The study of high-performance electrocatalysts for driving the oxygen evolution reaction (OER) is important for energy storage and conversion systems. As a representative of inverse-spinel-structured oxide catalysts, nickel ferrite (NiFe2O4) has recently gained interest because of its earth abundance and environmental friendliness. However, the gained electrocatalytic performance of NiFe2O4 for the OER is still far from the state-of-the-art requirements because of its poor reactivity and finite number of surface active sites. Here, we prepared a series of atomically thin NiFe2O4 catalysts with different lateral sizes through a mild and controllable method. We found that the atomically thin NiFe2O4 quantum dots (AT NiFe2O4 QDs) show the highest OER performance with a current density of 10 mA cm–2 at a low overpotential of 262 mV and a small Tafel slope of 37 mV decade–1. The outstanding OER performance of AT NiFe2O4 QDs is even comparable to that of commercial RuO2 catalyst, which can be attributed to its high reactivity and the high fraction of active edge sites resulting from the synergetic effect between the atomically thin thickness and the small lateral size of the atomically thin quantum dot (AT QD) structural motif. The experimental results reveal a negative correlation between lateral size and OER performance in alkaline media. Specifically speaking, the number of low-coordinated oxygen atoms increases with decreasing lateral size, and this leads to significantly more oxygen vacancies that can lower the adsorption energy of H2O, increasing the catalytic OER efficiency of AT NiFe2O4 QDs.Keywords: atomically thin quantum dots; catalytically active sites; nickel ferrite; oxygen evolution reaction; water electrolysis;

The exploration of highly efficient nonprecious metal-based electrocatalysts for the oxygen evolution reaction (OER) is of great importance for potential applications in sustainable energy conversion. Recently, the layered metal hydroxide (LMH) family is receiving extensive research attention owing to its unique structural properties. However, the gained electrocatalytic performance of LMH-based catalysts for the OER is still far from the state-of-the-art requirements because of its finite number and poor reactivity of exposed active sites. In response, we synthesized crystal lattice distorted ultrathin cobalt hydroxide (denoted as CLD-u-Co(OH)2) nanosheets with a great number of efficient catalytic active sites through the introduction of Ga into ultrathin Co(OH)2, followed by a selective removal of Ga, denoted as the “introduction @ removal” process. As a result, the crystal lattice distortion confined inside CLD-u-Co(OH)2 generates abundant elongated Co–OOH bonds on exposed (10) facets serving as efficient catalytic active sites for the OER. Besides, the optimized amount of “introduction @ removal” of Ga (4 at%) allows for an exquisite balance between distortion engineering and electrical conductivity, synergistically. The as-prepared CLD-u-Co(OH)2 achieves an overpotential of 265 mV at a current density of 10 mA cm−−2, an unexpectedly small Tafel slope of 47 mV dec−−1, and a long-term stability (beyond 20 h) in basic media. It is mainly attributed to abundant catalytic active sites, robust reactivity per site, and good electrical conductivity. Furthermore, the green and sustainable engineering of crystal lattice distortion to improve the intrinsic electrocatalytic activity of CLD-u-Co(OH)2 nanosheets presented in this work may provide a promising strategy to design and synthesize newly highly efficient LMH-based electrocatalysts for the OER.

Layered double hydroxides (LDHs) are increasingly being recognized as one of the most promising candidates to lower the oxygen evolution reaction (OER) barriers in water splitting. However, how to further increase catalytically active sites and then improve the corresponding catalytic activity is still a great challenge, which is critical for the design of an efficient OER electrocatalyst. Here, we present the in situ growth of ultrathin Ni–Fe LDH (u-Ni70Fe30 LDHs) nanosheets rich in catalytically active sites on the anodic polarized copper foil as an efficient electrocatalyst for OER through a mild electrodeposition approach (denoted as u-Ni70Fe30 LDHs/a-CF). We found that the u-Ni70Fe30 LDHs/a-CF shows a remarkable catalytic activity (the overpotential of 260 mV at 10 mA cm−2) and superior catalytic stability (beyond 50 h) in basic media. The achieved ultrahigh catalytic performance of u-Ni70Fe30 LDHs/a-CF is primarily attributed to its ultrathin nanosheet structure, the optimized 30 at% Fe-dopant concentration, and the synergetic effect between the nanosheets and the bumps on a-CF. These reasons lead to the high density of exposed active sites and the improved mass/electron transport capability. In comparison with the other developed LDH based electrocatalysts reported to date, we provide a controllable and mild strategy for designing highly-efficient ultrathin LDH based electrocatalysts with desirable kinetics for future large-scale application in water splitting.

Water splitting provides a potential path for producing clean, renewable H2 and O2. However, improving the overall efficiency of water splitting is a challenging issue. Here, we designed Co–Fe nanoparticle coupled nitrogen-enriched porous carbon (CoyFe10−yOx/NPC) nanosheets as highly efficient non-precious-metal electrocatalysts for the oxygen evolution reaction (OER). Nitrogen-enriched porous carbon (NPC) nanosheets were prepared using a Schiff-base network (SNW) as the precursor and the SNW was based on commercially available and inexpensive monomers which were terephthalaldehyde and melamine. The resulting SNW possessed a high nitrogen content, a high surface area and a high density of metal-coordination sites. In addition, when used as the catalyst for the OER, the Co–Fe nanoparticle catalyst containing 30% Co (Co3Fe7Ox/NPC) showed the highest activity, requiring 328 mV over-potential to achieve a stable current density of 10 mA cm−2 for at least 15 h and a small Tafel slope of 31.4 mV dec−1 in 1.0 M KOH solution, which were comparable even superior to those of many other non-noble metal catalysts. Consequently, the high efficiency and durability make these supported amorphous Co–Fe nanoparticles potentially applicable for improving the performance for electrolysis of water and energy storage applications. More importantly, the support of electrode materials comes from the pyrolysis of porous polymers and this idea offers a new possibility for exploring overall water splitting non-precious-metal catalysts.

The development of metal-free catalysts for efficient catalysis of both the oxygen evolution reaction (OER) and the oxygen reduction reaction (ORR) is extremely desirable in energy technologies. Herein, nitrogen-doped mesoporous carbon nanosheet/carbon nanotube (CNT) hybrids have been synthesized by the pyrolysis of glucose, urea and CNTs. Impressively, in 0.1 M KOH, the resulting hybrids afford remarkable OER activities with a low onset potential (1.50 V vs. RHE) and an exceptional over-potential (only 320 mV at 10 mA cm−2). Moreover, the same hybrids show comparable catalytic performance but better durability compared to the benchmark Pt/C (20 wt%) catalyst for ORR. The achieved ultrahigh catalytic performance of the hybrids originates from their large specific surface area (594.1 m2 g−1), high content percentage of N doping (8.5 wt%), and mesoporous structure, which leads to fully exposed active sites, improved mass/electron transport capability, easy adsorption/release of oxygen gas bubbles, and high structural stability. This work also provides a novel concept for fabricating heteroatom doped porous carbonaceous materials with integrated and improved catalytic performance for advanced applications.

•A self-assembly approach to synthesize Co-MOF@CNTs 3D hybrids.•CNTs can avoid the carbon corrosion at high positive potentials of OER.•The structure offers a large surface area and stable anchoring sites.•A synergistic interaction among Co(ΙΙ), organic ligands and CNTs.•The electrocatalyst achieves high performance in OER and ORR.Metal organic frameworks (MOF) derived carbonaceous materials have emerged as promising bifunctional oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) catalysts for electrochemical energy conversion and storage. But previous attempts to overcome the poor electrical conductivity of MOFs hybrids involve a harsh high-template pyrolytic process to in situ form carbon, which suffer from extremely complex operation and inevitable carbon corrosion at high positive potentials when OER is operated. Herein, a self-assembly approach is presented to synthesize a non-precious metal-based, high active and strong durable Co-MOF@CNTs bifunctional catalyst for OER and ORR. CNTs not only improve the transportation of the electrons but also can sustain the harsh oxidative environment of OER without carbon corrosion. Meanwhile, the unique 3D hierarchical structure offers a large surface area and stable anchoring sites for active centers and CNTs, which enables the superior durability of hybrid. Moreover, a synergistic catalysis of Co(II), organic ligands and CNTs will enhance the bifunctional electrocatalytic performance. Impressively, the hybrid exhibits comparable OER and ORR catalytic activity to RuO2 and 20 wt% Pt/C catalysts and superior stability. This facile and versatile strategy to fabricating MOF-based hybrids may be extended to other electrode materials for fuel cell and water splitting applications.Scheme of synthesis of the catalyst Co-MOF@CNTs.

•Polyhedral Pd nanoparticles with controlled sizes and shapes were prepared.•Pd nanocubes have high current density in HER.•The DFT shows electron density distribution of Pd {100} is better than Pd {111}.•The novel catalyst exhibits excellent HER activity comparable with that of Pt/C.•It opens new insights into design efficient Pd catalyst with low loading in HER.The catalytic activity of noble-metal nanoparticles (NPs) often has closely connection with their sizes and geometric shape. In the work, polyhedral NPs of palladium (Pd) with controlled sizes, shapes, and different proportions of {100} to {111} facets on the surface were prepared by a seed-mediated approach. Electrochemical experiment demonstrates that the catalytic performance of the Pd nanocubes (NCs) enclosed by {100} facets is more active than Pd octahedrons enclosed by {111} facets for the hydrogen evolution reaction (HER), which is consistent with density functional theory (DFT) calculation results. Meanwhile, with the assistance of a polyethyleneimine–reduced graphene oxide (PEI–rGO) support, the examined Pd cube/PEI–rGO50:1 (10 wt. %) electrocatalyst presents outstanding HER activity comparable with that of commercial Pt/C catalyst. This correlation between the HER catalytic activity and surface structure will contribute to the reasonable design of Pd catalysts for HER with high efficiency and low metal loading.

Cobalt-based nanomaterials are promising candidates as efficient, affordable, and sustainable alternative electrocatalysts for the oxygen evolution reaction (OER). However, the catalytic efficiency of traditional nanomaterials is still far below what is expected, because of their low stability in basic solutions and poor active site exposure yield. Here a unique hybrid nanomaterial comprising Co@Co3O4 core–shell nanoparticle (NP) encapsulated N-doped mesoporous carbon cages on reduced graphene oxide (denoted as Co@Co3O4@NMCC/rGO) is successfully synthesized via a carbonization and subsequent oxidation strategy of a graphene oxide (GO)-based metal–organic framework (MOF). Impressively, the special carbon cage structure is very important for not only leading to a large active surface area, enhanced mass/charge transport capability, and easy release of gas bubbles, but also preventing Co@Co3O4 NPs from aggregation and peeling off during prolonged electrochemical reactions. As a result, in alkaline media, the resulting hybrid materials catalyze the OER with a low onset potential of ∼1.50 V (vs. RHE) and an over-potential of only 340 mV to achieve a stable current density of 10 mA cm−2 for at least 25 h. In addition, metallic Co cores in Co@Co3O4 provide an alternative way for electron transport and accelerate the OER rate.

Active, stable and cost-effective electrocatalysts are the key to water splitting for hydrogen production through electrolysis. In this work, we report MoS2 quantum dots (MoS2 QDs) decorated on reduced graphene oxide (RGO) synthesized by a facile sonication method as highly effective electrocatalysts for the hydrogen evolution reaction (HER). Compared with MoS2 sheets, the zero-dimensional MoS2 QDs have a defect-rich structure rendering these quantum dots with plentiful active sites, which can further enhance the catalytic activity by a synergistic effect with RGO. Electrochemical experiments demonstrated that the catalyst exhibited large cathode currents (a small overpotential of 64 mV for 10 mA cm−2 current density) and a Tafel slope as small as 63 mV per decade, achieving high stability simultaneously. This work opens up possibilities for preparing non-noble metal electrocatalysts while achieving high HER performance similar to commercial Pt catalysts (Pt/C).

Highly efficient and non-precious metal electrocatalysts for oxygen evolution reactions (OERs) and oxygen reduction reactions (ORRs) are at the heart of key renewable-energy technologies. Nevertheless, developing highly active bi-functional catalysts at low cost for both OER and ORR still remains a huge challenge. In this paper, Co3O4 nanocrystals embedded in N-doped mesoporous graphitic carbon layer/multiwalled carbon nanotube (MWCNT) hybrids are prepared by a facile carbonization and subsequent oxidation process of MWCNT-based metal–organic frameworks (MOFs). As a result, in alkaline media, the hybrid material catalyzes OER with an onset potential of 1.50 V (vs. reversible hydrogen electrode) and an over-potential only of 320 mV to achieve a stable current density of 10 mA cm−2 for at least 25 h. The same hybrids also exhibit similar catalytic activity but superior stability to the commercial 20 wt% Pt/C catalyst for ORR, making it a high-performance cheap bi-catalyst for both OER and ORR. The design concept of nonmetal-doped and precious-metal-free electrocatalysts from MOFs can be extended to fabricate other novel, stable and easy to use catalyst systems for advanced applications.

In the present work, nickel (Ni) and palladium (Pd) core–shell structure nanospheres with various thicknesses have been facilely obtained via a one-pot synthesis process. These hybrid structures allow us to correlate the Pd thickness with their performance in the hydrogen evolution reaction (HER). The HER activity increases with a decrease of the Pd thickness, and it can be ascribed to the enhancement of electron donation of Pd (111) caused by the electron flow from Ni (111) to Pd (111) based on first-principles calculations. In this hybrid system, the difference in work functions of Pd and Ni results in surface polarization on the Pd surface, tuning its charge state for hydrogen reduction. Meanwhile, with the assistance of a polyethyleneimine–reduced graphene oxide (PEI–rGO) support, the examined Ni@Pd4:1/PEI–rGO50:1 (10 wt%) electrocatalyst presents outstanding HER activity comparable with that of platinum (Pt). This work opens up possibilities for reducing Pd usage while achieving high HER performance.

•In this study, the Cu–MoS2/rGO hybrid electrocatalyst was easily synthesized using hydrothermal method and chemical process.•This Non-noble Metal electrocatalyst preparation procedure is in accordance with the principles of green chemistry.•Edge-exposed MoS2 nanoflowers on rGO improves the catalytic activity by synergistic effect with Cu particles.•The Cu–MoS2/rGO hybrid catalyst exhibits excellent HER activity with a small Tafel slope value and large cathodic current.The process of water splitting by using efficient non-precious-metal catalyst to make the process economically viable has brought forward an extensive research. In this study, the composite of Cu and MoS2 on the reduced graphene oxide (rGO) (Cu–MoS2/rGO) with high catalytic activity toward hydrogen evolution reaction (HER) was easily synthesized using hydrothermal method and chemical process. It was found that the participation of rGO improved the electrical conductivity of the catalyst, and the MoS2 nanoflowers (NFs) with a higher number of exposed edges exhibited higher HER activity. In addition, the addition of Cu particles could not only enhance the electrical conductivity of the catalyst, but also further improved the catalytic activity by synergistic effect with MoS2 NFs. Electrochemical experiments demonstrated that the catalyst exhibited excellent HER activity with large cathode currents and a Tafel slope as small as 90 mV decade−1. Furthermore, it was discovered that the current density of this composite catalyst had a little decrease after the continual 700 cycling, which showed the catalyst had a high stability in the recycling process. These findings confirmed that this catalyst was a useful and earth-abundant material for water splitting.

•A facile method was put forward to prepare the catalyst MSS-SH-Au0.•The catalyst had mesoporous structure and Au NPs were well dispersed in the MSS.•The catalyst showed high activity in oxidations of cyclohexene and styrene by oxygen.•The catalyst was easily recovered with external magnetic field.•The catalytic could be reused for five times without any obvious loss of activity.We have synthesized a new catalyst based on thiol-functionalized silica-coated magnetic mesoporous nanocrystals as support and Au nanoparticles as active sites through a facile and environment-friendly approach and characterized by transmission electron microscopy (TEM), N2 adsorption–desorption, elemental analysis and inductive coupled plasma atomic emission spectrometer (ICP-AES), X-ray powder diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and vibrating sample magnetometry (VSM). The synthesized catalyst exhibited high catalytic activity in the oxidations of cyclohexene and styrene by molecular oxygen at atmospheric pressure. Furthermore, this catalyst had an excellent recyclability, evidenced by being extensively reused for five times without any substantial loss of activity, which was mainly attributed to the enough strong combination between the Au nanoparticles and thiol groups onto the surface of support. Meanwhile, the catalyst could be easily separated from the reaction solution by applying an external magnetic field.

New catalysts, consisting of perylene tetracarboxylic acid functionalized graphene sheets support-enhanced electrocatalytic Pd nanoparticles (Pd/PTCA–GS), were fabricated using different reducing agents, including H2, NaBH4 and ethylene glycol (EG). The graphene sheets (GS) were functionalized via π–π stacking and hydrophobic forces. The information of the morphologies, sizes, and dispersion of Pd nanoparticles (NPs) for the as-prepared catalysts was verified by transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), Raman spectra and X-ray diffraction (XRD). As the ethanol electro-oxidation anode catalysts, the new catalysts exhibited better kinetics, higher electrocatalytic activity, better tolerance and better electrochemical stability than the Pd/GS and Pd/C, which illustrated that the new catalysts had potential applications in direct ethanol alkaline fuel cells (DEAFCs). Most attractively, the role of the chemical reduction methods (the NaBH4, EG and H2 as reducing agents) were studied systematically for the ethanol electro-oxidation anode catalysts in DEAFCs. As expected, the chemical reduction method remarkably affected the electrochemical behavior. Among all the Pd/PTCA–GS catalysts tested, Pd/PTCA–GS(NaBH4) exhibited the highest catalytic activity and stability, which may be due to the Pd NPs for Pd/PTCA–GS(NaBH4) having a narrow size distribution, uniform distribution and more perfect crystal structure than that of other as-prepared nanocomposites. These Pd/PTCA–GS are promising catalysts for developing a highly efficient direct ethanol alkaline fuel cells system for power applications.

A very efficient, in situ growth approach has been developed to fabricate a graphene oxide (GO)-supported, high-performance Ni1−xFex loading electrocatalyst. Transition metal nanoparticles (NPs) such as Ni are quite attractive due to their advantages in cost, and excellent electrocatalytic activity for hydrazine electrooxidation. However, their strong aggregation propensity makes it difficult to develop them into highly efficient electrocatalysts. Polyethyleneimine (PEI), a branched polyelectrolyte, is devised to resolve this problem, and is utilized to uniformly anchor Ni–Fe NPs on GO. Meanwhile, partial GO can be reduced to reduced graphene oxide (rGO) in the presence of amine-containing molecules, which is favourable to enhance the electrical conductivity of supporting materials. Ni1−xFex NPs (50% loading) are prepared by a co-reduction method of metal precursors with hydrazine hydrate under 100 °C for 3 h. The content of PEI attached on GO significantly affects the dispersion and size of the resulting Ni1−xFex NPs. Transmission electron microscope (TEM) images reveal that the Ni1−xFex NPs with an average size of 8 nm are homogeneously dispersed on PEI–rGO10:1 sheet. The electrochemical measurements indicate that the Ni1−xFex/PEI–rGO10:1 hybrid with a Ni:Fe mass ratio of 80:20 exhibits the highest electrocatalytic activity toward hydrazine oxidation, while the PEI–rGO10:1 supporting material plays an excellent role in the synergistic catalytic effect. Under the same conditions, the electrocatalytic activity of Ni80Fe20 NPs on PEI–rGO10:1 for hydrazine is 2 times higher than that of the Ni80Fe20 NPs directly deposited on GO.

A simple method to decorate multiwalled carbon nanotubes (MWCNTs) with Ag nanoparticles by using dialdehyde starch (DAS) as complexant and reductant was developed. The Ag–DAS@MWCNTs were characterized by transmission electron microscopy (TEM), fourier transform infrared spectroscopy (FT-IR), X-ray diffraction (XRD), atomic absorption spectroscopy (AAS) and used as antibacterial materials against Escherichia coli (E. coli), Bacillus subtilis (B. subtilis), Bacillus megaterium (B. megaterium), Micrococcus tetragenus (M. tetragenus). The Ag–DAS@MWCNTs exhibited superior antibacterial activities.

In order to detect low concentration of glucose in cell sample conveniently, a glucose biosensor with high sensitivity and low detection limit was fabricated by modifying one-dimensional ultra-long Cu nanowires (Cu NWs) and multi-walled carbon nanotubes (MWCNTs) hybrid on the surface of glassy carbon electrode (GCE). Cu NWs with uniform diameter were synthesized by a facile approach in large scale and characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD). The electrocatalytic activity of the as-prepared biosensor towards glucose oxidation was investigated by cyclic voltammetry and amperometric measurement in alkaline media. The results showed that the biosensor exhibited a rapid response time of less than 1 s and high sensitivity of 1995 μAm M−1 cm−2 with a wide linear range (up to 3 mM) and low detection limit (0.26 μM at signal/noise ratio (S/N) = 3). For practical application, the biosensor was successfully applied to detect glucose in serum and study the relationship between the glucose concentrations in cultivated cells and the glucose concentrations in culture mediums, in which the cells were cultivated. All the experiment results showed that the Cu NWs–MWCNTs hybrid had great potential applications in the development of biosensors for enzyme-free detection of low concentration glucose.

A water-dispersible and supermagnetic nanocomposite (PAD-PEG-Fe3O4@PEI) has been successfully synthesized using polyethylenimine (PEI, Mol MW = 10000) coated supermagnetic Fe3O4–NH2 which was modified with 2, 2′-(phenylazanediyl) diacetic acid (PAD) through the bridge of poly(ethylene glycol) (PEG, Mol MW = 2000). The average particle size of PAD-PEG-Fe3O4@PEI was determined by TEM, and was about 50 nm. From magnetic hysteresis cycles for PAD-PEG-Fe3O4@PEI at room temperature, the saturation magnetization (Ms) was shown to be 58.14 emu g−1. Inductively coupled plasma spectrometry (ICP) analysis showed that the designed magnetic nanocomposite can remove 98% and 80% of Cd2+ from water and blood, respectively.

A simple and novel method to fill carbon nanotubes (CNTs) with Ni–Fe alloys by methylbenzene-oriented constant current electrodeposition is demonstrated. The method is based on the difference in the surface conductivity of CNTs inside and outside in electrodeposition process owing to the covering of methylbenzene. The Ni–Fe alloys filled multiwalled carbon nanotubes (MWCNTs) were characterized by transmission electron microscopy (TEM), Fourier transform infrared spectroscopy (FT-IR), X-ray diffraction (XRD), vibrating sample magnetometer (VSM) and atomic absorption spectroscopy (AAS), respectively. Then the Ni–Fe alloys filled MWCNTs were used as hydrazine oxidation electrocatalysts in direct hydrazine (N2H4)-air fuel cells. Cyclic voltammograms (CVs) indicated that Ni85Fe15-filled MWCNTs had superior electrocatalytic activity for hydrazine electrocatalysis than catalysts with other compositions.Graphical abstractThe process for the preparation of the Ni–Fe alloys filled MWCNTs.Highlights► A novel method is presented to fill multiwalled carbon nanotubes with Ni–Fe alloys. ► The method is methylbenzene-oriented constant current electrodeposition. ► The method is based on the different conductivity of MWCNTs inside and outside. ► The process is simple, environment friendly and proceeds at low temperature. ► The Ni85Fe15-filled MWCNTs showed high electrocatalytic activity for hydrazine.

New catalysts, consisting of perylene tetracarboxylic acid functionalized graphene sheets support-enhanced electrocatalytic Pd nanoparticles (Pd/PTCA–GS), were fabricated using different reducing agents, including H2, NaBH4 and ethylene glycol (EG). The graphene sheets (GS) were functionalized via π–π stacking and hydrophobic forces. The information of the morphologies, sizes, and dispersion of Pd nanoparticles (NPs) for the as-prepared catalysts was verified by transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), Raman spectra and X-ray diffraction (XRD). As the ethanol electro-oxidation anode catalysts, the new catalysts exhibited better kinetics, higher electrocatalytic activity, better tolerance and better electrochemical stability than the Pd/GS and Pd/C, which illustrated that the new catalysts had potential applications in direct ethanol alkaline fuel cells (DEAFCs). Most attractively, the role of the chemical reduction methods (the NaBH4, EG and H2 as reducing agents) were studied systematically for the ethanol electro-oxidation anode catalysts in DEAFCs. As expected, the chemical reduction method remarkably affected the electrochemical behavior. Among all the Pd/PTCA–GS catalysts tested, Pd/PTCA–GS(NaBH4) exhibited the highest catalytic activity and stability, which may be due to the Pd NPs for Pd/PTCA–GS(NaBH4) having a narrow size distribution, uniform distribution and more perfect crystal structure than that of other as-prepared nanocomposites. These Pd/PTCA–GS are promising catalysts for developing a highly efficient direct ethanol alkaline fuel cells system for power applications.

In the present work, nickel (Ni) and palladium (Pd) core–shell structure nanospheres with various thicknesses have been facilely obtained via a one-pot synthesis process. These hybrid structures allow us to correlate the Pd thickness with their performance in the hydrogen evolution reaction (HER). The HER activity increases with a decrease of the Pd thickness, and it can be ascribed to the enhancement of electron donation of Pd (111) caused by the electron flow from Ni (111) to Pd (111) based on first-principles calculations. In this hybrid system, the difference in work functions of Pd and Ni results in surface polarization on the Pd surface, tuning its charge state for hydrogen reduction. Meanwhile, with the assistance of a polyethyleneimine–reduced graphene oxide (PEI–rGO) support, the examined Ni@Pd4:1/PEI–rGO50:1 (10 wt%) electrocatalyst presents outstanding HER activity comparable with that of platinum (Pt). This work opens up possibilities for reducing Pd usage while achieving high HER performance.

Layered double hydroxides (LDHs) are increasingly being recognized as one of the most promising candidates to lower the oxygen evolution reaction (OER) barriers in water splitting. However, how to further increase catalytically active sites and then improve the corresponding catalytic activity is still a great challenge, which is critical for the design of an efficient OER electrocatalyst. Here, we present the in situ growth of ultrathin Ni–Fe LDH (u-Ni70Fe30 LDHs) nanosheets rich in catalytically active sites on the anodic polarized copper foil as an efficient electrocatalyst for OER through a mild electrodeposition approach (denoted as u-Ni70Fe30 LDHs/a-CF). We found that the u-Ni70Fe30 LDHs/a-CF shows a remarkable catalytic activity (the overpotential of 260 mV at 10 mA cm−2) and superior catalytic stability (beyond 50 h) in basic media. The achieved ultrahigh catalytic performance of u-Ni70Fe30 LDHs/a-CF is primarily attributed to its ultrathin nanosheet structure, the optimized 30 at% Fe-dopant concentration, and the synergetic effect between the nanosheets and the bumps on a-CF. These reasons lead to the high density of exposed active sites and the improved mass/electron transport capability. In comparison with the other developed LDH based electrocatalysts reported to date, we provide a controllable and mild strategy for designing highly-efficient ultrathin LDH based electrocatalysts with desirable kinetics for future large-scale application in water splitting.

Highly efficient and non-precious metal electrocatalysts for oxygen evolution reactions (OERs) and oxygen reduction reactions (ORRs) are at the heart of key renewable-energy technologies. Nevertheless, developing highly active bi-functional catalysts at low cost for both OER and ORR still remains a huge challenge. In this paper, Co3O4 nanocrystals embedded in N-doped mesoporous graphitic carbon layer/multiwalled carbon nanotube (MWCNT) hybrids are prepared by a facile carbonization and subsequent oxidation process of MWCNT-based metal–organic frameworks (MOFs). As a result, in alkaline media, the hybrid material catalyzes OER with an onset potential of 1.50 V (vs. reversible hydrogen electrode) and an over-potential only of 320 mV to achieve a stable current density of 10 mA cm−2 for at least 25 h. The same hybrids also exhibit similar catalytic activity but superior stability to the commercial 20 wt% Pt/C catalyst for ORR, making it a high-performance cheap bi-catalyst for both OER and ORR. The design concept of nonmetal-doped and precious-metal-free electrocatalysts from MOFs can be extended to fabricate other novel, stable and easy to use catalyst systems for advanced applications.

A very efficient, in situ growth approach has been developed to fabricate a graphene oxide (GO)-supported, high-performance Ni1−xFex loading electrocatalyst. Transition metal nanoparticles (NPs) such as Ni are quite attractive due to their advantages in cost, and excellent electrocatalytic activity for hydrazine electrooxidation. However, their strong aggregation propensity makes it difficult to develop them into highly efficient electrocatalysts. Polyethyleneimine (PEI), a branched polyelectrolyte, is devised to resolve this problem, and is utilized to uniformly anchor Ni–Fe NPs on GO. Meanwhile, partial GO can be reduced to reduced graphene oxide (rGO) in the presence of amine-containing molecules, which is favourable to enhance the electrical conductivity of supporting materials. Ni1−xFex NPs (50% loading) are prepared by a co-reduction method of metal precursors with hydrazine hydrate under 100 °C for 3 h. The content of PEI attached on GO significantly affects the dispersion and size of the resulting Ni1−xFex NPs. Transmission electron microscope (TEM) images reveal that the Ni1−xFex NPs with an average size of 8 nm are homogeneously dispersed on PEI–rGO10:1 sheet. The electrochemical measurements indicate that the Ni1−xFex/PEI–rGO10:1 hybrid with a Ni:Fe mass ratio of 80:20 exhibits the highest electrocatalytic activity toward hydrazine oxidation, while the PEI–rGO10:1 supporting material plays an excellent role in the synergistic catalytic effect. Under the same conditions, the electrocatalytic activity of Ni80Fe20 NPs on PEI–rGO10:1 for hydrazine is 2 times higher than that of the Ni80Fe20 NPs directly deposited on GO.

The development of metal-free catalysts for efficient catalysis of both the oxygen evolution reaction (OER) and the oxygen reduction reaction (ORR) is extremely desirable in energy technologies. Herein, nitrogen-doped mesoporous carbon nanosheet/carbon nanotube (CNT) hybrids have been synthesized by the pyrolysis of glucose, urea and CNTs. Impressively, in 0.1 M KOH, the resulting hybrids afford remarkable OER activities with a low onset potential (1.50 V vs. RHE) and an exceptional over-potential (only 320 mV at 10 mA cm−2). Moreover, the same hybrids show comparable catalytic performance but better durability compared to the benchmark Pt/C (20 wt%) catalyst for ORR. The achieved ultrahigh catalytic performance of the hybrids originates from their large specific surface area (594.1 m2 g−1), high content percentage of N doping (8.5 wt%), and mesoporous structure, which leads to fully exposed active sites, improved mass/electron transport capability, easy adsorption/release of oxygen gas bubbles, and high structural stability. This work also provides a novel concept for fabricating heteroatom doped porous carbonaceous materials with integrated and improved catalytic performance for advanced applications.

Cobalt-based nanomaterials are promising candidates as efficient, affordable, and sustainable alternative electrocatalysts for the oxygen evolution reaction (OER). However, the catalytic efficiency of traditional nanomaterials is still far below what is expected, because of their low stability in basic solutions and poor active site exposure yield. Here a unique hybrid nanomaterial comprising Co@Co3O4 core–shell nanoparticle (NP) encapsulated N-doped mesoporous carbon cages on reduced graphene oxide (denoted as Co@Co3O4@NMCC/rGO) is successfully synthesized via a carbonization and subsequent oxidation strategy of a graphene oxide (GO)-based metal–organic framework (MOF). Impressively, the special carbon cage structure is very important for not only leading to a large active surface area, enhanced mass/charge transport capability, and easy release of gas bubbles, but also preventing Co@Co3O4 NPs from aggregation and peeling off during prolonged electrochemical reactions. As a result, in alkaline media, the resulting hybrid materials catalyze the OER with a low onset potential of ∼1.50 V (vs. RHE) and an over-potential of only 340 mV to achieve a stable current density of 10 mA cm−2 for at least 25 h. In addition, metallic Co cores in Co@Co3O4 provide an alternative way for electron transport and accelerate the OER rate.

Active, stable and cost-effective electrocatalysts are the key to water splitting for hydrogen production through electrolysis. In this work, we report MoS2 quantum dots (MoS2 QDs) decorated on reduced graphene oxide (RGO) synthesized by a facile sonication method as highly effective electrocatalysts for the hydrogen evolution reaction (HER). Compared with MoS2 sheets, the zero-dimensional MoS2 QDs have a defect-rich structure rendering these quantum dots with plentiful active sites, which can further enhance the catalytic activity by a synergistic effect with RGO. Electrochemical experiments demonstrated that the catalyst exhibited large cathode currents (a small overpotential of 64 mV for 10 mA cm−2 current density) and a Tafel slope as small as 63 mV per decade, achieving high stability simultaneously. This work opens up possibilities for preparing non-noble metal electrocatalysts while achieving high HER performance similar to commercial Pt catalysts (Pt/C).

Water splitting provides a potential path for producing clean, renewable H2 and O2. However, improving the overall efficiency of water splitting is a challenging issue. Here, we designed Co–Fe nanoparticle coupled nitrogen-enriched porous carbon (CoyFe10−yOx/NPC) nanosheets as highly efficient non-precious-metal electrocatalysts for the oxygen evolution reaction (OER). Nitrogen-enriched porous carbon (NPC) nanosheets were prepared using a Schiff-base network (SNW) as the precursor and the SNW was based on commercially available and inexpensive monomers which were terephthalaldehyde and melamine. The resulting SNW possessed a high nitrogen content, a high surface area and a high density of metal-coordination sites. In addition, when used as the catalyst for the OER, the Co–Fe nanoparticle catalyst containing 30% Co (Co3Fe7Ox/NPC) showed the highest activity, requiring 328 mV over-potential to achieve a stable current density of 10 mA cm−2 for at least 15 h and a small Tafel slope of 31.4 mV dec−1 in 1.0 M KOH solution, which were comparable even superior to those of many other non-noble metal catalysts. Consequently, the high efficiency and durability make these supported amorphous Co–Fe nanoparticles potentially applicable for improving the performance for electrolysis of water and energy storage applications. More importantly, the support of electrode materials comes from the pyrolysis of porous polymers and this idea offers a new possibility for exploring overall water splitting non-precious-metal catalysts.