Proton Transfer Efficiency

In many MFCs, the proton transfer efficiency from the anode to the cathode is the rate-limiting step and a major cause of internal resistance. Although equivalent amounts of protons and electrons are produced at the anode in MFCs, the migration rate of protons to the cathode is much slower than that of the electrons. It arises from the fact that the migration of electrons is forced by the potential difference between the two electrodes, while the migration of protons is caused by diffusion. A proton exchange membrane (PEM), if present, functions as a proton transfer barrier, and further decreases the proton diffusion rate. Since proton transport inside the fuel cell is slower than its production rate in the anode and its consumption rate in the cathode, a pH difference between the two electrodes occurs without buffer. For example, in the absence of any buffer solution, Gil et al. [76] detected a pH difference of 4.1 (9.5 at cathode and 5.4 at anode) after a 5-h operation with an initial pH of 7 in both chambers. Accumulation of protons at the anode will suppress the microbial activity, thus the electricity production, whereas a limited proton concentration at the cathode may reduce the cathodic reduction rate. 

Proton-transfer efficiency depends on the distance between the two electrodes, the type of membrane used, and the type and concentration of buffer. At present, there is no standard method to measure the proton transfer efficiency. It can only be qualitatively determined by the pH difference between the anode and cathode chambers. According to the diffusion theory, reducing the anode-to-cathode distance facilitates the proton transfer. Should the electrodes be too close to each other, there will be adverse effects on the activities of microorganisms [60]. 

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The acido-basic properties of water molecules are greatly affected in restricted media such as the active sites of enzymes, reverse micelles, etc. The ability of water to accept or yield a proton is indeed related to its H-bonded structure which is, in a confined environment, different from that of bulk water. Water acidity is then best described by the concept of proton-transfer efficiency -characterized by the rate constants of deprotonation and reprotonation of solutes - instead of the classical concept of pH. Such rate constants can be determined by means of fluorescent acidic or basic probes. 

Excited-state proton transfer relates to a class of molecules with one or more ionizable proton, whose proton-transfer efficiency is different in the ground and excited states. The works of Forster [2-4] and Weller [5-7] laid the foundation for this area on which much of the subsequent work was based. Forster s work led to the understanding of the thermodynamics of ESPT. He constructed a thermodynamic cycle (Forster cycle) which, under certain acceptable approximations, provides the excited-state proton-transfer equilibrium constant (pK f,) from the corresponding ground-state value (pKa) and electronic transition energies of the acid (protonated) and base (deprotonated) forms of the ESPT molecule ... 

Up to date, performances of laboratory MFCs are still much lower than the ideal performance. Electricity generation in an MFC is a combined effect of (i) microbial catabolism, (ii) electron transfer from microbes to the anode (anode performance), (iii) reduction of electron acceptors at the cathode (cathode performance), and (iv) proton transfer from the anode to cathode. Factors affecting each of the above processes may have a great influence on the overall MFC performance, thus appropriate experimental conditions is very important. Many modifications have been carried out to improve each of these processes, leading to higher total power output of MFCs. As listed in Table 2.5, major factors affecting the power generation of MFCs include microbial catalytic activity, anode and cathode performances, proton transfer efficiency and solution chemistry. 

Proton transfer efficiency Reduce electrode spacing Use suitable membrane or eliminate membrane Use suitable buffer... 

The anodic microbial catalysts are expected to play a key role for the power density and efficiency of an MFC. The highest power generation of MFCs seems to be produced by those operating with a mixed culture, or a microbial community, rather than those operating with a pure culture. The structure and activity of the bacterial community are sensitive to various environmental conditions, such as solution pH, electrode potential, ionic strength, and temperature. Since these environmental parameters can also affect other processes (e.g. the proton transfer efficiency, and cathode performance), it is now more difficult to conclude a quantified relationship between the microbial community and these parameters. Moreover, the configuration and operating mode of MFCs also appear important for the composition and activity of anodic biofilms. 

Certain molecules that can permit concerted proton transfers are efficient catalysts for reactions at carbonyl centers. An example is the catalytic effect that 2-pyridone has on the aminolysis of esters. Although neither a strong base (pA aH+ = 0.75) nor a strong acid (pJsfa = 11.6), 2-pyridone is an effective catalyst of the reaction of -butylamine with 4-nitrophenyl acetate. The overall rate is more than 500 times greater when 2-pyridone acts... 

Acrylamides represent still another interesting class of monomers.6 Their anionic polymerization may be initiated by strong bases, like, e.g., amides. The growing chain contains the unit —CH2—CH —CO—NH2 and intramolecular proton transfer competes efficiently with its carbanionic growth. Since the rearrangement... 

Poly(aryl ether) branches of generation 1 to 3 have been appended to a pho-totautomerizable quinoHne core to investigate the effect of dendritic architecture on the excited state intramolecular proton transfer [45]. The changes observed in the absorption and emission spectra on increasing dendrimer generation indicate that the dendritic branches affect the planarity of the core and therefore the efficiency of the excited state intramolecular proton transfer and of the related fluorescence processes. 

For the simulation a correlation time 1 =0.1 ns is assumed for two protons at cOo=600MHz. (B) Maximum transfer efficiency for an isolated proton spin pair calculated using only dipolar relaxation processes. Note the sign change for the NOE cross-relaxation at cOo Uc=l -12. 

Fischer, M. Wan, P. Nonlinear solvent water effects in the excited-state (formal) intramolecular proton transfer (ESIPT) in m-hydroxy-1,1-diaryl alkenes efficient formation of m-quinone methides. J. Am. Chem. Soc. 1999, 121, 4555-4562. 

In the perfectly paired double strand 22, the yield of product PGgg> which indicates the amount of charge that has reached the hole trap GGG, is 68%. But if the intermediate G C base pair is exchanged by a G T mismatch, the efficiency of the charge transport drops to 23%. With an abasic site (H) opposite to G the hole transport nearly stops at this mismatched site (Fig. 15). We have explained this influence of a mismatch on the efficiency of the charge transport by a proton transfer from the guanine radical cation (G2 +)... 

These experiments demonstrate the importance of proton transfer processes during hole transfer through DNA. S. Steenken has already remarked that a proton shift between the G C bases stabilizes the positive charge [23]. If such a proton shift is coupled with the hole shift, a deuterium isotope effect should arise. Actually, H/D isotope effects are described by V. Shafiro-vich, M.D. Sevilla as well as H.H. Thorp in their articles of this volume. Experiments with our assay [22] also demonstrate (Fig. 16) that hole transfer in protonated DNA (H20 as solvent) is three times more efficient than in deuterated DNA (D20 as solvent). If this reflects a primary isotope effect, it shows that the charge transfer is coupled with a proton transfer. 

Epoxide derivatives of 2,5-bis-oxyphenyl-l,3,4-oxadiazoles and fluorescein, which are luminescent epoxide monomers, were synthesized and their luminescence properties were studied <1999CHE358>. The excited state intramolecular proton transfer reactions and luminescent properties of the ort j-hydroxy derivatives of 2,5-diphenyl-1,3,4-oxadiazole were elucidated to conclude that the proton phototransfer reaction is very efficient in the studied... 

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