Fundamentals of Electrochemistry Flashcards
What is electrochemistry?
What is an important characteristic?
Electrochemistry studies reactions, which involve an electrical current. The latter can either be generated by a spontaneous reaction or a reaction can be forced to proceed by applying a current.
Important characteristic: not Carnot limited! → high theoretical efficiencies
What is the Anode and what does it produce?
Anode: Electrode of an electrochemical cell through which net electric current flows and at which the predominating electrochemical reaction is an oxidation.
𝑨 → 𝑨(+) + 𝒆− or 𝑩(-) → 𝑩 + 𝒆−
Anodic current is denoted positive in the IUPAC convention
What is the Catode and what does it produce?
Cathode: Electrode of an electrochemical cell through which net electric current flows and at which the predominating electrochemical reaction is a reduction.
𝑨(+) + 𝒆− → 𝑨 or 𝑩 + 𝒆− → 𝑩(−)
Cathodic current is denoted negative in the IUPAC convention
What do we need for current from electrochemical reaction to flow?
- A driving force (electrochemical potential)
- Someone to give electrons
- Someone to receive electrons
- Something to conduct electrons
- Someone to give ions
Define Electric Current and Electric Charge and its formulas
Electric Current (I) : Rate at which electric charge is passed → I = dQ/dt
Electric Charge (Q) : quantity of electricity, integral of electric current over time → Basically to integer I over time
What does represent the Faraday’s Law?
Equation and what does each term mean
The Faraday’s Law represents how much charge is transferred.
Q = zFn = ZFm/M
Explanation of the terms: Presentation 1 slide 59
Why interfaces are really important?
Because they can allow the reaction to go easier or basically avoid the reaction. They need to be designed carefully
The resistance of the interface can be infinitely high.
Electrolytes:
1. Requirements
2. Characteristics
3. Resistivity equation
- Requirements:
A. Chemical stabillity
B. High ionic conductivity
C. Low mass
D. Low cost
E. Safe - Characteristics
Ions move due to electric field
sigma = ionic conductivity:
1. Parabola with the ion concentration, 2. Proportional to ion mobility and temperature
- Resistivity: rho = 1/sigma = A*R/L
R: Resistance: It is used to determine the potential drop over electrolyte via Ohm’s law.
Presentation 1 slide 61
Electrodes:
1. Requirements
2. Characteristics
- Requirements:
Apart from the classic things (chemical stability, low mass, low resistance…), they need to have as much area as possible. As we have a big area, there’s more space for the reaction to take place. - Characteristics:
The main characteristic is that as the T is higher, the electronic mobility is lower. However, the ionic conductiviry is higher. This means that there’s a trade-off between ionic conductivity and electronic
conductivity in which we will obtain the highest potential; this, therefore., occurs in a Temperature range.
Define ‘Electrochemical Cell’
- System that consists of at least two
electronic conductors (electrodes) in contact with an ionic conductor (electrolyte). - The ionic conductor acts as an electronic insulator.
- The half-reactions of the redox reaction (i.e.oxidation and reduction) are spatially separated.
- Difference between galvanic cells and electrolysis cells?
- Which types of galvanic cells do we have?
- Scheme chemical energy vs electrical energy
- Galvanic cells: a spontaneous reaction
occurs and electrical energy is released (gibbs free energy is therefore < 0) - Electrolysis cells: electrical energy is used
to drive non spontaneous reaction (gibbs free energy is therefore > 0) - The types of galvanic cells we have are fuel cells and batteries.
→ The scheme can be seen in Presentation 1 slide 65.
Define Electrochemical Potencial (Total Work), with the symbols and its meanings.
Definitions of Chemical work, Electrical work and gradient of the electrochemical potential
Total work = Chemical work + Electrical work
Chemical work: Bringing uncharged particle into the bulk of an uncharged phase.
Electrical work: Additionally accounts for effect of an electrical field on a charged particle. It can also be defined as a product of charge (q) and (cell) potential (E).
Gradient of the electrochemical potential = driving force for electrochemical reactions.
→ Symbols can be seen in Presentation 1 slide 66.
Definition of Electrode Potential [V], and path to obtain the ‘theoretical reversible potential’
It can be seen in Presentation 1 slide 68
Nernst equation: definition and equation
(With concentrations and with pressures)
Presentation 1 slide 69
(91 for the equation with pressures)
From which reaction is obtained the primary reference scale in electrochemistry, and which is the electrode made of?
2H+ + 2e- → H2
The electrode is made of Platinium
- Formula for the Potencial of a Cell
- Definition of Urev
- Ucell = Ecathode - Eanode
- Urev (reversible open circuit voltage) is the thermodynamic equilibrium potential at zero current flow. In the galvanic cells Umeasured < Urev, and in the electrolysis cells Umeasured > Urev.
How can we calculate the measured potential of a cell?
Why can we have losses in the cell?
The actual measured cell voltage is the sum of the thermidynamic standard potentials of both electrodes, but substracting the losses that occur in the cell:
Umeasured = Urev - losses.
Losses can be produced because of:
1. Ohmic resistances - Ohmic losses (Resistance to charge transfer)
2. Slow kinetics - Activation losses (kinetics of reactions taking place at the electrodes)
3. Ineffective mass transport - mass transport losses (reactant depletion at the electrode)
Measured Performance (V) = OCV - alog(i/i0) - iR - mexp(ni)
Tafel Equation: What does it describe?
Equation, graphic (example and what we have en the axis), and when it is valid
- It is an empirical description of electrochemical reaction kinetics.
- Overpotential [V] = +- a*log(i/i0)
- Plot is overpotential [V] vs log(i). It can be seen in presentation 1 slide 78.
- The equation is valid for: high overpotentials, uniform current distributions, no side reactions and no limitations due to mass transport.
Modes of mass transport in a electrochemical cell, why is the movement produced in each one and driving force.
- Migration: movement of charged species in electrical field (driving force: electrochemical potential gradient)
- Diffusion: movement of species due to concentration differences (driving force: chemical potential gradient)
- Convection (due to pressure gradients)
