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Conference paperAnthony D, Nguyen S, Senokos E, et al., 2019,
Hierarchical carbon aerogel modified carbon fiber composites for structural power applications
, 22nd International Conference on Composite Materials 2019 (ICCM22), Publisher: Engineers Australia, Pages: 1-7The desire to reduce overall weight in devices is a key driver for perpetual material development; the ability to combine composites with energy storage functions/capabilities which simultaneously provide structural integrity has the potential to supersede monofunctional components. To achieve this ambition, the multifunctional structure must perform both mechanical and energy storage functions sufficiently, but often there is a trade off in performance which is a significant challenge to overcome. Carbon aerogels have been shown to contribute positively to (electro-chemical double layer) capacitive performance due to an increased surface area in multifunctional carbon fiber based composite electrodes, but have also been shown to reduce mechanical properties; the addition of nanoscale reinforcers, such as carbon nanotubes, graphene or alike, with their superlative electrical and mechanical properties are proposed to address these concerns and create a truly hierarchical structure suitable for structural power applications.
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Journal articleMontelongo Y, Sikdar D, Ma Y, et al., 2019,
Author Correction: Electrotunable nanoplasmonic liquid mirror.
, Nat Mater, ISSN: 1476-1122In the version of this Article originally published, the last sentence of the acknowledgements incorrectly read 'L.V. acknowledges the support of a Marie Skodowska-Curie fellowship (N-SHEAD)'; it should have read 'L.V. and D.S. acknowledge the support of Marie Skłodowska-Curie fellowships, N-SHEAD and S-OMMs, respectively'.
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Conference paperSikdar D, Ma Y, Kucernak AR, et al., 2019,
Nanoplasmonic metamaterial devices as electrically switchable perfect mirrors and perfect absorbers
, Conference on Lasers and Electro-Optics (CLEO), Publisher: IEEE, Pages: 1-2, ISSN: 2160-9020We introduce nanoplasmonic metamaterial devices — electrically-switchable between perfect- mirror/absorber states — based on voltage-controlled assembly/disassembly of gold nanoparticles on silver films. These are investigated using effective-medium-theory, verified with simulations and experiments.
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Journal articleCastanheira L, Bedouet M, Kucernak A, et 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: 0378-7753The 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.
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Journal articleMalko D, Guo Y, Jones P, et al., 2019,
Heterogeneous iron containing carbon catalyst (Fe-N/C) for epoxidation with molecular oxygen
, Journal of Catalysis, Vol: 370, Pages: 357-363, ISSN: 0021-9517Pyrolized transition metal and nitrogen containing carbon materials (M-N/C) have shown promising activities as electrocatalysts for oxygen reduction reactions (ORR) in fuel cell cathodes. Similar materials have recently gained interest as heterogeneous catalysts. We report that ORR-active heterogeneous M-N/C materials can catalyze the chemical epoxidation of olefins with molecular oxygen and two equivalents of aldehyde at room temperature and ambient pressure. The observed yield and selectivity is higher than that for homogeneous analogues and the catalysts achieve TOF > 2700 h−1 and TON > 16,000. The ability to recycle the catalyst several times is also demonstrated.
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Journal articleRubio-Garcia J, Kucernak A, Zhao D, et al., 2019,
Hydrogen/manganese hybrid redox flow battery
, JPhys Energy, Vol: 1, Pages: 1-9, ISSN: 2515-7655Electrochemical energy storage is a key enabling technology for further integration of renewables sources. Redox flow batteries (RFBs) are promising candidates for such applications as a result of their durability, efficiency and fast response. However, deployment of existing RFBs is hindered by the relatively high cost of the (typically vanadium-based) electrolyte. Manganese is an earth-abundant and inexpensive element that is widely used in disposable alkaline batteries. However it has hitherto been little explored for RFBs due to the instability of Mn(III) leading to precipitation of MnO2 via a disproportionation reaction. Here we show that by combining the facile hydrogen negative electrode reaction with electrolytes that suppress Mn(III) disproportionation, it is possible to construct a hydrogen/manganese hybrid RFB with high round trip energy efficiency (82%), and high power and energy density (1410 mW cm−2, 33 Wh l−1), at an estimated 70% cost reduction compared to vanadium redox flow batteries.
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Journal articleMa Y, Zagar C, Klemme DJ, et al., 2018,
A tunable nanoplasmonic mirror at an electrochemical interface
, ACS Photonics, Vol: 5, Pages: 4604-4616, ISSN: 2330-4022Designing tunable optical metamaterials is one of the great challenges in photonics. Strategies for reversible tuning of nanoengineered devices are currently being sought through electromagnetic or piezo effects. For example, bottom-up self-assembly of nanoparticles at solid | liquid or liquid | liquid interfaces can be used to tune optical responses by varying their structure either chemically or through applied voltage. Here, we report on a fully reversible tunable-color mirror based on a TiN-coated Ag substrate immersed in an aqueous solution of negatively charged Au-nanoparticles (NPs). Switching electrode potential can be used to fully control the assembly/disassembly of NPs at the electrode | electrolyte interface within a 0.6 V wide electrochemical window. The plasmon coupling between the electrode and the adsorbed NP array at high positive potentials produces a dip in the optical reflectance spectrum, creating the "absorber" state. Desorption of NPs at low potentials eliminates the dip, returning the system to the reflective "mirror" state. The intensity and wavelength of the dip can be finely tuned through electrode-potential and electrolyte concentration. The excellent match between the experimental data and the theory of optical response for such system allows us to extract valuable information on equilibrium and kinetic properties of NP-assembly/disassembly. Together with modeling of the latter, this study promotes optimization of such meta-surfaces for building electrotunable reflector devices.
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Journal articleTariq F, Rubio-Garcia J, Yufit V, et 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 -
Conference paperRiedel R, Malko D, Seel A, et al., 2018,
Ion pairing and stability of alkalides in organic solutions
, 256th National Meeting and Exposition of the American-Chemical-Society (ACS) - Nanoscience, Nanotechnology and Beyond, Publisher: AMER CHEMICAL SOC, ISSN: 0065-7727 -
Journal articleRubio-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: 1388-2481A 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.
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Journal articleJackson C, Smith GT, Markiewicz M, et al., 2018,
Support induced charge transfer effects on electrochemical characteristics of Pt nanoparticle electrocatalysts
, Journal of Electroanalytical Chemistry, Vol: 819, Pages: 163-170, ISSN: 1572-6657The 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.
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Journal articleJaouen F, Jones D, Coutard N, et al., 2018,
Toward platinum group metal-free catalysts for hydrogen/airproton-exchange membrane fuel cells
, Johnson Matthey Technology Review, Vol: 62, Pages: 231-255, ISSN: 2056-5135The status, concepts and challenges toward catalysts free of platinum group metal (pgm) elements for proton-exchange membrane fuel cells (PEMFC) are reviewed. Due to the limited reserves of noble metals in the Earth’s crust, a major challenge for the worldwide development of PEMFC technology is to replace Pt with pgm-free catalysts with sufficient activity and stability. The priority target is the substitution of cathode catalysts (oxygen reduction) that account for more than 80% of pgms in current PEMFCs. Regarding hydrogen oxidation at the anode, ultralow Pt content electrodes have demonstrated good performance, but alternative non-pgm anode catalysts are desirable to increase fuel cell robustness, decrease the H2 purity requirements and ease the transition from H2 derived from natural gas to H2 produced from water and renewable energy sources.
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Journal articleLee M-R, Lee H-Y, Yim S-D, et al., 2018,
Effects of Ionomer Carbon Ratio and Ionomer Dispersity on the Performance and Durability of MEAs
, FUEL CELLS, Vol: 18, Pages: 129-136, ISSN: 1615-6846 -
Journal articlePark JY, Kwak DH, Ma KB, et 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.
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Journal articlezalitis C, Kucernak ARJ, sharman J, et 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: 2050-7496Improving 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.
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Journal articleBeruski O, Lopes T, Kucernak ARJ, et 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: 2469-990XRecent 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.
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Journal articleMontelongo Y, Sikdar D, Ma Y, et al., 2017,
Electrotunable nanoplasmonic liquid mirror
, Nature Materials, Vol: 16, Pages: 1127-1135, ISSN: 1476-1122Recently, 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.
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Journal articleMalko 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: 1388-2481Heat 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
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OtherBazant M, Bennewitz R, Bocquet L, et al., 2017,
Electrotunable wetting, and micro- and nanofluidics: general discussion
, Publisher: ROYAL SOC CHEMISTRY- Author Web Link
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Journal articleSymianakis 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: 0927-0256Embedded 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
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Journal articleCousens N, Kucernak ARJ, 2017,
Reversible ultralow-voltage liquid-liquid electrowetting without a dielectric layer
, Faraday Discussions, Vol: 199, Pages: 63-73, ISSN: 1364-5498Electrowetting-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.
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Journal articleKucernak 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: 1520-5126Although 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.
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Journal articleVelleman L, Sikdar D, Turek V, et al., 2016,
Tuneable 2D self-assembly of plasmonic nanoparticles at liquid | liquid interfaces
, Nanoscale, Vol: 8, Pages: 19229-19241, ISSN: 2040-3372Understanding 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.
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Journal articleKucernak 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: 1873-3859Poisoning 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.
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Journal articleChakrabarti BK, Nir DP, Yufit V, et al., 2016,
Studies of performance enhancement of rGO-modified carbon electrodes for Vanadium Redox Flow Systems
, ChemElectroChem, Vol: 4, Pages: 194-200, ISSN: 2196-0216Reduced 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.
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Journal articleMalko 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, Pages: 1-7, ISSN: 2041-1723The 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.
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Journal articleLopes T, Kucernak A, Malko D, et al., 2016,
Mechanistic Insights into the Oxygen Reduction Reactionon Metal–N–C Electrocatalysts under Fuel Cell Conditions
, ChemElectroChem, Vol: 3, Pages: 1580-1590, ISSN: 2196-0216Three 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.
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Journal articleMalko D, lopes T, Ticianelli E, et 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: 0378-7753The 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.
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Journal articleKucernak 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: 1932-7455Full 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
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Conference paperBeruski O, Lopes T, Kucernak A, et al., 2016,
Comparison between Darcy's law and Darcy-Brinkman formulation for reactant transport in PEFC porous media
, Publisher: AMER CHEMICAL SOC, ISSN: 0065-7727
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