Electrocatalytic Current Density Research Articles

(Invited) Development of Multifunctional Nanoscale Reactors for Electrocatalytic Oxidation of Dimethyl Ether Fuel

There has been growing interest in utilizing small (simple) organic molecules, as alternative fuels to hydrogen, for electrochemical energy conversion in fuel cells. Dimethyl ether (DME) is one of the possible choices in this respect. But electrooxidation of DME is rather slow at ambient conditions. Obviously, there is a need to develop novel electrocatalytic materials.Bimetallic PtSn catalysts were found to be almost as active as PtRu for methanol oxidation, and PtSn was even more efficient toward the electrooxidation of ethanol where breaking of C-C bond is required. While PtRu forms a single-phase homogeneous (intermetallic) alloy in which Ru is largely metallic, tin does not maintain its metallic state in PtSn (heterogenous alloy) and is transformed readily to such oxygenated species as Sn oxides or hydroxides. During operation of PtSn, the Pt component retains its mostly metallic character and act independently as Pt0 catalyst. By analogy to Ru, the Sn “bifunctional agent” tends to activate the water dissociative adsorption and increases population of chemisorbed hydroxyl groups to diminish the Pt-poisoning effect through enhanced oxidation of CO-type intermediates. Nevertheless, the DME oxidation on both PtRu or PtSn is still kinetically less favorable, relative to the systems’ performance toward methanol. While Sn, and particularly Ru alloyed atoms activate readily the water dissociative adsorption on Pt and permit the oxidative stripping of the poisoning CO-type adsorbates, the feasibility of the enhancement of the of the DME adsorption and activation through alteration of the electronic structure of Pt catalytic sites has not been fully explained. By analogy to the ethanol oxidation, Pt sites in the heterogeneous PtSn alloy are active toward dissociation of the DME molecule but the Sn additive is less efficient than Ru (from intermetallic PtRu) in the oxidative removal of poisoning adsorbates. But PtRu is probably the most active during oxidation of methanol, and this small organic molecule is also the DME-oxidation-reaction intermediate. Nevertheless, in comparison to bare Pt, electrooxidation of DME starts at PtRu and PtSn at less positive potentials.We pursue a concept of utilization of multicomponent hybrid electrocatalytic systems or nanoreactors utilizing zirconia oxide matrices for supporting and activation of primarily PtSn nanoparticles supported onto Vulcan XC-72 carbon black carriers. Electrocatalytic activity of Vulcan-supported PtSn (PtSn/V) nanostructured alloys supported onto ZrO2 matrices has been significantly enhanced toward electrooxidation of DME in acid medium (0.5 mol dm-3 H2SO4) by decorating PtSn/V with bimetallic PtRu nanoparticles. The enhancement effect concerns both shifting the onset potential for the DME-oxidation toward less positive values and increase of the DME electrocatalytic current densities recorded under both cyclic voltammetric and chronoamperometric conditions. The activating capabilities of ruthenium nanostructures seem to originate from the existence (even below 0.45 V vs. RHE) of reactive ruthenium oxo/ hydroxo groups on their surfaces capable of inducing the oxidative removal of poisoning (CO-type) adsorbates from the neighboring platinum catalytic sites. In this respect, the Ru-oxo species seem to support activity of Sn forming with Pt the PtSn heterogenous alloy. The PtSn/V admixed with PtRu decorated ZrO2 exhibit very high activity toward the oxidation of methanol which is also an important DME-oxidation intermediate. On the whole, the hybrid materials composed of Vulcan-supported PtSn decorated with Ru or PtRu nanoparticles decorated zirconia oxide seem to act as multifunctional nanoreactors inducing not only stripping of poisoning adsorbates but also catalyzing oxidation of the DME-reaction intermediates (methanol). Acknowledgement Authors acknowledge financial support of the National Science Center (Poland) under Opus Project 2018/29/B/ST5/02627.

Coordination of copper within a crystalline carbon nitride and its catalytic reduction of CO2.

Inherently disordered structures of carbon nitrides have hindered an atomic level tunability and understanding of their catalytic reactivity. Starting from a crystalline carbon nitride, poly(triazine imide) or PTI/LiCl, the coordination of copper cations to its intralayer N-triazine groups was investigated using molten salt reactions. The reaction of PTI/LiCl within CuCl or eutectic KCl/CuCl2 molten salt mixtures at 280 to 450 °C could be used to yield three partially disordered and ordered structures, wherein the Cu cations are found to coordinate within the intralayer cavities. Local structural differences and the copper content, i.e., whether full or partial occupancy of the intralayer cavity occurs, were found to be dependent on the reaction temperature and Cu-containing salt. Crystallites of Cu-coordinated PTI were also found to electrophoretically deposit from aqueous particle suspensions onto either graphite or FTO electrodes. As a result, electrocatalytic current densities for the reduction of CO2 and H2O reached as high as ∼10 to 50 mA cm-2, and remained stable for >2 days. Selectivity for the reduction of CO2 to CO vs. H2 increases for thinner crystals as well as for when two Cu cations coordinate within the intralayer cavities of PTI. Mechanistic calculations have also revealed the electrocatalytic activity for CO2 reduction requires a smaller thermodynamic driving force with two neighboring Cu atoms per cavity as compared to a single Cu atom. These results thus establish a useful synthetic pathway to metal-coordination in a crystalline carbon nitride and show great potential for mediating stable CO2 reduction at sizable current densities.

Direct Electrocatalytic Methanol Oxidation on MoO3/Ni(OH)2: Exploiting Synergetic Effect of Adjacent Mo and Ni.

Ni-based materials have been widely investigated as methanol oxidation reaction (MOR) catalysts. The formation of NiOOH and its reduction to Ni(OH)2 are generally regarded as essential steps for methanol oxidation. However, in such an indirect route, the efficiency of proton coupled electron transfer is fundamentally limited by the rate of transition from Ni(OH)2 to NiOOH back and forth. Herein we demonstrate a direct MOR pathway on MoO3/Ni(OH)2 without the formation of a NiOOH mediator. The MoO3/Ni(OH)2 exhibits a benchmark electrocatalytic MOR current density of 1000 mA cm-2 at 1.52 V vs. RHE with a nearly 100% faradic efficiency, outperforming all the state of art MOR electrocatalysts. In-situ Raman spectroscopy confirms that NiOOH is not formed during the electrocatalytic MOR process on the MoO3/Ni(OH)2. Density functional theory calculations suggest that Ni2+ in MoO3/Ni(OH)2 serves as the methanol adsorption site while the doped Mo6+ plays a key role in capturing the deprotonated H·. Benefiting from the Mo-Ni synergistic effect, the energy barrier of the CH2O* → CHO* + H* process is significantly reduced, avoiding the NiOOH formation and leading to the direct MOR. Our research unravels a direct electrochemical MOR pathway that does not rely on NiOOH formation and provides a facile strategy of regulating the intermediate process barrier for MOR.

Design of Carbon Dioxide Reduction Electrolysis Stacks and Trial Production of Electrolytic Cell Prototype

Currently, the electrolytic conversion of carbon dioxide (CO2) is a highly regarded and dynamic research field worldwide. The investigation and application of high-efficiency catalysts and ion-exchange membranes have emerged as prominent and sought-after research directions. This paper, in particular, centers on the design, trial production, and testing of carbon dioxide reduction electrolysis stacks. To optimize the stack performance, different plate materials were evaluated for their mechanical strength and corrosion resistance, resulting in the selection of aluminum alloy and 316L stainless steel. Furthermore, the choice of ionic membrane and carbon paper was based on Faraday efficiency and stability data, leading to the selection of DuPont Nafion anionic membrane and YLS carbon paper. Through the trial production of small electrolytic cells, the overall current density, Faraday efficiency, and durability of the chosen cell assemblies were validated. Subsequently, a KW-grade electrolysis stack was successfully manufactured, tested, and fine-tuned, achieving an impressive electrocatalytic current density of approximately 200 mA/cm2 and an impressive stability of around 400 hours.

Hybrid Electrocatalysts Composed of PtSn, Ru or PtRu Nanoparticles for Low-Temperature Oxidation of Dimethyl Ether Fuel

The science and technology of proton-exchange membrane (PEM) low-temperature fuel cells is a subject of intense during recent years, particularly with use of hydrogen gas due to its molecular simplicity and fast kinetics of its electrooxidation. However, there are still many fundamental problems to be resolved with respect to generation of pure hydrogen and related pollution side effects, the low density and safety of hydrogen storage and distribution. Thus, there are parallel studies involving such simple organic molecules as methanol, formic acid or ethanol as substitute fuels for a wide range of potential applications. Among important obstacles are slow kinetics of electrocatalytic oxidations of fuels and insufficient durability of operation of catalysts that includes poisoning of active sites by strong adsorption of the reaction intermediate products (e.g. CO-type adsorbates), in addition the fuel cross-over through the membrane, which depolarizes the cathode and decreases its activity. While methanol, which has been the most extensively studied, faces the problem of toxicity and challenges mentioned above, the low electrocatalytic efficiency of the formic acid oxidation (only 2-electron-process) is complicated by corrosion phenomena. When it comes to the electrooxidation of ethanol, extra problems concern difficulty of the strong C-C bond breaking and excessive formation of undesirable intermediate products, acetaldehyde and acetic acid. Dimethyl ether (DME), or methoxymethane having formula CH3OCH3, is the simplest ether, and it can be considered as an isomer of another small organic molecule, ethanol, but with a different functional group. DME is a colorless gas that has been demonstrated in a variety of fuel applications, e.g. as an aerosol propellant or substitute for propane in LPG. DME can be viewed as the synthetic second generation biofuel, which can be produced from lignocellulosic biomass biogas or methane from agricultural waste. With respect to potential applications in low-temperature fuel cells, DME could be an attractive simple fuel due to its high energy density, low toxicity, liquefied easy storage, as well as the molecule’s low dipole moment that should diminish crossover through Nafion membrane, when compared to methanol, ethanol and formic acid. The stoichiometric complete electrochemical oxidation of DME requires three H2O-molecules, produces two CO2-molecules, and yields as many as twelve electrons. Obviously the multi-electron and multi-proton electrode process are complex and composed of distinct and competing reaction pathways and, thus, they are subject to kinetic limitations. The main constraints related to application of common Pt-based catalysts (e.g. Pt and bimetallic PtRu) in direct DME fuel cells concern the system’s low practical electrooxidation efficiency.In the present study, electrocatalytic activity of Vulcan-supported PtSn (PtSn/V) nanostructured alloys toward electrooxidation of dimethyl ether (DME) has been significantly enhanced in acid medium (0.5 mol dm-3 H2SO4) by intentional and controlled decoration of PtSn/V with ruthenium black or bimetallic PtRu nanoparticles. The enhancement effect concerns both shifting the onset potential for the DME-oxidation toward less positive values and increase of the DME electrocatalytic current densities recorded under both cyclic voltammetric and chronoamperometric conditions. The activating capabilities of ruthenium nanostructures seem to originate from the existence of reactive ruthenium oxo/hydroxo groups (even below 0.45 V vs. RHE) on their surfaces capable of inducing the oxidative removal of poisoning (CO-type) adsorbates from the neighboring platinum catalytic sites. In that respect, the Ru-oxo species originating from Ru or PtRu additives seem to support activity of Sn co-catalytic sites interacting with Pt within the PtSn heterogenous alloy. The Ru-decorated PtSn/V and, in particular, PtSn/V admixed with PtRu exhibit very high activity toward the oxidation of methanol which is also an important DME-oxidation intermediate. On the whole, the hybrid materials composed of Vulcan-supported PtSn decorated with Ru or PtRu nanoparticles seem to act as multifunctional nanoreactors inducing not only stripping of poisoning adsorbates but also catalyzing oxidation of the DME-reaction intermediates (methanol). Despite possible differences in the DME-electrooxidation mechanisms at the hybrid and single component systems, the dissociation of H3C-O-CH3 molecule, which involves C-O bond breaking, formation of the methanol reaction intermediate and CO-type poisoning adsorbates should be operative.

Hybrid Electrocatalysts Composed of PtSn, Ru or PtRu Nanoparticles for Low-Temperature Oxidation of Dimethyl Ether Fuel

Electrocatalytic activity of Vulcan-supported PtSn (PtSn/C) nanostructured alloys toward electrooxidation of dimethyl ether (DME), a potential small-organic-molecule fuel, has been significantly enhanced in acid medium (0.5 mol dm-3 H2SO4) by decorating PtSn/C with bimetallic PtRu nanoparticles modified with phosphododecatungstic acid (PtRu/PW12). The enhancement effect concerns both shifting the onset potential for the DME-oxidation toward less positive values and increase of the DME electrocatalytic current densities recorded under both cyclic voltammetric and chronoamperometric conditions. In that respect, the Ru-oxo species originating from PtRu additives seem to support activity of Sn co-catalytic sites interacting with Pt within the PtSn heterogenous alloy. The polytungstate adsorbates on nanoparticles facilitate their distribution at the electrocatalytic interface, as well as they interact competitively with the CO adsorbates at noble metal catalytic sites. The hybrid materials composed of PtSn/C decorated with PtRu nanoparticles modified with PW12 seem to act as multifunctional nanoreactors during electrooxidation of DME.

Enhancement of oxidation of dimethyl ether through formation of hybrid electrocatalysts composed of Vulcan-supported PtSn decorated with Ru-black or PtRu nanoparticles

Electrocatalytic activity of Vulcan-supported PtSn (PtSn/C) nanostructured alloys toward electrooxidation of dimethyl ether (DME), a potential small-organic-molecule fuel, has been significantly enhanced in acid medium (0.5 mol dm−3 H2SO4) by decorating PtSn/C with ruthenium black or bimetallic PtRu nanoparticles. The enhancement effect concerns both shifting the onset potential for the DME-oxidation toward less positive values and increase of the DME electrocatalytic current densities recorded under both cyclic voltammetric and chronoamperometric conditions. The activating capabilities of ruthenium nanostructures seem to originate from the existence of reactive ruthenium oxo/hydroxo groups (even below 0.45 V vs. RHE) on their surfaces capable of inducing the oxidative removal of poisoning (CO-type) adsorbates from the neighboring platinum catalytic sites. In that respect, the Ru-oxo species originating from Ru or PtRu additives seem to support activity of Sn cocatalytic sites interacting with Pt within the PtSn heterogenous alloy. The Ru-decorated PtSn/C and, in particular, PtSn/C admixed with PtRu exhibit very high activity toward the oxidation of methanol which is also an important DME-oxidation intermediate. On the whole, the hybrid materials composed of Vulcan-supported PtSn decorated with Ru or PtRu nanoparticles seem to act as multifunctional nanoreactors inducing not only oxidative stripping of poisoning adsorbates but also catalyzing oxidation of the DME-reaction intermediates (methanol).

Efficient Electrochemical Water Oxidation Mediated by Pyridylpyrrole-Carboxylate Ruthenium Complexes.

Spurred by the rapid growth of Ru-based complexes as molecular water oxidation catalysts (WOCs), we propose novel ruthenium(II) complexes bearing pyridylpyrrole-carboxylate (H2ppc) ligands as members of the WOC family. The structure of these complexes has 4-picoline (pic)/dimethyl sulfoxide (DMSO) in [Ru(ppc)(pic)2(dmso)] and pic/pic in [Ru(ppc)(pic)3] as axial ligands. Another ppc2- ligand and one pic ligand are located at the equatorial positions. [Ru(ppc)(pic)2(dmso)] behaves as a WOC as determined by electrochemical measurement and has an ultrahigh electrocatalytic current density of 8.17 mA cm-2 at 1.55 V (vs NHE) with a low onset potential of 0.352 V (vs NHE), a turnover number of 241, a turnover frequency of 203.39 s-1, and kcat of 16.34 s-1 under neutral conditions. The H2O/pic exchange of the complexes accompanied by oxidation of a ruthenium center is the initial step in the catalytic cycle. The cyclic voltametric measurements of [Ru(ppc)(pic)2(dmso)] at various scan rates, Pourbaix diagrams (plots of E vs pH), and kinetic studies suggested a water nucleophilic attack mechanism. HPO42- in a phosphate buffer solution is invoked in water oxidation as the proton acceptor.

Online Electrochemistry–Mass Spectrometry Evaluation of the Acidic Oxygen Evolution Reaction at Supported Catalysts

Electrocatalytic current density and especially onset potential or overpotential, as the key parameters for evaluating electrocatalytic performance, are problematic metrics when electron-consuming side processes can take place. This includes the widely studied electrocatalytic oxygen evolution reaction (OER), which is often accompanied by anodic currents associated with the activation or degradation of the catalyst and/or support. Herein, we use an online chip-based electrochemistry–mass spectrometry (chip EC-MS) system to decouple the OER from the complex electrochemical reactions for a graphene-supported ruthenium (Ru) catalyst with high OER activity in an acidic medium. In this manner, we can quantitatively evaluate the current contribution from the OER during cyclic voltammetry (CV) tests and chronopotentiometry (CP) tests and are thus enabled to realize the accurate measurement of the onset potential and Faradaic efficiency (FE) toward the OER. The OER process, including the activation, steady-state, and degradation processes, as well as the side reaction, is also studied using such a chip EC-MS technique. This work is of practical importance to clarify some critical issues remaining in the study of OER electrocatalysis.

Development of a new biocathode for a single enzyme biofuel cell fuelled by glucose

In this study, we reported the development of Prussian blue (PB), poly(pyrrole-2-carboxylic acid) (PPCA), and glucose oxidase (GOx) biocomposite modified graphite rod (GR) electrode as a potential biocathode for single enzyme biofuel cell fuelled by glucose. In order to design the biocathode, the GR electrode was coated with a composite of PB particles embedded in the PPCA shell and an additional layer of PPCA by cyclic voltammetry. Meanwhile, GOx molecules were covalently attached to the carboxyl groups of PPCA by an amide bond. The optimal conditions for the biocathode preparation were elaborated experimentally. After optimization, the developed biocathode showed excellent electrocatalytic activity toward the reduction of H2O2 formed during GOx catalyzed glucose oxidation at a low potential of 0.1 V vs Ag/AgCl, as well as good electrochemical performance. An electrocatalytic current density of 31.68 ± 2.70 μA/cm2 and open-circuit potential (OCP) of 293.34 ± 15.70 mV in O2-saturated 10 mM glucose solution at pH 6.0 were recorded. A maximal OCP of 430.15 ± 15.10 mV was recorded at 98.86 mM of glucose. In addition, the biocathode showed good operational stability, maintaining 95.53 ± 0.15% of the initial response after 14 days. These results suggest that this simply designed biocathode can be applied to the construction of a glucose-powered single enzyme biofuel cell.

The Effect of pH and Anion Adsorption on Formic Acid Oxidation on Au(111) Electrodes

The impact of solution pH has been investigated for the electrooxidation of formic acid on Au(111), taking the mole fraction of formate in solution as the relevant parameter. The electrocatalytic current density is increasing with rising pH and reaches a plateau for pH > 4. The pH-dependence of characteristic potentials shows systematic trends for the onset of the reaction, the peak maximum, and the phase transition within the formate adlayer. After the addition of phosphate buffer, a substantial deactivation by specific adsorption of phosphates and a bell-shaped curve for the maximum current density as a function of pH is found. The general behavior of co-adsorbing anions is addressed at low pH using various oxyanions to get more insight into the role of adsorbed formate. While non-carboxylate anions are displaced by formate at high potentials, mixed phases are observed for formate and other carboxylates.

Recent Progress of Two‐Dimensional Metal‐Organic Frameworks and Their Derivatives for Oxygen Evolution Electrocatalysis

AbstractThe development of high‐performance transition metal−based electrocatalysts for the oxygen evolution reaction (OER) is paramount for electrochemical secondary metal‐air batteries and water splitting. Two‐dimensional (2D) metal‐organic framework (MOF) nanosheets have recently emerged as a novel class of efficient electrocatalysts for OER, due to a high specific surface area, ultrasmall thickness and abundance of active sites. Here, we summarize recent progress in the preparation and characterization of 2D transition metal−based MOFs (e. g. Ni, Co and Cu), as well as their composites and derivatives, including metals/alloys, metal oxides/chalcogenides and metal phosphides/hydroxides, as OER electrocatalysts reported over the past several years. Design principles and preparation routes are presented. The performance of the electrocatalysts is directly compared in terms of overpotential (η) (thermodynamics) and electrocatalytic current density (kinetics), as well as operational stability (economics). Many of these materials exhibit superior catalytic activity compared to the noble metal−based catalysts such as RuO2 (η<300 mV at 10 mA cm−2), and considerable stability with negligible decay with operation over several hours to days. Challenges and perspectives of applying 2D MOFs and their derivatives for OER electrocatalysis are also discussed.

Enhancement of Oxidation of Formic Acid through Application of Zirconia Matrix for Immobilization of Noble Metal Catalytic Nanoparticles

Electrocatalytic activity of common noble metal (platinum, bimetallic platinum-ruthenium and palladium) nanoparticles (both unsupported and supported on Vulcan carriers) toward electrooxidation of formic acid in acid medium (0.5 mol dm–3 H2SO4) can be significantly enhanced by dispersing them over thin film of zirconia (ZrO2). The enhancement effects concern increases of the electrocatalytic current densities (normalized against the mass of noble metal catalysts) recorded under both cyclic voltammetric and chronoamperometric conditions. The observations can be rationalized in terms of the ability of zirconia nanostructures to provide active hydroxyl groups capable of inducing the removal of the inhibiting COOH or CO type adsorbates from their surfaces. The role of Vulcan carriers is in improving electronic conductivity, in addition to facilitating distribution of catalytic metal sites at the electrocatalytic interface. Regarding dissimilar mechanisms for HCOOH oxidations at Pt, PtRu and Pd nanoparticles, the activating capabilities of zirconia differ depending on the potential applied thus reflecting distinct nature of electronic or chemical interactions. Specific interactions between the zirconia support and noble metal nanoparticles are also possible, particularly in the case of Pd-based catalysts where the enhancement effects are the most pronounced. Finally, it should be emphasized that, historically, representatives of Russian school of electrochemistry, including V.S. Bagotsky, were one of the first world-wide who investigated the formic acid oxidation and emphasized the importance of metal—support interactions during electrooxidation of small organic molecules at noble metal nanoparticles.

Nitrogen dopants in nickel nanoparticles embedded carbon nanotubes promote overall urea oxidation

Urea electro-oxidation is an attractive and alternative anodic reaction in the electrochemical generation of hydrogen using wastewater ascribing to the low theoretical voltage and non-precious metal (nickel) catalyst for urea oxidation reaction (UOR); however, the sluggish UOR and poisoning of catalyst impede the practical application. Here, in this work, we synthesize a series of nickel nanoparticles embedded nitrogen doped carbon nanotubes (Ni@NCNT) and study the effect of nitrogen dopants on UOR catalytic activity. The nitrogen dopants can weaken the binding strength between CO2 species and active sites resulting in alleviation of CO2 poisoning; simultaneously, nitrogen dopants also promote the in-situ conversion of Ni3+ species facilitating UOR catalysis; as a result, electrocatalytic current density of 45.8 mA cm−2 is recorded for Ni@NCNT in 1 M KOH electrolyte with 0.5 M urea at 1.5 V vs. RHE, which is 3.8 fold better than commercial Pt/C (11.8 mA cm−2). Moreover, Ni@NCNT, due to the more nitrogen dopants, exhibits a comparable overpotential to commercial Pt/C for driving hydrogen evolution reaction (HER) catalysis in 1 M KOH electrolyte at high current density (400 mA cm−2). Subsequently, 1.56 V is demanded for overall UOR catalysis on Ni@NCNT with current density of 10 mA cm−2. This work offers useful information for designing a stable and efficient electrocatalyst for not only UOR but also electrochemical generation of H2 from wastewater.

Enhancement of oxidation of dimethyl ether through application of zirconia matrix for immobilization of noble metal catalytic nanoparticles

Electrocatalytic activity of Pt and bimetallic PtRu nanoparticles (both Vulcan supported and unsupported) toward electrooxidation of dimethyl ether (DME), a potential small organic molecule fuel, in an acid medium (0.5 mol dm−3 H2SO4) has been significantly enhanced by dispersing them over a thin film of zirconia (ZrO2). The enhancement effects concern increases of the DME electrocatalytic current densities recorded under both cyclic voltammetric and chronoamperometric conditions. Similar effects have been observed for the oxidation of methanol. Regarding the dissimilar DME electrooxidation mechanisms at Pt and PtRu catalytic centers, the activating capabilities of zirconia seem to originate from the high population of reactive –OH groups favoring mobility of protons and the capability of inducing the oxidative removal of poisoning (CO-type) intermediates both at platinum and ruthenium catalytic sites. In the presence of the zirconia matrix, the onset potential for the oxidation of DME (particularly at PtRu) is shifted more than 50 mV toward less positive potentials. Mutual metal-support interactions are also postulated.

Flower-like CoNi2S4/Ni3S2 nanosheet clusters on nickel foam as bifunctional electrocatalyst for overall water splitting

Efficient electrocatalysts based on earth-abundant elements are urgently required for promoting the efficiency of electrochemical water splitting. In this work, heterostructures of CoNi2S4/Ni3S2 on nickel foam (CoNi2S4/Ni3S2@NF) are synthesized by a facile one-step hydrothermal method as promising bifunctional electrocatalysts. To achieve an electrocatalytic current density of 10 mA cm−2 in 1 M KOH solution, the CoNi2S4/Ni3S2@NF electrode requires overpotential as low as 171 mV and 243 mV for HER and OER, respectively. Furthermore, the overall water splitting device with CoNi2S4/Ni3S2@NF electrode as both cathode and anode requires 1.65 V to offer current density of 10 mA cm−2. Density functional theory calculations reveal that the strong coupling interactions between CoNi2S4 and Ni3S2 synergistically optimized the electronic structure, thus enhancing the electrochemical water splitting performances. Surfaces oxidation of CoNi2S4/Ni3S2@NF electrode is considered to hinder the electrochemical activities for OER. The present work not only provides a promising bifunctional electrocatalyst for water splitting but also expands the applications of CoNi2S4 based materials.

(Invited) Chimeric and Single-Site Mutant Enzymes for the Design of Nanostructured Bioelectrodes in Enzymatic Fuel Cells and Sensors

Control over redox enzyme immobilization and electrical interfacing at electrode is a major challenge when building an electrode for enzymatic sensor or fuel cell. An adequate environment has to be created in order to maximize the number of active enzymes and minimize electron transfer distances between enzyme active sites and conductive surfaces or redox partners. In this respect, the production of specific mutants is a powerful way to introduce a level of control over orientation and organization of redox enzymes at the electrode. In addition, the combination of these enzymes with nanomaterials is aimed at maximizing the number of wired enzymes per surface unit while also providing rapid electron transfer pathways and enhancing electrocatalytic current densities.1 A chimeric protein can be designed by combining a prion domain and a small iron-sulfur redox protein such as rubredoxin.2 Thanks to the ability of the prion domain to self-assemble in amyloid fibers, protein-only redox nanowires can be produced. These protein nanowires are able to entrap redox enzymes such as multicopper enzymes or [NiFe] hydrogenases, while ensuring interprotein electron transport between enzyme active sites and electrode surface.2,3 Chimeric laccases was also designed by combining this multicopper enzyme with a hydrophobin domain. In this case, hydrophobin strongly interacts with the pi-extended network of chemically-exfoliated graphene or carbon nanotubes. These biofunctionalized nanomaterials were used in electrochemical sensors for polyaromatic and phenolic compounds.4 Single-site mutation at the surface of enzymes is also a powerful means to control the immobilization and orientation of enzymes at the surface of nanostructured electrodes. We have especially investigated the covalent and non-covalent functionalization of single-site mutant laccases for the rational immobilization and direct wiring of the enzymes at the surface of carbon nanotubes and gold nanoparticles. 5 These nanomaterials are able to provide novel self-assembled nanostructured redox bioassembly or favor heterogeneous electron transfer to the enzyme by promoting specific orientations.1 A. Le Goff and M. Holzinger, Sustainable Energy Fuels, 2018, 2, 2555–2566.2 L. Altamura, C. Horvath, S. Rengaraj, A. Rongier, K. Elouarzaki, C. Gondran, A. L. B. Maçon, C. Vendrely, V. Bouchiat, M. Fontecave, D. Mariolle, P. Rannou, A. Le Goff, N. Duraffourg, M. Holzinger and V. Forge, Nat Chem, 2017, 9, 157–163.3 S. Rengaraj, R. Haddad, E. Lojou, N. Duraffourg, M. Holzinger, A. Le Goff and V. Forge, Angew. Chem. Int. Ed., 2017, 56, 7774–7778.4 I. Sorrentino, I. Stanzione, Y. Nedellec, A. Piscitelli, P. Giardina and A. Le Goff, submitted.5 N. Lalaoui, P. Rousselot-Pailley, V. Robert, Y. Mekmouche, R. Villalonga, M. Holzinger, S. Cosnier, T. Tron and A. Le Goff, ACS Catal., 2016, 6, 1894–1900.

Silver sulfide nanoparticles incorporated into graphene oxide: an efficient electrocatalyst for the oxygen reduction reaction

We report a facile one-step sonochemical synthesis of silver sulfide incorporated into reduced graphene oxide (Ag2S-RGO) for the oxygen reduction reaction (ORR). The Ag2S-RGO nanocomposite was characterized by field emission scanning electron microscopy, high-resolution transmission electron microscopy, energy dispersive X-ray spectroscopy with mapping, and X-ray photoelectron spectroscopy (XPS). To evaluate the electrochemical performance, the surface of a glassy carbon electrode (GCE) was modified with the Ag2S-RGO nanocomposite for the ORR. The Ag2S-RGO nanocomposite-modified GCE showed higher electrocatalytic performance toward the ORR in terms of the electrocatalytic current density and the shift in overpotential when compared with those of RGO and a commercial Pt/C catalyst. The maximum current density was found to be 0.554 mA cm−2 with an overpotential of − 0.39 V. Additionally, the proposed material showed excellent methanol tolerance, high stability, and good reproducibility and selectivity toward the reduction of O2.

Surface Engineering of Porphyrin Coordination on a Carbon Nanotube for Efficient Hydrogen Evolution

AbstractFunctionalised porphyrins have attracted considerable attention owing to their ability to coordinate with numerous metal ions. We design and synthesize a novel composite system based on functionalised Pt(II)‐porphyrin and multiwalled carbon nanotubes for hydrogen evolution reaction (HER) electrocatalysis. Rather than qualitatively integrating porphyrin with carbon nanotubes, the optimized Pt(II)‐porphyrin loading and precisely morphology control could be fine‐tuned and quantitatively premeditated by using fluorescence and UV‐vis titration based on the unique π–π interactions behaviour of the porphyrin for the first time. The as‐prepared composites exhibit superior HER catalytic activity with an overpotential of 35 mV for an electrocatalytic current density of j=−10 mA cm−2 and a Tafel slope of 32.3 mV dec−1. Moreover, such catalysts do not exhibit observable activity decay after a 1000‐cycle stability test, thus demonstrating their excellent durability. To further reduce Pt loading to achieve cost efficiency, we develop a Pt−Co bimetallic HER catalyst with an ultra‐low platinum content through a straightforward synthesis pathway with high catalytic activity and stability. This study provides a new approach for preparing a highly active and stable HER catalyst, which could potentially be extended to other reactions to improve the utilization efficiency of precious metal catalysts.

Exploring the effect of Ni/Cr contents on the sheet-like NiCr-oxide-decorated CNT composites as highly active and stable catalysts for urea electrooxidation

Abstract The developing high-efficiency urea fuel cells have an irreplaceable role in solving the increasingly severe environmental crisis and energy shortages. The sluggish six-electron dynamic anodic oxidation reaction is the bottleneck of the rapid progress of urea fuel-cell technology. To tackle this challenge, we select the NiCr bimetallic system due to the unique synergic effect between the Ni and the Cr. Moreover, better conductivity is assured using carbon nanotubes (CNTs) as the support. Most importantly, we use a simple hydrothermal method in catalyst preparation for easy scale-up at a low cost. The results show that the hybrid catalysts of NiCrx-oxide-CNTs with different Ni/Cr ratios show much better catalytic performance in terms of active surface area and current density as compared to that of Ni-hydro-CNTs. The optimized NiCr2-oxide-CNTs catalyst exhibits not only the largest electrochemically active surface area (ESA, 50.7 m2 g−1) and the highest urea electrocatalytic current density (115.6 mA cm−2), but also outstanding long-term stability. The prominent performance of the NiCr2-oxide-CNTs catalyst is due to the combined effect of the improved charge transfer between Ni and Cr species, the large ESA, along with an elegant balance between the oxygen-defect sites and hydrophilicity. Moreover, we have proposed a synergistically enhanced urea catalytic mechanism.