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  • Journal article
    Jackson C, Raymakers LFJM, Mulder MJJ, Kucernak ARJet al., 2020,

    Assessing Electrocatalyst Hydrogen Activity and CO Tolerance: Comparison of Performance obtained using the High Mass Transport ‘Floating Electrode’ Technique and in Electrochemical Hydrogen Pumps

    , Applied Catalysis B: Environmental, Pages: 118734-118734, ISSN: 0926-3373
  • Journal article
    Ma Y, Sikdar D, Fedosyuk A, Velleman L, Klemme DJ, Oh S-H, Kucernak ARJ, Kornyshev AA, Edel JBet al., 2020,

    Electrotunable nanoplasmonics for amplified surface enhanced Raman spectroscopy

    , ACS Nano, Vol: 14, Pages: 328-336, ISSN: 1936-0851

    Tuning the properties of optical metamaterials in real time is one of the grand challenges of photonics. Being able to do so will enable a new class of photonic materials for use in applications such as surface enhanced Raman spectroscopy and reflectors/absorbers. One strategy to achieving this goal is based on the electrovariable self-assembly and disassembly of two-dimensional nanoparticle arrays at a metal liquid interface. As expected the structure results in plasmonic coupling between NPs in the array but perhaps as importantly between the array and the metal surface. In such a system the density of the nanoparticle array can be controlled by the variation of electrode potential. Due to the additive effect, we show that less than 1 V variation of electrode potential can give rise to a dramatic simultaneous change in optical reflectivity from ~93 % to ~1 % and the amplification of the SERS signal by up to 5 orders of magnitude. The process allows for reversible tunability. These concepts are demonstrated in this manuscript, using a platform based on the voltage-controlled assembly of 40 nm Au-nanoparticle arrays at a TiN/Ag electrode in contact with an aqueous electrolyte. We show that all the physics underpinning the behaviour of this platform works precisely as suggested by the proposed theory, setting the electrochemical nanoplasmonics as a promising new direction in photonics research.

  • Journal article
    Javier R-G, Kucernak A, Liu R, Chakrabarti Bet al.,

    Hydrogen/functionalized benzoquinone for a high-performance regenerative fuel cell as a potential large-scale energy storage platform

    , Journal of Materials Chemistry A, ISSN: 2050-7488

    The redox flow battery (RFB) is a suitable option for electricity storage due to its high energy efficiency, scalability and relative safety. However, the limited metallic resources for redox materials and the high cost in systems such as the all-vanadium RFB are major challenges for wider application. Organics may be sourced more abundantly and have lower prices than metal based redox couples. In this work a regenerative fuel cell involving relatively inexpensive organic redox couples is demonstrated. The electrochemical properties of 1,2-dihydrobenzoquinone-3,5-disulfonic acid (BQDS) are characterised by cyclic voltammetry and linear-sweep voltammetry under hydrodynamic conditions. A regenerative fuel cell using 0.65 M BQDS in 1 M H2SO4 as positive electrolyte and gaseous hydrogen (1 bar) as negative redox-material results in an open circuit cell voltage of 0.86 V, a power density of 122 mW/cm2, and an energy density of 10.90 Wh L-1 without considering the volume occupied by the hydrogen. Very promising performance with an energy efficiency >60% at 100 mA cm-2 for 200 cycles is reported. New organic redox species resistant to side reactions could facilitate the use of this new system in practical applications. The use of hydrogen may also contribute to reduced side reactions of the organic redox associated with degradation in the presence of oxygen.

  • Journal article
    Parra-Puerto A, Ng KL, Fahy K, Goode AE, Ryan MP, Kucernak Aet al., 2019,

    Supported transition metal phosphides: Activity survey for HER, ORR, OER and corrosion resistance in acid and alkaline electrolytes

    , ACS Catalysis, Vol: 9, Pages: 11515-11529, ISSN: 2155-5435

    Carbon supported MxPy (M = Ni, Co, W, Cr and Mo) were prepared via pyrolysis using a very simple and scalable method utilizing non-toxic metal and phosphorous precursors. The electrochemical hydrogen evolution (HER), oxygen reduction (ORR), and oxygen evolution (OER) reactions and corrosion resistance under both acid and alkaline conditions were examined for all these catalysts and compared to the benchmark catalysts Pt/C (HER/ORR) and IrO2(OER). The highest activities were found in alkaline solutions for Co2P for HER and ORR and Ni2P for OER. Good activity for these was also found in acid for some of these reactions, although the catalysts suffered from susceptibility to corrosion. Co2P was further studied in an alkaline environment as it shows high catalytic activity towards the oxygen reduction reaction (ORR) without significant hysteresis. The onset potential (at 0.5 mA cm-2) obtained was 0.8 V and a Tafel slope value of 38 mV dec-1 with a maximum kinetic mass activity of 2870 A gCo-1 at 0.7 V (RHE). Utilising high resolution transmission electron microscopy (HRTEM) it is possible to observe high-surface area needle-like single crystal cobalt oxide structures on the surfaces of the Co2P particles at the beginning of the ORR. Hence the high rates of initial corrosion of the Co2P identified appear to be associated with the dissolution and precipitation of Cobalt oxide on the particle surface. The as-synthesised Co2P/C also shows good performance in an 8-hour stability test for the Oxygen Evolution Reaction (OER), carried out at 1.6 V vs. RHE in alkaline conditions, with negligible drop in current density over time. Interestingly, in an acidic environment the catalyst is very active towards 2-electron- oxygen reduction leading to H2O2 with high selectivity (85%). It is intriguing that the pH dependence on this catalyst towards the ORR is similar to that seen for gold.

  • Journal article
    Lopes T, Beruski O, Manthanwar A, Korkischko I, Pugliesi R, Stanojev M, Andrade M, Pistikopoulos E, Perez J, Fonseca R, Meneghini R, Kucernak Aet al., 2019,

    Spatially resolved oxygen reaction, water, and temperature distribution: Experimental results as a function of flow field and implications for polymer electrolyte fuel cell operation

    , Applied Energy, Vol: 252, ISSN: 0306-2619

    In situ and ex situ spatially-resolved techniques are employed to investigate reactant distribution and its impacts in a polymer electrolyte fuel cell. Temperature distribution data provides further evidence for secondary flows inferred from reactant imaging data, highlighting the contribution of convection in heat as well as reactant distribution. Water build-up from neutron tomography is linked to component degradation, matching the pattern seen in the reactant distribution and thus suggesting that high, non-uniform local current densities shape degradation patterns in fuel cells. The correlations shown between different techniques confirm the use of the versatile reactant imaging technique, which is used to compare commonly used flow field designs. Among serpentine-type designs, the single serpentine is superior in both equivalent current density and reactant distribution, showing large contributions from convective flow. On the other hand, the interdigitated design is shown to produce larger equivalent current densities, while showing a somewhat poorer reactant distribution. Considering the correlations drawn between the techniques, this suggests that the interdigitated design compromises durability in favour of power output. The results highlight how established techniques provide a robust background for the use of a new and flexible imaging technique toward designing advanced flow fields for practical fuel cell applications.

  • Journal article
    Castanheira L, Bedouet M, Kucernak A, Hinds Get al., 2019,

    Influence of microporous layer on corrosion of metallic bipolar plates in fuel cells

    , Journal of Power Sources, Vol: 418, Pages: 147-151, ISSN: 0378-7753

    The effect of the presence of a microporous layer on the propensity for corrosion of metallic bipolar plates in an operating polymer electrolyte membrane fuel cell is investigated using an in situ reference electrode array. The local potential at the surface of the cathode bipolar plate is significantly more negative in the presence of the microporous layer, which is attributed to the higher ionic resistance of the aqueous phase in the reactant transport layer associated with more effective removal of water from the catalyst layer/reactant transport layer interface. As a result the bipolar plate is effectively shielded from elevated potentials that may be present at the cathode electrode, even during start-up and shutdown of the cell. Revision of ex situ test protocols for candidate bipolar plate materials, surface treatments and coatings is recommended to reduce unnecessary conservatism in testing.

  • Journal article
    Tariq F, Rubio-Garcia J, Yufit V, Bertei A, Chakrabarti BK, Kucernak A, Brandon Net al., 2018,

    Uncovering the mechanisms of electrolyte permeation in porous electrodes for redox flow batteries through real time in situ 3D imaging

    , SUSTAINABLE ENERGY & FUELS, Vol: 2, Pages: 2068-2080, ISSN: 2398-4902
  • Journal article
    Rubio-Garcia J, Kucernak ARJ, Charleson A, 2018,

    Direct visualization of reactant transport in forced convection electrochemical cells and its application to Redox Flow Batteries

    , Electrochemistry Communications, Vol: 93, Pages: 128-132, ISSN: 1388-2481

    A novel, simple and low cost electrochemiluminescence imaging method for monitoring mass transport phenomena in a redox flow battery-like system is presented. Luminol solutions were pumped through a flow field (FF) with a given design. At the flowfield/electrode interface light is emitted upon dye oxidation allowing direct visualization of channels, U-bends and regions of poor wetting. Image analysis allows direct visualization of reactant distribution and poor mass transport through tortuous materials. These results were compared with the experimental performance of an all‑vanadium redox flow battery with different FFs as a function of flow and good correlation achieved.

  • Journal article
    Jackson C, Smith GT, Markiewicz M, Inwood DW, Leach AS, Whalley PS, Kucernak AR, Russell AE, Kramer D, Levecque PBJet al., 2018,

    Support induced charge transfer effects on electrochemical characteristics of Pt nanoparticle electrocatalysts

    , Journal of Electroanalytical Chemistry, Vol: 819, Pages: 163-170, ISSN: 1572-6657

    The electrokinetic properties of Pt nanoparticles supported on Carbon (Pt/C) and Boron Carbide-Graphite composite (Pt/BC) are compared over a wide potential range. The influence of the support on the electronic state of Pt was investigated via in-situ X-ray Absorption Spectroscopy. Pt d-band filling, determined from XANES white line analysis, was lower and nearly constant between 0.4 and 0.95V vs. RHE for Pt/BC, indicating more positively charged particles in the double layer region and a delay in the onset of oxide formation by about 0.2V compared to the Pt/C catalyst, which showed a marked increase in d-band vacancies above 0.8V vs. RHE. Moreover, δμ analysis of the XANES data indicated a lack of sub-surface oxygen for the Pt/BC catalyst compared to the Pt/C catalyst above 0.9V vs. RHE. Additional anion adsorption on the Pt/BC in the double layer region, detected by CO displacement, was also confirmed by XANES analysis of the d-band occupancy. The H 2 oxidation activities of electrodes with low catalyst loadings were assessed under high mass transport conditions using the floating electrode methodology. The metal-support interaction between the Pt and BC support improved the maximum hydrogen oxidation current density by 1.4 times when compared to Pt/C.

  • Journal article
    Park JY, Kwak DH, Ma KB, Han SB, Chai GS, Kim SK, Peck DH, Kim CS, Kucernak A, Park KWet al., 2018,

    Enhanced oxygen reduction reaction of Pt deposited Fe/N-doped bimodal porous carbon nanostructure catalysts

    , Journal of Catalysis, Vol: 359, Pages: 46-54, ISSN: 0021-9517

    © 2018 Elsevier Inc. For commercialization of proton exchange membrane fuel cells (PEMFCs), the loading amount of Pt-based cathode catalysts for oxygen reduction reaction (ORR) needs be significantly reduced. In this study, we propose Pt catalysts supported by an iron/nitrogen-doped porous carbon (FeNC) nanostructure having a catalytic activity for ORR in order to significantly reduce the utilization of Pt. The FeNC nanostructure was prepared using a template method with 50 and 500 nm SiO 2 beads and phthalocyanine as a dopant and carbon source. The nanosized Pt catalysts with different loading weights (5, 10, 20, 30 wt%) were uniformly deposited on the FeNC with a bimodal porous crystalline doped carbon nanostructure using an electron beam radiation method. In particular, the cathode catalyst having 5 wt% Pt on FeNC (Pt5/FeNC) exhibited enhanced ORR mass activities of 2.19 and 2.58 A mg Pt −1 at 0.9 V measured by electrochemical half cells in acidic and alkaline media, respectively, compared to a commercial Pt(20 wt%)/C (Pt20/C). Furthermore, Pt5/FeNC showed a higher mass activity of 18.76 A mg Pt −1 at 0.6 V as a unit cell performance than that of the commercial catalyst. The improved ORR activity of Pt/FeNC might be synergistically attributed to the homogeneous dispersion of Pt nanoparticles on the bimodal porous doped carbon nanostructure, the interaction (electronic effect) between the metallic catalyst and the doped support, and the dual catalytic effect of both Pt and the doped carbon nanostructure.

  • Journal article
    zalitis C, Kucernak ARJ, sharman J, wright Eet al., 2017,

    Design principles for platinum nanoparticles catalysing electrochemical hydrogen evolution and oxidation reactions: edges are much more active than facets

    , Journal of Materials Chemistry A, Vol: 5, Pages: 23328-23338, ISSN: 2050-7496

    Improving the performance of hydrogen evolution and oxidation reactions using precious metal catalysts is key in reducing the cost of electrolysers and fuel cells. By considering the performance of these reactions as a function of platinum particle size (2.1–15 nm) under high mass transport conditions in acids, we find that the activity is composed of two components which vary in a defined way with the particle size. Geometrical considerations and electrokinetic modelling suggest that these two components correspond to the response of edges/vertices and the response of facets (Pt(100) and Pt(111)). Edges and vertices are much more active towards the hydrogen reaction. This assignment also rationalises the poor performance of platinum in alkaline environments. We predict that “ideal” particles made up of only edges/vertices would allow fuel cells and electrolysers to operate with only 1 μgPt cm−2 – about two to three orders of magnitude lower than what is currently used.

  • Journal article
    Montelongo Y, Sikdar D, Ma Y, McIntosh AJS, Velleman L, Kucernak AR, Edel JB, Kornyshev AAet al., 2017,

    Electrotunable nanoplasmonic liquid mirror.

    , Nature materials, Vol: 16, Pages: 1127-1135, ISSN: 1476-1122

    Recently, there has been a drive to design and develop fully tunable metamaterials for applications ranging from new classes of sensors to superlenses among others. Although advances have been made, tuning and modulating the optical properties in real time remains a challenge. We report on the first realization of a reversible electrotunable liquid mirror based on voltage-controlled self-assembly/disassembly of 16 nm plasmonic nanoparticles at the interface between two immiscible electrolyte solutions. We show that optical properties such as reflectivity and spectral position of the absorption band can be varied in situ within ±0.5 V. This observed effect is in excellent agreement with theoretical calculations corresponding to the change in average interparticle spacing. This electrochemical fully tunable nanoplasmonic platform can be switched from a highly reflective 'mirror' to a transmissive 'window' and back again. This study opens a route towards realization of such platforms in future micro/nanoscale electrochemical cells, enabling the creation of tunable plasmonic metamaterials.

  • Journal article
    Beruski O, Lopes T, Kucernak ARJ, Perez Jet al., 2017,

    Investigation of convective transport in the so-called “gas diffusion layer” used in polymer electrolyte fuel cell

    , Physical Review Fluids, Vol: 2, ISSN: 2469-990X

    Recent experimental data on a fuel-cell-like system revealed insights into the fluid flow in both free and porous media. A computational model is used to investigate the momentum and species transport in such a system, solved using the finite element method. The model consists of a stationary, isothermal, diluted species transport in free and porous media flow. The momentum transport is treated using different formulations, namely, Stokes-Darcy, Darcy-Brinkman, and hybrid Stokes-Brinkman formulations. The species transport is given by the advection equation for a reactant diluted in air. The formulations are compared to each other and to the available experimental data, where it is concluded that the Darcy-Brinkman formulation reproduces the data appropriately. The validated model is used to investigate the contribution of convection in reactant transport in porous media of fuel cells. Convective transport provides a major contribution to reactant distribution in the so-called diffusion media. For a serpentine channel and flow with Re=260–590, convection accounts for 29–58% of total reactant transport to the catalyst layer.

  • Journal article
    Malko D, Kucernak ARJ, 2017,

    Kinetic isotope effect in the oxygen reduction reaction (ORR) over Fe-N/C catalysts under acidic and alkaline conditions.

    , Electrochemistry Communications, Vol: 83, Pages: 67-71, ISSN: 1388-2481

    Heat treated Fe-N/C materials which are highly effective oxygen reduction catalysts in alkaline and acid, show a significant kinetic isotope effect (KIE). The values in acid (~3.4) and alkaline (~2.5) are much larger than the value for the metal free catalyst in acid (~1.8) suggesting that the rate determining step (RDS) is a proton coupled electron transfer in acid with a significant pathway involving a proton independent step under an alkaline environment

  • Journal article
    Symianakis E, Kucernak A, 2017,

    Embedded atom method interatomic potentials fitted upon density functional theory calculations for the simulation of binary Pt-Ni nanoparticles

    , Computational Materials Science, Vol: 133, Pages: 185-193, ISSN: 0927-0256

    Embedded Atom Method (EAM) potentials have been fitted for the atomistic simulation of small, 2–5 nm, binary, PtANi, nanoparticles completely from Density Functional Theory (DFT) total energy calculations.The overall quality of the DFT calculations and the final potential is obtained through the independentcalculation of an array of properties of the pure metals and the stable alloys, which arenormally used for the fitting of interatomic potentials. The ability of the fitted potentials to simulatenanostructures is evaluated by the reproduction of binary nanoslabs with thickness 1 nm, and nanoparticlesin the extreme case of the smallest icosahedrons possible, with diameter 0.6 nm. The usedapproach requires high quality of convergence but otherwise low cost DFT as it is based on static totalenergy calculations. It also provides objective criteria for the evaluation of the fitted potentials during fittingand has been implemented with the open source code GULP

  • Journal article
    Cousens N, Kucernak ARJ, 2017,

    Reversible ultralow-voltage liquid-liquid electrowetting without a dielectric layer

    , Faraday Discussions, Vol: 199, Pages: 63-73, ISSN: 1364-5498

    Electrowetting-on-dielectric devices typically have operating voltages of 10-20 V. A reduction in the operating voltage could greatly reduce the energy consumption of these devices. Here, the fully reversible one electrolyte electrowetting of a droplet on a solid metal surface is reported for the first time. A reversible change of 29o for an 800 mV step is achieved. The effect of surface roughness, electrolyte composition, electrolyte concentration and droplet composition is investigated. It is found that there is a dramatic dependence of the reversibility and hysteresis of the system on these parameters, contrary to theoretical predictions. When a 3-chloro-1-propanol droplet is used, a system with no hysteresis and a 40o change in angle is achieved.

  • Journal article
    Velleman L, Sikdar D, Turek V, Kucernak A, Roser SJ, Kornyshev AA, Edel JBet al., 2016,

    Tuneable 2D self-assembly of plasmonic nanoparticles at liquid | liquid interfaces

    , Nanoscale, Vol: 8, Pages: 19229-19241, ISSN: 2040-3372

    Understanding the structure and assembly of nanoparticles at liquid | liquid interfaces is paramount to their integration into devices for sensing, catalysis, electronics and optics. However, many difficulties arise when attempting to resolve the structure of such interfacial assemblies. In this article we use a combination of X-ray diffraction and optical reflectance to determine the structural arrangement and plasmon coupling between 12.8 nm diameter gold nanoparticles assembled at a water | 1,2-dichloroethane interface. The liquid | liquid interface provides a molecularly flat and defect-correcting platform for nanoparticles to self-assemble. The amount of nanoparticles assembling at the interface can be controlled via the concentration of electrolyte within either the aqueous or organic phase. At higher electrolyte concentration more nanoparticles can settle at the liquid | liquid interface resulting in a decrease in nanoparticle spacing as observed from X-ray diffraction experiments. The coupling of plasmons between the nanoparticles as they come closer together is observed by a red-shift in the optical reflectance spectra. The optical reflectance and the X-ray diffraction data are combined to introduce a new ‘plasmon ruler’. This allows extraction of structural information from simple optical spectroscopy techniques, with important implications in understanding the structure of nanoparticle films at liquid interfaces and their self-assembly.

  • Journal article
    Kucernak ARJ, kakati, Fahy KF, 2016,

    Using corrosion-like processes to remove poisons from electrocatalysts: a viable strategy to chemically regenerate irreversibly poisoned polymer electrolyte fuel cells

    , Electrochimica Acta, Vol: 222, Pages: 888-897, ISSN: 1873-3859

    Poisoning of Pt/C catalysts due to SO2 on a rotating disk electrode (RDE), and as part of the cathode layer in a single cell fuel cell and fuel cell stack are studied in terms of the system performance, and the effect of electrochemical and chemical post treatment to remove the adsorbed sulphur containing species. It is found that external polarisation can only recover the ORR performance of catalyst on an RDE after SO2 poisoning when an applied potential of 1.6 V(RHE) is used for 1 ks. An alternative approach is to use ozone, as in the presence of this species, the electrode potential is raised to ~1.6V(RHE) due to the high potential of the ozone reduction reaction. The high open circuit potential leads to a mixed potential and was found also to be highly efficient at removing the poison via coupled ozone reduction and poison oxidation. The ozone process is found to work efficiently at the catalyst level as shown through rotating disk electrode studies and also in single cell fuel cells. Furthermore we demonstrate for the first time the recovery of a SO2 poisoned fuel cell stack using the mixed-potential approach and ozone as a reactant. The cleaning process is fast (~10 minutes), occurs at room temperature, and does not require any special modification to the fuel cell. The process may be applicable to a wide range of poisons which can be oxidatively removed from platinum at high potentials.

  • Journal article
    Kucernak A, Malko D, Lopes T, 2016,

    Performance of Fe-N/C oxygen reduction electrocatalysts towards NO−₂, NO, and NH₂OH electroreduction from fundamental insights into the active center to a new method for environmental nitrite destruction

    , Journal of the American Chemical Society, Vol: 138, Pages: 16056-16068, ISSN: 1520-5126

    Although major progress has recently been achieved through ex situ methods, there is still a lack of understanding of the behavior of the active center in non-precious metal Fe–N/C catalysts under operating conditions. Utilizing nitrite, nitric oxide, and hydroxylamine as molecular probes, we show that the active site for the oxygen reduction reaction (ORR) is different under acidic and alkaline conditions. An in-depth investigation of the ORR in acid reveals a behavior which is similar to that of iron macrocyclic complexes and suggests a contribution of the metal center in the catalytic cycle. We also show that this catalyst is highly active toward nitrite and nitric oxide electroreduction under various pH values with ammonia as a significant byproduct. This study offers fundamental insight into the chemical behavior of the active site and demonstrates a possible use of these materials for nitrite and nitric oxide sensing applications or environmental nitrite destruction.

  • Journal article
    Chakrabarti BK, Nir DP, Yufit V, Tariq F, Rubio Garcia J, Maher R, Kucernak A, Aravind PV, Brandon NPet al., 2016,

    Studies of performance enhancement of rGO-modified carbon electrodes for Vanadium Redox Flow Systems

    , ChemElectroChem, Vol: 4, Pages: 194-200, ISSN: 2196-0216

    Reduced graphene oxide (rGO) suspended in an N,N′-dimethylformamide (DMF) solvent underwent electrophoretic deposition (EPD) on carbon paper (CP) electrodes. X-ray computed micro-tomography (XMT) indicates a 24 % increase in the specific surface area of CP modified with rGO in comparison to the untreated sample. Furthermore, XMT confirms that the deposition also penetrates into the substrate. Raman analysis shows that the rGO deposited is more amorphous than the CP electrode. A significant reduction in charge-transfer resistance of the VO2+/VO2+ reaction is also observed (from impedance measurements) in modified samples in comparison to untreated CP electrodes.

  • Journal article
    Malko D, Kucernak A, Lopes T, 2016,

    In-situ electrochemical quantification of active sites in Fe-N/C non-precious metal catalysts

    , Nature Communications, Vol: 7, ISSN: 2041-1723

    The economic viability of low temperature fuel cells as clean energy devices is enhanced by the development of inexpensive oxygen reduction reaction catalysts. Heat treated iron and nitrogen containing carbon based materials (Fe–N/C) have shown potential to replace expensive precious metals. Although significant improvements have recently been made, their activity and durability is still unsatisfactory. The further development and a rational design of these materials has stalled due to the lack of an in situ methodology to easily probe and quantify the active site. Here we demonstrate a protocol that allows the quantification of active centres, which operate under acidic conditions, by means of nitrite adsorption followed by reductive stripping, and show direct correlation to the catalytic activity. The method is demonstrated for two differently prepared materials. This approach may allow researchers to easily assess the active site density and turnover frequency of Fe–N/C catalysts.

  • Journal article
    Lopes T, Kucernak A, Malko D, Ticianelli EAet al., 2016,

    Mechanistic Insights into the Oxygen Reduction Reactionon Metal–N–C Electrocatalysts under Fuel Cell Conditions

    , ChemElectroChem, Vol: 3, Pages: 1580-1590, ISSN: 2196-0216

    Three different transition metal-C-N catalysts are tested under a range of fuel cell conditions. It is found that common features of the polarisation curve can be explained by a change in electrocatalytic mechanism. Utilising a simple model to quantify the change in mechanisms, iR free results of the fuel cell experiments are fit and found to be represented by a common set of parameters. The change in mechanism is assumed to be a switch from four electron reduction of oxygen to water to a two electron reduction to hydrogen peroxide followed by disproportionation of the hydrogen peroxide to water and oxygen. The data is used to estimate a mass specific exchange current density towards the ORR in the range 10-11-10-13 A g-1 depending on the catalyst. For the reduction of oxygen to hydrogen peroxide, the mass specific exchange current density is estimated to be in the range 10-2-10-3 A g-1. Utilising the electrokinetic model, it is shown how the mass transport losses can be extracted from the polarisation curve. For all three catalyst layers studied, these mass transport losses reach about 100mV at a current density of 1 A cm-2. Finally a discussion of the performance and site density requirements of the non-precious metal catalysts are provided, and it is estimated that the activity towards the ORR needs to be increased by an order of magnitude, and the site density by two/three orders of magnitude in order to compete with platinum as an ORR electrocatalyst.

  • Journal article
    Malko D, lopes T, Ticianelli E, Kucernak Aet al., 2016,

    A catalyst layer optimisation approach using electrochemical impedance spectroscopy for PEM fuel cells operated with pyrolysed transition metal-N-C catalysts

    , Journal of Power Sources, Vol: 323, Pages: 189-200, ISSN: 0378-7753

    The effect of the ionomer to carbon (I/C) ratio on the performance of single cell polymer electrolyte fuel cells is investigated for three different types of non-precious metal cathodic catalysts. Polarisation curves as well as impedance spectra are recorded at different potentials in the presence of argon or oxygen at the cathode and hydrogen at the anode. It is found that a optimised ionomer content is a key factor for improving the performance of the catalyst. Non-optimal ionomer loading can be assessed by two different factors from the impedance spectra. Hence this observation could be used as a diagnostic element to determine the ideal ionomer content and distribution in newly developed catalyst-electrodes. An electrode morphology based on the presence of inhomogeneous resistance distribution within the porous structure is suggested to explain the observed phenomena. The back-pressure and relative humidity effect on this feature is also investigated and supports the above hypothesis. We give a simple flowchart to aid optimisation of electrodes with the minimum number of trials.

  • Journal article
    Kucernak ARJ, Zalitis CM, 2016,

    General Models for the Electrochemical Hydrogen Oxidation and Hydrogen Evolution Reactions – Theoretical Derivation and Experimental Results Under Near Mass-Transport Free Conditions

    , Journal of Physical Chemistry C, Vol: 120, Pages: 10721-10745, ISSN: 1932-7455

    Full derivations of Heyrovsky-Volmer (HV), Tafel-Volmer(TV), Heyrovsky-Tafel(HT), and Heyrovsky-Tafel-Volmer(HTV) mechanisms under steady state conditions are provided utilising a new theoretical framework which allows better understanding of the each of the mechanistic currents and part currents. Simple and easily implemented equations are presented, which provide both the hydrogen coverage and electrochemical current as a function of overpotential and relevant kinetic parameters. It is shown how these responses are governed by a set of dimensionless parameters associated with the ratio of electrokinetic parameters. For each of the different mechanisms, an “atlas” of Hads coverage with overpotential and corresponding current density is provided, allowing an understanding of all possible responses depending on the dimensionless parameters. Analysis of these mechanisms provides the limiting reaction orders of the exchange current density for protons and bimolecular hydrogen for each of the different mechanisms, as well as the possible Tafel slopes as a function of the molecular symmetry factor, . Only the HV mechanism is influenced by pH whereas the TV,HT, and HTV mechanisms are not. The cases where the equations simplify to limiting forms are discussed. Analysis of the exchange current density from experimental data is discussed, and it is shown that fitting the linear region around the equilibrium potential underestimates the true exchange current density for all of the mechanisms studied. Furthermore, estimates of exchange current density via back-extrapolation from large overpotentials is also shown to be highly inaccurate. Analysis of Tafel slopes is discussed along with the mechanistic information which can and cannot be determined. The new models are used to simultaneously fit sixteen experimental responses of Pt/C electrodes in acid towards the her/hor as a function of , pH, p(H2), and temperature, using a consistent set of electrokinetic parame

  • Journal article
    Kucernak ARJ, kakati, Unnikrishnan A, Rajalakshmi N, Jafri R, Dhathathreyan Ket al., 2016,

    Recovery of Polymer Electrolyte Fuel Cell exposed to sulfur dioxide

    , International Journal of Hydrogen Energy, Vol: 41, Pages: 5598-5604, ISSN: 1879-3487

    Sulfur dioxide (SO2) is a common atmospheric contaminant which has a deleterious effect on fuel cells. The performance of a Polymer Electrolyte Fuel Cell (PEFC) utilising a Pt on nitrogen doped graphene support as the cathode catalyst was studied in the presence of air contaminated with known levels of SO2. The nitrogen doped graphene supported platinum was synthesized by a hydrothermal method. At levels of 25ppm SO2 in air there was within 15 minutes a 28 % reduction in the PEFC performance at 0.5 V. The performance degradation was more severe at higher SO2 concentrations. At 100 ppm SO2 in air the performance degraded by 91% at the same potential. The power loss of the fuel cell could not be recovered by externally polarising the PEFC at 1.6 V. Even after continuous potential cycling of the cell for 9 h only 80% of the initial performance could be recovered. However, a 15 minute treatment with 0.4% O3 in air showed almost a 100% performance recovery of the 100ppm SO2 contaminated fuel cell. The enhanced recovery of the fuel cell is related both to the chemical reaction of O3 with the adsorbed sulphur contaminant, and an increase of cathode potential during the electrochemical treatment.

  • Journal article
    Edel JB, Kornyshev AA, Kucernak AR, Urbakh Met al., 2016,

    Fundamentals and applications of self-assembled plasmonic nanoparticles at interfaces

    , Chemical Society Reviews, Vol: 45, Pages: 1581-1596, ISSN: 1460-4744

    This tutorial review will introduce and explore fundamental and applied aspects of using electrolytic interfaces incorporating nanoscale building blocks for use in novel applications such as sensors, and tunable optics. In order to do this, it is important to be able to understand the principles behind even the simplest of immiscible interfaces such as that of the Liquid | Liquid and Solid | Liquid Qualitatively, the picture is simple however the complexity is easily compounded by the addition of electrolyte, and further compounded by addition of more complex entities such as nanoparticles. Nevertheless combining all these components surprisingly results in an elegant solution, where the nanoparticles have the ability to self assemble at the interface with a high level of control. Importantly, this opens up the door to development of new types of materials with a range of applications which have only recently been exploited. As such initially we begin with a description of the fundamentals related to liquid | Liquid and Solid | Liquid interfaces both with and without electrolyte. The discussions then shifts to a description of biasing the interface by application of an electric field. This is followed by an exploration of nanoparticle assembly and disassembly at the interface by controlling parameters such as ligand composition, charge, pH, and electric field. Finally a description of the state-of-the-art is given in terms of current applications and possible future directions. It is perhaps fair to say that these new frontiers have caused great excitement within the sensing community not only due to the simplicity of the technique but also due to the unprecedented levels of sensitivity

  • Journal article
    Malko D, Lopes T, Symianakis E, Kucernak ARJet al., 2016,

    The intriguing poison tolerance of non-precious metal oxygen reduction reaction (ORR) catalysts

    , Journal of Materials Chemistry A, Vol: 4, Pages: 142-152, ISSN: 2050-7496

    Electrochemical devices such as fuel cells are key to a sustainable energy future. However the applicability of such underrealistic conditions is not viable to date. Expensive precious metals are used as electrocatalysts and contaminants presentin the operating media poison the utilized catalysts. Here the one pot synthesis of a highly active, self-supporting andsurprisingly poison tolerant catalyst is reported. The polymerisation of 1,5-Diaminonaphthalene provides self-assemblednanospheres, which upon pyrolysis form a catalytically active high surface area material. Tolerance to a wide range ofsubstances that poison precious metal based catalysts combined with high electrocatalytic activity might enable numerousadditional technological applications. In addition to fuel cells these could be metal-air batteries, oxygen-depolarized chloralkalicathodes, oxygen sensors, medical implantable devices, waste water treatment and as counter electrodes for manyother sensors where the operating medium is a complex and challenging mixture.

  • Journal article
    Kucernak ARJ, Fahy KF, Naranammalpuram Sundaram VN, 2015,

    Facile synthesis of palladium phosphide electrocatalysts and their activity for the hydrogen oxidation, hydrogen evolutions, oxygen reduction and formic acid oxidation reactions

    , Catalysis Today, Vol: 262, Pages: 48-56, ISSN: 1873-4308

    We demonstrate a new approach for producing highly dispersed supported metal phosphide powders with small particle size, improved stability and increased electrocatalytic activity towards some useful reactions. The approach involves a one-step conversion of metal supported on high surface area carbon to the metal phosphide utilising a very simple and scalable synthetic process. We use this approach to produce PdP₂ and Pd₅P₂ particles dispersed on carbon with a particle size of 4.5–5.5 nm by converting a commercially available Pd/C powder. The metal phosphide catalysts were tested for the oxygen reduction, hydrogen oxidation and evolution, and formic acid oxidation reactions. Compared to the unconverted Pd/C material, we find that alloying the P at different levels shifts oxide formation on the Pd to higher potentials, leading to greater stability during cycling studies (20% more ECSA retained, 5k cycles) and in thermal treatment under air. Hydrogen absorption within the PdP₂ and Pd₅P₂ particles is enhanced. The phosphides compare favourably to the most active catalysts reported to date for formic acid oxidation, especially PdP₂, and there is a significant decrease in poisoning of the surface compared to Pd alone. The mechanistic changes in the reactions studied are rationalised in terms of increased water activation on the surface phosphorus atoms of the catalyst. One of the catalysts, PdP₂/C is tested in a fuel cell as anode and cathode catalyst and shows good performance.

  • Journal article
    Markiewicz M, Zalitis C, Kucernak A, 2015,

    Performance measurements and modelling of the ORR on fuel cell electrocatalysts - the modified double trap model

    , Electrochimica Acta, Vol: 179, Pages: 126-136, ISSN: 1873-3859

    Experimental results for the ORR (oxygen reduction reaction) in perchloric acid for ultra low loading Pt/Celectrodes have been fitted to a number of different ORR mechanisms. These were accomplished as afunction of temperature (280–330 K), oxygen partial pressure(0:01 < po2p0o2< 1) and potential (0.3–1.0V vs.RHE). A reaction exponent for oxygen of 1 0.1 across the potential range 0.3–0.85V vs. RHE is confirmed.From the experimental results it is clear that the surface becomes increasingly blocked towards the ORRas overpotential increases (i.e. as the potential decreases from 0.6 to 0.3 V vs. RHE). The double trap model[J.X. Wang, J. Zhang, R.R. Adzic, J. Phys. Chem. A, 111 (2007) 12,702] fails to account for this observation,although we have produced a modified version to include the formation of OOHad intermediates. Theseintermediates block the electrode at larger overpotentials and lead to a decrease in electrocatalystperformance compared to a Tafel type approximation. Furthermore these intermediates can lead to theformation of hydrogen peroxide at large overpotentials, an experimental observation which is currentlypoorly described by models.The decreased activity at large overpotentials suggests that blocking of active catalyst sites may be asimportant to catalyst activity in an operating fuel cell as the absolute performance of the electrode in thelow overpotential region as typically measured on an RDE. It may also offer an explanation to theincreased losses seen in fuel cell electrodes at lower catalyst loadings – i.e. the loses, which are typicallyascribed to increased mass transport loses, may instead result from decreased electrocatalyticperformance at high overpotentials.

  • Journal article
    Ahmad EA, Tileli V, Kramer D, Mallia G, Stoerzinger KA, Shao-Horn Y, Kucernak AR, Harrison NMet al., 2015,

    Optimizing Oxygen Reduction Catalyst Morphologies from First Principles

    , Journal of Physical Chemistry C, Vol: 119, Pages: 16804-16810, ISSN: 1932-7455

    Catalytic activity of perovskites for oxygen reduction (ORR) wasrecently correlated with bulk d-electron occupancy of the transition metal. Weexpand on the resultant model, which successfully reproduces the high activity ofLaMnO3 relative to other perovskites, by addressing catalyst surface morphology asan important aspect of the optimal ORR catalyst. The nature of reaction sites onlow index surfaces of orthorhombic (Pnma) LaMnO3 is established from FirstPrinciples. The adsorption of O2 is markedly influenced by local geometry andstrong electron correlation. Only one of the six reactions sites that result from experimentally confirmed symmetry-breakingJahn−Teller distortions is found to bind O2 with an intermediate binding energy while facilitating the formation of superoxide, animportant ORR intermediate in alkaline media. As demonstrated here for LaMnO3, rational design of the catalyst morphology topromote specific active sites is a highly effective optimization strategy for advanced functional ORR catalysts.

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