The recent synthetic and mechanistic progress in molecular and heterogeneous water oxidation catalysts highlights the new, overarching strategies for knowledge transfer and unifying design concepts.

The efficient transfer of photogenerated carriers and improved stability against corrosion are essential to maximize the performance of photoanodes. Herein, a reduced catalytic layer formed on a TiO2 protected BiVO4 (R-TiO2@BiVO4) photoanode has been prepared for progress on both fronts. Specifically, R-TiO2@BiVO4 photoanodes at pH 8 displayed a high photocurrent of 2.1 mA cm–2 at 1.23 VRHE and a more negative onset potential of 234 mVRHE compared to pristine BiVO4. We here discovered two surface states on BiVO4 photoanodes through photoelectrochemical impedance studies. In contrast, only one of them, located at higher potential, appeared on oxygen-vacancy-rich R-TiO2@BiVO4 photoanodes. For BiVO4 photoanodes, the first surface state (SS1) is located near the onset potential (∼0.45 VRHE), while the second surface state (SS2) sits near the water oxidation potential (∼1.05 VRHE). However, SS1 at lower energetics, which originated from water oxidation intermediates with slow kinetics, is passivated in R-TiO2@BiVO4 photoanodes. In contrast, the hole densities in SS2 at higher energetics were greatly enhanced in R-TiO2@BiVO4 photoanodes, due to the increased accumulation of intermediates with fast water oxidation kinetics. Therefore, SS2 is proposed as a reaction center, which is related to the amount and occupancy of oxygen vacancies. Additionally, surface recombination centers in BiVO4 photoanodes are passivated by TiO2, which prevents electron trapping into the irreversible surface conversion of VO2 + to VO2 +. These observations provide fundamental understanding of the role of surface states and of the function of oxygen vacancies in BiVO4 photoanodes. Our study offers detailed insight into key strategies for optimal photoelectrochemical performance through surface property tuning.

The development of efficient and noble-metal-free electrocatalysts for the challenging oxygen evolution reaction (OER) is crucial for sustainable energy solutions. In this work, a facile coprecipitation method, followed by thermal postsynthetic treatment in N2/air, was developed to synthesize molybdenum-doped -Mn2O3 materials (Mn2O3:1.72%Mo, Mn2O3:2.64%Mo, Mn2O3:32.23%Mo, and Mn2O3:49.67%Mo) as low-cost water-oxidizing electrocatalysts. Powder X-ray diffraction (PXRD), extended X-ray absorption fine structure (EXAFS), X-ray photoelectron spectroscopy (XPS), and highresolution transmission electron microscopy (HRTEM) investigations showed the presence of strong distortions in the molybdenum-doped -Mn2O3 host lattice (Mn2O3:2.64%Mo) and an average oxidation state of Mn2.8+. Several test assays demonstrated that these structural features significantly promote the OER activity. Mn2O3:2.64%Mo was found to exhibit very good activity among the series in cerium ammonium nitrate (CAN)-assisted water oxidation with a maximum turnover frequency (TOF) of 585 mol O2 m-2 h-1, which is a 15-fold improvement of the pure -Mn2O3 activity and higher than the value of the previously reported benchmark Mn-based catalyst, birnessite. The optimized catalyst (Mn2O3:2.64%Mo) excelled through a low onset potential (300 mV) and a promising overpotential of 570 mV for OER at a current density of 10 mA cm-2, which is only 20 mV above that of the noble metal benchmark RuO2 electrode and competitive with that of the most active Mn-based OER catalysts reported to date. Electrochemical impedance spectroscopy (EIS) studies demonstrated that the catalytically active surface area of Mn2O3:2.64%Mo is much higher than that of -Mn2O3 for the OER at the applied potential. In addition, stability during 30 h without degradation was achieved, which exceeds that of a wide range of current noble-metal-free electrocatalysts. Our study provides a facile and effective approach for the preparation of economical and high-performance manganese-based electrocatalysts for water oxidation.

The design of molecular oxygen-evolution reaction (OER) catalysts requires fundamental mechanistic studies on their widely unknown mechanisms of action. To this end, copper complexes keep attracting interest as good catalysts for the OER, and metal complexes with TMC (TMC = 1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane) stand out as active OER catalysts. A mononuclear copper complex, [Cu­(TMC)­(H2O)]­(NO3)2 (TMC = 1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane), combined both key features and was previously reported to be one of the most active copper-complex-based catalysts for electrocatalytic OER in neutral aqueous solutions. However, the functionalities and mechanisms of the catalyst are still not fully understood and need to be clarified with advanced analytical studies to enable further informed molecular catalyst design on a larger scale. Herein, the role of nanosized Cu oxide particles, ions, or clusters in the electrochemical OER with a mononuclear copper­(II) complex with TMC was investigated by operando methods, including in situ vis-spectroelectrochemistry, in situ electrochemical liquid transmission electron microscopy (EC-LTEM), and extended X-ray absorption fine structure (EXAFS) analysis. These combined experiments showed that Cu oxide-based nanoparticles, rather than a molecular structure, are formed at a significantly lower potential than required for OER and are candidates for being the true OER catalysts. Our results indicate that for the OER in the presence of a homogeneous metal complex-based (pre)­catalyst, careful analyses and new in situ protocols for ruling out the participation of metal oxides or clusters are critical for catalyst development. This approach could be a roadmap for progress in the field of sustainable catalysis via informed molecular catalyst design. Our combined approach of in situ TEM monitoring and a wide range of complementary spectroscopic techniques will open up new perspectives to track the transformation pathways and true active species for a wide range of molecular catalysts.

A tridentate benzoxazole-containing aminophenol ligand HL(BAP) was synthesized and complexed with Cu(II). The resulting Cu(II) complexes were characterized by X-ray, IR, UV-vis-NIR spectroscopies, and magnetic susceptibility studies, demonstrating that the ligand is oxidized to the o-iminosemiquinone form [L(BIS)](-) in the isolated complexes. L(BIS)Cu(II)Cl exhibits a distorted tetrahedral geometry, while L(BIS)Cu(II)OAc is square pyramidal. In both solid state structures the ligand is coordinated to Cu(II)via the benzoxazole, as well as the nitrogen and oxygen atoms from the o-iminosemiquinone moiety. The chloride, or acetate group occupies the fourth and/or fifth positions in L(BIS)Cu(II)Cl and L(BIS)Cu(II)OAc, respectively. Magnetic susceptibility measurements indicate that both complexes are diamagnetic due to antiferromagnetic coupling between the d(9) Cu(II) centre and iminosemiquinone ligand radical. Electrochemical studies of the complexes demonstrate both a quasi-reversible reduction and oxidation process for the Cu complexes. While L(BIS)Cu(II)X (X = Cl) is EPR-silent, chemical oxidation affords a species with an EPR signal consistent with ligand oxidation to form a d(9) Cu(II) iminoquinone species. In addition, chemical reduction results in a Cu(II) centre most likely bound to an amidophenoxide. Mild and efficient oxidation of alcohol substrates to the corresponding aldehydes was achieved with molecular oxygen as the oxidant and L(BIS)Cu(II)X-Cs2CO3 as the catalyst.