Nickel Complexes for Robust Light-Driven and Electrocatalytic Hydrogen Production from Water

Abstract

A series of nickel bis­(chelate) complexes having square planar coordination are studied for light-driven and electrocatalytic hydrogen production from water. The complexes Ni­(abt)2 (abt = 2-aminobenzenethiolate), Ni­(mp)2 (mp = 2-mercaptophenolate) and Ni­(mpo)2 (mpo = 2-mercaptopyridyl-N-oxide) are found to be active catalysts under light-driven conditions, using fluorescein (Fl) as the photosensitizer (PS) and triethanolamine (TEOA) as the sacrificial electron donor in water under basic pH (pH = 9.8). These molecular systems achieve a turnover number (TON) of ∼6000 (relative to catalyst) and are stable for more than 100 h under H2-generating conditions. When water-soluble CdSe quantum dots with tripodal S-donor capping agents are employed as PS and ascorbic acid (AA) is used as the sacrificial electron donor at pH 4.5, an active and robust system is obtained for the light-driven generation of H2 from aqueous protons. A TON of over 280 000 is achieved for the three active catalysts. These complexes are also examined electrochemically in organic solvents with weak organic acids as the proton source and in aqueous and aqueous/organic media for proton reduction. The most active photochemical catalysts also show excellent electrocatalytic activity in neutral pH water, achieving Faradaic yields close to 100% under anaerobic conditions and ∼80% under aerobic conditions.

Light to Hydrogen: Photocatalytic Hydrogen Generation from Water with Molecularly-Defined Iron Complexes

“…A stability of more than 100 h was obtained using this system with TEOA as the electron donor [39]. More than 280,000 turnovers were obtained by replacing the PS and SR by water-soluble CdSe quantum dots and ascorbic acid, respectively [39]. Very recently, also stabilized nickel [43], iron [44], zinc [45] and copper nanoparticles [46] were reported to constitute active WRC in the photocatalytic hydrogen generation from water.…”

“…However, in photocatalytic hydrogen generation, this system achieved only a TONNi of 2700 over 150 h with Ru photosensitizer A and ascorbic acid (SR) [37], while applying thiolate nickel complexes improved the TONNi up to 7300 after 30 h with fluorescein, a xanthene-type organic dye, as the photosensitizer and TEA as SR [40]. A stability of more than 100 h was obtained using this system with TEOA as the electron donor [39]. More than 280,000 turnovers were obtained by replacing the PS and SR by water-soluble CdSe quantum dots and ascorbic acid, respectively [39].…”

“…Recently, these systems have been outperformed by pentapyridyl cobalt complexes achieving a turnover number (TON) with respect to Co of up to 11,000 with a Re-photosensitizer (PS) and ascorbic acid (SR) [36]. Prominent examples of nickel catalysts constitute the DuBois catalyst [37], as well as tris(2-pyridylthiolate) [38], 2-aminobenzenethiolate, 2-mercaptophenolate and 2-mercaptopyridyl-N-oxide [39] nickel complexes by Eisenberg. The DuBois system showed excellent activity with a TOF up to 100,000 s −1 in the electrocatalytic hydrogen evolution reaction (HER) [41,42].…”

“…For this purpose, especially ruthenium complexes have played a key role since the 1970s [68][69][70][71][72][73][74], later followed by various iridium [75,76], platinum [77][78][79] and rhenium [80][81][82][83][84][85] complexes. In contrast, more abundant metals or even metal-free photocatalytic systems were reported: examples include, e.g., iron [86], zinc [24,25,[87][88][89] and magnesium-based [24,[90][91][92][93][94] photosensitizers, CdTe [95], CdSe [39] or carbon [96] quantum dots or organic dyes [38][39][40][97][98][99][100][101][102][103][104] together with either cobalt or nickel catalysts. Mostly, the reported activities and stabilities were still low.…”

“…However, Eisenberg recently reported a TON for a nickel WRC as high as 280,000 together with water-soluble CdSe quantum dots with tripodal S-donor capping agents as PS and ascorbic acid as the sacrificial electron donor at pH 4.5 [39].…”