J-E curves

For the same reason, Ru(OOOl) modihcation by Pt monolayer islands results in a pronounced promotion of the CO oxidation reaction at potentials above 0.55 V, which on unmodified Ru(OOOl) electrodes proceeds only with very low reaction rates. The onset potential for the CO oxidation reaction, however, is not measurably affected by the presence of the Pt islands, indicating that they do not modify the inherent reactivity of the O/OH adlayer on the Ru sites adjacent to the Pt islands. At potentials between the onset potential and a bending point in the j-E curves, COad oxidation proceeds mainly by dissociative H2O formation/ OHad formation at the interface between the Ru(OOOl) substrate and Pt islands, and subsequent reaction between OHad and COad- The Pt islands promote homo-lytic H2O dissociation, and thus accelerate the reaction. At potentials anodic of the bending point, where the current increases steeply, H2O adsorption/OHad formation and COad oxidation are proposed to proceed on the Pt monolayer islands. The lower onset potential for CO oxidation in the presence of second-layer Pt islands compared with monolayer island-modified Ru(OOOl) is assigned to the stronger bonding of a double-layer Pt film (more facile OHad formation). 

Figure 3.3.10 (A) The electrode potential dependence of the Gibbs free energy reaction pathway of the ORR. While the overall reaction has elementary steps that are energetically uphill at +1.23 V (red pathway), all elementary steps become downhill at +0.81 V (yellow pathway) (i.e. at an overpotential of approximately -0.42 V. At this point, the reaction is not limited by kinetics anymore. (B) The experimentally observed current-potential (j-E) relation of the ORR is consistent with the computational conclusions from (A) between +1.23 V and +0.81 V the j-E curve shows an exponential behavior, while at electrode potentials below +0.81 V, the ORR reaction rate becomes oxygen mass-transport limited, which is reflected by a flat ( j-E) profile. Figure adapted with permission from [19]. 

Figure 3.3.14 Experimental ORR activity of dealloyed Pt-Cu and Pt-Ni core-shell nanoparticle ORR catalysts compared to a pure-Pt nanoparticle catalyst. All three catalyst particles are supported on a high surface area carbon material indicated by the suffix 1C. The shift of the j-E curve of the core-shell catalysts indicates the onset of oxygen reduction catalysis at a more anodic electrode potential (equivalent to a lower overpotential) and hence represents improved ORR reactivity compared to pure Pt.

LaNi03 shows two-dimensional semiconducting character which would be responsible for the C 2 vs. E plots. Similar results have been reported for the reduction of Fe(CN)g" on Pb02 in 0.5MK2SO4 solutions by Lovrecek et. al [89]. While diffusion-controlled anodic oxidation currents on Pb02 are the same as on platinum, at potentials below 0.175 V vs. SCE where reduction of the oxide surface becomes noticeable, cathodic currents fall to very small values and the j-E curve shows a maximum. 

Fig. 18. Steady-state log j-E curves for H202 reduction and oxidation at a LaNi03 (1.2m2g 1) rotating disk electrode in 0.1 M KOH at 25°C after correction for mass transport in solution. H202 concentration = ImM [48],...

Fig. 19. Steady-state log j-E curves for oxygen reduction at iron oxide electrodes in 1M NaOH...

Fig. 10.35. Emission J-E curves from an oriented SiC nanowire emitter (emitting area 3.65 mm ). The average turn-on field and threshold field for this sample are about 0.9 V mm and 2.7 V mm respectively.

Figure 10.8 The toughness of a solid can be represented by the area under a engineering stress-engineering strain ((J-e) curve to fracture. Material A has the highest elastic modulus, but material C has the greatest toughness...

The j-E curve can be used not only to quantitatively describe the overall fuel cell performance but also to identify and quantify the activation loss, ohmic loss, and the mass transfer limited current density. At low current density, the ohmic loss is negligible and hence the activation loss can be directly obtained from the j-E curve at low current density. The semi-log plot of the/-E curve is linear for low current density and it can be fit to a Tafel equation (Equation 5.83) as shown in Figure 8.1 at low current density. Using the line fit to the Tafel equation. 

The linear nature of the j-E curve at low current density on a semi-log plot and the Tafel equation fitting. 

Fuel cell j-E curve, with activation loss line identifying ohmic loss and activation loss. 

As can be concluded from previous section, mass transfer rate has significant effects on recorded j-E curves. 

When working with RDE, thickness of the diffusion layer is determined by the rotational angular velocity (Eq. (12)). For every given rotation rate there is an upper limit in sweep rate for which maxima, characteristic for voltammetry in quiescent solution, are not present. For example, in aqueous solutions at room temperature, for angular velocity of 5 rps (revolutions per second) shape of the j-E curve remains unchanged up to sweep rates of 50 mV s. Conversely, for angular velocity of 2 rps, sweep rate of 50 mV s is too excessive, and characteristic current maxima in voltammetric curve can occur. 

In a typical experiment, the disc is held at a potential where intermediates or products are formed and the ring is maintained at a potential at which they undergo electron transfer. This allows quantitative kinetic measurements to be obtained. Alternatively, the disc is held at a potential where the reaction of interest takes place, and a j-E curve is recorded at the ring simultaneously. This allows the identification of intermediates and/or products. If j-E curve is recorded at the disc while the ring potential is held at a constant value where the intermediates or products are reduced or oxidized the exact potential range over which they are formed is identified. The ring current is related to the disk current by a quantity N, the collection efficiency, defined as ... 

Typical measurement is performed using RDE or RRDE voltammetry. In the first case one can control diffusion limitation, enabling more reliable extraction of kinetic currents. In the case of RRDE voltammetry selectivity is also assessed. Alternatively, one can estimate selectivity of O2 reduction to H2O (OH ) using K-L analysis (see Section 2.2) In order to obtain proper ORR j-E curves several steps are necessary (clearmess of electrochemical cell and the electrolyte is assumed, which in some case requires the use of specifically prepared cell) ... 

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