Identifying the impact of the covalent-bonded carbon matrix to FeN4 sites for acidic oxygen reduction

The atomic configurations of FeNx moieties are the key to affect the activity of oxygen rection reaction (ORR). However, the traditional synthesis relying on high-temperature pyrolysis towards combining sources of Fe, N, and C often results in the plurality of local environments for the FeNx sites. Unveiling the effect of carbon matrix adjacent to FeNx sites towards ORR activity is important but still is a great challenge due to inevitable connection of diverse N as well as random defects. Here, we report a proof-of-concept study on the evaluation of covalent-bonded carbon environment connected to FeN4 sites on their catalytic activity via pyrolysis-free approach. Basing on the closed π conjugated phthalocyanine-based intrinsic covalent organic polymers (COPs) with well-designed structures, we directly synthesized a series of atomically dispersed Fe-N-C catalysts with various pure carbon environments connected to the same FeN4 sites. Experiments combined with density functional theory demonstrates that the catalytic activities of these COPs materials appear a volcano plot with the increasement of delocalized π electrons in their carbon matrix. The delocalized π electrons changed anti-bonding d-state energy level of the single FeN4 moieties, hence tailored the adsorption between active centers and oxygen intermediates and altered the rate-determining step.

Unveiling the Degradation Nature of Pt/NbOx/C Catalysts for Oxygen Reduction Reaction via in situ X-ray Absorption Spectroscopy

Among various metal nanoparticles supported on metal oxide (MMO) catalysts, the Pt/NbOx/C system has promising oxygen reduction reaction (ORR) activity as cathode for proton exchange membrane fuel cells (PEMFCs). Herein, we study a series of Pt/NbOx/C catalysts with tunable structural and electronic properties via physical vapor deposition and unravel the nature of metal and metal oxide interaction (MMOI) by characterizing this system under reactive conditions. By conducting in situ X-ray absorption spectroscopy (XAS) experiments, we demonstrate the Pt preferably interacts with O but not Nb in the Pt/NbOx/C system and such Pt-O interaction benefits the ORR activity via electronic effect rather than strain effect. We also provide clear evidence for the formation of metallic Nb phase at the early stage of PEMFC operation and identify severe particle growth of Pt after long-term PEMFC operation. These findings deepen our understanding of the degradation mechanism of MMO catalysts during long-term PEMFC operation.

High surface area micro-mesoporous graphene for electrochemical applications

The manuscript presents results on the influence of external pressure on graphene exfoliation and subsequent 3D structuring by means of liquid-phase exfoliation. In contrast to known and applied exfoliation methods, the current study exploits the enhancement of splitting forces caused by the application of high pressure. The manufacturing pathway allowed to increase the surface area from 750 m2/g (nanoplatelets) to ca. 1100 m2/g (after 3D structuring). Electrochemical studies revealed that the 3D graphene materials were active in the oxygen reduction reaction (ORR). The outstanding ORR activity of 3D structured graphene materials should not be ascribed to heteroatom catalytic centers since such heteroatoms were successively removed upon increasing the carbonization temperature. XPS data showed that the presence of transition metals and nitrogen (usually regarded as catalytic centers) in G-materials was marginal. The results highlight the importance of structural factors of electrodes in the case of graphene-based materials for Zn–air batteries and ORR.

An Easily Prepared Monomeric Cobalt(II) Tetrapyrrole Complex that Efficiently Promotes the 4e–/4H+ Peractivation of O2 to Water

The selective 4e–/4H+ reduction of dioxygen to water is an important reaction that takes place at the cathode of fuel cells. Monomeric aromatic tetrapyrroles (such as porphyrins, phthalocyanines, and corroles) coordinated to Co(II) have been considered as oxygen reduction catalysts due to their low cost and relative ease of synthesis. How- ever, these systems have been repeatedly shown to be selective for O2 reduction by the less desired 2e –/2H+ pathway to yield hydrogen peroxide. Herein, we report the initial synthesis and study of a Co(II) tetrapyrrole complex based upon a non-aromatic isocorrole scaffold that is competent for 4e–/4H+ ORR. This Co(II) 10,10-dimethyl isocorrole (Co[10- DMIC]) is obtained in a just four simple steps and excellent yield from a known dipyrromethane synthon. Evaluation of the steady state spectroscopic and redox properties of Co[10-DMIC] against those of Co(II) porphyrin ([Co(TPFPP)]) and corrole ([Co(TPFPC)(PPh3)]) homologs demonstrated that the light harvesting and electrochemical properties of the isocorrole are distinct from those displayed by more traditional aromatic tetrapyrroles. Further, investigation of the ORR activity of Co[10-DMIC] using a combination of electrochemical and chemical reduction studies revealed that this simple, unadorned monomeric Co(II) tetrapyrrole is ~85% selective for the 4e–/4H+ reduction of O2 to H2O over the more kinetically facile 2e–/2H+ process that delivers H2O2. By contrast, the same ORR evaluations conducted for the Co(II) porphyrin and corrole homologs demonstrated that these traditional aromatic systems catalyze the 2e–/2H+ conversion of O2 to H2O2 with near complete selectivity. Despite being a simple, easily prepared, monomeric tetrapyrrole platform, Co[10-DMIC] supports an ORR catalysis that has historically only been achieved using elaborate porphyrinoid-based architectures that incorporate pendant proton-transfer groups, ditopic molecular clefts, or which impose cofacially ori- ented O2 binding sites. Accordingly, Co[10-DMIC] represents the first simple, unadorned, monomeric metalloisocorrole complex that can be easily prepared and which shows a privileged performance for the 4e–/4H+ peractivation of O2 to water as compared to other simple Co(II) tetrapyrroles.

High-performance AEM unitized regenerative fuel cell using Pt-pyrochlore as bifunctional oxygen electrocatalyst

The performance of fixed-gas unitized regenerative fuel cells (FG-URFCs) are limited by the bifunctional activity of the oxygen electrocatalyst. It is essential to have a good bifunctional oxygen electrocatalyst which can exhibit high activity for oxygen evolution reaction (OER) and oxygen reduction reaction (ORR). In this regard, Pt-Pb2Ru2O7-x is synthesized by depositing Pt on Pb2Ru2O7-x wherein Pt individually exhibits high ORR while Pb2Ru2O7-x shows high OER and moderate ORR activity. Pt-Pb2Ru2O7-x exhibits higher OER (η@10mAcm-2 = 0.25 ± 0.01 V) and ORR (η@-3mAcm-2 = -0.31 ± 0.02 V) activity in comparison to benchmark OER (IrO2, η@10mAcm-2 = 0.35 ± 0.02 V) and ORR (Pt/C, η@-3mAcm-2 = -0.33 ± 0.02 V) electrocatalysts, respectively. Pt-Pb2Ru2O7-x shows a lower bifunctionality index (η@10mAcm-2, OER− η@-3mAcm-2, ORR) of 0.56 V with more symmetric OER–ORR activity profile than both Pt (>1.0 V) and Pb2Ru2O7-x (0.69 V) making it more useful for the AEM (anion exchange membrane) URFC or metal-air battery applications. FG-URFC tested with Pt-Pb2Ru2O7-x and Pt/C as bifunctional oxygen electrocatalyst and bifunctional hydrogen electrocatalyst, respectively, yields a mass-specific current density of 715 ± 11 A/gcat-1 at 1.8 V and 56 ± 2 A/gcat-1 at 0.9 V under electrolyzer mode and fuel-cell mode, respectively. The FG-URFC shows a round-trip efficiency of 75% at 0.1 A/cm−2, underlying improvement in AEM FG-URFC electrocatalyst design.

Density functional theory study of active sites on nitrogen-doped graphene for oxygen reduction reaction

Oxygen reduction reaction (ORR) remains challenging due to its complexity and slow kinetics. In particular, Pt-based catalysts which possess outstanding ORR activity are limited in application with high cost and ease of poisoning. In recent years, nitrogen-doped graphene has been widely studied as a potential ORR catalyst for replacing Pt. However, the vague understanding of the reaction mechanism and active sites limits the potential ORR activity of nitrogen-doped graphene materials. Herein, density functional theory is used to study the reaction mechanism and active sites of nitrogen-doped graphene for ORR at the atomic level, focusing on explaining the important role of nitrogen species on ORR. The results reveal that graphitic N (GrN) doping is beneficial to improve the ORR performance of graphene, and dual-GrN-doped graphene can demonstrate the highest catalytic properties with the lowest barriers of ORR. These results provide a theoretical guide for designing catalysts with ideal ORR property, which puts forward a new approach to conceive brilliant catalysts related to energy conversion and environmental catalysis.

High electrical conductivity and oxidation reduction reaction activity of tungsten carbide/carbon nanocomposite synthesized from palm oil by solution plasma process

In this study, the enhancement of electrical conductivity and Oxidation Reduction Reaction (ORR) activity of tungsten carbide/carbon (WC/C) nanocomposite was successfully synthesized from palm oil by solution plasma process (SPP). For the synthesis, plasma fields with different frequency were applied the bipolar pulsed power supply connected with two tungsten electrodes. The properties of the synthesized WC/C nanocomposite were varied by using a different frequency. The electrical conductivity increased with the frequencies. The highest electrical conductivity was 4.27×10−2 S cm−1, which is higher than that of Ketjen Black (7.37 × 10−3 S cm−1). The WC/C nanocomposites were observed the surface area 160 m2 g−1, pore volume 0.53 cm3 g−1, average pore diameter 16.29 nm, basal plane crystallite size 18.0 nm, and the average compound granule diameter less than 100 nm. The cyclic voltammetry measurement was showed that the ORR activity of WC/C nanocomposites were obtained the good performance in alkaline solution for fuel cell application.