This work aims to reveal the origin of the large onset overpotential and sluggish kinetics of the oxygen reduction reaction (ORR) on Pt, a standing problem that hinders the progress of fuel cell technology. By investigating various possible reaction steps and pathways through detailed DFT calculations on Pt(111) surface covered by rationalized phase structures of oxygenated adsorbates, we show that the ORR overpotential and Tafel kinetics originate from the potential-dependent formation of a site-blocking spectator phase, √3 × √3-structured oxygen adatoms (O*), which coexists with a relatively weak blocking phase, (√3 × √3)R30°-patterned adsorption network of hydroxyl group (OH*) and water molecule (H2O*) at ORR relevant potentials. The OH*/H2O* phase provides sites for ORR to proceed through a dissociative pathway consisting of four proton/electron transfer (PET) steps. The first step, PET-coupled O2 adsorption, is identified as the activity-determining step. Different from the usual beliefs, we found the O2 and O* do not directly accept proton during the reduction steps; rather, OH* and H2O* act as PET mediators to facilitate the O2 adsorption and dissociation and the O* reduction. These findings unveil the distinctly multiple roles of various oxygenated adsorbates as intermediates, spectators, and PET mediators in ORR. The implications of these findings on designing Pt-based catalysts are discussed. It is concluded that the binding strength of O* impacts the ORR activity of Pt-based surface predominantly by modulating the number of the available active sites, rather than the activation barriers for the rate-determining step.

It has been shown previously that conventional voltammetric theories may become inapplicable at electrodes of nanometer scale due to enhanced effects of the diffuse double layer on the interfacial charge transport and transfer processes (Anal. Chem. 1993, 65, 3343; J. Phys. Chem. B 2006, 110, 3262). As well as the diffuse double layer effects, we show in present study that the voltammetric responses of nanometer-sized electrodes would differ from the macroscopic electrodes due to significant edge effects of dielectric screening and electron tunneling if the electrode has planar geometries. These nanoedge effects arise because of the comparable size of the electrode with the dipole molecules and the effective electron tunneling distance. Models for these nanoedge effects are developed and combined with Poisson−Nernst−Planck theory and Marcus electron-transfer theory to describe the voltammetric characteristics of nanometer-sized disk electrodes. Marcus theory instead of Butler−Volmer theory is used to describe the electrochemical electron transfer kinetics because the greatly increased mass transport rates at the nanoscale electrochemical interface would render most of the electron transfer reaction to become largely irreversible so that the kinetics-controlled voltammetric behavior would extend to very large overpotentials at which the Marcus inversion of the electron transfer rate may occur. The theoretical calculations based on present models show the pronounced radial heterogeneities of interfacial potential, concentrations, and rate constant of charge transfer at the electrochemical interface of nanodisk electrodes, which in turn can significantly alter the voltammetric responses of these electrodes. It is indicated that the radial extension of electron transfer at the nanodisk electrode overwhelmingly determines the limiting currents on the voltammetric responses of the nanodisk electrode, while the electrical double layer effects can severely impact the kinetic characteristics of voltammetric responses (e.g., half-wave potential).

Searching for non-noble-metal-based electrocatalysts with high efficiency and durability toward the hydrogen evolution reaction (HER) is vitally necessary for the upcoming clean and renewable energy systems. Here we report the synthesis of CoP nanoparticles encapsulated in ultrathin nitrogen-doped porous carbon (CoP@NC) through a metal–organic framework (MOF) route. This hybrid exhibits remarkable electrocatalytic activity toward the HER in both acidic and alkaline media, with good stability. Experiments and theoretical calculations reveal that the carbon atoms adjacent to N dopants on the shells of CoP@NC are active sites for hydrogen evolution and that CoP and N dopants synergistically optimize the binding free energy of H* on the active sites, which results in a higher electrocatalytic activity in comparison to its counterparts without nitrogen doping and/or CoP encapsulation.Keywords: catalytically active site; cobalt phosphide; DFT; hydrogen evolution; ultrathin nitrogen-doped carbon layer;

Boron doping can boost the catalytic activity of palladium for diverse reactions. Precise control of the doping content is crucial but remains difficult in current synthesis, which generally involves the use of instable and costly borane-organic compounds. Herein, by taking advantage of the relatively strong solvation of N,N-dimethylformamide (DMF) to Na+ and the increased stability BH4– in DMF, we synthesize B-Pd interstitial nanocrystals in DMF, with NaBH4 acting as a reductant and boron source. The boron content, which can be readily tuned by changing the reaction time and NaBH4 concentration, can reach up to 20 at. %. Such a high boron doping results in a great beneficial effect on the catalytic capability of Pd toward the oxygen reduction reaction (ORR). The synthesized B-Pd nanoalloy exhibits a mass and specific activity for ORR that are, respectively, ca. 14 and 14.6 times higher than those of the state-of-the-art commercial Pt catalyst in alkaline solution. Density functional theory (DFT) calculations reveal three types of surface sites that are responsible for the enhanced activity, namely, Pd-BO2 assemblies, Pd atoms neighbored by the assemblies, and the Pd atoms modified with subsurface B atoms. The Pd-BO2 assembly has a Pt-like activity, while the neighboring Pd-BO2 assembly and subsurface B-modified Pd atoms could catalyze ORR much more efficiently than Pt. The facile and controllable boron doping in palladium should strengthen the power of Pd-based catalysts and, therefore, provides great prospects for their widespread application.

A variety of carbon-based materials have been reported as electrocatalysts towards the oxygen evolution reaction. However, carbon oxidation during the OER was rarely considered or even neglected in most of the reports. Herein, using carbon black as a model material, we develop a method to estimate the contribution of carbon oxidation reactions (CORs) to the measured current during the OER test. It is shown that the CORs could result in significant overestimation of the OER activity of carbon black-based electrocatalysts.

•Facial hydrothermal synthesis of Pt-Pd nanodendrites.•Superior oxygen reduction activity and durability of the Pt-Pd NDs/C, which has the total metal loading of nearly 20 wt %.•High performance of Pt-Pd NDs/C in cathode of polymer electrolyte membrane fuel cells.•The inter-connected 3-D nanostructure of Pt-Pd NDs/C plays key role in enhanced electrocatalytic performance.Platinum-palladium (Pt-Pd) bimetallic alloys have shown prospect as electrocatalyst for the oxygen reduction reaction (ORR) in the cathode of polymer-electrolyte-membrane (PEM) fuel cells. This article reports a facile solvothermal synthesis of Pt-Pd bimetallic nanodendrites (Pt-Pd NDs). The characterization with a variety of spectroscopic techniques indicates that the Pt-Pd NDs possess a three-dimensional (3-D) porous structure consisting of interconnected branches of highly alloyed Pt-Pd nanorods (NR). The measurements using rotating disk electrode in electrolyte solution show that the catalyst of Pt-Pd NDs supported on carbon (Pt-Pd NDs/C) possesses a Pt mass activity for ORR that is more than 3 times higher than that of the state-of-the-art Pt/C catalyst, as well as the significantly improved stability due to the branched porous structure. The measurements using membrane-electrode-assembly (MEA) in a single PEM fuel cell indicate the 3-D interconnected dendrite structures make the Pt-Pd NDs/C catalyst significantly advantageous over the nanoparticle Pt/C catalyst in reducing the mass transport and ohmic polarization which would become significant at high current density in MEA.Download high-res image (238KB)Download full-size image

In this study, we report successful synthesis of ultrafine worm-like Ir-oriented nanocrystalline assemblies (ONAs) and their further use as efficient electrocatalysts towards the HOR/HER in an alkaline medium. Due to the fast mass/charge transfer rate as well as the appearance of the long segments of low-index crystalline planes, the as-synthesized Ir ONAs exhibit a remarkable performance for the HOR/HER in an alkaline electrolyte.

A number of important reactions such as the oxygen evolution reaction (OER) are catalyzed by transition metal oxides (TMOs), the surface reactivity of which is rather elusive. Therefore, rationally tailoring adsorption energy of intermediates on TMOs to achieve desirable catalytic performance still remains a great challenge. Here we show the identification of a general and tunable surface structure, coordinatively unsaturated metal cation (MCUS), as a good surface reactivity descriptor for TMOs in OER. Surface reactivity of a given TMO increases monotonically with the density of MCUS, and thus the increase in MCUS improves the catalytic activity for weak-binding TMOs but impairs that for strong-binding ones. The electronic origin of the surface reactivity can be well explained by a new model proposed in this work, wherein the energy of the highest-occupied d-states relative to the Fermi level determines the intermediates’ bonding strength by affecting the filling of the antibonding states. Our model for the first time well describes the reactivity trends among TMOs, and would initiate viable design principles for, but not limited to, OER catalysts.

A simple Zn/Fe bimetallic zeolitic–imidazolite framework (ZIF) carbonization method is developed to synthesize an Fe–N–C hybrid with hierarchical nitrogen-doped porous carbons crossed by carbon nanotubes. Both the specific ratios of Zn/Fe in the bimetallic metal–organic framework (MOF) precursors and the selected annealing temperature are essential for the formation of this unique hybrid structure with good conductivity and exposure of more active sites. The resulting FeNC-20-1000 hybrid electrocatalyst exhibits excellent oxygen reduction reaction (ORR) activity, with a half-wave potential of 0.770 V comparable to that of the commercial Pt/C catalysts in acidic media, and a half-wave potential of 0.880 V, ca. 50 mV more positive than that of Pt/C for ORR in alkaline solution. More importantly, the as-prepared Fe–N–C hybrid exhibits much more stability for the ORR in both acidic and alkaline solutions than Pt/C, which makes it among the best non-noble-metal catalysts ever reported for ORR under acidic and alkaline conditions.

Ternary Fe/N/C catalysts are regarded as the most promising candidates for low-cost alternatives to Pt for catalyzing the oxygen reduction reaction (ORR). High-temperature pyrolysis of N-containing carbon precursors in the presence of Fe salts has been a common and efficient route to prepare Fe/N/C catalysts. However, simple pyrolysis usually leads to a large weight loss of N-containing precursors, which not only increases the overall cost of the catalysts, but also results in the catalysts having a low density of active sites. We show herein that this problem can be effectively overcome by dispersing a highly hydrosoluble polymer precursor, namely, polyvinylpyrrolidone (PVP), in NaCl crystallites. The use of the hydrosoluble polymer precursor enables efficient dispersion and confinement of the precursor in NaCl crystallites in a simple solvent evaporation process. The subsequent pyrolysis process not only shows little weight loss, but also produces a mesoporous Fe/N/C ORR catalyst with a remarkably higher specific surface area (414.5 m2 g−1) and ORR activity (half-wave potentials, E1/2 = 0.793 V and 0.878 V vs. RHE, respectively, in acidic and alkaline media) than those of the material prepared without using NaCl confinement (72.9 m2 g−1, E1/2 = 0.634 V and 0.785 V, respectively, in acidic and alkaline media).

Here, we, for the first time, synthesized Pd nanotubes covered by high-density Au-islands, in which abundant exposed Pd–Au heterojunction interfaces are present and the content of element Au can be easily controlled. The optimized nanostructures show remarkably enhanced activity for catalyzing different alcohols in alkaline media than commercial Pd/C. The mass activity is 9.66, 1.83, and 4.60 A mgmetal–1 toward the electrooxidation of ethanol, glycerol, and ethylene glycol in alkaline media, which is about 6, 4, and 7 times that of Pd/C, respectively. Additionally, a model was proposed to explain the relationship between the structure and the catalytic activity.

The electrochemical oxygenation processes of Pt(111) surface are investigated by combining density functional theory (DFT) calculations and Monto Carlo (MC) simulations. DFT calculations are performed to construct force-field parameters for computing the energy of (√3 × √3)R30°-structured OH*-H2O* hydrogen-bonding networks (differently dissociated water bilayer) on the Pt(111) surface, with which MC simulations are conducted to probe the reversible H2O* ↔ OH* conversion in OH*-H2O* networks. The simulated isotherm (relation between electrode potential and OH* coverage) agrees well with that predicted by the experimental cyclic voltammetry (CV) in the potential region of 0.55–0.85 V (vs RHE). It is suggested that the butterfly shape of CV in this region is due to different variation trends of Pt-H2O* distance in low and high OH* coverages. DFT calculation results indicate that the oxidative voltammetry in the potential region from 0.85 V to ca. 1.07 V is associated with the dissociation of OH* to O*, which yields surface structures consisting of OH*-H2O* networks and (√3 × √3)-structured O* clusters. The high stability of the half-dissociated water bilayer (OH*-H2O* hydrogen-bonding network with equal OH* and H2O* coverages) formed in the butterfly region makes OH* dissociation initially very difficult in energetics, but become facile once starts due to the destabilization of OH* by the formed O* nearby. This explains the experimentally observed nucleation and growth behavior of O* phase formation and the high asymmetry of oxidation–reduction voltammetry in this potential region.Keywords: density functional theory; Monto Carlo simulations; nucleation and growth; oxygenated adsorbates; phase transition; Pt(111)

Active and stable electrocatalysts based on earth-abundant elements for oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) are crucially important for the utilization of renewable energy. Herein, we reported the synthesis of Fe3C nanorod encapsulated, N-doped carbon nanotubes grown on N-doped porous carbon sheets (Fe3C@NCNT/NPC) by simply annealing a Fe-based MOF (MIL-88B) loaded with melamine at 800 °C in N2. Thanks to the synergistic effect of the high density of Fe–N active sites and electric conductance offered by the unique hybrid structure, the as-prepared Fe3C@NCNT/NPC hybrid exhibited a half-wave potential ca. 60 mV more positive and a durability performance much better than that of Pt/C for ORR, and an overpotential ca. 20 mV lower and a Tafel slope much smaller than that of IrO2 for OER, which make it one of the best reported nonprecious metal bifunctional electrocatalysts for oxygen electrode reactions.

•Hybrids of Fe3O4 and nitrogen-doped carbon as efficient ORR electrocatalyst.•Hybridization of Fe3O4 and nitrogen-doped carbon through low-temperature annealing.•Urea as reductant and carbon/nitrogen precursors at the same time.Cost-effective electrocatalyst as alternative to Pt for the oxygen reduction reaction (ORR) is one of the key requirements for viable fuel cells. Hybrids of metal oxides and nitrogen (N)-doped carbon (NC) are promising candidates, while effective synthesizing methods are highly expected. We report an efficient hybridization of Fe3O4 and NC by simply annealing FeOOH nanorods together with urea at a relatively low temperature (500 °C). The urea is ingeniously chosen to act as the reductant to convert FeOOH into Fe3O4 and as the source to form NC on Fe3O4 nanorods at the same time; while Fe species in FeOOH/Fe3O4 catalyze the reaction from urea to NC. Such synergetic effects are vital for the hybrid formation. The as-synthesized hybrid exhibits attractive ORR activity, with an onset potential of ∼0.915 V (vs. RHE) and a half-wave potential of 0.806 V (vs. RHE) in alkaline media, which are superior to that reported so far for the Fe3O4-based materials and N-doped carbons.

A facile one-step synthesis of the hybrid of hierarchical nitrogen-doped porous carbon and Fe3C nanoparticles encapsulated a nitrogen-doped carbon nanotube through simply annealing the mixture of FeCl3, o-phthalic anhydride, and melamine at 800 °C in Ar was proposed. Both the specific ratio of these precursors and the selected annealing temperature are key factors for the formation of the unique hybrid structure, while any subtle modulation will result in different morphologies. Thanks to the good conductivity and hierarchical porous diversity, Fe–N–C material obtained at 800 °C exhibits a half-wave potential of 0.880 V, ca. 50 mV more positive than Pt/C, for oxygen reduction reaction (ORR) and an overpotential of 0.41 V, ca. 36 mV lower than IrO2 black, at the current of 10 mA cm–2 for oxygen evolution reaction (OER).

Cost-effective non-precious metal electrocatalysts for the oxygen reduction reaction (ORR) is the key for fuel cells to become a viable electricity generation technology. Metal macrocyclic compounds such as Fe/Co porphyrins and phthalocyanines are excellent molecular catalysts for O2 reduction; but they are still considerably less competitive than Pt-based materials when catalyzing the ORR in electrochemical environments. Using Fe phthalocyanine (FePc) as a model compound, we show that the electrocatalytic activity of metal macrocyclic compounds for the ORR can be greatly enhanced through tailoring assembling architectures on high-surface-area nanocarbons. By simply ball-milling FePc with nanocarbons, such as graphene nanosheets and carbon-black nanoparticles, molecular architectures of FePc from nanorods to uniform thin shells are obtained. The resulting carbon-supported FePc composites exhibit ORR performance much superior to the state-of-the-art carbon-supported Pt in alkaline solution, with up to a 60 mV positive shift in the half-wave potential and more than 5 times increase in the mass activity. As well as showing that the molecule–support interaction provides a degree of control on the molecular architectures of metal macrocyclic compounds, the present work reveals that the FePc molecule is intrinsically much more efficient than Pt in catalyzing the ORR in alkaline media, and therefore has great prospects as a cathode electrocatalyst in alkaline fuel cells.

A straightforward one-step spontaneous deposition approach for growth of Pt atomic shell on Au nanoparticles and the superior activity and durability of the resulted Pt-on-Au nanoparticles for the oxygen reduction reaction (ORR) are reported. Transmission electron microscopy, X-ray photoelectron spectroscopy, energy-dispersive spectrometry, and electrochemical measurements indicate that Pt can be spontaneously deposited on Au surface upon simply dispersing carbon-supported Au nanoparticles in PtCl42–-containing solution, without introducing any extraneous reducing agents or any pre/post-treatments. The deposited Pt atoms are uniformly distributed on the surface of Au nanoparticles, with coverage tunable by the concentration of PtCl42– and temperatures. An approximate monolayer of Pt forms at temperature of ca. 80 °C and PtCl42– concentrations of above 10–4 mol/L. The obtained Pt-on-Au core–shell nanoparticles catalyze the ORR with specific and mass activities of Pt that are 3.5 times higher than that of pure Pt nanoparticles. Moreover, they exhibit no visible activity degradation after undergoing long-term oxidization/reduction cycling in O2-saturated acid media, therefore showing great prospect as durable cathode electrocatalysts in proton-exchange membrane fuel cells.Keywords: core−shell structure; gold; low-platinum electrocatalyst; oxygen reduction reaction; spontaneous deposition

•Facile surfactant-free synthesis of carbon-supported monodisperse Pd (Pd/C).•Excellent electrocatalytic performance for formic acid and ethanol oxidation.•Different particle size effects of Pd on the formic acid and ethanol oxidation.•Adsorbed hydroxyl groups play important roles in the ethanol oxidation on Pd.Electrocatalysts of ultrafine and surface-clean nanoparticles uniformly dispersed on high-surface-area conducting supports are among the prime requests for large-scale application of fuel cells. We report a facile one-pot synthesis of carbon-supported monodisperse and ultrafine Pd nanoparticles (Pd/C) in N,N-dimethylformamide by using amine–borane as reducing agent in the absence of any surfactants. The ultrasmall size, monodispersity and surfactant-free surface of Pd nanoparticles make the resulted Pd/C catalyst exhibit excellent electrocatalytic performance toward the oxidation of organic fuels. For formic acid oxidation reaction, it exhibits a similar onset potential to that of the commercial Pd/C (BASF) but about 2.6 times increase in current density, which can be attributed to the increased electrochemically surface area (ECSA); while for ethanol oxidation, it shows about 140 mV negative shift of the onset potential as well as the current increase much beyond that expected by the increased ECSA, indicating that the reduction of Pd particle size not only increases the ECSA, but also enhances the intrinsic activity of surface Pd atoms. The different activity enhancing behaviors for the formic acid and ethanol oxidation have been discussed in terms of the reaction mechanisms. Chronoamperometric measurements show that the home-made Pd/C is also much more durable.Carbon-supported monodisperse and ultrafine Pd nanoparticles synthesized in a ambient, one-pot and surfactant-free process exhibit excellent electrocatalytic activity for ethanol and formic acid oxidation.

The stability and oxygen reduction reaction (ORR) activity of the Pt-segregated surface in various Pt-M alloys (M: transition metals) are investigated through systematic DFT calculations on the thermodynamic (alloy formation energy and Pt surface segregation energy), surface chemical property (oxygen binding energy) and electronic (d-band center) properties. Factors affecting these properties, such as the atomic radii and surface energy of M and the electronic ligand interaction between Pt and M are analyzed as a function of outmost d electron numbers of M. It is shown that the electronic ligand interaction plays determining role in the alloy formation energy of various Pt-M alloys; the formation of Pt-segregated surface in Pt-M alloys is favored when alloying metals have higher surface energy and smaller radii than Pt; the oxygen binding energy on the Pt-segregated surface in Pt-M alloys varies approximately linearly with the d-band center of surface Pt atoms; the lattice strain and electronic ligand effects are simply additive in Pt-M alloys; the stain effect in Pt-M alloys nearly linearly affects the d-band center of the Pt-segregated surface in Pt-M alloys; transition metals with less than 10 d electrons mostly exhibit electron ligand effects which result in downshift of the d-band center of the segregated surface Pt atoms, while those with ten d electrons exhibit electron ligand effect upshifting the d-band center of the segregated Pt atoms.

Heterogeneous electron-transfer (ET) processes at solid electrodes play key roles in molecular electronics and electrochemical energy conversion and sensing. Electrode nanosization and/or nanostructurization are among the major current strategies for performance promotion in these fields. Besides, nano-sized/structured electrodes offer great opportunities to characterize electrochemical structures and processes with high spatial and temporal resolution. This review presents recent insights into the nanoscopic size and structure effects of electrodes and electrode materials on heterogeneous ET kinetics, by emphasizing the importance of the electric double-layer (EDL) at the electrode/electrolyte interface and the electronic structure of electrode materials. It is shown, by general conceptual analysis and recent example demonstrations of representative electrode systems including electrodes of nanometer sizes and gaps and of nanomaterials such as sp2 hybridized nanocarbons and semiconductor quantum dots, how the heterogeneous ET kinetics, the electronic structures of electrodes, the EDL structures at the electrode/electrolyte interface and the nanoscopic electrode sizes and structures may be related.

Nanocarbons doped with nitrogen (N) and/or metal-N coordination structures hold great promise in replacing Pt for catalyzing the oxygen reduction reaction (ORR) in fuel cells. The lack of clear views on the natures of ORR active sites in these materials has hindered the progress in reducing their activity gap to Pt through a rational desire of doping structures. Using 14 types of N and Fe–N doping structures in graphene as model systems, systematic density-functional-theory (DFT) calculations are performed within a unified electrochemical thermodynamic framework and the same reaction mechanism to gain insights into ORR active sites in doped nanocarbons. Scaling relations are obtained between the calculated adsorption free energy of key ORR intermediates at surface sites associated with various graphene doping structures. Reaction free energy analysis indicates that the proton–electron transfer coupled O2 adsorption and/or reduction of adsorbed hydroxyl group (*OH) are the activity-determining steps in the ORR on most doped graphenes and that the ORR activity of various graphene doping structures can be described with a single thermodynamic descriptor, namely, the adsorption free energy of *OH (ΔG*OH). A model volcano plot of ORR activity as a function of ΔG*OH is established for active sites in doped graphenes, which indicates that the surface sites associated with a few edge N-doping structures, such as armchair graphitic N, zigzag pyridinic N, and zigzag pyridinic N oxide, offer optimized binding strength of oxygenated species for catalyzing the ORR. Some other structures, such as in-plane graphitic N and the Fe–N4 complex and hydrogenated zigzag pyridinic N, are also expected to form ORR activity sites. The possible electronic structure origin of the differing binding strength of oxygenated species on various graphene doping structures is analyzed in terms of the density of pz states near the Fermi level of active carbon atoms. These results may serve as guidance for designing ORR electrocatalysts of doped nanocarbons. Especially, it is revealed that merely N doping indeed can produce highly active electrocatalytic sites for the ORR in nanocarbons.Keywords: active sites; density functional theory; doped graphenes; oxygen reduction reaction; scaling relationship; volcano plot

Wavy palladium (Pd) nanorods were obtained by controlled synthesis by using amine–boranes as the reducing agents. Thanks to the unique structure and strong interaction with graphene, the as-synthesized Pd nanorods supported on graphene exhibit much enhanced electrocatalytic activity towards formic acid oxidation as compared with Pd nanoparticles.

The large-scale application of acidic water electrolysis as a viable energy storage technology has been hindered by the high demand of precious metal oxides at anode to catalyze the oxygen evolution reaction (OER). We report an Ir–Co binary oxide electrocatalyst for OER fabricated by a multistep process of selective leaching of Co from Co-rich composite oxides prepared through thermal decomposition. The stepwise leaching of the Co component from the composites leads to the formation of macro- and mesoscale voids walled by a cross-linked nanoporous network of rod- and wedge-like building units of Ir–Co binary oxide with a rutile phase structure and an Ir-enriched surface. In comparison, Ir–Co binary oxide with similar composition prepared by direct thermal decomposition method exhibits a loose nanoparticle aggregation morphology with a Co-enriched surface. The cross-linked porous Ir–Co binary oxide from selective leaching is about 3-fold more active for the OER than that from direct thermal decomposition. Compared with pure IrO2 from thermal decomposition, the Co-leached binary oxide is ca. two times more active and is much more durable during continuous oxygen evolution under a constant potential of 1.6 V, thus showing a possibility of reducing the demand of the expensive and scarce Ir in OER electrocatalyst for acidic water splitting.Keywords: acidic media; Ir-enriched; oxygen evolution; selective leaching; water oxidation

Electrodes of nanometer sizes provide a model approach to study the nanoscale electrochemical properties and processes, which are of fundamental and applied significance in a variety of areas including energy and environmental science, scanning probe microscopies, nanofabrication as well as electrochemistry itself. This Perspective reviews recent developments in conceptual understanding, theoretical modelling and simulation, and experimental observation of nanosize-induced properties and phenomena at interfaces between nanometer-sized electrodes and electrolytes. The aim is to provide a view on how the dimension comparability of nanoelectrodes with the electric double layer and the effective electron-tunnelling distance may raise distinct features in interfacial structure and reactivity. The strong coupling between the electrostatic field, the concentration field and the dielectric field of solvent at nanoelectrode/electrolyte interfaces is highlighted. The effects of this coupling on the voltammetric responses of nanoelectrodes are evaluated. Electron transfer kinetics at the nanoelectrode/electrolyte interface is discussed by emphasizing the inappropriateness of the Butler–Volmer (BV) and classic Marcus–Hush (MH) theories at potentials largely departing from the formal potential of the redox moieties and the importance of the long-distance electron tunnelling. The conditions for using the mathematically more straightforward BV and classic MH formalisms as an alternative to the physically more realistic but mathematically unfriendly Marcus–Hush–Chidsey model are analysed.

•Synthesis of Nb-doped TiO2 carrier with high surface area.•IrO2 nanoparticles are supported on Nb-doped TiO2.•IrO2/Nb0.05Ti0.95O2 (26 wt%) exhibits the optimal mass activity for OER.•IrO2/Nb0.05Ti0.95O2 (26 wt%) has significantly improved stability.A corrosion-resistant Nb0.05Ti0.95O2 material with high surface area was prepared by a sol–gel process. IrO2 nanoparticles (about 16–33 wt%) were successfully loaded on Nb0.05Ti0.95O2 powders as the electrocatalyst for oxygen evolution reaction (OER) in acidic medium. The IrO2/Nb0.05Ti0.95O2 catalyst with the IrO2 loading of 26 wt% exhibits the best mass normalized cyclic voltammetry charge and mass normalized activity among all the IrO2/Nb0.05Ti0.95O2 catalysts because IrO2 nanoparticles were uniformly supported on the surface of Nb0.05Ti0.95O2 providing conductive channels to reduce the grain boundary resistance. Due to the anchoring effect of carrier on the catalyst, the stability of the supported IrO2 was significantly improved as compared to the unsupported one. The IrO2/Nb0.05Ti0.95O2 catalyst with 26 wt% IrO2 loading demonstrates the best effectiveness of the OER activity and cost.

Redox cycling in nanometer-wide thin-layer cells holds great promise in ultrasensitive voltammetric detection and in probing fast heterogeneous electron-transfer kinetics. Quantitative understanding of the influence of the nanometer gap distance on the redox processes in the thin-layer cells is of crucial importance for reliable data analysis. We present theoretical consideration on the voltammetric behaviors associated with redox cycling of electroactive molecules between two electrodes separated by nanometer widths. Emphasis is placed on the weakness of the commonly used Butler–Volmer theory and the classic Marcus–Hush theory in describing the electrochemical heterogeneous electron-transfer kinetics at potentials significantly removed from the formal potential of redox moieties and, in addition, the effect of the electric-double-layer on the electron-transfer kinetics and mass transport dynamics of charged redox species. The steady-state voltammetric responses, obtained by using the Butler–Volmer and Marcus–Hush models and that predicted by the more realistic electron-transfer kinetics formulism, which is based on the alignments of the density of states between the electrode continuum and the Gaussian distribution of redox agents, and by inclusion of the electric-double-layer effect, are compared through systematic finite element simulations. The effect of the gap width between the electrodes, the standard rate constant and reorganization energy for the electron-transfer reactions, and the charges of the redox moieties are considered. On the basis of the simulation results, the reliability of the conventional voltammetric analysis based on the Butler–Volmer kinetic model and diffusion transport equations is discussed for nanometer-wide thin-layer cells.Keywords: electric double layer; finite element simulation; heterogeneous electron transfer; nanogap effects; thin-layer cells; voltammetric responses;

The fabrication and electrochemistry of a new class of graphene electrodes are presented. Through high-temperature annealing of hydrazine-reduced graphene oxides followed by high-speed centrifugation and size-selected ultrafiltration, flakes of reduced graphene oxides (r-GOs) of nanometer and submicrometer dimensions, respectively, are obtained and separated from the larger ones. Using n-dodecanethiol-modified Au ultramicroelectrodes of appropriately small sizes, quick dipping in dilute suspensions of these small r-GOs allows attachment of only a single flake on the thiol monolayer. The electrodes thus fabricated are used to study the heterogeneous electron transfer (ET) kinetics at r-GOs and the nanoscopic charge transport dynamics at electrochemical interfaces. The r-GOs are found to exhibit similarly high activity for electrochemical ET reactions to metal electrodes. Voltammetric analysis for the relatively slow ET reaction of Fe(CN)63– reduction produces slightly higher ET rate constants at r-GOs of nanometer sizes than at large ones. These ET kinetic features are in accordance with the defect-dominant nature of the r-GOs and the increased defect density in the nanometer-sized flakes as revealed by Raman spectroscopic measurements. The voltammetric enhancement and inhibition for the reduction of Ru(NH3)63+ and Fe(CN)63–, respectively, at r-GO flakes of submicrometer and nanometer dimensions upon removal of supporting electrolyte are found to significantly deviate in magnitude from those predicted by the electroneutrality-based electromigration theory, which may evidence the increased penetration of the diffuse double layer into the mass transport layer at nanoscopic electrochemical interfaces.

Low cost alternatives to the expensive and scarce Pt to catalyze the oxygen reduction reaction (ORR) in acid media are essential for the proton-exchange-membrane (PEM) fuel cells to become economically viable. Chemically doped nanocarbons are among the most promising candidates in this regard. We report the facile synthesis and superior electrocatalytic activity of an Fe–N doped nanocarbon composite of carbon nanotubes (CNTs) grown on/between graphene sheets. The structure and composition of the composite is characterized by using a variety of techniques including SEM, TEM, N2 adsorption/desorption isotherms, XPS, XRD, and Mössbauer spectroscopy. It is shown that the in situ growth of CNTs in the presence of graphene sheets not only produces a tubes-on/between-sheets architecture that enhances the dispersion of CNTs and graphene sheets, but also leads to optimized doping and coordination of nitrogen and Fe which favour the ORR. The composite can catalyze the ORR much more efficiently than either of the single materials containing only CNTs or graphene synthesized under similar conditions, and similarly to Pt/C in both alkaline and acid media.

Pt–W nanoalloys with compositions ranging from Pt3W to PtW2 were explored as electrocatalysts for hydrogen oxidation reaction (HOR). It is shown that alloying Pt with W can lead to significantly enhanced electrocatalytic activity for HOR, with nearly 4 times increase in the exchange current density as compared with pure Pt. What's more, these Pt–W alloys possess superior CO tolerance to Pt and PtRu, mainly due to the weakened bonding of CO on their Pt-enriched surfaces.Highlights► Pt-enriched surface even at low Pt composition such as PtW2. ► Much higher exchange current density of Pt–W alloys for HOR than pure Pt. ► Much better CO-tolerance of Pt–W alloys than PtRu. ► CO-tolerant hydrogen oxidation of Pt–W alloys through a detoxification mechanism rather than a bifunctional mechanism.

A Fe/N co-doped ternary nanocarbon hybrid, with uniform bamboo-like carbon nanotubes (CNTs) in situ grown on/between the single/few-layer graphene sheets interspaced by carbon nanosphere aggregates, was prepared through a one-pot heat treatment of a precursor mixture containing graphene oxide, Vulcan XC-72 carbon nanospheres, nitrogen rich melamine and small amounts of Fe ions. Physical characterization including electron microscopic images, N2 adsorption–desorption isotherms, pore size distribution, XPS, XRD, Mössbauer spectra, and EDX revealed that the 0-D/1-D/2-D ternary hybrid architecture not only offered an optimized morphology for high dispersion of each nanocarbon moiety, while the carbon nanosphere interspaced graphene sheets have provided a platform for efficient reaction between Fe ions and melamine molecules, resulting in uniform nucleation and growth of CNTs and formation of high density Fe–N coordination assemblies that have been believed to be the active centers for the oxygen reduction reaction (ORR) in carbon-based nonprecious metal electrocatalysts. In the absence of graphene oxides or carbon nanospheres, a similar heat treatment was found to result in large amounts of elemental Fe and Fe carbides and entangled CNTs with wide diameter distributions. As a result, the ternary Fe/N-doped nanocarbon hybrid exhibits ORR activity much higher than the Fe–N doped single or binary nanocarbon materials prepared under similar heat treatment conditions, and approaching that of the state-of-the-art carbon-supported platinum catalyst (Pt/C) in acidic media, as well as superior stability and methanol tolerance to Pt/C.

DFT calculations are combined with thermodynamic and kinetic modeling to study the effect of temperature on the electrochemical adsorption of hydrogen and the exchange current density (j0) of the hydrogen electrode reactions (HERs) on Pt(1 1 1). The nature of the underpotential and overpotential deposited hydrogen (UPD H and OPD H), the potential dependence of their coverage and the corresponding voltammetric curves for hydrogen adsorption are evaluated for temperatures from 273 K to 373 K. By introducing a coverage-dependent correction term in the free energy expression to account for the effect of the interaction between hydrogen ad-atoms on the configurational entropy, the DFT calculations give isotherms and voltammetric curves for hydrogen adsorption agreeing reasonably with those observed in experiments. It is shown that both the UPD and OPD H on Pt(1 1 1) surface could be the H atoms adsorbed at the 3-fold fcc hollow sites. A linear dependence of lnj0 on T−1 is found under temperatures from 273 K to 373 K, which, however, deviates from that predicted by the Arrhenius relation. The values of j0 and activation enthalpy for HERs on Pt(1 1 1) surface are estimated.Highlights► DFT calculations reveal the nature and coverage of UPD and OPD H. ► UPD H atoms predominantly adsorb at fcc sites on Pt(1 1 1). ► OPD H atoms also adsorb at fcc sites at potentials positive to −0.2 V. ► The temperature dependence of the exchange current density deviates from the Arrhenius relation.

Combined with air annealing, rutile-structured IrO2 nanoparticles with various sizes were prepared using colloidal method. The nanoparticles were used as the electrocatalysts for the oxygen evolution reaction (OER) in acidic media, and their grain size effect was studied. The results show that with the increase in annealing temperature, the grain size of the catalyst increases, and the voltammetric charges (the electroactive areas) and apparent activity for the OER decrease. The relationship between the intrinsic activity and the annealing temperature exhibits a volcano-type curve and the catalyst annealed at 550 °C achieved the best result.

Three-dimensional ordered macroporous (3-DOM) IrO2 was synthesized by using the silica colloidal crystal template method and explored as electrocatalyst for oxygen evolution reaction (OER) in acidic medium. X-Ray diffraction (XRD) patterns indicate that the prepared 3-DOM IrO2 has a rutile structure. Images of scanning and transmission electron microscopies suggest that the 3-DOM IrO2 possesses a hierarchical pore structure, in which the honeycomb array of 300 nm primary macropores were cross-linked by secondary mesopores on the walls. The presence of mesopores is also indicated by the N2 adsorption/desorption isotherms and the small-angle XRD patterns. As compared with IrO2 prepared by conventional colloidal method, the 3-DOM IrO2 exhibited much larger BET area and voltammetric charges. Accordingly, about two and half times enhancement in OER activity was achieved by using 3-DOM IrO2 as the electrode materials, showing prospect of 3-DOM materials in reducing the demand of the expensive IrO2 electrocatalyst for OER in water electrolysis.

A non-precious metal electrocatalyst based on nitrogen-doped graphene (NG) was synthesized through a single step heat-treatment of a precursor mixture containing graphene oxide, urea, carbon black (CB) and small amount of iron species. The structure, morphology and composition of the prepared materials were characterized with a variety of techniques. XRD and Raman measurements showed the presence of distorted graphene layers. BET, TEM and cyclic voltammagram results indicated that CB served as spacer to prevent NG sheets from agglomerating, leading to enhanced dispersion of NG sheets. XPS analysis gave a total surface nitrogen concentration of ∼4 at.%, with the pyridinic nitrogen being the main component. Rotating electrode measurements revealed that the NG electrocatalyst can efficiently catalyze the oxygen reduction reaction (ORR), with activities equivalent to Pt/C in alkaline medium and approaching to Pt/C in acid medium, and with nearly 4-electron pathway selectivity.Highlights► N-doped graphene/carbon composite electrocatalyst synthesized through a single-step heat-treatment. ► Solid urea is more suitable than gaseous ammonia as nitrogen source for preparing N-doped graphene electrocatalyst. ► Carbon black as spacer and support inhibits the restacking of the graphene sheets. ► The ORR activity of the composite rival that of Pt/C in alkaline and approach that of Pt/C in acid.

By using a catalyst-lean thin-film RDE method, the fast kinetics of the hydrogen oxidation reaction (HOR) on highly dispersed Pt nanoparticle electrocatalysts can be determined, free from the interference of the mass transport of H2 molecules in solution. Measurements with carbon-supported Pt nanoparticles of different sizes thus allow revealing the particle size effect of Pt for the HOR. It is shown that there is a “negative” particle size effect of Pt on the kinetics of HOR, i.e., the exchange current density j0 decreases with the increased dispersion (i.e. decreased mean particle size). A maximum mass activity of Pt for the HOR is found at particle sizes of 3–3.5 nm. The observed particle size effect is interpreted in terms of the size dependent distribution of surface atoms on the facets and edges, which is implied by the voltammetric responses of Pt/C catalysts with differently sized Pt particles. The accompanied decrease in the HOR activity with the increase in the edge atom fraction suggests that the edge atoms on the surface of Pt nanoparticles are less active for the HOR than those on the facets.

Efficient electron transfer (ET) between microbes and electrodes is a key factor for electricity generation in microbial fuel cell (MFC). The utilization of reversible redox electron-mediator can enhance such extracellular ET but could result in environmental contamination and low cost-effectiveness. These limitations may be overcome by immobilizing electron-mediator molecules on electrode surface. In this paper, we present a stepwise amidation procedure to covalently immobilize neutral red (NR), which has been proved to be an appropriate mediator to harvest microbial metabolic electrons due to its excellent electrochemical reversibility and compatible redox potential to the major metabolic electron carriers (e.g., of NADH/NAD+), on carbon electrodes. In this procedure, immobilization of NR is realized by acylchlorination of the carboxylated carbon surface with thionyl chloride followed by amidation reaction with NR. It is shown that such a stepwise amidation procedure can significantly increase the amounts of NR molecules immobilized on carbon surface without altering their redox properties. In addition, the use of NR-immobilized carbon electrodes as MFC anode can significantly increase the power output and the utilization of carbon sources (organic fuel).

The commercial viability of polymer electrolyte membrane fuel cells in transportation applications will rely on significant reductions in the amount and great improvements in the stability of Pt in their cathodes to catalyze the oxygen reduction reaction (ORR). We demonstrated through first-principle theoretical calculations and experimental measurements that alloying Pt with 5d transition metal W could help to accomplish this aspiration. It was shown that the strong surface segregation tendency of Pt in Pt−W alloys and the peculiar electronic effect of W to Pt allow facile formation of stable Pt-enriched surfaces with likely better-than-Pt ORR activity at Pt-lean alloys such as PtW2, in contrast to the previously reported Pt alloy ORR electrocatalysts which mostly have to be Pt-rich in total composition (e.g., Pt3M). The prepared PtW2 alloy catalysts exhibited a Pt mass activity nearly 4 times higher than the pure Pt catalysts and almost no changes in the activity and surface area in over 30,000 cycles of potential cycling under oxidizing conditions of ORR, in contrast to significant losses seen with the pure Pt catalyst. In addition, the present study also showed a density functional theory-based multiple-descriptor strategy for screening durable and efficient bimetallic catalysts of low precious metal contents.

Microkinetic modeling and density functional theory (DFT) calculations are combined to understand the surface structure and nanoparticle size effects of Pt on the kinetics of hydrogen electrode reactions (HERs). The microkinetic modeling leads to a mechanism-free volcano relation between the exchange current density of HERs (j0) and the surface coverage of the reactive H adatoms at the equilibrium potential (θ0), making the activity trend of various catalytic surfaces for HERs predictable with θ0. It is shown that catalytic surfaces with θ0 values closer to 0.5 monolayer will have higher j0. A DFT calculation scheme is developed to determine the nature of the reactive H atoms and the corresponding θ0 values. The calculated results on Pt single crystal electrodes predict that j0 follows a trend that Pt(110) ≈ Pt(100) > Pt(111), whereas for Pt nanoparticles the j0 follows a trend that (100) facets > (111) facets ≈ edge rows, which in turn suggests a decrease of j0 with the decreasing particle size of Pt. It is shown that, although the individual edge atom rows on Pt fcc nanoparticles are similar in structures to the top atom rows on the Pt(110) surface, the catalytic properties of the nanoparticle edges are not simply equivalent to the extended (110) surfaces since some of the reactive sites for a reaction on extended (110) surfaces (e.g., the long-bridge sites) are absent at nanoparticle edges. The results presented here not only throw new insights into the long-standing problem about the Pt surface structure and particle size effects in hydrogen electrocatalysis but also provide a general theoretical scheme to identify the most active catalytic surfaces for HERs. More importantly, we demonstrate that not only the thermodynamic data like the adsorption energy but also a detailed nature of the reactive sites and their coverage are very important for the proper prediction of the activity trend of catalytic surfaces.

Core–shell nanoparticulate catalysts with a nonprecious metal core and a thin precious metal shell not only save precious metals but also could enhance the catalytic performance of precious metals through properly tuned strain and ligand effects. In this study, we show that, in addition to the nature and composition of core metals, the electrocatalytic properties of the precious metal shell can be tuned by varying its surface coverage. Carbon-supported Ni–Pt core–shell nanoparticle catalysts (Ni–Pt/C) with a series of Pt coverages are synthesized by depositing different amounts of Pt on Ni nanoparticles ca. 5 nm in size. It is found that Pt approximately forms an extended layer on Ni particles as its coverage does not exceed that required for a monolayer formation. The electrocatalytic properties of the Ni–Pt/C catalysts, such as the stripping potential for the oxygenated adsorbates, the activity for the oxygen reduction reaction (OPR), and the electrochemical stability under continuous potential cycling, exhibit a volcano type of dependence on Pt coverage, with the apex occurring near the monolayer. This suggests that core–shell nanoparticles with a monolayer Pt shell would be active and durable catalysts for the ORR and that extra Pt benefits neither the ORR activity nor the durability of the core–shell structured electrocatalysts.

The effect of the supporting electrolyte concentration on the interfacial profiles and voltammetric responses of nanometer-sized disk electrodes have been investigated theoretically by combining the Poisson–Nernst–Planck (PNP) theory and Butler–Volmer (BV) equation. The PNP-theory is used to treat the nonlinear couplings of electric field, concentration field and dielectric field at electrochemical interface without the electroneutrality assumption that has been long adopted in various voltammetric theories for macro/microelectrodes. The BV equation is modified by using the Frumkin correction to account for the effect of the diffuse double layer potential on interfacial electron-transfer (ET) rate and by including a distance-dependent ET probability in the expression of rate constant to describe the radial heterogeneity of the ET rate constant at nanometer-sized disk electrodes. The computed voltammetric responses for disk electrodes larger than 200 nm in radii in the absence of the excess of the supporting electrolyte using the present theoretical scheme show reasonable agreements with the predications of the conventional microelectrode voltammetric theory which uses the combined Nernst–Planck equation and electroneutrality equation to describe the mixed electromigration-diffusion mass transport without including the possible effects of the diffuse double layer (Amatore et al. [25]). For electrodes smaller than 200 nm, however, the voltammetric responses predicated by the present theory exhibit significant deviation from the microelectrode theory. It is shown that the deviations are mainly resulted from the overlap between the diffuse double layer and the concentration depletion layer (CDL) at nanoscale electrochemical interfaces in weakly supported media, which will result in the invalidation of the electroneutrality condition in CDL, and from the radial inhomogeneity of ET probability at nanometer-sized disk electrodes.

A density functional theory (DFT)-calculation scheme for constructing the modified embedded atom method (MEAM) potentials for face-centered cubic (fcc) metals is presented. The input quantities are carefully selected and a more reliable DFT approach for surface energy determination is introduced in the parameterization scheme, enabling MEAM to precisely predict the surface and nanoscale properties of metallic materials. Molecular dynamics simulations on Pt and Au crystals show that the parameterization employed leads to significantly improved accuracy of MEAM in calculating the surface and nanoscale properties, with the results agreeing well with both DFT calculations and experimental observations. The present study implies that rational DFT parameterization of MEAM may lead to a theoretical tool to bridge the gap between nanoscale theoretical simulations and DFT calculations.

The voltammetric responses of Pt disk electrodes 5−50 nm in radii in the presence of excess inert electrolyte were investigated to verify the applicability of the conventional diffusion-based voltammetric theory to nanoscale electrochemical interfaces. A so-called “inverted heat-sealing” procedure was introduced in the electrode fabrication process to eliminate the possible tiny interstice between the glass sheath and electrode wire that could severely distort the voltammetric curves of nanometer-szied electrodes. Linear relations between the limiting currents (iL) and the concentrations of electroactive ions (ca) were found at electrodes as small as 5 nm, seemingly inferring that the classic voltammetric theory is applicable at such small electrodes. However, a delicate analysis on the dependences of iL on the electroactive size of electrode and the charge carried by the electroactive ions revealed that the iL ∼ ca linearity is altered from the predication of the conventional voltammetric theory as the size of electrode approaches nanometer scales (e. g., <10 nm). The altered iL ∼ ca linearity at nanoelectrodes is explainable in terms of size-induced merging of electric double layer (EDL) and concentration depletion layer (CDL) and is well-predictable from the previous dynamic double layer model for nanoelectrode based on Poisson−Nernst−Planck theory (J. Phys. Chem. B 2006, 110, 3262). It is thus concluded that the enhanced EDL effects at nanoscale electrochemical interfaces do cause deviations from the predication of the conventional voltammetric theory, but the deviations are quantitatively small (e.g., within 20% even at electrodes of a few nanometers) and in most cases might be hardly distinguished with the experimental uncertainties.

The adsorption and dissociation of O2 on the Pt(111) surface in both the absence and the presence of the hydrated proton were investigated using ab initio DFT calculations to evaluate the role of the proton in the initial steps of the Pt-catalyzed oxygen reduction reaction (ORR) in acid solutions. The results from geometric optimization and electronic structure and minimum energy path calculations indicated that, although in both cases, a t-b-t configured chemisorption state serves as the most stable molecular precursor for the dissociation of O2, the formation of this precursor state and its dissociation are substantially altered in the presence of the hydrated proton. The interactions of O2 with the hydrated proton inhibit the formation of the t-b-t precursor state but facilitate its dissociation. In the presence of the hydrated proton, the t-b-t molecular chemisorption of O2 is preceded by a metastable end-on chemisorption state that is protonated while the t-b-t state itself is not protonated. That is, the chemisorption of O2 on Pt in acid solution may undergo a sequential protonation and deprotonation process. It is also shown that the transformation from the end-on state to the t-b-t state is nearly a nonactivated process with the reaction energy larger in amount than the activation energy required for the subsequent dissociation. The formation of the end-on state via a proton-coupled electron-transfer process is, therefore, identified as the rate-determining step in the adsorption and dissociation processes of O2 on the Pt(111) surface in acidic media. The present calculation results may provide a link between the long disputed Damjanovic’s view and the Yeager’s view on the mechanism of the initial steps in ORR.

Previous reports showed that Escherichia coli (E. coli) undergone an electrochemical activation process can catalyze glucose oxidation in microbial fuel cell (MFC) in the absence of extraneous mediators [T. Zhang, C.Z. Cui, S.L. Chen, X.P. Ai, H.X. Yang, P. Shen, Z.R. Peng, Chem. Commun. (2006) 2257; T. Zhang, Y.L. Zeng, S.L. Chen, X.P. Ai, H.X. Yang, Electrochem. Commun. 9 (2007) 349]. This paper investigates the electron transfer mechanism associated with the direct bioelectrocatalysis of electroactivated E. coli in MFC. It is shown that covering the electrode surface with a filter membrane with pore smaller than the size of E. coli cells hardly changes the fuel cell discharge behaviors and the voltammetric responses in the culture of electroactivated E. coli, indicating that the electron transfer between the electrode and E. coli cells is carried out by soluble compounds in the culture. The GC–MS results indicate that these compounds are excreted by E. coli in MFC, i.e., they are exploitable metabolites.

A mediatorless microbial fuel cell based on the direct biocatalysis of Escherichia coli shows significantly enhanced performance by using bacteria electrochemically-evolved in fuel cell environments through a natural selection process and a carbon/PTFE composite anode with an optimized PTFE content.

•The spontaneous Pt deposition and redox replacement of UPD Cu were combined.•Pt atomic layer was deposit on Au nanoparticles with high Pt coverage.•AuPt core-shell nanoparticles had high electrocatalytic activity for ORR.Core-shell structured AuPt nanoparticles are promising electrocatalysts for oxygen reduction reaction (ORR) in fuel cells due to the superior durability as well as high activity. We report the fabrication of AuPt core-shell nanoparticles by combining the spontaneous Pt deposition and redox replacement of underpotential-deposited (UPD) Cu. It is shown that redox replacement of UPD Cu, a commonly used approach to fabricate Pt monolayer catalysts, only can produce Pt atomic layer partially covering the Au nanoparticles, leading to insufficient Au utilization in ORR. By pre-modifying the Au nanoparticles with Pt through spontaneous deposition, which produces relatively uniformly dispersed Pt atoms on the Au nanoparticles at moderate and low Pt contents, the subsequent UPD Cu and redox replacement yield a Pt overlayer with a high coverage. The resulted core-shell nanoparticles show higher precious metal (Pt + Au) utilization than that obtained either through the single approach of the spontaneous Pt deposition or redox replacement of UPD Cu, or double redox replacement of UPD Cu.Download high-res image (371KB)Download full-size image

A Fe/N co-doped ternary nanocarbon hybrid, with uniform bamboo-like carbon nanotubes (CNTs) in situ grown on/between the single/few-layer graphene sheets interspaced by carbon nanosphere aggregates, was prepared through a one-pot heat treatment of a precursor mixture containing graphene oxide, Vulcan XC-72 carbon nanospheres, nitrogen rich melamine and small amounts of Fe ions. Physical characterization including electron microscopic images, N2 adsorption–desorption isotherms, pore size distribution, XPS, XRD, Mössbauer spectra, and EDX revealed that the 0-D/1-D/2-D ternary hybrid architecture not only offered an optimized morphology for high dispersion of each nanocarbon moiety, while the carbon nanosphere interspaced graphene sheets have provided a platform for efficient reaction between Fe ions and melamine molecules, resulting in uniform nucleation and growth of CNTs and formation of high density Fe–N coordination assemblies that have been believed to be the active centers for the oxygen reduction reaction (ORR) in carbon-based nonprecious metal electrocatalysts. In the absence of graphene oxides or carbon nanospheres, a similar heat treatment was found to result in large amounts of elemental Fe and Fe carbides and entangled CNTs with wide diameter distributions. As a result, the ternary Fe/N-doped nanocarbon hybrid exhibits ORR activity much higher than the Fe–N doped single or binary nanocarbon materials prepared under similar heat treatment conditions, and approaching that of the state-of-the-art carbon-supported platinum catalyst (Pt/C) in acidic media, as well as superior stability and methanol tolerance to Pt/C.

Three-dimensional ordered macroporous (3-DOM) IrO2 was synthesized by using the silica colloidal crystal template method and explored as electrocatalyst for oxygen evolution reaction (OER) in acidic medium. X-Ray diffraction (XRD) patterns indicate that the prepared 3-DOM IrO2 has a rutile structure. Images of scanning and transmission electron microscopies suggest that the 3-DOM IrO2 possesses a hierarchical pore structure, in which the honeycomb array of 300 nm primary macropores were cross-linked by secondary mesopores on the walls. The presence of mesopores is also indicated by the N2 adsorption/desorption isotherms and the small-angle XRD patterns. As compared with IrO2 prepared by conventional colloidal method, the 3-DOM IrO2 exhibited much larger BET area and voltammetric charges. Accordingly, about two and half times enhancement in OER activity was achieved by using 3-DOM IrO2 as the electrode materials, showing prospect of 3-DOM materials in reducing the demand of the expensive IrO2 electrocatalyst for OER in water electrolysis.

Cost-effective non-precious metal electrocatalysts for the oxygen reduction reaction (ORR) is the key for fuel cells to become a viable electricity generation technology. Metal macrocyclic compounds such as Fe/Co porphyrins and phthalocyanines are excellent molecular catalysts for O2 reduction; but they are still considerably less competitive than Pt-based materials when catalyzing the ORR in electrochemical environments. Using Fe phthalocyanine (FePc) as a model compound, we show that the electrocatalytic activity of metal macrocyclic compounds for the ORR can be greatly enhanced through tailoring assembling architectures on high-surface-area nanocarbons. By simply ball-milling FePc with nanocarbons, such as graphene nanosheets and carbon-black nanoparticles, molecular architectures of FePc from nanorods to uniform thin shells are obtained. The resulting carbon-supported FePc composites exhibit ORR performance much superior to the state-of-the-art carbon-supported Pt in alkaline solution, with up to a 60 mV positive shift in the half-wave potential and more than 5 times increase in the mass activity. As well as showing that the molecule–support interaction provides a degree of control on the molecular architectures of metal macrocyclic compounds, the present work reveals that the FePc molecule is intrinsically much more efficient than Pt in catalyzing the ORR in alkaline media, and therefore has great prospects as a cathode electrocatalyst in alkaline fuel cells.

Ternary Fe/N/C catalysts are regarded as the most promising candidates for low-cost alternatives to Pt for catalyzing the oxygen reduction reaction (ORR). High-temperature pyrolysis of N-containing carbon precursors in the presence of Fe salts has been a common and efficient route to prepare Fe/N/C catalysts. However, simple pyrolysis usually leads to a large weight loss of N-containing precursors, which not only increases the overall cost of the catalysts, but also results in the catalysts having a low density of active sites. We show herein that this problem can be effectively overcome by dispersing a highly hydrosoluble polymer precursor, namely, polyvinylpyrrolidone (PVP), in NaCl crystallites. The use of the hydrosoluble polymer precursor enables efficient dispersion and confinement of the precursor in NaCl crystallites in a simple solvent evaporation process. The subsequent pyrolysis process not only shows little weight loss, but also produces a mesoporous Fe/N/C ORR catalyst with a remarkably higher specific surface area (414.5 m2 g−1) and ORR activity (half-wave potentials, E1/2 = 0.793 V and 0.878 V vs. RHE, respectively, in acidic and alkaline media) than those of the material prepared without using NaCl confinement (72.9 m2 g−1, E1/2 = 0.634 V and 0.785 V, respectively, in acidic and alkaline media).

A simple Zn/Fe bimetallic zeolitic–imidazolite framework (ZIF) carbonization method is developed to synthesize an Fe–N–C hybrid with hierarchical nitrogen-doped porous carbons crossed by carbon nanotubes. Both the specific ratios of Zn/Fe in the bimetallic metal–organic framework (MOF) precursors and the selected annealing temperature are essential for the formation of this unique hybrid structure with good conductivity and exposure of more active sites. The resulting FeNC-20-1000 hybrid electrocatalyst exhibits excellent oxygen reduction reaction (ORR) activity, with a half-wave potential of 0.770 V comparable to that of the commercial Pt/C catalysts in acidic media, and a half-wave potential of 0.880 V, ca. 50 mV more positive than that of Pt/C for ORR in alkaline solution. More importantly, the as-prepared Fe–N–C hybrid exhibits much more stability for the ORR in both acidic and alkaline solutions than Pt/C, which makes it among the best non-noble-metal catalysts ever reported for ORR under acidic and alkaline conditions.

Heterogeneous electron-transfer (ET) processes at solid electrodes play key roles in molecular electronics and electrochemical energy conversion and sensing. Electrode nanosization and/or nanostructurization are among the major current strategies for performance promotion in these fields. Besides, nano-sized/structured electrodes offer great opportunities to characterize electrochemical structures and processes with high spatial and temporal resolution. This review presents recent insights into the nanoscopic size and structure effects of electrodes and electrode materials on heterogeneous ET kinetics, by emphasizing the importance of the electric double-layer (EDL) at the electrode/electrolyte interface and the electronic structure of electrode materials. It is shown, by general conceptual analysis and recent example demonstrations of representative electrode systems including electrodes of nanometer sizes and gaps and of nanomaterials such as sp2 hybridized nanocarbons and semiconductor quantum dots, how the heterogeneous ET kinetics, the electronic structures of electrodes, the EDL structures at the electrode/electrolyte interface and the nanoscopic electrode sizes and structures may be related.

Active and stable electrocatalysts based on earth-abundant elements for oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) are crucially important for the utilization of renewable energy. Herein, we reported the synthesis of Fe3C nanorod encapsulated, N-doped carbon nanotubes grown on N-doped porous carbon sheets (Fe3C@NCNT/NPC) by simply annealing a Fe-based MOF (MIL-88B) loaded with melamine at 800 °C in N2. Thanks to the synergistic effect of the high density of Fe–N active sites and electric conductance offered by the unique hybrid structure, the as-prepared Fe3C@NCNT/NPC hybrid exhibited a half-wave potential ca. 60 mV more positive and a durability performance much better than that of Pt/C for ORR, and an overpotential ca. 20 mV lower and a Tafel slope much smaller than that of IrO2 for OER, which make it one of the best reported nonprecious metal bifunctional electrocatalysts for oxygen electrode reactions.

Wavy palladium (Pd) nanorods were obtained by controlled synthesis by using amine–boranes as the reducing agents. Thanks to the unique structure and strong interaction with graphene, the as-synthesized Pd nanorods supported on graphene exhibit much enhanced electrocatalytic activity towards formic acid oxidation as compared with Pd nanoparticles.

Low cost alternatives to the expensive and scarce Pt to catalyze the oxygen reduction reaction (ORR) in acid media are essential for the proton-exchange-membrane (PEM) fuel cells to become economically viable. Chemically doped nanocarbons are among the most promising candidates in this regard. We report the facile synthesis and superior electrocatalytic activity of an Fe–N doped nanocarbon composite of carbon nanotubes (CNTs) grown on/between graphene sheets. The structure and composition of the composite is characterized by using a variety of techniques including SEM, TEM, N2 adsorption/desorption isotherms, XPS, XRD, and Mössbauer spectroscopy. It is shown that the in situ growth of CNTs in the presence of graphene sheets not only produces a tubes-on/between-sheets architecture that enhances the dispersion of CNTs and graphene sheets, but also leads to optimized doping and coordination of nitrogen and Fe which favour the ORR. The composite can catalyze the ORR much more efficiently than either of the single materials containing only CNTs or graphene synthesized under similar conditions, and similarly to Pt/C in both alkaline and acid media.

Electrodes of nanometer sizes provide a model approach to study the nanoscale electrochemical properties and processes, which are of fundamental and applied significance in a variety of areas including energy and environmental science, scanning probe microscopies, nanofabrication as well as electrochemistry itself. This Perspective reviews recent developments in conceptual understanding, theoretical modelling and simulation, and experimental observation of nanosize-induced properties and phenomena at interfaces between nanometer-sized electrodes and electrolytes. The aim is to provide a view on how the dimension comparability of nanoelectrodes with the electric double layer and the effective electron-tunnelling distance may raise distinct features in interfacial structure and reactivity. The strong coupling between the electrostatic field, the concentration field and the dielectric field of solvent at nanoelectrode/electrolyte interfaces is highlighted. The effects of this coupling on the voltammetric responses of nanoelectrodes are evaluated. Electron transfer kinetics at the nanoelectrode/electrolyte interface is discussed by emphasizing the inappropriateness of the Butler–Volmer (BV) and classic Marcus–Hush (MH) theories at potentials largely departing from the formal potential of the redox moieties and the importance of the long-distance electron tunnelling. The conditions for using the mathematically more straightforward BV and classic MH formalisms as an alternative to the physically more realistic but mathematically unfriendly Marcus–Hush–Chidsey model are analysed.

By using a catalyst-lean thin-film RDE method, the fast kinetics of the hydrogen oxidation reaction (HOR) on highly dispersed Pt nanoparticle electrocatalysts can be determined, free from the interference of the mass transport of H2 molecules in solution. Measurements with carbon-supported Pt nanoparticles of different sizes thus allow revealing the particle size effect of Pt for the HOR. It is shown that there is a “negative” particle size effect of Pt on the kinetics of HOR, i.e., the exchange current density j0 decreases with the increased dispersion (i.e. decreased mean particle size). A maximum mass activity of Pt for the HOR is found at particle sizes of 3–3.5 nm. The observed particle size effect is interpreted in terms of the size dependent distribution of surface atoms on the facets and edges, which is implied by the voltammetric responses of Pt/C catalysts with differently sized Pt particles. The accompanied decrease in the HOR activity with the increase in the edge atom fraction suggests that the edge atoms on the surface of Pt nanoparticles are less active for the HOR than those on the facets.