In this paper, a mathematical model for the all-vanadium battery is presented and analytical solutions are derived. The model is based on the principles of mass and charge conservation, incorporating the major resistances, the electrochemical reactions and recirculation of the electrolyte through external reservoirs. Comparisons between the model results and experimental data show good agreement over practical ranges of the vanadium concentrations and the flow rate. The model is designed to provide accurate, rapid solutions at the unit-cell scale, which can be used for control and monitoring purposes. Crucially, the model relates the process time and process conditions to the state of charge via vanadium concentrations. Electrochemical energy storage systems are crucial to the regulation and transmission of intermittent power derived from wind, solar and tidal sources. One promising example of such a storage system is the redox flow battery (RFB), which is suitable for both medium and large scales storage needs.1 Other important applications of RFB technology include power balancing and peak shaving for incumbent power generation methods. Unisearch Limited, University of New South Wales (UNSW) Australia as the applicant. The success of the VRFB is largely attributable to its high energy efficiencies (between 80 and 90% in large installations), the soluble state of the active species (no metal deposition), its potentially low cost per kilo watt hour for large storage capacities, the minimal gas evolution during normal operation, and use of the same element in both half-cells, avoiding problems associated with cross-contamination during long-term use. The main electrode reactions for the VFRB are as followswith further side reactions (notably gas evolution) when the cells are overcharged. The electrolytes for each cell are circulated through the electrochemical cell and external reservoirs/tanks. The half cells are separated by an ion-selective membrane, typically Nafion, to transport protons. In theory, the energy storage capacity increases with the volume of the reservoirs and the concentrations of vanadium species, while the power output depends on the active electrode surface area and number of cells (when placed in a stack). Laboratory-based investigations (considering materials, operating conditions, additives and cell structure) can be highly costly, as well as time-and labour-intensive. In order to reduce costs and timescales, modelling and simulation can be employed during the design and test cycles, and used to control and monitor systems in real time, [10][11][12][13] provided of course that model parameters are available from suitable experimental data. Mathematical models of the VRFB system have been developed by Shah et al. [14][15][16][17] and by Li and Hikihara.18 These models incorporate the fundamental modes of transport, the electrochemical kinetics (including hydrogen and oxygen evolution 16,17 ) and heat losses. 15 It is not feasible, however, to incorporate this level of detail in control/mon...

Oxygen reduction reaction (ORR) in acidic media is investigated at various potentials in a thin-film rotating disk electrode (TF-RDE) configuration using electrochemical impedance spectroscopy (EIS). The ionomer-free and ionomer-containing thin-film catalyst layers are composed of Pt black and carbon-supported Pt catalysts of different metal loadings (5 and 20 wt%). The simplest EI spectrum consisting of an arc or a semi-circle is obtained at high potentials with ionomer-free Pt catalyst layers. The most complex spectrum consisting of a high frequency (HF) arc and two semi-circles is observed in the mixed diffusion-controlled region of the ionomer-containing catalyst layer with high loading of carbon-supported Pt. The nature of the EI spectrum is decided by the constituents of the thin-film catalyst layer and by the operating potential. The evolution of the EI spectra with ionomer and carbon contents is underlined. The effect of rotation rate (rpm) of the electrode on the impedance spectrum is also investigated. A series of equivalent circuits is required to completely describe the EI spectra of ORR. The kinetic parameters and the electrochemical surface area of the catalysts are derived from the impedance spectrum. Oxygen reduction reaction (ORR) is one of the most important reactions at the cathode side in low-temperature fuel cells (e.g., polymer electrolyte fuel cells (PEFCs) and direct methanol fuel cells (DMFCs)) and metal-air batteries.1-14 Because of the sluggish ORR kinetics and the stability issues of the catalyst in the electrochemical environment, expensive precious metal catalysts are often used in these electrochemical devices to catalyze the ORR. 15 Conventionally, the performance of the catalyst is evaluated in an operating fuel cell mode using the DC methods. [16][17][18] The information gathered from a DC analysis usually provides the sum of various polarizations of the electrode, which is difficult to separate into individual contributions. 19 On the other hand, electrochemical impedance spectroscopy (EIS), one of the AC methods, is a sensitive tool to investigate electrode-electrolyte interface and it allows the simultaneous resolution of various charge-transfer and mass-transfer processes (kinetic, ohmic, and diffusion). It involves a small sinusoidal electrical perturbation around a steady-state value and measures the impedance along with the phase angle. However, the interpretation of the EI spectra is difficult. Often, simple fitting models based on equivalent circuit analogues and physical models are used to extract the parameters those represent the underlying cell processes. [20][21][22][23][24][25][26] Springer et al. proposed the theoretical impedance spectrum of ORR on porous gas-diffusion electrode using the flooded-agglomerate electrode model in series with a thin electrolyte film. 20,25 The model predicted by Raistrick shows three semi-circles in the spectrum attributed to the charge-transfer process (ORR); agglomerate diffusion (depletion of the oxygen concentration in the pores...

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Shape-controlled Pt nanoparticles (cubic, tetrahedral, and cuboctahedral) are synthesized using stabilizers and capping agents. The nanoparticles are cleaned thoroughly and electrochemically characterized in acidic (0.5 M H2SO4 and 0.1 M HClO4) and alkaline (0.1 M NaOH) electrolytes, and their features are compared to that of polycrystalline Pt. Even with less than 100% shape-selectivity and with the truncation at the edges and corners as shown by the ex-situ TEM analysis, the voltammetric features of the shape-controlled nanoparticles correlate very well with that of the respective single-crystal surfaces, particularly the voltammograms of shape-controlled nanoparticles of relatively larger size. Shape-controlled nanoparticles of smaller size show somewhat higher contributions from the other orientations as well because of the unavoidable contribution from the truncation at the edges and corners. The Cu stripping voltammograms qualitatively correlate with the TEM analysis and the voltammograms. The fractions of low-index crystallographic orientations are estimated through the irreversible adsorption of Ge and Bi. Pt-nanocubes with dominant {100} facets are the most active toward oxygen reduction reaction (ORR) in strongly adsorbing H2SO4 electrolytes, while Pt-tetrahedral with dominant {111} facets is the most active in 0.1 M HClO4 and 0.1 M NaOH electrolytes. The difference in ORR activity is attributed to both the structure-sensitivity of the catalyst and the inhibiting effect of the anions present in the electrolytes. Moreover, the percentage of peroxide generation is 1.5-5% in weakly adsorbing (0.1 M HClO4) electrolytes and 5-12% in strongly adsorbing (0.5 M H2SO4 and 0.1 M NaOH) electrolytes.

Due to the sluggish kinetics of the hydrogen oxidation reaction (HOR) in alkaline electrolytes, the development of more efficient HOR catalysts is essential for the next generation of anion‐exchange membrane fuel cells (AEMFCs). In this work, CeOx is selectively deposited onto carbon‐supported Pd nanoparticles by controlled surface reactions, aiming to enhance the homogenous distribution of CeOx and its preferential attachment to Pd nanoparticles, to achieve highly active CeOx‐Pd/C catalysts. The catalysts are characterized by inductively coupled plasma–atomic emission spectroscopy, X‐ray diffraction, high‐resolution transmission electron microscopy, scanning transmission electron microscopy (STEM), electron energy loss spectroscopy, and X‐ray photoelectron spectroscopy to confirm the bulk composition, phases present, morphology, elemental mapping, local oxidation, and surface chemical states, respectively. The intimate contact between Pd and CeOx is shown through high‐resolution STEM maps. The oxophilic nature of CeOx and its effect on Pd are probed by CO stripping. The interfacial contact area between CeOx and Pd nanoparticles is calculated for the first time and correlated to the electrochemical performance of the CeOx‐Pd/C catalysts. Highest recorded HOR specific exchange current (51.5 mA mg−1Pd) and H2–O2 AEMFC performance (peak power density of 1,169 mW cm−2 mgPd−1) are obtained with a CeOx‐Pd/C catalyst with Ce0.38/Pd bulk atomic ratio.

The urea oxidation reaction (UOR) is technologically important for the development of a renewable energy infrastructure. Urea electrolysis (UE) can be used to produce hydrogen much more cost‐effectively than water electrolysis, as it theoretically requires 93% less energy. Urea can also be used as fuel in direct urea fuel cells (DUFCs), instead of H2, and thus serve as an efficient hydrogen carrier. This review addresses the UOR in neutral, acidic, and alkaline electrolytes, with special emphasis on the latter. Recent developments in Ni‐based catalysts for urea oxidation (UO) in alkaline electrolytes are discussed in detail, highlighting proposed reaction mechanisms and intermediates, based on experimental and computational results. Various catalytic designs used to mitigate the UO kinetic barriers, including the use of transition metal oxides and alloys, as well as tailored surface support materials, and discuss their application in UE and DUFCs are presented. The significant challenges impeding advances in urea electrocatalysis, in addition to emerging research areas in this field, are also discussed.

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Oxygen reduction reaction (ORR) is investigated on bulk PdO-based catalysts (oxides of Pd and Pd3Co) in oxygen-saturated 0.1 M HClO4 to establish the role of surface oxides and adsorbed hydrogen in the activity and product selectivity (H2O/H2O2). The initial voltammetric features suggest that the oxides are inactive toward ORR. The evolution of the ORR voltammograms and potential-dependent H2O2 generation features on the PdO catalyst suggest gradual and parallel in situ reduction of the bulk PdO phase below ∼0.4 V in the hydrogen underpotential deposition (Hupd) region; the reduction of the bulk PdO catalyst is confirmed from the X-ray photoelectron spectra (XPS) and X-ray diffraction (XRD) patterns. The potential-dependent H2O2 generation features originate due to the presence of surface oxides and adsorbed hydrogen; this is further confirmed using halide ions (Cl(-) and Br(-)) and peroxide as the external impurities.