A Study Of Molecular Electrocatalysis For Artificial-Photosynthesis Reactions

Exploiting sustainable energy is a key to rescuing the approaching crisis of energy and environment. As an important technology of sustainable energy, artificial photosynthesis aims at converting the solar energy into chemical energies, which are storable and transportable. In the study of artificial photosynthesis based on water splitting, tremendous research efforts have been devoted to the molecular electrocatalysis, in which organometallic complexes are employed as the catalyst to mimic the biocatalytic process of enzyme, and the water molecules are split electrochemically or photoelectrochemically into H2and O2. The theme of the present work is molecular electrocatalysis for reactions relevant to artificial photosynthesis, including the water oxidation reaction (WOR), the hydrogen evolution reaction (HER), and the oxygen reduction reaction (ORR). In this work, about27organometallic complexes were synthesized and studied; according to their electrochemical and catalytic behaviors in organic and aqueous media, high-performance molecules were screened and integrated with metal-oxide photoelectrodes for photoelectrochemical study. Major achievements of this research are summarized as follows:1. Half-sandwich Ru complexes for electrocatalysis of WORAt present, most molecular catalysts for the WOR are still inefficient and have rarely been applied in photoelectrochemical water splitting. We synthesized5types of pseudo-tetrahedral Ru complexes ([Cp*RuL2X]n+, abbreviated as Rp*-L) with different electronic configuration in ligand and different charge density at the Ru site. All the molecules are catalytic toward the WOR in basic media, but for those soluble in aqueous solutions, the more active in catalysis the less resistant to the hydrolysis. The [Cp*Ru(PPh3)2Cl]Cl molecule (Rp*-P2), which is relatively active and stable, was fixed onto the electrode surface for heterogeneous catalysis of WOR. Only with350mV in potential polarization, O2can be detected on the Rp*-P2/Au electrode surface by using differential electrochemical mass spectrometry (DEMS). This is among the best molecular catalysts for WOR reported thus far.2. Polypyridyl cyclometalated Ru complexes for photoelectrocatalysis of WORTo enhance the stability of molecular catalyst for photoelectrolysis, we synthesized pseudo-octahedral Ru complexes [Ru(tpy)(dcbpy)Cl]Cl (Rt-dcb) and [Ru(tpy)(pba)Cl](Rt-pba) which are oxidation tolerant and can be bound onto the surface of a-Fe2O3. Unlike the traditional "inorganic catalyst/semiconductor" contact, such a "molecular catalyst/semiconductor" interface is free from any grain boundary, and an electronic heterojunction can also be formed in between. These features have benefited the separation of the light-induced charges and thus increased the plateau photocurrent density. Due to the employment of electron-donating C-∧N ligand in Rt-pba, the active Ruv intermediate is more accessible, resulting in an enhanced catalytic activity toward the WOR. Specifically, upon bonding Rt-pba onto the a-Fe2O3surface, the onset potential of photoelectrolysis has shifted from0.9V to0.7V (vs. RHE) and the photocurrent at1.23V increased by ca.140%.3. Polypyridyl Ni and Co complexes for electrocatalysis of HERThe molecular electrocatalysis for HER is a very active field in recent years, however, most molecular catalysts can only be used in organic media for the electroreduction of organic acid. We investigated a series of polypyridyl Ni and Co complexes, and6molecules, including [Ni(tpy)2](BF4)2,[Ni(tpy)(bpy)Cl]Cl,[Co(tpy)(bpy)Cl]Cl,[Co(tpy)(dmbpy)Cl]Cl,[Co(tpy)(dcbpy)Cl]Cl, and [Co(tpy)(phen)Cl]Cl, were found to be able to catalyze the HER in aqueous solutions, ranging from acid to weak base. In organic system, the turnover frequency (TOF) of the HER catalyzed by these molecules is about110～289s-1, greater than that of the traditional cobaloxime catalyst. In aqueous electrolytes, the catalytic activity of polypyridyl Co is much higher than that of the Ni molecules, and can even be increased by using stronger electron-withdrawing ligands. The overpotential of the HER is about500mV, comparable to the best record in the literature.4. Polypyridyl Co complex for photoelectrocatalysis of HERAmong the challenges in molecular electrocatalysis is how to increase the catalyst loading on electrode surface. We found that, through the electrochemical polymerization of the1,10-phenanthroline (phen) ligand, the [Co(tpy)(phen)Cl]Cl catalyst can be fabricated on the electrode surface with a very high loading. The DEMS measurements showed that, on such a poly-[Co(tpy)(phen)Cl]Cl electrode (PCo), the overpotential of HER is only370mV in a buffer solution of pH=3, with the TOF being10s-1and a TON of ca.2.2×106, which is among the best molecular catalysts thus found for HER. Characterized by XPS, the active site in PCo was found to be still in molecular state, rather than in metallic or oxide states. By polymerizing the [Co(tpy)(phen)Cl]Cl onto a Cu2O photocathode, we have realized, for the first time, a molecular catalyst/p-type semiconductor photocathode for the HER, which can stably work at0V (vs. RHE) under illumination, with a HER photocurrent increased by50%, in comparison to that on a bare CU2O photocathode.5. Polypyridyl Cu complexes for electrocatalysis of ORRThe ORR catalyzed by Fe and Co porphyrine or phthalocyanine has been intensively studied in comparison to Cu complexes, though the latter is known to play a key role in relevant biocatalytic process. We synthesized7types of polypyridyl Cu complexes, including [Cu(bpy)2](C104)2,[Cu(dcbpy)2](C104)2,[Cu(bpy)3](C104)2,[Cu(tpy)(C104)](ClO4),[Cu(tpy)(bpy)(H20)](ClO4)2,[Cu(tpy)(dcbpy)(H20)](ClO4)2, and [Cu(tpy)2](C104)2, and used them to catalyze the ORR in aqueous solutions. It turned out that the catalytic activity of these molecules toward the ORR was majorly affected by the electrode potential of the CuⅢ/Ⅰ couple, rather than the electronic property of the ligands. The reaction electron number of the ORR was2at low [Cu] and increases with the [Cu], indicating that the studied Cu complexes seemed to act as a redox shuttle for the ORR, rather than a catalyst of inner-sphere reaction.