Crucially, the soft pyrolysis of the COF enables the formation of porous carbon and simultaneous growth of small RuO2 particles without aggregation.

Two-Dimensional Materials on the Rocks: Positive and Negative Role of Dopants and Impurities in Electrochemistry

Two-dimensional (2D) materials, such as graphene and transition-metal chalcogenides, were shown in many works as very potent catalysts for industrially important electrochemical reactions, such as oxygen reduction, hydrogen and oxygen evolution, and carbon dioxide reduction. We critically discuss here the development in the field, showing that not only dopants but also impurities can have dramatic effects on catalysis. Note here that the difference between dopant and impurity is merely semantic-dopant is an impurity deliberately added to the material. We contest the general belief that all doping has a positive effect on electrocatalysis. We show that in many cases, dopants actually inhibit the electrochemistry of 2D materials. This review provides a balanced view of the field of 2D materials electrocatalysis.

Enhanced oxygen evolution reaction on amine functionalized graphene oxide in alkaline medium

Development of highly efficient oxygen evolution reaction (OER) electrocatalysts is a critical challenge in the cost-effective generation of clean fuels. Here, a metal-free tyramine functionalized graphene oxide (T-GO) electrocatalyst is proposed to use in alkaline electrolytes for enhanced OER. Moreover, the T-GO and GO nanomaterials are well characterized by SEM, XRD, FTIR, XPS and Raman spectroscopy. T-GO exhibits an electrocatalytic OER with a current density of 2 mA cm^-2 at a low onset potential of ∼1.39 V and a small Tafel slope of about 69 mV dec^-1 and GO exhibits an onset potential of 1.51 V and Tafel slope of about 92 mV dec^-1. Additionally, the current stability and RRDE based diffusion controlled response of the T-GO electrocatalyst are outstanding compared to GO. This study establishes metal free T-GO as an efficient electrocatalyst for the OER and used for cathodic production of hydrogen as a counter reaction in the field of water splitting.

Efficacious Electrochemical Oxygen Evolution from a Novel Co(II) Porphyrin/Pyrene-Based Conjugated Microporous Polymer

Oxygen evolution reaction (OER) is energetically challenging from the platform of making many photovoltaic devices such as metal-air batteries and water splitting systems because of its poor kinetics even when precious metals are used. Herein, a Co(II)-porphyrin/pyrene-comprised conjugated microporous polymer Co-MPPy-1 has been developed which shows efficient OER in alkaline medium. The material was characterized by Fourier transform infrared, solid-state ¹³C cross-polarization magic angle spinning nuclear magnetic resonance, N₂ volumetric adsorption/desorption analysis, scanning electron microscopy, ultra high resolution-transmission electron microscopy, X-ray photoelectron spectroscopy, and other physical studies. Co-MPPy-1 showed Brunauer-Emmett-Teller surface area of ∼501 m² g^-1. Co-MPPy-1 achieved a current density of 1 and 10 mA/cm^-2 at 340 and 420 mV, respectively. The turnover frequency calculated for the OER is 0.43 s^-1. The heterogeneity of this electrocatalyst was tested by chronoamperometric measurement and 1000 cycle recyclability test with retainment of the excellent electrochemical catalytic activity. This can be attributed to the presence of high density of Co(II) porphyrin unit and efficient charge transport in the π-conductive conjugated polymeric backbone.

Monitoring compositional changes in Ni(OH)2 electrocatalysts employed in the oxygen evolution reaction

Relating morphology and compositional changes spatially across a catalyst is important for understanding the active site involved in a reaction which is studied here for the OER at Ni(OH)₂.

Recent Advances of Cobalt-Based Electrocatalysts for Oxygen Electrode Reactions and Hydrogen Evolution Reaction

This review summarizes recent progress in the development of cobalt-based catalytic centers as the most potentially useful alternatives to noble metal-based electrocatalysts (Pt-, Ir-, and Ru-based) towards the oxygen reduction reaction (ORR), oxygen evolution reaction (OER), and hydrogen evolution reaction (HER) in acid and alkaline media. A series of cobalt-based high-performance electrocatalysts have been designed and synthesized including cobalt oxides/chalcogenides, Co–Nx/C, Co-layered double hydroxides (LDH), and Co–metal-organic frameworks (MOFs). The strategies of controllable synthesis, the structural properties, ligand effect, defects, oxygen vacancies, and support materials are thoroughly discussed as a function of the electrocatalytic performance of cobalt-based electrocatalysts. Finally, prospects for the design of novel, efficient cobalt-based materials, for large-scale application and opportunities, are encouraged.