Hydrogen production in artificial photosynthesis

In recent years, many studies have reported various heterogeneous and homogeneous molecular systems for hydrogen evolution based on transition metal complexes.  Many of these systems that sometimes exhibit high turnover (TON) and efficiency for hydrogen production only operate in organic or aqueous-organic media (5-50% H2O). Moreover, molecular systems working in pure water are necessary for actual application in large-scale artificial photosynthesis.

The transfer of protons is central to artificial photosynthesis. Proton activity management in an environment of catalytic water oxidation is essential to efficient catalysis. Producing proton currents over low-resistance pathways to and from redox sites might be a step towards a fuel cell/reversible water electrolyzer construction for artificial photosynthesis.

An artificial photosynthetic system requires two electrons to reduce protons into molecular hydrogen. All redox couples with a reductive potential at pH 7 can produce hydrogen in such a system. However, some proton reduction processes are too sluggish to occur without an appropriate catalyst. Transition metal complexes can store electrons across various redox states.

As such, transition metal complexes with their multiple redox states can act as efficient catalysts for this reaction. This article discusses the proton reduction mechanisms in an artificial photosynthetic system catalyzed by metallic complexes and the best molecular hydrogen evolving catalysts (HECs) explored so far, focusing on their performance and those that function in hydro-organic media or aqueous conditions. 

Proton reduction mechanisms 

Researchers have conducted numerous experiments and theoretical analyses to explore the proton reduction catalytic process at metallic centers, particularly with cobalt, nickel, and diiron catalysts. Although the majority of these experiments have been done in an organic environment, they offer valuable knowledge for engineering more efficient catalysts that will be utilized in an aqueous environment. 

In the typical proton reduction reaction for an artificial photosynthetic system, the reduction of Mn+ followed by protonation, furnishes the crucial intermediate hydride H–Mn+, which can progress in three different directions. 

The first is the protonation and liberation of hydrogen, restoring the original Mn+ catalyst (the heterolytic pathway). An alternate path is a reaction with a second hydride molecule to form M(n-1)+ and release hydrogen (homolytic pathway). A third option is the additional reduction of the hydride to form a low valent hydride H–M(n-1)+, which can proceed either heterolytically or homolytically. 

Homolytic and heterolytic pathways can occur independently or simultaneously, depending on factors like catalyst concentration and pH. Bimetallic complexes can be useful to accelerate the catalysis process and favor one pathway in certain cases.

Proton Coupled Electron Transfer (PCET) activity in hydrogen-generating catalysts can be enhanced by including proton relays. It has been established that proton relays encourage the creation of the H–Mn+ hydride intermediate and the building of the H–H bond. 

Platinum and Rhodium Catalysts – Hydrogen Production

Early studies on the practical photo-induced hydrogen production for artificial photosynthesis utilized a mix of [Ru(bpy)3] 2+ (P2) as a light harvester, a heterogeneous (platinum colloid) or a homogenous (metal complex) proton reduction catalyst, and a sacrificial electron donor (SED). One can link these systems to a redox mediator such as methyl viologen (MV2+) or metal complexes.

A recent study at the University of Rochester, United States, revealed that a P2 photosensitized aqueous solution of colloidal platinum, with hydrogen evolving catalysts (HEC1) as an electron relay, can proficiently generate hydrogen under visible light irradiation (l > 400 nm, 10.8 h-1 turnover frequency (TOF)). 

An analogous solution at pH 5.2 without platinum colloids produced 6 TON of hydrogen production in 3 hours. HEC1 is the first fully molecular photocatalytic mechanism for proton reduction, using the rhodium complex as the hydrogen evolving catalyst.

A polypyridyl catalyst, known as HEC2 was used in another research conducted at the University of Rochester and it was capable of achieving a yield of 5000 TON with an astounding 34% quantum yield (fH2).Researchers coupled this complex catalyst with a cyclometalated iridium photosensitizer. TEA acted as SED in 80:20 THF-water mixture, illuminated with monochromatic light (l = 460 nm). A downside of this system is the requirement for an organic co-solvent besides water. 

Photocatalytic Proton Reduction Systems

Rhodium catalyst, P2 photosensitizer, and sodium ascorbate/ascorbic acid buffer can construct HEC3 proton reduction system. Catalyst reached 100 TON in 3 hours under visible light irradiation (l > 430 nm) under optimal conditions.

Optimum pH in this system results from a balance between catalyst reactivity to protons and photosensitizer quenching efficiency. However, this system requires the accurate selection of the sodium ascorbate/ascorbic acid ratio to maximize the system’s efficacy. 

Recent research has developed various photocatalytic proton reduction systems. Most effective systems for pure water involve platinum complexes with nitrogen-rich coordination spheres, such as mono- and bi-nuclear.

H2 production achieved 31% quantum yield and 100 average turnover with HEC5 under visible light irradiation.

HEC5 with P2, MV2+, and EDTA in pH 5 acetate buffer is the most effective proton reduction system. The closer the distance and the greater the attraction between the two metal centers, the more active the catalyst is. 

Cobalt and Iron Catalysts – Hydrogen Production

Water-reducing catalysts with first-row transition metals have improved in effectiveness in the past decade, including iron, cobalt, and nickel. Several studies have described cobaloxime-type complex [Co(dmgH)2] (dmgH2 = dimethylglyoxime) as promoting photocatalytic proton reduction when P2 is used as a photosensitizer. 

Researchers have revealed that a broad range of dioxime/diimine cobalt HECs have high activity in organic environments. Low stability in acidic, reductive conditions restricts the application of first-row transition metals in aqueous systems.

HEC8 and natural PSI with ascorbic acid as an SED have been successful in homogenous photocatalysis in pure water. The heterogeneous approach is an efficient strategy to upgrade the performance of cobaloxime complexes using catalysts attached to solid surfaces. 

Recently, a very active cathode with the catalyst HEC14 was fabricated that achieved up to 5.5 x 104 TON at 0.59 V vs. RHE. HEC15 applied to a glassy carbon electrode can produce an electrocatalytic material with up to 5 x 106 TON at 0.61 V vs. NHE. Researchers expect to create more cost-effective cathodes using these molecular catalysts to facilitate proton reduction. 

Examples of Alternative Catalysts for HECs

HEC9, HEC11, and HEC16 have shown remarkable stability and activity in water-based photochemical and electrochemical systems.

Polypyridyl-based ligands were used to create efficient molecular electrocatalysts for hydrogen production, such as HEC17 and HEC18, by reducing water.

Complex HEC18, with a turnover number ranging from 3.5 x 103 to 1.9 x 107, is thought to be a molecular equivalent of the edge sites of two-dimensional bulk MoS2, which has also been employed in the water-reduction process.

Researchers have widely investigated iron-based catalysts as HECs. These catalysts usually form dimeric iron(I) complexes similar to the [FeFe]-hydrogenase enzyme.However, it is possible to circumvent the low solubility of these catalysts in water by encapsulating them into micelles or cyclodextrins, securing them to a solid support, or using ligands with water solubility.

A recent study reported one example of this latter approach, which used a ligand with trimeric ethylene glycol chains. This study demonstrated 505 TON under visible light irradiation with CdTe nanocrystals as photosensitizers and ascorbic acid as an SED in pure water.

Future research focus 

Artificial photosynthesis for hydrogen production has witnessed an increase in research over the years. To compete with or complement other renewable energy systems as a replacement for fossil fuels, the system needs to address several issues.

Future research should center on the clever design and formation of more economical metal-based molecular catalysts for electrochemical hydrogen production, eliminating the need for additional acids and organic solvents. Researchers are expected to create more cost-effective cathodes using these molecular catalysts to facilitate proton reduction. Future research should also focus on anchoring molecular catalysts for hydrogen production on the surfaces of cathode electrodes to take the next step towards designing the whole water-splitting photoelectrochemical cells by joining water oxidation and proton reduction.