Kinetics overpotential

The rational design of improved electrocatalyst materials requires first and foremost a fundamental understanding of the origin of the kinetic overpotential. Over... 

July 13,1916 in Berlin, Germany - Dec. 12,1974 in Berlin, Germany) Study of chemistry and physics in Gottingen and Berlin from 1955 professor of physical chemistry at the Free University of Berlin. Cooperation with K. F. Bonhoeffer, - Gerischer, and J. W. Schultze, almost 100 publications (fundamentals of electrode kinetics, overpotential, redox reactions, ion transfer, passivity and corrosion, especially of iron), most important is his book [i] ... 

In Eq. 2, F is the Faraday constant (96485 C mof ) and the negative sign denotes the thermodynamically non-spontaneous nature of the water splitting process. The actual voltage required for electrolysis will depend on the fugacities of the gaseous products in Reaction 1 as well as on the electrode reaction kinetics (overpotentials)... 

Oxidation of CO is also important in fuel cell applications. By combining the half reaction for the CO2/CO couple, equation (a), with electrochemical reduction of O2, fuel cells may achieve a maximum open circuit potential given by IlSlfCOj/CO) - (02/H20) = 1.34 . In practice, electrocatalysts are required to lessen the normally high kinetic overpotentials for electrodic CO oxidation. An example of a CO/O2 fuel cell which operates at the relatively low T of 80°C by employing [Rh(CO)2Br2] as the electrocatalyst and the Rh couple to mediate CO oxidation is shown below . 

Enhancing the catalysis at the surface of PEC electrodes results in a lower kinetic overpotential and an increase in photocurrent. The effectiveness of the catalysts after surface treatment can be determined by utilizing three-electrode j-V measurements (see Section Three-Electrode j-V and Photocurrent Onset ) as well as IPCE measurements (see Chapter Incident Photon-to-Current Efficiency and Photocurrent Spectroscopy ). It may also useful to perform Mott-Schottky (see Section Mott-Schottky ) to determine any impacts these catalysts may have on the band structure (e.g., due to Eermi level pinning). 

When illuminated with energy equal to or above the band gap hv > Eg) at these operating potentials, minority hole carriers in n-type electrodes drive the OER at the electrode-electrolyte interface while minority electron carriers in p-type electrodes drive the HER at this interface. The potential at which this phenomenon begins to occur is the photocurrent onset potential (Eonset). which is offset relative to the flat-band potential (Efb) by the required kinetic overpotentials for the reaction of interest. The difference between the photocurrent onset potential (Eonset) and the reversible redox potential of interest (E°) is the onset voltage (Eonset)- A band diagram of a n-type photoanode and its hypothetical j-V response is shown in Fig. 6.7. 

The Efb is a property of the semiconductor interface that depends on the electrolyte in which the measurement is made. The onset of photocurrent does not necessarily define the potential because other interfacial effects may delay the onset to a point beyond the transition from accumulation to depletion. The error from such interfacial effects could be on the order of a few millivolts to over a volt. One such interfacial effect might be the kinetic overpotential required to drive the reaction. This overpotential shifts the photocurrent onset in the cathodic direction for p-type samples and in the anodic direction for n-type samples. Therefore, catalysts are often deposited onto the electrode surface to minimize the overpotentials (see section Catalyst Surface Treatments ). However, the modification of electrode surfaces with catalysts may influence the semiconductor/electrolyte junction and surface states and additionally shift the Efb in unexpected ways. Ideally, the catalyst treatment will improve the accuracy of the measured by this technique, although effects such as Fermi level pinning may introduce a change in the band structure at the surface which may negate the improvement from a reduced kinetic overpotential. 

The generated potential via exergonic reactions in direct fuel cells is partly used to promote electrode reactions (kinetic overpotential), while in indirect fuel cells, the fuel is first processed into simpler fuels (to reduce the kinetic overpotential) via conventional catalytic reactors, or CMRs, in which temperature is used as the key operating parameter to accomplish the desired kinetics and equilibrium conversions. Among the direct fuel cells, e.g., PEMFCs use hydrogen, direct methanol fuel cell (DMFC) uses methanol, while the SOFC can operate directly on natoral gas (Figiue 15.3). However, in indirect fuel cells, a complex fuel must be suitably reformed into simpler molecules such as H2 and CO before it can be used in a fuel ceU. 

In comparison to Eqn (15.34), however, this scheme has the following drawbacks (1) it requires expensive catalysts such as Pt—Ir, as compared to cheaper catalysts for alkaline electrolytes, such as Ni, Ag, or C (2) it is a vpe = 4-electron oxygen reduction and (3) the low exchange-current density of ORR means that the kinetic overpotential terms would be dominant in polarization expression ... 

If the reference electrode is exposed to the conditions at the reaction site, then a surface or kinetic overpotential can be defined... 

For jS = 1/2, the ratio j/jo must be greater than 4.95 for the single-term Butler-Volmer equation to be valid. The single-term cathode and anode kinetic overpotentials are... 

Equations 25 and 26 are used to measure Tafel slopes. The equations apply only when the criteria of Eq. 23 or 24 are met. Otherwise, the full Butler-Volmer equation, Eq. 21, must be solved iteratively to obtain kinetic overpotential as a function of current. 

Generally, any electrochemical half-cell conversion requires protons, electrons, and neutral molecules. These species must be transported to catalyst sites, where the reaction events take place. Electrons that are transported typically do not pose any problems, as electron conductivity of fuel cell layers could be made sufficiently high. However, transport of protons and neutral species is not that simple. In a fuel cell, any hurdle in the transport of reaction participants transforms into potential loss, that is, any transport costs some potential. The transport expenses sum up with the kinetic overpotential required to activate the conversion reaction. Metaphorically speaking, while the open-circuit potential is the total capital, the cell polarization curve displays the money (potential) left at our disposal for a given value of current in the external load. 

Viswanathan V, Norskov JK, Speidel A et al (2013) Li-02 kinetic overpotentials Tafel plots from experiment and first-principles theory. J Phys Chem Lett 4 556-560. doi 10.1021/ jz400019y... 

Hummelshoj JS, Luntz AC, Narskov JK (2013) TheOTetical evidence for low kinetic overpotentials inLi-02 electrochemistry. J Chem Phys 138 034703. doi 10.1063/1.4773242... 

The position of the electrodes in the reactor can be optimized as a function of hydrodynamic parameters and current density (j). Complementary rules should include the influences of electrode gap (e) and operating conditions on voltage U (and consequently on energy consumption). The measured potential is the sum of three contributions, namely the kinetic overpotential, the mass transfer overpotential and the overpotential caused by solution ohmic resistance. Kinetic and mass transfer overpotentials increase with current density, but mass transfer is mainly related to mixing conditions if mixing is rapid enough, mass transfer overpotential should be negligible. In this case, the model described by Chen et al., (2004) is often recommended for non-passivated electrodes ... 

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