A Keggin Polyoxometalate Shows Water Oxidation Activity at Neutral pH: POM@ZIF‐8, an Efficient and Robust Electrocatalyst

Abstract

Keggin-type polyoxometalate anions [XM 12 O 40 ] nÀ are versatile,a st heir applications in interdisciplinary areas show.T he Keggin anion [CoW 12 O 40 ] 6À turns into an efficient and robust electrocatalyst upon its confinement in the welldefined void space of ZIF-8, am etal-organic framework (MOF). [H 6 CoW 12 O 40 ]@ZIF-8 is so stable to water oxidation that it retains its initial activity even after 1000 catalytic cycles. The catalyst has at urnover frequency (TOF) of 10.8 mol O 2 (mol Co) À1 s À1 ,o ne of the highest TOFs for electrocatalytic oxygen evolution at neutral pH. Controlled experiments rule out the chances of formation and participation of CoO x in this electrocatalyic water oxidation.Photosynthesis,one of the most fundamental and essential processes to sustain life on earth, works on the principle of trapping solar energy via electron-hole pair formation. [1,2] This energy is ultimately utilized in splitting of water molecules into H 2 and O 2 .W ater splitting (WS) has emerged as apromising source of clean and sustainable energy. [3] Water oxidation (WO; 2H 2 O!4H + + O 2 + 4e À ; E 0 = 1.23 V), being the bottleneck process of WS owing to its high thermodynamic potential and high overpotential h,requires an efficient and stable WO catalyst (WOC). [4,5] In last few decades, various research groups have contributed towards understanding of the process of WO and designing of robust and efficient WOCs. [1][2][3][4][5][6][7] Recently enormous efforts have been devoted to prepare first-row transition-metal-ion (particularly,c obalt)-based inexpensive WOCs. [8][9][10][11][12][13][14][15][16][17][18] Development of polyoxometalates (POMs) as WOCs came as am ajor breakthrough in this regard. [19][20][21][22][23][24][25][26][27] Thek ey structural features of POMs for POM-WOCs are their complete inorganic skeleton and the scope of fast, reversible electron transfer (resulting into fast WO kinetics). [14,28] In last few years,a long with remarkable progress in this field, questions also arose about the true catalyst species (formation and participation of CoO x as true catalyst). [28][29][30][31][32] Meanwhile,H ill, Geletii, and their coworkers [23] confirmed that their starting Co-POM compound [Co 4 (H 2 O) 2 (a-PW 9 O 34 ) 2 ] 10À was atrue molecular WO catalyst (not CoO x ). Keggin-type POMs have the general formula of [XM 12 O 40 ] nÀ (X = Co 2+ ,P 5+ ,S i 4+ ,e tc.;M= W 6+ ,M o 6+ ,e tc.) and are the most stable structural variant among all POMs. [33][34][35][36] Despite ac onsiderable development in the field of POM-WOC,n ot much attention has been paid to Keggin (as such)-WOC systems as most of them do not possess suitable WOC-active site (unless it has one or more substitution of Mb yaWO-active transition-metal ion). [28,[37][38][39] Recently,S ong et al. [37] showed K 6 [CoW 12 O 40 ]t ob ei nactive to photocatalytic WO,w hile K 7 [Co III Co II (H 2 O)W 11 O 39 ], was found to be active owing to ap eripheral Co III -aqua coordination complex. We thus made an attempt to prepare WOC from [CoW 12 ...

MOF-templated cobalt nanoparticles embedded in nitrogen-doped porous carbon: a bifunctional electrocatalyst for overall water splitting

“…Given these aspects, there is an essential necessity for the development of stable, cost-effective, durable and highly efficient bifunctional electrocatalysts for sustainable overall water splitting. A variety of alternatives including, heteroatom-doped nanostructured carbon materials 14 and hybrid electrocatalysts based on transition metal oxides 15 /sulphides 16 /selenides 17 / phosphides, 18,19 supported by carbon materials were developed that demonstrated promising electrocatalytic activity for the oxygen reduction reaction (ORR), 20 OER [21][22][23][24][25][26] and HER. [27][28][29] Interestingly, among transition metals, Co-, Ni-and Fe-based hybrid electrocatalysts such as CoSe, 30 NiCo 2 S 4 , 31 FeNiOH/ NF, 32 iron phosphide nanotubes (IPNTs), 33 Co-P, 34 cobalt-cobalt oxide/N-doped carbon hybrids (CoO x @CN), 35 porous Co phosphide/phosphate thin lms (PCPTFs) 36 and CoP NA/CC 37 were explored as bifunctional electrocatalyst for overall water splitting.…”

Reactivity and Stability of Polyoxometalate Water Oxidation Electrocatalysts

“…Prime examples were reported by Sakai and colleagues who used the species [H6Co III Mo6O24H6] 3− ({CoMo6}) and [H4Co III 2Mo10O38] 6− ({Co2Mo10}) as homogeneous, light-driven WOCs [42] and by Das and colleagues who explored [Co II W12O40] 6-embedded in the metal organic framework ZIF-8 as electrochemical WOC. [31] Remarkably, both systems show significant oxygen evolution activity while the expected cobalt reaction sites are "locked" within the POM framework and are not accessible for the binding of external water ligands. While for the cobalt molybdate systems, initial electrochemical data is required to rationalize possible water oxidation pathways, this data is available for the co-tungstate [Co II W12O40] 6-.…”

“…Zeolitic imidazolate framework (ZIF) supports: as described above, pioneering studies have used ZIFs for the immobilization of the POM-WOC [CoW12O40] 6-(see Section 2). [31] The resulting POM-WOC@ZIF composite was coated on glassy carbon electrodes for water oxidation studies in aqueous 0.1 M Na2SO4 (pH 8). In addition to the results discussed above, chronoamperometry showed that the catalytic stability was sustained for at least 8 h (at E = 1.2 V vs NHE) and the initial activity was retained even after 1000 CV cycles between 0.2 V and 1.2 V vs NHE).…”

“…[26] While some reports recently discussed the use of POMs without water binding sites for WOC, further studies are required to fully understand the oxygen evolution processes reported for these systems. [29][30][31] (3) The proton-coupled electron transfer reactions required for water oxidation can be facilitated by nearby proton transfer sites, i.e. Bronsted bases capable of supporting the release and transfer of protons from the water substrate.…”

Synthesis and functionality of transition metal-centered polyoxometalate-based crystalline framework materials

“…效水氧化电催化剂 [25] Figure 1 (Color online) POM was encapsulated in ZIF-8 for construction of high performance electrocatalyst for water oxidation [25] 配位的四面体、五配位的四方锥或扭曲的三角双锥、 六配位的扭曲八面体或四方双锥), 使得金属钒原子 被广泛地用于构筑结构多样的多金属氧簇 [27] .…”