Proton recombination

The main components of a PEM fuel cell are the flow channels, gas diffusion layers, catalyst layers, and the electrolyte membrane. The respective electrodes are attached on opposing sides of the electrolyte membrane. Both electrodes are covered with diffusion layers, and the flow channels/current collectors. The flow channels collect current from the electrodes while providing the fuel or oxidant with access to the electrodes. The gas diffusion layer allows gases to diffuse to the electro-catalysts and provides electrical contact throughout the catalyst layers. Within the anode catalyst layer, the fuel (typically H2) is oxidized to produce electrons and protons. The electrons travel through an external circuit to produce electricity, while the protons pass through the proton conducting electrolyte membrane. Within the cathode catalyst layer, the electrons and protons recombine with the oxidant (usually 02) to produce water. 

Let us consider the possible events following excitation of an acid AH that is stronger in the excited state than in the ground state (pK < pK). In the simplest case, where there is no geminate proton recombination, the processes are presented in Scheme 4.6, where t0 and Tq are the excited-state lifetimes of the acidic (AH ) and basic (A- ) forms, respectively, and ki and k i are the rate constants for deprotonation and reprotonation, respectively, kj is a pseudo-first order rate constant, whereas k i is a second-order rate constant. The excited-state equilibrium constant is K = k /k 7 ... 

The second independent method for finding K is by direct time-resolved measurements of the proton-dissociation and proton-recombination reaction rates of the excited photoacid. These measurements have traditionally utilized time-resolved fluorescence and absorption spectroscopy. They were originally developed by Weller [7-11] and Forster [3-6] and have been widely in use in photoacid research [17,18, 27]. 

Assuming homogenous proton recombination and unidirectional dissociation reaction one has, for the excited-state equilibrium constant ... 

The advantage of Eqs. (12.13)-(12.15) and (12.17) was that they allowed the direct determination of the excited-state equilibrium constant by a single kinetic measurement. The proton dissociation rate constant and hence also the proton recombination rate constant may also be found from the same measurement. Although this method has been applied successfully in only a few cases [60, 61], the values thus found have been in very good agreement with values independently estimated from the Forster cycle or by steady-state titrations. 

Figure 12.14 (a) Fluorescence spectra of 1-hydroxypyrene at 80°C measured under the excitation of the photoacid at pH 6 (circles) and at pH 3 where negligible photoacid dissociation occurs due to much faster proton recombination process with bulk protons (dashes), and under direct excitation of the photobase at pH 12 (solid line), (b) Dashed... 

There are some similarities between these two sites the lifetime of 0-, the rate of proton escape (k23), and even the apparent rate of proton recombination (A2i-[H+]). The implication of these values will be discussed below. What markedly differentiates the two sites is the rate of proton dissociation (ki2). In both sites, the rate of proton dissociation is significantly slower than in water, implying that in these sites the water molecules are at a state that is not suitable for rapid (sub-picosecond) hydration of the discharged proton. The equivalent water activity coefficients, as estimated from the kinetic method described in Section III. are... 

The rate of proton recombination has been measured either from the time-resolved kinetics or using the steady-state Equation (5). The values calculated by both methods are shown in Figure 22, which relates the rate of the reaction, as calculated by Equation (21) with the radius of the proton-permeable space. The experimental results cluster on the theoretical curve in the range of R = 72 7 A, a good approximation with the internal radius of a small liposome (Brauillette et al., 1982). 

Figure 22. The correlation between the rate of proton recombination with its conjugate base and the internal diameter of the liposome. Rates of proton recombination were determined either by steady-state fluorescence ( ) or time-resolved (O) measurements. The line is drawn according to Equation (21).

The rate constant of proton recombination, either in a microspace or in a liposome is fast, k 1 (r sec-1 (Table 11, Figure 22) and measurable with a high degree of accuracy ( 20%). In our formalism, this rate is treated as a product of two terms k2i[H+]. This might be misleading, especially when the probed space is very small and a(H2>[Pg.40]

Figure 25. The dependence of the macroscopic parameters on the rate constant of proton recombination with the proton emitter anion. The macroscopic parameters were calculated for simulations describing the experimental conditions defined in Figure 23. The frames represent y, (A), y3 (B), Tmax (C), and Fmax (D) as a function of the rate of protonation of CT. In each figure, there are three curves calculated for k3 with the values of 3.2 x lO10 Af"1 sec-1 ( ), 4.2 x lO10 Af"1 sec-1 (—), and 6.2 x 1010 Af"1 sec 1 (—). The experimentally determined macroscopic parameters are indicated as parallel horizontal lines. The vertical lines denote the range of A, values that will yield macroscopic parameters compatible with the measured ones.

In order to prove that geminate recombination is Indeed responsible for the nonexponential luminescence decay of R OH, we introduced proton scavengers to the aqueous solution. According to our model, it is expected that the contribution to the fluorescence intensity due to geminate recombination will be reduced. In the case of HPTS, the luminescence after few hundred picoseconds is mainly due to geminate recombination. The obvious scavenger for HPTS is its.ground state anionic form RO with a proton recombination rate of 2 x 10 s... 

At low pH values, when additional protons are present, the separation step becomes reversible and one observes homogeneous proton recombination. The reaction under these conditions undergoes a transition from unimolecular (correlated pairs) to a bimolecular (or pseudo-unimolecular) reaction. The rate of this recombination reaction is expected to diminish with increasing concentration of inert salt, which screens the Coulombic attraction between the proton and the anion. In fact, the classical Bronsted-Bjerrum theory of salt effects puts all of the effect in the recombination reaction while predicting zero salt effect on the dissociation direction [7]. 

The aim of this lecture is to provide a qualitative description of reversible proton transfer reactions in the excited-state, using the extended theory of diffusion influenced reactions. The complete equations and numerical procedures may be found in the literature [10-14]. Major results include (i) the asymptotic power-law decay and the evidence for diffusive kinetics [10] (ii) The salt effect [11] and the Naive Approximation for the screening function [17, 11] and (iii) an extension [18] of the theory for approximating the effect of competing geminate and homogeneous proton recombination expected atdow pH values. 

Ultrafast proton transfer including geminate proton recombination in the excited states... 

---