Study on Catalytic Water Oxidation Properties of Polynuclear Manganese Containing Polyoxometalates

: Splitting of water to produce hydrogen and oxygen is a green and effective method to produce clean energy. Finding an efficient water decomposition catalyst is the key step to realize water decomposition. In this work, by choosing from the literature, six polynuclear manganese (Mn) containing polyoxometalates (Mn-POMs) with different Mn-O clusters and oxidation states of Mn, [Mn II Mn III SiW 10 O 37 (OH)(H 2 O)] 6 − (Mn 2 -POM), [Mn II3 Mn III (H 2 O) 2 (PW 9 O 34 ) 2 ] 9 − (Mn 4 -POM), [Mn II4 Mn III2 Ge 3 W 24 O 94 (H 2 O) 2 ] 18 − (Mn 6 -POM-1), [Mn III2 Mn II4 ( µ 3 -O) 2 (H 2 O) 4 (B- β SiW 8 O 31 )(B- β -SiW 9 O 34 )( γ -SiW 10 O 36 )] 18 − (Mn 6 -POM-4), [{Mn III 3 Mn IV 4 O 4 (OH) 2 (OH 2 )} 2 (W 6 O 22 ) (H 2 W 8 O 32 ) 2 (H 4 W 13 O 46 ) 2 ] 26 − (Mn 14 -POM), [Mn II19 (OH) 12 (SiW 10 O 37 ) 6 ] 34 − (Mn 19 -POM) were prepared. First, the catalytic performance towards the water oxidation of six Mn-POMs was investigated in solution for the first time. Second, six Mn-POMs were fabricated on the surface of ITO electrode using layer-by-layer self-assembly (LBL) to form the composite films, which were characterized by UV-vis spectroscopy and cyclic voltammetry, and then the catalytic water oxidation performance of the composite films was studied and compared with that in solution via a series of controlled experiments, the results indicate that the Mn-POMs with three-dimensional structures, which contain variable valence Mn-O cluster similar to the structure of photocatalytic active center (PSII) exhibit better catalytic performance.


Introduction
In recent years, global warming has led to the continuous deterioration of the human living environment. COVID-19 has also impacted global trade, exacerbating the impact of the energy crisis on social life. The attention and demand of the international community for clean energy are increasing. Hydrogen energy is recognized as a clean energy source, and its combustion product is water. At present, hydrogen production by cracking fossil fuels is widely used, which also causes secondary emissions of greenhouse gases and a decrease in the quantity of fossil fuels. Therefore, if water (the largest potential hydrogen energy provider) can be split to produce hydrogen, a green and recyclable clean energy system will be established. However, there are some problems to be solved, such as high energy consumption and high operation cost. Therefore, it is very necessary to develop fast, economical and reliable catalysts.
POMs are metal oxygen cluster compounds composed of transition metal atoms (M) and oxygen atoms (O) connected by MO x polyhedra (tetrahedra, octahedra and icosahedra) in the way of common angle and common edge [1]. Because of this, the structures of POMs are changeable. The properties of POMs can be changed by adjusting the type and structure of transition metals. Heteropoly acids containing heteroatoms have richer structures due to the introduction of heteroatoms. POMs have excellent catalytic performance and reversible redox activity. They can be the potential catalysts for water cracking. At present, heteropoly acids containing ruthenium [2], manganese [3], cobalt, copper [4] have been proved to catalyze the decomposition of water. Compared with precious metal substituted polyacids, manganese, cobalt, nickel, iron and other metal substituted POMs are cheap and have a wider range of applications. Among them, the Mn-O clusters in Mn-POM have the same composition or structure as the photosynthetic center of green plants [5]. Therefore, we focus on the research of Mn containing POM catalysts. There are dozens of Mn containing POMs ranging from 1 to 40 [3,6]. At present, the catalytic water oxidation properties of some manganese containing POMs have been studied separately [7][8][9][10][11][12], the effects of amount, oxidation state of Mn ions and Mn-O cluster structure on the catalytic performance for water decomposition have hardly been reported. In order to select Mn-POMs with good catalytic performance, and find the influence of amount, oxidation state and Mn-O cluster structure, we selected six MnxPOMs as representative for comparison, including two manganese containing POMs: [Mn II Mn III SiW10O37(OH)(H2O)] 6− (Mn2-POM) [13] and four manganese containing POM: [Mn II 3Mn III (H2O)2(PW9O34)2] 9− (Mn4-POM) [14], six manganese containing POMs: [Mn II 4Mn III 2Ge3W24O94(H2O)2] 18− (Mn6-POM-1) [15], [Mn III [17] and 19 manganese containing POM [Mn II 19 (OH)12(SiW10O37)6] 34− (Mn19-POM) [18]. It is hoped that certain laws can be found for researchers' reference.
Mn2-POM is synthesized by one pot method from Mn(CH3COO)2·4H2O,Na10[a-SiW9O34]·15H2O and KCl at 80 °C [13]. The compound is a one-dimensional chain structure formed by Keggin type units containing two manganese, in which one manganese is divalent and one manganese is trivalent. The structure is shown in Figure 1a.
Mn6-POM-1 was synthesized from Na2WO4, GeO2, HCl, Mn(CH3COO)2·4H2O and imidazole at 90 °C [15]. The compound is composed of two Keggin units with three manganese substituted [Mn3GeW9O34] 4− and a six-vacancy bridged cluster [GeW6O26] 12− . The structure is shown in Figure 2a, in which each Keggin unit species substituted by [Mn3GeW9O34] 4− contains two divalent manganese and one trivalent manganese.  6 octahedra, green = MnO 6 octahedra, yellow = SiO 4 tetrahedra). Reprinted with permission from ref. [13]. Copyright 2013 Elsevier Ltd.; (b) The tetramanganese unit of [Mn 4 (H 2 O) 2 (PW 9 O 34 ) 2 ] 9− showing atomic labels. Reprinted with permission from ref. [14]. Copyright 1995 Elsevier Science Ltd. Mn  and {Mn4O4} cubane core with an appended "pendant" Mn ion. The structure of {Mn4O4} cubane core is very similar to that of PSII [16]. The compound was previously confirmed by our research group to have the ability of photo-electrocatalytic water decomposition [19]. The structure is shown in Figure 3. The compound contains four divalent manganese and two trivalent manganese.    [16]. The compound was previously confirmed by our research group to have the ability of photo-electrocatalytic water decomposition [19]. The structure is shown in Figure 3. The compound contains four divalent manganese and two trivalent manganese. and {Mn4O4} cubane core with an appended "pendant" Mn ion. The structure of {Mn4O4} cubane core is very similar to that of PSII [16]. The compound was previously confirmed by our research group to have the ability of photo-electrocatalytic water decomposition [19]. The structure is shown in Figure 3. The compound contains four divalent manganese and two trivalent manganese.    14 Mn centers, which are divided into a pair of Mn 7 cores. Each core is embedded in the shell of diamagnetic isopolytungstate ligand and well separated from the other [17]. The structure of Mn 7 is shown in Figure 4. Each Mn 7 includes a [Mn IV 4 O 4 ] 8+ cubic unit, and three Mn III are connected at the periphery. The compound contains 8 tetravalent manganese and 6 trivalent manganese. Mn14-POM was synthesized from precursors Mn12-acetate, Na2WO4, NaOAc·3H2O, HCl and dimethylamine hydrochloride at 100 °C. The compound contains a total of 14 Mn centers, which are divided into a pair of Mn7 cores. Each core is embedded in the shell of diamagnetic isopolytungstate ligand and well separated from the other [17]. The structure of Mn7 is shown in Figure 4. Each Mn7 includes a [Mn IV 4O4] 8+ cubic unit, and three Mn III are connected at the periphery. The compound contains 8 tetravalent manganese and 6 trivalent manganese.  Mn19-POM was synthesized from MnCl2·4H2O, Na10[A-α-SiW9O34], NaOH, Na3PO4 and HCl at 70 °C [18]. The structure is shown in Figure 5. All 19 Mn II ions are located on the same plane to form a hexagonal structure based on edge shared MnO6 octahedron. In order to more intuitively understand the valence of Mn ions in MnxPOMs, we summarize it in Table 1.  Mn 19 -POM was synthesized from MnCl 2 ·4H 2 O, Na 10 [A-α-SiW 9 O 34 ], NaOH, Na 3 PO 4 and HCl at 70 • C [18]. The structure is shown in Figure 5. All 19 Mn II ions are located on the same plane to form a hexagonal structure based on edge shared MnO 6 octahedron.
Mn14-POM was synthesized from precursors Mn12-acetate, Na2WO4, NaOA HCl and dimethylamine hydrochloride at 100 °C. The compound contains a total o centers, which are divided into a pair of Mn7 cores. Each core is embedded in the diamagnetic isopolytungstate ligand and well separated from the other [17]. The s of Mn7 is shown in Figure 4. Each Mn7 includes a [Mn IV 4O4] 8+ cubic unit, and thr are connected at the periphery. The compound contains 8 tetravalent mangane trivalent manganese.  Mn19-POM was synthesized from MnCl2·4H2O, Na10[A-α-SiW9O34], NaOH, and HCl at 70 °C [18]. The structure is shown in Figure 5. All 19 Mn II ions are loc the same plane to form a hexagonal structure based on edge shared MnO6 octahe In order to more intuitively understand the valence of Mn ions in MnxPO summarize it in Table 1. In order to more intuitively understand the valence of Mn ions in Mn x POMs, we summarize it in Table 1. In order to verify that the Mn x POMs were synthesized according to the literature, their infrared spectra were measured. As shown in Figure 6, we can observe the main characteristic vibration frequencies at 991, 945, 898, 779 and 710 cm −1 (as shown in Figure 6a 19 -POM [18] in the literature, which proves that we have successfully prepared the above six Mn x POMs. There are some small differences between the measured and reported values, which may be caused by the different instrument used. In order to verify that the MnxPOMs were synthesized according to the literature, their infrared spectra were measured. As shown in Figure 6, we can observe the main characteristic vibration frequencies at 991, 945, 898, 779 and 710 cm −1 (as shown in Figure  6a), four weak peaks at 662, 642, 538 and 480 (internal diagram in Figure 6a); the main characteristic vibration frequencies at 1105, 1060, 944, 778, 620, 592 and 500 cm −1 (as shown in Figure 6b); the main characteristic vibration frequencies at 941, 875, 770, 696 and 458 and the weak peaks at 509 cm −1 (as shown in Figure 6c); the main characteristic vibration frequencies at 1626, 940, 877, 775, 725 and 509 cm −1 (as shown in Figure 6d); the main characteristic vibration frequencies at 1629, 1465, 1438, 1408, 1385, 1020, 943 and 750 cm −1 ; and the weak peaks at 676, 631, 597, 569, 487 and 467 cm −1 in the illustration (as shown in Figure 6e); The main characteristic vibration frequencies at 988, 945, 895, 793, 711, 646 and 536 cm −1 (as shown in Figure 6f) are consistent with the infrared characteristic peaks of Mn2-POM [13], Mn4-POM [14,20], Mn6-POM-1 [15], Mn6-POM-4 [16], Mn14-POM [17] and Mn19-POM [18] in the literature, which proves that we have successfully prepared the above six MnxPOMs. There are some small differences between the measured and reported values, which may be caused by the different instrument used.

X-ray Transmission Spectroscopy
In order to deeply understand the composition of Mn X -POMs and prove that they were successfully assembled on ITO, the XPS of six Mn X -POMs films electrodes were tested. As shown in Figures Figure S7c is the Mn2p diagram of [Mn 14 -POM] 5 . There is a narrow peak at 644 eV binding energy, corresponding to Mn 4+ [21], which proves the existence of Mn 4+ in Mn 14 -POM. Figures S3-S6c have a wide Mn2p 3/2 peak at 638 to 644 eV, which is different from the narrow peak shown in Figure S8c at 640.5 eV, which indicates the presence of is Mn 2+ . For the former, there is a wide peak due to the presence of Mn 2+ and Mn 3+ in a compound.

Cyclic Voltammetry
In order to further verify that Mn x POMs were correctly prepared, the electrochemical behavior of Mn x POMs in solution was studied by cyclic voltammetry (CV). In Figure 7a, it is found that there are two oxidation peaks of Mn 2 -POM at 0.5 and 1.2 V potentials, which are the oxidation peaks of Mn 2+/3+ and Mn 3+/4+ respectively. At the same time, there are three reduction peaks at 1.0, 0.85 and 0.4 V, which are Mn 4+/3+ , Mn 4+/2+ and Mn 3+/2+ respectively. It is consistent with the literature. In Figure 7b, it is found that there a peak of Mn 4 -POM at 0.9 V of oxidation peaks of Mn 2+/4+ and two reduction peaks at 1.1, and 0.7 V which are Mn 4+/3+ and Mn 4+/2+ respectively. In Figure 7c, it is found that an oxidation peaks of Mn 6

pH
In order to determine the optimal pH value of the subsequent catalytic experiment in aqueous solution and study the stability of the MnxPOMs at different pH, we tested the CVs of six MnxPOMs at pH 5/6/7. As shown in Figure 8, most MnxPOMs have obvious redox peaks at pH 5/6/7, indicating that the MnxPOMs are stable within this range, and the redox peak of Mn of the MnxPOMs moves to the negative potential direction with the increase of pH, indicating that there is proton migration in the redox process of Mn. The redox peak of compound Mn19-POM decreases with the increase of pH, indicating that the compound is relatively stable under acidic conditions.

pH
In order to determine the optimal pH value of the subsequent catalytic experiment in aqueous solution and study the stability of the Mn x POMs at different pH, we tested the CVs of six Mn x POMs at pH 5/6/7. As shown in Figure 8, most Mn x POMs have obvious redox peaks at pH 5/6/7, indicating that the Mn x POMs are stable within this range, and the redox peak of Mn of the Mn x POMs moves to the negative potential direction with the increase of pH, indicating that there is proton migration in the redox process of Mn. The redox peak of compound Mn 19 -POM decreases with the increase of pH, indicating that the compound is relatively stable under acidic conditions.

pH
In order to determine the optimal pH value of the subsequent catalytic experiment in aqueous solution and study the stability of the MnxPOMs at different pH, we tested the CVs of six MnxPOMs at pH 5/6/7. As shown in Figure 8, most MnxPOMs have obvious redox peaks at pH 5/6/7, indicating that the MnxPOMs are stable within this range, and the redox peak of Mn of the MnxPOMs moves to the negative potential direction with the increase of pH, indicating that there is proton migration in the redox process of Mn. The redox peak of compound Mn19-POM decreases with the increase of pH, indicating that the compound is relatively stable under acidic conditions.

Assembly of Thin Film Electrode
We use the layer by layer assembly method to assemble the MnxPOMs to the ITO electrode surface. See Section 3.3 for the assembly method. In order to verify the repeatability of the assembly method, we monitored the UV-Vis absorption of the films during the assembly process. At the same time, we also carried out secondary verification by electrochemical means.

UV-Vis Absorption Spectrum Test
As can be seen from Figure S1a, MnxPOMs has an absorption peak in the ultraviolet range of 243 to 257 nm. The growth process of [MnxPOMs] n was monitored by UV-vis spectroscopy. As shown in Figure 9 when comparing the absorption spectra of components MnxPOMs in the composite film with components MnxPOMs in the solution, it can be observed that the characteristic absorption peak of MnxPOMs appears in the spectrum of each layer, indicating that MnxPOMs has been assembled on the composite film. In addition, as shown in Figure S1c the absorbance value of MnxPOMs at the characteristic absorption peak is taken to draw the relationship with the number of composite film layers. It is found that there is a highly linear relationship between them, which proves that MnxPOMs grows evenly on the composite film.

Assembly of Thin Film Electrode
We use the layer by layer assembly method to assemble the Mn x POMs to the ITO electrode surface. See Section 3.3 for the assembly method. In order to verify the repeatability of the assembly method, we monitored the UV-Vis absorption of the films during the assembly process. At the same time, we also carried out secondary verification by electrochemical means.

UV-Vis Absorption Spectrum Test
As can be seen from Figure S1a, Mn x POMs has an absorption peak in the ultraviolet range of 243 to 257 nm. The growth process of [Mn x POMs] n was monitored by UV-vis spectroscopy. As shown in Figure 9 when comparing the absorption spectra of components Mn x POMs in the composite film with components Mn x POMs in the solution, it can be observed that the characteristic absorption peak of Mn x POMs appears in the spectrum of each layer, indicating that Mn x POMs has been assembled on the composite film. In addition, as shown in Figure S1c the absorbance value of Mn x POMs at the characteristic absorption peak is taken to draw the relationship with the number of composite film layers. It is found that there is a highly linear relationship between them, which proves that Mn x POMs grows evenly on the composite film.

Assembly of Thin Film Electrode
We use the layer by layer assembly method to assemble the MnxPOMs to the ITO electrode surface. See Section 3.3 for the assembly method. In order to verify the repeatability of the assembly method, we monitored the UV-Vis absorption of the films during the assembly process. At the same time, we also carried out secondary verification by electrochemical means.

UV-Vis Absorption Spectrum Test
As can be seen from Figure S1a, MnxPOMs has an absorption peak in the ultraviolet range of 243 to 257 nm. The growth process of [MnxPOMs] n was monitored by UV-vis spectroscopy. As shown in Figure 9 when comparing the absorption spectra of components MnxPOMs in the composite film with components MnxPOMs in the solution, it can be observed that the characteristic absorption peak of MnxPOMs appears in the spectrum of each layer, indicating that MnxPOMs has been assembled on the composite film. In addition, as shown in Figure S1c the absorbance value of MnxPOMs at the characteristic absorption peak is taken to draw the relationship with the number of composite film layers. It is found that there is a highly linear relationship between them, which proves that MnxPOMs grows evenly on the composite film.

OER of MnxPOMs Solution
Firstly, we compared the catalytic water oxidation performance of MnxPOMs under the same solution concentration using cyclic voltammetry. From Figure 10 Table 2 for specific values.   Table 2 for specific values.

OER of MnxPOMs Solution
Firstly, we compared the catalytic water oxidation performance of MnxPOMs under the same solution concentration using cyclic voltammetry. From Figure 10 Table 2 for specific values.   In order to fix the Mn x POMs on the electrode surface to reuse the catalyst, we chose the layer by layer assembly method to fabricate Mn x POMs on the ITO electrode surface. See Section 3.3 for detailed operation steps. We assembled multilayer thin films and monitored the electrochemical behavior of thin film electrodes. It can be seen from Figure 11a that the redox peak current density of Mn x POMs on the electrode surface increases with the increase of the number of assembly layers. It can be seen from the inner graph that the oxidation peak current density has a linear relationship with the number of assembly layers. Combined with the regular changes of ultraviolet spectra in Figure 9b,c, we can confirm the effectiveness of the layer by layer assembly method, and Mn x POMs multilayer thin film electrodes were successfully prepared. At the same time, we monitored the catalytic water oxidation performance of the thin film electrode by Linear Scanning Voltammetry (LSV). It can be seen from Figure 11b that the current density of the electrode catalytic water oxidation increases with the increase of the number of assembly layers. In order to facilitate comparison, we selected a representative [Mn x POM] 3 for horizontal comparison, as shown in Figure 12.  In order to fix the MnxPOMs on the electrode surface to reuse the catalyst, we chose the layer by layer assembly method to fabricate MnxPOMs on the ITO electrode surface. See Section 3.3 for detailed operation steps. We assembled multilayer thin films and monitored the electrochemical behavior of thin film electrodes. It can be seen from Figure 11a that the redox peak current density of MnxPOMs on the electrode surface increases with the increase of the number of assembly layers. It can be seen from the inner graph that the oxidation peak current density has a linear relationship with the number of assembly layers. Combined with the regular changes of ultraviolet spectra in Figure 9b,c, we can confirm the effectiveness of the layer by layer assembly method, and MnxPOMs multilayer thin film electrodes were successfully prepared. At the same time, we monitored the catalytic water oxidation performance of the thin film electrode by Linear Scanning Voltammetry (LSV). It can be seen from Figure 11b that the current density of the electrode catalytic water oxidation increases with the increase of the number of assembly layers. In order to facilitate comparison, we selected a representative [MnxPOM]3 for horizontal comparison, as shown in Figure 12. As shown in Figure 12a, we compared the OER performance of [Mn x POM] 3 thin film electrode. At E = 1.4 V, Mn 14 -POM has the highest current density (250.97 µA cm −2 ), followed by Mn 19 -POM. See Table 3 for specific values. We believe that the catalytic current of Mn x POM may depend on the amount of Mn in Mn x POM, Mn valence and material structure. However, according to our experimental results, Mn 14 -POM current density is greater than Mn 19 -POM, and Mn 2 -POM current density is greater than Mn 6 -POM. The amount of Mn in Mn x POM is not a decisive factor. We believe that among the six manganese containing POMs, Mn 14 -POM has the largest current density, the smallest catalytic peak starting potential (0.92 V) and the highest catalytic performance, which should be attributed to its Mn-O cluster cubic structure and its manganese in a higher oxidation state. The Mn-O cluster cubic structure is similar to that of the PSII active center, [Mn 4 CaO 5 ]. In fact, there are two trivalent Mn and two tetravalent Mn in [Mn 4 CaO 5 ], and Mn 14 -POM is similar to it. Among them, the view of Mn-O cluster cubic structure is also confirmed in the comparison of two Mn 6 -POM with the same number and the same valence of Mn. The current density of Mn 6 -POM-4 (29.73 µA cm −2 )with Mn-O cluster cubic structure is more than twice that of Mn 6 -POM (13.22 µA cm −2 ) at 1.4 V potential, and the peak onset potential is 1.07 V, which is 0.15 V ahead of Mn 6 -POM-1.
In Table 4, some study reports regarding the electrocatalytic activity of manganese containing POMs for water decomposition are listed for comparison. They all adopt the layer by layer assembly method, which is comparable. In contrast, Mn 14 -POM with Mn-O cubic structure shows better catalytic performance.
Catalysts 2022, 12, x FOR PEER REVIEW 12 of 18 As shown in Figure 12a, we compared the OER performance of [MnxPOM]3 thin film electrode. At E = 1.4 V, Mn14-POM has the highest current density (250.97 μA cm −2 ), followed by Mn19-POM. See Table 3 for specific values. We believe that the catalytic current of MnxPOM may depend on the amount of Mn in MnxPOM, Mn valence and material structure. However, according to our experimental results, Mn14-POM current density is greater than Mn19-POM, and Mn2-POM current density is greater than Mn6-POM. The amount of Mn in MnxPOM is not a decisive factor. We believe that among the six manganese containing POMs, Mn14-POM has the largest current density, the smallest catalytic peak starting potential (0.92 V) and the highest catalytic performance, which should be attributed to its Mn-O cluster cubic structure and its manganese in a higher oxidation state. The Mn-O cluster cubic structure is similar to that of the PSII active center, In Table 4, some study reports regarding the electrocatalytic activity of manganese containing POMs for water decomposition are listed for comparison. They all adopt the layer by layer assembly method, which is comparable. In contrast, Mn14-POM with Mn-O cubic structure shows better catalytic performance.  It can be seen from Table 4 that Mn 14 -POM in this paper has better OER catalytic properties compared with other manganese containing POMs tested by the same method. At the same time, in order to understand the practical prospect of our materials, we have compared them with precious metal commercial reference materials in Table 5. The experimental environment is neutral. It is found that there are still some gaps that can be improved. The durability and stability of materials have also been studied., taking Mn 14 POM as an example. As shown in Figure 13, we compared the electrocatalytic oxidation performance of a thin film electrode before and after standing for 48 days. We can observe that after standing for 48 days, the peak potential of electrocatalytic oxidation of the thin film electrode moves slightly forward, but the catalytic current density at 1.5 V potential does not change significantly, it shows that the catalytic components in the thin film electrode can exist stably on the electrode surface for a long time. Then, the electrode was electrolyzed at a constant potential for 1200 s, and the LSV test was carried out again. It was found that the peak potential of electrocatalytic oxidation of the thin film electrode moved slightly forward, and the catalytic current density decreased by 22%, which was caused by the partial falling off of the main catalytic components during the test. The stability of other Mn x POMs materials can be found in the supporting information.

LBL Assembled Composite Film Modified Electrode
Before assembly, the substrate (ITO electrode) needed to be cleaned according to the literature [23]. The ITO was immersed in 1 M NaOH: CH 3 CH 2 OH (V:V = 1:1) mixed solution, ultrasonically for 20 min, then cleaned with water, and then dried with nitrogen for standby. Then, the composite film was assembled on the clean substrate by the LbL assembly method. First, the substrate was immersed in the positively charged PDDA (5%) solution and left to stand for 20 min. Second, the substrate was immersed in the negatively charged PSS solution for 20 min. Third, the substrate was immersed in the PDDA (5%) aqueous solution for 20 min. Fourth, the substrate was immersed in 0.5 mM Mn x POMs solution with a negative charge for 20 min. After each assembly, the substrate was washed with water 3-5 times to remove the physical adsorption substances, and then dried with nitrogen flow. After the above steps, the monolayer [PDDA/PSS/PDDA/Mn x POM] composite films were prepared. Different layers of [PDDA/PSS/PDDA/Mn x POM]n-ITO (n = 1-4) composite film modified electrodes were prepared by repeating steps 3 and 4.

UV-Vis Absorption Spectra
The fabrication process of the samples is the same as that of the LBL assembling the multilayer film modified electrode in Section 3.3. The only difference is that ITO is replaced by a quartz slide. Firstly, the quartz glass was cleaned according to the literature method [24] (different from ITO), and then assembled according to the film assembly steps, monitoring the assembly process of composite films by UV-vis absorption spectrum.

Electrochemical Test
A series of experiments were performed to study the PEC catalytic activity for water oxidation, ITO/PDDA/PSS/[PDDA/Mn x POM]n was used as the working electrode, using Ag/AgCl as a reference electrode, and a platinum wire as a counter electrode. The buffer solution was obtained from 0.5 M pH 7 CH 3 COONa (NaAc) solutions adjusted by CH 3 COOH (HAc). Before the electrochemical experiments, the oxygen in the electrolyte was removed by purging high-purity nitrogen. All the electrochemical measurements were performed on an electrochemical workstation (CHI 611E, CH Instruments Co., Ltd., Shanghai, China) at room temperature.

Conclusions
Via appropriate choice, six Mn-containing POMs, with different structures, composition, Mn-O clusters and oxidation state of Mn, were prepared and their electrocatalytic water oxidation performance was investigated under the same experiment conditions in solution and composite film. The fabrication of the composite films was characterized by UV-vis spectra and CVs, confirming that the Mn x POMs have been fabricated on the composite films and their electrochemical properties have been maintained very well in the composite films. Through a series of controlled experiments, the following rules were obtained: the amount and the oxidation state of Mn ions in Mn x POM affect the electrocatalytic water oxidation performance; at the same time, the structure of Mn-O clusters in Mn x POM also affects the electrocatalytic water oxidation performance. After the comparison, it is found that among the six Mn x POMs, Mn 14 -POM displays the highest electrocatalytic performance towards water oxidation. Such observation indicates the oxidation state of Mn and the structure of Mn-O cluster cubic structure are very important factors impacting electrocatalytic performance for the water oxidation as the oxidation state of Mn and the structure of Mn-O cluster in Mn 14 -POM are the same as that in the photocatalytic active center (PSII). Therefore, Therefore, this study provides a reference for further screening water oxidation catalysts with high performance.

Supplementary Materials:
The following supporting information can be downloaded at https: //www.mdpi.com/article/10.3390/catal12020160/s1, Figure S1: (a) UV-vis absorption spectra of MnxPOMs (5 µM) in the aqueous solutions; (b) UV-vis spectra for the [MnxPOMs]n film on quartz slide; (c) relationship of the absorbance value and layer number at λ = 243~257 nm; Figure S2: LSV curves of OER of [Mn2-POM]1 and [MnxPOM]3 film electrode in NaAc/HAc buffer (0.5 M) at pH = 7, initial, after 48 days, and after potentiostatic electrolysis;; Table S1: Summary of the radio of current density and the difference of initial potential of OER;; Figure

Data Availability Statement:
The data presented in this study are available on request from the corresponding author. The data are not publicly available due to the need of follow-up research.