Using nickel manganese oxide catalysts for eﬃcient water oxidation †

Nickel–manganese oxides with variable Ni:Mn ratios, synthesised from heterobimetallic single-source precursors, turned out to be efficient water oxidation catalysts. They were subjected to oxidant-driven, photo-and electro-catalytic water oxidation showing superior activity and remarkable stability. In addition, a structure– activity relation could be established.

The detailed bonding states of Ni, Mn and O were further characterised by X-ray Photoelectron Spectroscopy (XPS).The XPS core level spectra of Ni2p 3/2 and Ni2p 1/2 for Ni 6 MnO 8 , NiMn 2 O 4 and NiO exhibited peaks at binding energy (BE) of B854.5 eV and 872.2 eV corresponding to Ni 2+ while the peaks of MnNi 2 O 4 shifted to the higher energy of 856 eV and 873.8 eV which can be assigned as the mixture of Ni 2+/3+ (Fig. S16, ESI †). 21The Mn2p spectra of Ni 6 MnO 8 and MnNi 2 O 4 displayed two major peaks for Mn2p 3/2 and Mn2p 1/2 at BE B643.5 eV and B655.0 eV that are consistent Mn 4+ species whereas for NiMn 2 O 4 , the peak positions were shifted to lower energy of 642.4 eV and 653.9 eV and are characteristic for Mn 3+ (Fig. S17, ESI †). 22The O1s spectrum for all oxides exhibited a major O 2À peak assignable to bridging oxides with two smaller ones that could be attributed to the surface oxygen, physi-and chemisorbed water at or near the surface and to the hydroxide species (Fig. S18, ESI †). 19xidant-driven water oxidation experiments (see ESI † for details) were conducted with all catalysts (Fig. S19, ESI †) in deoxygenated aqueous solution of 0.5 M ceric ammonium nitrate (CAN) and the rate of the oxygen evolution was calculated from the slope of the linear fitting for the first 60 s.The Ni 6 MnO 8 was found to be extremely active with a maximum rate of 1.41 mmol O2 mol M À1 s À1 considering both nickel and manganese atoms are active, and was approximately thrice higher than the MnNi 2 O 4 (0.52 mmol O2 mol M À1 s À1 ).
However, for NiMn 2 O 4 , the rate was far lesser with the value of 0.19 mmol O2 mol M À1 s À1 and is comparable with the pure NiO (0.15 mmol O2 mol M À1 s À1 ).The surface area and the total number of active sites present on the catalyst play a crucial role in water oxidation.Therefore, the correlation of surface area normalised plots is shown in Fig. S20 (ESI †) and follows the same trend as that of total mass activity.The photo-catalytic water oxidation was performed in a phosphate buffer solution of pH 7 in the presence of [Ru(bpy) 3 ] 2+ (bpy = 2,2bipyridine) as a photosensitiser and S 2 O 8 2À as two electron acceptor (Scheme S1, ESI †).In a similar trend to the oxidant-driven water oxidation, the highest rate of oxygen evolution was exhibited by the nickel-rich Ni 6 MnO 8 with a value of 1.00 mmol O2 mol M À1 s À1 that was again 1.5 times higher than the other nickel-rich MnNi 2 O 4 phase (0.69 mmol O2 mol M À1 s À1 ) (Fig. 2).The rate of oxygen evolution for the nickel-diluted NiMn 2 O 4 was 0.44 mmol O2 mol M À1 s À1 while NiO showed only a limited activity (0.07 mmol O2 mol M À1 s À1 ).[10]20 After the Clark electrode experiments, a set of experiments for longer duration was also carried out separately (see ESI †) and the oxygen gas was collected in the head space of the reaction mixture was quantitatively analysed by a gas chromatograph (GC).A maximum oxygen yield of 0.08 mL h À1 of O 2 was detected for Ni 6 MnO 8 and 0.07 mL h À1 for MnNi 2 O 4 (Table S5, ESI †).Moreover, it is not enough to have catalysts that are extremely active but one of the indispensible criteria is also to know the fate of the catalyst after the photo-catalytic experiments, and therefore, PXRD and HRTEM investigation were conducted on high performance Ni 6 MnO 8 and MnNi 2 O 4 catalysts.From PXRD and HRTEM images (Fig. S23, ESI †),  it is clear that the crystallinity and the morphology of the nickel-rich manganese oxides catalysts were preserved and stayed intact unveiling the enhanced stability.
The electro-catalytic measurements (see ESI †) were performed in alkaline 0.1 M KOH solution using Cyclic Voltammetry (CV) at a scan rate of 20 mV s À1 .The current for all electrodes were initially increased during the first few cycles and reached a steady value after 50 cycles (Fig. S24, ESI †), and then stayed stable and were unchanged even after additional cycling.As shown in Fig. 3, For Ni 6 MnO 8 , the anodic current started growing at 1.54 (vs. the reversible hydrogen electrode, RHE).The maximum current density of 5.85 mA cm À2 was attained at 1.87 V. Similarly, for the MnNi 2 O 4 and NiMn 2 O 4 , the current started increasing at 1.58 and 1.60 V, and the highest current density was achieved at 2.83 mA cm À2 and 1.25 mA cm À2 at 1.87 V, respectively.Interestingly, for NiO, the current started growing at 1.40 V itself and the CV's featured a pair of anodic and cathodic peaks centred B1.5 V vs. RHE corresponding to the oxidation of NiO (NiO + OH À À 1e À -NiOOH), followed by a current due to O 2 evolution. 23Electrodes were also preconditioned and forward and backward scans were performed, with respect to the NiO/NiOOH redox reaction of NiO (Fig. S25, ESI †). 16,24It could also be seen that for the  S6, ESI †).25,26 Also similar trend was extended when normalised with the surface area suggesting that more active sites are available on the surface of NiO than the nickel manganese oxides (Fig. S26).Tafel slopes were extracted in the potential range of 1.55 to 1.80 V and a Tafel slope of 65 mV per decade was achieved for NiO whereas 88 mV per decade for Ni 6 MnO 8 associated with a rate determining chemical step preceded by a reversible electrochemical step at equilibrium (Fig. S27, ESI †).Increase in the apparent Tafel slope values were seen for MnNi 2 O 4 and NiMn 2 O 4 that could correspond to a change in the reaction mechanism but would also be expected if mass or ion transport limitations became significant. 27,28However, from the above electrochemical behaviour, it can be inferred that a higher content of Ni ions in the structure leads to lower Tafel slopes and thus, beneficial electro-catalytical properties.
Furthermore, to test the stability of all catalysts, chronoamperometric experiments were carried out (Fig. S28, ESI †) and the current values for NiO and NiMn 2 O 4 were maintained over the period of 15 hours.In the case of Ni 6 MnO 8 , increased current values were achieved demonstrating the exceptional stability of catalysts on a long run.On the other hand, a slight decrease in currents was observed for MnNi 2 O 4 .
After the long-term stability tests, the electrodes were further characterised by TEM and CV.HRTEM images of NiO, Ni 6 MnO 8 and MnNi 2 O 4 revealed that an amorphous shell of NiOOH appears on the surface of the catalysts, which has already been well described for the Ni based catalysts (Fig. S29, ESI †). 16,23fter chronoamperometry, the NiO electrode was subjected to CV attaining a lower overpotential (370 mV at 1 mA cm À1 ) with slightly lower current density, which unveils the impressive nature of the catalyst with prolonged durability (Fig. S30, ESI †).In addition to the alkaline media, the NiO catalyst was also studied in neutral (pH 7) and slightly basic (pH 9) conditions using phosphate and borate buffers and in KOH solution of pH 11, but only resulting into lower activity (Fig. S31, ESI †).The determined Tafel slope at pH 11 was lower than pH 13 elucidating slower kinetics at lower pH (Fig. S32, ESI †).
Based on the higher activity of nickel-rich manganese oxides for oxidant-driven and photo-catalytic water oxidation (Ni 6 MnO  ) and may be considered as rock-salt structure where 6/8 of octahedral sites are occupied by Ni 2+ atoms and 1/8 by Mn 4+ atoms, and by vacancies. 29The vacancies are ordered in the alternative (111) planes (Fig. 4a).Under oxidant-driven and photochemical conditions, not only the Ni 6 MnO 8 provides more active sites due to the presence of higher number of Ni 2+ as active centres that are supported and stabilised by Mn 4+ but also an additional (extra) hole density drives this reaction efficiently.However, both MnNi 2 O 4 and NiMn 2 O 4 crystallise in cubic (space group Fd3m) system and belong to the spinel type (AB 2 O 4 ) structure (Fig. 4b). 30,31The Mn 4+ ions occupy the tetrahedral sites and the octahedral sites are preferred by Ni 2+/3+ for MnNi 2 O 4 whereas the tetrahedral sites are occupied by Ni 2+ and octahedral by Mn 3+ for NiMn 2 O 4 .3][34] The NiO (cubic Fm3m) adopts a rock-salt structure 35 similar to Ni 6 MnO 8 (Fig. 4c) with octahedral Ni 2+ and O 2À and perhaps because of unavailable support of manganese, displays limited activity.The situation is partly reversed in the electrochemical water oxidation.The NiO exhibits highest activity followed by nickel manganese oxides.This is due to a large amount of amorphous NiOOH, where Ni 3+ is hexa-coordinated (Fig. 4d), 36 is formed on the surface of the electrodes (as shown by TEM and XPS) under electrochemical conditions and the resulting amorphous phase is known to be active for water oxidation by making the system very efficient and has been already well established for other nickel oxide and hydroxides. 16,23,25Interestingly, for Ni 6 MnO 8 , MnNi 2 O 4 , and NiMn 2 O 4 lesser amount of NiOOH is generated depending on the amount of nickel present in the catalysts.Therefore, it can be concluded that for oxidant-driven and photo-catalytic water oxidation Ni 6 MnO 8 is efficient due to the higher amount of Ni active sites stabilised by manganese and higher structural-hole density whereas amorphous NiOOH seems to be crucial for electro-catalytic water oxidation due to its structural features.
In conclusion, we investigated for the first time, the oxidantdriven, photochemical and electrochemical water oxidation employing nickel manganese oxide-based catalysts (Ni 6 MnO 8 , MnNi 2 O 4 , NiMn 2 O 4 ) with various Ni : Mn ratios and morphologies, starting from well-defined heterobimetallic nickel manganese SSPs; their activities were compared with NiO.Nickel-rich manganese oxides were found to be highly efficient with very high activity for oxidant-driven and photo-catalytic water oxidation whereas NiO exhibited higher performance and remarkable stability for electrocatalytic water oxidation.Based on the crystallographic aspects, a structure-activity relationship could be deduced from structural features of the oxide systems.The latter relationship deduced here can help to predesign new material to boost efficiency of water oxidation.
Financial support by the BMBF (L2H project) and the DFG (Cluster of Excellence UniCat) is gratefully acknowledged.O. Levy would like to thank Einstein Foundation Berlin for the financial support and Prof. David Avnir for the helpful discussions.
Fig. S21, ESI †).Surface normalisation discloses that the values for MnNi 2 O 4 are superior to Ni 6 MnO 8 4 NiMn 2 O 4 c NiO due to their lower surface area (Fig. S22, ESI †).Comparison of the mass and surface normalised activity with other reported catalysts confirmed that the diluted-manganese oxide based Ni systems produced higher oxygen evolution than most of the known active nickel and manganese based catalysts (Table