Anionic Regulated NiFe (Oxy)Sulfide Electrocatalysts for Water Oxidation

The construction of active sites with intrinsic oxygen evolution reaction (OER) is of great significance to overcome the limited efficiency of abundant sustainable energy devices such as fuel cells, rechargeable metal–air batteries, and in water splitting. Anionic regulation of electrocatalysts by modulating the electronic structure of active sites significantly promotes OER performance. To prove the concept, NiFeS electrocatalysts are fabricated with gradual variation of atomic ratio of S:O. With the rise of S content, the overpotential for water oxidation exhibits a volcano plot under anionic regulation. The optimized NiFeS‐2 electrocatalyst under anionic regulation possesses the lowest OER overpotential of 286 mV at 10 mA cm^?2 and the fastest kinetics being 56.3 mV dec^?1 to date. The anionic regulation methodology not only serves as an effective strategy to construct superb OER electrocatalysts, but also enlightens a new point of view for the in‐depth understanding of electrocatalysis at the electronic and atomic level.     

Design of Ultrathin Pt‐Based Multimetallic Nanostructures for Efficient Oxygen Reduction Electrocatalysis

Nanocatalysts with high platinum (Pt) utilization efficiency are attracting extensive attention for oxygen reduction reactions (ORR) conducted at the cathode of fuel cells. Ultrathin Pt‐based multimetallic nanostructures show obvious advantages in accelerating the sluggish cathodic ORR due to their ultrahigh Pt utilization efficiency. A focus on recent important developments is provided in using wet chemistry techniques for making/tuning the multimetallic nanostructures with high Pt utilization efficiency for boosting ORR activity and durability. First, new synthetic methods for multimetallic core/shell nanoparticles with ultrathin shell sizes for achieving highly efficient ORR catalysts are reviewed. To obtain better ORR activity and stability, multimetallic nanowires or nanosheets with well‐defined structure and surface are further highlighted. Furthermore, ultrathin Pt‐based multimetallic nanoframes that feature 3D molecularly accessible surfaces for achieving more efficient ORR catalysis are discussed. Finally, the remaining challenges and outlooks for the future will be provided for this promising research field.     

Graphene Composites with Cobalt Sulfide: Efficient Trifunctional Electrocatalysts for Oxygen Reversible Catalysis and Hydrogen Production in the Same Electrolyte

Nitrogen and sulfur‐codoped graphene composites with Co₉S₈ (NS/rGO‐Co) are synthesized by facile thermal annealing of graphene oxides with cobalt nitrate and thiourea in an ammonium atmosphere. Significantly, in 0.1 m KOH aqueous solution the best sample exhibits an oxygen evolution reaction (OER) activity that is superior to that of benchmark RuO₂ catalysts, an oxygen reduction reaction (ORR) activity that is comparable to that of commercial Pt/C, and an overpotential of only ?0.193 V to reach 10 mA cm^?2 for hydrogen evolution reaction (HER). With this single catalyst for oxygen reversible electrocatalysis, a potential difference of only 0.700 V is observed in 0.1 m KOH solution between the half‐wave potential in ORR and the potential to reach 10 mA cm^?2 in OER; in addition, an overpotential of only 450 mV is needed to reach 10 mA cm^?2 for full water splitting in the same electrolyte. The present trifunctional catalytic activities are markedly better than leading results reported in recent literature, where the remarkable trifunctional activity is attributed to the synergetic effects between N,S‐codoped rGO, and Co₉S₈ nanoparticles. These results highlight the significance of deliberate structural engineering in the preparation of multifunctional electrocatalysts for versatile electrochemical reactions.     

Synthesis of size-controlled CoMn2O4 quantum dots supported on carbon nanotubes for electrocatalytic oxygen reduction/evolution

A combined hot-injection and heat-up method was developed to synthesize monodisperse and uniform CoMn2O4 quantum dots (CMO QDs).CMO QDs with average size of 2.0,3.9,and 5.4 nm were selectively obtained at 80,90,and 105 ℃,respectively.The CMO QDs supported on carbon nanotubes (CNTs) were employed as catalysts for the oxygen reduction/evolution reaction (ORR/OER) in alkaline solution to investigate their size-performance relationship.The results revealed that the amount of surface-adsorbed oxygen and the band gap energy,which affect the charge transfer in the oxygen electrocatalysis processes,strongly depend on the size of the CMO QDs.The CMO-3.9/CNT hybrid,consisting of CNT-supported CMO QDs of 3.9 nm size,possesses a moderate amount of surfaceadsorbed oxygen,a lower band gap energy,and a larger charge carrier concentration,and exhibits the highest electrocatalytic activity among the hybrid materials investigated.Moreover,the CMO-3.9/CNT hybrid displays ORR and OER performances similar to those of the benchmark Pt/C and RuO2 catalysts,respectively,due to the strong carbon-oxide interactions and the high dispersion of CoMn2O4 QDs on the carbon substrate;this reveals the huge potential of the CMO-3.9/CNT hybrid as a bifunctional OER/ORR electrocatalyst.The present results highlight the importance of controlling the size of metal oxide nanodots in the design of active oxygen electrocatalysts based on spinel-type,nonprecious metal oxides.     

Molybdenum Carbide‐Decorated Metallic Cobalt@Nitrogen‐Doped Carbon Polyhedrons for Enhanced Electrocatalytic Hydrogen Evolution

Electrocatalytic hydrogen evolution reaction (HER) based on water splitting holds great promise for clean energy technologies, in which the key issue is exploring cost‐effective materials to replace noble metal catalysts. Here, a sequential chemical etching and pyrolysis strategy are developed to prepare molybdenum carbide‐decorated metallic cobalt@nitrogen‐doped porous carbon polyhedrons (denoted as Mo/Co@N–C) hybrids for enhanced electrocatalytic hydrogen evolution. The obtained metallic Co nanoparticles are coated by N‐doped carbon thin layers while the formed molybdenum carbide nanoparticles are well‐dispersed in the whole Co@N–C frames. Benefiting from the additionally implanted molybdenum carbide active sites, the HER performance of Mo/Co@N–C hybrids is significantly promoted compared with the single Co@N–C that is derived from the pristine ZIF‐67 both in alkaline and acidic media. As a result, the as‐synthesized Mo/Co@N–C hybrids exhibit superior HER electrocatalytic activity, and only very low overpotentials of 157 and 187 mV are needed at 10 mA cm^?2 in 1 m KOH and 0.5 m H₂SO₄, respectively, opening a door for rational design and fabrication of novel low‐cost electrocatalysts with hierarchical structures toward electrochemical energy storage and conversion.     

Revealing the Role of Electrocatalyst Crystal Structure on Oxygen Evolution Reaction with Nickel as an Example

Establishing a correlation between the crystal structure and electrocatalytic activity is crucial to the rational design of high performance electrocatalysts. In this work, taking the widely investigated nickel (Ni) based nonprecious oxygen evolution reaction (OER) catalyst as an example, for the first time, it is reported that the crystal structure plays a critical role in determining the OER performance. Similar‐sized nickel nanoparticles but in different hexagonal close‐packed phase and face‐centered cubic phase coated with N‐doped carbon shells, noted as hcp‐Ni@NC and fcc‐Ni@NC, are successfully prepared, respectively, in which the N‐coated carbon shell structures were also similar. Surprisingly, a dramatically enhanced OER performance of hcp‐Ni@NC in comparison with fcc‐Ni@NC is observed. The hcp‐Ni@NC only requires 305 mV overpotential to achieve the current density of 10 mA cm^?2, which is 55 mV lower than that of fcc‐Ni@NC, which can be ascribed to the influence of nickel crystal phase on the electron structure of N‐doped carbon shell. This finding will bring new thinking toward the rational design of high performance non‐noble metal electrocatalysts.     

A catalytic study of formic acid oxidation on preferentially oriented platinum electrodes

The oxidation of formic acid was examined by cyclic voltammetry and chronoamperometry in order to determine the rate of catalytic activity (reaction turnover) as a function of surface crystallography on preferentially oriented (electrochemically modified) platinum electrodes. The resulting turnover rates indicated a maximum fourfold current enhancement for an approximately 60% (111)-oriented surface versus a polycrystalline surface, suggesting that preferentially oriented electrodes are of potential practical significance.