Non-Noble Metal Catalysts As Oxygen Depolarized Cathodes in Chlor Alkali Electrolyzers

Chlorine is an essential element of chemical industries, and especially has paramount importance in polymer industry. A typical route for chlorine generation is electrolysis of brine solution where chlorine is generated at the anode; producing caustic soda and hydrogen at cathode. However, the relevant state of the art membrane reactors is one of the most energy intensive processes in industry. Moreover, the membrane technology is believed to have reached the theoretical limit, and energy consumption cannot be reduced further. In the present chlor-alkali technology, sodium chloride solution is electrolyzed at a constant current density, and the decomposition voltage ΔE of the overall reaction is 2.186 V as per thermodynamic calculation. If the hydrogen evolution reaction at the cathode side is replaced with oxygen reduction reaction (ORR), the decomposition voltage will be significantly reduced, and, as a consequence, the energy consumption will be reduced simultaneously. Introduction of such oxygen depolarized cathode results in the following chemical reactions:

Anode: 2Cl^- —— Cl₂+2e^- (E=1.358 V vs. NHE)

Cathode: 4Na⁺ + 2H₂O + O₂ +4e^- —— 4NaOH (E=0.401 V vs. NHE)

Overall Reaction: 4NaCl + 2H₂O + O₂ —— 4NaOH + 2Cl₂ (ΔE=-0.957 V)

Even though the voltage requirement can be minimized by oxygen depolarized cathode, there is still significant overpotential loss from the cathodic reaction. This is due to the fact that ORR is kinetically a very sluggish reaction. Platinum is the best catalyst known for ORR in acid media. However, Pt is poisoned in concentrated NaOH solution resulting in significant shifts in onset potential and limiting current values. Early in 1970s, heating of non-noble metal along with carbon and a source of nitrogen atoms in presence of ammonia resulted in formation of electrocatalysts with ability to reduce oxygen. Since then, there has been a gradual development of such “non-PGM” catalysts, where PGM stands for Precious Group Metals. The introduction of metal organic frameworks (MOFs) as sacrificial templates has brought a revolution in electrocatalysis research. Owing to the large surface area, preferable porosity and high nitrogen content, the non-PGM catalysts obtained from MOF precursors have shown excellent ORR activity. However, most of the MOF synthesis methods are based on the traditional solution reactions, which need either excessive amount of ligand or a huge amount of solvent, or both. Thus, the low yielding synthesis route and comprehensive separation methods of MOF seriously constrain the application of the MOF based catalysts for ORR.

In the presentation, we will be discussing about the new in-house synthesized MOF based catalyst using solid-state reaction. In addition to advantages like simple preparatory route and high reproducibility, this catalyst has high yield and scalability which are uncommon traits for the more conventional MOF based catalysts. This catalyst has high ORR onset potential even in extreme corrosive conditions such as concentrated alkali and acid. Herein, we propose a facile, simplified approach to obtain the MOF based catalysts for ORR with low cost and high reproducibility. The as-prepared catalysts have been tested as oxygen depolarized cathodes in chlor-alkali electrolysis cells and have been operated at significantly lower voltages compared to  commercial catalysts like Pt/C and Ag/C.

References

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