Skip to main content
NCBI home page
JACS Au logoLink to JACS Au
. 2023 Jan 24;3(2):488–497. doi: 10.1021/jacsau.2c00624

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

Zhengxin Zhu 1, Taoli Jiang 1, Jifei Sun 1, Zaichun Liu 1, Zehui Xie 1, Shuang Liu 1, Yahan Meng 1, Qia Peng 1, Weiping Wang 1, Kai Zhang 1, Hongxu Liu 1, Yuan Yuan 1, Ke Li 1, Wei Chen 1,*
PMCID: PMC9975835  PMID: 36873693

Abstract

graphic file with name au2c00624_0007.jpg

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

Keywords: decoupling water electrolysis, H2 production, electrocatalytic H2, pH-universal, mass production of H2

Currently, fossil fuels such as oil, coal, and natural gas are still the major consumed resources in global energy supply.1,2 However, these non-renewable resources do not comply with the development of a low-carbon society.3 Hydrogen gas (H2) has been advocated as a green energy carrier for many chemical transformations compared with fossil fuels due to its high energy density and clean products during utilization.4−6 In addition, H2 as an electrocatalytic electrode has recently been developed in rechargeable H2 battery systems to produce H2 during the charging process.7−11 Water electrolysis powered by renewable energy is a sustainable technology for the mass production of H2 because water is one of the largest reservoirs of H2 on earth.12,13 Typically, in conventional water electrolysis (CWE), which involves a H2 electrode, an oxygen gas (O2) electrode, and an ion-exchange membrane (IEM), the H2 evolution reaction (HER) and O2 evolution reaction (OER) happen simultaneously and thus are tightly coupled.14,15 The simultaneous H2 and O2 generation by the electrocatalytic electrodes are also applied in flow-electrode capacitive deionization systems, which contributes to the improvement of desalination efficiency.16,17 However, such a compact coupling of H2 and O2 is hard to separate, resulting in gas crossover through the membrane to give rise to hazardous H2/O2 mixtures.18 To overcome these difficulties, it is very crucial to develop novel water electrolysis for decoupling HER and OER.

Recently, the introduction of auxiliary electrodes during water electrolysis allows the decoupling of HER and OER in time or in space.19−28 For example, Cronin et al. successfully developed a concept of an electron-coupled proton buffer by introducing a soluble silicotungstic acid redox electrode, which can produce hydrogen gas faster than CWE.29 However, an expensive IEM was employed in the above strategy to restrain the crossover of the soluble redox electrode. To overcome this obstacle, Wang et al. designed a series of inorganic and organic solid-state auxiliary electrodes to temporally decouple HER and OER in an acidic or alkaline water electrolysis process, as illustrated in Figure 1a.30,31 For instance, a solid-state Ni(OH)2 electrode as an auxiliary electrode was used in an alkaline electrolyte for water electrolysis with the reversible transformation of Ni(OH)2/NiOOH.30 Moreover, an organic pyrene-4,5,9,10-tetraone (PTO) proton-transfer electrode with a reversible H+ reaction as an auxiliary electrode was deployed to decouple acidic water electrolysis.31 However, the above strategies have the limitation of the complicated operation of an frequent on/off switch in three-electrode configurations (a H2 electrode, an O2 electrode, and an auxiliary electrode). Meanwhile, Grader et al. introduced a solid-state NiOOH/Ni(OH)2 auxiliary electrode into two-cell water electrolysis, each with a Pt-coated electrode and a NiOOH/Ni(OH)2 electrode, in which the two Pt electrodes were connected to a power source and the nickel electrodes were wired together.20 The H2 and O2 generation can be separated using the NiOOH/Ni(OH)2 electrode, as illustrated in Figure 1b, but in a complicated multi-cell operation. Afterward, they optimized the cell design by changing the water electrolysis process through a two-step electrochemical–chemical cell.32,33 In the first electrochemical step, H2 production was achieved by water reduction along with the oxidation of Ni(OH)2 to NiOOH. In the subsequent chemical step, O2 was liberated by the spontaneous reduction of NiOOH back to Ni(OH)2 at an elevated temperature of ∼95 °C. However, either a Ni(OH)2 or a PTO solid-state electrode is required to comply with strict working conditions for the pH of electrolytes to maintain the stability of auxiliary electrodes, which induces large limitations in the application scenarios for H2 production.21,23,30,31,34,35 Therefore, it is highly desirable to develop new strategies to decouple water electrolysis ideally in all pH conditions to make H2 production much universal and effective.

Figure 1.

Figure 1

Open in a new tab

Cell structure and working mechanisms of various decoupled water electrolyzers. (a) Two-step, three-electrode water electrolysis with a frequent on/off switch. (b) Two-cell, four-electrode water electrolysis. (c) Our proposed pH-universal two-electrode electrolytic cell for the decoupling of H2 and O2 generation. The electrolytes can be acidic, neutral, and alkaline solutions.

In this work, a proof-of-concept demonstration of pH-universal decoupled water electrolysis enabled by electrocatalytic H2 capacitive chemistry, referred to as an all-pH capacitive decoupled water electrolyzer (all-pH-CDWE), is proposed to overcome the abovementioned challenges. The proposal of all-pH-CDWE is inspired by our recently developed electrocatalytic H2 capacitors, where H2 is only generated in the charging process for efficient gas separation.36Figure 1c shows the structural composition of the designed all-pH-CDWE with a two-electrode configuration of a carbon capacitive electrode and an electrocatalytic gas (H2 and O2) electrode, which is fundamentally different from the previous technologies of decoupled water electrolysis by multi-electrode (≥3 electrodes) or multi-cell (≥2 cells) operations (Figure 1a,b).20,30Table S1 compares our all-pH-CDWE with other reported CWEs and decoupled water electrolyses in terms of cell components, operational modes, and electrochemical performance. It is obvious that in our all-pH-CDWE architecture, the H2 production involves cathodic H+ or H2O reduction and adsorption of anodic anions (SO42– in an acidic electrolyte, H2PO4–/HPO42– in a neutral electrolyte, or OH– in an alkaline electrolyte) by the capacitive electrode in pH-universal electrolytes (0–14). The subsequent O2 production is dependent on the cathodic anion desorption process and the anodic H2O oxidization reaction. Furthermore, the alternative H2 and O2 generation can be cycled with high stability for over 800 consecutive cycles in high electrolyte utilization ratios of nearly 100%. The demonstrated all-pH-CDWE technology shows promises for large-scale H2 production in a facile and cost-effective pathway.

To verify our designed all-pH-CDWE, a typical Swagelok cell was first designed to decouple water electrolysis with a commercial Pt-coated Ti-mesh electrode for H2 and O2 generation and an activated carbon (AC)-coated carbon paper electrode.8,11,36 As a comparison, CWE with two commercial Pt-coated Ti-mesh electrodes was chosen as the direct water electrolysis. In this all-pH-CDWE, there is a small energy loss by the oxidation and reduction reactions of Pt during the process of alternative H2 and O2 generation, which was also reported by previous reports on catalytic gas electrodes.37−39 As shown in Figure S1, a symmetric capacitor in two AC electrodes was assembled with a capacitance of 1017 mF cm–2 at a current density of 5 mA cm–2. Even at a high current density of 20 mA cm–2, a considerable capacitance of 893 mF cm–2 was retained (Figure S1), indicating an ultrafast ion adsorption/desorption of the AC electrode.36,40−43 The water electrolysis performance in the acidic electrolyte was first investigated by chronopotentiometry measurements at various current densities ranging from 5 to 20 mA cm–2 with an applied capacity of 3600 mC cm–2, as shown in Figure 2a. In the H2 generation process, the average voltages of acidic-CDWE are 0.84, 0.84, 0.83, and 0.84 V at current densities of 5, 10, 15, and 20 mA cm–2, respectively, indicating a negligible change of the H2 generation voltage with the current density and confirming the robustness of the acidic-CDWE. In the O2 generation process, the acidic-CDWE exhibits an increased voltage tendency with average voltages of 1.13, 1.21, 1.25, and 1.29 V at current densities of 5, 10, 15, and 20 mA cm–2, respectively, which is ascribed to the inferior OER performance of the Pt electrode. In addition, the average voltage during H2 generation process of the acidic-CDWE (0.84 V) is much lower than that of the acidic-CWE (1.98 V) at a large current density of 20 mA cm–2 (Figure 2a). Furthermore, the energy efficiency of the acidic-CDWE can be evaluated by comparing its total driving voltage (HER and OER) to that of acidic-CWE, according to a previous report.30 As shown in Figure 2b, the energy efficiency of the acidic-CDWE with a total driving voltage of 1.97 V is about 94% of that of acidic-CWE with an electrolytic voltage of 1.85 V at a current density of 5 mA cm–2. When the current density increases from 10 to 20 mA cm–2, the energy efficiency of the acidic-CDWE slightly changes from 94 to 93%, which is ascribed to the excellent rate performance of the AC electrode (Figure S1). The achieved energy efficiency is comparable to that of two-step, three-electrode water electrolysis by Wang et al.21,30 It should be noted that as a notably mature capacitive material, AC not only exhibits high energy efficiency but also delivers an ultralong cycle life, which were demonstrated by the previous literature on an AC electrode-based electrocatalytic hydrogen gas capacitor and electric double-layer capacitors.36,44−46 These characteristics are important in facilitating the cycling capability of H2 and O2 generation. To demonstrate, the cycle performance of H2 and O2 generation was investigated with an applied current of 10 mA cm–2 for a process time of 360 s. After 800 consecutive cycles (over 160 h), the HER/OER average voltages of acidic-CDWE show a negligible increase, indicating the excellent stability of the H2 and O2 generation with a limited electrolyte of 300 μL (Figure 2c). Figure 2c also indicates that the acidic-CDWE delivers a lower operating voltage of H2 generation than that of acidic-CWE. For the acidic-CDWE, the H2 generation average voltage slightly decreases and the O2 generation average voltage slightly increases upon cycling due to the difference between Pt catalytic HER and OER performance (Figure 2d,e). Highly stable energy efficiencies are also observed over the course of 800 round-trip cycles, as shown in Figure 2e. The energy efficiency improves slightly in the initial cycles, which is associated with the activation of the AC electrode.

Figure 2.

Figure 2

Open in a new tab

Electrochemical performance of acidic-CDWE in a Swagelok cell. (a) Voltage curves of acidic-CDWE and acidic-CWE at various current densities with an operational capacity of 3600 mC cm–2. (b) HER/OER average voltage and energy efficiency as a function of current density. (c) Voltage curves of acidic-CDWE and acidic-CWE during cycling at a current density of 10 mA cm–2 with an operational time of 360 s. (d) Voltage curves at different cycles. (e) HER/OER average voltage and energy efficiency as a function of cycle number. (f) Cycle performance of H2/O2 generation by consuming up and repeatedly adding a 100 μL electrolyte into the acidic-CDWE.

Further, we tested the water electrolytic performance of the acidic-CDWE with a limited electrolyte volume of 100 μL, as shown in Figure 2f. At an initial amount of 100 μL, the acidic-CDWE is cycled at a constant current density of 10 mA cm–2. The cell delivers stable average voltages for 350 cycles (over 70 h) until the charge voltage increases to 2 V. This indicates that the water solution in our designed electrolytic cell has achieved a high utilization ratio of nearly 100% (the detailed calculation is shown in the Methods section). In the subsequent process, the cell voltage can revert to the original state and continually maintain stable cycling for over another 350 cycles, when an additional 100 μL of the electrolyte was added into the cell. The cycling of the acidic-CDWE with a high electrolyte utilization ratio of nearly 100% can be achieved multiple times by repeatedly adding 100 μL of the electrolyte into the cell as demonstrated in Figure 2f. Figure S2 indicates that the designed acidic-CDWE not only delivers stable H2 and O2 generation but also achieves highly efficient utilization of the water solution. The micromorphology, energy-dispersive X-ray spectroscopy (EDX), and X-ray photoelectron spectroscopy (XPS) of the AC electrode (Figure S3) show no obvious change after 800 cycles, which confirms the excellent stability of the AC electrode in the acidic-CDWE. Meanwhile, after a long-term cycling test, the phase composition of the Pt electrode displays no obvious change (Figure S4a). The XPS spectra of Pt show that slight oxidization occurred in the Pt electrode after 800 cycles (Figure S4b,c). These characterizations imply the high structural stability of the AC electrode and Pt electrode during the repeated H2 and O2 generation processes, proving the robustness of our acidic-CDWE.

To demonstrate pH-universal capability, the water electrolytic performances of the CDWE were further evaluated in neutral and alkaline electrolytes with a Swagelok cell. The feasibility of the AC auxiliary electrode in neutral- and alkaline-CDWEs was verified by chronopotentiometry measurements at different current densities, as shown in Figure 3. A similarly good electrolytic capability was observed in the neutral-CDWE (Figure 3a). As a comparison, the neutral-CDWE delivers lower HER/OER average voltages than the neutral-CWE. Figure 3b reveals that the HER voltage and energy efficiency of neutral-CDWE at 5 mA cm–2 are 0.95 V and 90%, respectively. The neutral-CDWE investigated at the other current densities of 10, 15, and 20 mA cm–2 further illustrates the operational flexibility of this electrolyzer (Figure 3b). The cycle performance of the H2 and O2 generation was investigated with an applied current density of 10 mA cm–2 and a process time of 216 s for 500 cycles. After 100 consecutive cycles of this electrolytic process, the HER average voltage tends to stabilize at ∼0.8 V with an energy efficiency of ∼90%, demonstrating the excellent stability of the alternating H2 and O2 generation in the neutral-CDWE (Figure 3c and Figure S5). In addition, the alkaline-CDWE delivers a very low HER average voltage of 0.68 V, a cell average voltage of 1.74 V, and a high energy efficiency of 97% at a current density of 5 mA cm–2 (Figure 3d,e), which are superior to those from the previous literature on decoupling water electrolysis.29,30 When the current densities increase to 10, 15, and 20 mA cm–2, the HER average voltages can remain as 0.7, 0.73, and 0.75 V, respectively (Figure 3e). Furthermore, the alkaline-CDWE delivers a lower operational voltage than alkaline-CWE (Figure 3d). The alkaline-CDWE also shows high stability in alkaline water electrolysis for over 200 cycles (Figure 3f and Figure S6). Therefore, it can be concluded that the strategy of the two-electrode design could decouple pH-universal water electrolysis with excellent activity and stability, which further verified the operational flexibility of this all-pH-CDWE with the potential to be applied broadly in the water electrolysis industry.

Figure 3.

Figure 3

Open in a new tab

Electrochemical performances of neutral- and alkaline-CDWEs in Swagelok cells. (a) Voltage curves of a neutral-CDWE and neutral-CWE at various current densities with an operational capacity of 2160 mC cm–2. (b) HER/OER average voltage and energy efficiency as a function of current density. (c) HER/OER average voltage and energy efficiency as a function of cycle number at a current density of 10 mA cm–2 with an operational time of 216 s. (d) Voltage curves of an alkaline-CDWE and alkaline-CWE at various current densities with an operational capacity of 2160 mC cm–2. (e) HER/OER average voltage and energy efficiency as a function of current density. (f) HER/OER average voltage and energy efficiency as a function of cycle number at a current density of 10 mA cm–2 with an operational time of 216 s.

Based on the above analysis, to further demonstrate the advances and practicality of this novel CDWE for large-scale application in decoupling of H2 and O2, we amplified the electrolytic cell by employing a low-cost gas electrode in the CDWE. Accordingly, we took inspiration from the chloride corrosion of metal to prepare a low-cost bifunctional HER/OER electrocatalyst of NiMoCo in alkaline electrolytes.47 The facilely prepared NiMoCo catalyst exhibited excellent performances toward water splitting with overpotentials of 108 mV at a current density of 20 mA cm–2 for HER and 330 mV (20 mA cm–2) for OER, better than those of nickel foam and noble metal Pt (Figure S7). In addition, the NiMoCo catalyst was stable for more than 100 h at a constant current density of 50 mA cm–2 toward HER and OER (Figure S8), which is important toward long-term overall water splitting. When the NiMoCo catalyst was used as the electrode for the overall water-splitting device, a current density of 20 mA cm–2 could be observed at an applied potential of 1.66 V, which is superior to that of the Pt catalyst (1.95 V) (Figure S9).

To test the feasibility of the electrolytic cell, a scaled-up alkaline-CDWE was constructed with a low-cost NiMoCo-coated nickel foam gas electrode (5 × 10 cm2), an AC-coated nickel foam electrode (5 × 10 cm2), and a poly(methyl methacrylate) (PMMA) box with a volume of 0.48 L (10 × 10 × 4.8 cm3), as shown in Figure 4a. The morphology of the NiMoCo catalyst was observed as nanosheet arrays growing on nickel foam (Figure 4a). In addition, nickel-foam-supported AC particles were used as the active material to prepare a dense and thick carbon electrode with a thickness of 1.4 mm and a capacity of 720 C, as shown in Figure 4a. The two electrode plates were fixed upright and parallel with each other with a gap of 1 mm (Figure 4b). The electrode plates were put inside the PMMA box, and then a 2 M KOH alkaline electrolyte of 380 mL was added into this box. The testing results demonstrate that when the scaled-up alkaline-CDWE was cycled at an applied current of 1 A and an operational time of 720 s, the water electrolytic process stably produced H2 and O2 in several consecutive cycles (Figure 4c). Highly stable HER/OER average voltages of 0.99/1.35 V were also observed over the course of several round-trip cycles. The micromorphology and XPS spectra of the AC electrode also show no obvious change after cycles in the scaled-up alkaline-CDWE, which confirms the excellent stability of the AC electrode in large-scale water electrolysis (Figure S10). As shown in Figure S11, the energy efficiency of the scaled-up alkaline-CDWE using a NiMoCo-coated nickel foam gas electrode is 80% as compared to the alkaline-CWE counterpart. It is noticed that the energy efficiency can be further enhanced if optimized AC electrodes are employed. The corresponding photo profile of large H2 bubbles in the scaled-up alkaline-CDWE at a current of 1 A is shown in the inset of Figure 4c. The video evidence also clearly demonstrates the separated H2 and O2 generation in the amplified electrolyzer (Videos S1 and S2, respectively). In addition, we constructed a photovoltaic-powered water electrolysis device that could directly convert water and sunlight into green H2 without a membrane, as shown in Figure 4d. The working voltage of HER in the photovoltaic-powered water electrolysis under natural sunshine is about 1.47 V, as displayed in the photo profile (Figure 4d) and the video for H2 generation (Video S3).

Figure 4.

Figure 4

Open in a new tab

Electrochemical performance of the scaled-up alkaline-CDWE and its practical demonstration. (a) Digital photos and SEM images of the NiMoCo electrode (5 × 10 cm2) and carbon electrode (5 × 10 cm2). (b) Digital photo of the scaled-up alkaline-CDWE (10 × 10 × 4.8 cm3). (c) Voltage curves of the H2/O2 generation cycle at a current of 1 A with an operational time of 720 s for each cycle. The inset is the corresponding photo of H2 generation at a current of 1 A. (d) Photo profile of the HER of photovoltaic-powered water electrolysis under natural sunlight. The rated power, voltage, and current of each solar panel are 1.2 W, 3 V, and 400 mA, respectively.

We carried out additional experiments by taking the acidic-CDWE as a typical example to characterize the decoupled water electrolyzers. The photo profiles of H2 and O2 generation are shown in Figure S12 to confirm the decoupled characteristics of the acidic-CDWE. It is clearly observed that a large number of bubbles were only produced on the gas electrode, while no bubble was generated on the AC electrode (Figure S12c,d). In addition, the video record confirmed the directly separated H2 and O2 generation (Videos S4 and S5). The purity of the H2 and O2 produced by decoupling water electrolysis was evaluated by ex situ gas chromatography (GC) and in situ differential electrochemical mass spectrometry (DEMS). The GC and corresponding chronopotentiometry data of voltage curves for H2 and O2 generation are shown in Figure 5a,b, respectively. As displayed in Figure 5a, the recorded data showed that only H2 was detected, indicating no O2 generation during the HER process. After the charging process, a rest step of 20 min was performed to remove the produced H2 in the system. Then, O2 was detected by GC analysis during the subsequent discharging process with no H2 production (Figure 5b). The results indicated the effectiveness of the capacitive decoupling water electrolysis for the production of high-purity H2 and O2.

Figure 5.

Figure 5

Open in a new tab

Purity analysis of the generated H2/O2 in the CDWE. Gas chromatography for (a) HER and (b) OER. The insets show the corresponding voltage curves of H2 and O2 generation by water electrolysis, where the asterisk represents the sampling point. In situ DEMS curves of (c) the acidic-CDWE and (d) acidic-CWE for H2 evolution (red line) and O2 evolution (blue line) in the total water electrolysis process at an applied current density of 10 mA cm–2 for 360 s. Voltage curves of (e) the acidic-CDWE and (f) acidic-CWE corresponding to the in situ DEMS tests.

In addition, in situ DEMS was used to analyze the process of gas production by decoupled water electrolysis at a constantly applied current density of 10 mA cm–2. The obtained in situ DEMS and the corresponding chronopotentiometry data of the acidic-CDWE are given in Figure 5c,e, respectively. As shown in Figure 5c, in parallel with the beginning of charging, H2 evolution was detected in the online analysis record, whereas O2 remained at the background level, indicating no O2 generation during the charging process. After the end of charging, purging with a high-purity N2 stream was performed for a few minutes to eliminate the remaining H2 in the system. Then, O2 evolution was observed in the online analysis record of in situ DEMS during the discharging process but without any H2 evolution. As a comparison, in situ DEMS and the corresponding chronopotentiometry data of acidic-CWE are shown in Figure 5d,f, respectively. In contrast, H2 and O2 evolution were simultaneously observed in the online analysis record, which indicates that H2 and O2 generation in acidic-CWE cannot be decoupled. The above ex situ GC and in situ DEMS proved the effectiveness and robustness of the developed capacitive decoupled water electrolyzers as an advanced H2 and O2 production and separation technology.

In summary, we have successfully designed and demonstrated a novel all-pH-CDWE in a two-electrode single-cell configuration by utilizing a low-cost capacitive electrode and an electrocatalytic gas electrode for decoupling water electrolysis. The experimental analysis revealed that the interval generation of H2 and O2 can be achieved simply by reversing the current polarity of the all-pH-CDWE, which can be cycled with high stability over 800 consecutive cycles in a high electrolyte utilization ratio of nearly 100%. As compared to the driving voltage of CWE, the all-pH-CDWE showed high energy efficiencies of 94% in the acidic-CDWE and 97% in the alkaline-CDWE at a current density of 5 mA cm–2. Ex situ GC and in situ DEMS tests were conducted to confirm the high purity of the decoupling-generated H2 and O2. This designed all-pH-CDWE in a highly rechargeable process can be scaled up and well applied in practical water electrolysis to facilitate the utilization of renewable energy resources to generate H2 in mass production.

Methods

Fabrication of a Low-Cost NiMoCo Bifunctional Catalyst

Prior to the synthesis of the NiMoCo electrode, a 5 × 10 cm2 nickel foam (NF) was ultrasonicated with HCl (1 M), acetone, and ethanol for 5 min to ensure that there was no oxide or other purities on the surface. The synthesis of a low-cost NiMoCo bifunctional catalyst was modified on the basis of a previous report.47 Specifically, a piece of NF was put into a beaker containing NaCl (0.5 M), CoCl2·6H2O (1 mM), and MoCl5 (1 mM) solutions. Then, the beaker was frequently stirred for 12 h to ensure that the surface of NF can be completely reacted. Finally, the resulting catalyst-coated NF was dried naturally after washing.

Electrode Preparation

For the AC electrode, commercial AC (YEC-8, Fuzhou Yihuan Carbon, Ltd.), acetylene black (MTI), and poly(vinylidene fluoride) (MTI) were well mixed at a weight ratio of 8:1:1 to form a homogeneous slurry by adding the solvent N-methylpyrrolidone (Aladdin). After that, the slurry was coated onto carbon paper (Fuel cell store, USA) and formed a dense film with an activated mass loading of ∼15 mg cm–2. For the scaled-up experiment, the AC slurry was injected into an NF with an activated mass loading of ∼80 mg cm–2.

Water Electrolysis Investigation

In a beaker cell, galvanostatic measurements of the all-pH-CDWE with an AC electrode and a commercial Pt-coated Ti mesh (Suzhou Shure Tai Industrial Technology Co. Ltd.) as the gas electrode were carried out on a Landhe battery-testing instrument, a Neware battery-testing system, and a VMP-3 multi-channel electrochemical workstation. The all-pH-CDWE was designed in a Swagelok cell that was used to test H2 batteries previously,8,11,36 which can effectively collect H2 and O2. The typical Swagelok cell was made of an AC electrode, a Pt gas electrode, and a glass fiber (GF/D, Whatman) separator with a certain amount of electrolyte. The Swagelok device was made up of stainless-steel inlet and outlet valves with a Klein flange in a polytetrafluoroethylene-centered O-ring that was assembled with a clamp, as shown in Figure S13. The electrolyte solutions were 100 or 300 μL of 0.5 M H2SO4 for the acidic-CDWE, 200 μL of 1 M KH2PO4/K2HPO4 (phosphate buffer solution) for the neutral-CDWE, and 200 μL of 2 M KOH for the alkaline-CDWE. All experiments were performed at room temperature.

Calculation of Utilization of the Electrolyte

For the acidic-CDWE, the amount of transferred charge is calculated by the equation n = It/F, where I is the current density, t is the electrolytic time, and F is the Faraday constant. According to the reaction of water splitting, 2H2O → 2H2 + O2, the amount of transferred charge is twice as much as that of the consumed water. Therefore, the theoretical volume of the consumed water can be calculated by the equation VT = 0.5ItM/Fρ, where M is the molecular mass of water and ρ is the density of water.

Calculation of Energy Efficiency of the CDWE

For the CDWE, the energy efficiency (EE) can be calculated by the equation EE = ECWE/ECDWE, where ECWE is the input energy of CWE and ECDWE is the input energy of the CDWE for the overall water splitting. According to the equation E = C × U, where C is the capacity of the cells, the EE of the CDWE can be expressed as EE = UCWE/(UHER + UOER), where UHER is the average voltage in the H2 generation process of the CDWE, UOER is the average voltage in the O2 generation process of the CDWE, and UCWE is the average voltage of the CWE.

Gas Product Analysis

Gas products from the water electrolytic cell during HER and OER were detected and analyzed using ex situ GC (GC-2014c) and in situ DEMS (HPR-40 DEMS). The water electrolytic cell was sealed in the Swagelok cell, which was connected to a GC-2014c by two tubes as the gas inlet and outlet (Figure S14). High-purity Ar was used as the carrier gas and purified gas. Before testing, the sealed electrolytic cell was purified with Ar gas at a flow rate of 5 mL min–1 for 20 min to eliminate the air inside the cell. After that, HER and OER were performed using an SP-150 single-channel electrochemical workstation (BioLogic) with an applied current density of 20 mA (2 cm2) in an acidic electrolyte. After about 5.5 min, the H2 produced in the charging process was collected and analyzed by the GC instrument. Using the same method, a high-purity Ar gas stream was purged again for 20 min to blow out the H2 in the cell. A current density of 20 mA (2 cm2) was applied for 5.5 min, and the O2 produced in the discharging process was also collected and analyzed by the GC instrument. In addition, the mass spectrometer was connected to the Swagelok water electrolytic cell with two tubes as the gas inlet and outlet (Figure S14). A high-purity N2 gas stream was used as the purging gas before electrolysis and the carrier gas during the electrolysis process. The gas flows were controlled to be 4 mL min–1. Before the online gas analysis, the system was purged with a high-purity N2 stream for a few minutes. HER and OER were performed using a Chenhua electrochemical workstation with an applied current density of 20 mA (2 cm2) in the acidic-CDWE. The operational duration of the charge was set to be 6 min. After the end of the charging step, purging with a high-purity N2 stream was performed for a few minutes to eliminate any remaining H2 in the system. Then, the discharging step was started and continued until the cell voltage sharply increased. The experiment was completed when almost no remaining O2 in the electrolytic cell can be detected.

Physicochemical Characterizations

The morphologies of the AC electrodes and the NiMoCo catalytic electrodes were characterized with a scanning electron microscope (SEM) instrument (Hitachi 8220). Energy-dispersive X-ray spectroscopy was collected by the SEM instrument. Powder X-ray diffraction (PXRD) patterns were collected with a Philips X’Pert Pro Super X-ray diffractometer equipped with graphite monochromatized Cu Kα radiation. X-ray photoelectron spectroscopy (XPS) measurements of the carbon electrode and Pt electrode were performed with an ESCALAB 250Xi.

Acknowledgments

This research work was supported by startup funds from the USTC (Grant No. KY2060000150) and the Fundamental Research Funds for the Central Universities (WK2060000040). We thank the support from the USTC Center for Micro and Nanoscale Research and Fabrication.

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/jacsau.2c00624.

  • (Figure S1) Charge/discharge curves and rate capacitance of the AC symmetric capacitor; (Figure S2) voltage curves of the acidic-CDWE in different cycles when repeatedly adding 100 μL electrolyte into the cell; (Figure S3) SEM images, EDX analyses, and XPS spectra of the fresh AC electrode and the AC electrode after 800 cycles in an acidic-CDWE; (Figure S4) XRD patterns and XPS spectra of the Pt electrode in the initial state and after 800 cycles in an acidic-CDWE; (Figures S5 and S6) voltage curves of the neutral-CDWE and alkaline-CDWE during cycling; (Figure S7) HER and OER performances of Ni foam, PtTi, and NiMoCo catalysts; (Figure S8) HER and OER stability tests of the NiMoCo catalyst; (Figure S9) polarization curves of NiMoCo and Pt catalysts for overall water splitting; (Figure S10) SEM image and XPS spectrum of the AC electrode after cycling in the scaled-up alkaline-CDWE; (Figure S11) voltage curves of CWE cycles in the scaled-up electrolytic cell; (Figure S12) characterization of the acidic-CDWE in a beaker testing cell; (Figures S13 and S14) digital photographs of the testing Swagelok device of the all-pH-CDWE and the all-pH-CDWE connected to the DEMS device; (Table S1) comparison of different water electrolysis technologies; (Table S2) EDX elemental contents of the fresh AC electrode and AC electrode after 800 cycles (PDF)

  • (Video S1) H2 generation in the scaled-up alkaline-CDWE (MP4)

  • (Video S2) O2 generation in the scaled-up alkaline-CDWE (MP4)

  • (Video S3) H2 generation in the photovoltaic-powered water electrolysis device (MP4)

  • (Video S4) H2 generation in the acidic-CDWE (MP4)

  • (Video S5) O2 generation in the acidic-CDWE (MP4)

Author Contributions

CRediT: Zhengxin Zhu data curation, investigation, methodology, project administration, writing-original draft, writing-review & editing; Taoli Jiang visualization; Jifei Sun visualization; Zaichun Liu visualization; Zehui Xie visualization; Shuang Liu visualization; Yahan Meng visualization; Qia Peng visualization; Weiping Wang visualization; Hongxu Liu visualization; Yuan Yuan visualization; Ke Li visualization; Wei Chen funding acquisition, project administration, writing-original draft, writing-review & editing.

The authors declare the following competing financial interest(s): A patent based on this work was filed on June 3, 2021.

Supplementary Material

au2c00624_si_001.pdf (1.2MB, pdf)
au2c00624_si_002.mp4 (19.7MB, mp4)
au2c00624_si_003.mp4 (16.2MB, mp4)
au2c00624_si_004.mp4 (15.5MB, mp4)
au2c00624_si_005.mp4 (14.9MB, mp4)
au2c00624_si_006.mp4 (15MB, mp4)

References

  1. Turner J. A. A Realizable Renewable Energy Future. Science 1999, 285, 687–689. 10.1126/science.285.5428.687. [DOI] [PubMed] [Google Scholar]
  2. Zhu Z.; Jiang T.; Ali M.; Meng Y.; Jin Y.; Cui Y.; Chen W. Rechargeable Batteries for Grid Scale Energy Storage. Chem. Rev. 2022, 122, 16610–16751. 10.1021/acs.chemrev.2c00289. [DOI] [PubMed] [Google Scholar]
  3. Wang M.; Zheng X.; Zhang X.; Chao D.; Qiao S.-Z.; Alshareef H. N.; Cui Y.; Chen W. Opportunities of Aqueous Manganese-Based Batteries with Deposition and Stripping Chemistry. Adv. Energy Mater. 2021, 11, 1704378. 10.1002/aenm.202002904. [DOI] [Google Scholar]
  4. Ball M.; Weeda M. The hydrogen economy – Vision or reality?. Int. J. Hydrogen Energy 2015, 40, 7903–7919. 10.1016/j.ijhydene.2015.04.032. [DOI] [Google Scholar]
  5. Wei D.; Sang R.; Moazezbarabadi A.; Junge H.; Beller M. Homogeneous Carbon Capture and Catalytic Hydrogenation: Toward a Chemical Hydrogen Battery System. JACS Au 2022, 2, 1020–1031. 10.1021/jacsau.1c00489. [DOI] [PMC free article] [PubMed] [Google Scholar]
  6. Wang Y.; Jin F.; Zeng X.; Ma C.; Wang F.; Yao G.; Jing Z. Catalytic activity of Ni3S2 and effects of reactor wall in hydrogen production from water with hydrogen sulphide as a reducer under hydrothermal conditions. Appl. Energy 2013, 104, 306–309. 10.1016/j.apenergy.2012.11.014. [DOI] [Google Scholar]
  7. Turner J. A. Sustainable Hydrogen Production. Science 2004, 305, 972–974. 10.1126/science.1103197. [DOI] [PubMed] [Google Scholar]
  8. Chen W.; Li G.; Pei A.; Li Y.; Liao L.; Wang H.; Wan J.; Liang Z.; Chen G.; Zhang H.; et al. A manganese–hydrogen battery with potential for grid-scale energy storage. Nat. Energy 2018, 3, 428–435. 10.1038/s41560-018-0147-7. [DOI] [Google Scholar]
  9. Zhu Z.; Meng Y.; Cui Y.; Chen W. An Ultrastable Aqueous Iodine-Hydrogen Gas Battery. Adv. Funct. Mater. 2021, 31, 2101024. 10.1002/adfm.202101024. [DOI] [Google Scholar]
  10. Zhu Z.; Meng Y.; Yin Y.; Liu Z.; Jiang T.; Peng Q.; Yin T.; Li M.; Chen W. High performance aqueous Prussian blue analogue-hydrogen gas hybrid batteries. Energy Storage Mater. 2021, 42, 464–469. 10.1016/j.ensm.2021.07.050. [DOI] [Google Scholar]
  11. Zhu Z.; Wang W.; Yin Y.; Meng Y.; Liu Z.; Jiang T.; Peng Q.; Sun J.; Chen W. An Ultrafast and Ultra-Low-Temperature Hydrogen Gas-Proton Battery. J. Am. Chem. Soc. 2021, 143, 20302–20308. 10.1021/jacs.1c09529. [DOI] [PubMed] [Google Scholar]
  12. Berger A.; Segalman R. A.; Newman J. Material requirements for membrane separators in a water-splitting photoelectrochemical cell. Energy Environ. Sci. 2014, 7, 1468–1476. 10.1039/C3EE43807D. [DOI] [Google Scholar]
  13. Jiang X.; Liu Q.; Cheng C.; Xing F.; Chen C.; Huang C. In situ photodeposition of metalloid Ni2P co-catalyst on Mn0.5Cd0.5S for enhanced photocatalytic H2 evolution with visible light. Int. J. Hydrogen Energy 2021, 46, 5197–5206. 10.1016/j.ijhydene.2020.11.011. [DOI] [Google Scholar]
  14. Paul A.; Symes M. D. Decoupled electrolysis for water splitting. Curr. Opin. Green Sustainable Chem. 2021, 29, 100453. 10.1016/j.cogsc.2021.100453. [DOI] [Google Scholar]
  15. Chen G.; Wang T.; Zhang J.; Liu P.; Sun H.; Zhuang X.; Chen M.; Feng X. Accelerated Hydrogen Evolution Kinetics on NiFe-Layered Double Hydroxide Electrocatalysts by Tailoring Water Dissociation Active Sites. Adv. Mater. 2018, 30, 1706279. 10.1002/adma.201706279. [DOI] [PubMed] [Google Scholar]
  16. Zhao X.; Wei H.; Zhao H.; Wang Y.; Tang N. Electrode materials for capacitive deionization: A review. J. Electroanal. Chem. 2020, 873, 114416. 10.1016/j.jelechem.2020.114416. [DOI] [Google Scholar]
  17. Pan Z.; An J.; Wang P.; Fan X.; Shen T.; Xu R.; Song Y.; Song C. Novel strategy to enhance the desalination performance of flow-electrode capacitive deionization process via the assistance of electro-catalytic water splitting. Sep. Purif. Technol. 2021, 279, 119753. 10.1016/j.seppur.2021.119753. [DOI] [Google Scholar]
  18. Liu X.; Chi J.; Dong B.; Sun Y. Recent Progress in Decoupled H2 and O2 Production from Electrolytic Water Splitting. ChemElectroChem 2019, 6, 2157–2166. 10.1002/celc.201801671. [DOI] [Google Scholar]
  19. McHugh P. J.; Stergiou A. D.; Symes M. D. Decoupled Electrochemical Water Splitting: From Fundamentals to Applications. Adv. Energy Mater. 2020, 10, 2002453. 10.1002/aenm.202002453. [DOI] [Google Scholar]
  20. Landman A.; Dotan H.; Shter G. E.; Wullenkord M.; Houaijia A.; Maljusch A.; Grader G. S.; Rothschild A. Photoelectrochemical water splitting in separate oxygen and hydrogen cells. Nat. Mater. 2017, 16, 646–651. 10.1038/nmat4876. [DOI] [PubMed] [Google Scholar]
  21. Ma Y.; Dong X.; Wang Y.; Xia Y. Decoupling Hydrogen and Oxygen Production in Acidic Water Electrolysis Using a Polytriphenylamine-Based Battery Electrode. Angew. Chem., Int. Ed. 2018, 57, 2904–2908. 10.1002/anie.201800436. [DOI] [PubMed] [Google Scholar]
  22. Huang J.; Xie Y.; Yan L.; Wang B.; Kong T.; Dong X.; Wang Y.; Xia Y. Decoupled amphoteric water electrolysis and its integration with Mn–Zn battery for flexible utilization of renewables. Energy Environ. Sci. 2021, 14, 883–889. 10.1039/D0EE03639K. [DOI] [Google Scholar]
  23. Liang S.; Jiang M.; Luo H.; Ma Y.; Yang J. A High-Rate Electrode with Grotthuss Topochemistry for Membrane-Free Decoupled Acid Water Electrolysis. Adv. Energy Mater. 2021, 11, 2102057. 10.1002/aenm.202102057. [DOI] [Google Scholar]
  24. Zhang F.; Wang Q. Redox-Mediated Water Splitting for Decoupled H2 Production. ACS Mater. Lett. 2021, 3, 641–651. 10.1021/acsmaterialslett.1c00074. [DOI] [Google Scholar]
  25. Wallace A. G.; Symes M. D. Decoupling Strategies in Electrochemical Water Splitting and Beyond. Joule 2018, 2, 1390–1395. 10.1016/j.joule.2018.06.011. [DOI] [Google Scholar]
  26. Landman A.; Halabi R.; Dias P.; Dotan H.; Mehlmann A.; Shter G. E.; Halabi M.; Naseraldeen O.; Mendes A.; Grader G. S.; et al. Decoupled Photoelectrochemical Water Splitting System for Centralized Hydrogen Production. Joule 2020, 4, 448–471. 10.1016/j.joule.2019.12.006. [DOI] [Google Scholar]
  27. Wang F.; Sheng H.; Li W.; Gerken J. B.; Jin S.; Stahl S. S. Stable Tetrasubstituted Quinone Redox Reservoir for Enhancing Decoupled Hydrogen and Oxygen Evolution. ACS Energy Lett. 2021, 6, 1533–1539. 10.1021/acsenergylett.1c00236. [DOI] [PMC free article] [PubMed] [Google Scholar]
  28. Chen J.; Xiao R.; Fu K.; Wu Y.; Guo Y.; Yang S.; Li H.; Zheng J.; Li X. Metal hydride mediated water splitting: Electrical energy saving and decoupled H2/O2 generation. Mater. Today 2021, 47, 16–24. 10.1016/j.mattod.2021.02.019. [DOI] [Google Scholar]
  29. Rausch B.; Symes M. D.; Chisholm G.; Cronin L. Decoupled catalytic hydrogen evolution from a molecular metal oxide redox mediator in water splitting. Science 2014, 345, 1326–1330. 10.1126/science.1257443. [DOI] [PubMed] [Google Scholar]
  30. Chen L.; Dong X.; Wang Y.; Xia Y. Separating hydrogen and oxygen evolution in alkaline water electrolysis using nickel hydroxide. Nat. Commun. 2016, 7, 11741. 10.1038/ncomms11741. [DOI] [PMC free article] [PubMed] [Google Scholar]
  31. Ma Y.; Guo Z.; Dong X.; Wang Y.; Xia Y. Organic Proton-Buffer Electrode to Separate Hydrogen and Oxygen Evolution in Acid Water Electrolysis. Angew. Chem., Int. Ed. 2019, 58, 4622–4626. 10.1002/anie.201814625. [DOI] [PubMed] [Google Scholar]
  32. Dotan H.; Landman A.; Sheehan S. W.; Malviya K. D.; Shter G. E.; Grave D. A.; Arzi Z.; Yehudai N.; Halabi M.; Gal N.; et al. Decoupled hydrogen and oxygen evolution by a two-step electrochemical–chemical cycle for efficient overall water splitting. Nat. Energy 2019, 4, 786–795. 10.1038/s41560-019-0462-7. [DOI] [Google Scholar]
  33. Landman A.; Hadash S.; Shter G. E.; Ben-Azaria A.; Dotan H.; Rothschild A.; Grader G. S. High Performance Core/Shell Ni/Ni(OH)2 Electrospun Nanofiber Anodes for Decoupled Water Splitting. Adv. Funct. Mater. 2021, 31, 2008118. 10.1002/adfm.202008118. [DOI] [Google Scholar]
  34. Wang Q.; Xu C. Q.; Liu W.; Hung S. F.; Bin Yang H.; Gao J.; Cai W.; Chen H. M.; Li J.; Liu B. Coordination engineering of iridium nanocluster bifunctional electrocatalyst for highly efficient and pH-universal overall water splitting. Nat. Commun. 2020, 11, 4246. 10.1038/s41467-020-18064-w. [DOI] [PMC free article] [PubMed] [Google Scholar]
  35. Chen J.; Qian G.; Zhang H.; Feng S.; Mo Y.; Luo L.; Yin S. PtCo@PtSn Heterojunction with High Stability/Activity for pH-Universal H2 Evolution. Adv. Funct. Mater. 2022, 2107597. 10.1002/adfm.202107597. [DOI] [Google Scholar]
  36. Zhu Z.; Liu Z.; Yin Y.; Yuan Y.; Meng Y.; Jiang T.; Peng Q.; Wang W.; Chen W. Production of a hybrid capacitive storage device via hydrogen gas and carbon electrodes coupling. Nat. Commun. 2022, 13, 2805. 10.1038/s41467-022-30450-0. [DOI] [PMC free article] [PubMed] [Google Scholar]
  37. Ruge M.; Drnec J.; Rahn B.; Reikowski F.; Harrington D. A.; Carlà F.; Felici R.; Stettner J.; Magnussen O. M. Structural Reorganization of Pt(111) Electrodes by Electrochemical Oxidation and Reduction. J. Am. Chem. Soc. 2017, 139, 4532–4539. 10.1021/jacs.7b01039. [DOI] [PubMed] [Google Scholar]
  38. Wei J.-X.; Cao M.-Z.; Xiao K.; Guo X.-P.; Ye S.-Y.; Liu Z.-Q. In Situ Confining Pt Clusters in Ultrathin MnO2 Nanosheets for Highly Efficient Hydrogen Evolution Reaction. Small Struct. 2021, 2, 2100047. 10.1002/sstr.202100047. [DOI] [Google Scholar]
  39. Wei J.; Xiao K.; Chen Y.; Guo X.-P.; Huang B.; Liu Z.-Q. In situ precise anchoring of Pt single atoms in spinel Mn3O4 for a highly efficient hydrogen evolution reaction. Energy Environ. Sci. 2022, 15, 4592–4600. 10.1039/D2EE02151J. [DOI] [Google Scholar]
  40. Yu M.; Lu Y.; Zheng H.; Lu X. New Insights into the Operating Voltage of Aqueous Supercapacitors. Chem. – Eur. J. 2018, 24, 3639–3649. 10.1002/chem.201704420. [DOI] [PubMed] [Google Scholar]
  41. Sun W.; Lipka S. M.; Swartz C.; Williams D.; Yang F. Hemp-derived activated carbons for supercapacitors. Carbon 2016, 103, 181–192. 10.1016/j.carbon.2016.02.090. [DOI] [Google Scholar]
  42. Yang I.; Kwon D.; Kim M.-S.; Jung J. C. A comparative study of activated carbon aerogel and commercial activated carbons as electrode materials for organic electric double-layer capacitors. Carbon 2018, 132, 503–511. 10.1016/j.carbon.2018.02.076. [DOI] [Google Scholar]
  43. Yu L.; Hu L.; Anasori B.; Liu Y.-T.; Zhu Q.; Zhang P.; Gogotsi Y.; Xu B. MXene-Bonded Activated Carbon as a Flexible Electrode for High-Performance Supercapacitors. ACS Energy Lett. 2018, 3, 1597–1603. 10.1021/acsenergylett.8b00718. [DOI] [Google Scholar]
  44. Xu Y.; Chang L.; Hu Y. H. KOH-assisted microwave post-treatment of activated carbon for efficient symmetrical double-layer capacitors. Int. J. Energy Res. 2017, 41, 728–735. 10.1002/er.3676. [DOI] [Google Scholar]
  45. Itoi H.; Maki S.; Ninomiya T.; Hasegawa H.; Matsufusa H.; Hayashi S.; Iwata H.; Ohzawa Y. Electrochemical polymerization of pyrene and aniline exclusively inside the pores of activated carbon for high-performance asymmetric electrochemical capacitors. Nanoscale 2018, 10, 9760–9772. 10.1039/C8NR01529E. [DOI] [PubMed] [Google Scholar]
  46. Zhong M.; Zhang M.; Li X. Carbon nanomaterials and their composites for supercapacitors. Carbon Energy 2022, 4, 950–985. 10.1002/cey2.219. [DOI] [Google Scholar]
  47. Hao S.; Chen L.; Yu C.; Yang B.; Li Z.; Hou Y.; Lei L.; Zhang X. NiCoMo Hydroxide Nanosheet Arrays Synthesized via Chloride Corrosion for Overall Water Splitting. ACS Energy Lett. 2019, 4, 952–959. 10.1021/acsenergylett.9b00333. [DOI] [Google Scholar]

Associated Data

This section collects any data citations, data availability statements, or supplementary materials included in this article.

Supplementary Materials

au2c00624_si_001.pdf (1.2MB, pdf)
au2c00624_si_002.mp4 (19.7MB, mp4)
au2c00624_si_003.mp4 (16.2MB, mp4)
au2c00624_si_004.mp4 (15.5MB, mp4)
au2c00624_si_005.mp4 (14.9MB, mp4)
au2c00624_si_006.mp4 (15MB, mp4)

Articles from JACS Au are provided here courtesy of American Chemical Society

RESOURCES