电动势与热动力学燃料电池

1、电动势与热动力学燃料电池第1页，共18页，2022年，5月20日，18点44分，星期四Reversibility1)Chemical ReversibilityPt is 0.222V more negative than Ag Shorted External batterychemically reversible: Reversing the cell current merely reverses the cell reaction. No new reactions appear, thus the cell is termed;chemically irreversible: di

2、fferent electrode reactions as well as a different net process upon current reversal;第2页，共18页，2022年，5月20日，18点44分，星期四ReversibilityThermodynamically reversible: when an infinitesimal reversal in a driving force causes it to reverse directionIn electrochemistryIf a system follows the Nernst equation or

3、 an equation derived from it, the electrode reaction is often said to be thermodynamically or electrochemically reversible (or nernstian).第3页，共18页，2022年，5月20日，18点44分，星期四Free Energy and Cell emf第4页，共18页，2022年，5月20日，18点44分，星期四Reduction PotentialsThe primary reference, chosen by convention, is the norm

4、al hydrogen electrode (NHE), also called the standard hydrogen electrode (SHE):第5页，共18页，2022年，5月20日，18点44分，星期四emf and Concentration第6页，共18页，2022年，5月20日，18点44分，星期四Reference Electrodes第7页，共18页，2022年，5月20日，18点44分，星期四第8页，共18页，2022年，5月20日，18点44分，星期四Impedance Spectroscopy(Or, how a sinusoidally varying vo

5、ltage is usedto probe multiple electrical properties of materials)第9页，共18页，2022年，5月20日，18点44分，星期四IntroductionIS measures the resistance of a circuit to an applied voltageZ(t) = E(t)/I(t), Z()=E()/I()When E is applied as a sinusoidal function in a linear system, I response can be represented by a sum

6、 of sinusoidal fuctions with phase shifts.If an equivalent circuit for the system being probed can be constructed, then the resistance or capacitance values for each circuit element can be backed out from Z.Especially in electrochemical impendance spectroscopy (EIS), the the voltage probe signal is

7、small (1-10mV) so the current response is pseudolinear (side benefit: it doesnt perturb the sample very much)A Bard and L Faulkner, Electrochemical Methods: Fundamentals and Applications (2000)第10页，共18页，2022年，5月20日，18点44分，星期四Background Impedance and Circuit ElementsE(t) = E0cos(t), =2fI(t) = I0 cos(

8、t-)Or, if one writes in complex notation:E(t) = E0 exp(it)I(t) = I0 exp(it - i)Z(t) = Z0 exp(i) = Z0 (cos + isin )E/I response for a resistor (=0)E/I response for a capacitor (=-90)E/I response for an inductor (=90)http:/www.bath.ac.uk/chsacf/solartron/circp/html/acrci.htm第11页，共18页，2022年，5月20日，18点44

9、分，星期四 =-j Xc= j Xlhttp:/www.bath.ac.uk/chsacf/solartron/circp/html/acrci.htmRFor more complex circuits, use Kirchoffs rules第12页，共18页，2022年，5月20日，18点44分，星期四Plotting Impedance A Cole-Cole (or Nyquist) plot of the real and imaginary components of Zfor a circuit containing a capacitor and resistor in pa

10、rallel (real worldexample: a charged layer at an interface). = 1/ = 1/RCNamed after KS Cole and RH Cole, J Chem Phys 9, 341 (1941)第13页，共18页，2022年，5月20日，18点44分，星期四Plotting ImpedanceA Bode plot of a circuit containing a capacitor and resistor in parallel.log |Z| and vs. log are plotted, showing explic

11、itly the impact of impedance.第14页，共18页，2022年，5月20日，18点44分，星期四Circuit Elements from ElectrochemistryElectrolyte resistanceDouble layer capacitance (30F/cm2 for aqueous systems)Polarization resistanceCharge transfer resistanceCoating capacitance Constant phase element (a fudge factor)Virtual inductor

12、(adsorption?)AndlAWE, RE, CER = l / A第15页，共18页，2022年，5月20日，18点44分，星期四Diffusion (aka Warburg Impedance)Warburg impedance = diffusion of chemical species to a large planar electrodeInfinite diffusion layer thicknessDiffusion layer thickness Also, in the Warburg region, = 45第16页，共18页，2022年，5月20日，18点44分

13、，星期四Equivalent Circuit Model for Electrochemical Cells Mixed Kinetic + Diffusion modelCole-Cole plotBode plot第17页，共18页，2022年，5月20日，18点44分，星期四Some Applications of ISDetermine corrosion rates of materials*Measure capacitance and resistance of SAMs to ascertain thickness and packing qualityProbe linearity of electrical/electrochemical reactionsCharacterize electrical activity across interfaces to determine carrier concentration, etc.Determine diffusion rate of counter ions in conducting polymer/carbon nanotube compositesASL