Ortho to para hydrogen conversion catalyst (O-P catalyst) is integral for large-scale hydrogen liquefaction projects. However, factors that influence catalyst performance remain preliminary and unclear. In the mean time, the mechanical strength of the O-P catalyst is crucial for its efficacy and longevity, yet most related research has paid sufficient attention to the catalytic activity. In this work, an iron-based O-P catalyst was synthesized using a straightforward precipitation method. And effects of catalyst activation method, drying temperature, particle size, concentration ratio, and doping element on catalytic activity and mechanical strength were studied. Furthermore, the catalytic performance and structural characterization of the prepared catalyst and commercial catalyst were compared. The prepared catalyst achieved a para hydrogen (p-H₂) content of 46.49% post-conversion at 77 K with a hydrogen flow rate of 1200 mL/min, surpassing the commercial catalyst by 2.9%. The maximum single particle crushing force of the prepared catalyst reached 4.75 N. Therefore, a preliminary mechanism for enhancing catalytic activity optimization was elucidated, offering valuable insights into ortho to para hydrogen conversion, and this study provides foundational data supporting the scaled production of domestic catalysts.

Ammonia decomposition is a promising approach for on-site hydrogen generation for fuel cells, and the development of a cost-effective and efficient catalyst is highly desired. In this study, a series of Ni_xMg_75-xAl₂₅ hydrotalcite-like compounds (HTlc) with different Ni contents were synthesized by co-precipitation, followed by calcination and reduction treatments. Influences of Ni content and ammonia reduction on the catalytic performance for ammonia decomposition were investigated. The characterization results of the as-prepared samples showed that HTlc was decomposed into Mg(Ni, Al)O solid solution by calcination, which displayed a strong interaction between Ni species and support, while upon reduction with ammonia at 750 ℃, well-dispersed Ni metal nanoparticles with an average crystallite size range of 5.9-7.7 nm were formed. No nitrogen oxides (NO_x) were produced during the NH₃ reduction process as indicated by mass spectrometry analysis, and the catalyst reduced with ammonia showed comparable activity with that reduced with hydrogen, suggesting that ammonia can be used as a reductant gas. The catalyst activity increased with the increase of Ni content and reduction temperature. Among the catalysts, the Ni₂₀Mg₅₅Al₂₅ catalyst reduced with ammonia at 750 ℃ showed the best activity, which afforded 98% ammonia conversion at 600 ℃ at a space velocity of 30000 mL·g_cat^-1·h^-1, and no evident deactivation was observed during a 100 h test, demonstrating good activity, stability, and sintering resistance.

Elemental doping is an effective strategy for tuning the band structure of graphite carbon nitride (CN) to enhance its photocatalytic performance. In this study, sodium (Na) and oxygen (O) co-doped carbon nitride (Na/O-CN_x, x=1.0, 2.0, 3.0, 4.0) was synthesized via solid-phase reaction of sodium citrate (NaCA) and pure CN powder in the Teflon-sealed autoclave under air conditions at 180 ℃. Surface area of Na/O-CN_3.0 is measured to be 18.8 m²/g, increasing by 60.7% compared to that of pure CN (11.7 m²/g). Bandgap energy of Na/O-CN_3.0 is determined to be 2.68 eV, marginally lower than that of pure CN (2.70 eV), thereby enhancing its capacity for sunlight absorption. Meanwhile, the incorporation of Na and O atoms into Na/O-CN_x is found to effectively reduce recombination rates of photogenerated electron-hole pairs. As a result, Na/O-CN_x samples exhibit markedly enhanced photocatalytic hydrogen evolution activity under visible light irradiation. Notably, the optimal Na/O-CN_3.0 sample achieves a photocatalytic hydrogen production rate of 103.2 μmol∙g^-1∙h^-1, which is 8.2 times greater than that of pure CN (11.2 μmol∙g^-1∙h^-1). Furthermore, a series of Na/O-CN_x-yO₂ (y=0, 20%, 40%, 60%, 80%, 100%) samples were prepared by modulating the oxygen content within reaction atmosphere. The catalytic performance evaluations reveal that the incorporation of both Na and O atoms in Na/O-CN_3.0 enhances photocatalytic activity. This study also introduces novel methodologies for synthesis of metal atom-doped CN materials at lower temperature, highlighting the synergistic effect of Na and O atoms in photocatalytic hydrogen production of Na/O-CN_x samples.

Methane pyrolysis is a technology that utilizes fossil energy to produce high added value carbon materials and hydrogen. However, traditional methods, such as chemical vapor deposition (CVD) and molten metal catalysis, face challenges in the production of graphene, including catalyst deactivation, difficulty in separating graphene from the catalyst, and high reaction temperatures (≥1100 ℃), which limit their industrial applications. This study proposes an innovative approach to produce graphene by catalyzing methane pyrolysis using Cu and metal oxides-KCl molten medium. By adding metal oxides (Al₂O₃, TiO₂, ZrO₂, MgO, SiO₂) as dispersants, the dispersion of active Cu sites is enhanced. Notably, Cu/ZrO₂ with a Cu content of 50% (in volume) and Cu/MgO with a Cu content of 75% (in volume) catalysts enable the efficient production of few-layer graphene. Cu/ZrO₂ catalyst with a Cu content of 50% (in volume) exhibits the highest activity, achieving a methane conversion rate of 22%, a hydrogen production yield of 21.5 mmol/h, and formation of large-area and smooth few-layer graphene. This study provides a new technical route for co-production of graphene and hydrogen via methane pyrolysis, offering potential for large-scale graphene production in the future.

To solve the existing energy crisis and achieve continuous seawater electrolysis, it is necessary to design efficient electrocatalysts to deal with the problems of slow anodic oxygen evolution and chloride ion (Cl^-) corrosion. In this study, a unique nanostructural modified Ce-FeHPi/NF electrode was prepared by a one-step hydrothermal method on a nickel foam (NF) skeleton. The experimental results show that Ce doping regulates the surface morphology of FeHPi/NF, forming amorphous nanospheres, which not only enables the catalytic layer to grow into a compact nanostructure, but also greatly increases the active surface area of the electrode, significantly improving the electrocatalytic activity. In addition, the presence of phosphoric acid group can effectively repel Cl^- on surface of the electrode, which enhances its corrosion resistance, and stabilizes it in seawater for a long time. The 10%Ce-FeHPi/NF electrode in alkaline simulated seawater (1 mol·L^-1 KOH + 0.5 mol·L^-1 NaCl) electrolyte requires only a low overpotential of 296 mV to reach a current density of 100 mA·cm^-2. In 1 mol·L^-1 KOH + 1 mol·L^-1 NaCl, the 10%Ce-FeHPi/NF electrode runs stably for more than 130 h at a constant potential of 1.774 V (vs. RHE). Therefore, the modified nanostructured material prepared in this study can effectively improve the oxygen evolution activity of electrodes, and provide a new way for the development of seawater electrolytic anode catalytic materials.

NiFeOH/CoP/NF composite electrode was fabricated by constructing a metal hydroxide layer on the surface of cobalt phosphide via hydrothermal, phosphating, and electrodeposition methods. The electrolytic water splitting to hydrogen performance by as-prepared electrode was investigated in 1.0 mol/L KOH medium. NiFeOH/CoP/NF composite electrode exhibited excellent water electrolysis performance, and the required overpotentials for HER and OER at 100 mA/cm² current density were 141 and 372 mV, respectively. When NiFeOH/CoP/NF electrode served as both cathode and anode for water splitting, only 1.61 V voltage was required to reach current density of 10 mA/cm². Because NiFeOH protection layer enhanced the electrocatalytic activity and stability of CoP for water splitting, NiFeOH/CoP/NF composite electrode exhibited high stability during the galvanostatic electrolysis in the HER and OER, and its activity could maintain 60000 s without significant performance degradation. The photovoltaic-electrolytic water cell constructed with two NiFeOH/CoP/NF electrodes and GaAs solar cell showed 18.0% efficiency of solar to hydrogen under 100 mW/cm² simulated solar irradiation and worked stably for 200 h.

In the process of electrolyzing water to produce hydrogen, the sluggish electrocatalytic kinetics of the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) limit the energy conversion efficiency. High-entropy materials have been considered as potential catalysts due to their unique structural features and excellent performance, which could potentially replace traditional metal oxides and precious metals for energy conversion and water electrolysis. Due to the incompatibility between different metals and non-metals, there have been few reports on the synthesis of high-entropy compounds, especially high-entropy metal phosphides. In this study, a series of carbon-based high-entropy alloy phosphide nanoparticles were synthesized using citric acid as complexing agent and ammonium dihydrogen phosphate as phosphorus source via a low-temperature Sol-Gel method with different elemental metals. In 1 mol·L^-1 KOH solution, FeCoNiMoCeP/C exhibited good water electrolysis performance at a current density of 10 mA·cm^-2, with overpotentials of 119 and 240 mV for the HER and OER, respectively. Similarly, in overall water splitting studies, FeCoNiMoCeP/C also showed excellent catalytic activity. When operating at a current density of 10 mA·cm^-2, FeCoNiMoCeP/C required only 1.53 V as the combined anode and cathode voltage for electrolyzing water. This is due to the synergistic effects among the atoms of high-entropy phosphide catalysts which provide more reaction sites to increase reaction activity and selectivity. This study is expected to expand the potential applications of high-entropy alloys in the field of electrocatalysis.

Hydrogen generation from electrolyzed water has received extensive attention in the scientific community due to its green and environmentally friendly properties, as well as the high purity of hydrogen produced. However, the slow oxygen evolution reaction (OER) during electrocatalytic water splitting has significantly hampered the development of hydrogen production, posing numerous challenges in its practical application. In this study, a novel three-dimensional (3D) core-shell heterostructure catalyst with crystalline NiMoO₄ nanorods as “core” and amorphous CoFe-LDH nanosheets as “shell” was successfully fabricated on a conductive nickel foam (NF) substrate by using a combination of hydrothermal and electrodeposition strategy. This special 3D core-shell structure fully stimulates the electrocatalytic potential of NiMoO₄ and CoFe-LDH, which greatly enhances the efficiency of the overall water-splitting. Through the synergistic interaction of NiMoO₄ and amorphous CoFe-LDH, the NiMoO₄@CoFe-LDH/NF nanocatalysts generates more active sites and exhibits highly efficient electron transfer ability and excellent OER electrocatalytic activity. Electrochemical tests show that NiMoO₄@CoFe-LDH/NF exhibits the most excellent electrochemical performance when the electrodeposition time is 60 s. The overpotentials η₁₀ and η₁₀₀ at 10 and 100 mA·cm⁻² are only 168 and 216 mV, respectively, which shows a very small Tafel slope and excellent long-term stability. Meanwhile, the overall water-splitting system of NiMoO₄@CoFe-LDH||NiMoO₄ exhibits a low driving voltage, which can produce a current density of 10 mA·cm⁻² at 1.57 V. In conclusion, this work provides new ideas for design and development of efficient catalytic materials for electrolyzed water.

By using renewable electricity proton exchange membrane (PEM), water electrolysis can produce “green hydrogen” of which the rate-determining step is oxygen evolution reaction. Considering the problems of stability, activity and cost, manganese oxides (MnO_x) doped with transition metals as a kind of electrocatalysts (Fe-MnO_x, Co-MnO_x, and Ni-MnO_x) were directly synthesized in one-step by gliding arc warm plasma. Complementary characterizations of the crystal structure, morphology, element composition, and surface valence state on this kind of synthetic catalysts were conducted. The results show that MnO_x mostly consists of crystalline Mn₂O₃ and amorphous Mn₃O₄. Although the transition metal doped catalysts possess the very approximate crystalline compositions to MnO_x, they showed a remarkable decrease in particle size and an increase in specific surface area are achieved. Introducing Co to form Co-MnO_x can increase surface density of electrons as compared to pure MnO_x. We further reveal a unique current step of MnO_x-based catalysts in acidic medium which appears in three consecutive potential regions (low I: 1.4-1.8 V, low II: 1.8-2.4 V, and high III: 2.4-2.7 V) under cyclic voltammetry (CV) measurements. This current step process is quite consistent with the electrode dynamics curve (as reference) which is reduced by a simplified version of Bulter-Volmer equation, during which the electrocatalytic behavior involves manganese at multi-valence states. At low potential regions Fe-MnO_x achieves the highest activity among all catalysts, while at high potential region only Co-MnO_x achieves. Furthermore, Co-MnO_x onset potential is 160 mV lower than that of pure MnO_x, and three times of the ending current density of pure MnO_x during bulk electrolysis under potentiostat. Consistent with the trend in activity, at low potential regions Fe-MnO_x is the most stable, while at high potential region Co-MnO_x is. Consequently, the significant increase in oxygen evolution reaction activity and stability of manganese oxides doped with transition metals is attributed to transition metal doping, which obviously optimizes the particle size, specific surface area, and electronic structure of MnO_x.

Developing low-cost, high-activity non-precious metal electrocatalysts is of great significance for the practical application of water electrolysis. Rare earth (RE) elements have become a research hotspot for the modification of metal catalysts due to their unique electronic structures. However, the methods for preparing rare earth composite catalysts on nickel foam (NF) substrates are demanding, presenting issues such as high cost, complex processes, and long production time. This research employed a straightforward chemical deposition technique to fabricate rare earth composite electrodes on NF substrates. The structure and morphology of the catalytic electrodes were characterized, and their hydrogen evolution performances in 1 mol·L^-1 KOH solution were investigated. The results revealed that adding Sm, Dy, and Tb could alter the electronic structure of the electrodes, improve the intrinsic properties of the catalyst materials, and enhance the catalytic performance for hydrogen evolution reaction (HER). Ni-Co-B-Tb/NF displayed superior hydrogen evolution performance with an overpotential of only 58 mV at a current density of 10 mA·cm^-2 and a Tafel slope of 65 mV·dec^-1. HER was controlled by the Volmer-Heyrovsky step. It was found that the variation of rare earth concentration had great effect on the electrocatalytic performance. When the Tb concentration was 3 g·L^-1, smaller size and uniformer distribution of particles on the surface of Ni-Co-B-Tb/NF exposed more active sites, which was favorable for HER charge transfer, and the best performance of hydrogen evolution was achieved. Furthermore this catalyst exhibited remarkable electrochemical stability following an extensive 100 h stability test and 2000 cyclic voltammetry (CV) testing.

S-scheme heterojunction has been extensively investigated for hydrogen evolution and environmental pollution issues. In this study, a monoclinic WO₃/hydrothermally treated red phosphorus (HRP) S-scheme composite was prepared by hydrothermal method. XPS and EPR characterization confirmed that the monoclinic WO₃/HRP composite formed S-scheme heterojunction. 5%WO₃/HRP composite displayed the optimal photocatalytic activity under visible light irradiation, and its degradation rate of Rhodamine B (RhB) reached 97.6% after 4 min of visible light irradiation, while its hydrogen evolution rate reached 870.69 μmol·h^-1·g^-1 which was 3.62 times of that of pure HRP. This could be ascribed to the tight interfacial bonding between WO₃ and HRP, and the formation of S-scheme heterostructure, enabling rapid separation of photogenerated carriers and therefore improving the strong redox capacity. This study provided a promising RP-based photocatalyst to meet the demand for clean energy and drinking water.

Bismuth vanadate (BVO) can be used for photoelectrochemical (PEC) water splitting to hydrogen. However, suffering from its high charge-recombination and slow surface catalytic reaction, the PEC performance is far below the expectation, and the modification of the co-catalysts only on the electrode cannot overcome this disadvantage. Here, we report FeNiO_x cocatalyst decorated on the BVO photoanode, which can restrict the onset potential and improve the PEC performance. Moreover, a more effective dual modified-BVO photoanode is formed, with the loading of g-C₃N₄ before decoration of FeNiO_x cocatalyst. The type-II p-n heterojunction composed by g-C₃N₄ nanosheets and BVO, can inhibit recombination of photogenerated charge, and promote the separation of charge effectively at the electrode. Results show that the charge separation efficiency of the electrode reaches 88.2% after the insertion of g-C₃N₄, which is nearly 1.5 times that of BVO/FeNiO_x (60.6%). Moreover, surface charge injection efficiency of the dual-modified BVO/g-C₃N₄/FeNiO_x electrode reaches 90.2%, while the current density reaches 4.63 mA∙cm^-2 at 1.23 V (vs. RHE). This work provides a facile approach to develope high performance photoanodes for PEC water splitting.

Nickel-based electrocatalytic material is considered one of the cost-optimal transition metal catalysts in alkaline water electrolysis due to its accessible industrial-applicability. Nevertheless, slow hydrogen evolution kinetics and low activation are still the grand challenges. Herein, we report a three-dimentional porous cluster structure vanadium oxide implanting into nickel-copper alloy electrocatalyst with phase-separation metallic nickel and copper as the main crystal phase mixed up with amorphous vanadium oxide phase, which is fabricated in situ on nickel foam (NF) by one-step cyclic voltammetry. The tri-hierarchical porous micro-nano structure of VO_x-NiCu/NF was constructed by nanoparticles of whichmicropores were created by clusters. This nickel foam micropores endows the target catalyst with a 28-fold increased electrochemically active surface area (ECSA), comparable to Pt-like catalytic activity towards hydrogen evolution reaction (HER). Encouragingly, VO_x-NiCu/NF needs merely 35 mV (η₁₀) to drive -10 mA·cm^-2 towards HER in alkaline medium. In addition, the as-prepared VO_x-NiCu/NF exhibits excellent long-time stability and durability. These data suggest that the formation of cluster structure, piled by nanoparticles, creates a large number of surface micropores, which greatly enhance the active sites and provide abundant material transfer channels for HER. Formation of NiCu alloy and amorphous V₂O₅ phase synergically boost the intrinsic HER activity to a certain extent. Simultaneously, the ideal composition and unique structural characteristics of VO_x-NiCu/NF contribute to the superior catalytic performance with the structural advantage responsible for the predominant effect. On the basis of kinetic analysis, the HER at VO_x-NiCu/NF proceed via a Volmer-Heyrovsky mechanism, where chemical desorption of hydrogen adsorbed is regarded as the rate-limiting step. Therefore, this study lays a foundation for promotion electrocatalytic hydrogen production.

Oxygen evolution reaction (OER) is the key reaction for water splitting, but its slow kinetics limitsits application. Therefore, rational design and construction of efficient OER catalysts are crucial for water splitting. Here, a Co²⁺ ion doped NiFe layered double hydroxides coupled Ti₆C_3.75 (NiFeCo-LDH-Ti₆C_3.75) catalyst was prepared by a simple one-step hydrothermal method using cobalt nitrate, nickel nitrate, iron nitrate, urea, and Ti₆C_3.75 as raw materials. NiFeCo-LDH-Ti₆C_3.75 catalyst showed a lamellar stacking structure, which is facilitating exposing more active sites. Introduction of Co²⁺ and Ti₆C_3.75 reduced the electronic density of Ni and Fe sites of NiFeCo-LDH-Ti₆C_3.75 catalyst. Benefiting from these features, the NiFeCo-LDH-Ti₆C_3.75 catalyst exhibits excellent OER activity with an overpotential of merely 290 mV at a current density of 20 mA·cm^-2 and a Tafel slope of 87.84 mV·dec^-1 with faster reaction kinetics. NiFeCo-LDH-Ti₆C_3.75 catalyst shows a relatively low charge transfer resistance, which means a fast charge transfer efficiency. Furthermore, after 6000 cycles of accelerated aging test at 20 mA·cm^-2, the overpotential only increased ~7 mV, indicating excellent cycle stability of NiFeCo-LDH-Ti₆C_3.75.