Potential carbon phase

Fig. 8.21 Left) schematic of a DMFC with strong methanol depletion in the second half of the channel (the normalized methanol concentration, membrane potential and carbon phase potentials are shown in the bottom) (right) calculated cell potential and carbon and Ru overpotentials as function of the fraction of methanol-depleted domain (Source [195] reproduced with permission...

The driving forces for charged particles in this environment are modelled by two continuous potentials the electrolyte phase potential and the carbon phase potential ip. The gradient of (pm drives protons in the electrol de phase while the gradient of cp induces electron current in the Pt-carbon phase. 

Let the cathode side be grounded, i.e. the carbon phase potential is zero. Thanks to the high electronic conductivity of this phase, the variation... 

Returning to the real fuel cell, we see that the real cell voltage is simply Vceii = Voc — Vioss and the real carbon phase potential on the anode side is = Vceii = Voa — p ° (Figure 1.3). The introduction of is convenient in modelling to calculate the losses we may ignore the OCV and use p ° instead of the real carbon phase potential on the anode side. The cell voltage is then obtained from the relation Vceii = Voc — ... 

In one-dimensional electrode modeling, (x) denotes the metal phase potential and (x) the electrolyte phase potential. The gradient of the metal (carbon) phase potential drives the electron flux, while protons move along the potential gradient of the electrolyte (ionomer) phase. At equilibrium, these gradients are zero and the potentials in the distinct phases are constant, (p (x) = and O (x) = 4) . The potential distribution of a working PEFC with porous electrodes of finite thickness is shown in Figure 1.9. For illustrative purposes, a simple assembly of anode catalyst layer, PEM and cathode catalyst layer is displayed. 

The simplest way of modeling the CCL performance is to consider the structure depicted in Figure 3.1 as a homogeneous medium with effective parameters for electron, proton, and oxygen transport (macrohomogeneous model, MHM). Particularly, the membrane phase potential and carbon-phase potential 4> are assumed to be continuous functions of coordinates. Physically, this means that each representative volume of the CCL contains many carbon and Pt particles, Nafion phase domains, and voids, so that one may speak about volume-averaged concentrations, potentials, transport coefficients, currents, reaction rates, and so on. 

The electrode E serves as a second anode, which converts the flux of methanol in the membrane into useful current. Indeed, if R is small, the carbon phase potential of E is almost equal to the carbon phase potential of the cathode, while the electrolyte phase potential in E is not far from that potential in the anode. Thus, the MOR overpotential in E, would be large, that is, the auxiliary electrode would efficiently convert the flux of methanol in the membrane into the usefiil ionic and electron currents. Moreover, thanks to E, no methanol would arrive at the cathode, thereby no poisoning of the ORR electrode by the MOR products would occur. 

The anode side of the cell is grounded and, hence, = 0. Solution of Equation 5.217, subject to Equation 5.218, is obtained by varying the only free parameter in this problem, the carbon phase potential of the cathode 4> . Eor simplicity, 5 in Equations 5.207 and 5.213 is taken to be the same. 

From Table 5.11, it can be seen that the expression for the ORR overpotential contains yet undefined parameter, the carbon phase potential of the cathode cj) (the cell potential). The solution to Equation 5.228 must obey the following condition ... 

Regarding the first problem, the most elemental treatment consists of focusing on a few points on the gas-phase potential energy hypersurface, namely, the reactants, transition state structures and products. As an example, we will mention the work [35,36] that was done on the Meyer-Schuster reaction, an acid catalyzed rearrangement of a-acetylenic secondary and tertiary alcohols to a.p-unsaturatcd carbonyl compounds, in which the solvent plays an active role. This reaction comprises four steps. In the first, a rapid protonation takes place at the hydroxyl group. The second, which is the rate limiting step, is an apparent 1, 3-shift of the protonated hydroxyl group from carbon Ci to carbon C3. The third step is presumably a rapid allenol deprotonation, followed by a keto-enol equilibrium that leads to the final product. 

Equations (18-20) are discretized by the control volume method53 and solved numerically to obtain distributions of species (H2, 02, and N2) concentration, phase potential (solid and electrolyte), and the current resulting from each electrode reaction, in particular, carbon corrosion and oxygen evolution currents at the cathode catalyst layer, with the following initial and boundary conditions ... 

Here / is the current density with the subscript representing a specific electrode reaction, capacitive current density at an electrode, or current density for the power source or the load. The surface overpotential (defined as the difference between the solid and electrolyte phase potentials) drives the electrochemical reactions and determines the capacitive current. Therefore, the three Eqs. (34), (35), and (3) can be solved for the three unknowns the electrolyte phase potential in the H2/air cell (e,Power), electrolyte phase potential in the air/air cell (e,Load), and cathode solid phase potential (s,cath), with anode solid phase potential (Sjan) being set to be zero as a reference. The carbon corrosion current is then determined using the calculated phase potential difference across the cathode/membrane interface in the air/air cell. The model couples carbon corrosion with the oxygen evolution reaction, other normal electrode reactions (HOR and ORR), and the capacitive current in the fuel cell during start-stop. 

Copper, lead, cadmium and zinc have been found predominantly in potentially mobile forms by sequential extraction of material collected during road cleaning (Colandini et al, 1995). Cadmium and zinc were found to be more labile than lead and copper. A study of street dust and gully pot sediments confirmed this order of potential availability (Striebel and Gruber, 1997) and also suggested that lead levels in material of the types studied had decreased since the introduction of unleaded fuel in Germany. Lead has also been studied in street dusts from Brisbane, Australia. The element was found mainly in the carbonate phase and in the smaller particle size fraction (Al-Chalabi and Hawker, 1996) except where resuspension caused particle aggregation. 

Potentials The potential drop across the metal/electrolyte interface is the driving force for the electrochemical reactions. The rates of electrochemical reactions depend on the potential in the membrane electrolyte phase, ipe, and on the potentials of carbon phase cpa and [Pg.511]

Since the conductivity of the carbon phase is large, the electrode potentials keep track of the variation of membrane phase potential (pe. Note that catalyst layer/membrane interface (cf. Fig. 24). 

A dense carbon phase with a calculated density of 4.1 gcm was predicted by N. N. Matyusenko and V. E. Strel nitzkii in 1979 [71]. Due to this high density value, ultrahigh hardness of this carbon material is expected. In addition, several different hypothetical three-dimensional polymeric carbon networks with interesting materials properties have been proposed. The most relevant ones with respect to the potential of high hardness are the following carbon networks. 

Figure 6.2. Sketch of voltage losses in a PEFC. ACL and CCL abbreviate anode and cathode catalyst layers, respectively. Solid line - potential of the membrane phase, dashed lines - potential of the carbon phase. Dotted areas display local polarization voltage tj.

The atomic partial charge of the model phase and hydrocarbon does not change significantly after optimization of the eomplex form, while the electron potential is slightly shifted toward the molecular interaction side. These results clearly indicate the existence of different retention mechanisms on graphitized carbon phases, a hydrophobic interaction and hydrogen bonding. 

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