Electrochemistry Flashcards
What is electrochemistry?
Electrochemistry is the study of the relatioship between electricity and chemical change
What are oxidatiopn, reduction, redox and explain oxidation states using combustion of methane
Oxidation is loss of electrons, Reduction is the gain of electrons
Redox: chemical reaction that involves a reduction and oxidation process
CH4 + 2O2 —> CO2 + 2H2O
C is more electronegative than H so oxidation state is -4 then +4
H oxidation state is +1 then +1
O is more electronegative than carbon sp oxidation state =0 then -2
Which species is oxidised and which is reduced in combustion of methane?
C oxidation state -4 — +4 = oxidation (loss e-)
H oxidation state +1 — +1
O oxidation state 0 — -2 = reduction (gain of e-)
O2 is actimg as an oxidising agent
What are the half cell reactions and net reaction for combustion of hydrogen?
H2 oxidation: 2H2 —> 4H+ + 4e-
O2 reduction: O2 + 4H+ + 4e- —> 2H2O
Net: 2H2 + O2 —> 2H2O
reaction in forward direction is spontaneous and releasing energy
reaction in backwards direction is energy storing in the form of chemical bonds
What is the difference between a fuel cell and a battery?
A fuel cell is anm open reactive system - requires the influx of fuel and the outflux of products
Battery closed reactive system
How do we release energy from a fuel cell
Hydrogen is ionised at platinum electrode to 4H+
4e- flow through electrode then through wire to other platinum electrode.
4H+ flow through salt bridge (membrane separating two half cells permeable to H+
O2 combine with electrons and protons to form water
e- flowing through the circuit can be made to do work such as drive a motor in a car
How do we capture energy in a solar panel?
Sun releases photons
Panel converts photons absorbed to electricity
Electricity uis used to drive water splitting
Knows as an artificial leaf - chemical/ synthetic photosynthesis
Apllications of electrochemistry to synthesis
Use an electrical driving force to activate otherwise challenging chemical bond formation reactions
tertiary hydricarbon with half O2 using C5H12N and electrical system with RBC anode and Ni cathode can form tertiary alcohol
What is an anode, cathode, net reaction and cell potential using lithium ion fuel cell?
Anode - where oxidation occurs (anions attracted):
LiC6 —> Li+ + e- + C6 - oxidation
Cathode: where reduction occurs:
CoO2 + Li+ + e- —> LiCoO2 - cobalt reduction
Net reaction: LiC6 + CoO2 —> LiCoO2 +C6
Cell potential E cell = Ecathode - Eanode
What is the equation for Gibbs free energy?
ΔGcell = -nFEcell
n = no. electrons transferred
F = Faraday constant (96485 C mol-1)
Ecell = cell potential
What is the equation for the equilibrium constant?
ΔG = -RTlnK
K = exp^ΔG/RT
T = temp in Kelvin
K = equlibrium constant
How do you predict if an acid base reaction is spontanteous?
ΔGreaction = ΔG1 - ΔG2
-RTlnKrxn = -RTlnK1 + RTlnK2
lnKrxn = lnK1 - lnK2 = ln(K1/K2)
Krxn = K1/K2 = 10^-pKa1 / 10^-pKa2
What is the general equation used for redox reactions
For generic reaction where oxidied species is reversibly recuced by unspecified number of electrons we use the equation: Ox + ne- <—> Red
Why do we define standard electrode potentials and what are they?
We use standard electrode potentials to quantify the thermodynamics of reactions
They are a measure of the individual potentials of a reversible electrode under standard consitions:
Stated temp - usually 298 K
Unit activity of every ion in the reaction - 1 mol dm-3
Partial pressure of 1 bar for each gas in the reaction
Metals in their pure state
What do the signs of standard electrode potentials show?
What are standard elevctrode potantials defined against?
Very -ve = loses electrons easily
Very +ve = gains electrons easily
Defined against standartd hydrogen electrode
What is the equation for th standard electrode potential of a cell?
Ecell = Ecathode - Eanode
Which way do electrons spontaneously flow?
from the anode to the cathode
What are the parameters for a reaction being spontaneous?
DG must be negative
For this to be:
Ecathode > Eanode
What are the rules for writing cell diagrams?
Anode described first, then the cathode
Reagents described first then products
Single line drawn between two chemical species and double line represent salt bridge
Phase shows in brackets
If electrolytes are not at standard consitions concentration/pressure are specified in the brackets
Zn(s) | Zn(aq)2+ || Cu(aq)2+ | Cu(s)
What is a latimer diagram?
a summary of teh standard electrode potential data of an element
The most highluy oxidised form of the element is on the left with lower oxidation states to the right
Species are connected by arows - each arrow labelled with the standard potential for that reduction reaction
What is the latimer diagram for Cl and what are the oxidation states
ClO4- = +7
ClO3- = +3
HClO2 = +3
HClO = +1
Cl2 = 0
Cl- = -1
How do you calculate the standard electrode potential for the cinversion of the mist oxidised species to the most reduced ina latimer diagram?
Thestandard electrode potentials for each small reaction cannot be added.
Using DG = nFE the DG value for each small reaction must be determined then added
This value can be converted back to E
What is a comproportion reaction?
When two equivalents of the same elements differing in oxidation state combine to form an intermediate compound with intermediate oxidation state
ClO3- + 5Cl- + 6H+ <—> 3Cl2 + 3H2O
What is a Frost diagram?
summarises the energetics of the half-reaction in which species X with oxidation state N is conveted to its elemental form for all oxidatin states accessible to element X
ON the y axis the NE value for converting a given oxidation state to the elememental form
The oxidation state os potted on the x axis
y axis of proportional to the gibbs energy of the conversion
The gradient of a line between two species is the electrode potential for that reduction
How do you determine whether a species os unstable with respect to disproportionation?
A species that lies above the gradient between two points shows that is unstable with respect to disproportionation
How do you determine whether comproportionation is favourable?
If a the middle species in a set of three is in a ‘valley’ on the Frost diagram
How do you construct a Frost diagram?
Write down all the species in the latimer diagram
Write down the oxidation states of each
Write down the electrode potentials for each step
Multiply each electrode potential by the nukber of electrons
Add the NE values that would be required to get the species to the elemental form
How do you find the mopst stable oxidation state from a Frost diagram
Lowest point on the graph
What is the Nernst equation?
E = Eo - RT/nF lnQ
Q = reaction quotient - supposed to be acitivity of products/reactats but we simplify to concentration
The activity of a solid is 1
What is the equation for dE/dpH?
For m-proton and n-electron processes
dE/dpH = -m/n * (RT ln10) / F
How much does the potential of a reaction change with pH unit?
59 mV
How do we decide which pH to run a reaction at?
If the line of the latimer diagram is steeper at one pH this means the reduction potential is larger and therefroe the reaction is more feasible at this pH
What are Pourbaix diagrams?
plots of electrode potentials vs pH
They map out the possible stable phases of an electrochemical system and compare this to the redox reactivity
What is the Pourbaix diagram for water?
Water is only stable within a restricted potential window
At suffieciently negative pH water is susceptible to reduction:
2H+ + 2e- H2 Eo = 0V
Moving to higher pHs the electrodempotential changes with gradient -59 mV
At sufficiently high potantials water is susceptible to oxidation:
O2 (g) + 4H+ + 4e- 2H20
What is activation energy?
The energy difference between the starting materials and the transition state
If the products are at a lower energy than the reactants the reaction is usually exothermic - this gives negative delta G value
What is the overpotential in the oxidation process?
Theoretically if an applied voltage is greater than Ered this should drive oxidation
In reality the applied voltage must be greater than the overpotential for oxidation to occur
What s the overpotential in recution process?
Theoretically if an applied voltage is more negative than Ered this should drive reduction
In reality it must be more negative than the overpotential
How do overpotentials affect batteries?
A kinetic barrier to the reaction manifests itself as an Ecell that is smaller than the thermodynamic value
This qualifies as an overpotential
Ecell = Ecathode - Eanode - Eoverpotential
If there is an overpotential the energy released is lower than the theortical maximum
What does the theoretical Ecell look like if the anodic process has an overpotential and if the cathodic process has an overpotential?
How does an overpotential manifest itself in an energy consuming process?
The requirement for greater input of energy
What is the overpotential in photosynthesis?
- 0.3V is the potential for NAD+/NADH at pH7
0. 8V is the potential for O2/H2O at pH7
Photosynthesis has an overpotential of approx. 0.3V
Ephotosynthesis = -0.3 - (0.8 + 0.3) = -1.4V
DGphotosynthesis = -nFE = -4 x 96485 x -1.4
= 540 kJ mol-1
In a zinc copper cell how do we determine how the concentrations of each half cell have changed at equilibrium?
How do we determine the total charge that flows in a cell?
Total charge (C) = amount of e- (mol) x F (Cmol-1)
For the zinc copper cell2 electrons flow for every mol of Cu consumed (2 x 0.4 = 0.8 mol of electrons)
So 0.8 x 96485 = 77188 C
This gives no information about how quickly the equilibrium is reached
What is dynamic electrochemistry?
monitoring the rate and energetics of an electrochemical reaction
All basic dynamic electrochemistry experiments measure electrical current (rate of flow of e-) while controlling the electrode potential Cthermodynamic driving force
What is the most common way to make a dynmaic electrochemistry experiment?
The 3 electrode cell
The potential of the working electrode is measured relative to that of the reference electrode while current flows between the working electrode and the counter electrode
What does current tell us about the working electrode?
Negative current = electrons moving out of the working elevctrode surface
Positive current = electrons moving into the working electrode surface
What is faradaic current?
Current arising from a redox process occurring at a working electrode
Positive faradaic current indicates an oxidative process
Negative faradaic current indicates a reduction reaction is occurring
Current due to a non redox potential is described as non faradaic
What is the reference electrode used for?
Standard electrode potentials are measured relative to the standard hydrgen electrode at pH0, 1 bar H2 and Pt electrpocatalyst
This is not always practical
Reference electrodes are used instead which have highly stable redox potentials vs the hydrigen electrode
reference electrode potential is measured V vs SHE so the potential of a cell measured is also defined as V vs SHE
Why do we use a counterelectrode?
If current flows through the reference electrode this will perturb the ratio of oxidesed:reduced species
This would change the redox potential
What is the supporting electrolyte?
The solvent system
Must have suffiecient conductivity so that the solution resistance does not convolute experiment results
In aq solution NaCl is commonly used
In organic solvents tetrabutylammonium hexafluorophosphate is a common salt to use.
What is chronoamperometry?
Measure current as a function of time while constant voltage is applied
applied voltage is Eapp (V vs Ref)
Time of hold - thold
What is voltammetry?
Voltage is modulated as a function of time while current is monitored - plot current vs voltage
What is linear sweep voltammetry?
Linear voltage sweep is applied with either a positive (anodic) or negative (cathodic) gradient
Control the potential of the working electode (gradient of plot against time is the scan rate)
Increasing the potential increases the driving force to remove electrons
Measure current vs voltage
What is cyclic voltammetry?
A forward and backwards sweep is applied with both a positive and negative gradient
Repeated voltage cycles are called scans or sweeps or CVs
Control potential of the working electrode with Einitial, Eend and Eturn (tutning point voltage)
Measure current
What do we measure in cyclic voltammetry when there is a lack of redox activity?
With non-faradaic current (absence of redox active species) a baseline current response is recorded
This can be thought of as eqivalent to the solvent ad cuvetter absorbance in UV vis
This current is referred to as non-faradaic or capacitive current
It arises because of double layer rearrangements at th wokring elecrode surface-solution interface
How does changing current affect the baseline current?
Steadily decreasing the voltage of the woring electrode charges the surface of the electrode with cations because of the electrostatic attraction and anions move away resulting in negative current. The rate of movement of ions is directly proportional to the rate of change of potential so a constant -ve gradient os measured
Reversing the direction of the voltage sweep reverses the direction of ion movement resulting in an equal current of opposite sign as anions move towards the electrode surace and cations move away
How does scan rate affect ion movement?
Scan rate directly controls rate of movement of ions
There is a linear relationship between capacitive current and scan rate
What is the difference between film and solution electrochemistry?
Film electrochemistry is when the analyste is adsorbed onto the working electrode
Solution electrochemistry is when the analyte is dissolved in the electrochemical cell solution
What is the set up of film electrochemical experiment?
What happens in the recution of CO2 with a Co porphyrin catalyst when constant voltage sufficient to reduce the Co is applied?
The co is reduced from Co2+ to Co+
The eletrode transfers electrons to the Co catalyst
The co catalust transfers its electrons to CO2 and H+
This converts them to CH4 and H2O
This is thought to be a spontaneous potential scale
What does a chronoamperometry experiement tell us?
For the reduction of CO2 the measured current is -10mA
8 electrons are used per CH4 produced
- 10 mA = 10 x 10-3 Cs-1
- ve current means electrons flowing out of the electrode
We can find the maximum ate of production of CH4
How do we know this is the maximum current?
We assumed 100% faradaic efficiency
assume every 8 electrons produces CH4 with no side reactions or inefficiencies
How do we calculate the maximum yield of methane production in the reduction of CO2?
- 10 mA current flows for 60 seconds
0. 01 Cs-1 x 60 s = 0.6 C - total charge flowing out of the electrode
0. 6 C / 96485 C (mol e-)-1 x 8 electrons (CH4)-1
= 7.7 x 10-7 mol CH4
How do we calculate the rate of catalysis?
kcat = icat / n x F x mcat
kcat = rate of catalysis
icat = current of catalysis
mcat = molar amount of catalyst
Can voltammetry be used to monitor catalytic redox chmistry?
Yes catalytic redox chemistry can be intergrated as a function of potential using cyclic voltammetry
The voltage of the working electrode will change
It is important to consider both faradaic and non-faradaic current
How do we find the molar amounts of Co+ and Co2+ in the recution of CO2?
What shape would the purely faradaic catakytic current have?
Sigmoidal
How can purely faradaic current be thought of in terms of windows?
Three windows:
1 - zero faradaic current - potential of the working electrode so positivr that none of the Co2+ is reduced and catalysis does not proceed
2 - constant faradaic current - potential of the working electrode os so negative that all the cobalt of converted to Co+ so the rate of catalysis is independent of potential
3 - dramatic change in current - equilibrium ratio of MCO2+ : MCO+ is highly dependent on potential
What does real data look like?
Real data is the sum of faradaic and non-faradaic current
How do we calculate the overpotential of the catalyst?
Look at the m (no. protons) and n (no. electrons) values
For Co catalyst in reduction of CO2 m= 8 and n=8
When m = n E drops by 59 mV per pH unit
If the Eo value is -0.25 V vs SHE at pH0 then at pH 1
Eo = -0.25 - 0.0059 = -0.31 V
How does the cyclic voltammogram change with a higher turnovr rate?
When kcat increases icat increases
The plot expands
In what situations would an electlyte salt not be necessary?
If the solution includes concentrated acid the ionic strenght will be high enough
How do we calculate the time of an experiment?
Voltage / scan rate
What happens in film voltammetry of non catalytic reversible redox process?
electrode reversibly pushes/pulls electrons into a species adsorbed onto the surface
Copper electron transfer proteons can be adsorbed onto graphite or gold electrodes
What would the theortical faradaic current response look like in film voltammetry on non catalytic reverible redox process?
Eox = Ered
These peak potentials equal the potential of the redox couple being investigated
The peak area is equal to the charge passed and provides a measure of the muber of moles of redox active material
For film voltammetry of a reversible redox process what is the relationship between current and scan rate?
Linear relationship as the same amount of redox active species is on the surfact of the electrode in all experiments
What is the total current response?
Totsal current response = sum of faradaic and non faradaic contributions
In real film voltammetry of reversible redox process what effect is visible that is not predicted by theory?
Eox and Ered shift with scan rate
If Eox and Ered are plotted against scan rate a trumpet plot of formed
This is because the faradaic current is smaller and therefore non-faradaic contributions to the total curretn are more significant
The shift reflects kinetics of the electron transfer - rate info contained
What is solution cyclic voltammetry used for?
Determining HOMO LUMO gap
Evaluating homogenous electrocatalytic processes
Observing electron transfer through molecules
What is the set up of solution cyclic voltammetry?
Same as for film electrochemistry but analyte is in solution
Auxilary electrode - completes circuit
Reference electrode - defines x axis on voltammogram
Working electrode - disc of platinum / gold / glassy carbon
All electrodes must be submerged in the solution
Electrochemistry is only happening at the surface of the electrode The bulk solution remains unaffected
Why is the solvent choice important?
At a certain negative anmd positive potential the solvent will be reduced and oxidised respectively
Between these two potentials is called the solvent window
The analyte must undergo redox chemistry within this window
What is the shape of the voltammogram in solution voltammetry?
Duck shaped
The peak anodic potential is Epa
The peak cathodic potential is Epc
The average of these points is E1/2
The max current upon oxidation (difference between peak and baseline) is ipa
The max current upon reduction is ipc
Why is the voltammogram duck shaped?
In the beginning there is no current response - the voltage is not high enough toreove and electron
The current peaks as the electrons are removed from the anion
The current decreases - fewer molecules to oxide in the diffusion sphere
Current drops to non zero baseline because molecules are diffusimg into the diffusion sphere and instantly oxidised
Current peaks in opposite ciection as electrons are added to the newly oxidised analyte
Returns to zero baseline
What is diffusion control?
The solution is under diffusion control
The faradaic signal is dominated by loinear diffuciom effects
Diffusion is only significant in the direction normal to the electrode surface - reaction and transports are uniform across the surface
current is proportional to concentration of analyte and scales with squ root of scan rate
How does scan rate affect diffusion control?
Faster scan rate:
shirter diffusion distance
greater concentration gradient
local diffusion rates increase
higher current response
What is the Randles-Sevcik equation?
How does the concentration at the electrode change - using Fc<—> Fc+ + e- as example?
high conc of Fc low conc of Fc+
Fc conc decreases and Fc+ incerases to a max at the duck’s bill
From here Fc+ decreases and Fc incerases
What affects the redox potential of a species?
Electron withdrawing and donating groups
EDG - easier to oxidise, harer to reduce, stabalise cation
EWG - Harder to oxidise, easier to reduce, destabalise cation
What are the different types of reversibility
Chemical reversibility
Electrochemical reversibility
How does electrochemical reversibility change?
The Nernst equation tells us that for a 1 electron transfer process
DE = | Epa - Epc | = 59 mV
This should not change with scan rate
This shift in peak potential is an incrase in DE which implies slow electron transfer amd therefore poor electrochemical reversibility
When peak potentials shift at higher scan rates it shows it is less reversible at higher scan rates
How does chemical reversibility change?
Reversibility decreases if the product formed after the first redox reaction undergoes a chemical reaction
In a fully reversible process | ipc/ipa | = 1
In a non reversible process the return wave could be diminished or shifted
What mechanisms can electron transfer and chemical steps occur by?
E = electron transfer process
C = chemical change
A –(-e-)–> A+ —(C)—> B EC mechanisms
A —(C)—> B —(E)—> B+
What happens in an EC mechamism?
electron transfer process is followed by a chemical process where the new species is redox inert
Leads to peak in forward scan and none in return scan
Sometimes chemical step is slow enough that at fast scan rates the electrochemical measurement occurs faster than the chemical reaction so the CV looks reversible
WHat happens in a CE mechanism?
electron transfer step occurs after there has been a chemical reaction
Return wave is much larger than forward scan
What are multiple redox processes?
Some molecules can undergo multiple redox processes
e.g Naphthalenediimide - two 1-electron reductions
Shows up as two combined ducks
What are mixed valence complexes?
A discrete species in which two or more of the same redox active fragments exist in different oxidation states
Three clases
Class I: valence trapped - no electronic comunication - no e- transfer
Class II: moderately coupled - some electronic comunication - barrier to electron transfer
Class III: fully delocalised - complete electronic communication - electrons spread over both centres
What happens in a CV of a mixed valence molecule?
First the molecule is neutral
First 1 electron transfer occurs
At this pooint a mixed valence species is generated - one centre is oxidised and the other is not
After the second oxidation there is no longer a mixed values species
How can we study mixed valence species?
Comparing DE1/2 values - can evaluate the stability of MV state
Can infer the extent of electron transfer
How can the mixed valence species be seen to exist?
In equilibrium with the neutral starting material and over-oxidised species
Represented as comproprtionation constant - Kc
The higher the value of kc the more stable the MV ion is with repect to conversion to the neutral or doubly oxidised species
Greater stability is inferred to be due to greater electonic coupling
How can we calculate Kc?
Variation of the Nernst equation:
Kc = e^(DE1/2 n1n2F/RT)
