Use of “depolarizers” in electrolytic reactions helps lower input electrical energy in electrochemical processes. For example, in batteries depolarizers help prevent buildup of hydrogen gas bubbles thereby preventing the voltage, and thereby current, from being reduced. Among the known depolarizers, including sulfur dioxide which is used for lowering energy use in production of hydrogen, oxygen consuming cathodes have gained in importance due to the high onset potential of the oxygen reduction reaction (ORR). Oxygen depolarized cathodes are used in electrolytic production of chlorine from brine solution, and in its recovery from hydrochloric acid, a byproduct of various chemical processes, e.g., production of polymers notably polyvinyl chloride, polyurethanes and polycarbonate, chloroaromatics, and many other components. However, electrochemical chlorine production is currently one of the most energy-intensive processes in the chemical industry, Manufacture of chlorine using oxygen depolarized cathode (ODC) promises to reduce power consumption by as much as 30% compared to standard membrane technology, and is also accompanied with a cut in indirect carbon dioxide emissions. The process integrates use of oxygen with the reaction taking place at the cathode in the manufacture of chlorine by electrolysis of sodium chloride or HCl to produce water instead of hydrogen gas. Oxygen is pumped into the cathode compartment, which reacts with hydrogen to produce water, and the voltage needed for the electrolysis process is reduced by approximately a third. Thus, feeding of gaseous oxygen enables electrolysis to be performed at a lower voltage.
In this process, the conventional cathodic reduction of protons to H2 (g):2H++2e−H2(g)E=0.00 V vs. RHE  (1)is replaced with the Oxygen Reduction Reaction (ORR):O2(g)+4H++4e−2H2O(l)E=1.23 V  (2)Thus, overall process is the following:2HCl+½O2Cl2(g)+H2O(l)E=−0.13 V  (3)as compared to conventional:2HCl2H2(g)+2Cl2(g)E=−1.36 V  (4)with a theoretical energy savings of ˜700 kWh per ton of Cl2 (g). Thus, the much lower ORR overpotential associated with oxygen consuming gas diffusion electrode (GDE) is expected to result in significant cost and energy savings.
Operation of an electrolytic cell for chlorine production involves use of aqueous solution of hydrochloric acid at concentration as high as 20% (˜5 M) and temperature as high as 60° C., which creates a highly corrosive environment. The presence of anions (chloride ions) in such corrosive environment causes poisoning of the catalyst, thereby reducing the efficiency of the cell. Therefore, there is a need to develop ORR catalysts that can resist anion poisoning.
The oxygen reduction reaction (ORR) is one of the most studied reactions in energy conversion systems due to the large overpotential caused by the slow kinetics. Due to the lower proton conductivity and permeability of oxygen in the phosphoric acid environment, high loadings of metal are required. Traditionally platinum based electrocatalyst have been used to facilitate ORR in acidic media, but the scarcity of these materials significantly increases the cost of the system. In addition to the economic disadvantage of platinum based electrocatalyst, there is also the issue of poisoning of these materials by the adsorption of the dihydrogen phosphate anion. The number of electrons transferred per active site of Pt per second is diminished by approximately 70% in the presence of moderate concentrations of phosphoric acid. Phosphoric acid fuel cells (PAFC) are successfully commercialized and presently operate at 80% combined heat and power efficiency, but the cathode materials suffer poisoning effects from the dihydrogen phosphate ion adsorption (H2O4−) limiting the performance. Anion adsorption is structure dependent and it has been shown that some Pt-alloys exhibit a heightened tolerance to phosphate poisoning (He, Q. PhysChemChemPhys. 2010. 12, 12544) but cost is still an issue. However, non-platinum group metal catalysts (NPMC) for ORR containing Fe and/or Co have been investigated and progress has been made in developing synthetic strategy for preparing these NPMC have now made these materials viable contenders with Pt-based catalysts for use in acid based systems. (Jaouen, F. et al., 2011)
Current state of the art catalyst for ODC is rhodium based chalcogenite (RhxSy/C), currently produced by DeNora. The RhxSy/C catalyst outperforms the extremely ORR active carbon supported platinum (Pt/C) catalysts in resisting anion poisoning. The Pt/C catalyst is easily poisoned by chloride ions. Despite a lower activity for oxygen reduction relative to state of the art Pt-based electrocatalysts in most systems, RhxSy is not severely depolarized by contaminants such as chloride ions and assorted organics. However, Rh is a precious metal and the cost of RhxSy/C is a drawback for successful commercialization of RhxSy/C.
Anion poisoning, which is a common problem in electrocatalysis in aqueous media, is a result of strong interaction of catalytic metal nanoparticles (Pt, Rh, Ru, etc.) with impurities at potentials above potential of zero charge (PZC). The poisoning blocks access of the reactants (e.g., oxygen in ORR reactions) to the active centers on the metal surface, resulting in increased overpotential. Chemisorption of any species, e.g., anions, on the metal surface depends on the free energy of adsorption and free energy of solvation of that species. In acidic environment water molecules act as weak anionic species and interact with the metallic surface through the oxygen atoms of hydroxide ions. More electronegative moieties such as chloride or bromide ions, or other anions when present replace the hydroxide ions. The metal-anion interaction grows in strength with increased positive potentials, which is specifically challenging for oxygen reduction reactions as the ORR onset is desired to occur at high potentials. As shown for adsorption of chloride anions on platinum nanoparticles (FIG. 1) even small concentrations of anions result in significant losses in the activity of the catalyst.
Most reported nonplatinum group metal (non-pgm) catalysts consist of biomimetic Fe—Nx centers which are frequently wrapped within protective graphene layers, and contain non-coordinated metal nanoparticles (FeNPs). The overall catalytic performance and stability of these materials depends upon the structure and distribution of the metal centers throughout the catalyst surface. While the better performing MNC catalyst in this group exhibit promising durability in standard fuel cell environment, most of the protected Fe-consisting nanoparticles are susceptible to oxidation by strong anions like a chloride ion, especially at high concentrations, as in case of catastrophic cathode flooding with concentrated hydrochloric acid in chlorine recovery HCl electrolyzers.