Modular Electrochemical Processes Mediated by Redox Reservoirs

Electrochemical synthesis of valuable chemicals using renewable electricity could bring sustainability advantages over conventional chemical manufacturing. The most successful industrial electrochemical manufacturing processes pair useful half-reactions and could produce high-value chemicals at both the cathode and anode simultaneously;1-2 however, the tight coupling between paired half-reactions requires identical conditions (e.g., reaction conditions and rates) and constrains the products and efficiency. Inspired by decoupled water splitting systems,3-4 we are developing modular electrochemical synthesis (ModES) using redox reservoirs (RRs),which are solid energy-storage materials that can store/release electrons and desired ions, to pair multiple independent half-reactions that are not comparable in conventional processes. Analogous to the water reservoirs used in the pumped hydroelectric storage (Fig. 1a), redox reservoirs allow temporary storage of the electrons and ions to redirect them for carrying out different electrochemical half-reactions (Fig. 1b), potentially at different times, locations, and/or scales of the reactions. These new strategies can not only improve energy efficiency and reduce waste of electrochemical manufacturing without the use of membranes, but also enable sustainable resource recovery and electrosynthesis using ion-selective RRs.

 

Figure 1. (a) Water reservoirs used in the pumped hydroelectric storage. (b) Illustration of the RR enabled ModES processes for sequential synthesis of H2O2 and Na2S2O8 or active chlorine (AC) in a cyclic fashion.

Sustainable modular electrochemical synthesis using ion-selective redox reservoirs.

Conventional electrochemical synthesis commonly features a working electrode that performs a desirable synthetic reaction, while the counter-electrode only serves to balance the redox stoichiometry without interfering with the desired reaction. The sacrificial reactions at this auxiliary electrode, however, often produce less valuable or undesirable products, resulting in inefficiency and waste. The RRs can serve as “counter-electrodes” for either anodic or cathodic half-reactions. In our first demonstration of the ModES strategy,5 we developed and used nickel hexacyanoferrate (NiHCF) as the RR for modular production of several strong oxidants, hydrogen peroxide (H2O2), sodium persulfate (Na2S2O8) and active chlorine (AC), with a stable operation and a high voltage efficiency. Cathodic H2O2 production reaction is coupled with the oxidation of the RR from RRred to RRox (Fig. 1b), while independent anodic production of Na2S2O8 or active chlorine is achieved by coupling these processes to the reduction of the RR from RRox to RRred. Reversible oxidation/reduction of the RR electrode stores and releases electrons and ions, bypassing the less valuable generation of O2 and H2 as counter electrode reactions.

Achieving stable ModES operation critically depends on successful management of ion migration using ion-selective RRs to maintain the ion balance between the paired half-reactions that generate or consume the same ions during electrosynthesis. To overcome the bottlenecks in the initial ModES demonstration (pH swing and ion imbalance), we have further developed an ion-balanced ModES process using a hydroxide-ion selective Ni(OH)2 RR to produce H2O2 and NaClO disinfectants without undesired byproducts and appreciable pH swings (Fig. 2a).6 The redox cycle of the Ni(OH)2 RR effectively transports the OH- ions from the cathodic cell in which the H2O2 production releases OH- to the anodic cell where the production of NaClO consumes OH- to balance the ion generation and consumption associated with the paired half-reactions.

 

Figure 2. (a) Illustration of the OH--balanced ModES process to co-produce H2O2 and NaClO using a Ni(OH)2 RR. (b) Illustration of RR-enabled ModES for paired oxidation of 4-t-butyltoluene in methanol and reduction of oxygen to H2O2 in water.

In collaboration with Prof. Shannon Stahl’s group, we also have extended the ModES strategy to pair different aqueous and non-aqueous electrosynthesis and achieve sustainable production across different solvents (methanol, acetonitrile, and water) (Fig. 2b).7 Two electrochemical oxidation reactions in organic solvents, the conversion of 4-t-butyltoluene to benzylic dimethyl acetal and aldehyde in methanol or the oxidative C–H amination of naphthalene in acetonitrile, were paired with the reduction of oxygen to hydrogen peroxide in water using nickel hexacyanoferrate as an RR that can selectively store and release protons (and electrons) while serving as the counter electrode for these reactions.

Simultaneous resource recovery and electrosynthesis using ion-selective redox reservoirs. The development of ion-selective RRs provides new opportunities to recover resources from waste streams. For example, recovering ammonia and other nutrients from manure wastewater to produce fertilizer can improve the sustainability of the livestock systems. We have developed a new electrochemical strategy to achieve simultaneous ammonium (NH4+) and potassium (K+) ion recovery and electrochemical synthesis using potassium nickel hexacyanoferrate (KNiHCF) as ion-selective redox material to mediate the nutrient recovery (Fig. 3),8 in collaboration with Prof. Mohan Qin’s group. The KNiHCF electrode spontaneously oxidizes the organic matter and uptakes NH4+ and K+ ions from manure wastewater with a nutrient selectivity of ~100%. Subsequently, NH4+- and K+-rich fertilizers are generated alongside the electrosynthesis of value-added chemicals, such as H2 (green fuel) or H2O2 (disinfectant), without expensive ion-exchange membranes. Preliminary analysis by Prof. Fikile Brushett and Prof. Rebecca Larson shows that the integrated process holds economic potential for dairy farms and could mitigate NH3 emissions by up to 70%. These results provide a new conceptual strategy for distributed electrochemical resource recovery and on-demand electrochemical manufacturing that can improve agricultural sustainability.