Industrial Electrochemistry (Electrolysis: ))))G is positive, calculated E is negative)

Why electrochemistry?• clean and green (not always true!)• often no or few byproducts• electrons the cheapest redox “reagents”

Industrial electrochemical processes• Primary metal production (electrowinning): Al, Mg, Na, and

recently developed methods for Cu, Ni

• Metal refining: Cu, Ni – also electroplating

• Inorganic chemicals: Cl2/NaOH; H2O2 and other peroxycompounds; O3; chromic acid; KMnO4

• Organic compounds: adiponitrile

• Waste water purification and recycling

In North America:• Al production = 10% of total electrical energy production• Cl2 production = 3% of total electrical energy production

Differences between laboratory set-up and industrial processes• importance of mass transfer

S stirred/unstirred solutionsS batch vs continuousS planar vs 3-D electrodes

mass transport occurs by:S diffusion = migration due to concentration gradientS migration of charged species due to potential gradientS convection (mechanical stirring or agitation)

Other issues• undivided vs divided cells• distance between electrodes• potentiostatic vs amperostatic control• importance of current efficiency• what is the reaction at the other electrode?

Under laboratory conditions of electrochemical synthesis of a fewgrams of material, these issues are of minor importance. They are ofimmense importance industrially because of their impact on processcosts, when thousands of kg – or even tonnes – of material are to beproduced.

Mass transport and overpotential

The Tafel equation links (over)potential (a thermodynamic concept)with current (a kinetic concept)

For an anodic process:

log i = log i0 + "AnF0/(2.303RT) "A = anode transfercoefficient0 = overpotentiali0 = equilibrium exchangecurrent density

i.e. log iexp " 0

Eventually, the current (rate) levels off because of mass transportlimitation (example: limiting currents in voltammetry) —> no benefitfrom further increase in cell potential

Optimizing mass transport therefore allows the use of appliedpotentials —> higher current densities (faster reactions) before thelimiting current is reached.

Note that the x axis here is overpotential

Current efficiency, NNNN (expressed here as a fraction, not a %)

% yield = n(product)/n(reactant converted)

I am using n for moles of substance and n for moles of electrons

N = charge used to form product/total charge passed

= n(product) × (nF)/ (Ii dt) for n moles of product

For constant-current conditions, this is

N = n(product) × (nF/i.t)

Electrical energy consumption (EEC):

EEC = !nF Ecell/N per mole

= !nF Ecell/NM per gram

EEC as defined above has the units J g!1. To convert to morepractical units:

EEC (kW h/kg) = EEC (J/g) × (1000 g/1 kg) × (1 W s/1 J) × (1 kW/1000 W) × (1 h/3600 s)

Thus EEC (kWh/kg) = (2.78×10!4) × EEC(J/g)

The objective is to minimize EEC• maximize NNNN• minimize Ecell

What contributes to Ecell? It is NOT just E calculated by the Nernst Equation!!

Ecell = Ecathode ! Eanode* ! 0cell** ! iRcell

* expressed as a reduction potential** overpotentials beyond the equilibrium (calculated)

potentials E for the individual anode and cathode reactions(different from the 0 needed to drive the reaction byinducing current through the cell)

Notice that all these terms make Ecell more negative, so a largerapplied potential must be applied to drive electrolysis forward.

iRcell may have several components due to: S resistance of the catholyte and anolyte solutions;

minimized by small distance between anode andcathode

S resistance of the separator of a divided cell (e.g., ionexchange membrane); absent in an undivided cell

S resistance in the external circuitS electrode materials and design

For an electrolytic process, !Ecell is the smallest potential(voltage) that allows current to flow through the cell. To drivean electrolytic process, the applied potential must be greaterthan Ecell, i.e., there must be an overpotential 0.

Eventually, if 0 is too large, a competing reaction intervenes. Note the optimum 0 is near 01 on the diagram.

% energy efficiency of the cell (%EE):

%EE = 100 × (Ecathode ! Eanode)/(Ecathode ! Eanode !0cell ! iRcell)

Both 0cell and iRcell increase numerically with current density. Therefore high current density is a source of inefficiency BUThigh current density increases the rate of formation of product(since q = i.t)

Heat transfer

• iRcell dissipates electrical energy as heat

• the need for heat to be vented may oppose the desire for acompact cell design

Current and current density• current i is the measured current through the cell (amps)

• current density j is current per unit area of the electrodesurface (A m!2)

• What really counts is j in terms of mass transport

• Hence the attraction of 3-D and other high-surface areaelectrodes such as carbon cloth and RVC, despite theirphysical fragility, tendency to plugging, inhomogeneous j

Electrode materials

• they should be cheap, long-lasting, free from fouling

Types of electrochemical reactor

Recirculating systems are also used, especially (b)