The development of new energy materials that can be utilized to make renewable and clean fuels from abundant and easily accessible resources is among the most challenging and demanding tasks in science today. Solar-powered catalytic water-splitting processes can be exploited as a source of electrons and protons to make clean renewable fuels, such as hydrogen, and in the sequestration of CO2 and its conversion into low-carbon energy carriers. Recently, there have been tremendous efforts to build up a stand-alone solar-to-fuel conversion device, the "artificial leaf", using light and water as raw materials. An overview of the recent progress in electrochemical and photo-electrocatalytic water splitting devices is presented, using both molecular water oxidation complexes (WOCs) and nano-structured assemblies to develop an artificial photosynthetic system.

In situ Raman and surface-enhanced Raman scattering (SERS) are established vibrational spectroscopic techniques with a wide range of applications in the field of chemical, material and life sciences. Their particular characteristics make them especially useful when dealing with catalytic water oxidation at anodes. The in situ characterization of the fate of electrocatalysts (whether molecular or oxide materials) employed under reaction conditions is crucial to determine the chemical identity and the physical state of the actual catalytic species. Such studies also help in both, attaining mechanistic insights underlying the catalytic reaction and confirming/discarding the possibility of molecular to colloidal or heterogeneous phase conversions taking place prior or under turnover conditions. This perspective article highlights the use of in situ Raman and SERS as principal spectroscopic tools in the electrocatalysis field by means of recent contributions where they are employed to in operando characterize both molecular and oxide-based water oxidation electrocatalysts. These in situ spectroscopic measurements support in assessing both the progressive oxidation and the structural evolution of the employed catalytic species under electrochemical conditions. Therefore, this article provides an informative guideline for developing in situ spectroelectrochemical methods to study and characterize water oxidation catalysis at working anodes.

Catalytic water splitting using solar energy represents an attractive potential solution for affordable and renewable energy. [1,2] To construct a (photo)electrochemical H 2 /O 2 evolution system, oxygen evolving catalysts (OECs) need to be immobilized on a conducting surface.[1] Many metal complexes containing single or more catalytic sites have been tested for water oxidation; [2][3][4][5][6][7] however, the design and implementation of a stable and efficient molecular water oxidation system that operates at high catalytic turnover number (TON) and frequency (TOF) for extended periods of controlled-potential electrolysis (CPE), with moderate overpotential and high current density, are challenging. [8][9][10] Herein we disclose robust immobilized [(L 2 bpy)Ir2+ (L is -PO 3 H 2 or -COOH, bpy is 2,2'-bipyridine, Cp* is pentamethylcyclopentadiene) complexes on ITO (indium tin oxide) surface (ITO/Cat) for electrocatalytic water oxidation ( Figure 1). The aqua complexes were obtained by Cl to H 2 O ligand exchange before immobilization on the ITO surfaces. The mono-iridium catalysts are modified with carboxylate and phosphonate linkers that are known to anchor covalently on ITO.[1, 9] At 1.75 V (vs. NHE; NHE = normal hydrogen electrode) the system operates with a high TOF of 6.7 s À1 and has TONs of more than 210 000, well in excess of the maximum TON of 28 000 reported before.[9] The oxygen generation current densities are higher than 1.70 mA cm À2 , which is one to two orders of magnitude higher than the reported densities of approximately 50 mA cm À2 .[ 2+ molecular complex is a highly competent catalytic system for electrochemical oxygen evolution.The synthesis and characterization of the complexes are detailed in the Supporting Information ( Figure S1). [12,13] In situ ligand exchange from Cl to H 2 O and deprotonation are monitored by UV/Vis spectroscopy ( Figure S2 in the Supporting Information). CV of an ITO/Cat.Ir-COOH in aqueous acid (pH 1) shows a catalytic current wave at approximately 1.22 V that sharply grows until approximately 1.31 V, and leads to an O 2 evolution current with tiny oxygen 2+ (Cat.Ir-PO 3 H 2 and Cat.Ir-COOH for L = PO 3 H 2 and COOH, respectively) on ITO for electrochemical water oxidation. The metalstabilizing Cp* ligand is highlighted in brown-red.

near-neutral pH (pH = 6.7-6.8), the cobalt-bicarbonate-derived electrocatalyst (Co-Ci) is readily generated on conducting oxide electrode surfaces such as FTO (fl uorine-doped tin oxide) and ITO (indium tin oxide), or on a glassy carbon (GC) anode and show remarkable activity for water oxidation during extended period operation (Figure 1 ). At 1.37 V (vs NHE) in a HCO 3 − / CO 2 system at near-neutral pH, a stable oxygen evolution current density ( J ) of ≈2.0 mA cm −2 was obtained during constantpotential electrolysis (CPE) of water. The anodic current was sustained for many hours of electrochemical operation with no noticeable decrease in the performance. We also show that under neutral pH conditions, bicarbonate is an excellent electrolyte system that provides more stability to the electrodeposited Co-oxide derived water oxidation catalyst in comparison with the neutral phosphate buffer. As the Co-Ci is assembled in a CO 2 enriched environment, the catalytic system can operate along with a carbon dioxide reduction module in the same electrochemical setup to make liquid fuel products (such as formic acid or methanol). The catalytic system also shows remarkable activity in a clean bicarbonate electrolyte (no CO 2 bubbling) without having Co 2+ in the solution. Energy-dispersive X-ray (EDX) spectroscopy shows the presence of carbon (about 30%)

An effcient electrocatalytic Pd system, prepared via the AACVD method, is presented executing high activity water oxidation at 1.43 V vs RHE; η = 200 mV while exceeding the benchmark performance of IrO2.

Ab initio molecular dynamics simulations with an adaptive biasing potential are carried out to study the reaction path in mononuclear Ru catalysts for water oxidation of the type [(Ar)Ru(X)(bpy)](+) with different aromatic ligands (Ar). The critical step of the O-O bond formation in the catalytic cycle starting from the [(Ar)Ru(O)(bpy)](2+) intermediate is analyzed in detail. It is shown that an explicit inclusion of the solvent environment is essential for a realistic description of the reaction path. Clear evidence is presented for a concerted reaction in which the O-O bond formation is quickly followed by a proton transfer leading to a Ru-OOH intermediate and a hydronium ion. An alternative path in which the approaching water first coordinates to the metal centre is also investigated, and it is found to induce a structural instability of the catalyst with the breaking of the aromatic ligand coordination bond.

We demonstrate here for the first time the photoelectrochemical properties of a BiVO 4 photoanode in conjunction with a molecular catalyst. When the Ru-based molecular catalyst (RuCat) is coupled to a BiVO 4 light-absorber the performance of this photoanode improves particularly in the low-bias region (<1.0 V vs RHE). The RuCat-BiVO 4 photoanode shows a higher photocurrent than CoP i -BiVO 4 under front illumination, and a 0.1 V more cathodic onset potential. The former can be partly explained by the low light absorption of the RuCat (<5% light absorption in the UV−vis− NIR range). For the latter, we propose that the linkers in the RuCat reduce the surface recombination in BiVO 4 to a greater extent than CoP i . Finally, we observe that the fill factor of the RuCat-BiVO 4 JV characteristic improves after the stability test. The results presented herein not only show the feasibility and potential of the solid state/molecular heterojunctions but also represent a proof of principle to improve conventional all-solidstate systems such as CoP i -BiVO 4 .

that combine the anodic water oxidation products (protons and electrons) with a CO 2 reduction catalytic system at the cathode to generate liquid fuels such as formic acid or methanol ( Scheme 1 ). [16][17][18] Such a scheme represents a potentially attractive route for direct conversion of solar energy to produce carbon-based liquid fuels on a large scale for power generation or transportation applications and avoids the problem related to hydrogen storage and safety. [ 19 ] For the development of such systems, self-assembling and self-repairing water oxidation electrocatalysts that operate in a carbon dioxide enriched environment with good catalytic stability and effi ciency will be helpful. [ 18,20 ] A CO 2 saturated solution of a bicarbonate system (pH = 6.7-6.8) is an optimal environment to reduce and convert CO 2 into formic acid or desired products. [ 20,21 ] We report here that it allows for the in situ formation of an effi cient water oxidation electrocatalyst from easily available nickel(II) in a bicarbonate electrolyte. The nickel-bicarbonate type electrocatalysts self-assemble on a glassy carbon (GC) anode or ITO (indium tin oxide) surface via anodic electro-deposition from a Ni 2+ solution with (Ni/ HCO 3 /CO 2 ) or without (Ni/HCO 3 ) carbon dioxide saturation. The electrocatalytic system exhibits a remarkable activity for anodic oxygen evolution in a CO 2 enriched electrolyte. This new catalytic design also offers a direct solution for the extraction of protons and electrons from water for the generation of liquid fuels in combination with a suitable CO 2 reduction electrocatalytic module. [ 2,21 ] These nickel derived electrocatalysts (Ni/ HCO 3 /CO 2 and Ni/HCO 3 ) also show excellent performance in other neutral (phosphate) or near-neutral (borate) buffers and carbonate solution (pH > 10). Moreover, the Ni-derived electrocatalysts presented here do not require the proton abstracting phosphate or borate buffers for electrodeposition and for anodic water oxidation as recently shown to be essential for the generation and activity of Co-Pi and Ni-Bi based catalysts. [ 9,23,24 ] The Ni/HCO 3 /CO 2 type electrocatalyst is readily formed at the CO 2 /HCO 3 − phase boundary in the Pourbaix diagram on a freshly polished glassy carbon surface while scanning the potential between 0.97 and 0.85 V (vs. NHE) in a CO 2 saturated 0.2 M bicarbonate electrolyte near-neutral solution that contains 1.0 m M Ni 2+ . After the formation of the electrocatalyst fi lm on a glassy carbon anode, oxidative currents appear at about 1.05 V (vs. NHE) in the cyclic voltammetry (CV) curves ( Figure 1 ). This is ascribed to the formation of oxides of the nickel metal ions in carbon dioxide enriched bicarbonate electrolyte system. [22][23][24][25][26] The oxidative current wave is followed by a sharp catalytic current rise at >1.3 V (vs. NHE) that is accompanied by the generation of tiny oxygen bubbles at the anode. The oxygen evolution current density rises with further potential increase. The backward sweep generates the corresp...