Towards continuous ammonia electro-oxidation reaction on Pt catalysts with weakened adsorption of atomic nitrogen

One of the key challenges for ammonia-fed anion exchange membrane fuel cells is to the ammonia electro-oxidation reaction (AEOR) at anode, which has sluggish kinetics and generates atomic nitrogen (N_ads) poisoning the Pt catalyst. In this study, a comparative study on Pt/Ta₃N₅, Pt/Ta₂O₅, Pt/carbon black, and Pt plate are conducted in order to clarify the promoting effect of the support materials for Pt catalysts. X-ray photoelectron spectroscopy analysis and density functional theory calculations reveal that the support materials significantly affect the electron condition of the Pt, resulting in the tuned adsorption energy of N_ads on Pt surface. The electrochemical analysis demonstrates that the weakened adsorption of N_ads lowers the coverage of N_ads on Pt surface, resulting in the enhanced performance and stability of Pt catalysts for AEOR. In particular, Pt/Ta₃N₅ catalyst exhibits a current density of 5.92 mA cm⁻² of AEOR at −0.34 V vs. SCE, which is higher than that of Pt/Ta₂O₅ (2.56 mA cm⁻², −0.35 V vs. SCE) and Pt/C (4.45 mA cm⁻², −0.26 V vs. SCE). The achievements in this study demonstrate the importance of controlling the type of supports for the development of an active electrocatalyst for continuous AEOR.     

Trimetallic NiFeCr-LDH/MoS2composites as novel electrocatalyst for OER

The development of efficient and stable oxygen evolution reaction (OER) catalysts is an ongoing challenge. In order to solve the problem of low oxygen evolution efficiency of the current OER catalysts, a novel material was synthesized by the incorporation of NiFeCr-LDH and MoS₂, and its structural and electrochemical properties were also investigated. The introduction of MoS₂ improves the electrochemical performance of NiFeCr-LDH. The polarization curve shows that the potential of composite material is only 1.50 V at a current density of 10 mA cm^?2, which is far superior to commercial precious metal catalysts. In addition, the stability experiment shows that the composite material has excellent stability, and the current density has little change after 500 cycles. Furthermore, we found that some metal ions, such as Ni, Cr and Mo, exist in the form of high valence on the surface of NiFeCr-LDH@MoS₂, which is also conducive to the occurrence of oxygen evolution reaction.     

Template-assisted polymerization-pyrolysis derived mesoporous carbon anchored with Fe/Fe3C and Fe−NX species as efficient oxygen reduction catalysts for Zn-air battery

Constructing highly efficient and durable non-noble metal modified carbon catalysts for oxygen reduction reaction (ORR) in the whole pH range is essential for energy conversion devices but still remains a challenge. Herein, the Fe/Fe₃C nanoparticles and Fe-N_X species anchored on the interconnected mesoporous carbon materials are fabricated through an economical and facile template-assisted polymerization-pyrolysis strategy. The catalyst exhibits unique features with the electronic interaction between Fe/Fe₃C and Fe−N_X, large specific surface area and high mesoporous structure as well as nitrogen doping in porous carbon skeletons, which can effectively catalyze ORR over the full pH range. In an alkaline electrolyte, the optimized catalyst displays favorable ORR performance with positive onset potential (E_onset = 0.91 V), half-wave potential (E_1/2 = 0.83 V), long-term cycles stability and methanol tolerance, exceeding those for the commercial Pt/C. Furthermore, the prepared catalyst could be directly assembled into the alkaline Zn−air battery that exhibits the open-circuit voltage of 1.44 V, high power density of 96.0 mW cm⁻² and long-term durability. Therefore, the template-assisted polymerization-pyrolysis strategy provides a promising route for designing high-performance non-noble metal decorated ORR electrocatalysts.     

Conversion of dinitrogen to ammonia by rhenium doped graphyne

Developing efficient catalysis for dinitrogen (N₂) fixation is a very important and challenging issue in chemistry. Herein, by means of density functional theory (DFT) computations, we reported that the potential of single-atom Rhenium (Re) supported on the graphyne (Re-GY) for activating N₂ and reducing it into ammonia (NH₃). It is found that the adsorption energies of N₂ on Re-GY for end-on/side-on configuration are −1.05/-0.56 eV, together with the N–N bond length are stretched to 1.14/1.21 Å, respectively. The hydrazine intermediate is difficult to release with adsorption energies of −1.71/1.49 eV on Re-GY for different pathways, which ensures it is sequentially hydrogenated and eventually form NH₃. All the mechanisms considered are energetically favorable and the most favorable pathway is the mixed mechanism. It is worth noting that there are relatively rare researches on Re-based catalysts for N₂ fixation, our findings provide a new option for the design and synthesis of catalysts for N₂ reduction reaction.     

Biomass derivative-based fibrous perovskite electrocatalysts with a hierarchical porous structure for oxygen reduction in alkaline media

The CaMnO₃ perovskites have been proposed as promising electrocatalysts for the oxygen reduction reaction (ORR) as substitutes for the noble metal. However, their ORR catalytic activities still need to be further improved. Herein, fibrous CaMnO₃ catalysts with hierarchical mesoporous/macroporous structures for the ORR were prepared through a simple ion exchange process with biomass derivative calcium alginate as a precursor. Optimization of the synthesis conditions, in particular, the La doping content, is conducted. The as-prepared CaMnO₃ shows superior ORR performance in alkaline media with a limited diffusion density of 6.13 mA cm⁻². Furthermore, the ORR performance was further promoted by La-doping. The optimal La-doped CaMnO₃ sample exhibits the most positive onset potential of −0.01 V, the most positive half-wave potential of −0.28 V (VS. Ag/AgCl), and a greatly enhanced limited diffusion density of 6.76 mA cm⁻². The improvement of catalytic activity for ORR can be attributed to the porous structure, the rich surface chemisorbed oxygen and the redox couple Mn³⁺/Mn⁴⁺. The as-prepared perovskite fibers have potential as low cost and efficient ORR catalysts.     

Vapor-assisted engineering heterostructure of 1D Mo3N2 nanorod decorated with nitrogen-doped carbon for rapid pH-Universal hydrogen evolution reaction

Reasonable construction of heterostructure is of significance yet a great challenge towards efficient pH-universal catalysts for hydrogen evolution reaction (HER). Herein, a facial strategy coupling gas-phase nitridation with simultaneous heterogenization has been developed to synthesize heterostructure of one-dimensional (1D) Mo₃N₂ nanorod decorated with ultrathin nitrogen-doped carbon layer (Mo₃N₂@NC NR). Thereinto, the collaborative interface of Mo₃N₂ and NC is conducive to accomplish rapid electron transfer for reaction kinetics and weaken the Mo–H_ads bond for boosting the intrinsic activity of catalysts. As expected, Mo₃N₂@NC NR delivers an excellent catalytic activity for HER with low overpotentials of 85, 129, and 162 mV to achieve a current density of 10 mA cm^?2 in alkaline, acidic, and neutral electrolytes, respectively, and favorable long-term stability over a broad pH range. As for practical application in electrocatalytic water splitting (EWS) under alkaline, Mo₃N₂@NC NR || NiFe-LDH-based EWS also exhibits a low cell voltage of 1.55 V and favorable durability at a current density of 10 mA cm^?2, even surpassing the Pt/C || RuO₂-based EWS (1.60 V). Consequently, the proposed suitable methodology here may accelerate the development of Mo-based electrocatalysts in pH-universal non-noble metal materials for energy conversion.     

Atomic metal,N, S co-doped 3D porous nano-carbons: Highly efficient catalysts for HT-PEMFC

A new strategy for fabricating atomically dispersed heteroatom-doped nanoporous carbon materials is reported. Through the self-assembly of dopamine, triblock copolymer F127, and metal ions, different three-dimensional atomic metal-N/S doped carbon catalysts are obtained after pyrolysis. Noble metal salts with ferrous sulfate induce the bimetallic monatomic FeM (M = Pd, Pt) N, S-doped carbon catalysts. Minute amounts of Pt or Pd single atoms in the catalysts greatly improve the oxygen reduction reaction (ORR) activity both in acidic and alkaline conditions. Typically, the obtained Fe, Pt–N/S co-doped carbon (FePt-NSC) catalyst exhibits superior ORR performance with positive half-wave potentials (E_1/2) of 0.89 and 0.80 V in alkaline and acidic solutions, respectively. In addition, FePt-NSC displays dominant four electron catalytic process and excellent electrocatalytic stability. The high temperature proton exchange membrane fuel cell (HT-PEMFC) test (160 °C) illustrates that FePt-NSC reaches 0.67 V at 400 mA/cm² and achieves the peak power density of 628 mW/cm², better than most of the catalysts reported at the similar conditions. These results indicate the atomic metal-N/S doped porous carbon catalysts to be highly promising low-Pt catalysts for HT-PEMFC.     

Zeolitic-imidazolate-framework-derived Fe-NC catalysts towards efficient oxygen reduction reaction

The development of non-precious metal catalysts to replace scarce and expensive Pt-based catalysts is critical for oxygen reduction reactions (ORR), where zeolitic-imidazolate-framework-derived (ZIF-derived) iron-based electrocatalysts hold a promising prospect. Herein, Fe₃O₄ was used as Fe source, and ZIF-8 was used as C and N source to prepare Fe-NC catalysts. Specifically, the half-wave potential (E_1/2) of the Fe-NC reached 0.90 V, which was higher than commercial Pt/C catalysts (0.87 V), and the overpotential of OER reached 327 mV. In addition, the power density tested in Zn-air batteries upped to 129.59 mW cm⁻², surpassing that of the Pt/C (108.93 mW cm⁻²). The superior performance was attributed to the effective introduction of Fe, the large specific surface area (851.6 m² g⁻¹), relatively regular porous structure and the high degree of graphitization.     

Performance evaluation of molybdenum dichalcogenide (MoX2; X= S,Se, Te) nanostructures for hydrogen evolution reaction

Hydrogen evolution reaction (HER) using transition metal dichalcogenides (TMDs) have gained interest owing to their low-cost, abundancy and predominant conductivity. However, forthright comparisons of transition metal chalcogenides for HER are scarcely conducted. In this work, we report the synthesis of series of molybdenum chalcogenide nanostructures MoX₂ (X = S, Se, Te) via a facile hydrothermal method. Used as an electrocatalyst for HER, MoS₂ nanograins, MoSe₂ nanoflowers and MoTe₂ nanotubes could afford the benchmark current densities of 10 mA cm⁻² at the overpotentials of −173 mV, −208 mV and −283 mV with the measured Tafel slope values of 109.81 mV dec⁻¹, 65.92 mV dec⁻¹ and 102.06 mV dec⁻¹, respectively. Besides other factors influencing HER, the role of electronic conductivity in HER of these molybdenum dichalcogenides are elucidated. In addition, the presented molybdenum dichalcogenides in this work are also complimented with robustness as determined from high-current density stability measurements.     

Interfacial coupling effect of CoO/CoMoO4 promotes alkaline hydrogen electrode for hydrogen energy storage system

Herein, CoO and CoMoO₄ heterostructure supported on nickel foam (CoO/CoMoO₄@NF) are proposed as an effective bifunctional hydrogen evolution reaction (HER) and hydroxide reaction (HOR) electrocatalyst. The electron density distribution at the interface can be optimized by coupling CoO and CoMoO₄, thereby improving conductivity and regulating the hydrogen binding energy (HBE) and hydroxyl binding energy (OHBE). CoO/CoMoO₄@NF exhibits high stability and activity with an exchange current density of ∼3.67 mA cm⁻². Co/CoMoO₄@NF reaches the current density of −10 mA cm⁻² at only −29 mV and the corresponding Tafel slope of 40.2 mV dec⁻¹. This work provides a promising solution for non-precious metal catalyst for hydrogen reaction in energy storage.     

Cobalt-molybdenum disulfide supported on nitrogen-doped graphene towards an efficient hydrogen evolution reaction

Herein, the cobalt-molybdenum bimetallic sulfide catalysts supported on nitrogen-doped graphene (CoMoS₂/NGO) were facilely synthesized by a one-pot hydrothermal route. These composites exhibit various special nanostructures, rich in abundant edge site exposure and defects, which play an important role in providing active sites for catalyzing hydrogen evolution reaction (HER). When hydrogen peroxide (H₂O₂) was used as an additive in hydrothermal process, the as-fabricated composite exhibited more efficiency towards HER, showing as low onset overpotential (η_on) as −54 mV in 0.5 M H₂SO₄. Typical H₂O₂-assisted composite realized a remarkable cathodic current density of 30 mA cm⁻² at an overpotential η = −137 mV and it possessed a small Tafel slope of 34.13 mV dec⁻¹. Moreover, it exhibited an excellent cycling stability and superior electronic exchange rate. The results prove that CoMoS₂/NGO catalysts have great potential for electrochemical HER.     

Facile synthesis of PdAu/C by cold plasma for efficient dehydrogenation of formic acid

Decomposition of formic acid biomass to generate hydrogen is vital for coping with fossil energy depletion, environmental pollution, and developing clean, efficient, safe, and sustainable modern energy system. In this study, a PdAu/C−C bimetallic catalyst was prepared by the co-impregnation method followed by an atmospheric pressure (AP) cold plasma treatment to synthesize PdAu/C−P catalysts. The resulting PdAu/C−P showed excellent catalytic activity for the formic acid dehydrogenation (FAD) reaction. The total volume of H₂ and CO₂ released from the FAD reaction was about 375 mL after 4 h at 50 °C, and the initial turnover frequency (TOF_initial) was 808.6 h⁻¹. We used X−ray diffractometry (XRD), temperature programmed reduction (TPR) and high-resolution transmission electron microscopy (HRTEM) to show that plasma can effectively promote the redispersion of Pd−Au particles on the surface of the support. The average particle size of PdAu/C−P (3.5 ± 1.5 nm) was less than PdAu/C−C (4.4 ± 1.9 nm) and uniformly distributed. X-ray photoelectron spectroscopy (XPS), TPR, and HRTEM showed that PdAu/C−P has a higher degree of alloying. In addition, the strong electric field in the plasma facilitated more metal sites located on the outer surface of the support in PdAu/C−P, and the atomic ratio of M/C (M = Pd and Au) (0.0134) was much larger than that of PdAu/C−C (0.0060). The apparent activation energy (E_a) of PdAu/C−P for the FAD reaction was only 27.25 kJ mol⁻¹, and it had much higher activity and stability than the commercial Pd/C (Sigma−Aldrich). The total volume of H₂ and CO₂ produced over the PdAu/C−P for three cycles was 1.33, 5.87, and 8.56 times that of commercial Pd/C. Overall, the cold plasma enhanced the degree of alloying, promoted the redispersion of agglomerated particles, and regulated the surface enrichment of the active metal components. This is of great significance for guiding the preparation of high−performance multi-metal catalysts by cold plasma.     

Design of PdxIr/g-C3N4 modified FTO to facilitate electricity generation and hydrogen evolution in alkaline media

In current work, the performance of Pd_xIr hybridized with g-C₃N₄ (Pd_xIr/g-C₃N₄) onto a fluorine-doped tin oxide (FTO) glass was investigated for alcohols oxidation reaction and hydrogen evolution reaction in alkaline media. The nanostructures were synthesized with convenient one-step solvothermal method and characterized with ICP-AES, EDX, XRD and TEM techniques. For comparison, Pd/g-C₃N₄ and Pt/C catalyst-coated FTO were also investigated. Higher current densities of 250 for methanol oxidation and 2570 mA mg⁻¹ for glycerol oxidation and better stability in the presence of Pd₃Ir/g-C₃N₄ compared to the other catalysts were proven by cyclic voltammetry and chronoamperometry. Electro-oxidation mechanism was investigated using linear sweep voltammetry method. Also, Pd₃Ir/g-C₃N₄ showed the Tafel slope of 72 mV dec⁻¹ and good stability in alkaline media which is comparable to that of other catalysts for hydrogen evolution reaction.     

Mechanisms insight into oxygen reduction reaction on sulfur-doped Fe–N2 graphene electrocatalysts

The density functional theory (DFT) calculations were performed to investigate the stability of the S-doped Fe–N₂G electrocatalysts, as well as ORR mechanism and activity. The most stable configuration is the Fe–N₂S1G because of forming a strong bond structure of Fe–S. In addition, the structures of Fe–N₂S3G and Fe–N₂S4G also exhibit the higher stability compared to the undoped Fe–N₂G. According to the distinct charge difference on the surface, the O-contained intermediates would like to adsorb on the active sites of Fe–N₂ complex active sites. The binding strength of OH on these different catalysts follows the increasing order of Fe–N₂S4G < Fe–N₂S3G < FeN₂G < Fe–N₂S1G < Fe–N₂S5G < Fe–N₂S2G < Fe–N₂S6G < Fe–N₂S7G, implying the opposite order of the catalytic activity. The calculations of the free energy diagrams show that all elementary reaction steps on Fe–N₂S4G, Fe–N₂S3G, FeN₂G and Fe–N₂S1G are downhill. Besides, the rate determining step (RDS) for these catalysts (excluded Fe–N₂S4G) is the fourth reduction step (OH*+H⁺+e⁻→H₂O+*). The study of the reaction mechanisms predicted that the direct 4-electron reduction process is the favorable ORR pathway, and the alternative reaction pathways containing the formation of OH* + OH* co-adsorbate also process without the formation of H₂O₂ for these catalysts. Particularly, Fe–N₂S4G also exhibits the outstanding performance for H₂O₂ reduction. In general, since the higher stability and working potential for ORR, Fe–N₂S4G is predicted to be the prior candidate site for ORR among S-doped FeN₂G catalysts.     

One-step synthesis of carbon-encapsulated nickel phosphide nanoparticles with efficient bifunctional catalysis on oxygen evolution and reduction

As a promising and cost-efficient alternative to noble metal catalysts, transition metal phosphides (TMPs) show highly catalytic performance toward oxygen reduction and evolution reactions (ORR and OER). Mesoporous carbon-coated nickel phosphide (NiP) nanoparticles were successfully synthesized by thermal decomposition at 500 °C under N₂/H₂ (95:5) atmosphere. The NiP/C hybrid exhibits excellent OER/ORR activity. It can generate an OER current density of 10 mA cm^?2 at the overpotential of 0.26 V with a low Tafel slope of 43 mV dec^?1, and produce a limited ORR current density of 5.10 mA cm^?2 at 1600 rpm with a half-wave potential of 0.82 V via a 4-electron pathway. In addition, the OER/ORR catalytic currents remain considerable stable without significant loss for more than 25 h polarization. This work will open up a new avenue to design a bifunctional catalyst with a superior OER/ORR activity and stability, and this cost-efficient strategy will pave the way for the industrial application of the renewable energy technologies.     

Selenium vacancy and phosphorus-doping-induced phase transition engineering of cobalt diselenide as bi-functional catalyst for water electrolysis

This study demonstrates a two-step approach for the synthesis of a cobalt phosphoselenide nanobelt (H–CoSe_(2−x)P_x NB) that has excellent activity for the hydrogen evolution reaction over a wide pH range (0–14), exhibiting low overpotentials of 112, 261, and 391 mV at a current density of 10 mA cm⁻² in 0.5 M H₂SO₄, 1 M KOH, and 1 M phosphate-buffered solution, respectively. Conversely, the H–CoSe_(2−x)P_x NB can be used for the oxygen evolution reaction in basic media, for which its electrochemical performance is superior to that of a platinum catalyst. When a H–CoSe_(2−x)P_x NB is used on both sides of a single electrolysis cell, almost no degradation occurs at various constant potentials for 12 h period. Its high performance, electrode stability, and easy synthesis suggest that the H–CoSe_(2−x)P_x NB is an efficient and economic electrocatalyst for water electrolysis.     

Tuning morphology and structure of Fe–N–C catalyst for ultra-high oxygen reduction reaction activity

Exploring efficient and durable non-precious metal catalysts for oxygen reduction reaction (ORR) has long been pursued in the field of metal-air batteries, fuel cells, and solar cells. Rational design and controllable synthesis of desirable catalysts are still a great challenge. In this work, a novel approach is developed to tune the morphologies and structures of Fe–N–C catalysts in combination with the dual nitrogen-containing carbon precursors and the gas-foaming agent. The tailored Fe–N₁/N₂–C-A catalyst presents gauze-like porous nanosheets with uniformly dispersed tiny nanoparticles. Such architectures exhibit abundant Fe-N_x active sites and high-exposure surfaces. The Fe–N₁/N₂–C-A catalyst shows extremely high half-wave potential (E_1/2, 0.916 V vs. RHE) and large limiting current density (6.3 mA cm⁻²), far beyond 20 wt% Pt/C catalyst for ORR. This work provides a facile morphological and structural regulation to increase the number and exposure of Fe-N_x active sites.     

In-situ synthesis of porous Ni2P nanosheets for efficient and stable hydrogen evolution reaction

The design and development of highly efficient and stable non-noble metal electrocatalysts for hydrogen evolution reaction (HER) have attracted increasing attention. However, some key issues related to large overpotential, high cost and poor stability at high current density still remains challenging. In this work, we report a facile in-situ integration strategy of porous Ni₂P nanosheet catalysts on 3D Ni foam framework (PNi₂P/NF) for efficient and stable HER in alkaline medium. The two-step method can creates high density of ultra-thin porous Ni₂P nanosheets firmly rooted into Ni foam substrate which can guarantee excellent electrical contacts, strong substrate adherence and large amount of active sites. Such a binder-free flexible HER cathode exhibits superior electrocatalytic performance with an overpotential of 134 mV at current density of 10 mA cm⁻². It also shows superior stability at higher current densities of 100 and 500 mA cm⁻² for at least 48 h and negligible performance degradation is observed.     

Precise control of π-conjugated polymer/carbon nanotubes heterointerfaces for oxygen reduction reactions

Nitrogen-coordinated metal catalyst has been regarded as a promising candidate for precious platinum for oxygen reduction reaction (ORR). However, controlling the structure and composition of coordinated metals in heterogeneous catalysts remains a synthetic bottleneck. Here, we design and fabricate π-conjugated polymer/CNTs heterointerfaces by polymerizing Co-BTA on CNTs. Co-BTA contains abundant Co–N₄ moieties and provides catalytic sites for ORR. CNT acts as a support and constructs the network for electron transport. Therefore, Co-BTA/CNT exhibits outstanding catalytic activity for ORR with comparable half-wave potential to commercial Pt/C. Furthermore, Co-BTA/CNT demonstrates better durability and methanol tolerance compared with Pt/C. Importantly, zinc-air batteries with Co-BTA/CNT have a maximum discharge power of 94.5 mW cm⁻² and a high energy density of 985 Wh kg⁻¹, superior to that with commercial Pt/C (51.5  mW cm⁻², 930 Wh kg⁻¹). This work paves a new avenue for precisely controlling nitrogen-coordinated metal catalysts for electrochemical energy conversion and storage.     

Hollow NH2-MIL-101@TA derived electrocatalyst for enhanced oxygen reduction reaction and oxygen evolution reaction

Non-precious metal-based electrocatalysts with excellent activity and stability are highly desired for the sluggish oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). Herein, a tannic acid (TA) etching strategy is used to inhibit the metal aggregation and achieve muti-metal doping. The hollow NH₂-MIL-101@TA derived Fe–N–C catalyst exhibits superior ORR catalytic activity with an E_1/2 of 0.872 V and a maximum output power density of 123.4 mW cm⁻² in Zn-air battery. Since TA can easily chelate with metal ions, Fe/Co–N–C and Fe/Ni–N–C are also synthesized. Fe/Ni–N–C manifests exceptional bifunctional activity with an E_j = 10 of 1.67 V and a potential gap of 0.833 V between E_j = 10 and E_1/2 in alkaline electrolyte, which is 45 mV smaller than Pt/C–IrO₂. The improvement of ORR and OER performance of the catalysts via the simple TA etching and chelation method provides a novel strategy for the design and synthesis of efficient electrocatalysts.