Stable Tetrasubstituted Quinone Redox Reservoir for Enhancing Decoupled Hydrogen and Oxygen Evolution

In situ electrosynthesis of quinone-based redox-active molecules coupling with high-purity hydrogen production

Clean hydrogen production via conventional water splitting involves sluggish anodic oxygen evolution, which can be replaced with more valuable electrosynthesis reactions. Here, we propose one novel strategy for coupling in situ organic electrosynthesis with high-purity hydrogen production. A benzoquinone-derivative disodium 4,5-dihydroxy-1,3-benzenedisulfonate (Tiron)-o1 and a naphthoquinone-derivative 2,6,8-trismethylaminemethylene-3,5-dihydroxy-1,4-naphthoquinone (TANQ) were in situ electrosynthesized and directly used in a flow battery without any further purification treatment. Constant, simultaneous production of TANQ and hydrogen was demonstrated for 61 hours, while stable charge-discharge capacities were retained for 1000 cycles. The work provided a new avenue for achieving in situ redox-active molecule synthesis and high-purity hydrogen.

Hydrogen Sulfide Splitting into Hydrogen and Sulfur through Off-Field Electrocatalysis

Hydrogen sulfide (H2S), a toxic gas abundant in natural gas fields and refineries, is currently being removed mainly via the Claus process. However, the emission of sulfur-containing pollutants is hard to be prevented and the hydrogen element is combined to water. Herein, we report an electron-mediated off-field electrocatalysis approach (OFEC) for complete splitting of H2S into H2 and S under ambient conditions. Fe(III)/Fe(II) and V(II)/V(III) redox mediators are used to fulfill the cycles for H2S oxidation and H2 production, respectively. Fe(III) effectively removes H2S with almost 100% conversion during its oxidation process. The H+ ions are reduced by V(II) on a nonprecious metal catalyst, tungsten carbide. The mediators are regenerated in an electrolyzer at a cell voltage of 1.05 V, close to the theoretical potential difference (1.02 V) between Fe(III)/Fe(II) and V(II)/V(III). In a laboratory bench-scale plant, the energy consumption for the production of H2 from H2S is estimated to be 2.8 kWh Nm-3 H2 using Fe(III)/Fe(II) and V(II)/V(III) mediators and further reduced to about 0.5 kWh Nm-3 H2 when employing well-designed heteropolyacid/quinone mediators. OFEC presents a cost-effective approach for the simultaneous production of H2 and elemental sulfur from H2S, along with the complete removal of H2S from industrial processes. It also provides a practical platform for electrochemical reactions involving solid precipitation and organic synthesis.

Decoupled supercapacitive electrolyzer for membrane-free water splitting

Green hydrogen production via water splitting is vital for decarbonization of hard-to-abate industries. Its integration with renewable energy sources remains to be a challenge, due to the susceptibility to hazardous gas mixture during electrolysis. Here, we report a hybrid membrane-free cell based on earth-abundant materials for decoupled hydrogen production in either acidic or alkaline medium. The design combines the electrocatalytic reactions of an electrolyzer with a capacitive storage mechanism, leading to spatial/temporal separation of hydrogen and oxygen gases. An energy efficiency of 69% lower heating value (48 kWh/kg) at 10 mA/cm2 (5 cm-by-5 cm cell) was achieved using cobalt-iron phosphide bifunctional catalyst with 99% faradaic efficiency at 100 mA/cm2. Stable operation over 20 hours in alkaline medium shows no apparent electrode degradation. Moreover, the cell voltage breakdown reveals that substantial improvements can be achieved by tunning the activity of the bifunctional catalyst and improving the electrodes conductivity. The cell design offers increased flexibility and robustness for hydrogen production.

Investigation of Weak Noncovalent Interactions Directed by the Amino Substituent of Pyrido- and Pyrimido-[1,2-a]benzimidazole-8,9-diones

Quinones are small redox-active molecules that are able to form intra- and intermolecular interactions both in the solid state and in solution. On the basis of 6-amino-substituted pyrido- and pyrimido-[1,2-a]benzimidazole-8,9-diones, weak interactions were investigated by single-crystal X-ray and 1H NMR spectroscopy methods. Crystallization of quinone derivatives containing a -NH-CH2- fragment led to the formation of both chiral and achiral crystals. The presence of two forms with (endo form) and without (exo form) an intramolecular hydrogen bond was experimentally detected by X-ray crystallography analysis and variable-temperature (VT) 1H NMR experiments in the cases of isopentylamino- and benzylamino-substituted derivatives. Interestingly, the exo form dominates both in the solid state and in solution.

Density Functional Theory Approximations and Experimental Investigations on Co1-xMoxTe2 Alloy Electrocatalysts Tuning the Overall Water Splitting Reactions

Understanding the relationship between electronic structure, surface characteristic, and reaction process of a catalyst helps to architect proficient electrodes for sustainable energy development. The highly active and stable catalysts made of earth-abundant materials provide a great endeavor toward green hydrogen production. Herein, we assembled the Co_1-xMo_xTe (x = 0-1) nanoarray structures into a bifunctional electrocatalyst to achieve high-performance hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) kinetics under alkaline conditions. The designed Co_0.75Mo_0.25Te and Co_0.50Mo_0.50 electrocatalysts require minimum overpotential and Tafel slope for high-efficacy HER and OER, respectively. Furthermore, we constructed a Co_0.50Mo_0.50Te₂∥Co_0.50Mo_0.50Te₂ device for overall water splitting with an overpotential of 1.39 V to achieve a current density of 10 mA cm^-2, which is superior to noble electrocatalyst performance, with stable reaction throughout the 50 h continuous process. Density functional theory approximations and Gibbs free energy calculations validate the enhanced water splitting reaction catalyzed by the Co_0.50Mo_0.50Te₂ nanoarrays. The partial replacement of Co atoms with Mo atoms in the Co_0.50Mo_0.50Te₂ structure substantially enhances the water electrolysis kinetics through the synergistic effects between the combined metal atoms and the bonded chalcogen.

pH-Universal Decoupled Water Electrolysis Enabled by Electrocatalytic Hydrogen Gas Capacitive Chemistry

In conventional water electrolysis (CWE), the H₂ and O₂ evolution reactions (HER/OER) are tightly coupled, making the generated H₂ and O₂ difficult to separate, thus resulting in complex separation technology and potential safety issues. Previous efforts on the design of decoupled water electrolysis mainly concentrated on multi-electrode or multi-cell configurations; however, these strategies have the limitation of involving complicated operations. Here, we propose and demonstrate a pH-universal, two-electrode capacitive decoupled water electrolyzer (referred to as all-pH-CDWE) in a single-cell configuration by utilizing a low-cost capacitive electrode and a bifunctional HER/OER electrode to separate H₂ and O₂ generation for decoupling water electrolysis. In the all-pH-CDWE, high-purity H₂ and O₂ generation alternately occur at the electrocatalytic gas electrode only by reversing the current polarity. The designed all-pH-CDWE can maintain a continuous round-trip water electrolysis for over 800 consecutive cycles with an electrolyte utilization ratio of nearly 100%. As compared to CWE, the all-pH-CDWE achieves energy efficiencies of 94% in acidic electrolytes and 97% in alkaline electrolytes at a current density of 5 mA cm^-2. Further, the designed all-pH-CDWE can be scaled up to a capacity of 720 C in a high current of 1 A for each cycle with a stable HER average voltage of 0.99 V. This work provides a new strategy toward the mass production of H₂ in a facilely rechargeable process with high efficiency, good robustness, and large-scale applications.

The Use of Redox Mediators in Electrocatalysis and Electrosynthesis

Electrocatalysis and electrosynthesis, which convert the electrical energy and store them in the chemical forms, have been considered as promising technologies to utilize green renewable energy sources. Most of the studies focused on developing novel active molecules or advanced electrodes to improve the performance. However, the direct acquisition and electron transferring will be limited by the intrinsic characters of the electrodes. The introduce of redox mediators, which are served as the intermediate electron carriers or reservoirs without changing the final products, provide a unique approach to accelerate the electrochemical performance of these energy conversions. This review provides an overview of the recent development of electrocatalysis and electrosynthesis by using redox mediators, and provides a comprehensive discussion toward focusing on the principles and construction of these systems.

Pairing of Aqueous and Nonaqueous Electrosynthetic Reactions Enabled by a Redox Reservoir Electrode

Paired electrolysis methods are appealing for chemical synthesis because they generate valuable products at both electrodes; however, development of such reactions is complicated by the need for both half-reactions to proceed under mutually compatible conditions. Here, a modular electrochemical synthesis (ModES) strategy bypasses these constraints using a "redox reservoir" (RR) to pair electrochemical half-reactions across aqueous and nonaqueous solvents. 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, and the reduction of oxygen to hydrogen peroxide in water were paired using nickel hexacyanoferrate as an RR that can selectively store and release protons (and electrons) while serving as the counter electrode for these reactions. Selective proton transport through the RR is optimized and confirmed to enable the ion balance, and thus the successful pairing, between redox half-reactions that proceed with different rates, on different scales, and in different solvents (methanol, acetonitrile, and water).

In situ electrochemical recomposition of decomposed redox-active species in aqueous organic flow batteries

Aqueous organic redox flow batteries offer a safe and potentially inexpensive solution to the problem of storing massive amounts of electricity produced from intermittent renewables. However, molecular decomposition represents a major barrier to commercialization-and although structural modifications can improve stability, it comes at the expense of synthetic cost and molecular weight. Now, utilizing 2,6-dihydroxy-anthraquinone (DHAQ) without further structural modification, we demonstrate that the regeneration of the original molecule after decomposition represents a viable route to achieve low-cost, long-lifetime aqueous organic redox flow batteries. We used in situ (online) NMR and electron paramagnetic resonance, and complementary electrochemical analyses to show that the decomposition compound 2,6-dihydroxy-anthrone (DHA) and its tautomer, 2,6-dihydroxy-anthranol (DHAL) can be recomposed to DHAQ electrochemically through two steps: oxidation of DHA(L)^2- to the dimer (DHA)₂^4- by one-electron transfer followed by oxidation of (DHA)₂^4- to DHAQ^2- by three-electron transfer per DHAQ molecule. This electrochemical regeneration process also rejuvenates the positive electrolyte-rebalancing the states of charge of both electrolytes without introducing extra ions.

Electrochemically Mediated Oxidation of Sensitive Propargylic Benzylic Alcohols

The electrochemical oxidation of sensitive propargylic benzylic alcohols having varying substituents is reported. We describe the preparation and characterization of N-hydroxytetrafluorophthalimide (TFNHPI) and pseudo-high-throughput development of a green electrochemical oxidation protocol for sensitive propargylic benzylic alcohols that employs TFNHPI as a stable electrochemical mediator. The electrochemical oxidation of propargylic benzylic alcohols was leveraged to develop short synthetic pathways for preparing gram quantities of resveratrol natural products such as pauciflorols.

Chemical and Electrochemical O2 Reduction on Earth-Abundant M-N-C Catalysts and Implications for Mediated Electrolysis

M-N-C catalysts, incorporating non-precious-metal ions (e.g. M = Fe, Co) within a nitrogen-doped carbon support, have been the focus of broad interest for electrochemical O₂ reduction and aerobic oxidation reactions. The present study explores the mechanistic relationship between the O₂ reduction mechanism under electrochemical and chemical conditions. Chemical O₂ reduction is investigated via the aerobic oxidation of a hydroquinone, in which the O-H bonds supply the protons and electrons needed for O₂ reduction to water. Mechanistic studies have been conducted to elucidate whether the M-N-C catalyst couples two independent half-reactions (IHR), similar to electrode-mediated processes, or mediates a direct inner-sphere reaction (ISR) between O₂ and the organic molecule. Kinetic data support the latter ISR pathway. This conclusion is reinforced by rate/potential correlations that reveal significantly different Tafel slopes, implicating different mechanisms for chemical and electrochemical O₂ reduction.

Di(Thioether Sulfonate)-Substituted Quinolinedione as a Rapidly Dissoluble and Stable Electron Mediator and Its Application in Sensitive Biosensors

The commonly required properties of diffusive electron mediators for point-of-care testing are rapid dissolubility, high stability, and moderate formal potential in aqueous solutions. Inspired by nature, various quinone-containing electron mediators have been developed; however, satisfying all these requirements remains a challenge. Herein, a strategic design toward quinones incorporating sulfonated thioether and nitrogen-containing heteroarene moieties as solubilizing, stabilizing, and formal potential-modulating groups is reported. A systematic investigation reveals that di(thioether sulfonate)-substituted quinoline-1,4-dione (QLS) and quinoxaline-1,4-dione (QXS) display water solubilities of ≈1 m and are rapidly dissoluble. By finely balancing the electron-donating effect of the thioethers and the electron-withdrawing effect of the nitrogen atom, formal potentials suitable for electrochemical biosensors are achieved with QLS and QXS (-0.15 and -0.09 V vs Ag/AgCl, respectively, at pH 7.4). QLS is stable for >1 d in PBS (pH 7.4) and for 1 h in tris buffer (pH 9.0), which is sufficient for point-of-care testing. Furthermore, QLS, with its high electron mediation ability, is successfully used in biosensors for sensitive detection of glucose and parathyroid hormone, demonstrating detection limits of ≈0.3 × 10^-3 m and ≈2 pg mL^-1 , respectively. This strategy produces organic electron mediators exhibiting rapid dissolution and high stability, and will find broad application beyond quinone-based biosensors.

Sustainable Coproduction of Two Disinfectants via Hydroxide-Balanced Modular Electrochemical Synthesis Using a Redox Reservoir

Challenges posed by the sacrificial auxiliary reactions and expensive ion-exchange membranes in conventional electrosynthesis necessitate developing new electrochemical processes to enable efficient and sustainable distributed electrochemical manufacturing. Modular electrochemical synthesis (ModES) using a redox reservoir (RR) offers a promising membrane-free approach to improve energy efficiency and reduce waste through the pairing of multiple independent oxidative and reductive half-reactions; however, undesired ion-imbalance and induced pH changes in the existing ModES process limit sustained production. Here we present Ni(OH)₂ as a heterogeneous RR that can selectively store and transport the hydroxide ions involved in the target half-reactions by reversible conversion with NiOOH to enable an ion-balanced ModES of two common disinfectants, hydrogen peroxide (H₂O₂) and sodium hypochlorite (NaClO). This hydroxide-balanced ModES realizes stable operation without appreciable pH swing to accumulate practically useful concentrations of H₂O₂ and NaClO up to 251 and 481 ppm, respectively. These results illustrate additional design principles for electrosynthesis without sacrificial auxiliary reactions and the need for ion-selective RRs for modular electrochemical manufacturing.