svoboda summary lecture 1

7/28/2019 Svoboda Summary Lecture 1

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Summary

The Basic Electrochem istry

Lecture1


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Summary: Faradic process

The current flow to and from electrodes is the results of

Faradaic and Non-Faradaic processes. These processes

can occur alone or simultaneously.

A Faradaic process involves the transfer of electrons

across the electrode-electrolyte interface.

A Non-Faradaic process involves charging of the interface

(double layer), much like that of capacitor, without the

transfer of electrons.

Basic Electrochemistry: Lecture 1 Summary Vojtech Svoboda, January 2010

CPE690: Power Sources for Portable, Automotive, and Renewable Energy Systems


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Summary: Faradic process

Charge Transfer Electrodes: electrodes at which Faradaic

process occur. Electrons are either gained or given up

depending on their energy.

By driving the potential of the electrode more negative, the energy of

the electrons in the electrode increases thereby given the electrons

sufficient energy to occupy vacant states on the species in the

electrolyte.

By driving the potential more positive, the electrons in the electrode

loose energy, thereby making it possible for electrons from species in

the electrolyte to lower their energy by transferring to the electrode.




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Summary: Faradays Law of Electrolysis

Faradays Law of Electrolysis: The extent of the electroc hemical

react ion is p roport ional to the charge passed to the electrode.

This can be demonstrated by the dissolution of metal: M Mn+ + ne-

The charge Q(in coulombs) passed to the electrode is represented as:I= current (amps) to the electrode

t= duration (seconds) of the current

Moles of electrons passed are obtained by dividing Qby

Faraday constant: F= 96487 C/mol e

Mols of metal dissolved are obtained by dividing the mols of electron by n:

(mol) Then mass: (g) AW = atomic weight (g)

t

IdtQ0

Fn

tIM

*

* AW

Fn

tIM *

*

*




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Summary: Faradays Law of Electrolysis

Electrolysis is a Faradaic process where electrons are

transferred across the electrode-electrolyte interface

resulting in transformation of chemical spices.

Water electrolysis is electrochemical water splitting, where

Reduction reaction proceeds on Cathode, in which

electrons are consumed: 4H+ + 4e 2H2

Oxidation reaction proceeds on Anode, in which electronsare given: 2H2O O2 + 4H

+ + 4e

Overall reaction: 2H2O O2 + 2H




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Summary: Galvanic & Electrolytic Cell

Galvanic cell: Reactions occur spontaneously, chemical

energy can be converted into electrical energy.

Examples: Discharging batteries.

Electrolytic cell: Reactions driven by external power

source. A sufficient potential is applied across the two

electrodes to induce the electrochemical reactions.

Cathode potential is more negative than anode potential.

Examples: Charging batteries, electrolysis, electroplating.




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Summary: Non-Faradaic process & Double Layer

In some cases, it might be thermodynamically or kinetically

unfavorable for Faradaic charge transfer processes to

occur even if electrode is polarized through a range of

potentials. Current, however may still flow to the electrode.

Non-Faradaic Current - Double Layer Charging

The current originates from charging of the electrode,

which is analogous to charging of a capacitor. During

charging, species in the electrolyte may absorb or desorbfrom the interface, and therefore, the structure of the

interface may change.




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Summary: Double Layer

Interphase region: The transitional region going from the

electrode phase to the electrolyte phase. The interphase

region is also called the electric double layerdue to the

two layers of excess charge.

Double layerrepresents a capacitor is describes by

capacitance Cdranging from 10 to 40 mF/cm2. However,

since the capacitance is a function of potential, the doublelayer does not behave as an ideal capacitor.




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Summary: Double LayerDouble layer models:

Helmholtz model

Helmholtz proposed there is one layer of excesscharge in the electrode and another layer of excess

charge in the electrolyte separated by a fixed

distance to give a linear potential profile.

Goy-Chapman model

Stern model: Stern proposed a hybrid ofHelmholtz and Goy-Chapman models.

Hamann, C. et al.: Electrochemistry,

Wiley-VCH Verlag, 2007, page 117



svoboda summary lecture 1

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