Decoupled Redox Catalytic Hydrogen Production with a Robust Electrolyte-Borne Electron and Proton Carrier

Abstract

Electrolytic water splitting is an effective approach for H 2 mass production. A conventional water electrolyzer concurrently generates H 2 and O 2 in neighboring electrode compartments separated by a membrane, which brings about compromised purity, energy efficiency, and system durability. On the basis of distinct redox electrochemistry, here, we report a system that enables the decoupling of both the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) from the electrodes to two spatially separated catalyst bed reactors in alkaline solutions. Through a pair of close-loop electrochemical− chemical cycles, the system operates upon 7,8-dihydroxy-2-phenazinesulfonic acid (DHPS) and ferricyanide-mediated HER and OER, respectively, on Pt/ Ni(OH) 2 and NiFe(OH) 2 catalysts. Near unity faradaic efficiency and sustained production of hydrogen has been demonstrated at a current density up to 100 mA/cm 2 . The superior reaction kinetics, particularly the HER reaction mechanism of DHPS as a robust electrolyte-borne electron and proton carriers, were scrutinized both computationally and experimentally. We anticipate the system demonstrated here would provide an intriguing alternative to the conventional water electrolytic hydrogen production.

Redox-Mediated Water Splitting for Decoupled H2 Production

“…Recently, an alkaline system with considerably reduced cell voltage based on 7,8-dihydroxy-2-phenazinesulfonate (DHPS, −0.91 V vs SHE at pH 14.6) and [Fe(CN) 6 ] 3−/4− (0.51 V vs SHE), respectively, as HER and OER redox mediators has been reported. 36 Upon operation, the reduced DHPS acts both as proton and electron carriers, which circumvents the sluggish water dissociation step of HER in conventional alkaline water electrolysis. The hydrogenation of DHPS on the cathode generates DHPS-2H, which is kinetically faster than the direct H 2 O reduction, and then initiates HER reaction while it flows through a Pt catalyst bed in the reactor.…”

“…Reversible redox mediators with fast reaction kinetics and optimized device structures are thus desired to minimize these losses. Besides, one potential development for redox mediators could be the ECPB species, which operate as both charge and proton carriers in the (P)EC−C cyclethere remains intriguing opportunities that if the redox-mediated HER or OER process proceeds via a different route, as the hydrogenation and dehydrogenation processes of DHPS does for HER (note a Tafel mechanism still prevails on Pt in this system), 36 the prevailing scaling effect for the conventional electrocatalysts could be circumvented for the development of low-cost and efficient HER and OER catalysts. Here, for a fair and consistent comparison across different approaches, standardized evaluations of the performance are desired, considering the inconsistency in reported energy efficiency and stability (duration versus cycle number).…”

“…Upon operation, V 3+ and Ce 3+ are electrochemically charged on the electrode in the central cell, and then chemically discharged through catalytic HER and OER reactions in the Mo 2 C and RuO 2 catalyst tanks, respectively (Figure d). Recently, an alkaline system with considerably reduced cell voltage based on 7,8-dihydroxy-2-phenazinesulfonate (DHPS, −0.91 V vs SHE at pH 14.6) and [Fe­(CN) 6 ] 3–/4– (0.51 V vs SHE), respectively, as HER and OER redox mediators has been reported . Upon operation, the reduced DHPS acts both as proton and electron carriers, which circumvents the sluggish water dissociation step of HER in conventional alkaline water electrolysis.…”

“…In order to take the productions of H 2 and O 2 apart in time or in place, redox mediators (either in solid-state or dissolved in solution) are generally employed in these approaches, which bring along an additional reversible electrochemical reaction pairing with the HER or OER, so that the latter could proceed either electrochemically or chemically in separate steps (Figure c). As such, the one-step electrochemical HER (or OER) reaction is translated to a two-step electrochemical–electrochemical (EC–EC) reaction ,− or an electrochemical–chemical (EC-C) reaction. − When sunlight is introduced as a direct energy source, the above decoupled HER (or OER) could proceed via a two-step photoelectrochemical–electrochemical (PEC–EC) cycle, − a photoelectrochemical–chemical (PEC–C) cycle, or a photocatalytic–electrochemical (PC–EC) cycle. , Such a seemingly more complicated reaction cycle endows the H 2 production process with additional features, such as enhanced gas purity, operation flexibility, direct solar energy harnessing, energy storage for on-demand H 2 generation, safety, etc.…”

Controllable and facile preparation of Co9S8–Ni3S2 heterostructures embedded with N,S,O-tri-doped carbon for electrocatalytic oxidation of 5-hydroxymethylfurfural

“…It is well-known that electrocatalytic oxidation can be efficiently and selectively carried out under ambient conditions. Thus, the electrochemical oxidation of furan compounds has shown great prospects, [11][12][13][14][15][16] owing to the integration of biomass transformation and utilization of renewable electricity. More importantly, Earth-abundant metal-based catalysts can provide competitive performance in comparison with noble metal-based catalysts in electrocatalysis.…”