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Processes for Electrochemical Production of Electrolyte-free Hydrogen Peroxide

2019; Elsevier BV; Volume: 3; Issue: 12 Linguagem: Inglês

10.1016/j.joule.2019.11.017

2542-4785

Colin W. Anson, Shannon S. Stahl,

Advanced Photocatalysis Techniques

Hydrogen peroxide is the focus of growing attention for distributed manufacturing, reflecting its decentralized applications and shipping hazards. Two recent reports, one in Science by Wang and colleaugues and one in this issue of Joule by Surendranath, Hatton, and colleaugues, introduce innovative electrochemical methods to produce electrolyte-free aqueous solutions of hydrogen peroxide. Hydrogen peroxide is the focus of growing attention for distributed manufacturing, reflecting its decentralized applications and shipping hazards. Two recent reports, one in Science by Wang and colleaugues and one in this issue of Joule by Surendranath, Hatton, and colleaugues, introduce innovative electrochemical methods to produce electrolyte-free aqueous solutions of hydrogen peroxide. Hydrogen peroxide (H2O2) is an important environmentally benign oxidant with a large global market (>4.5 million metric tons in 20151Solvay (2015). Peroxides: A Growing and Resilient Cash Generator. Capital Markets Day, London, June 10–11. https://solvay.gcs-web.com/static-files/32db218f-52eb-4fa9-93bf-aeb671c0d60f.Google Scholar) and diverse industrial applications.2Campos-Martin J.M. Blanco-Brieva G. Fierro J.L.G. Hydrogen Peroxide Synthesis: An Outlook beyond the Anthraquinone Process.Angew. Chem. Int. Ed. 2006; 45: 6962-6984Crossref PubMed Scopus (1551) Google Scholar,3Ciriminna R. Albanese L. Meneguzzo F. Pagliaro M. Hydrogen Peroxide: A Key Chemical for Today's Sustainable Development.ChemSusChem. 2016; 9: 3374-3381Crossref Scopus (225) Google Scholar The majority of H2O2 is produced in concentrated solutions through the anthraquinone process in dedicated chemical plants. This centralized process consumes large amounts of energy, resulting in a significant carbon footprint, and the distribution of concentrated H2O2 solutions presents safety hazards and high transportation costs. Primary markets for H2O2 include the paper and pulp industry, commodity and fine chemical industries, and wastewater treatment. Some applications of H2O2, such as propylene oxide production, are well aligned with large-scale, centralized sources of H2O2 because they can benefit from economies of scale and, in some cases, co-location. Other processes, such as water treatment, feature delocalized operation and would benefit from modular, distributed approaches to H2O2 production. Many of the latter applications do not require the highly concentrated solutions of H2O2 (70 wt %) produced from the anthraquinone process. For example, "advanced oxidation processes" employed in water treatment4Miklos D.B. Remy C. Jekel M. Linden K.G. Drewes J.E. Hübner U. Evaluation of advanced oxidation processes for water and wastewater treatment - A critical review.Water Res. 2018; 139: 118-131Crossref Scopus (1372) Google Scholar use solutions of 100 h. High rates of H2O2 production were accessible. By using H2 as a fuel, a rate of 0.53 mmol cm−2 h–1 was obtained when no bias was applied to the cell (0 V cell potential), and this rate increased to 3.4 mmol cm–2 h–1 with an applied voltage of 0.61 V. Similar rates were obtained at a cell voltage of 2.13 V when H2O was oxidized at the anode (i.e., as the source of electrons and protons). The H2O2 solutions generated in this process were shown to be directly compatible with wastewater treatment in tests with rainwater samples. These demonstrations showed that the total organic content of rainwater decreased from ∼5 ppm to 2 ppm, corresponding to a treatment rate of up to 0.22 L rainwater h–1 cm–2electrode. In this issue of Joule, Surendranath, Hatton, and coworkers report a conceptually different approach for the electrochemical production of electrolyte-free aqueous H2O2, using solution-phase redox mediators and phase transfer principles.10Murray A.T. Voskian S. Schreier M. Hatton T.A. Surendranath Y. Electrosynthesis of Hydrogen Peroxide by Phase-Transfer Catalysis.Joule. 2019; (this issue, ▪▪▪–▪▪▪)Scopus (51) Google Scholar The strategy takes inspiration from the industrial anthraquinone process in which reduced anthrahydroquinones undergo facile autoxidation to the corresponding anthraquinones, with concomitant conversion of O2 into H2O2. The anthraquinone-2,6-disulfonate (AQDS2–) mediator selected by the authors exhibits high solubility in acidic aqueous solutions and has been widely employed in organic redox flow batteries.11Winsberg J. Hagemann T. Janoschka T. Hager M.D. Schubert U.S. Redox-Flow Batteries: From Metals to Organic Redox-Active Materials.Angew. Chem. Int. Ed. 2017; 56: 686-711Crossref PubMed Scopus (598) Google Scholar Electrochemical reduction of this quinone under aerobic conditions has been used for mediated H2O2 production, but the resulting solution contains the H2O2 in a mixture of acid and mediator electrolyte that would be difficult to separate. The phase-transfer process illustrated in Figure 1C overcomes this problem. Electrochemical reduction of the AQDS2– mediator is conducted under anaerobic conditions generating an aqueous solution of the dianionic hydroquinone (AQDSH22–) charge-balanced with H+ or Na+ ions. This solution is delivered to the first mixing-settling reservoir, which contains 1-hexanol, a water-immiscible organic solvent, and tetrabutylammonium chloride (NBu4Cl), an organic soluble salt. Ion exchange at the aqueous/organic interface results in transfer of the reduced mediator into the organic phase as a NBu4+ salt (i.e., AQDSH22–·2 NBu4+). The organic solution is then flowed into the second mixing-settling reservoir, where it is combined with electrolyte-free water and O2. Autoxidation of the reduced mediator generates the quinone, which is retained in the organic phase as the NBu4+ salt (i.e., AQDS2–·2 NBu4+), and H2O2, which partitions into the water layer. The oxidized mediator is then returned to the first mixing tank, where it can partition back into the aqueous phase and undergo electrochemical reduction. This innovative process produces a high-purity dilute H2O2 stream with high selectivity and low cost. A device prototype was constructed using a nickel foam anode to catalyze alkaline water oxidation and a bipolar membrane to enable the cathode to operate under acidic conditions. The device displayed stable performance and good faradic efficiencies (typically >95%), and H2O2 was produced at a rate of 2–3 μmol min−1 cm−2 (0.12 mmol cm−2 h−1). H2O2 concentrations up to 33 mM were produced. Future development of this system should enable improvements, such as decreased cell resistance and optimized mediator performance. For example, the AQDS2– mediator features a substantial overpotential for H2O2 production. Use of a higher-potential mediator will improve the overall energy efficiency of the system by lowering the overpotential, but it will also lead to slower rates of autoxidation and H2O2 production. Such issues, together with optimization of the phase transfer properties of the mediator, represent opportunities for future investigation and development. Together, these recent reports highlight two complementary strategies for the electrochemical production of H2O2. The principal breakthrough achieved by these methods is the ability to generate moderate-to-high concentrations of H2O2 in electrolyte-free water, thereby bypassing a difficult (or impossible) purification step and greatly expanding the scope of applications for the resulting product. Such processes are ideally suited for distributed production of hydrogen peroxide. They could find near-term use for water treatment or environmental remediation applications and set the stage for broader electrification of the chemical industry. The authors are grateful to the Center for Molecular Electrocatalysis, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, for support of their work on mediated electrocatalysis and electrochemical energy conversion. Electrosynthesis of Hydrogen Peroxide by Phase-Transfer CatalysisMurray et al.JouleOctober 24, 2019In BriefNegatively charged anthraquinones are reduced electrochemically and can generate hydrogen peroxide (H2O2) by partial oxygen reduction. The use of phase-transfer catalysis allows the production of the product far from the site of electrolysis, facilitating the precious metal-free synthesis of H2O2 in an electrolyte-free medium. A proof-of-concept device is designed to perform this process in continuous flow. 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