Efficient oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) are the determinants of the realization of a hydrogen-based society, as sluggish OER and ORR are the bottlenecks for the production and utilization of H2, respectively. A Co complex of 5,15-bis­(pentafluorophenyl)-10-(4)-(1-pyrenyl)­phenylcorrole (1) bearing a pyrene substituent was synthesized. When it was immobilized on multiwalled carbon nanotubes (MWCNTs), the 1/MWCNT composite displayed very high electrocatalytic activity and durability for both OER and ORR in aqueous solutions: it catalyzed a direct four-electron reduction of O2 to H2O in 0.5 M H2SO4 with an onset potential of 0.75 V vs normal hydrogen electrode (NHE), and it catalyzed the oxidation of water to O2 in neutral aqueous solution with an onset potential of 1.15 V (vs NHE, η = 330 mV). Control studies using a Co complex of 5,10,15-tris­(pentafluorophenyl)­corrole (2) demonstrated that the enhanced catalytic performance of 1 was due to the strong noncovalent π–π interactions between its pyrene moiety and MWCNTs, which were considered to facilitate the fast electron transfer from the electrode to 1 and also to increase the adhesion of 1 on carbon supports. The noncovalent immobilization of molecular complexes on carbon supports through strong π–π interactions appears to be a simple and straightforward strategy to prepare highly efficient electrocatalytic materials.

Metrics & More Article Recommendations * sı Supporting Information CONSPECTUS: The hydrogen evolution reaction (HER), oxygen evolution reaction (OER), and oxygen reduction reaction (ORR) are involved in biological and artificial energy conversions. H−H and O−O bond formation/cleavage are essential steps in these reactions. In nature, intermediates involved in the H−H and O−O bond formation/cleavage are highly reactive and short-lived, making their identification and investigation difficult. In artificial catalysis, the realization of these reactions at considerable rates and close to their thermodynamic reaction equilibria remains a challenge. Therefore, the elucidation of the reaction mechanisms and structure−function relationships is of fundamental significance to understand these reactions and to develop catalysts. This Account describes our recent investigations on catalytic HER, OER, and ORR with metalloporphyrins and derivatives. Metalloporphyrins are used in nature for light harvesting, energy conversion, electron transfer, O 2 activation, and peroxide degradation. Synthetic metal porphyrin complexes are shown to be active for these reactions. We focused on exploring metalloporphyrins to study reaction mechanisms and structure−function relationships because they have stable and tunable structures and characteristic spectroscopic properties. For HER, we identified three H−H bond formation mechanisms and established the correlation between these processes and metal hydride electronic structures. Importantly, we provided direct experimental evidence for the bimetallic homolytic H−H bond formation mechanism by using sterically bulky porphyrins. Homolytic HER has been long proposed but rarely verified because the coupling of active hydride intermediates occurs spontaneously and quickly, making their detection challenging. By blocking the bimolecular mechanism through steric effects, we stabilized and characterized the Ni III −H intermediate and verified homolytic HER by comparing the reaction behaviors of Ni porphyrins with and without steric effects. We therefore provided an unprecedented example to control homolytic versus heterolytic HER mechanisms through tuning steric effects of molecular catalysts.For the OER, the water nucleophilic attack (WNA) on high-valent terminal Mn-oxo has been proposed for the O−O bond formation in natural and artificial water oxidation. By using Mn tris(pentafluorophenyl)corrole, we identified Mn V (O) and Mn IVperoxo intermediates in chemical and electrochemical OER and provided direct experimental evidence for the Mn-based WNA mechanism. Moreover, we demonstrated several catalyst design strategies to enhance the WNA rate, including the pioneering use of protective axial ligands. By studying Cu porphyrins, we proposed a bimolecular coupling mechanism between two metal-hydroxide radicals to form O−O bonds. Note that late-transition metals do not likely form terminal metal-oxo/oxyl. For the ORR, we presented several strategies to improve activity and selectivity, including providi...

Few-layer two-dimensional (2D) molybdenum oxide nanoflakes are exfoliated using a grinding assisted liquid phase sonication exfoliation method. The sonication process is carried out in five different mixtures of water with both aprotic and protic solvents. We found that surface energy and solubility of mixtures play important roles in changing the thickness, lateral dimension, and synthetic yield of the nanoflakes. We demonstrate an increase in proton intercalation in 2D nanoflakes upon simulated solar light exposure. This results in substoichiometric flakes and a subsequent enhancement in free electron concentrations, producing plasmon resonances. Two plasmon resonance peaks associated with the thickness and the lateral dimension axes are observable in the samples, in which the plasmonic peak positions could be tuned by the choice of the solvent in exfoliating 2D molybdenum oxide. The extinction coefficients of the plasmonic absorption bands of 2D molybdenum oxide nanoflakes in all samples are found to be high (ε > 10(9) L mol(-1) cm(-1)). It is expected that the tunable plasmon resonances of 2D molybdenum oxide nanoflakes presented in this work can be used in future electronic, optical, and sensing devices.

Metal oxides as electrode materials are of great potential for rechargeable aqueous batteries. However, they suffer from inferior cycle stability and rate capability because of poor electronic and ionic conductivities. Herein, taking vertically orientated Bi 2 O 3 nanoflakes on Ti substrates as examples, we find that the δ-Bi 2 O 3 electrode with plenty of positively charged oxygen defects show remarkably higher specific capacity (264 mA h g −1 ) and far superior rate capability than that of α-Bi 2 O 3 with less oxygen vacancies. Through pinpointing the existence form and the role of oxygen vacancies within the electrochemical processes, we demonstrate that oxygen vacancies in δ-Bi 2 O 3 can not only promote electrical conductivity but also serve as central entrepots collecting OH − groups via electrostatic force effect, which has boosted the oxidation reaction and enhanced the electrochemical properties. Our work merits an excellent Bi 2 O 3 negative electrode material via giving full play to the role of oxygen vacancies in electrochemical energy storage.