Decoupling charge‒discharge electrolysis for accelerating hydrogen evolution and organic oxidation reactions

Decoupling charge‒discharge electrolysis for accelerating hydrogen evolution and organic oxidation reactions | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Article Decoupling charge‒discharge electrolysis for accelerating hydrogen evolution and organic oxidation reactions Bin Zhang, Yi Huang, Hongyu Zhou, Jiajun Wang, Jingfang Zhang, and 2 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-6716751/v1 This work is licensed under a CC BY 4.0 License Status: Under Review Version 1 posted You are reading this latest preprint version Abstract The substitution of an oxygen evolution reaction with more thermodynamically favourable organic oxidation reactions (OORs) offers a promising strategy for energy-efficient hydrogen evolution and hydrogenation reactions. However, the kinetics of the cathodic reduction reaction is still hindered by sluggish OORs. Herein, we report a decoupled electrolysis strategy to realize a kinetically accelerated hydrogen evolution reaction (HER), a model reduction reaction, for the simultaneous production of H 2 and valuable chemicals via the use of a solid redox mediator. The decoupled system with a rechargeable capability features a HER paired with redox mediator oxidation to store electricity and subsequently OORs to value-added chemicals coupled with the reduction of the oxidized redox mediator to generate electricity. Owing to the rapid kinetics of redox mediator oxidation and operation in a membrane-free cell, the kinetics of the HER are notably accelerated compared with those of conventional overall water splitting and OORs-paired HER systems. The value-added chemicals and electricity are cocreated during the discharging process, offering more economic benefits. Importantly, the design of such decoupled systems is universally applicable to OORs-paired electrocatalytic reduction systems (e.g., acetylene semihydrogenation) to synthesize various chemicals for electricity storage and generation, paving a sustainable avenue for energy-efficient H 2 and chemical manufacturing. Physical sciences/Chemistry/Electrochemistry/Electrocatalysis Physical sciences/Chemistry/Green chemistry/Sustainability Physical sciences/Energy science and technology/Renewable energy/Hydrogen energy Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Introduction Water-participating electrocatalytic hydrogen evolution and hydrogenation reactions are the promising approaches for producing green hydrogen (H 2 ) and various other chemicals, 1 – 3 but a high overpotential is usually required to drive the kinetically sluggish oxygen evolution reaction (OER) at the anode to match the rate of the cathodic reduction reaction (CRR). 4 – 6 This requirement results in a high cell voltage input and significantly increases energy consumption, such as > 1.8 V of cell voltage with a high electricity consumption of 4.5–6 kWh m – 3 H 2 in an overall water splitting (OWS) system. 7 Moreover, the OER yields low-value O 2 and poses risks such as the formation of explosive H 2 /O 2 gas mixtures such that an ion-exchange membrane is required to prevent gas crossover. The membrane also suffers from degradation due to the generation of reactive oxygen species. 8 Recently, the electrooxidation reactions of organic small molecules (e.g., urea, alcohols, aldehydes, and amines), which exhibit lower theoretical equilibrium potentials but generate value-added chemicals (Supplementary Table 1), have been reported as alternatives to the OER under reduced cell voltages. 4 – 6 , 9 – 15 Despite significant efforts, elevated cell voltages are still required at high current densities within this coupled system. This is attributed to the close coupling between organic oxidation reactions (OORs) and the CRR, where the CRR rate is explicitly dependent on the rate of the OORs. 16 The latter typically exhibit slow reaction kinetics due to their multielectron-coupled proton transfer processes. 17 , 18 This mismatch in reaction rates consequently hinders cathodic kinetics. Furthermore, the kinetic loss may be further exacerbated by the mass transport of organic reactants and products on the surface of the anode. 19 Therefore, the development of a strategy to achieve efficient conversion of organic molecules and simultaneously accelerate CRR kinetics is highly desirable but remains a considerable challenge. Decoupling electrolysis, achieved by pairing individual half-reactions with electrochemical reactions of redox mediators (RMs), has spurred innovative approaches to overcome the inherent limitations of electrolysis, as first reported by Symes and Cronin in 2013. 8 , 18 – 25 For example, a nickel (oxy)hydroxide electrode was employed as a RM in an alkaline electrolyte to decouple the one-step OWS process into two steps, thereby enhancing the promotion of H 2 generation. 26 This decoupling strategy allows for flexibly matching diverse half-reactions at desired reaction rates across different timescales, offering a promising solution to the mismatched rate issue between reduction reactions and OORs. 16 , 27 Given that OORs can be paired with a reduction reaction possessing a more positive potential to construct batteries/cells for simultaneous electricity generation, 28 , 29 the simultaneous production of value-added chemicals on both the cathode and anode along with electricity storage/generation (i.e., energy input in one step and output in another) presents an appealing prospect. However, this topic within decoupled systems has been largely overlooked in current research. Herein, by selecting the hydrogen evolution reaction (HER) as a model CRR, we present a decoupled process to realize kinetically accelerated H 2 generation and the simultaneous production of valuable chemicals by using nickel‒cobalt hydroxide (named NCOH) as a RM. The ethylene glycol oxidation reaction (EGOR) into value-added glycolic acid (GA) was selected as a model OOR to demonstrate the feasibility and efficiency of the decoupled system, which comprises cathodic HER alongside anodic oxidation of NCOH to nickel‒cobalt oxyhydroxide (NCOOH), followed by subsequent GA production through cathodic reduction of NCOOH back to NCOH and anodic EGOR. The kinetic advantages of the HER in this system are discussed in comparison with those of conventional electrolysis systems. Moreover, the decoupled system has the potential to be expanded to other OORs paired cathodic reduction reaction systems to increase the HER/hydrogenation reactions rates and synthesize various chemicals with electricity storage and generation ability. Results Principle of two-step decoupling electrolysis The cell incorporates a RM (NCOH/NCOOH) positioned between a HER cathode and an OOR anode (Fig. 1 a). The HER cathode and RM are situated within the same chamber, and are isolated from the OOR anode by an ion-exchange membrane. H 2 is generated on the HER cathode while simultaneously undergoing anodic oxidation of NCOH to store electricity energy (NCOH to NCOOH) (Step 1, charging process). The subsequent generation of chemicals involves the anodic OORs, with the cathodic NCOOH being reduced back to NCOH, yielding energy output due to the positive potential difference between the reduction of NCOOH and the OORs (Step 2, discharging process). Notably, the separation of the catholyte and anolyte by a membrane in this process is essential, given that OORs may be catalysed by the charged state of the RM (NCOOH). 14 , 22 The temporally and spatially distinct Steps 1 and 2 result in an alternating operating sequence, and the periodic interchange between Steps 1 and 2 enables the continuous operation of the system. This approach yields a device architecture that offers several significant advantages in contrast to the conventionally configured OWS and ORRs-paired HER systems (Supplementary Fig. 1). First, the kinetics of H 2 generation can be accelerated because of the rapid kinetics of paired oxidation reactions (NCOH to NCOOH), which involve a single-electron transfer process. 19 This solves the rate mismatch issue between the HER and OORs. Second, H 2 generated within a single cell without a membrane can minimize energy loss caused by membrane resistance, thereby increasing energy efficiency. Third, value-added chemicals and electricity are cocreated during the discharging process, offering more economic benefits. Finally, the periodic cycle between Steps 1 and 2 is similar to a rechargeable system to produce H 2 with electricity storage (charging process) and generate valuable chemicals with electricity output (discharging process). This implies that we can flexibly store electricity during the daytime or at low or even negative electricity prices and generate electricity at night or at high electricity prices to optimize power supply and demand to reduce electricity costs. 27 Given that the premise of energy output in Step 2 is the greater potential of the reduction of the charged RM than that of the OORs, the RM plays a significant role in the decoupled system. The RM should satisfy both high capacity and moderate redox potential to match the oxidation potential of ORRs and avoid competition with the OER when NCOH is charged. Here, nickel hydroxide (Ni(OH) 2 ) was selected as a model RM because of its high theoretical capacity and suitable redox potential of 1.45 V relative to the reversible hydrogen electrode (RHE; hereafter, the potentials were recorded relative to the RHE if not specified). 22 , 26 , 30 Notably, this potential is higher than the oxidation potentials of several OORs (Supplementary Table 1). Doping Co into Ni(OH) 2 to produce NCOH enhances the capacity and negatively shifts the redox potential, avoiding competition with the OER during the charging of NCOH (Supplementary Figs. 2 and 3, Supplementary Notes 1 and 2). 22 , 30 The electrochemical profile of NCOH in an alkaline electrolyte was investigated via cyclic voltammetry (CV). As illustrated in Fig. 1 b, a pair of redox peaks were distinctly observed at 1.09 and 1.27 V. This is attributed to the reversible cycling between NCOH and NCOOH. 22 , 26 EGOR, as a value-added GA, was selected as a model OOR to demonstrate the feasibility and efficiency of the decoupled system for the following reasons: 1) EGOR has garnered significant attention because of its potential for upgrading ethylene glycol (EG, ∼ US $ 1.1 kg −1 ) sourced from biomass and poly(ethylene terephthalate) (PET) plastic waste into valuable products, including C 2 and C 1 chemicals (Supplementary Fig. 4); 31 2) the selective oxidation of EG-to-GA (US $ 100–300 kg −1 ) is particularly appealing because of its much lower equilibrium potential than that of the OER (0.57 vs. 1.23 V) 32 and the considerable promise of GA in various industrial applications, especially as a biodegradable polymer with high market demand. 33 Given the activity of Pd in the selective electrooxidation of EG to GA at low potential, 34 , 35 combined with the greater number of exposed active surfaces and increased mass diffusion efficiency of porous structures, 36 we designed Pd porous nanosheet arrays (PNAs) to serve as model electrocatalysts for the selective conversion of EG to GA (discussed in detail below). Obviously, the potential window for the EGOR on Pd PNAs lies between the redox potential of NCOH/NCOOH and the onset potential for the HER (Fig. 1 b). This result indicates that it is theoretically possible to achieve a decoupled process, as illustrated in Fig. 1 a, using NCOH as a RM and Pd PNAs as an EGOR electrocatalyst. Synthesis of Pd PNAs for converting EG to GA Pd PNAs were synthesized via a self-template strategy (Fig. 2 a). Specifically, Co(OH) 2 nanosheet arrays were synthesized on Ni foam by electrodeposition, followed by a galvanic reaction between Co(OH) 2 and PdCl 4 2– . 37 The area of the electrode can be easily scaled up (Supplementary Fig. 5). Scanning electron microscopy (SEM) images revealed that uniform nanosheet arrays grew on the Ni foam, which were maintained after the galvanic reaction between Co(OH) 2 and PdCl 4 2– (Fig. 2 b and Supplementary Fig. 6). The porous nanosheet structures of the Pd PNAs transformed from the solid nanosheets of Co(OH) 2 were observed from typical transmission electron microscopy (TEM) images (Fig. 2 c, d and Supplementary Fig. 7). The lattice fringes measuring 0.23 and 0.20 nm align well with the (111) and (200) lattice spacings of the face-centered cubic (fcc) Pd crystal, as observed in the high-resolution TEM (HRTEM) image (Fig. 2 e). Energy-dispersive X-ray (EDX) element mapping images reveal a uniform distribution of Pd in the Pd PNAs (Supplementary Fig. 8). X-ray diffraction (XRD) patterns display that the main diffraction peaks were well indexed to Pd, and the peaks attributed to Co(OH) 2 are not detected (Supplementary Fig. 9). The EGOR activity of the Pd PNAs was evaluated in 1 M KOH containing 0.5 M EG with commercial Pd/C as a comparison. All the electrochemical tests were performed without iR correction unless otherwise specified. As shown in Fig. 2 f, the linear sweep voltammetry (LSV) curves reveal that the apparent oxidation current starts from an applied potential of ∼0.49 V, and the current density can reach ~ 808 mA cm − 2 at 1.13 V. In the absence of EG, no current is observed in the above potential windows, except for an oxidation peak centred at ∼0.90 V assigned to the oxidation of Pd (Supplementary Fig. 10). 34 For comparison, Pd/C shows a very low oxidation current starting from an applied potential of ∼0.49 V for the EGOR because of the limited number of active sites. After electrolysis, the oxidation products were analysed via 1 H nuclear magnetic resonance (NMR) spectroscopy (Supplementary Fig. 11). 17 The Faradic efficiency (FE) of GA can reach above 90% in the potential range of 0.62 V to 1.02 V (Fig. 2 g), indicating that one hydroxyl group in EG is selectively oxidized to form GA. The GA generation rate is as high as 5.5 mmol h – 1 cm – 2 at 1.02 V ( j > 600 mA cm −2 ). The high current density with high selectivity for GA generation at such a low potential represents the superior performance of the as-prepared Pd PNAs compared with reported electrocatalysts for the EGOR (Fig. 2 h and Supplementary Table 2). 17 , 31 – 33 , 35 , 38 – 41 The stability of the Pd PNAs was also evaluated for five continuous batches of reaction. As shown in Fig. 2 i, the generation rate and FE of the GA are largely maintained, together with the preservation of the original nanosheet array structure (Supplementary Figs. 12 and 13), demonstrating the high stability of the Pd PNAs. To investigate the mechanism underlying the enhanced activity of EGOR-to-GA on Pd PNAs, a series of experiments were carried out. Considering that the generation of adsorbed hydroxyl (OH*) groups plays an important role in the EGOR, 31 , 42 CV measurements were first used to study the adsorption/activation of OH – species. As displayed in Fig. 2 j, the Pd PNAs clearly exhibit OH – adsorption peaks. Moreover, the onset potential of OH – adsorption peaks on Pd PNAs is lower than that on Pd/C, which is in line with that of EGOR, implying that the OH* species is the main active species participating in EGOR to rapidly convert *OC−CH 2 OH intermediates to the target GA product. 39 The open-circuit potential (OCP), which reflects absorption in the inner Helmholtz layer, 43 was further measured to assess the EG adsorption behaviour on the catalysts. Upon the addition of 0.5 M EG, a more significant decrease in the OCP for Pd PNAs is observed than that for Pd/C, suggesting favourable adsorption of EG on Pd PNAs (Supplementary Fig. 14). Operando electrochemical Fourier transform infrared (FTIR) spectroscopy was then carried out to further probe the intermediates and understand the reaction pathway during EGOR on Pd PNAs. As displayed in Fig. 2 k, downwards enhancement bands at 1076 cm − 1 and 1590 cm − 1 , which are attributed to the stretching vibration of aldehydes and the antisymmetric stretching vibrations of the carboxyl group in GA, respectively, 39 can be observed with increasing potential, indicating that the OH group of EG is first oxidized to glycolaldehyde (GD). More favourable GD oxidation than EG oxidation and no GA oxidation at a potential of < 1.0 V further indicate that GD may be the intermediate product that rapidly converts to GA (Supplementary Fig. 15). Thus, we deduce that the enhanced ability of OH* species generation and EG adsorption greatly contributes to the enhanced activity of EGOR on Pd PNAs, and the OH* active species participate in the cascade oxidation pathway of EGOR to generate GA with high selectivity instead of C–C bond cleavage into CO (Supplementary Fig. 16). Cell assembly and performance investigation We then assembled a cell to verify the feasibility of a decoupling system to realize kinetically accelerated H 2 generation and simultaneous production of valuable GA along with electricity storage/generation. The detailed cell assembly is illustrated in Fig. 3 a and Supplementary Fig. 17, comprising the as-prepared Pd PNAs, a Pt plate, and an RM (NCOH/NCOOH). Here, a commercially available Pt plate was used as the HER electrode, and the RM electrode was placed in an electrolytic cell with no membrane. The ion-exchange membrane was used to separate the RM electrode and Pd PNAs, avoiding the oxidation of EG by NCOOH. In Step 1, an energy input facilitates the generation of H 2 on the Pt cathode coupled with the oxidation reaction of NCOH (NCOR) to NCOOH to store energy. The advantages of the HER in Step 1 (named HER ‖ NCOR) were evaluated compared with those of conventional OWS and EGOR-paired HER systems (named HER ‖ OER and HER ‖ EGOR, respectively; see Supplementary Fig. 1 for detail). As demonstrated in Fig. 3 b, the LSV curves reveal that the HER ‖ NCOR system results in the fastest current density increase among the three systems, and its onset potential is lower than that of the OWS system. Although the HER ‖ EGOR system has a lower onset potential than the HER ‖ NCOR system because of the lower oxidation potential of the EGOR than that of the NCOR (as discussed in Fig. 1 b), the HER ‖ NCOR system shows a greater increase in current density than the HER ‖ EGOR system with increasing applied cell voltage. Operando electrochemical impedance spectroscopy (EIS) was performed to examine the electron transfer kinetics of the three systems. Compared with that of the HER ‖ EGOR and OWS systems, the resistance of the HER ‖ NCOR system rapidly decreases at the onset potential of the HER, indicating fast electron transfer (Fig. 3 c, Supplementary Fig. 18 and Supplementary Note 3). Rapid electron transfer enables fast reaction kinetics of the HER, resulting in a fast increase in the current density with increasing applied potential, as shown in Fig. 3 b. In fact, both the EGOR and OER are thermodynamically more favourable than the NCOR. However, the coupled transfer of four electrons and protons results in a kinetically slow process for the EGOR and OER. Additionally, the evolution of molecular oxygen gas and liquid organic products may contribute to additional kinetic loss due to mass transport. 19 Conversely, NCOR, a process involving the transfer of a single electron/proton without generating any products, outperforms the OER and EGOR in kinetically accelerating the HER. The kinetically favoured NCOR renders the cell architecture versatile and robust in terms of voltage efficiency for the HER. At a current density of 50 mA cm −2 , the HER ‖ NCOR system displays a cell voltage of only 1.38 V, which is lower than that of the HER ‖ EGOR and OWS systems (1.87 and 1.99 V, respectively) (Fig. 3 d). In addition, the generation rate of H 2 for HER ‖ NCOR system is 3.2 mmol h −1 cm −2 , which is markedly higher than that of conventional systems at a cell voltage of 1.5 V. A steady-state study was also performed over an extended period. At a current density of 50 mA cm − 2 , the average electrolysis voltage is ~ 1.6 V, translating to a voltage efficiency of 84.4% (Fig. 3 e). The complete conversion of NCOH is indicated by a remarkable voltage increase to 1.8 V, indicating the occurrence of the OER. It is imperative for the continuous operation of the HER to integrate the EGOR in Step 2, as discussed below. Moreover, H 2 generation increases with increasing current density without detectable O 2 at current densities ranging from 0 to 200 mA cm −2 (Supplementary Fig. 19, Supplementary Note 4, and Supplementary Movie 1). The current density reaches 200 mA cm −2 , and the Faradaic efficiency (FE) of the HER is near 100%, indicating high-rate generation of H 2 with high purity in Step 1. Observations also reveal that the corresponding cell voltage fluctuated between 1.25 and 1.62 V under the aforementioned current densities (Supplementary Fig. 20). This implies the adaptability of H 2 production to variable power outputs of sustainable energy, such as solar or wind energy. Impressively, the cell architecture of the HER ‖ NCOR within a single cell has a lower internal resistance than those of the HER ‖ EGOR and OWS systems with a membrane, minimizing energy loss caused by membrane resistance (Supplementary Fig. 21). The HER ‖ NCOR system results in a 33% reduction in energy consumption (Fig. 3 f). When the NCOH was fully converted to NCOOH in Step 1, the reduction of NCOOH back to NCOH and the oxidation of EG to GA released energy in Step 2 (discharging) when the switch was toggled. The polarization and power density curves of the battery show that the peak power density reaches 14 mW cm −2 (Fig. 4 a), which is close to that reported for liquid fuel cells. 28 , 29 The open circuit voltage (OCV) of the aqueous battery is 0.89 V (Supplementary Fig. 22), which is equal to the potential difference between the cathodic reduction of NCOOH (NCOOH → NCOH) and the anodic EGOR (Δ U discharging in Fig. 1 b). When the battery is discharged with different current densities, it delivers a similar discharge capacity at current densities ranging from 20 to 100 mA cm −2 , indicating its excellent rate capability (Fig. 4 b and Supplementary Fig. 23). The capacity of the battery can be further improved by an electrode with a relatively high mass loading of NCOH. 29 The product of the EGOR in the anode chamber during the discharging process was collected. Interestingly, the FEs of GA generation reach 90% at current densities ranging from 20 to 100 mA cm −2 , and the generation rate of GA is 3.3 mmol h −1 cm −2 at a high discharge current density of 100 mA cm −2 (Fig. 4 c). In contrast, the FE of GA is only 8.2% because of the generation of O 2 and formic acid (FA) byproducts at the same current density for the HER/EGOR system with high energy input (Supplementary Fig. 24). Moreover, at a current density of 50 mA cm −2 , the discharge time (1200 s) of the cell in Step 2 is the same as the electrolysis time in Step 1, indicating a Coulombic efficiency of ~ 100% (Fig. 4 d). At the end of discharge, the cell voltage sharply decreases, indicating that all of the NCOOH has been converted into NCOH, which is then used for H 2 production in Step 1. The periodic cycle between Steps 1 and 2 to produce H 2 with electricity storage and generate a GA with electricity output fabricates a rechargeable system. After 50 cycles, no obvious degradation was observed, and the FE of GA generation remained at ~ 90% with intervals for H 2 production (Fig. 4 e and Supplementary Fig. 25). Hence, the decoupled configuration has superior energy efficiency, demonstrating significant potential for practical application compared with conventional one-step electrolysis with a membrane in OWS and OOR-paired HER systems. Universality and practicality of the decoupled system As previously mentioned, if the reduction potential of NCOOH is higher than the oxidation potential of ORRs, which is a positive potential difference between the cathode and anode, electricity will be generated in Step 2. Therefore, we further investigated other ORRs to demonstrate the universality of the decoupled system. We selected glycerol (GLY) oxidation to lactic acid (LA), formaldehyde (FD) oxidation to formic acid (FA), and ascorbic acid (AA) oxidation to dehydroascorbic acid (DHA) as additional model reactions. 10 , 31 , 44 The oxidation potentials of the above three reactions are lower than the reduction potential of NCOOH (Fig. 5 a, Supplementary Fig. 26, and Supplementary Note 5). After the RM was charged in Step 1, GLY, FD and AA were added to the anode chamber to fabricate batteries that combined with the charged RM (NCOOH). The OCVs are measured at 0.609, 0.985, and 0.236 V, and the corresponding products are LA, FA and DHA, respectively, for the above three aqueous batteries (Fig. 5 b). This implies that the decoupled strategy can be expanded to other OORs paired with HER systems to increase the HER kinetics and synthesize various chemicals with electricity storage and generation. The use of variable OORs with various oxidation potentials allows flexible adjustment of the output voltage in the discharging process, rendering the decoupled system suitable for a wide range of scenarios. More importantly, other RMs can also be used instead of NCOH, such as Mn-based oxides, which have also been widely used in rechargeable batteries. 21 , 45 , 46 For example, Na 0.44 MnO 2 can be used as a RM to boost the alkaline HER under a low input voltage in Step 1 and concurrent GA and electricity in Step 2 (Supplementary Fig. 27−29 and Supplementary Notes 6 and 7). 21 Moreover, the acid HER could also be increased by using Mn 2+ /MnO 2 as a RM in 1 M H 2 SO 4 (Fig. 5 c and Supplementary Fig. 30). 45 Interestingly, the OCV of the charging process in Step 2 is measured at 1.78 V (Fig. 5 d), which is high compared with the potential difference between the cathodic reduction of RM (MnO 2 → Mn 2+ ) and the anodic EGOR because of the contribution of the electrochemical neutralization energy of 0.828 V in the acid‒base asymmetric electrolyte. 47 The polarization and power density curves of the acid–base asymmetric battery show that the peak power density reached as high as 133 mW cm −2 , outperforming some reported hybrid fuel cells (Fig. 5 e and Supplementary Table 3). 48 – 50 In addition to the HER, the electrocatalytic hydrogenation reaction coupled with the OOR can also be decoupled by our proposed strategy. Selecting the electrocatalytic semihydrogenation of coal-derived acetylene to ethylene (ESAE) as an example, 51 we can achieve the promoted kinetics of ESAE and a high ethylene FE of 97% at a current density of 500 mA cm − 2 under a low cell voltage in Step 1 (Supplementary Fig. 31 and Supplementary Note 8). It is imperative to exercise caution in the selection of RM, as this will help prevent the hydrogenation products from undergoing oxidation at the surface of the RM anode. To further assess the practical viability of the decoupled system, three reaction cells were linked in series to increase the overall efficiency (Fig. 6 a and the inset of Fig. 6 b). Here, high-performance nickel‒cobalt phosphide (NiCoP) was synthesized and employed as an HER electrocatalyst (Supplementary Fig. 32), 52 with the objective of reducing catalyst costs. In Step 1, the electrolysis process used to generate H 2 and charge the RM operates in parallel. This can be powered by a low-voltage photovoltaic cell or other sustainable energy. In contrast, the discharge process in Step 2, which is responsible for generating chemicals and electricity, is configured in tandem to effectively increase the working voltage. The charging curve of the series cell shows that the current density could reach an industrial current density of ~ 400 mA cm − 2 at a very low voltage of 2.0 V (Fig. 6 b). Powered by a photovoltaic cell with an output of 1.52 V (Supplementary Fig. 33), the electrolysis cell is able to produce 38.2 mL of H 2 in 16 min (Fig. 6 c and Supplementary Fig. 34). After charging, we evaluate the performance of the discharging process in Step 2. The aqueous battery pack had a high power density (a maximum value of up to 38.8 mW cm − 2 ) and a high OCV of 2.12 V (Fig. 6 d and Supplementary Fig. 35) so that it could supply stable power for a timer (Supplementary Fig. 36). Moreover, 0.72 mmol of GA was collected after a discharging process of 20 min at a current density of 50 mA cm − 2 (Fig. 6 e), accompanied by the generation of 135 J of electricity (0.68 kWh of electricity per kg of GA generation). Preliminary technoeconomic analysis (TEA) was then conducted to investigate the feasibility of this decoupled system via a model adapted from the literature (Supplementary Fig. 37 and Supplementary Note 9). In addition to the cost of renewable electricity and FEs of target products, the profitability of the process largely depends on the operating current density. 52 , 53 In this context, our system is economically feasible even when the current density of the HER in Step 1 is 50 mA cm −2 because of the low input voltage and highly valuable GA generation in Step 2 (Fig. 6 f). The net revenue reaches ~ $ 3578 for generating liquid H 2 per m 3 under a commercially relevant current density (> 200 mA cm − 2 ), indicating the economic potential of our proposed decoupled system. In terms of application scenarios, we envision that a decoupled system with a rechargeable capability will be suitable for distributed energy storage and chemical production (Fig. 6 g). For example, it could store intermittent renewable electricity (such as from solar or wind sources) during the daytime or oversupply electricity and then release electricity for household use at night or during high electricity price periods. Concurrently, this process enables the kinetically accelerated generation of H 2 , along with the simultaneous production of valuable chemicals. These chemicals can then be collected and separated for subsequent use. Discussion In summary, we have realized kinetically accelerated H 2 generation and simultaneous production of valuable chemicals alongside electricity storage/generation via decoupled electrolysis using nickel‒cobalt hydroxide as a redox mediator. The kinetics of the HER in the decoupled system without a membrane are largely accelerated because of the rapid reaction kinetics of the paired redox mediator oxidation and the extremely low internal resistance. We select the EGOR into the value-added GA as a model OOR to demonstrate the feasibility and efficiency of the decoupled system. High-performance Pd porous nanosheet arrays were designed as EGOR electrocatalysts, enabling a high GA generation rate and power density during the discharging process. Importantly, this decoupled strategy has potential for broad application in other OORs paired with HER/hydrogenation reaction systems to increase the cathodic reduction reaction rate and facilitate the synthesis of various chemicals alongside electricity storage and generation. Our proposed decoupled electrolysis method allows better utilization of intermittent renewable sources, such as solar or wind, for chemical manufacturing, presenting a promising approach to enhancing renewable-to-chemical conversion while also offering innovative concepts for constructing a hybrid energy conversion/storage system. Considering the chemical properties and species diversity of redox mediator materials, this work provides a universality and adaptability decoupled strategy for broad application across diverse scenarios. Methods Materials All the chemicals used in the experiments were analytically pure and were used without further purification (see Supplementary Note 10 for details). Deionized water (DIW) was used in all the experimental processes. Synthesis of Pd PNAs electrodes Co(OH) 2 nanosheet arrays grown on Ni foam were first synthesized by electrodeposition. A Ni foam (1×2 cm 2 ), a saturated calomel electrode (SCE, saturated KCl, aqueous), and a platinum plate were used as the working electrode, reference electrode and counter electrode, respectively. All the samples were placed into a 25 mM Co(NO 3 ) 2 solution, and the immersed size of the Ni foam in the solution was maintained at 1×1 cm 2 . The blue Co(OH) 2 was obtained after working at -1 V vs. SCE for 240 s and washing with DIW and ethanol several times. The Co(OH) 2 electrode was subsequently immersed in a Na 2 PdCl 4 solution (3.6 mg/mL) at room temperature for 40 minutes. The Pd PNAs electrodes were obtained after washing with DIW and ethanol several times and drying in a vacuum oven overnight. Synthesis of Ni(OH) 2 or Co 1–x Ni x (OH) 2 RMs Ni(OH) 2 or Co 1 − x Ni x (OH) 2 was prepared via an electrodeposition method in a three-electrode electrochemical configuration in a 0.1 M Ni(NO 3 ) 2 solution or Ni(NO 3 ) 2 /Co(NO 3 ) 2 mixed solution with a Ag/AgCl electrode as the reference electrode. The nickel foams were subjected to ultrasonic cleaning in an acetone solution, followed by ethanol and deionized water for 15 minutes each. Two pieces of Ni foams of equal size (2×2 cm 2 ) were used as the working electrode and the counter electrode. The electrodeposition lasted 300 s at a constant current density of -10 mA cm − 2 . The loading mass of Ni(OH) 2 or Co 1 − x Ni x (OH) 2 was ~ 2 mg cm − 2 . Synthesis of NiCoP electrodes First, the Co 0.4 Ni 0.6 (OH) 2 electrode was prepared via the above method. Subsequently, NaH 2 PO 2 ·H 2 O (0.8 g) and the as-prepared Co 0.4 Ni 0.6 (OH) 2 were heated at 300°C at a heating rate of 2°C min − 1 under an Ar atmosphere, and the temperature was held at 300°C for 2 h. The NiCoP electrodes were synthesized after cooling to room temperature. Synthesis of Na 0.44 MnO 2 Na 0.44 MnO 2 was prepared according to previous work. 21 A solid-state reaction method was adopted to prepare Na 0.44 MnO 2 using Na 2 CO 3 and Mn 3 O 4 as the precursors. Stoichiometric amounts of Na 2 CO 3 and Mn 3 O 4 were ground by ball milling at 300 RPM for 5 h and then sintered in a muffle furnace at 775°C for 10 h. Na 0.44 MnO 2 was synthesized after cooling to room temperature. General characterizations X-ray diffraction (XRD) patterns of all the samples were obtained on a Bruker D8 Advance diffractometer with monochromatized Cu K α radiation (λ = 0.15418 nm). Scanning electron microscopy (SEM) images were acquired with a JEOL 6700-F field-emission scanning electron microscope. Transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) images, high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) images and corresponding energy dispersive spectroscopy (EDS) images were obtained with a transmission electron microscope (JEOL JEM-2100F) operating at an acceleration voltage of 200 kV. 1 H NMR spectra were recorded on a Bruker 400 MHz NMR spectrometer. Fourier transform infrared spectroscopy (FTIR) was performed on a Nicolet IS50 instrument. Electrochemical measurements Electrochemical measurements were performed via a CHI 760E electrochemical workstation (Shanghai Chenhua Instrument Company, China) in a standard three-electrode cell configuration. The as-prepared catalysts were used as the working electrode, while a Hg/HgO electrode (KCl, aqueous) and a platinum plate were used as the reference and counter electrodes, respectively. The potentials were converted to voltages via the following formula: E RHE = E Hg/HgO + 0.098 + 0.059×pH, with reference to the reversible hydrogen electrode (RHE). The polarization curves were recorded at a scan rate of 5 mV s − 1 . O perando electrochemical impedance spectroscopy (EIS) tests were performed, measuring a frequency range of 0.01 Hz to 100 kHz with an AC amplitude of 10 mV. The FEs of all the products were calculated on the basis of their corresponding electron transfer per oxidation molecule via the following equations. $$\:FE=\frac{{n}_{e}\times\:{n}_{products}\times\:F}{Q}\times\:100\%$$ where n e is the number of electrons from reactants to products, n products is the productivity of products, F is the Faraday constant ( F = 96485), and Q is the quantity of electric charge. The electricity input for cathodic H 2 production in the stacked membrane-free electrolyzer for the electrooxidation of Ni(OH) 2 /NiOOH was calculated via the following equations. $$\:{W}_{{H}_{2}}=\frac{I\times\:U\times\:t}{{V}_{{\text{H}}_{2}}\times\:{10}^{3}}$$ where is the electricity consumption per unit of hydrogen production (kWh/m 3 H 2 ), I is the electrolyzer output current (A), U is the electrolyzer input voltage (V), t is the reaction time (h), I × t is the integral area of the I-t curve, and V H2 is the hydrogen production (m 3 ). The productivity of H 2 in the membrane-free electrolyzer was calculated on the basis of the charge transfer of NCOH/NCOOH oxidation. Quantification analysis of oxidation products The formic acid (FA) was identified and quantified via anion chromatography (Thermo Scientific, IC-900, Dionex IonPac AS23). The concentration of HCOOH produced was ascertained by diluting the electrolyte sample to a level that corresponded with the range of the calibration curve. Glycolic acid (GA), lactic acid (LA), and dehydroascorbic acid (DHA) were quantified via NMR spectroscopy. The products in the anolyte sample were quantified via 1 H NMR spectroscopy via water suppression techniques. Each sample was acquired with a total of 32 transient scans. The internal standard used was DMSO with a chemical shift at 2.71 ppm in D 2 O. The NMR samples were prepared by adding 200 µL of DMSO in D 2 O to 500 µL of the liquid sample. The relationship between the analyte and internal standard was quantified via the following equation: $$\:\frac{{n}_{x}}{{n}_{std}}=\frac{{I}_{\text{x}}}{{I}_{std}}\times\:\frac{{N}_{std}}{{N}_{x}}$$ where n represents the number of moles of the analyte (x), N represents the number of hydrogen atoms, and I represents the integrated area of both the analyte and internal standard (std). The FE was calculated at a given potential as follows: $$\:FE\left(\text{\%}\right)=\frac{nFc\text{V}}{Q}$$ where the concentration c represents the electrooxidation products (g L −1 ). V represents the volume of the electrolyte (L), and n represents the number of electrons transferred for product formation (mol). F is the Faraday constant (96485 C), and Q represents the quantity of electric charge integrated by the i – t curve (C). Declarations Data availability The data that support other plots within this paper are available from the corresponding author upon reasonable request. Competing Financial Interests The authors declare no competing interests. Author Contributions Y.H. and B.Z. conceived the idea and directed the project. Y.H., H.Z., and J.Z. designed the experiments. H.Z. and B.H.Z. carried out the experiments and characterization. J.G. assisted in some experiments. Y.H. and J.Z. wrote the paper. B.Z. revised the paper. All the authors discussed the results and commented on the paper. Acknowledgements We acknowledge the National Key Research and Development Program of China (2023YFA1507400 and 2024YFA1510100 to B.Z.), the National Natural Science Foundation of China (U21A20286 to Y.H., 22206054 to Y.H., and 22478310 to J.Z.) and the Fundamental Research Funds for the Central China Normal University (CCNU). References Chatenet M et al (2022) Water electrolysis: from textbook knowledge to the latest scientific strategies and industrial developments. Chem Soc Rev 51:4583–4762 Shi Y, Zhang B (2016) Recent advances in transition metal phosphide nanomaterials: synthesis and applications in hydrogen evolution reaction. Chem Soc Rev 45:1529–1541 Liu C, Chen F, Zhao B-H, Wu Y, Zhang B (2024) Electrochemical hydrogenation and oxidation of organic species involving water. 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Nat Sustain 6:827–837 Zhou H et al (2021) Electrocatalytic upcycling of polyethylene terephthalate to commodity chemicals and H 2 fuel. Nat Commun 12:4679 Leow WR et al (2020) Chloride-mediated selective electrosynthesis of ethylene and propylene oxides at high current density. Science 368:1228–1233 Additional Declarations There is NO Competing Interest. Supplementary Files HYHERSI0521.pdf Supplementary Information Cite Share Download PDF Status: Under Review Version 1 posted You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. We do this by developing innovative software and high quality services for the global research community. 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Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-6716751","acceptedTermsAndConditions":true,"allowDirectSubmit":false,"archivedVersions":[],"articleType":"Article","associatedPublications":[],"authors":[{"id":533084305,"identity":"340dbc9e-9a5d-45e1-9be5-e5d1bc32f6a0","order_by":0,"name":"Bin 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1","display":"","copyAsset":false,"role":"figure","size":205216,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003ePrinciple of two-step decoupling electrolysis. a\u003c/strong\u003e Schematic of the operation mechanism of the cell. \u003cstrong\u003eb\u003c/strong\u003e CV curves of the NCOH electrode at a scan rate of 5 mV s\u003csup\u003e-1\u003c/sup\u003e in 1 M KOH (yellow line), and linear sweep voltammetry (LSV) curves of the commercial Pt plate for the HER (red line) and Pd PNAs for EGOR (blue line) in 1 M KOH with a sweep rate of 5 mV s\u003csup\u003e-1\u003c/sup\u003e. \u003cstrong\u003ec\u003c/strong\u003e, Electrode reactions of charging and discharging processes.\u003c/p\u003e","description":"","filename":"image1.png","url":"https://assets-eu.researchsquare.com/files/rs-6716751/v1/2b7c4597688f2be667527aaa.png"},{"id":94161865,"identity":"206d2675-a078-4510-bbe6-2a2eae1fc1e6","added_by":"auto","created_at":"2025-10-23 05:03:07","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":867344,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eSynthesis and performance evaluation of Pd PNAs. a\u003c/strong\u003e Schematic diagram illustrating the synthetic process of Pd PNAs. \u003cstrong\u003eb-e \u003c/strong\u003eSEM (\u003cstrong\u003eb\u003c/strong\u003e), TEM (\u003cstrong\u003ec\u003c/strong\u003e) and HRTEM (\u003cstrong\u003ed\u003c/strong\u003e and \u003cstrong\u003ee\u003c/strong\u003e) images of Pd PNAs. \u003cstrong\u003ec \u003c/strong\u003eLSV curves of the Pd PNAs, Pd/C and NF in the presence of 0.5 M EG. \u003cstrong\u003eg\u003c/strong\u003e FE and generation rate of the GA under different potentials over Pd PNAs. \u003cstrong\u003eh\u003c/strong\u003e Current densities of the EGOR over reported electrocatalysts and Pd PNAs in this work. \u003cstrong\u003ei\u003c/strong\u003e FEs and generation rates of the GA for electrolysis cycles. \u003cstrong\u003ej\u003c/strong\u003e CV curves of Pd PNAs and Pd/C in 1 M KOH. \u003cstrong\u003ek\u003c/strong\u003e \u003cem\u003eOperando\u003c/em\u003e electrochemical FTIR spectra during EGOR on Pd PNAs.\u003c/p\u003e","description":"","filename":"image2.png","url":"https://assets-eu.researchsquare.com/files/rs-6716751/v1/dedd217fa60a0e0e20459725.png"},{"id":94161869,"identity":"4a8662e9-0d13-4e42-8be5-daa56b415840","added_by":"auto","created_at":"2025-10-23 05:03:07","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":320611,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eHER kinetics investigations. a\u003c/strong\u003e Schematic diagram illustrating the decoupled cell. \u003cstrong\u003eb\u003c/strong\u003e LSV curves, \u003cstrong\u003ec\u003c/strong\u003e operando EIS, \u003cstrong\u003ed\u003c/strong\u003e cell voltages at −50 mA cm\u003csup\u003e-2\u003c/sup\u003e and H\u003csub\u003e2\u003c/sub\u003e generation rate at a cell voltage of -1.5 V for HER in the systems of HER ‖ NCOR, HER ‖ OER and HER ‖ EGOR. The \u003cem\u003eoperando\u003c/em\u003e EIS was measured at different frequencies over potentials ranging from -1.3 to -1.8 V. The frequency ranged from 1 Hz to 10 Hz, and the black arrow indicates a decrease in the imaginary part of the resistance (−\u003cem\u003eZ\u003c/em\u003e\"). \u003cstrong\u003ee\u003c/strong\u003e Chronopotentiometry curve of Step 1 (HER ‖ NCOR) for HER at 50 mA cm\u003csup\u003e-2\u003c/sup\u003e. \u003cstrong\u003ef\u003c/strong\u003e Comparison of energy consumption between HER ‖ NCOR and conventional systems at various current densities.\u003c/p\u003e","description":"","filename":"image3.png","url":"https://assets-eu.researchsquare.com/files/rs-6716751/v1/3a4c2d5296eb33153fa66704.png"},{"id":94162522,"identity":"f9e5fa2f-b200-4e2e-9e76-67984c583923","added_by":"auto","created_at":"2025-10-23 05:11:07","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":210110,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eBattery performance assessment. a\u003c/strong\u003e Polarization and power density curves of Step 2. \u003cstrong\u003eb\u003c/strong\u003e Discharge curves and \u003cstrong\u003ec\u003c/strong\u003e FE of GA generation with different current densities. \u003cstrong\u003ed\u003c/strong\u003e Chronopotentiometry curves of Steps 1 and 2 at a current density of 50 mA cm\u003csup\u003e-2\u003c/sup\u003e. \u003cstrong\u003ee\u003c/strong\u003e Multiswap test between Step 1 and Step 2 at 50 mA cm\u003csup\u003e-2\u003c/sup\u003e for GA and H\u003csub\u003e2\u003c/sub\u003e generation.\u003c/p\u003e","description":"","filename":"image4.png","url":"https://assets-eu.researchsquare.com/files/rs-6716751/v1/14db6059f26bec82611312c7.png"},{"id":94162685,"identity":"b339d275-111c-4702-8b56-a234c3084220","added_by":"auto","created_at":"2025-10-23 05:19:07","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":256523,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eUniversality of the decoupled system. a \u003c/strong\u003eLSV curves of\u003cstrong\u003e \u003c/strong\u003eFOR, AAOR and GOR at a scan rate of 5 mV s\u003csup\u003e-1\u003c/sup\u003e in 1 M KOH and the corresponding reactions (inset). The red and blue lines are the LSV curves of the commercial Pt plate for the HER and the CV curve of the NCOH electrode, respectively. \u003cstrong\u003eb\u003c/strong\u003e OCV and yields of the oxidative products of FOR, GOR and AAOR in the discharging process of Step 2. \u003cstrong\u003ec\u003c/strong\u003e Schematic diagram illustrating the acid‒base asymmetric decoupled cell. \u003cstrong\u003ed\u003c/strong\u003e OCV curve of Step 2, \u003cstrong\u003ee\u003c/strong\u003e Charging‒discharging and power density curves for the acid‒base asymmetric decoupled cell.\u003c/p\u003e","description":"","filename":"image5.png","url":"https://assets-eu.researchsquare.com/files/rs-6716751/v1/29b8758463a680304d914dea.png"},{"id":94161866,"identity":"d454b313-f11a-4095-a0ba-aa0cc9d0c992","added_by":"auto","created_at":"2025-10-23 05:03:07","extension":"png","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":392627,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003ePracticality of the decoupled system. a \u003c/strong\u003eSchematic diagram illustrating three reaction cells linked in series. \u003cstrong\u003eb\u003c/strong\u003e LSV curve of Step 1 and photographs of a series of reaction cells (inset). \u003cstrong\u003ec\u003c/strong\u003e Yield of H\u003csub\u003e2\u003c/sub\u003e generation powered by a photovoltaic cell in Step 1. \u003cstrong\u003ed\u003c/strong\u003e Polarization and power density curves of Step 2 for series reaction cells. \u003cstrong\u003ee\u003c/strong\u003e GA yield at a discharge current density of 50 mA cm\u003csup\u003e-2\u003c/sup\u003e in Step 2. \u003cstrong\u003ef\u003c/strong\u003e TEA of the decoupled cell at different current densities. \u003cstrong\u003eg\u003c/strong\u003e Potential application scenario of the rechargeable decoupled cell.\u003c/p\u003e","description":"","filename":"image6.png","url":"https://assets-eu.researchsquare.com/files/rs-6716751/v1/d48572c22096af6855a7d03c.png"},{"id":94163223,"identity":"04a0ff0a-0ece-458a-b7fb-97802b09ac24","added_by":"auto","created_at":"2025-10-23 05:27:09","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":3152565,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-6716751/v1/9cbbb398-a82f-4950-acf0-474eb01f8567.pdf"},{"id":94161870,"identity":"3531c9e8-34d0-4e6a-b053-ed9dd99391b8","added_by":"auto","created_at":"2025-10-23 05:03:07","extension":"pdf","order_by":1,"title":"","display":"","copyAsset":false,"role":"supplement","size":2857908,"visible":true,"origin":"","legend":"Supplementary Information","description":"","filename":"HYHERSI0521.pdf","url":"https://assets-eu.researchsquare.com/files/rs-6716751/v1/f05dc17f7897f4e736e8d7f3.pdf"}],"financialInterests":"There is \u003cb\u003eNO\u003c/b\u003e Competing Interest.","formattedTitle":"Decoupling charge‒discharge electrolysis for accelerating hydrogen evolution and organic oxidation reactions","fulltext":[{"header":"Introduction","content":"\u003cp\u003eWater-participating electrocatalytic hydrogen evolution and hydrogenation reactions are the promising approaches for producing green hydrogen (H\u003csub\u003e2\u003c/sub\u003e) and various other chemicals,\u003csup\u003e\u003cspan additionalcitationids=\"CR2\" citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e\u003c/sup\u003e but a high overpotential is usually required to drive the kinetically sluggish oxygen evolution reaction (OER) at the anode to match the rate of the cathodic reduction reaction (CRR).\u003csup\u003e\u003cspan additionalcitationids=\"CR5\" citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e\u003c/sup\u003e This requirement results in a high cell voltage input and significantly increases energy consumption, such as \u0026gt;\u0026thinsp;1.8 V of cell voltage with a high electricity consumption of 4.5\u0026ndash;6 kWh m\u003csup\u003e\u0026ndash;\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e\u003c/sup\u003e H\u003csub\u003e2\u003c/sub\u003e in an overall water splitting (OWS) system.\u003csup\u003e\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e\u003c/sup\u003e Moreover, the OER yields low-value O\u003csub\u003e2\u003c/sub\u003e and poses risks such as the formation of explosive H\u003csub\u003e2\u003c/sub\u003e/O\u003csub\u003e2\u003c/sub\u003e gas mixtures such that an ion-exchange membrane is required to prevent gas crossover. The membrane also suffers from degradation due to the generation of reactive oxygen species.\u003csup\u003e\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e\u003c/sup\u003e Recently, the electrooxidation reactions of organic small molecules (e.g., urea, alcohols, aldehydes, and amines), which exhibit lower theoretical equilibrium potentials but generate value-added chemicals (Supplementary Table\u0026nbsp;1), have been reported as alternatives to the OER under reduced cell voltages.\u003csup\u003e\u003cspan additionalcitationids=\"CR5\" citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e, \u003cspan additionalcitationids=\"CR10 CR11 CR12 CR13 CR14\" citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e\u003c/sup\u003e Despite significant efforts, elevated cell voltages are still required at high current densities within this coupled system. This is attributed to the close coupling between organic oxidation reactions (OORs) and the CRR, where the CRR rate is explicitly dependent on the rate of the OORs.\u003csup\u003e\u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e\u003c/sup\u003e The latter typically exhibit slow reaction kinetics due to their multielectron-coupled proton transfer processes.\u003csup\u003e\u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e, \u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e\u003c/sup\u003e This mismatch in reaction rates consequently hinders cathodic kinetics. Furthermore, the kinetic loss may be further exacerbated by the mass transport of organic reactants and products on the surface of the anode.\u003csup\u003e\u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e\u003c/sup\u003e Therefore, the development of a strategy to achieve efficient conversion of organic molecules and simultaneously accelerate CRR kinetics is highly desirable but remains a considerable challenge.\u003c/p\u003e\u003cp\u003eDecoupling electrolysis, achieved by pairing individual half-reactions with electrochemical reactions of redox mediators (RMs), has spurred innovative approaches to overcome the inherent limitations of electrolysis, as first reported by Symes and Cronin in 2013.\u003csup\u003e\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e, \u003cspan additionalcitationids=\"CR19 CR20 CR21 CR22 CR23 CR24\" citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e25\u003c/span\u003e\u003c/sup\u003e For example, a nickel (oxy)hydroxide electrode was employed as a RM in an alkaline electrolyte to decouple the one-step OWS process into two steps, thereby enhancing the promotion of H\u003csub\u003e2\u003c/sub\u003e generation.\u003csup\u003e\u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e26\u003c/span\u003e\u003c/sup\u003e This decoupling strategy allows for flexibly matching diverse half-reactions at desired reaction rates across different timescales, offering a promising solution to the mismatched rate issue between reduction reactions and OORs.\u003csup\u003e\u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e, \u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e27\u003c/span\u003e\u003c/sup\u003e Given that OORs can be paired with a reduction reaction possessing a more positive potential to construct batteries/cells for simultaneous electricity generation,\u003csup\u003e\u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e28\u003c/span\u003e, \u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e29\u003c/span\u003e\u003c/sup\u003e the simultaneous production of value-added chemicals on both the cathode and anode along with electricity storage/generation (i.e., energy input in one step and output in another) presents an appealing prospect. However, this topic within decoupled systems has been largely overlooked in current research.\u003c/p\u003e\u003cp\u003eHerein, by selecting the hydrogen evolution reaction (HER) as a model CRR, we present a decoupled process to realize kinetically accelerated H\u003csub\u003e2\u003c/sub\u003e generation and the simultaneous production of valuable chemicals by using nickel‒cobalt hydroxide (named NCOH) as a RM. The ethylene glycol oxidation reaction (EGOR) into value-added glycolic acid (GA) was selected as a model OOR to demonstrate the feasibility and efficiency of the decoupled system, which comprises cathodic HER alongside anodic oxidation of NCOH to nickel‒cobalt oxyhydroxide (NCOOH), followed by subsequent GA production through cathodic reduction of NCOOH back to NCOH and anodic EGOR. The kinetic advantages of the HER in this system are discussed in comparison with those of conventional electrolysis systems. Moreover, the decoupled system has the potential to be expanded to other OORs paired cathodic reduction reaction systems to increase the HER/hydrogenation reactions rates and synthesize various chemicals with electricity storage and generation ability.\u003c/p\u003e"},{"header":"Results","content":"\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e\u003ch2\u003ePrinciple of two-step decoupling electrolysis\u003c/h2\u003e\u003cp\u003eThe cell incorporates a RM (NCOH/NCOOH) positioned between a HER cathode and an OOR anode (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ea). The HER cathode and RM are situated within the same chamber, and are isolated from the OOR anode by an ion-exchange membrane. H\u003csub\u003e2\u003c/sub\u003e is generated on the HER cathode while simultaneously undergoing anodic oxidation of NCOH to store electricity energy (NCOH to NCOOH) (Step 1, charging process). The subsequent generation of chemicals involves the anodic OORs, with the cathodic NCOOH being reduced back to NCOH, yielding energy output due to the positive potential difference between the reduction of NCOOH and the OORs (Step 2, discharging process). Notably, the separation of the catholyte and anolyte by a membrane in this process is essential, given that OORs may be catalysed by the charged state of the RM (NCOOH).\u003csup\u003e\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e, \u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e\u003c/sup\u003e The temporally and spatially distinct Steps 1 and 2 result in an alternating operating sequence, and the periodic interchange between Steps 1 and 2 enables the continuous operation of the system.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eThis approach yields a device architecture that offers several significant advantages in contrast to the conventionally configured OWS and ORRs-paired HER systems (Supplementary Fig.\u0026nbsp;1). First, the kinetics of H\u003csub\u003e2\u003c/sub\u003e generation can be accelerated because of the rapid kinetics of paired oxidation reactions (NCOH to NCOOH), which involve a single-electron transfer process.\u003csup\u003e\u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e\u003c/sup\u003e This solves the rate mismatch issue between the HER and OORs. Second, H\u003csub\u003e2\u003c/sub\u003e generated within a single cell without a membrane can minimize energy loss caused by membrane resistance, thereby increasing energy efficiency. Third, value-added chemicals and electricity are cocreated during the discharging process, offering more economic benefits. Finally, the periodic cycle between Steps 1 and 2 is similar to a rechargeable system to produce H\u003csub\u003e2\u003c/sub\u003e with electricity storage (charging process) and generate valuable chemicals with electricity output (discharging process). This implies that we can flexibly store electricity during the daytime or at low or even negative electricity prices and generate electricity at night or at high electricity prices to optimize power supply and demand to reduce electricity costs.\u003csup\u003e\u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e27\u003c/span\u003e\u003c/sup\u003e\u003c/p\u003e\u003cp\u003eGiven that the premise of energy output in Step 2 is the greater potential of the reduction of the charged RM than that of the OORs, the RM plays a significant role in the decoupled system. The RM should satisfy both high capacity and moderate redox potential to match the oxidation potential of ORRs and avoid competition with the OER when NCOH is charged. Here, nickel hydroxide (Ni(OH)\u003csub\u003e2\u003c/sub\u003e) was selected as a model RM because of its high theoretical capacity and suitable redox potential of 1.45 V relative to the reversible hydrogen electrode (RHE; hereafter, the potentials were recorded relative to the RHE if not specified).\u003csup\u003e\u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e, \u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e26\u003c/span\u003e, \u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e30\u003c/span\u003e\u003c/sup\u003e Notably, this potential is higher than the oxidation potentials of several OORs (Supplementary Table\u0026nbsp;1). Doping Co into Ni(OH)\u003csub\u003e2\u003c/sub\u003e to produce NCOH enhances the capacity and negatively shifts the redox potential, avoiding competition with the OER during the charging of NCOH (Supplementary Figs.\u0026nbsp;2 and 3, Supplementary Notes 1 and 2).\u003csup\u003e\u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e, \u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e30\u003c/span\u003e\u003c/sup\u003e The electrochemical profile of NCOH in an alkaline electrolyte was investigated via cyclic voltammetry (CV). As illustrated in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003eb, a pair of redox peaks were distinctly observed at 1.09 and 1.27 V. This is attributed to the reversible cycling between NCOH and NCOOH.\u003csup\u003e\u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e, \u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e26\u003c/span\u003e\u003c/sup\u003e\u003c/p\u003e\u003cp\u003eEGOR, as a value-added GA, was selected as a model OOR to demonstrate the feasibility and efficiency of the decoupled system for the following reasons: 1) EGOR has garnered significant attention because of its potential for upgrading ethylene glycol (EG, \u0026sim; US\u003cspan\u003e$\u003c/span\u003e 1.1 kg\u003csup\u003e\u0026minus;1\u003c/sup\u003e) sourced from biomass and poly(ethylene terephthalate) (PET) plastic waste into valuable products, including C\u003csub\u003e2\u003c/sub\u003e and C\u003csub\u003e1\u003c/sub\u003e chemicals (Supplementary Fig.\u0026nbsp;4);\u003csup\u003e\u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e31\u003c/span\u003e\u003c/sup\u003e 2) the selective oxidation of EG-to-GA (US\u003cspan\u003e$\u003c/span\u003e 100\u0026ndash;300 kg\u003csup\u003e\u0026minus;1\u003c/sup\u003e) is particularly appealing because of its much lower equilibrium potential than that of the OER (0.57 vs. 1.23 V)\u003csup\u003e\u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e32\u003c/span\u003e\u003c/sup\u003e and the considerable promise of GA in various industrial applications, especially as a biodegradable polymer with high market demand.\u003csup\u003e\u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e33\u003c/span\u003e\u003c/sup\u003e Given the activity of Pd in the selective electrooxidation of EG to GA at low potential,\u003csup\u003e\u003cspan citationid=\"CR34\" class=\"CitationRef\"\u003e34\u003c/span\u003e, \u003cspan citationid=\"CR35\" class=\"CitationRef\"\u003e35\u003c/span\u003e\u003c/sup\u003e combined with the greater number of exposed active surfaces and increased mass diffusion efficiency of porous structures,\u003csup\u003e\u003cspan citationid=\"CR36\" class=\"CitationRef\"\u003e36\u003c/span\u003e\u003c/sup\u003e we designed Pd porous nanosheet arrays (PNAs) to serve as model electrocatalysts for the selective conversion of EG to GA (discussed in detail below). Obviously, the potential window for the EGOR on Pd PNAs lies between the redox potential of NCOH/NCOOH and the onset potential for the HER (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003eb). This result indicates that it is theoretically possible to achieve a decoupled process, as illustrated in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ea, using NCOH as a RM and Pd PNAs as an EGOR electrocatalyst.\u003c/p\u003e\u003c/div\u003e\n\u003ch3\u003eSynthesis of Pd PNAs for converting EG to GA\u003c/h3\u003e\n\u003cp\u003ePd PNAs were synthesized via a self-template strategy (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ea). Specifically, Co(OH)\u003csub\u003e2\u003c/sub\u003e nanosheet arrays were synthesized on Ni foam by electrodeposition, followed by a galvanic reaction between Co(OH)\u003csub\u003e2\u003c/sub\u003e and PdCl\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e2\u0026ndash;\u003c/sup\u003e.\u003csup\u003e\u003cspan citationid=\"CR37\" class=\"CitationRef\"\u003e37\u003c/span\u003e\u003c/sup\u003e The area of the electrode can be easily scaled up (Supplementary Fig.\u0026nbsp;5). Scanning electron microscopy (SEM) images revealed that uniform nanosheet arrays grew on the Ni foam, which were maintained after the galvanic reaction between Co(OH)\u003csub\u003e2\u003c/sub\u003e and PdCl\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e2\u0026ndash;\u003c/sup\u003e (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003eb and Supplementary Fig.\u0026nbsp;6). The porous nanosheet structures of the Pd PNAs transformed from the solid nanosheets of Co(OH)\u003csub\u003e2\u003c/sub\u003e were observed from typical transmission electron microscopy (TEM) images (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ec, d and Supplementary Fig.\u0026nbsp;7). The lattice fringes measuring 0.23 and 0.20 nm align well with the (111) and (200) lattice spacings of the face-centered cubic (fcc) Pd crystal, as observed in the high-resolution TEM (HRTEM) image (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ee). Energy-dispersive X-ray (EDX) element mapping images reveal a uniform distribution of Pd in the Pd PNAs (Supplementary Fig.\u0026nbsp;8). X-ray diffraction (XRD) patterns display that the main diffraction peaks were well indexed to Pd, and the peaks attributed to Co(OH)\u003csub\u003e2\u003c/sub\u003e are not detected (Supplementary Fig.\u0026nbsp;9).\u003c/p\u003e\u003cp\u003eThe EGOR activity of the Pd PNAs was evaluated in 1 M KOH containing 0.5 M EG with commercial Pd/C as a comparison. All the electrochemical tests were performed without iR correction unless otherwise specified. As shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ef, the linear sweep voltammetry (LSV) curves reveal that the apparent oxidation current starts from an applied potential of \u0026sim;0.49 V, and the current density can reach\u0026thinsp;~\u0026thinsp;808 mA cm\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e at 1.13 V. In the absence of EG, no current is observed in the above potential windows, except for an oxidation peak centred at \u0026sim;0.90 V assigned to the oxidation of Pd (Supplementary Fig.\u0026nbsp;10).\u003csup\u003e\u003cspan citationid=\"CR34\" class=\"CitationRef\"\u003e34\u003c/span\u003e\u003c/sup\u003e For comparison, Pd/C shows a very low oxidation current starting from an applied potential of \u0026sim;0.49 V for the EGOR because of the limited number of active sites. After electrolysis, the oxidation products were analysed via \u003csup\u003e\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e\u003c/sup\u003eH nuclear magnetic resonance (NMR) spectroscopy (Supplementary Fig.\u0026nbsp;11).\u003csup\u003e\u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e\u003c/sup\u003e The Faradic efficiency (FE) of GA can reach above 90% in the potential range of 0.62 V to 1.02 V (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003eg), indicating that one hydroxyl group in EG is selectively oxidized to form GA. The GA generation rate is as high as 5.5 mmol h\u003csup\u003e\u0026ndash;\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e\u003c/sup\u003e cm\u003csup\u003e\u0026ndash;\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e\u003c/sup\u003e at 1.02 V (\u003cem\u003ej\u003c/em\u003e\u0026thinsp;\u0026gt;\u0026thinsp;600 mA cm\u003csup\u003e\u0026minus;2\u003c/sup\u003e). The high current density with high selectivity for GA generation at such a low potential represents the superior performance of the as-prepared Pd PNAs compared with reported electrocatalysts for the EGOR (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003eh and Supplementary Table\u0026nbsp;2).\u003csup\u003e\u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e, \u003cspan additionalcitationids=\"CR32\" citationid=\"CR31\" class=\"CitationRef\"\u003e31\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e33\u003c/span\u003e, \u003cspan citationid=\"CR35\" class=\"CitationRef\"\u003e35\u003c/span\u003e, \u003cspan additionalcitationids=\"CR39 CR40\" citationid=\"CR38\" class=\"CitationRef\"\u003e38\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR41\" class=\"CitationRef\"\u003e41\u003c/span\u003e\u003c/sup\u003e The stability of the Pd PNAs was also evaluated for five continuous batches of reaction. As shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ei, the generation rate and FE of the GA are largely maintained, together with the preservation of the original nanosheet array structure (Supplementary Figs.\u0026nbsp;12 and 13), demonstrating the high stability of the Pd PNAs.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eTo investigate the mechanism underlying the enhanced activity of EGOR-to-GA on Pd PNAs, a series of experiments were carried out. Considering that the generation of adsorbed hydroxyl (OH*) groups plays an important role in the EGOR,\u003csup\u003e\u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e31\u003c/span\u003e, \u003cspan citationid=\"CR42\" class=\"CitationRef\"\u003e42\u003c/span\u003e\u003c/sup\u003e CV measurements were first used to study the adsorption/activation of OH\u003csup\u003e\u0026ndash;\u003c/sup\u003e species. As displayed in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ej, the Pd PNAs clearly exhibit OH\u003csup\u003e\u0026ndash;\u003c/sup\u003e adsorption peaks. Moreover, the onset potential of OH\u003csup\u003e\u0026ndash;\u003c/sup\u003e adsorption peaks on Pd PNAs is lower than that on Pd/C, which is in line with that of EGOR, implying that the OH* species is the main active species participating in EGOR to rapidly convert *OC\u0026minus;CH\u003csub\u003e2\u003c/sub\u003eOH intermediates to the target GA product.\u003csup\u003e\u003cspan citationid=\"CR39\" class=\"CitationRef\"\u003e39\u003c/span\u003e\u003c/sup\u003e The open-circuit potential (OCP), which reflects absorption in the inner Helmholtz layer,\u003csup\u003e\u003cspan citationid=\"CR43\" class=\"CitationRef\"\u003e43\u003c/span\u003e\u003c/sup\u003e was further measured to assess the EG adsorption behaviour on the catalysts. Upon the addition of 0.5 M EG, a more significant decrease in the OCP for Pd PNAs is observed than that for Pd/C, suggesting favourable adsorption of EG on Pd PNAs (Supplementary Fig.\u0026nbsp;14). \u003cem\u003eOperando\u003c/em\u003e electrochemical Fourier transform infrared (FTIR) spectroscopy was then carried out to further probe the intermediates and understand the reaction pathway during EGOR on Pd PNAs. As displayed in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ek, downwards enhancement bands at 1076 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e and 1590 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, which are attributed to the stretching vibration of aldehydes and the antisymmetric stretching vibrations of the carboxyl group in GA, respectively,\u003csup\u003e\u003cspan citationid=\"CR39\" class=\"CitationRef\"\u003e39\u003c/span\u003e\u003c/sup\u003e can be observed with increasing potential, indicating that the OH group of EG is first oxidized to glycolaldehyde (GD). More favourable GD oxidation than EG oxidation and no GA oxidation at a potential of \u0026lt;\u0026thinsp;1.0 V further indicate that GD may be the intermediate product that rapidly converts to GA (Supplementary Fig.\u0026nbsp;15). Thus, we deduce that the enhanced ability of OH* species generation and EG adsorption greatly contributes to the enhanced activity of EGOR on Pd PNAs, and the OH* active species participate in the cascade oxidation pathway of EGOR to generate GA with high selectivity instead of C\u0026ndash;C bond cleavage into CO (Supplementary Fig.\u0026nbsp;16).\u003c/p\u003e\n\u003ch3\u003eCell assembly and performance investigation\u003c/h3\u003e\n\u003cp\u003eWe then assembled a cell to verify the feasibility of a decoupling system to realize kinetically accelerated H\u003csub\u003e2\u003c/sub\u003e generation and simultaneous production of valuable GA along with electricity storage/generation. The detailed cell assembly is illustrated in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ea and Supplementary Fig.\u0026nbsp;17, comprising the as-prepared Pd PNAs, a Pt plate, and an RM (NCOH/NCOOH). Here, a commercially available Pt plate was used as the HER electrode, and the RM electrode was placed in an electrolytic cell with no membrane. The ion-exchange membrane was used to separate the RM electrode and Pd PNAs, avoiding the oxidation of EG by NCOOH.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eIn Step 1, an energy input facilitates the generation of H\u003csub\u003e2\u003c/sub\u003e on the Pt cathode coupled with the oxidation reaction of NCOH (NCOR) to NCOOH to store energy. The advantages of the HER in Step 1 (named HER ‖ NCOR) were evaluated compared with those of conventional OWS and EGOR-paired HER systems (named HER ‖ OER and HER ‖ EGOR, respectively; see Supplementary Fig.\u0026nbsp;1 for detail). As demonstrated in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003eb, the LSV curves reveal that the HER ‖ NCOR system results in the fastest current density increase among the three systems, and its onset potential is lower than that of the OWS system. Although the HER ‖ EGOR system has a lower onset potential than the HER ‖ NCOR system because of the lower oxidation potential of the EGOR than that of the NCOR (as discussed in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003eb), the HER ‖ NCOR system shows a greater increase in current density than the HER ‖ EGOR system with increasing applied cell voltage.\u003c/p\u003e\u003cp\u003e\u003cem\u003eOperando\u003c/em\u003e electrochemical impedance spectroscopy (EIS) was performed to examine the electron transfer kinetics of the three systems. Compared with that of the HER ‖ EGOR and OWS systems, the resistance of the HER ‖ NCOR system rapidly decreases at the onset potential of the HER, indicating fast electron transfer (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ec, Supplementary Fig.\u0026nbsp;18 and Supplementary Note 3). Rapid electron transfer enables fast reaction kinetics of the HER, resulting in a fast increase in the current density with increasing applied potential, as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003eb. In fact, both the EGOR and OER are thermodynamically more favourable than the NCOR. However, the coupled transfer of four electrons and protons results in a kinetically slow process for the EGOR and OER. Additionally, the evolution of molecular oxygen gas and liquid organic products may contribute to additional kinetic loss due to mass transport.\u003csup\u003e\u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e\u003c/sup\u003e Conversely, NCOR, a process involving the transfer of a single electron/proton without generating any products, outperforms the OER and EGOR in kinetically accelerating the HER.\u003c/p\u003e\u003cp\u003eThe kinetically favoured NCOR renders the cell architecture versatile and robust in terms of voltage efficiency for the HER. At a current density of 50 mA cm\u003csup\u003e\u0026minus;2\u003c/sup\u003e, the HER ‖ NCOR system displays a cell voltage of only 1.38 V, which is lower than that of the HER ‖ EGOR and OWS systems (1.87 and 1.99 V, respectively) (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ed). In addition, the generation rate of H\u003csub\u003e2\u003c/sub\u003e for HER ‖ NCOR system is 3.2 mmol h\u003csup\u003e\u0026minus;1\u003c/sup\u003e cm\u003csup\u003e\u0026minus;2\u003c/sup\u003e, which is markedly higher than that of conventional systems at a cell voltage of 1.5 V. A steady-state study was also performed over an extended period. At a current density of 50 mA cm\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e, the average electrolysis voltage is ~\u0026thinsp;1.6 V, translating to a voltage efficiency of 84.4% (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ee). The complete conversion of NCOH is indicated by a remarkable voltage increase to 1.8 V, indicating the occurrence of the OER. It is imperative for the continuous operation of the HER to integrate the EGOR in Step 2, as discussed below. Moreover, H\u003csub\u003e2\u003c/sub\u003e generation increases with increasing current density without detectable O\u003csub\u003e2\u003c/sub\u003e at current densities ranging from 0 to 200 mA cm\u003csup\u003e\u0026minus;2\u003c/sup\u003e (Supplementary Fig.\u0026nbsp;19, Supplementary Note 4, and Supplementary Movie 1). The current density reaches 200 mA cm\u003csup\u003e\u0026minus;2\u003c/sup\u003e, and the Faradaic efficiency (FE) of the HER is near 100%, indicating high-rate generation of H\u003csub\u003e2\u003c/sub\u003e with high purity in Step 1. Observations also reveal that the corresponding cell voltage fluctuated between 1.25 and 1.62 V under the aforementioned current densities (Supplementary Fig.\u0026nbsp;20). This implies the adaptability of H\u003csub\u003e2\u003c/sub\u003e production to variable power outputs of sustainable energy, such as solar or wind energy. Impressively, the cell architecture of the HER ‖ NCOR within a single cell has a lower internal resistance than those of the HER ‖ EGOR and OWS systems with a membrane, minimizing energy loss caused by membrane resistance (Supplementary Fig.\u0026nbsp;21). The HER ‖ NCOR system results in a 33% reduction in energy consumption (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ef).\u003c/p\u003e\u003cp\u003eWhen the NCOH was fully converted to NCOOH in Step 1, the reduction of NCOOH back to NCOH and the oxidation of EG to GA released energy in Step 2 (discharging) when the switch was toggled. The polarization and power density curves of the battery show that the peak power density reaches 14 mW cm\u003csup\u003e\u0026minus;2\u003c/sup\u003e (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ea), which is close to that reported for liquid fuel cells.\u003csup\u003e\u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e28\u003c/span\u003e, \u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e29\u003c/span\u003e\u003c/sup\u003e The open circuit voltage (OCV) of the aqueous battery is 0.89 V (Supplementary Fig.\u0026nbsp;22), which is equal to the potential difference between the cathodic reduction of NCOOH (NCOOH \u0026rarr; NCOH) and the anodic EGOR (Δ\u003cem\u003eU\u003c/em\u003e\u003csub\u003edischarging\u003c/sub\u003e in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003eb). When the battery is discharged with different current densities, it delivers a similar discharge capacity at current densities ranging from 20 to 100 mA cm\u003csup\u003e\u0026minus;2\u003c/sup\u003e, indicating its excellent rate capability (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003eb and Supplementary Fig.\u0026nbsp;23). The capacity of the battery can be further improved by an electrode with a relatively high mass loading of NCOH.\u003csup\u003e\u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e29\u003c/span\u003e\u003c/sup\u003e\u003c/p\u003e\u003cp\u003eThe product of the EGOR in the anode chamber during the discharging process was collected. Interestingly, the FEs of GA generation reach 90% at current densities ranging from 20 to 100 mA cm\u003csup\u003e\u0026minus;2\u003c/sup\u003e, and the generation rate of GA is 3.3 mmol h\u003csup\u003e\u0026minus;1\u003c/sup\u003e cm\u003csup\u003e\u0026minus;2\u003c/sup\u003e at a high discharge current density of 100 mA cm\u003csup\u003e\u0026minus;2\u003c/sup\u003e (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ec). In contrast, the FE of GA is only 8.2% because of the generation of O\u003csub\u003e2\u003c/sub\u003e and formic acid (FA) byproducts at the same current density for the HER/EGOR system with high energy input (Supplementary Fig.\u0026nbsp;24). Moreover, at a current density of 50 mA cm\u003csup\u003e\u0026minus;2\u003c/sup\u003e, the discharge time (1200 s) of the cell in Step 2 is the same as the electrolysis time in Step 1, indicating a Coulombic efficiency of ~\u0026thinsp;100% (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ed). At the end of discharge, the cell voltage sharply decreases, indicating that all of the NCOOH has been converted into NCOH, which is then used for H\u003csub\u003e2\u003c/sub\u003e production in Step 1. The periodic cycle between Steps 1 and 2 to produce H\u003csub\u003e2\u003c/sub\u003e with electricity storage and generate a GA with electricity output fabricates a rechargeable system. After 50 cycles, no obvious degradation was observed, and the FE of GA generation remained at ~\u0026thinsp;90% with intervals for H\u003csub\u003e2\u003c/sub\u003e production (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ee and Supplementary Fig.\u0026nbsp;25). Hence, the decoupled configuration has superior energy efficiency, demonstrating significant potential for practical application compared with conventional one-step electrolysis with a membrane in OWS and OOR-paired HER systems.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\n\u003ch3\u003eUniversality and practicality of the decoupled system\u003c/h3\u003e\n\u003cp\u003eAs previously mentioned, if the reduction potential of NCOOH is higher than the oxidation potential of ORRs, which is a positive potential difference between the cathode and anode, electricity will be generated in Step 2. Therefore, we further investigated other ORRs to demonstrate the universality of the decoupled system. We selected glycerol (GLY) oxidation to lactic acid (LA), formaldehyde (FD) oxidation to formic acid (FA), and ascorbic acid (AA) oxidation to dehydroascorbic acid (DHA) as additional model reactions.\u003csup\u003e\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e, \u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e31\u003c/span\u003e, \u003cspan citationid=\"CR44\" class=\"CitationRef\"\u003e44\u003c/span\u003e\u003c/sup\u003e The oxidation potentials of the above three reactions are lower than the reduction potential of NCOOH (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ea, Supplementary Fig.\u0026nbsp;26, and Supplementary Note 5). After the RM was charged in Step 1, GLY, FD and AA were added to the anode chamber to fabricate batteries that combined with the charged RM (NCOOH). The OCVs are measured at 0.609, 0.985, and 0.236 V, and the corresponding products are LA, FA and DHA, respectively, for the above three aqueous batteries (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003eb). This implies that the decoupled strategy can be expanded to other OORs paired with HER systems to increase the HER kinetics and synthesize various chemicals with electricity storage and generation. The use of variable OORs with various oxidation potentials allows flexible adjustment of the output voltage in the discharging process, rendering the decoupled system suitable for a wide range of scenarios.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eMore importantly, other RMs can also be used instead of NCOH, such as Mn-based oxides, which have also been widely used in rechargeable batteries.\u003csup\u003e\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e, \u003cspan citationid=\"CR45\" class=\"CitationRef\"\u003e45\u003c/span\u003e, \u003cspan citationid=\"CR46\" class=\"CitationRef\"\u003e46\u003c/span\u003e\u003c/sup\u003e For example, Na\u003csub\u003e0.44\u003c/sub\u003eMnO\u003csub\u003e2\u003c/sub\u003e can be used as a RM to boost the alkaline HER under a low input voltage in Step 1 and concurrent GA and electricity in Step 2 (Supplementary Fig.\u0026nbsp;27−29 and Supplementary Notes 6 and 7).\u003csup\u003e\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e\u003c/sup\u003e Moreover, the acid HER could also be increased by using Mn\u003csup\u003e2+\u003c/sup\u003e/MnO\u003csub\u003e2\u003c/sub\u003e as a RM in 1 M H\u003csub\u003e2\u003c/sub\u003eSO\u003csub\u003e4\u003c/sub\u003e (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ec and Supplementary Fig.\u0026nbsp;30).\u003csup\u003e\u003cspan citationid=\"CR45\" class=\"CitationRef\"\u003e45\u003c/span\u003e\u003c/sup\u003e Interestingly, the OCV of the charging process in Step 2 is measured at 1.78 V (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ed), which is high compared with the potential difference between the cathodic reduction of RM (MnO\u003csub\u003e2\u003c/sub\u003e → Mn\u003csup\u003e2+\u003c/sup\u003e) and the anodic EGOR because of the contribution of the electrochemical neutralization energy of 0.828 V in the acid‒base asymmetric electrolyte.\u003csup\u003e\u003cspan citationid=\"CR47\" class=\"CitationRef\"\u003e47\u003c/span\u003e\u003c/sup\u003e The polarization and power density curves of the acid–base asymmetric battery show that the peak power density reached as high as 133 mW cm\u003csup\u003e−2\u003c/sup\u003e, outperforming some reported hybrid fuel cells (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ee and Supplementary Table\u0026nbsp;3).\u003csup\u003e\u003cspan additionalcitationids=\"CR49\" citationid=\"CR48\" class=\"CitationRef\"\u003e48\u003c/span\u003e–\u003cspan citationid=\"CR50\" class=\"CitationRef\"\u003e50\u003c/span\u003e\u003c/sup\u003e In addition to the HER, the electrocatalytic hydrogenation reaction coupled with the OOR can also be decoupled by our proposed strategy. Selecting the electrocatalytic semihydrogenation of coal-derived acetylene to ethylene (ESAE) as an example,\u003csup\u003e\u003cspan citationid=\"CR51\" class=\"CitationRef\"\u003e51\u003c/span\u003e\u003c/sup\u003e we can achieve the promoted kinetics of ESAE and a high ethylene FE of 97% at a current density of 500 mA cm\u003csup\u003e− 2\u003c/sup\u003e under a low cell voltage in Step 1 (Supplementary Fig.\u0026nbsp;31 and Supplementary Note 8). It is imperative to exercise caution in the selection of RM, as this will help prevent the hydrogenation products from undergoing oxidation at the surface of the RM anode.\u003c/p\u003e\u003cp\u003eTo further assess the practical viability of the decoupled system, three reaction cells were linked in series to increase the overall efficiency (Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003ea and the inset of Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003eb). Here, high-performance nickel‒cobalt phosphide (NiCoP) was synthesized and employed as an HER electrocatalyst (Supplementary Fig.\u0026nbsp;32),\u003csup\u003e\u003cspan citationid=\"CR52\" class=\"CitationRef\"\u003e52\u003c/span\u003e\u003c/sup\u003e with the objective of reducing catalyst costs. In Step 1, the electrolysis process used to generate H\u003csub\u003e2\u003c/sub\u003e and charge the RM operates in parallel. This can be powered by a low-voltage photovoltaic cell or other sustainable energy. In contrast, the discharge process in Step 2, which is responsible for generating chemicals and electricity, is configured in tandem to effectively increase the working voltage. The charging curve of the series cell shows that the current density could reach an industrial current density of ~ 400 mA cm\u003csup\u003e− 2\u003c/sup\u003e at a very low voltage of 2.0 V (Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003eb). Powered by a photovoltaic cell with an output of 1.52 V (Supplementary Fig.\u0026nbsp;33), the electrolysis cell is able to produce 38.2 mL of H\u003csub\u003e2\u003c/sub\u003e in 16 min (Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003ec and Supplementary Fig.\u0026nbsp;34). After charging, we evaluate the performance of the discharging process in Step 2. The aqueous battery pack had a high power density (a maximum value of up to 38.8 mW cm\u003csup\u003e− 2\u003c/sup\u003e) and a high OCV of 2.12 V (Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003ed and Supplementary Fig.\u0026nbsp;35) so that it could supply stable power for a timer (Supplementary Fig.\u0026nbsp;36). Moreover, 0.72 mmol of GA was collected after a discharging process of 20 min at a current density of 50 mA cm\u003csup\u003e− 2\u003c/sup\u003e (Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003ee), accompanied by the generation of 135 J of electricity (0.68 kWh of electricity per kg of GA generation).\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003ePreliminary technoeconomic analysis (TEA) was then conducted to investigate the feasibility of this decoupled system via a model adapted from the literature (Supplementary Fig.\u0026nbsp;37 and Supplementary Note 9). In addition to the cost of renewable electricity and FEs of target products, the profitability of the process largely depends on the operating current density.\u003csup\u003e\u003cspan citationid=\"CR52\" class=\"CitationRef\"\u003e52\u003c/span\u003e, \u003cspan citationid=\"CR53\" class=\"CitationRef\"\u003e53\u003c/span\u003e\u003c/sup\u003e In this context, our system is economically feasible even when the current density of the HER in Step 1 is 50 mA cm\u003csup\u003e−2\u003c/sup\u003e because of the low input voltage and highly valuable GA generation in Step 2 (Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003ef). The net revenue reaches ~\u003cspan\u003e$\u003c/span\u003e3578 for generating liquid H\u003csub\u003e2\u003c/sub\u003e per m\u003csup\u003e\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e\u003c/sup\u003e under a commercially relevant current density (\u0026gt; 200 mA cm\u003csup\u003e− 2\u003c/sup\u003e), indicating the economic potential of our proposed decoupled system. In terms of application scenarios, we envision that a decoupled system with a rechargeable capability will be suitable for distributed energy storage and chemical production (Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003eg). For example, it could store intermittent renewable electricity (such as from solar or wind sources) during the daytime or oversupply electricity and then release electricity for household use at night or during high electricity price periods. Concurrently, this process enables the kinetically accelerated generation of H\u003csub\u003e2\u003c/sub\u003e, along with the simultaneous production of valuable chemicals. These chemicals can then be collected and separated for subsequent use.\u003c/p\u003e"},{"header":"Discussion","content":"\u003cp\u003eIn summary, we have realized kinetically accelerated H\u003csub\u003e2\u003c/sub\u003e generation and simultaneous production of valuable chemicals alongside electricity storage/generation via decoupled electrolysis using nickel‒cobalt hydroxide as a redox mediator. The kinetics of the HER in the decoupled system without a membrane are largely accelerated because of the rapid reaction kinetics of the paired redox mediator oxidation and the extremely low internal resistance. We select the EGOR into the value-added GA as a model OOR to demonstrate the feasibility and efficiency of the decoupled system. High-performance Pd porous nanosheet arrays were designed as EGOR electrocatalysts, enabling a high GA generation rate and power density during the discharging process. Importantly, this decoupled strategy has potential for broad application in other OORs paired with HER/hydrogenation reaction systems to increase the cathodic reduction reaction rate and facilitate the synthesis of various chemicals alongside electricity storage and generation. Our proposed decoupled electrolysis method allows better utilization of intermittent renewable sources, such as solar or wind, for chemical manufacturing, presenting a promising approach to enhancing renewable-to-chemical conversion while also offering innovative concepts for constructing a hybrid energy conversion/storage system. Considering the chemical properties and species diversity of redox mediator materials, this work provides a universality and adaptability decoupled strategy for broad application across diverse scenarios.\u003c/p\u003e\u003cdiv id=\"Sec8\" class=\"Section2\"\u003e\u003cdiv id=\"Sec9\" class=\"Section3\"\u003e\u003c/div\u003e\u003c/div\u003e"},{"header":"Methods","content":"\u003ch2\u003eMaterials\u003c/h2\u003e\u003cp\u003eAll the chemicals used in the experiments were analytically pure and were used without further purification (see Supplementary Note 10 for details). Deionized water (DIW) was used in all the experimental processes.\u003c/p\u003e\n\u003ch3\u003eSynthesis of Pd PNAs electrodes\u003c/h3\u003e\n\u003cp\u003eCo(OH)\u003csub\u003e2\u003c/sub\u003e nanosheet arrays grown on Ni foam were first synthesized by electrodeposition. A Ni foam (1\u0026times;2 cm\u003csup\u003e\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e\u003c/sup\u003e), a saturated calomel electrode (SCE, saturated KCl, aqueous), and a platinum plate were used as the working electrode, reference electrode and counter electrode, respectively. All the samples were placed into a 25 mM Co(NO\u003csub\u003e3\u003c/sub\u003e)\u003csub\u003e2\u003c/sub\u003e solution, and the immersed size of the Ni foam in the solution was maintained at 1\u0026times;1 cm\u003csup\u003e\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e\u003c/sup\u003e. The blue Co(OH)\u003csub\u003e2\u003c/sub\u003e was obtained after working at -1 V vs. SCE for 240 s and washing with DIW and ethanol several times. The Co(OH)\u003csub\u003e2\u003c/sub\u003e electrode was subsequently immersed in a Na\u003csub\u003e2\u003c/sub\u003ePdCl\u003csub\u003e4\u003c/sub\u003e solution (3.6 mg/mL) at room temperature for 40 minutes. The Pd PNAs electrodes were obtained after washing with DIW and ethanol several times and drying in a vacuum oven overnight.\u003c/p\u003e\u003cdiv id=\"Sec11\" class=\"Section2\"\u003e\u003ch2\u003eSynthesis of Ni(OH)\u003csub\u003e2\u003c/sub\u003e or Co\u003csub\u003e1\u0026ndash;x\u003c/sub\u003eNi\u003csub\u003ex\u003c/sub\u003e(OH)\u003csub\u003e2\u003c/sub\u003e RMs\u003c/h2\u003e\u003cp\u003eNi(OH)\u003csub\u003e2\u003c/sub\u003e or Co\u003csub\u003e1\u0026thinsp;\u0026minus;\u0026thinsp;x\u003c/sub\u003eNi\u003csub\u003ex\u003c/sub\u003e(OH)\u003csub\u003e2\u003c/sub\u003e was prepared via an electrodeposition method in a three-electrode electrochemical configuration in a 0.1 M Ni(NO\u003csub\u003e3\u003c/sub\u003e)\u003csub\u003e2\u003c/sub\u003e solution or Ni(NO\u003csub\u003e3\u003c/sub\u003e)\u003csub\u003e2\u003c/sub\u003e/Co(NO\u003csub\u003e3\u003c/sub\u003e)\u003csub\u003e2\u003c/sub\u003e mixed solution with a Ag/AgCl electrode as the reference electrode. The nickel foams were subjected to ultrasonic cleaning in an acetone solution, followed by ethanol and deionized water for 15 minutes each. Two pieces of Ni foams of equal size (2\u0026times;2 cm\u003csup\u003e\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e\u003c/sup\u003e) were used as the working electrode and the counter electrode. The electrodeposition lasted 300 s at a constant current density of -10 mA cm\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e. The loading mass of Ni(OH)\u003csub\u003e2\u003c/sub\u003e or Co\u003csub\u003e1\u0026thinsp;\u0026minus;\u0026thinsp;x\u003c/sub\u003eNi\u003csub\u003ex\u003c/sub\u003e(OH)\u003csub\u003e2\u003c/sub\u003e was ~\u0026thinsp;2 mg cm\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e.\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec12\" class=\"Section2\"\u003e\u003ch2\u003eSynthesis of NiCoP electrodes\u003c/h2\u003e\u003cp\u003eFirst, the Co\u003csub\u003e0.4\u003c/sub\u003eNi\u003csub\u003e0.6\u003c/sub\u003e(OH)\u003csub\u003e2\u003c/sub\u003e electrode was prepared via the above method. Subsequently, NaH\u003csub\u003e2\u003c/sub\u003ePO\u003csub\u003e2\u003c/sub\u003e\u0026middot;H\u003csub\u003e2\u003c/sub\u003eO (0.8 g) and the as-prepared Co\u003csub\u003e0.4\u003c/sub\u003eNi\u003csub\u003e0.6\u003c/sub\u003e(OH)\u003csub\u003e2\u003c/sub\u003e were heated at 300\u0026deg;C at a heating rate of 2\u0026deg;C min\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e under an Ar atmosphere, and the temperature was held at 300\u0026deg;C for 2 h. The NiCoP electrodes were synthesized after cooling to room temperature.\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec13\" class=\"Section2\"\u003e\u003ch2\u003eSynthesis of Na\u003csub\u003e0.44\u003c/sub\u003eMnO\u003csub\u003e2\u003c/sub\u003e\u003c/h2\u003e\u003cp\u003eNa\u003csub\u003e0.44\u003c/sub\u003eMnO\u003csub\u003e2\u003c/sub\u003e was prepared according to previous work.\u003csup\u003e\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e\u003c/sup\u003e A solid-state reaction method was adopted to prepare Na\u003csub\u003e0.44\u003c/sub\u003eMnO\u003csub\u003e2\u003c/sub\u003e using Na\u003csub\u003e2\u003c/sub\u003eCO\u003csub\u003e3\u003c/sub\u003e and Mn\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e as the precursors. Stoichiometric amounts of Na\u003csub\u003e2\u003c/sub\u003eCO\u003csub\u003e3\u003c/sub\u003e and Mn\u003csub\u003e3\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e were ground by ball milling at 300 RPM for 5 h and then sintered in a muffle furnace at 775\u0026deg;C for 10 h. Na\u003csub\u003e0.44\u003c/sub\u003eMnO\u003csub\u003e2\u003c/sub\u003e was synthesized after cooling to room temperature.\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec14\" class=\"Section2\"\u003e\u003ch2\u003eGeneral characterizations\u003c/h2\u003e\u003cp\u003eX-ray diffraction (XRD) patterns of all the samples were obtained on a Bruker D8 Advance diffractometer with monochromatized Cu \u003cem\u003eK\u003c/em\u003eα radiation (λ\u0026thinsp;=\u0026thinsp;0.15418 nm). Scanning electron microscopy (SEM) images were acquired with a JEOL 6700-F field-emission scanning electron microscope. Transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) images, high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) images and corresponding energy dispersive spectroscopy (EDS) images were obtained with a transmission electron microscope (JEOL JEM-2100F) operating at an acceleration voltage of 200 kV. \u003csup\u003e\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e\u003c/sup\u003eH NMR spectra were recorded on a Bruker 400 MHz NMR spectrometer. Fourier transform infrared spectroscopy (FTIR) was performed on a Nicolet IS50 instrument.\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec15\" class=\"Section2\"\u003e\u003ch2\u003eElectrochemical measurements\u003c/h2\u003e\u003cp\u003eElectrochemical measurements were performed via a CHI 760E electrochemical workstation (Shanghai Chenhua Instrument Company, China) in a standard three-electrode cell configuration. The as-prepared catalysts were used as the working electrode, while a Hg/HgO electrode (KCl, aqueous) and a platinum plate were used as the reference and counter electrodes, respectively. The potentials were converted to voltages via the following formula: \u003cem\u003eE\u003c/em\u003e\u003csub\u003eRHE\u003c/sub\u003e = \u003cem\u003eE\u003c/em\u003e\u003csub\u003eHg/HgO\u003c/sub\u003e + 0.098\u0026thinsp;+\u0026thinsp;0.059\u0026times;pH, with reference to the reversible hydrogen electrode (RHE). The polarization curves were recorded at a scan rate of 5 mV s\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e. \u003cem\u003eO\u003c/em\u003eperando electrochemical impedance spectroscopy (EIS) tests were performed, measuring a frequency range of 0.01 Hz to 100 kHz with an AC amplitude of 10 mV. The FEs of all the products were calculated on the basis of their corresponding electron transfer per oxidation molecule via the following equations.\u003cdiv id=\"Equa\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equa\" name=\"EquationSource\"\u003e\n$$\\:FE=\\frac{{n}_{e}\\times\\:{n}_{products}\\times\\:F}{Q}\\times\\:100\\%$$\u003c/div\u003e\u003c/div\u003e\u003c/p\u003e\u003cp\u003ewhere \u003cem\u003en\u003c/em\u003e\u003csub\u003ee\u003c/sub\u003e is the number of electrons from reactants to products, \u003cem\u003en\u003c/em\u003e\u003csub\u003eproducts\u003c/sub\u003e is the productivity of products, \u003cem\u003eF\u003c/em\u003e is the Faraday constant (\u003cem\u003eF\u003c/em\u003e\u0026thinsp;=\u0026thinsp;96485), and \u003cem\u003eQ\u003c/em\u003e is the quantity of electric charge.\u003c/p\u003e\u003cp\u003eThe electricity input for cathodic H\u003csub\u003e2\u003c/sub\u003e production in the stacked membrane-free electrolyzer for the electrooxidation of Ni(OH)\u003csub\u003e2\u003c/sub\u003e/NiOOH was calculated via the following equations.\u003cdiv id=\"Equb\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equb\" name=\"EquationSource\"\u003e\n$$\\:{W}_{{H}_{2}}=\\frac{I\\times\\:U\\times\\:t}{{V}_{{\\text{H}}_{2}}\\times\\:{10}^{3}}$$\u003c/div\u003e\u003c/div\u003e\u003c/p\u003e\u003cp\u003ewhere is the electricity consumption per unit of hydrogen production (kWh/m\u003csup\u003e3\u003c/sup\u003e H\u003csub\u003e2\u003c/sub\u003e), \u003cem\u003eI\u003c/em\u003e is the electrolyzer output current (A), \u003cem\u003eU\u003c/em\u003e is the electrolyzer input voltage (V), \u003cem\u003et\u003c/em\u003e is the reaction time (h), \u003cem\u003eI\u003c/em\u003e \u0026times; \u003cem\u003et\u003c/em\u003e is the integral area of the I-t curve, and \u003cem\u003eV\u003c/em\u003e\u003csub\u003e\u003cem\u003eH2\u003c/em\u003e\u003c/sub\u003e is the hydrogen production (m\u003csup\u003e\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e\u003c/sup\u003e). The productivity of H\u003csub\u003e2\u003c/sub\u003e in the membrane-free electrolyzer was calculated on the basis of the charge transfer of NCOH/NCOOH oxidation.\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec16\" class=\"Section2\"\u003e\u003ch2\u003eQuantification analysis of oxidation products\u003c/h2\u003e\u003cp\u003eThe formic acid (FA) was identified and quantified via anion chromatography (Thermo Scientific, IC-900, Dionex IonPac AS23). The concentration of HCOOH produced was ascertained by diluting the electrolyte sample to a level that corresponded with the range of the calibration curve. Glycolic acid (GA), lactic acid (LA), and dehydroascorbic acid (DHA) were quantified via NMR spectroscopy. The products in the anolyte sample were quantified via \u003csup\u003e\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e\u003c/sup\u003eH NMR spectroscopy via water suppression techniques. Each sample was acquired with a total of 32 transient scans. The internal standard used was DMSO with a chemical shift at 2.71 ppm in D\u003csub\u003e2\u003c/sub\u003eO. The NMR samples were prepared by adding 200 \u0026micro;L of DMSO in D\u003csub\u003e2\u003c/sub\u003eO to 500 \u0026micro;L of the liquid sample. The relationship between the analyte and internal standard was quantified via the following equation:\u003cdiv id=\"Equc\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equc\" name=\"EquationSource\"\u003e\n$$\\:\\frac{{n}_{x}}{{n}_{std}}=\\frac{{I}_{\\text{x}}}{{I}_{std}}\\times\\:\\frac{{N}_{std}}{{N}_{x}}$$\u003c/div\u003e\u003c/div\u003e\u003c/p\u003e\u003cp\u003ewhere \u003cem\u003en\u003c/em\u003e represents the number of moles of the analyte (x), \u003cem\u003eN\u003c/em\u003e represents the number of hydrogen atoms, and \u003cem\u003eI\u003c/em\u003e represents the integrated area of both the analyte and internal standard (std).\u003c/p\u003e\u003cp\u003eThe FE was calculated at a given potential as follows:\u003cdiv id=\"Equd\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equd\" name=\"EquationSource\"\u003e\n$$\\:FE\\left(\\text{\\%}\\right)=\\frac{nFc\\text{V}}{Q}$$\u003c/div\u003e\u003c/div\u003e\u003c/p\u003e\u003cp\u003ewhere the concentration c represents the electrooxidation products (g L\u003csup\u003e\u0026minus;1\u003c/sup\u003e). \u003cem\u003eV\u003c/em\u003e represents the volume of the electrolyte (L), and \u003cem\u003en\u003c/em\u003e represents the number of electrons transferred for product formation (mol). \u003cem\u003eF\u003c/em\u003e is the Faraday constant (96485 C), and \u003cem\u003eQ\u003c/em\u003e represents the quantity of electric charge integrated by the \u003cem\u003ei\u003c/em\u003e\u0026ndash;\u003cem\u003et\u003c/em\u003e curve (C).\u003c/p\u003e\u003c/div\u003e"},{"header":"Declarations","content":"\u003cdiv id=\"Sec17\" class=\"Section2\"\u003e\u003ch2\u003eData availability\u003c/h2\u003e\u003cp\u003eThe data that support other plots within this paper are available from the corresponding author upon reasonable request.\u003c/p\u003e\u003c/div\u003e\u003cp\u003e\u003ch2\u003eCompeting Financial Interests\u003c/h2\u003e\u003cp\u003eThe authors declare no competing interests.\u003c/p\u003e\u003c/p\u003e\u003ch2\u003eAuthor Contributions\u003c/h2\u003e\u003cp\u003eY.H. and B.Z. conceived the idea and directed the project. Y.H., H.Z., and J.Z. designed the experiments. H.Z. and B.H.Z. carried out the experiments and characterization. J.G. assisted in some experiments. Y.H. and J.Z. wrote the paper. B.Z. revised the paper. All the authors discussed the results and commented on the paper.\u003c/p\u003e\u003ch2\u003eAcknowledgements\u003c/h2\u003e\u003cp\u003eWe acknowledge the National Key Research and Development Program of China (2023YFA1507400 and 2024YFA1510100 to B.Z.), the National Natural Science Foundation of China (U21A20286 to Y.H., 22206054 to Y.H., and 22478310 to J.Z.) and the Fundamental Research Funds for the Central China Normal University (CCNU).\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\u003cli\u003e\u003cspan\u003eChatenet M et al (2022) Water electrolysis: from textbook knowledge to the latest scientific strategies and industrial developments. 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Science 368:1228\u0026ndash;1233\u003c/span\u003e\u003c/li\u003e\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":true,"hideJournal":false,"highlight":"","institution":"","isAcceptedByJournal":true,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"nature-portfolio","isNatureJournal":true,"hasQc":false,"allowDirectSubmit":false,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"","title":"Nature Portfolio","twitterHandle":"","acdcEnabled":false,"dfaEnabled":false,"editorialSystem":"ejp","reportingPortfolio":"","inReviewEnabled":true,"inReviewRevisionsEnabled":false},"keywords":"","lastPublishedDoi":"10.21203/rs.3.rs-6716751/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-6716751/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eThe substitution of an oxygen evolution reaction with more thermodynamically favourable organic oxidation reactions (OORs) offers a promising strategy for energy-efficient hydrogen evolution and hydrogenation reactions. However, the kinetics of the cathodic reduction reaction is still hindered by sluggish OORs. Herein, we report a decoupled electrolysis strategy to realize a kinetically accelerated hydrogen evolution reaction (HER), a model reduction reaction, for the simultaneous production of H\u003csub\u003e2\u003c/sub\u003e and valuable chemicals via the use of a solid redox mediator. The decoupled system with a rechargeable capability features a HER paired with redox mediator oxidation to store electricity and subsequently OORs to value-added chemicals coupled with the reduction of the oxidized redox mediator to generate electricity. Owing to the rapid kinetics of redox mediator oxidation and operation in a membrane-free cell, the kinetics of the HER are notably accelerated compared with those of conventional overall water splitting and OORs-paired HER systems. The value-added chemicals and electricity are cocreated during the discharging process, offering more economic benefits. Importantly, the design of such decoupled systems is universally applicable to OORs-paired electrocatalytic reduction systems (e.g., acetylene semihydrogenation) to synthesize various chemicals for electricity storage and generation, paving a sustainable avenue for energy-efficient H\u003csub\u003e2\u003c/sub\u003e and chemical manufacturing.\u003c/p\u003e","manuscriptTitle":"Decoupling charge‒discharge electrolysis for accelerating hydrogen evolution and organic oxidation reactions","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2025-10-23 05:03:02","doi":"10.21203/rs.3.rs-6716751/v1","editorialEvents":[],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"nature-communications","isNatureJournal":true,"hasQc":false,"allowDirectSubmit":false,"externalIdentity":"NCOMMS","sideBox":"Learn more about [Nature Communications](http://www.nature.com/ncomms/)","snPcode":"","submissionUrl":"https://mts-ncomms.nature.com/","title":"Nature Communications","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"ejp","reportingPortfolio":"Nature Communications","inReviewEnabled":true,"inReviewRevisionsEnabled":false}}],"origin":"","ownerIdentity":"61108e61-e4a0-4b43-b680-e662781e90ff","owner":[],"postedDate":"October 23rd, 2025","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"under-review","subjectAreas":[{"id":56671387,"name":"Physical sciences/Chemistry/Electrochemistry/Electrocatalysis"},{"id":56671388,"name":"Physical sciences/Chemistry/Green chemistry/Sustainability"},{"id":56671389,"name":"Physical sciences/Energy science and technology/Renewable energy/Hydrogen energy"}],"tags":[],"updatedAt":"2025-12-16T08:21:26+00:00","versionOfRecord":[],"versionCreatedAt":"2025-10-23 05:03:02","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-6716751","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-6716751","identity":"rs-6716751","version":["v1"]},"buildId":"8U1c8b4HqxoKbykW_rLl7","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}