Low-Voltage Gaseous HCl Electrolysis with Iron Redox-Mediated Cathode for Chlorine Regeneration
Authors: Zhao Yun , Gu Shuang , Wang Junhua , Yan Yushan 
Published: February 2020

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 peryear (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 VSHE) 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.