Specific Electrocatalytic Activity Research Articles

Even partially amorphous Pd2Ni2P metallic glass significantly promotes hydrogen evolution electrocatalysis

Metallic glasses are expected to be endowed with higher electrocatalytic activity, with respect to their crystalline counterparts, due to the presence of a high density of under-coordinated sites. However, glasses made of metals, as opposed to metal oxide/sulfides are harder to synthesize, with the challenge increasing with amorphousness. In light of these issues, we calibrate the increase in hydrogen evolution reactivity using the Pd2Ni2P bulk metallic glass composition as a model system. This composition has a good glass-forming ability and is interesting from a catalytic point of view, as Ni and P lie on the opposite leg of the HER volcano plot with respect to Pd. Partially amorphous (PA) Pd2Ni2P alloy displayed a five-fold higher specific electrocatalytic activity on per unit electrochemical surface area (ECSA) basis compared to its crystalline (C) counterpart. This magnitude of specific electrocatalytic activity, which is on par with that for pure Pd, has been achieved with just 40% of the precious metal, leading to a considerable saving in cost. The homogeneous single-phase structure of the highly electro-active partially amorphous alloy leads to higher electrochemical stability than its polycrystalline counterpart. This finding implies that many compositions ignored traditionally due to their poor electrocatalytic activity or stability can now be reconsidered in amorphous forms, thus expanding the material space of valuable catalysts.

Effect of porosity and active area on the assessment of catalytic activity of non-precious metal electrocatalyst for oxygen reduction

We describe a method to study porous thin-films deposited onto rotating disc electrodes (RDE) applied to non-platinum group electrocatalyst obtained by pyrolysis of iron phthalocyanine and carbon, FePc/C. The electroactive area and porous properties of the thin film electrodes were obtained using electrochemical impedance spectroscopy under the framework of de Levie impedance model. The electrocatalytic activity of different electrodes was correlated to the total electroactive area (A p) and the penetration ratio parameter through the film under ac current. The cylindrical pore model was extended to the RDE boundary conditions and derived in a Koutecky–Levich type expression that allowed to separate the effect of the electroactive area and structural properties. The resulting specific electrocatalytic activity of FePc/C heat treated at different temperatures was correlated to FePc surface concentration.

Electrocatalytic Oxidation of Water by OH−‐ and H2O‐Capped IrOx Nanoparticles Electrophoretically Deposited on Graphite and Basal Plane HOPG: Effect of the Substrate Electrode**

AbstractIridium oxide (IrOx) is one of the most efficient electrocatalysts for water oxidation reaction (WOR). Here, WOR electrocatalysis by 1.6 nm IrOx nanoparticles (NPs) electrophoretically deposited onto spectroscopic graphite (Gr) and basal plane highly ordered pyrolytic graphite (HOPG) was studied as a function of NPs’ capping ligands and electrodeposition substrate. On Gr, OH−‐ and H2O‐capped NPs exhibited close sub‐monolayer surface coverages and specific electrocatalytic activity of 18.9‐23.5 mA nmol−1 of IrIV/V sites, at 1 V and pH 7. On HOPG, OH−‐capped NPs produced films with a diminished WOR activity of 5.17±2.40 mA nmol−1. Electro‐wettability‐induced changes impeded electrophoretic deposition of H2O‐capped NPs on HOPG, WOR currents being 25‐fold lower than observed for OH−‐capped ones. The electrocatalysis efficiency correlated with hydrophilic properties of the substrate electrodes, affecting morphological and as a result catalytic properties of the formed IrOx films. These results, important both for studied and related carbon nanomaterials systems, allow fine‐tuning of electrocatalysis by a proper choice of the substrate electrode.

Improved electrocatalytic activity of ethanol oxidation over Pd/TiO2 nanofibers-carbon black nanocomposite

Herein, titanium dioxide nanofibers (TiO2 NFs) were successfully prepared using sol-gel combined electrospinning method and subsequent calcination at 600 °C. The TiO2 NFs were used for preparation of TiO2 NFs-carbon black composite including Pd nanoparticles (Pd/TiO2 NFs-C). The prepared samples were characterized by field-emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), Fourier transform infrared (FT-IR), thermal gravimetric analysis (TGA) and N2 adsorption-desorption isotherm (BET). The FE-SEM showed that the polyvinyl pyrrolidone/titanium tetraisopropoxide composite (average fiber diameter: 125 nm) and TiO2 NFs (average fiber diameter: 48 nm) were obtained. TEM micrograph showed a relatively good dispersion of the Pd nanoparticles with average diameters at about 31 nm over TiO2 NFs-C composite. Using XRD, anatase and rutile crystalline phases for the TiO2 NFs and face center cubic (fcc) structure for Pd were identified in the nanocomposite. The dispersed Pd nanoparticles onto the TiO2 NFs-C showed a specific electro-catalytic activity in EOR with j = 17.6 mA cm−2, 6.4-fold improvement than that of traditional Pd/C catalyst.

Enhanced Mass Activity and Stability of Bimetallic Pd‐Ni Nanoparticles on Boron‐Doped Diamond for Direct Ethanol Fuel Cell Applications

AbstractIn this work, electrochemical deposition of Pd and bimetallic Pd−Ni nanoparticles on an oxygen‐terminated boron‐doped diamond (BDD) substrate is described for use as an electrocatalyst in a direct ethanol fuel cell. A potentiostatic two‐step electrochemical method involving the electrodeposition of Ni nanoparticles on BDD followed by mono‐dispersed Pd nanoparticles was used for the fabrication of a Pd−Ni/BDD electrode. The electrocatalytic activity of the bimetallic Pd−Ni nanoparticles was evaluated in an alkaline solution containing ethanol and compared to that of the Pd nanoparticles alone. The bimetallic Pd−Ni nanoparticles showed 2.4 times higher mass activity than similar systems in the literature as well as stability when operated in alkaline media. The higher electrochemical response towards the electrooxidation of ethanol observed for the bimetallic electrocatalysts was attributed to the synergistic effects of the electron interaction at the interface of the two metals. Chronopotentiometric measurements revealed that Pd is more stable when anchored to the Ni nanoparticles. The optimised loading of mono‐dispersed Pd on a foreign Ni metal as nanoparticles plays a crucial role in achieving a high mass (3.63×106 mA/g) and specific (10.53 mA/cm2) electrocatalytic activity of Pd towards ethanol electrooxidation in alkaline media.

Electrochemistry and electrocatalysis of covalent hemin-G4 complexes on gold

Peroxidase-mimicking catalytic DNAzyme labels comprising hemin-guanine quadruplex (G4) complexes are widely used in electrochemical DNA and aptamer assays. However, their catalytic activity and thus the assay performance may be limited by the complex stability. Here, electrochemical properties of the hemin-G4 complexes with hemin covalently attached to the G4 sequence (covalent complexes) and of the non-covalent hemin-G4 complexes were studied and compared in the reaction of electrocatalytic reduction of H2O2. At pH7, non-covalently and covalently bound hemin-G4 complexes immobilized on gold electrodes showed formal potentials of −242 and −220mV vs. Ag/AgCl, being less negative than −316mV exhibited by hemin attached to the alkanethiol SAM. Both complexes were able to electrocatalytically reduce H2O2 starting from 0.4V, and O2 – from 0.15V. In the reaction of H2O2 reduction at 0.2V, within the potential window of no interference from O2, the covalent hemin-G4 complex showed a 20 fold higher specific electrocatalytic activity compared to the non-covalent analogue and the KM (towards H2O2) of 0.58mM (2mM for the non-covalent complex). Both the enhanced catalytic activity and sensitivity for H2O2 were ascribed to the higher integrity/stability of the covalent complexes. The results open new biosensor perspectives for the covalent hemin-G4 complexes as more stable and active labels for DNA and aptamer assays.

An Amperometric Flow Injection Analysis of Dissolved Hydrogen Molecule Using Tightly Immobilized Electrodeposited Platinum Particles on Nitrogen-containing Functional Groups Introduced Glassy Carbon Electrodes.

An amperometric sensor based on flow injection analysis (FIA) of dissolved hydrogen molecules was first developed using electrodeposited platinum particles on glassy carbon electrodes modified with nitrogen-containing functional groups (Pt-NGC) as the working electrode. A glassy carbon (GC) electrode was covalently modified by electrochemical oxidation/reduction procedures. The redox waves between hydrogen ions and hydrogen molecules at highly positive potential range in the hydrodynamic voltammogram were obtained by using a Pt-NGC electrode. The specific electrocatalytic activity for the electrode oxidation of hydrogen molecules has successfully been applied to the FIA of dissolved hydrogen. The typical current vs. time curve was obtained by the repetitive measurement of dissolved hydrogen, and the measurement of dissolved hydrogen was fully completed in a short time (∼15 s). A linear relationship was obtained between the oxidation current of hydrogen molecules and dissolved hydrogen concentration. This indicates that our proposed technique can be used for the determination of the dissolved hydrogen concentration. The fabrication method of the present sensor is very simple because the direct modification of the glassy carbon electrode surface can be performed, differing from the tedious fabrication method in which electrocatalytic carbon powder prepared must be immobilized to the surface of the glassy carbon electrode using Nafion coating and high temperature treatment.

Electrochemically modified boron-doped diamond electrode with Pd and Pd-Sn nanoparticles for ethanol electrooxidation

The modification of hydrogen terminated boron-doped diamond (HBDD) electrode with pure palladium (Pd) and Pd-Sn (tin) nanoparticles is described in this study. For synthesis of Sn/HBDD and Pd-Sn/HBDD electrode, a potentiostatic two-step electrochemical method involving the electrodeposition of Sn followed by Pd was used, respectively. The modification of the HBDD electrode with Sn and noble metal Pd by forming bimetallic Pd-Sn nanoparticle leads to a higher electrocatalytic activity. The electrocatalytic activity of the bimetallic Pd-Sn nanoparticles was evaluated towards the electrooxidation of ethanol in alkaline media and compared with that of the Pd nanoparticles alone. The bimetallic Pd-Sn nanoparticles modified HBDD electrode exhibits higher current densities and less poisoning effects during ethanol electrooxidation compared to Pd/HBDD. The proper tuning of the Pd loading on a foreign metal along with the surface termination effects of the BDD electrode plays a crucial role in achieving a high mass (4.26×106mA/g) and specific (12.37mA/cm2) electrocatalytic activity of Pd towards ethanol electrooxidation. The aforementioned catalysts of this research possess a high poisoning resistance (If/Ib=1.63) and stability towards ethanol electrooxidation in alkaline media.

Determination of the Resistance of Cone-Shaped Solid Electrodes

A cone-shaped electrode pressed into an electrolyte can with advantage be utilized to characterize the electro-catalytic properties of the electrode, because it is less dependent on the electrode microstructure than e.g. thin porous composite electrodes, and reactions with the electrolyte occurring during processing can be avoided. Newman's formula for current constriction in the electrolyte is then used to deduce the active contact area based on the ohmic resistance of the cell, and from this the surface specific electro-catalytic activity. However, for electrode materials with low electrical conductivity (like Ce1-xPrxO2-δ), the resistance of the cell is significantly influenced by the ohmic resistance of the cone electrode, wherefore it must be included. In this work the ohmic resistance of a cone is modelled analytically based on simplified geometries. The two analytical models only differ by a model specific pre-factor, which is consequently determined by a finite element model. The model was applied to measurements on cones of Ce1-xPrxO2-δ characterized on an YSZ electrolyte. Conclusively, the finite element model was used to obtain a formula for the resistance for different cone angles with a small contact area. This reproduces Newman's formula for a cone angle equal to 90°, i.e. a semi-infinite body.

Development of tailored high-performance and durable electrocatalysts for advanced PEM fuel cells

A family of novel carbon materials with intermediate surface area and varying morphology and surface chemistry were used to prepare Pt/C catalysts by two different preparation procedures; a chemical impregnation method and a microwave-assisted polyol method. The catalysts were thoroughly characterized, and their electrochemical performance and stability were investigated with rotating disc electrode (RDE) cyclic voltammetric (CV) measurements. The intermediate-surface-area carbon supports gave catalysts with much greater support stability than a widely used standard catalyst. The novel catalysts had lower electrochemical surface area than the reference, but their specific electrocatalytic activity towards the oxygen-reduction reaction (ORR) was much higher, and some of them also featured higher mass-specific ORR activity than the reference. The series of catalysts prepared by the microwave-assisted polyol method featured smaller Pt nanoparticles and higher activities than those prepared by impregnation. On the other hand, the impregnated catalysts showed better durability of the Pt particles. The most promising catalysts were selected and elaborated in further optimized preparation procedures to obtain quantities sufficient for their use in proton-exchange membrane fuel cells (PEMFCs).

Rationalizing the Influence of the Mn(IV)/Mn(III) Red-Ox Transition on the Electrocatalytic Activity of Manganese Oxides in the Oxygen Reduction Reaction

Knowledge on the mechanisms of oxygen reduction reaction (ORR) and descriptors linking the catalytic activity to the structural and electronic properties of transition metal oxides enable rational design of more efficient catalysts. In this work ORR electrocatalysis was studied on a set of single and complex Mn(III) oxides with a rotating disc electrode method and cyclic voltammetry. We discovered an exponential increase of the specific electrocatalytic activity with the potential of the surface Mn(IV)/Mn(III) red-ox couple, suggesting the latter as a new descriptor for the ORR electrocatalysis. The observed dependence is rationalized using a simple mean-field kinetic model considering availability of the Mn(III) centers and adsorbate-adsorbate interactions. We demonstrate an unprecedented activity of Mn2O3, ca. 40 times exceeding that of MnOOH and correlate the catalytic activity of Mn oxides to their crystal structure.

Photochemically modified ATO NPs as conductive support of Pt electrocatalysts for proton exchange membrane fuel cells

Antimony-doped tin oxide (ATO) nanoparticles (NPs) were covalently modified with a benzophenone-silicate photoreactive organic molecule to enable the UV-mediated photoreduction of Pt(IV) on the surface of the ATO NPs to give Pt(0) NPs. The successfully synthesized Pt/ATO nanocomposites (NCs) that were based on these novel hybrid photoreactive ATO NPs showed a much better Pt dispersion than Pt/ATO NCs prepared by traditional methods. The size of the Pt NPs was below 2.8 nm for all the NCs. The prepared NCs were studied with respect to their properties as durable and active electrocatalysts for proton exchange membrane fuel cells. They were subjected to fuel-cell-relevant electrochemical characterization by rotating disc electrode cyclic voltammetry. The electrochemically active surface area was found to be significantly lower for the novel NCs than for the standard Pt/C catalyst, while on the other hand, their specific electrocatalytic activity towards the oxygen reduction reaction (ORR) was found to exceed that of the reference Pt/C by several times. The ORR activity in terms of the mass of Pt was comparable to, or greater than, that of the Pt/C. The stability towards electrochemical ageing was greatly improved for Pt/ATO NCs relative to Pt/C.

Catalytic Activity of Platinum Monolayer on Iridium and Rhenium Alloy Nanoparticles for the Oxygen Reduction Reaction

A new type of electrocatalyst with a core–shell structure that consists of a platinum monolayer shell placed on an iridium–rhenium nanoparticle core or platinum and palladium bilayer shell deposited on that core has been prepared and tested for electrocatalytic activity for the oxygen reduction reaction. Carbon-supported iridium–rhenium alloy nanoparticles with several different molar ratios of Ir to Re were prepared by reducing metal chlorides dispersed on Vulcan carbon with hydrogen gas at 400 °C for 1 h. These catalysts showed specific electrocatalytic activity for oxygen reduction reaction comparable to that of platinum. The activities of PtML/PdML/Ir2Re1, PtML/Pd2layers/Ir2Re1, and PtML/Pd2layers/Ir7Re3 catalysts were, in fact, better than that of conventional platinum electrocatalysts, and their mass activities exceeded the 2015 DOE target. Our density functional theory calculations revealed that the molar ratio of Ir to Re affects the binding strength of adsorbed OH and, thereby, the O2 reduction activity of the catalysts. The maximum specific activity was found for an intermediate OH binding energy with the corresponding catalyst on the top of the volcano plot. The monolayer concept facilitates the use of much less platinum than in other approaches. The results with the PtML/PdML/Ir2Re electrocatalyst indicate that it is a promising alternative to conventional Pt electrocatalysts in low-temperature fuel cells.

Au-supported Pt–Au mixed atomic monolayer electrocatalyst with ultrahigh specific activity for oxidation of formic acid in acidic solution

Au-supported Pt-Au mixed atomic monolayer electrocatalyst was prepared by underpotential deposition of Cu on Au and then redox replacement with noble metal atoms, which shows an ultrahigh Pt-mass (or Pt-area) normalized specific electrocatalytic activity of 102 mA μg(Pt)(-1) (124 mA cm(Pt)(-2)) for oxidation of formic acid in acidic aqueous solution.

Stabilization and compressive strain effect of AuCu core on Pt shell for oxygen reduction reaction

A great challenge in fuel cell development involves improving the durability and electrocatalytic activity of Pt-based electrocatalysts, while reducing the Pt loading. Herein, we report for the first time the synthesis of core–shell AuCu@Pt nanoparticles, which exhibit superior electrocatalytic activity and excellent stability towards oxygen reduction reaction (ORR). The Au component in the AuCu alloy core is crucial toward stabilizing the Pt shell during ORR. The extraordinary electrocatalytic activity of the AuCu@Pt nanoparticles for ORR is attributed to the compressive strain effect exerted by the AuCu alloy core on the Pt shell, which is induced by the slightly smaller lattice parameter of the AuCu core. In contrast, pure Au core with a larger lattice parameter than Pt would induce a tensile strain effect on the Pt shell, decreasing the electrocatalytic activity of Pt for ORR. This study illustrates that tuning the surface strain in Pt-based nanomaterials can be an effective way to manipulate the specific electrocatalytic activity. Moreover, the replacement of precious Pt core with the less expensive AuCu alloy can significantly reduce Pt loading and the associated catalyst cost, while achieving a superior electrocatalytic activity.

Electrochemical Detection of Hydrogen Peroxide at Carbon Nanotubes-Poly(Styrene-Co-N-Isopropylacrylamide) Nanoparticles Modified Electrode

Electrochemical sensors for the sensitive detection of H2O2 are for the first time described using carbon nanotubes/poly(styrene-co-N-isopropylacrylamide) nanoparticles (CNT/pS-co-NIPAm) modified glassy carbon electrode (GCE). The fabricated GCE is characterized using scan electron microscopy (SEM) and cyclic voltammetry (CV). The CNT/pS-co-NIPAm modified electrode has larger specific surface area and excellent electrocatalytic activity. The peak current of H2O2 obviously improves at modified GCE compared to bare electrode. The proposed electrode provides a promising avenue to develop novel electrochemical sensors.

Nanostructure and formation mechanism of Pt-WO3/C nanocatalyst by ethylene glycol method

Pt-WO3 nanoparticles uniformly dispersed on Vulcan XC-72R carbon black were prepared by an ethylene glycol method. The morphology, composition, nanostructure, electrochemical characteristics and electrocatalytic activity were characterized, and the formation mechanism was investigated. The average particle size was 2.3 nm, the same as that of Pt/C catalyst. The W/Pt atomic ratio was 1/20, much lower than the design of 1/3. The deposition of WO3·xH2O nanoparticles on Vulcan XC-72R carbon black was found to be very difficult by TEM. From XPS and XRD, the Pt nanoparticles were formed in the colloidal solution of Na2WO4, the EG insoluble Na2WO4 resulted in the decreased relative crystallinity and increased crystalline lattice constant compared with those of Pt/C catalyst and, subsequently, the higher specific electrocatalytic activity as determined by CV. The Pt-mass and Pt-electrochemically-active-specific-surface-area based anodic peak current densities for ethanol oxidation were 422.2 mA·mg−1Pt and 0.43 mA·cm−2Pt, 1.2 and 1.1 times higher than those of Pt/C catalyst, respectively.

Carbonized polyaniline nanotubes/nanosheets-supported Pt nanoparticles: Synthesis, characterization and electrocatalysis

Nitrogen-containing carbonized polyaniline nanotubes/nanosheets were used to synthesize a novel type of supported Pt nanoparticles electrocatalyst PtNPs/Carb-nanoPANI, with the Pt nanoparticles of ~ 9 nm in diameter. PtNPs/Carb-nanoPANI nanocomposite was characterized by transmission electron microscopy, cyclovoltammetry, thermogravimetric and XRD analyses. Its electrocatalytic activity towards the oxygen reduction reaction in both the alkaline and acidic solutions was studied by a rotating disc technique. In acidic media, this electrocatalyst was compared to both smooth platinum and commercial C-supported Pt-based electrocatalyst. Its higher specific electrocatalytic activity, which amounted to ~ 1 mA cm − 2 in the region of diffusion control, was proved. High electrocatalytic activity of PtNPs/Carb-nanoPANI towards the ethanol oxidation reaction in acidic medium was also evidenced.

Electrochemical quartz crystal microbalance study on Au-supported Pt adlayers for electrocatalytic oxidation of methanol in alkaline solution

Underpotential deposition (UPD) of Cu on an Au electrode followed by redox replacement reaction (RRR) of CuUPD with a Pt source (H2PtCl6 or K2PtCl4) yielded Au-supported Pt adlayers (for short, Pt(CuUPD-Pt4+)n/Au for H2PtCl6, or Pt(CuUPD-Pt2+)n/Au for K2PtCl4, where n denotes the number of UPD-redox replacement cycles). The electrochemical quartz crystal microbalance (EQCM) technique was used for the first time to quantitatively study the fabricated electrodes and estimate their mass-normalized specific electrocatalytic activity (SECA) for methanol oxidation in alkaline solution. In comparison with Pt(CuUPD-Pt2+)n/Au, Pt(CuUPD-Pt4+)n/Au exhibited a higher electrocatalytic activity, and the maximum SECA was obtained to be as high as 35.7 mA μg−1 at Pt(CuUPD-Pt4+)3/Au. The layer-by-layer architecture of Pt atoms on Au is briefly discussed based on the EQCM-revealed redox replacement efficiency, and the calculated distribution percentages of bare Au sites agree with the experimental results deduced from the charge under the AuO x -reduction peaks. The EQCM is highly recommended as an efficient technique to quantitatively examine various electrode-supported catalyst adlayers, and the highly efficient catalyst adlayers of noble metals are promising in electrocatalysis relevant to biological, energy and environmental sciences and technologies.

Preparation of Pt/multiwalled carbon nanotubes modified Au electrodes via Pt–Cu co-electrodeposition/Cu stripping protocol for high-performance electrocatalytic oxidation of methanol

Pt nanoparticles well dispersed on multiwalled carbon nanotubes (MWCNTs) were prepared for high-performance electrocatalytic oxidation of methanol in both acidic and alkaline media via the co-electrodeposition/stripping (CS) protocol, namely, co-electrodeposition of Pt and Cu followed by electrochemical stripping of Cu, as examined by cyclic voltammetry (CV), electrochemical quartz crystal microbalance (EQCM), scanning electron microscopy (SEM), and X-ray diffraction (XRD). The Pt catalyst prepared by the CS protocol on MWCNTs (Pt cs/MWCNTs/Au) exhibited a specific electrocatalytic activity of 519 and 2210 A g −1 toward cyclic voltammetric electrooxidation (50 mV s −1) of methanol in 0.5 M CH 3OH + 0.5 M H 2SO 4 and 0.5 M CH 3OH + 1.0 M NaOH media, respectively, which are larger than those prepared by conventional electrodeposition from chloroplatinic acid on Au and MWCNTs/Au, as well as that by a CS protocol on Au. The Pt cs/MWCNTs/Au electrode also possessed the highest stability, which maintained 91% and 90% of its initial catalytic activity after 120-cycle CV in 0.5 M CH 3OH + 0.5 M H 2SO 4 and 0.5 M CH 3OH + 1.0 M NaOH, respectively. The electrode kinetics of methanol oxidation is also briefly discussed. The nanosubstrate-based CS protocol is simple, convenient and efficient, which is expected to find wide applications in film electrochemistry and electrocatalysis.