Population growth and living standards are creating an increase on the global energy consumption projected to be at least 2-fold in the following decades relative to the present. With no doubt meeting this huge demand of energy in a competitive, reliable, clean, and sustainable manner is so far the most important scientific and technical challenge that humanity is facing in the 21st century. The use of fossil fuels for energy production supposes the 80.9% of global energy consumption. Unfortunately, consumption of fossil fuels is producing a potentially significant global issue, since it leads to carbon emissions in the form of CO2. There are scientific evidences that indicate that atmospheric CO2 concentration during the past 650000 years has kept stable between 210 and 300 ppm, while an evident increase has been recorded during the last 50 years due to anthropogenic CO2 emissions from fossil fuels consumption, until an excess of 380 ppm. Evidence of these high levels has been observed in climate change, as there is an obvious increase in the intensity, frequency, and duration of heat waves globally, the number of heavy rainfall events in many regions, changes in atmospheric circulation patterns, and the rate of combined mass loss from the Greenland and Antarctic ice sheets. Thus, it is of imperious importance the invention, development, and deployment of carbon-neutral energy sources and technology. Solar energy is the most exploitable source of renewable energy, due to being clean, abundant, economically affordable, and the only one with sufficient capacity to meet energy demands for the entire planet. It is necessary a robust, and cost-effective system to convert and store solar energy (batteries or fuels).
Mimicking photosynthesis, an artificial photosynthesis platform should convert sunlight into spatially separated electron/hole pairs and use the electric potential to mediate water splitting into H2 (reduction) and O2 (oxidation) at catalytic centers. So, solar energy is stored in the form of chemical bonds. The water oxidation reaction is considered the bottleneck in such water splitting scheme, because a large potential (overpotential) is typically needed to overcome the activation barriers required by the four-electron oxidation of two water molecules coupled to the removal of four protons to form a relatively weak oxygen-oxygen bond. The main challenge to realize artificial photosynthesis is still the search for a robust, efficient, and inexpensive water oxidation catalyst.
A large number of metal complexes have been described as homogeneous water oxidation catalysts since Meyer, in 1982, reported the catalytic oxidation of water by a μ-oxo-bridged ruthenium dimer coordinated by polypyridil ligands, known as blue dimer. These complexes have shown high performance in terms of rates and efficiency, and they are easy to optimize and process. However, long-term stability is an important issue, since the organic ligands are unstable toward oxidative deactivation due to the strong oxidizing conditions during the water oxidation process. Therefore, metal complexes containing organic ligands do not appear as a viable solution for water-splitting devices. The state-of-the art in terms of feasible industrial applications is lead by heterogeneous transition metal oxides. Their poor processing capabilities and their pH-dependent degradation still makes too costly their implementation in water electrolysis, when compared with hydrogen prices from natural gas reforming (still fossil fuels technology).
Polyoxometalates (POMs) are very interesting compounds in the field of water oxidation catalysis, since they can serve as all-inorganic multidentate ligands with high stability under strong oxidizing conditions to stabilize catalytic transition metal oxide clusters. POMs are also known as strong Brœnsted acids and as fast reversible multielectron oxidants, stabilizing high-valent intermediates and assisting deprotonation equilibria on the polyoxygenated surface. Moreover, they have the ability of stabilize adjacent d-electrons centers through multiple-μ-hydroxo/oxo bridging units, which is one of the most important features that natural enzymes posses to effect multiple electron/cascade transformations. Hence, polyoxometalates can be employed as bifunctional (acid and redox) catalysts, with the stability of heterogeneous catalysts and with the tunability and processing of homogeneous catalysts. In 2012 our group reported the water oxidation catalytic activity under homogeneous conditions of a high nuclearity cobalt-containing polyoxometalate: the nonanuclear [Co9(H2O)6(OH)3(HPO4)2(PW9O34)3]16− cluster (Co9). In this Thesis we analyzed its activity in the solid state as the first step towards its implementation into an artificial photosynthesis plartform.
In Chapter 2 we show that precipitation of the POM with Cs+ leads to a water insoluble salt (CsCo9) which, blended with a solid-state conducting matrix (i.e. carbon paste), promotes heterogeneous electrocatalytic water oxidation in mild conditions (pH 7). The electrochemical data indicates that a current density of 1 mA/cm2 needs an overpotential of ca. 525 mV in optimum conditions for this catalyst. These electrodes show a remarkable stability, with no sings of fatigue for at least 8 hours with Faradaic O2 evolution. Interestingly, these modified electrodes are also active even in strong acidic conditions (pH 1), where cobalt oxide is unstable. Comparison with Co3O4/Carbon Paste modified electrodes show different electrochemical behavior, than that observed with the POM modified electrode. Multiple experimental evidences support the stability of the CsCo9 catalyst under working conditions inside the carbon paste matrix, with consistent analytical and structural data obtained before and after water electrolysis.
In Chapter 3 we present the heterogeneous catalytic activity of CsCo9 for light-driven water oxidation at pH 7 employing [Ru(bpy)3]2+ as photosensitizer, and (S2O8)2- as sacrificial electron acceptor. Our catalyst yields a maximum turnover number (TON) of 14.2 and an maximum turnover frequency (TOF) of 10.8 per hour. Characterization of the collected catalyst after running the experiments confirms that the structure of the Co9 remains stable under experimental conditions, whereas cation exchange occurs during the experiment. Indeed, a suspension of the insoluble salt of Co9 with the chromophore [Ru(bpy)3]2+
(RuCo9) in a solution containing an electron acceptor, yields improved oxygen evolution under the same conditions. The maximum TON and TOF obtained with the photosensitizer/catalyst solid (RuCo9) increase to 27.3 and 19.1 per hour, respectively. However, degradation of the organic ligand of the photosensitizer, due to the highly oxidation environment, leads to its deactivation. Addition of fresh [Ru(bpy)3]2+ into the same solution restarts the oxygen evolution with TONs and TOFs comparable to those obtained with the CsCo9, supporting that the POM is highly stable under turnover conditions. RuCo9 exhibits superior activity when compared with Co3O4 under the same working conditions. The later, combined with the characterization of the catalyst, supports the genuine catalytic activity of Co9 for water oxidation, ruling out the in situ formation of cobalt oxide.
Finally, in Chapter 4 we study the reaction mechanism of water oxidation catalyzed with cobalt-containing polyoxometalates, treated as single-site catalysts, by means of DFT calculations. It is worthy to say that the computational modeling of these cobalt-multisubstituted polyoxometalates is quite challenging because of their large size and multiple magnetic cobalt centers. Moreover, the lack of supporting experimental data makes the study even more complicated. However, we have attempted to rationalize/characterize the water oxidation catalysis mechanism. In these calculations we compute the proton-coupled electro transfer (PCET) events leading to the formation of the active Co(IV)−oxo or Co(III)−oxyl species, and “water-assisted” O-O bond formation mechanism. Because of bulkiness and structural stability of Co-POM anions one can eliminate the O−O bond formation mechanism by direct coupling between two Co groups. We propose a reaction mechanism of the water oxidation catalyzed by the well known [Co(II)4(H2O)2(PW9O34)2]10− (PCo4) POM, identifying all the intermediate species and the overpotential-limiting step. Then we expand this study to the analogous system [Co(II)4(H2O)2(VW9O34)2]10− (VCo4). In our proposed mechanism, we identify the active species as Co(III)-O·, which is a radical intermediate. Formation of this Co(III)-oxyl species determines the overpotential-limiting step of the mechanism. The radical formation has also been observed in Ru-containing catalysts, where the Ru(VI)=O species are closer to be Ru(V)-O·. Therefore, the formation of Co(IV)=O species seems to be not realistic in POMs. Once the active species is formed the O-O bond is formed via a water nucleophilic attack. In the case of the analogous VCo4 POM, the reaction is proposed to follow a similar pathway with somewhat higher potential for the initial step. However, the formation of O2 in the transition state happens with a lower energy barrier because of the presence of orbital coupling between the d-empty orbitals of V(V) and orbitals of the reactive Co site. This orbital coupling has also been observed experimentally in the UV-vis spectra. In addition, we also study the effect of the nucleation on the formation of the active species, where we compare the computed energetics required for the formation of the Co(III)-oxyl species for the PCo4 with a cobalt-monosubstituted POM and a model of the nonanuclear cluster Co9.