Structure evolution and durability of Metal-Nitrogen-Carbon (M = Co, Ru, Rh, Pd, Ir) based oxygen evolution reaction electrocatalyst: A theoretical study.

Developing low-cost, high activity and stability oxygen evolution reaction (OER) catalysts is significantly important but still challenging for water electrolyzers. In this work, we calculated the OER activity and stability of Metal-Nitrogen-Carbon (MNC, M = Co, Ru, Rh, Pd, Ir) based electrocatalyst with different structures (MN4C8, MN4C10, MN4C12) using density functional theory (DFT) method. These electrocatalysts were divided into three groups based on the value of ΔG*OH, that is ΔG*OH > 1.53 eV (PdN4C8, PdN4C10, PdN4C12), ΔG*OH < 1.23 eV (RuN4C8, RuN4C10, RuN4C12, CoN4C8, CoN4C10) and 1.23 eV < ΔG*OH < 1.53 eV (RhN4C8, RhN4C10, RhN4C12, IrN4C8, IrN4C10, IrN4C12, CoN4C12), and ΔG*OH determine whether the structure evolution will appear. The results proved that MNC (M = Rh, Ir) with 1.23 eV < ΔG*OH < 1.53 eV shows higher OER activity due to moderate binding energy between reaction intermediates and MNC. Furthermore, these catalysts could maintain MNC structure without further oxidation and structural evolution under working conditions (high temperature, dynamic condition, local electric field and strong specific adsorption), therefore show excellent stability. However, MNC electrocatalyst with ΔG*OH > 1.53 eV or ΔG*OH < 1.23 eV revealed less stability under working conditions, due to their low intrinsic stability or structural evolution under working conditions, respectively. In conclusion, we proposed a comprehensive evaluation method for MNC electrocatalysts by taking ΔG*OH as the screening criterion for OER activity and stability, as well as ΔEb under working condition as descriptor of stability. This is of great significance for the design and screening of ORR, OER and HER electrocatalysts under working conditions.

Uneven phosphoric acid interfaces with enhanced electrochemical performance for high-temperature polymer electrolyte fuel cells.

Ultrahigh mass transport resistance and excessive coverage of the active sites introduced by phosphoric acid (PA) are among the major obstacles that limit the performance of high-temperature polymer fuel cells, especially compared to their low-temperature counterparts. Here, an alternative strategy of electrode design with fibrous networks is developed to optimize the redistribution of acid within the electrode. Via structural tailoring with varied electrospinning parameters, uneven migration of PA with dispersed droplets is observed, subverting the immersion model of conventional porous electrode. Combining with experimental and calculation results, the microscaled uneven PA interfaces could not only provide extra diffusion pathways for oxygen but also minimize the thickness of PA layers. This electrode architecture demonstrates enhanced electrochemical performance of oxygen reduction within the PA phase, resulting in a 28% enhancement of the maximum power density for the optimally designed electrode as cathode compared to that of a conventional one.

Chemically stable piperidinium cations for anion exchange membranes.

The chemical stability of the anion exchange membranes (AEMs) is determinative towards the engineering applications of anion exchange membrane fuel cells (AEMFCs) and other AEM-based electrochemical devices, yet remains a challenge due to deficiencies in the structural design of cations. In this work, an effective design strategy for ultra-stable piperidinium cations is presented based on the systematic investigation of the chemical stability of piperidinium in harsh alkaline media. Firstly, benzyl-substituted piperidinium was degraded by about 23% in a 7 M KOH solution at 100 °C after 1436 h, which was much more stable than pyrrolidinium due to its lower ring strain. The introduction of substituent effects at the α-C position was proved to be an effective strategy for enhancing the chemical stability of the piperidinium functional group. As a result, the butyl-substituted piperidinium cation showed no obvious structural changes after being treated in the 7 M KOH solution at 100 °C for 1050 h. Afterwards, GC-MS and NMR analysis indicated that the α-C atoms in the substituents of piperidinium are fragile to the nucleophilic attack of OH-. Based on the above results, the electronic and steric effects of different alkyl substitutions were analyzed. This work provides critical insights into the structural design of chemically stable piperidinium functional groups for the AEM and boosts its application in electrochemical devices, such as fuel cells and alkaline water electrolysis.

Fe-N-C with Intensified Exposure of Active Sites for Highly Efficient and Stable Direct Methanol Fuel Cells.

Fe-N-C catalysts are promising candidates to replace expensive and scarce Pt-based catalysts for oxygen reduction reaction (ORR) in fuel cell devices. Herein, simultaneous improvement of activity and stability of Fe-N-C is achieved through exposing active sites via a surface modification strategy. Concretely, EDTAFe groups are anchored on the external surface of zeolitic imidazolate framework-8 (ZIF-8) through size limitation, followed by pyrolysis to obtain ZIF@EDTAFe-1%-950, whose surface active site density increases more than 1.7 times as detected by X-ray photoelectron spectroscopy (XPS) and 57Fe Mössbauer spectra. Consequently, 1.7 times improvement of active site utilization efficiency in electrochemical measurements and more than 2 times performance enhancement in direct methanol fuel cells (DMFCs) are achieved due to facilitated mass transport as revealed by oxygen gain voltage and electrochemical impedance spectroscopy (EIS). Furthermore, through engineering robust drainage channels around exposed active sites to alleviate flooding, the assembled DMFC exhibits better stability than that of Pt/C in the first 3 h and remains 83.9% voltage after 24 h at 100 mA cm-2.

Effect of an external electric field, aqueous solution and specific adsorption on segregation of PtML/MML/Pt(111) (M = Cu, Pd, Au): a DFT study.

The oxygen reduction reaction (ORR) that occurs on the outermost layer of electrocatalysts is significantly affected by the composition and structure of the electrocatalysts. During the preparation of PtM alloy electrocatalysts, high-temperature annealing in an inert or reducing atmosphere could promote the segregation of M toward the core, forming a highly active Pt-skin structure. However, under fuel cell operating conditions, the adsorption of oxygen-containing groups could stimulate the easily dissolved M to segregate to the surface, reducing the activity and stability of the electrocatalysts. In this work, we conducted segregation energy calculation of PtM (M = Cu, Pd, Au) electrocatalysts under specific adsorption (SA), aqueous solution (AS) and an external electric field (EEF) with a density functional theory method. It was found that different factors have different effects on the segregation energy: ΔΔESA ≫ ΔΔEEEF > ΔΔEAS. The coupling effects have also been considered and compared: ΔΔESA+EEF > ΔΔESA+AS > ΔΔEEEF+AS. When including all three factors, the change of segregation energy could reach 1.63 eV. Therefore, operating conditions have a noteworthy influence on the segregation behavior of PtM ORR electrocatalysts, which should be considered in the further design of PtM ORR electrocatalysts.

Optimizing Platinum Location on Nickel Hydroxide Nanosheets to Accelerate the Hydrogen Evolution Reaction.

Rational electrode design is crucial to promote the performance of the hydrogen evolution reaction (HER) via further enhancing the activity, stability, and utilization of platinum (Pt) in an alkaline electrolyte. Herein, a binder-free low-Pt-content HER electrode, Pt (∼20 µg cm-2) decorated on nickel hydroxide grown on nickel foam (Pt-Ni(OH)2-2h-NF20), is fabricated at near room temperature in a test tube. To lower the ohmic resistance, for the first time, the Pt nanoparticles were location-selectively anchored on the bottom of height-controlled vertical Ni(OH)2 nanosheets via utilizing the mass transfer resistance of the dense Ni(OH)2 film for chloroplatinate. Furthermore, the excellent mass transfer, high specific surface area of Pt, synergistic effect between Pt with Ni(OH)2, and stable structure together prompt the resulting electrode with a special structure to exhibit superior HER electrocatalytic activity and stability in 1 M KOH. Typically, this electrode reaches a current density of 35.9 mA cm-2 at an overpotential of 100 mV, which is over 8 times higher than that of commercial Pt/C, and the overpotential only increases by 20 mV at 100 mA cm-2 over 150,000 s of stability test. Benefiting from the simple fabrication process, the electrode with an area of 840 cm2 was successfully prepared with a steady overpotential of 370 mV at 1000 mA cm-2 and increased potential of 23 mV over 50 h of stability test.

Size-dependence of the electrochemical performance of Fe-N-C catalysts for the oxygen reduction reaction and cathodes of direct methanol fuel cells.

Comprehension of the structure-activity relationship is of great importance for the rational design of electrocatalysts for the oxygen reduction reaction (ORR). Herein, Fe-N-C catalysts obtained from zeolitic imidazolate framework-8 (ZIF-8) with a tunable size ranging from 30 to 400 nm are precisely synthesized. Structural investigation indicates that the catalyst with smaller size possesses a higher proportion of mesopores originating from particle stacking, which leads to enhanced catalyst utilization and accelerated mass transport. The size effect of the catalyst on ORR activity is systematically investigated by rotation disk electrode (RDE) and direct methanol fuel cell (DMFC) tests. The electrochemical performance of the Fe-N-C catalyst is found to be increased with the reduction of its particle size. The correlation among size, mesoporosity and catalyst performance is discussed, giving new inspiration for the development of rational design strategies of non-precious metal catalysts.

Efficient Design for a High-Energy and High-Power Capability Hybrid Electric Power Device with Enhanced Electrochemical Interfaces.

Fabrication of novel electrode architectures with tailored electrochemical interfaces (EI) is an effective strategy for enhancing charge and mass transport processes within electrochemical devices. Here, we design and fabricate a well-hybrid electrode based on the coupling of polyaniline (PANI) nanowires and Pt-based electrocatalysts to manufacture a hybrid electric power device (HEPD) combining the advantages of supercapacitors and fuel cells. Because of the boosted charge transfer between PANI nanowires and Pt-based materials via enhanced EIs, the HEPD assembled with hybrid electrodes shows remarkable performance with a peak power density of 222 mW cm-2, a specific power of 3810 W kg-1, and a specific energy of 2100 Wh kg-1, normalized to the mass of membrane electrode assemblies. The in situ Raman spectra and extended electrochemical studies demonstrate the intrinsic mechanism of charge transfer processes within hybrid electrodes, shedding light on the alternative progress of electrochemical energy conversion systems and storage devices.

The mechanism and activity of oxygen reduction reaction on single atom doped graphene: a DFT method.

Heteroatom doped graphene as a single-atom catalyst for oxygen reduction reaction (ORR) has received extensive attention in recent years. In this paper, the ORR activity of defective graphene anchoring single heteroatom (IIIA, IVA, VA, VIA and VIIA) was systematically investigated using a dispersion-corrected density functional theory method. For all of the 34 catalysts, 14 of which were further analyzed, and the Gibbs free energy of each elementary reaction was calculated. According to the scaling relationship between ΔG OOH* and ΔG OH*, we further analyzed the rate-determining step of the remaining 20 catalysts. The results show that when the ORR reaction proceeds in the path O2 → OOH → O → OH → H2O, the reaction energy barriers are lower than 0.8 eV for Te-SV, Sb-DV, Pb-SV, Pb-DV, As-SV, As-DV, B-SV, Sn-SV and N-SV. Our result provides a theoretical basis for further exploration of carbon-based single-atom catalysts for ORR.

Carbon-Supported Divacancy-Anchored Platinum Single-Atom Electrocatalysts with Superhigh Pt Utilization for the Oxygen Reduction Reaction.

Maximizing the platinum utilization in electrocatalysts toward oxygen reduction reaction (ORR) is very desirable for large-scale sustainable application of Pt in energy systems. A cost-effective carbon-supported carbon-defect-anchored platinum single-atom electrocatalysts (Pt1 /C) with remarkable ORR performance is reported. An acidic H2 /O2 single cell with Pt1 /C as cathode delivers a maximum power density of 520 mW cm-2 at 80 °C, corresponding to a superhigh platinum utilization of 0.09 gPt kW-1 . Further physical characterization and density functional theory computations reveal that single Pt atoms anchored stably by four carbon atoms in carbon divacancies (Pt-C4 ) are the main active centers for the observed high ORR performance.

Electrochemical Dynamics of a Single Platinum Nanoparticle Collision Event for the Hydrogen Evolution Reaction.

Chronoamperometry was used to study the dynamics of Pt nanoparticle (NP) collision with an inert ultramicroelectrode via electrocatalytic amplification (ECA) in the hydrogen evolution reaction. ECA and dynamic light scattering (DLS) results reveal that the NP colloid remains stable only at low proton concentrations (1.0 mm) under a helium (He) atmosphere, ensuring that the collision events occur at genuinely single NP level. Amperometry of single NP collisions under a He atmosphere shows that each discrete current profile of the collision event evolves from spike to staircase at more negative potentials, while a staircase response is observed at all of the applied potentials under hydrogen-containing atmospheres. The particle size distribution estimated from the diffusion-controlled current in He agrees well with electron microscopy and DLS observations. These results shed light on the interfacial dynamics of the single nanoparticle collision electrochemistry.

Chemical Foaming Coupled Self-Etching: A Multiscale Processing Strategy for Ultrahigh-Surface-Area Carbon Aerogels.

Due to the unique structure, carbon aerogels have always shown great potential for multifunctional applications. At present, it is highly desirable but remains challenging to tailor the microstructures with respect to porosity and specific surface area to further expand its significance. A facile chemical foaming coupled self-etching strategy is developed for multiscale processing of carbon aerogels. The strategy is directly realized via the pyrolysis of a multifunctional precursor (pentaerythritol melamine phosphate) without any special drying process and multiple steps. In the micrometer scale, the macroporous scaffold structures with interconnected and strutted carbon nanosheets are built up by chemical foaming from decomposition of melamine, whereas the meso/microporous nanosheets are formed via self-etching by phosphorus-containing species. The delicately hierarchical structures and record-breaking specific surface area of 2668.4 m2 g-1 render the obtained carbon aerogels great potentials for absorption (324.1-593.6 g g-1 of absorption capacities for varied organic solvents) and energy storage (338 F g-1 of specific capacitance). The construction of such novel carbon nanoarchitecture will also shed light on the design and synthesis of multifunctional materials.

Erratum: High performance platinum single atom electrocatalyst for oxygen reduction reaction.

This corrects the article DOI: 10.1038/ncomms15938.

High performance platinum single atom electrocatalyst for oxygen reduction reaction.

For the large-scale sustainable implementation of polymer electrolyte membrane fuel cells in vehicles, high-performance electrocatalysts with low platinum consumption are desirable for use as cathode material during the oxygen reduction reaction in fuel cells. Here we report a carbon black-supported cost-effective, efficient and durable platinum single-atom electrocatalyst with carbon monoxide/methanol tolerance for the cathodic oxygen reduction reaction. The acidic single-cell with such a catalyst as cathode delivers high performance, with power density up to 680 mW cm-2 at 80 °C with a low platinum loading of 0.09 mgPt cm-2, corresponding to a platinum utilization of 0.13 gPt kW-1 in the fuel cell. Good fuel cell durability is also observed. Theoretical calculations reveal that the main effective sites on such platinum single-atom electrocatalysts are single-pyridinic-nitrogen-atom-anchored single-platinum-atom centres, which are tolerant to carbon monoxide/methanol, but highly active for the oxygen reduction reaction.

Hybrid Polymer Nanoarrays with Bifunctional Conductance of Ions and Electrons and Enhanced Electrochemical Interfaces.

Ion migration and electron transfer are crucial phenomena in electrochemistry and interfacial sciences, which require effective coupling and integration of separated charge pathways within medium materials. Here, in this work, we fabricated an ordered nanowire material based on hybrid polymers of polypyrrole, with electronic conductance, and perfluorosulfonic acid ionomers, with ionic conductance, via a facile one-step electrochemical route. Because of the nanoconfined effects for the different charge-transfer channels within the nanowire polymer matrix, the electronic and ionic conductivities of the hybrid polymer are surprisingly enhanced, being 26.4 and 0.096 S cm-1, respectively. Such an improvement in the formation of charge pathways also leads to an increased electrochemical capacitance through enlargement of the area of ion/electron transport boundaries, which may show great potential in the applications of supercapacitors, fuel cells, rechargeable batteries, and other electrochemical devices.

A "copolymer-co-morphology" conception for shape-controlled synthesis of Prussian blue analogues and as-derived spinel oxides.

The morphologically and compositionally controlled synthesis of coordination polymers and spinel oxides is highly desirable for realizing new advanced nanomaterial functionalities. Here we develop a novel and scalable strategy, containing a "copolymer-co-morphology" conception, to shape-controlled synthesis of various types of Prussian blue analogues (PBAs). Three series of PBAs MyFe1-y[Co(CN)6]0.67·nH2O (MyFe1-y-Co, M = Co, Mn and Zn) with well-controlled morphology have been successfully prepared through this strategy. Using MnyFe1-y-Co PBAs as the model, by increasing the relative content of Mn, flexible modulation of the morphology could be easily realized. In addition, a series of porous MnxFe1.8-xCo1.2O4 nano-dices with well-inherited morphologies and defined cation distribution could be obtained through a simple thermal treatment of the PBAs. All these results demonstrate the good universality of this novel strategy. When evaluated as an electrocatalyst, the octahedral-site Mn(III)/Mn(IV) content in MnxFe1.8-xCo1.2O4, mainly determined by sensitive (57)Fe Mössbauer in combination with X-ray photoelectron spectroscopic techniques, was discovered to be directly correlated with the oxygen reduction/evolution reaction (ORR/OER) activity.

Bio-inspired Construction of Advanced Fuel Cell Cathode with Pt Anchored in Ordered Hybrid Polymer Matrix.

The significant use of platinum for catalyzing the cathodic oxygen reduction reactions (ORRs) has hampered the widespread use of polymer electrolyte membrane fuel cells (PEMFCs). The construction of well-defined electrode architecture in nanoscale with enhanced utilization and catalytic performance of Pt might be a promising approach to address such barrier. Inspired by the highly efficient catalytic processes in enzymes with active centers embedded in charge transport pathways, here we demonstrate for the first time a design that allocates platinum nanoparticles (Pt NPs) at the boundaries with dual-functions of conducting both electrons by aid of polypyrrole and protons via Nafion(®) ionomer within hierarchical nanoarrays. By mimicking enzymes functionally, an impressive ORR activity and stability is achieved. Using this brand new electrode architecture as the cathode and the anode of a PEMFC, a high mass specific power density of 5.23 W mg(-1)Pt is achieved, with remarkable durability. These improvements are ascribed to not only the electron decoration and the anchoring effects from the Nafion(®) ionomer decorated PPy substrate to the supported Pt NPs, but also the fast charge and mass transport facilitated by the electron and proton pathways within the electrode architecture.

Ionic liquids as precursors for efficient mesoporous iron-nitrogen-doped oxygen reduction electrocatalysts.

A ferrocene-based ionic liquid (Fe-IL) is used as a metal-containing feedstock with a nitrogen-enriched ionic liquid (N-IL) as a compatible nitrogen content modulator to prepare a novel type of non-precious-metal-nitrogen-carbon (M-N-C) catalysts, which feature ordered mesoporous structure consisting of uniform iron oxide nanoparticles embedded into N-enriched carbons. The catalyst Fe(10) @NOMC exhibits comparable catalytic activity but superior long-term stability to 20 wt % Pt/C for ORR with four-electron transfer pathway under alkaline conditions. Such outstanding catalytic performance is ascribed to the populated Fe (Fe3 O4 ) and N (N2) active sites with synergetic chemical coupling as well as the ordered mesoporous structure and high surface area endowed by both the versatile precursors and the synthetic strategy, which also open new avenues for the development of M-N-C catalytic materials.

Catalyst-free synthesis of crumpled boron and nitrogen co-doped graphite layers with tunable bond structure for oxygen reduction reaction.

Two-dimensional materials based on ternary system of B, C and N are useful ranging from electric devices to catalysis. The bonding arrangement within these BCN nanosheets largely determines their electronic structure and thus chemical and (or) physical properties, yet it remains a challenge to manipulate their bond structures in a convenient and controlled manner. Recently, we developed a synthetic protocol for the synthesis of crumpled BCN nanosheets with tunable B and N bond structure using urea, boric acid and polyethylene glycol (PEG) as precursors. By carefully selecting the synthesis condition, we can tune the structure of BCN sheets from s-BCN with B and N bond together to h-BCN with B and N homogenously dispersed in BCN sheets. Detailed experiments suggest that the final bond structure of B and N in graphene depends on the preferentially doped N structure in BCN nanosheets. When N substituted the in-plane carbon atom with all its electrons configured into the π electron system of graphene, it facilitates the formation of h-BCN with B and N in separated state. On the contrary, when nitrogen substituted the edge-plane carbon with the nitrogen dopant surrounded with the lone electron pairs, it benefits for the formation of B-N structure. Specially, the compound riched with h-BCN shows excellent ORR performance in alkaline solution due to the synergistic effect between B and N, while s-BCN dominant BCN shows graphite-like activity for ORR, suggesting the intrinsic properties differences of BCN nanosheets with different dopants bond arrangement.

Iron encapsulated within pod-like carbon nanotubes for oxygen reduction reaction.

Chainmail for catalysts: a catalyst with iron nanoparticles confined inside pea-pod-like carbon nanotubes exhibits a high activity and remarkable stability as a cathode catalyst in polymer electrolyte membrane fuel cells (PEMFC), even in presence of SO(2). The approach offers a new route to electro- and heterogeneous catalysts for harsh conditions.