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Electrochemical nitrogen reduction reaction (NRR) is a promising approach for NH3 production to take place of the traditional Haber-Bosch process, which is still limited by the low NH3 yield rate and low Faradaic efficiency. Herein, Ti was post-synthetic exchanged into Zr-based metal organic frameworks to synthesize UiO-Zr-Ti as NRR electrocatalysts. The incorporated Ti is found to function as active sites for NRR and benefit the improved charge-transfer efficiency, which has a positive effect on the high NH3 yield rate. Moreover, the existence of Zr and Ti species can effectively suppress the competing HER, thus leading to high Faradaic efficiency. Therefore, the modified UiO-Zr-Ti-5d shows the highest NH3 yield rate of 1.16 × 10−10 mol cm−2 s−1 and the highest Faradaic efficiency of 80.36%, which is comparable to recently reported NRR electrocatalysts.

To meet the demand of producing hydrogen at low cost, a molybdenum (Mo)-doped cobalt oxide (Co3O4) supported on nitrogen (N)-doped carbon (x%Mo–Co3O4/NC, where x% represents Mo/Co molar ratio) is developed as an efficient bifunctional electrocatalyst for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). This defect engineering strategy is realized by a facile urea oxidation method in nitrogen atmosphere. Through X-ray diffraction (XRD) refinement and other detailed characterizations, molybdenum ion (Mo4+) is found to be doped into Co3O4 by substituting cobalt ion (Co2+) at tetrahedron site, while N is doped into carbon matrix simultaneously. 4%Mo–Co3O4/NC is the optimized sample to show the lowest overpotentials of 91 and 276 mV to deliver 10 mA cm−2 for HER and OER in 1 M potassium hydroxide solution (KOH), respectively. The overall water splitting cell 4%Mo–Co3O4/NC||4%Mo–Co3O4/NC displays a voltage of 1.62 V to deliver 10 mA cm−2 in 1 M KOH. The Mo4+ dopant modulates the electronic structure of active cobalt ion (Co3+) and boosts the water dissociation process during HER, while the increased amount of lattice oxygen and formation of pyridinic nitrogen due to Mo doping benefits the OER activity. Besides, the smaller grain size owing to Mo doping leads to higher electrochemically active surface area (ECSA) on 4%Mo–Co3O4/NC, resulting in its superior bifunctional catalytic activity.

NiCo

In this letter, a novel acousto-optical (AO) Q-switch is designed by using $\alpha $ -BaTeMo $_{\mathbf {2}}~\text{O}_{\mathbf {9}}$ crystal grown by the flux method. The acousto-optical modulator (AOM) exhibits a high polarization extinction ratio of 65 dB with a rise time of 32.8 ns and insertion loss of 3.66 dB. An actively Q-switched ytterbium doped all fiber laser at 1035 nm is successfully realized by using the prepared AOM. Under a pump power of 404.9 mW, a maximum output power of 1.66 W is obtained with pulse width of 167 ns at the modulation rate of 10 kHz. The results show that the $\alpha $ -BaTeMo $_{\mathbf {2}}~\text{O}_{\mathbf {9}}$ AOM is a potential Q-switch for all fiber lasers.

Co-Nd2O3/N-doped carbon (NC) as bifunctional oxygen electrocatalyst was synthesized by annealing a bimetal precursor (Co, Nd). It exhibits a nanostructure of Co-Nd2O3 hybrid nanoparticles coated with NC shells. The introduced Nd2O3 is beneficial to obtain high ratios of O vacancy/total O and graphitic-N/total N and modify the Lewis acidic property of Co-Nd2O3/NC-700, thus benefiting the OER activity. Besides, not only high Co2+/total Co and pyridinic-N/total N ratios, but also the enhanced Lewis base property is achieved after the introduction of Nd2O3, which is favorable for the ORR activity. As a result, the optimal Co-Nd2O3/NC-700 exhibits the enhanced OER/ORR activity. Especially, the OER activity is remarkably improved, thus leading to the excellent bifunctional catalytic activity with low ΔE of ∼0.86 V. Furthermore, the Zn-air battery with Co-Nd2O3/NC-700 shows a high specific capacity of ∼613.6 mA h g−1Zn and superior charge-discharge cycling durability.

In this work, the fabrication and sensing performance of fusion structures based on single-mode fiber (SMF) and multimode fiber (MMF) with different cladding diameters are discussed, and the effects of different lengths of MMF and fiber etching on sensing performance are analyzed. First, the transmitted intensity measurement experiment is performed, and the results indicate that the performance of the SMF-MMF-SMF(SMS)-based structure is better for sensing purposes. In addition, the results demonstrate that the performance of etched fiber is better than that of non-etched fiber. The etched fiber structure with lower fiber diameters produces more evanescent waves and is better for sensing purposes. Therefore, the proposed structure has certain development potential as an application of future optical fiber sensors.

In this paper, three S-tapered fiber (STF) structures with different diameters (40, 60, and 80 µm) are fabricated using conventional single-mode fiber. First, the reproducibility of the proposed S-tapered structure is confirmed through an analysis of the diameter distribution. Considering the transmitted intensities of the three various diameter, S-tapered structures reveal that the STF with a 40 µm diameter produces more evanescent waves and is more sensitive to external refractive index variations. Therefore, the STF structure with a 40 µm diameter was evaluated for the detection of different concentration of glucose solutions, demonstrating that the structure has the potential to be utilized to develop a highly sensitive fiber sensor.

Histamine is a biologically active molecule that serves as a reliable predictor of the quality of fish. In this work, authors have developed a novel humanoid-shaped tapered optical fiber (HTOF) biosensor based on the localized surface plasmon resonance (LSPR) phenomenon to detect varying histamine concentrations. In this experiment, a novel and distinctive tapering structure has been developed using a combiner manufacturing system and contemporary processing technologies. Graphene oxide (GO)/multi-walled carbon nanotubes (MWCNTs) are immobilized on the HTOF probe surface to increase the biocompatibility of biosensor. In this instance, GO/MWCNTs are deployed first, then gold nanoparticles (AuNPs). Consequently, the GO/MWCNTs help to give abundant space for the immobilization of nanoparticles (AuNPs in this case) as well as increase surface area for the attachment of biomolecules to the fiber surface. By immobilizing AuNPs on the surface of the probe, the evanescent field can stimulate the AuNPs and excite the LSPR phenomena for sensing the histamine. The surface of the sensing probe is functionalized with diamine oxidase enzyme in order to enhance the histamine sensor's particular selectivity. The proposed sensor is demonstrated experimentally to have a sensitivity of 5.5 nm/mM and a detection limit of 59.45 µM in the linear detection range of 0-1000 µM. In addition, the probe's reusability, reproducibility, stability, and selectivity are tested; the results of these indices show that the probe has a high application potential for detecting histamine levels in marine products.

This paper discusses the details about the fabrication of single-mode fiber (SMF)- and multi-mode fiber (MMF)-based core-offset sensor structures for biomolecules detection. SMF-MMF-SMF (SMS) and SMF-core-offset MMF-SMF (SMS structure with core-offset) are proposed in this paper. In the conventional SMS structure the incident light is introduced from the SMF to the MMF and then passes through the MMF to the SMF. However, in the SMS-based core offset structure (COS) the incident light is introduced from the SMF to the core offset MMF and then passes through the MMF to the SMF, and more incident light leaks at the fusion point between the SMF and the MMF. This structure causes more incident light to leak out from the sensor probe, forming evanescent waves. By analyzing the transmitted intensity, the performance of COS can be improved. The results show that the structure of the core offset has great potential for the development of fiber-optic sensors.

In this Letter, using a Yb,Gd:CaSrF

This paper primarily discusses the structural performance analysis of a single/double/quadruple tapered optical fiber (TOF) structure based on single-mode fiber (SMF) and multi-mode fiber (MMF). Furthermore, the TOF’s performance, including its diameter distribution, transmitted intensity, and reproducibility, is also evaluated. According to the experimental results, it can be concluded that the quadruple TOF structure based on SMF has a higher density of evanescent waves (EWs) on the surface of the tapered area, which is essential for the fabrication of high-sensitivity optical fiber sensors. The structure proposed in this article is feasible, and it can be used for optical fiber sensing while offering significant practical and promising applications as well.

A new multi-functional crystal has been implemented by combining the acousto-optic (A/O) modulation with Raman laser output, which has never reported before. Notably, the α-BTM displaying an adequate A/O figure of merit of 10.18 × 10 -18 s 3 /g using the comparison method is originally tailored to fabricate an A/O modulator that exhibits a high diffraction angle and diffraction efficiency of 1.432° and 82.1% at 1064 nm, respectively, which can be competitive with that of TeO 2 . Pumped by a pulsed laser, sequentially, the self-Q-switch Raman laser working at 1178.1 nm is successfully realized based on α-BTM crystal. A maximum output power of 86 mW operating at 1178.1 nm and the repetition frequency of 15 kHz were obtained with the pump power of 27 W. Our works not only enrich the family of A/O materials, but also opens up a viable route towards exploiting the design of multi-functional crystal.

To alleviate the sluggish oxygen evolution reaction (OER) kinetics, it's urgent to develop electrocatalysts with high activity and low cost. In this work, Fe doped metal organic frameworks (Ni)/carbon black composites were synthesized via a facile hydrothermal method. Benefiting from the direct use of metal organic frameworks (MOFs) for OER, numerous and highly dispersed active sites are exposed to the electrolyte and reactants. By regulating Ni/Fe ratios, a high electrochemical active surface area (ECSA) and high relative surface content of active Ni3+ species are obtained, which mainly contribute to the high OER activity. Besides, the introduced carbon black (CB) was found to enhance the charge-transfer efficiency of the electrocatalysts, which is also favorable for OER. The optimal Ni9Fe1-BDC-0.15CB electrocatalyst shows excellent OER activity with the low overpotential of ~290 mV at 10 mA cm−2 and the Tafel slope of ~76.1 mV dec−1, which is comparable to RuO2 and other MOFs-based OER electrocatalysts reported in recent years.

A metal-organic framework-derived method was developed to synthesize highly efficient non-noble metal electrocatalyst for alkaline hydrogen evolution reaction (HER). Zn2+, phosphomolybdic acid were coordinated with 2-methylimidazole, and zinc (Zn) and phosphorus (P) species were removed by annealing at 850 °C in N2 atmosphere, resulting in micro/mesoporous molybdenum carbide (Mo2C) composited with nitrogen-doped carbon (denoted as ZIF8-xMo-850). The optimized sample ZIF8-12Mo-850 displayed a low overpotential of ~85.7 mV to deliver a current density of 10 mA cm−2, with a corresponding Tafel slope of ~69.7 mV dec−1 in 1 M KOH. This HER catalytic activity was competitive with the most recently developed Mo2C-based HER electrocatalysts. From further investigation, the high HER catalytic activity of ZIF8-12Mo-850 is owing to three aspects: (i) The appropriate Mo feeding amount of ZIF8-12Mo-850 resulted in the highest surface content of Mo2+ active site; (ii) The evaporation of Zn and P in the ZIF8-12Mo precursor formed its largest average pore diameter of 32.3 nm, which leaded to the highest electrochemically active surface area (ECSA) of 64.29 cm2 (iii) The 2-methylimidazole in the precursor resulted in the highest surface content of pyridinic N in ZIF8-12Mo-850 (14.63%), which efficiently improved its conductivity and charge transfer efficiency.

Non-noble-metal based materials with high activity for oxygen reduction reaction (ORR) are urgently required to substitute Pt-based materials. Herein, metallic Co/N-doped carbon electrocatalysts were synthesized via a facile pyrolysis method of bimetal ZIF(ZnCo)/polypyrrole-polyaniline (ppy-pani) precursors. The evaporation of Zn and the introduced ppy-pani can lead to the porous structure and effectively hinder the aggregation of Co species, which results in the small nanoparticles uniformly distributed on carbon matrices. A high ECSA and a high content of Co species are obtained after the introduction of ppy-pani, thus resulting in the abundant Co2+ species to enhance ORR. Therefore, the optimized ZIF/ppy-pani-750 exhibits a high E1/2 (~0.86 V) and a low Tafel slope (~43.6 mV dec−1).

The global NH3 production is dominated by Haber–Bosch process, requiring high temperature and pressure. Electrochemical N2 reduction reaction (NRR) under ambient conditions is a greener path for artificial N2 fixation to NH3 but calling for efficient catalyst to increase activity and selectivity. Herein, we report the iron-based metal–organic frameworks (MOFs), i.e., MIL-88B–Fe and amine-functionalized MIL-88B–Fe (NH2–MIL-88B–Fe) as efficient catalysts for electrochemical NRR under ambient temperature and pressure in neutral electrolyte. NH2–MIL-88B–Fe shows higher NH3 yield rate of 1.205 × 10–10 mol s−1 cm−2 than MIL-88B–Fe (3.575 × 10–11 mol s−1 cm−2). Furthermore, NH2–MIL-88B–Fe exhibits the highest Faradaic efficiency of 12.45% at 0.05 V versus RHE. The control experiments prove that NH3 is produced through electrocatalytic NRR. This work may trigger the interest of using MOFs as highly efficient catalysts for electrochemical NH3 production.

Two-dimensional (2D) mono-elemental materials (Xenes) show remarkable potential in the fields of fundamental science and technology, have been regarded as a wide range of building blocks for electronic technologies due to their unique chemical, physical, electrical, and optical properties. Here, 2D tellurene/black phosphorus (Te/BP) heterojunctions are successfully fabricated through liquid-phase exfoliation (LPE) method. Their nonlinear optical absorption properties at 1.0, 2.0, and 2.8 μm have been studied by an open-aperture Z-scan method. The results revealed the excellent broadband saturable absorption responses of the prepared BP/Te heterojunctions, which are further confirmed by using them as saturable absorbers (SAs) for passively Q-switched all-solid-state lasers operating at 1.0, 2.0, and 2.8 μm, respectively. In particular, 1.04 μm continuous-wave (CW) mode-locked lasers with a pulse width of 404 fs is realized for the first time, to our best knowledge. Our work indicates that 2D Xenes especially 2D Xenes based heterojunctions have great potential in the fields of pulsed laser generation/modulation and other optoelectronic and photonic devices.

Highly active noble-metal-free catalysts for hydrogen evolution reaction (HER) are essential for the sustainable production of hydrogen. MoS2 based HER catalysts are potentially competitive to noble metals but facing the challenges of low conductivity, density of active sites and intrinsic activity in basal planes. Herein, oxygen incorporated V–MoS2 supported on partially sulfurized nickel foam (V–Mo(x)S/NF) are synthesized as binder-free HER catalysts via a one-step in-situ hydrothermal method. By controlling the fed atomic ratios of V/Mo, the optimal V–Mo(0.05)S/NF shows the low HER overpotential of ~31 and ~115 at 10 mA cm−2 and 100 mA cm−2, respectively in alkaline electrolyte, which is among the most active noble-metal and noble-metal-free HER catalysts reported. In V–Mo(0.05)S/NF, the introduced V and partially sulfurized nickel foam increases the conductivity. The hedgehog-like morphology ensures the exposure of high-density active sites. More importantly, the incorporated surface oxygen can be readily tuned by the fed atomic ratios of V/Mo, which plays the primary role in promoting the intrinsic HER activity for V–Mo(x)S/NF. This work demonstrates the feasibility of boosting HER through the precise control of incorporated surface oxygen in MoS2 based catalysts.

Developing earth-abundant highly efficient catalysts for the oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) is indispensable for the widespread implementation of electrochemical water splitting to store renewable energy. Herein, amorphous bimetallic selenide (Ni-Fe-Se) hollow nanospheres electrodeposited on nickel foam (Ni-Fe-Se/NF) are developed as a bifunctional catalyst for the HER and OER. The HER and OER bifunctional activity of Ni-Fe-Se/NF outperforms those of monometallic Ni-Se/NF and Fe-Se/NF owing to the synergy of Ni and Fe in Ni-Fe-Se/NF. Moreover, the amorphous hollow spherical morphology of Ni-Fe-Se/NF increases the active site density and facilitates the mass transfer of electrolytes and H2/O2 products. Ni-Fe-Se/NF drives a current density of 10 mA cm-2 with an overpotential of ∼85 mV for the HER and 100 mA cm-2 with an overpotential of ∼222 mV for the OER. As the HER and OER bifunctional catalyst, Ni-Fe-Se/NF can split alkaline water with total voltages of ∼1.52 V and ∼1.66 V at 10 mA cm-2 and 100 mA cm-2, respectively, and remain stable over 50 hours of operation in 1 M KOH.

This work develops CoNi alloys with slight oxidation@N,O co-doped carbon (NOC) as multifunctional catalysts for the oxygen reduction reaction (ORR), oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) and Zn–air batteries. Their outer NOC shells have direct contributions to ORR, OER and HER activity that is dominated by N and O dopants that closely correlate with active adjacent C atoms. Meanwhile, the inner cores (CoNi alloys with slight oxidation) can provide indirect and important contributions. Relying on enhanced collective contributions of outer NOC shells and inner cores that are tuned by Co/Ni molar ratios, CoNi(1 : 1)-TB-800N2 with a Co/Ni molar ratio of 1 : 1 affords optimized multifunctional activity. Furthermore, it also brings about optimal Zn–air battery performance: ∼98.6% and ∼74.8% of theoretical specific capacity and energy density, respectively, and more than 100 h of stable charge–discharge operation at high current densities.

In this work, a passively mode-locked erbium-doped fiber laser with layers of molybdenum disulfide saturable absorber (MoS2 -SA) immobilized etched single mode fiber (SMF) is developed. MoS2-SA are synthesized via liquid phase exfoliation then transferred to etched fiber (EF). SA is used to achieve mode-locked operation in an erbium-doped fiber laser. With a pump power of 250 mW, a repetition rate of 11.07 MHz produces a pulse width of 388 fs and a maximum average output power of 16.9 mW. This work demonstrates the potential of SA based on EF in C and L band mode-locked fiber lasers.

Niobium carbide (Nb2CTx), a type of MXene with high optical transparency, large specific surface area, and good electrical conductivity, is expected to perform as an excellent medium in the field of optical fiber biosensing. Here, we fabricated double S-tapered fiber sensors functionalized with gold nanoparticles/graphene oxide/tyrosinase (AuNPs/GO/tyrosinase) and AuNPs/Nb2CTx/tyrosinase, respectively. The double S-tapered structure can provide more evanescent wave leakage and enhance light–matter interaction. By implementing transmittance experiment, the sensitivity of the two probes were tested to be 17 and 34 pm/ μM over 0–300 μM tyramine concentrations. The comparative results demonstrate that Nb2CTx-enhanced localized surface plasmon resonance (LSPR) sensor has more excellent performance due to the existence of surface functional groups and large specific surface area of Nb2CTx. Our work provides a research platform for improving the sensitivity of LSPR sensors.

In this work, a series of metallic Co-ZnO encapsulated in N-doped carbon/carbon nanotubes (Co-ZnO@NC/CNT-T) for the electrochemical oxygen reduction reaction (ORR) were synthesized by annealing bimetallic metal-organic frameworks (MOFs) containing Co and Zn in N2 atmosphere. Within the hybrid, metallic Co was integrated with ZnO and encapsulated by NC, while coral-like CNTs grew out of the obtained Co-ZnO@NC nanoparticles. When applied in ORR catalysis, the optimized Co-ZnO@NC/CNT-700 obtained in 700 °C displayed a half-wave potential of ~0.86 V and a limiting current density of ~−5.98 mA cm−2, which were even superior to 20 wt% Pt/C. Further results demonstrated that the presence of ZnO was closely related with the high ORR catalytic activity from two aspects. (i) ZnO catalyzed the formation of Co0, Co2+, pyridinic and graphitic N as the main active species for ORR. (ii) ZnO promoted the growth of CNTs and micro/mesoporous structure, which improved the electrochemical active surface area (ECSA) and mass transfer during the ORR process.

Bifunctional catalysts are vital for oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) in metal-air batteries. In this work, Co–Co3O4/N-doped carbon nanosheets (NCNs) were developed as highly efficient bifunctional oxygen catalysts via the pyrolysis of a hybrid ZIF-67/CNs precursor. It is found that the introduced CNs play important roles. On one hand, the introduced CNs can tune the surface contents of Co, N and/or O species that are closely correlated with OER and ORR activity. On the other hand, they also facilitate to achieve high specific surface areas for the catalysts. In addition, the introduced CNs helps the formed Co–Co3O4 hybrid nanoparticles with uniform and small sizes to be well-distributed on the NCNs substrates. Despite such important roles, it should be noted that a moderate content of the introduced CNs is required to achieve optimal oxygen catalytic activity. As a result, the optimized ZIF-67/CNs(1)-600 exhibits a low value of η10 (~350 mV) for OER and a high value of E1/2 (~0.85 V) for ORR. Its overall bifunctional activity (ΔE) is as low as ~0.73 V, which is comparable to the recent reported Co-based catalysts.

In this work, electrochemical nitrogen reduction reaction (NRR) on metallic Co@N-doped carbon (NC) (Co@NC) were investigated. The Co@NC catalysts obtained via pyrolysis of a metal-organic framework precursor (ZIF-67) showed high Faraday efficiency under ambient conditions in neutral electrolytes. At −0.9 V versus Ag/AgCl, the Faraday efficiency of Co@NC achieved as high as 21.79%, surpassing most of reported catalysts under comparable conditions, while the NH3 yield rate of Co@NC reached 1.57 × 10−10 mol s−1 cm−2 at the same potential. This work demonstrated that Co@NC had outstanding electrocatalytic performance for NRR. Thus it was a promising transition-metal-based catalyst in electrochemical ammonia synthesis.

The development of highly efficient electrocatalysts with low cost for oxygen reduction reaction (ORR) is urgently required for metal-air batteries and fuel cells. In this work, Fe Co/MIL-101(Cr) with various molar ratios of Fe/Co as hybrid catalysts was prepared by a facile and mild impregnation method. MIL-101(Cr) increased the specific surface areas of the hybrid catalysts, thus improving the dispersion of Fe and Co species at their surfaces. The effects of Fe and Co species on ORR activity of the hybrid catalysts were investigated. It is found that the synergistic effects between the well-dispersed Fe and Co species contribute mainly to ORR activity. More specifically, Fe species exert a partial-charge-transfer-activation effect on Co ones, which reduces the charge transfer resistance and thus improves the catalytic activity. As a result, Fe Co/MIL-101(Cr) showed the excellent ORR activity, in which Co50Fe50/MIL-101(Cr) exhibited the superior ORR activity to the other prepared hybrid catalysts.

Molybdenum sulfide (MoS2) is considered as low-cost catalyst with great potential for hydrogen evolution reaction (HER). In this contribution, a promising Mo-precursor was first designed and prepar...

Electrocatalysts with high activity toward oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) are essential and desired for metal-air batteries, fuel cells, water-splitting and CO2 reduction. Here, a series of Co-CoxO/N-doped Carbon (NC) is developed as bifunctional electrocatalysts via the pyrolysis of a simple physical mixture of cobalt-based metal-organic framework (MOF) and carbon black (CB). The obtained electrocatalysts exhibit considerably enhanced bifunctional activity as compared to the pure MOF derived counterpart. The essential roles of added CB in boosting the bifunctional activity are investigated. With the addition of CB, the surface concentrations of active pyridinic-N and graphitic-N for ORR and the surface CoIII/CoII ratios for OER are increased. Furthermore, the charge transfer efficiency and the electrochemically active surface areas (ECSAs) are also enhanced for the resulting electrocatalyst. The optimized Co-CoO-Co3O4/NC exhibits excellent bifunctional activity and durability, surpassing the commercial 20 wt% Pt/C (for ORR) and IrO2 (for OER). When applied in an air electrode, the corresponding Zn-air battery with Co-CoO-Co3O4/NC presents the better performance than that with 20 wt% Pt/C.

A ternary transitional metal dichalcogenides alloy material, molybdenum sulphoselenide (MoSSe), has been successfully fabricated and used as a saturable absorber (SA) for a passively Q-switched (PQS) Tm:YAP bulk laser operating at $2.0~\mu \text{m}$ for the first time, to best of our knowledge. The open aperture Z-scan method was used to characterize the saturable absorption properties of the prepared MoSSe SA. The maximum average output power and pulse repetition rate, shortest pulse width, highest single pulse energy, and highest pulse peak power of this laser were 2.26 W, 95 kHz, 303 ns, $24.2~\mu \text{J}$ , and 80 W, respectively. The results are distinguished from other two-dimensional materials-based $2.0~\mu \text{m}$ PQS lasers, indicating that MoSSe could be a promising material for generating high power and short pulse width lasers in the $2.0~\mu \text{m}$ spectral range.

Non-precious bifunctional electrocatalysts for the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) play a pivotal role in regenerative fuel cells and rechargeable metal–air batteries. In this contribution, less active CeO2 is introduced for regulating the ORR/OER bifunctional activity of Co3O4@N-doped carbon (NC). The introduced CeO2 plays essential roles in increasing the surface contents of Co2+, Co3+ and N species in CeO2/Co3O4@NC for ORR/OER processes, while still maintaining hierarchical porosity and large specific surface area. Furthermore, the surface acid–base properties for the adsorption of oxygen molecules (for the ORR) and activation of water molecules (for the OER) and the charge transfer efficiency are also improved by the introduced CeO2. As a result, significantly enhanced ORR/OER bifunctional activity is achieved for the optimal and desired electrocatalyst, i.e., CeO2/Co3O4@NC. Its bifunctional activity is comparable or even superior to that of the benchmarked 20 wt% Pt/C (for the ORR) and IrO2 (for the OER), respectively. In the applications of Zn–air batteries, the battery with CeO2/Co3O4@NC still exhibits higher performance than the battery with mixed 20 wt% Pt/C + IrO2, due to its outstanding bifunctional activity toward the ORR and OER.

In this work, a series of metallic cobalt-molybdenum carbide@graphitic carbon (CoMo(x:y)-T@GC) nanocomposites for the electrochemical hydrogen evolution reaction (HER) were synthesized by a sol-gel method. In the as-prepared nanocomposites, β-Mo2C and metallic Co coexisted and were encapsulated by graphitic carbon. The presence of metallic Co effectively enhanced the crystallinity of β-Mo2C, charge transfer efficiency and electrochemical active surface area (ECSA), thus resulting in the improved HER catalytic activities of the CoMo(x:y)-T@GC nanocomposites. The optimized electrocatalyst CoMo(0.5:0.5)-800@GC required the lowest overpotential of ∼165 mV to deliver a current density of 10 mA cm−2 in 0.1 M KOH, which was at the forefront compared with recently reported Mo2C-based electrocatalysts.

Niobium carbide (