Tuning Redox Active Polyoxometalates for Efficient Electron-Coupled Proton-Buffer-Mediated Water Splitting.

We present strategies to tune the redox properties of polyoxometalate clusters to enhance the electron-coupled proton-buffer-mediated water splitting process, in which the evolution of hydrogen and oxygen can occur in different forms and is separated in time and space. By substituting the heteroatom template in the Keggin-type polyoxometalate cluster, H6 ZnW12 O40 , it is possible to double the number of electrons and protonation in the redox reactions (from two to four). This increase can be achieved with better matching of the energy levels as indicated by the redox potentials, compared to the ones of well-studied H3 PW12 O40 and H4 SiW12 O40 . This means that H6 ZnW12 O40 can act as a high-performance redox mediator in an electrolytic cell for the on-demand generation of hydrogen with a high decoupling efficiency of 95.5 % and an electrochemical energy efficiency of 83.3 %. Furthermore, the H6 ZnW12 O40 cluster also exhibits an excellent cycling behaviour and redox reversibility with almost 100 % H2 -mediated capacity retention during 200 cycles and a high coulombic efficiency >92 % each cycle at 30 mA cm-2 .

Water splitting is a promising process that could provide a route to a clean and inexhaustible renewable energy, but significant issues remain for establishing scalable systems and also coping with intermittent power.1 In this regard proton exchange membrane electrolysis (PEME) is a prime candidate to achieve this transformation but gas crossover through the membrane is both a safety concern and affects the purity of H2, as well as degrading the membrane.2, 3 Thus, it is highly important to develop new strategies to coordinate the intermittent renewable power sources that meet the requirements of H2 large‐scale production, transportation and storage simultaneously with safe and economic approaches.

Previously, we introduced the electron‐coupled‐proton‐buffer (ECPB) to separate the process of water splitting in time and space.4, 5, 6, 7 In this way, the hydrogen evolution reaction (HER) is no longer coupled with the rate‐limiting step of the oxygen evolution reaction (OER). Electric energy can be stored in a soluble H2‐mediated redox couple, which can be chemically or electrochemically converted into gaseous H2 on‐demand. Such a system is ideal as a flexible and scalable option to mitigate the intermittent output of renewably generated energy. It will also prevent the crossover issue of O2/H2 gas through the membrane. Similarly, Xia et al. decoupled the hydrogen and oxygen production system in a membrane‐free electrolyser based on the reversible solid‐state Ni(OH)2 and polytriphenylamine redox mediator.8, 9 Integrated battery‐electrolysers have also been constructed to decouple hydrogen production driven by the electrochemical energy storage system, such as an all‐vanadium dual circuit redox flow battery or a nickel‐iron battery.10, 11 Furthermore, such hydrogen‐mediated redox species also show potential applications in high‐purity H2 storage and generation on demand. Hydrogen could be spontaneously evolved from the reduced‐ECPB on a conventional HER‐catalyst if its redox potential is more negative than the HER onset potential of the catalyst.12, 13 By exploiting the overpotentials related to hydrogen evolution on carbon, an ECPB can be reduced past the point of the normal hydrogen electrode (NHE) without any H2 evolution. Once the reduced ECPB encounters the HER catalyst, it can then be oxidized with concomitant high‐purity H2 release—that is, without any additional energy input.

Soluble redox mediators with the ability to buffer protons are the cornerstone of the promising applications described above, but the electron‐storage capacity of the mediators considered hitherto are limited to only 1–2 electrons per molecule. Thus, there is a great need to develop new redox mediators that can store as many electrons per molecule as possible. Polyoxometalates (POMs) show tremendous promise in this regard, due to their ability to perform reversible multielectron reactions with high structural stability in aqueous media.14] Despite many successful demonstrations of polyoxometalates in solid‐state energy storage systems15, 16, 17, 18, 19, 20, 21 and redox flow batteries,22, 23 it is still difficult to develop strategies to modify the POMs’ structure to tune the number of reversible electrons in aqueous states, and thus to increase the volumetric capacities for the applications in ECPB water splitting and hydrogen‐mediated redox couples for aqueous energy storage. Moreover, the energetic inputs required for ECPB‐based water splitting depends on the redox species’ electrochemical potential. For example, silicotungstic acid, can undergo a 2‐electron redox reaction, but only one of the two electrons can be decoupled to release H2 spontaneously by exposure over Pt/C catalysts, leading to a low H2 decoupling efficiency around 67 % and an electrochemical energy efficiency of 79.3 %.7 Thus, it is important to increase the electron storage capacity and to modify the redox potential to improve energy efficiency in the overall water splitting.

Herein, we successfully illustrate the strategies to tune redox properties of Keggin‐type tungsten POMs with different central heteroatoms. The number of reversible electrons with protonation could be doubled by changing the heteroatom from P5+or Si4+ to Zn2+. The desired redox potentials of H6ZnW12O40 enhance its ECPB performance for on‐demand storage and generation of H2 with a high decoupling efficiency of 95.5 % and an electrochemical energy efficiency of 83.3 %.

The molecular structure in this work is based on the typical Keggin‐type heteropolyoxotungstates {XW12O40}. As shown in the CV results in Figure 1 a, {XW12O40} exhibits distinguished differences of redox chemistry by changing their central heteroatoms (X=P5+, Si4+, and Zn2+) within the voltage range of HER onset potentials between glassy carbon electrode and Pt electrode. This allows {XW12O40} to be reduced firstly on carbon without any competing H2 evolution and then evolve H2 spontaneously through simple exposure to Pt later. H3PW12O40 undergoes two redox steps at the potentials +0.222 and −0.052 V vs. NHE. While the redox potentials of H4SiW12O40 shift negatively to +0.019 and −0.212 V vs. NHE compared to those of H3PW12O40. These two separated redox peaks in both H3PW12O40 and H4SiW12O40 have been confirmed to be two one‐electron redox processes.24 Intriguingly, due to the prominent effect of ionic charge on the pristine anion clusters, the midpoint potential of the first redox wave shift negatively when the central heteroatom changes from P5+ or Si4+ to Zn2+. H6ZnW12O40 (prepared by following the modified literature method,25 see the Supporting Information, Section SI‐2 and Figures S1 and S2) exhibits the redox potentials of −0.078 and −0.198 V vs. NHE under the same conditions. This results are consistent with previous studies of the energy levels.26, 27, 28, 29, 30 Moreover, the number of electrons stored by H6ZnW12O40 at each redox potential was increased to 2, respectively.

Figure 1

a) Crystal structure of {XW12O40} (phosphorous, grey; silicon, orange; zinc, blue; tungsten, cerulean; oxygen, red) and the corresponding cyclic voltammograms (CVs) of 100 mm HnXW12O40 (X=P5+, Si4+, and Zn2+) solution on glassy carbon electrode at a scan rate of 50 mV s−1. b) pH changing during the reduction process of 100 mm HnXW12O40 (X=P5+, Si4+, and Zn2+) solution. c) CVs of a 1 mm HXW12O40 (X=P5+, Si4+, and Zn2+) under different pH buffer solutions at a scan rate of 50 mV s−1. For more details, see the Supporting Information, Sections SI‐3, SI‐4, and SI‐5.

Changing the heteroatom type inside the {XW12O40} also leads to a significant difference in ability of the cluster to buffer under reducing conditions. Bulk electrolysis in an airtight H‐cell was performed to electrochemically reduce {XW12O40} with different equivalents of electrons per cluster (see the details in the Supporting Information, Section SI‐5). As shown in Figure 1 b, a continuous decrease of pH could be observed when 100 mm H3PW12O40 and H4SiW12O40 solution are reduced to two electrons per cluster, while 100 mm H6ZnW12O40 solution has a negligible pH change up to four electron reductions per cluster. This can be ascribed to the enhanced protonation ability of {ZnW12O40} that accompany reduction in acid solution, keeping the overall ionic charge of the reduced Keggin {XW12O40} cluster constant at −6.31, 32, 33 This is also confirmed by the CV investigation under different pH values. As shown in Figure 1 c, the redox potential of H6ZnW12O40 has a dependence on the pH of the solution, which means that protons are involved in the electrochemical redox reaction of H6ZnW12O40 according to the Nernst equation. However, the redox potential of H3PW12O40 and H4SiW12O40 has no dependence on pH. The CVs of H3PW12O40 at pH 2.0 and 3.0 cannot be obtained because of its instability in the solution with pH>1.5.32

Four‐electron redox reactions of H6ZnW12O40 were further proved by passing charge equivalents to different numbers of electrons per cluster at a current density of 5 mA cm−2 and measuring the coulombic efficiency between the reduction and reoxidation process. As shown in Figure 2, the coulombic efficiency remains above 94 % even up to a reduction level of four electrons per cluster molecule. But more than four electrons, reduction of H6ZnW12O40 produces a brown solution, which indicates that some irreversible reactions have occurred (Figure S5, Supporting Information). XPS of the W in the {ZnW12O40} cluster under the various reduced states are displayed in Figure S6 (see more details of sample preparation in the Supportin Information, Section SI‐7). The peaks at 34.1 and 36.28 eV are assigned to WV 4f7/2 and WⅤ 4f5/2, whereas the peaks in 35.39 and 37.57 eV belong to WⅥ 4f7/2 and WⅥ 4f5/2.34 The content of WⅤ increases obviously with the number of reduction electrons, while the peaks of Zn 2p3/2 and Zn 2p1/2 at 1021.82 and 1044.88 eV, respectively remain unchanged (Supporting Information, Figure S7), indicating that the negative charges are mainly localized around tungsten sites. Thus, taken all together, H6ZnW12O40 could exhibit a four‐electron redox reaction with accompanying protons and the detailed electrochemical reactions that can occur are presented in the Supporting Information (Section SI‐6). Moreover, the modulated redox potentials make it suitable as an ECPB for high‐performance water splitting.

Figure 2

Reduction and reoxidation of a 20 mm H6ZnW12O40 solution with a constant current density of 5 mA cm−2 with different equivalents of electrons per cluster.

Regarding the hypothesis of application for on‐demand H2 storage and generation, the spontaneous H2 evolution experiments were carried out to test the hydrogen‐mediated capacity of H6ZnW12O40. The schematic of this ECPB‐mediated water‐splitting system was illustrated in Figure 3 a. At the anode, H2O is split into O2, protons, and electrons, whereas the mediator is reversibly reduced and protonated at the cathode in preference to direct production of H2. In other words, the electrons were stored in the H6ZnW12O40 by means of electron‐coupled protons rather than direct H2 evolution at the cathode. The reduced ECPB is then transferred to a separate chamber for H2 evolution over a suitable catalyst. According to their CVs in Figure 1 a, H6ZnW12O40 will be reduced by four electrons and then couple with four protons at the constant potential of −0.36 V vs. NHE. Silicotungstic acid was used as the control experiment. The theoretical H2 volume stored in the 2 e−‐reduced H4SiW12O40 or 4 e−‐reduced H6ZnW12O40 solution could be calculated to 44.8 and 89.6 mL by the charge passed in the first‐step electroreduction process (the calculated equation is given in the Supporting Information, Section SI‐9). As shown in Figure 3 b, about 30 mL H2, 67 % of the theoretical H2 volume, can be collected by physically mixing the 2 e−‐reduced H4SiW12O40 solution with 5 mg 20 % Pt/C, which is consistent with previous reported result.7 The incomplete release of hydrogen stored is caused by the first redox potential of H4SiW12O40 being more positive than the HER onset potential of Pt/C catalyst (Figure 1 a). Nevertheless, 95.5 % of the theoretical H2 volume stored could be released in the H6ZnW12O40‐mediated system (see movie S1) and the H2 initial decoupling rate could reach a value of 121 mmol h−1 mg−1 Pt, which is much higher than that of the 2 e−‐reduced H4SiW12O40. The filtered H6ZnW12O40 solution after hydrogen release shows the same UV/Vis spectrum as the initial solution (Supporting Information, Figure S9), which indicates the structural stability of the H6ZnW12O40 mediator. Moreover, the H6ZnW12O40‐mediated system has a higher electrochemical energy efficiency of 83.3 % than the H4SiW12O40‐mediated system due to its modulated redox potentials and redox electron number (for the detailed calculation process see the Supporting Information, Section SI‐8).

Figure 3

a) Schematic of the ECPB‐mediated water‐splitting system and the spontaneous hydrogen evolution system. b) Spontaneous H2 evolution from a 20 mL sample of 100 mm 4 e−‐reduced H6ZnW12O40 or 2 e−‐reduced H4SiW12O40 over 5 mg 20 % Pt/C.

Another potential application of this ECPB in the production of H2 and O2 at separated time and space were evaluated by the flow cells system (Figure 4 a). In this system, water is oxidized to produce O2 on the anode in cell‐a with an iridium oxide catalyst. The concomitant electrons and protons would be applied to reduce and protonate the H6ZnW12O40 solution on a carbon cathode (12.96 cm2 of geometric area) in cell‐a. Once a full 4‐electron reduction per H6ZnW12O40 cluster has been reached by passing corresponding amount of charge, the 4 e−‐reduced H6ZnW12O40 solution would be then pumped to a carbon anode in cell‐b to be reoxidized electrochemically and H2 is produced on the cathode in cell‐b with a Pt/C catalyst at the same time. The charge passed in the reoxidation process is denoted as practical ECPB storage capacity and the coulombic efficiency could be gauged by comparing it with the charge initially used to reduce H6ZnW12O40 solution. As a result, H6ZnW12O40 exhibits excellent ECPB cycling behaviors and redox reversibility. Under a constant electrolysis current density of 30 mA cm−2, it delivers an almost 100 % H2‐mediated capacity retention during 200 cycles with a high coulombic efficiency >92 % each cycle (Figure 4 b).

Figure 4

a) Schematic of the flow cells system for long‐term separated time and space water electrolysis. b) Long‐term electrochemical reduction and re‐oxidation cycling of a 20 mm H6ZnW12O40 solution at a current density of 30 mA cm−2 under Ar atmosphere.

In summary, this work shows the strategies to tune the properties of H2‐mediated redox couples by the adjustment of Keggin‐type tungsten polyoxometalates’ molecular structure with different heteroatoms. When the central heteroatom changes from P5+or Si4+ to Zn2+, H6ZnW12O40 exhibits an increased number of reversible electrons with enhanced protonation ability and more favorable redox potentials. As an ECPB for H2 storage and generation, it also delivers a high decoupling efficiency of 95.5 % and an electrochemical energy efficiency of 83.3 % for on‐demand catalytic H2 evolution and an excellent long‐term time and space separated, water electrolysis. This work illustrates how to optimize practical application issues by using fundamental approaches, and we believe this will lead to new flexible and safe H2 production, storage and transportation systems to mitigate the challenges inherent in present renewable energy systems.

Conflict of interest

The authors declare no conflict of interest.

As a service to our authors and readers, this journal provides supporting information supplied by the authors. Such materials are peer reviewed and may be re‐organized for online delivery, but are not copy‐edited or typeset. Technical support issues arising from supporting information (other than missing files) should be addressed to the authors.

Supplementary

Click here for additional data file.

Supplementary

Click here for additional data file.