Chlorine (Cl₂) is used to synthesize numerous consumer products and useful intermediates.^[1] Chlorination of alkanes or aromatics is a substitution reaction with hydrogen, for which only one chlorine atom is actually used to produce chloro-organics, while the other is lost as gaseous hydrogen chloride (HCl) by-product, which is either disposed as waste gas or recovered as low-value HCl aqueous solution. Today, HCl by-product totals globally 9.3 million tons per year, when only considered is the synthesis of polyurethane, chloromethane, and polycarbonate. At present, only 15% of this HCl by-product is recycled, leaving an unrealized Cl₂ regeneration market of $2.4 billion per year (based on the current price of Cl₂: $315/ton).^[2] The need to regenerate Cl₂ from HCl has become urgent in recent years as the demand for Cl₂ is increasing drastically. Conversion from HCl to Cl₂ can be achieved via chemical reactions in the Deacon process using a heterogeneous catalyst, but high temperature (e.g., 250–350 ^oC) and complex reactor design are needed to mitigate intrinsically low reactivity and yield.

Alternatively, an electrochemical process can be used that as a simple modular design and mild operating conditions. ^[3] To complete the electrolysis process, anodic regeneration of Cl₂ from HCl must be balanced by a cathodic reaction. There are two major types of cathodes used in HCl electrolysis process: the hydrogen evolution cathode (HEC) (Eqn. 1), and the oxygen reduction cathode, also commonly known as the oxygen depolarized cathode (ODC) (Eqn. 2). HEC-based electrolysis can produce H₂as a valuable by-product but ODC-based electrolysis offers significantly lower standard cell voltage due to its high redox potential (1.23 V vs. SHE)

HCl → Cl₂ + H₂ (1)

4HCl → 2Cl₂ + 2H₂O (2)

Herein, we demonstrate a gaseous HCl electrolyzer with Fe³⁺/Fe²⁺ redox-mediated cathode (IRC) for the first time to regenerate Cl₂. At the anode, gaseous HCl is oxidized to generate chlorine and protons, at the cathode, Fe³⁺ is electrochemically reduced to form Fe²⁺. Subsequently, Fe²⁺ is chemically oxidized to Fe³⁺ by oxygen in a reactor external to the electrolytic cell. Regeneration Fe²⁺ is recycled to the electrolyzer. Due to high standard potential (p^o, 0.77 V_SHE) and fast kinetics (exchange current density, i₀, of ~10⁻² A/cm² on glassy carbon, no catalyst was required),^[4] IRC offers substantial benefits over alternative commercial HEC and ODC cathodes. Taking advantage of IRC, a much lower cell voltage is achieved: 0.67 V vs. 1.16 V (with ODC) and 1.22 V (HEC) at a typical current density of 4 kA/cm². Compared to the commercial HEC or ODC-based HCl electrolysis processes, it will save 45–50% of energy consumption approximately. Moreover, without the need for a precious metal cathode catalyst and a costly thick membrane, the captital cost can be reduced by 40%–50% (IRC: $2,640/m² vs. HEC: $4,339/m² and ODC: $5,034/m², estimated with 4 kA/m²and current materials prices).

[1] J. Perez-Ramirez, C. Mondelli, T. Schmidt, et al, Energy & Environmental Science, 2011, 4, 4786.

[2] ThyssenKrupp, HCl electrolysis brochure.

[3] I. G. Martinez, T. Vidaković-Koch, R. Kuwertz, et al, Electrochimica Acta, 2014, 123, 387.

[4] Y. H. Wen, H. M. Zhang, P. Qian, et al, Electrochimica Acta, 2006, 51, 3769.