In 1886, an American Charles Martin Hall and a Frenchman Paul Heroult independently developed and patented a process for electrowinning primary Al (from an aluminum oxide feed). The key discovery was that the fused salt cryolite (Na3AlF6) was a suitable solvent for the dissolution of alumina Al2O3.
Today, we understand that this relatively high solubility (about 13 wt.% at ∼1000°C) results from the favorable stereochemistry of forming three different, stable oxyfluoride complex anions with octahedral or tetrahedral coordination of the large O2− and F− anions about the small Al3 cation. These solute polyhedra are stable because of the nearly identical ionic diameters for oxygen and fluorine anions. In fact, alumina solubility varies with the cryolite bath molar ratio of NaF/AlF3, the presence of various additives (CaF2, MgF2, LiF), and temperature.
But the continued use of the Hall-Heroult electrowinning cell (HHC) is beset by many environmental and economic problems that have not been satisfactorily solved even after 120 years.
Today, liquid Al is deposited by an electrochemical reduction reaction and is contained in a carbon hearth with side walls protected by a frozen crust of the cryolite electrolyte. The modern version of the HHC runs the anodic oxidation reaction at expensive prebaked and refined carbon anodes, resulting in the oxidation and consumption of the carbon and the release of CO2 product gas.
The stoichiometric reaction of the consumable carbon anode would occur according to the reaction:
1/2 Al2O3 3/4 C = Al 3/4 CO2
This would require that 0.33 #C/#Al be consumed in the evolution of the greenhouse gas CO2. In practice, some direct oxidation of carbon and other losses result in the consumption of 0.45 #C/#Al, amounting to the release of 1.65 #CO2/#Al, and accounting for ∼14% of the total cost for producing Al.
Further, from an environmental standpoint, the fabrication and oxidation of carbon anodes also evolve objectionable HF, CO, perfluorocarbon volatiles, and other volatile organic compounds (VOCs). The equipment and associated maintenance and labor necessary to minimize these emissions represent a significant problem and cost for the primary Al producers.
As for all electrochemical processes, through Faraday's Law, the rate of Al production is established (for 100% current efficiency) by the cell current. In the HHC, the maximum anodic current density is limited by the arrival of the oxygen-containing solutes at the anode by diffusion and convection (in the bubbling salt). Thus, Al production is necessarily a slow process, requiring a huge acreage for cells to realize the desired output.
The significant cost of electrical energy (about 15 kWh/kg Al, nearly 25% of the total cost) is proportional to the impressed cell voltage. This comprises the thermodynamic (open circuit) voltage (about 1.2 V), the anodic and cathodic polarization overvoltages, bubble effects, the ohmic (IR) drop in the fused salt electrolyte, plus other drops in the electrodes and collector bars external to the cell. Thus, the modern HHC operates at about 4.4 V with a current efficiency exceeding 95%. The heat balance for the cell is designed so that the I2R heat generated in the electrolyte maintains the operating temperature.
For decades and decades, the worldwide Al industry has attempted to step up production (e.g. using vertical electrodes), reduce pollution (evolving O2 rather than CO2), and reduce electrical and material costs (replacing the carbon anode).
The two dominant programs have involved the development of an ‘inert anode’ and a dimensionally stable ‘inert cathode’. The ‘holy grail’ of an inert anode requires a conductive high-temperature material that electrocatalytically evolves molecular oxygen (instead of CO2) while resisting oxidation and dissolution in the excellent solvent cryolite. If the first condition, oxygen evolution, could be met, the required cell voltage would be increased and the heat balance for the cell would become difficult. But in fact, no material has yet demonstrated the necessary insolubility in cryolite, and no customers want to purchase impure primary Al.
An inert cathode envisions a material that would be compatible (insoluble) in both liquid Al and fused cryolite. If such a material existed, some cathodically charged rods could poke up through the Al/cryolite interface and serve as dimensionally stable sites for the reduction reaction (instead of the wavy liquid Al bath). In particular, in combination with an (unavailable) inert anode, the electrode space could be reduced and stabilized, reducing the IR drop. While TiB2 and ZrB2 materials have shown some promise for this application, they are not a part of the modern HHC.
The realization of both inert anodes and cathodes would permit a conversion of the cell geometry to vertical electrodes with a much higher production rate per unit acreage.
As the materials community rushes forth in education and research toward new frontiers (especially nanomaterials), the community should not forget the tremendous payoff promised by the solution of certain problems remaining in ‘trailing-edge technology’.