Structure of redox-active ligands, electrocatalysts, and photocatalysts.
The design and modification of metal–organic complexes for hydrogen (H2) gas production by water splitting have been intensively investigated over the recent decades. In most reported mechanistic pathways, metal hydride species are considered as crucial intermediates for H2 formation where the metal present at the active site plays an imperative role in the transfer of electron and proton. In the last few decades, much consideration has been done on the development of non-precious metal–organic catalysts that use solar energy to split water into hydrogen (H2) and oxygen (O2) as alternative fossil fuels. This review discussed the design, fabrication, and evaluation of the catalysts for electrocatalytic and photocatalytic hydrogen production. Mechanistic approach is addressed here in order to understand the fundamental design principle and structural properties relationship of electrocatalysts and photocatalysts. Finally, we discuss some challenges and opportunities of research in the near future in this promising area.
- first-row transition metals
- hydrogen evolution
- catalytic cycle
Climate change and increasing energy demand have emphasized research on sustainable energy source [1, 2]. Day-by-day increase of human population and global requirements has compelled researchers to develop new renewable sustainable energy sources in replacement of hydrocarbon deposits . Renewable sources such as solar power, wind, and water, storage of these energies for on-demand utilization, and transportation are the major challenges for researchers. To develop a clean and eco-friendly environment, splitting of water into hydrogen and oxygen is a tremendous way to produce sustainable energy. Hydrogen gas emerged as a green energy fuel due to its high-energy density and zero carbon dioxide (CO2) emission [4, 5, 6]. In this regard, electrocatalytic and photocatalytic H2 generation from water has been considered as one of the most striking approaches [7, 8, 9]. In recent years, a substantial number of artificial photosynthesis have been developed, exploited solar power as electron and proton source to make a clean renewable fuel [10, 11, 12, 13, 14]. Light-induced splitting of water is a suitable process because the production of hydrogen is used as green fuel in future and even used for the synthesis of other chemicals [15, 16, 17, 18].
Literature reports suggested more than 500 billion cubic meters (44.5 million tons) of hydrogen gas is produce yearly worldwide [19, 20]. In the current scenario, steam methane refining, coal gasification, and water electrolysis are the major way for hydrogen production. Nowadays 95% hydrogen gas is produced from steam methane reforming and coal gasification, however only 4% hydrogen from water electrolysis. Steam methane is a high-energy-intensive process maintained at high temperature with the formation of carbon dioxide and carbon monoxide: (i) CH4 + H2O = CO + 3H2 (ii) CO + H2O = CO2 + H2. Hence, it is not an eco-friendly method for hydrogen production. Water electrolysis is the most sustainable and clean approach for hydrogen production because its source is abundant. Since the most suitable way of light-driven energy conversion is water electrolysis, artificial photosynthesis (PS II) has been considered as primary goal to produce electron and proton [21, 22]. Water splitting is a redox reaction in which aqueous protons are reduced into H2 at cathode and water is oxidized to O2 at anode . Both H2 (HER) and O2 (OER) reactions are rigorously coupled, which may lead to the formation of explosive H2/O2 mixtures due to gas crossover [24, 25, 26]. By far, only a few stable metal complexes as catalysts are achieved that can decompose water into H2 and O2 [27, 28, 29, 30, 31]. Water-splitting reactions are split into two half-reactions: water oxidation to O2 evolution and water reduction to H2 production:
The limitation of OER is that it takes place after the successive accumulation of four oxidized electrons and protons in Kok cycle (catalytic cycle of the water oxidation in PS II) that require much higher overpotential input than that of HER . Thermodynamic potential is different for H+/H2 (0 V vs. NHE) and OH-/O2 (1.23 V vs. NHE), and the overall solar energy conversion efficiency is only ∼15% in OER . The hydrogen evolution reaction (HER, 2H+ + 2e- = H2) is the cathodic reaction with the two-electron transfer in one catalytic intermediate and offers the potential to hydrogen production. However, hydrogen production technology requires proficient electrocatalysts and photocatalysts which support two key electrode reactions (OER and HER) at lower overpotentials.
Moreover discussion on the mechanism of HER, H+ adsorption on the hydrogen evolution catalyst surface is the first step, known as Volmer step, followed by Heyrovsky or Tafel steps shown in Figure 1. A suitable HER catalyst always binds H+ very fast and releases the product. Hence, electrochemical hydrogen evolution reaction (HER) facilitates for H2 production on large-scale.
Afterwards, H2 evolution may occur via two different reaction mechanisms depending on the action of catalyst . Hydronium cation (H3O+) is the proton source in acidic solution, and in alkaline condition H2O is the proton source. In Volmer-Tafel mechanism, two protons absorbed on the catalytic surface can combine to form H-H bond to yield H2. In Heyrovsky reaction route, a second electron and another proton from the solution are transferred to the catalyst surface which reacts with the absorbed H atom and generate H2. This is an electrochemical desorption pathway. Precious metal like Pt-based electrocatalysts is highly reactive for HER and is usually pursuing Volmer-Tafel mechanism. Lately few literatures [35, 36, 37] have been reported on Ni-based electrocatalysts which follows Volmer-Heyrovsky path.
1.1 Fundamental aspects of photocatalytic and electrocatalytic hydrogen production process
Electrocatalytic water splitting is driven by passing the electric current through the water; conversion of electrical energy to chemical energy takes place at electrode through charge transfer process. During this process, water reacts at the anode form O2 and hydrogen (proton) produce at the cathode as we mentioned earlier. Suitable electrocatalysts can maximally reduce the overpotential which is highly desirable for driving a specific electrochemical reaction. However, the process of surface catalytic reactions in electrocatalysis is very similar to photocatalysis . Photocatalytic is a simple water-splitting reaction in which H2 and O2 are produced from water by utilizing the energy of sunlight. Figure 2(a) shows the process of photocatalysis in which a metal catalyst contains chromophores that immersed solar energy and triggered the electron transfer reaction. The most important criteria for the solar-driven water-splitting reaction are electronic band gap matching of the photosensitive material to the redox potential of water . Metal complexes act as chromophore associated with mainly three types of electron transfer: metal center (MC), ligand center (LC), and metal–ligand center transition (MLCT). The MLCT state of the metal complex plays a crucial role in photocatalytic reactions. In octahedral complexes with conjugated ligand system, the highest occupied molecular orbital (HOMO) corresponds to the metal-localized t2g-orbitals, and the lowest unoccupied molecular orbital (LUMO) is associated with anti-bonding π*-orbital localized on the ligands. On the absorption of UV–visible light, an electron is promoted from one of the metal-centered t2g orbitals to a ligand-centered π* orbital, resulting in the MLCT state shown in Figure 2(b). As a result, the redox properties of the metal complexes are dramatically changed. The excited metal complexes behave as better oxidants and better reductants than their electronic ground state and can hold more thermodynamic driving force for the charge transfer reactions. Based on the photo-induced redox potential changes and the long-lived lifetime of the excited state, many metal complexes have been intensively investigated as chromophores for this photocatalytic H2 production purpose . Zou and coworkers have described various photocatalytic systems for H2 production, which exposed that most of the photocatalytic systems suffer photodecomposition and instability . Hence, for long-term use, it is imperative to build up highly proficient H2 generation systems with long lifetimes and high durability. Many reviews have been published on solar H2 evolution systems based on photocatalysts [41, 42, 43, 44].
1.2 Mechanistic pathway
So far, extensive theoretical study has been revealed, the possible mechanistic process of proton reduction to hydrogen evolution through transition metal molecular catalyst. A generalized mechanistic scheme depicting the homolytic and heterolytic path is shown in Figure 3. The homolytic mechanism involves bimetallic route, where a metal hydride species ([Mn+–H]) react with another metal hydride to release one H2 via reductive elimination. Instead, heterolytic is a monometallic pathway, where the metal hydride [Mn+–H] is further reduce and protonated for H2 evolution . Both pathways function simultaneously, two protons and two electrons are delivered to the metal center, and in few cases, the pH, catalytic concentration, and proton source decide the dominant route . During the past decade, a number of review articles emphases on the structural property relationship and mechanistic study [45, 47, 48, 49]. Among all research on catalyzed H2 evolution, the mechanistic investigation on proton reduction catalysis is essential because it can give us a significant idea to design better molecular catalysts in the future .
1.3 Metal and redox-active ligands for HER
Here we start by describing the fundamental concept of metal and organic ligand system which gives a strong influence on the performance of H2 evolution. Transition metal cations with partial filled d-electronic configurations are considered as catalyst. The characteristic feature of this type of catalyst is that the metal ions can exist in higher oxidation state . There are several literatures reported with partially filled d-orbital which show high stability toward water-splitting reactions [50, 51, 52, 53]. However, the most catalytic system requires very high temperature and precious metal at the active site; therefore, it will be a challenge for researchers to develop a photocatalytic and electrocatalytic system at low temperature with low-cost metal [54, 55, 56, 57]. Indeed, first-row transition metal complexes (Co, Ni, Cu, Zn) have been exploited in the last few decades for this purpose. Beyond the reactivity of metal, the redox activity of organic ligands has also received continuous attention. The redox-active ligand works as electron sink in the complexes and maintains the metal in its original oxidation state. Redox-active ligands convey a novel reactivity to the complex by loss or gain of electrons . In addition, the redox-active property of the ligand can also be influenced by the modification of the substituents by σ and π donating ability, π accepting ability, and conjugation . A highly conjugated system such as bpy, porphyries, and ortho-phenylenediamine (opda) having anti-bonding π*-orbital localized on the ligands is considered for hydrogen production due to its multielectron or multiproton pooling ability which is responsible for dramatically changing the potential of redox properties [60, 61]. Substituents attached to redox-active ligands, electron density, and charge on the metal ions also effect the standard electrode potential.
So far, considerable advancement has been done in the field of electrocatalytic and photocatalytic water-splitting reaction for hydrogen production, and several advance review papers have been reported by scientists [40, 51, 62, 63, 64]. However, very limited comprehensive tutorial has published on only first-row transition metal-based catalysts. This chapter describes electrocatalytic as well photocatalytic properties of inorganic catalysts and their structural and mechanistic features. Here we put an effort elucidate the direction of fundamental mechanistic aspects during electrocatalytic and photocatalytic hydrogen (H2) production reaction (HER).
2. Photochemical hydrogen production from a series of 3D transition metal complexes bearing o-phenylenediamine ligand
Masaki Yoshida et al.  developed a series of 3D-transition metal complexes with
a. Photochemistry of aromatic amine (opda)
b. Redox properties of opda complex
In the past M. Yoshida and coworkers proposed [Fe-opda] for photochemical HER mechanism at photoirradiation of λ = 298 nm; ππ* excitation occurred in complex with N-H bond homolysis process, followed by H2 elimination . After this process, the opda ligands in the complex were partially oxidized to bqdi or s-bqdi ligands. This mechanism is based on the deep-rooted photochemical N-H bond activation of aromatic amines. Theoretical study and ultrafast spectroscopic studies of amino benzene support that the photochemical N-H fission occurs by the photoexcitation to higher-lying ππ* level which leads to the formation of the πσ* state . Photochemical mechanism for HER of all complexes is shown in Figure 4.
All opda-based metal complexes display photochemical HER activities with the formation of almost one equivalent of H2 gas. However, the HER was not observed at all in the dark in all complexes, which suggests that the HER was obsessed by photochemical reaction. Moreover, they observed remarkable decrease in hydrogen evolution reaction, while the ligand is replaced with aromatic amines. This experiment suggested that the photo-induced HER activities of the complexes in this case are weakly dependent on the central metal ion and strongly dependent on the redox-active ligand. Further to check the metal ion dependency, examine the catalytic hydrogen production in the presence of hydroquinone (HQ; 10 equiv) as a sacrificial electron-proton donor. The photochemical H2 production from [M-opda] (7.98 × 10−2 mmol) with HQ (7.98 × 10−1 mmol) in THF (4 mL) under an N2 atmosphere at 20°C for 190 h turns over the number for all the complexes given in Table 2. Difference in TON may be caused by the stability of each complexes.
|Catalysts||Redox-active organic ligands||Catalytic potential (Ep)||Solvent||TON(H2 mol cat−1)||Ref.|
|b||Diimine-dioxime||−0.68 V vs. Fc+/0||H2O/CH3CN||300|||
|||Diimine-dioxime||−0.96 V vs. Fc+/0||H2O/CH3CN||50|||
|||Bis(thiosemicarbazone)||−1.7 V vs. Fc/Fc+||CH3CN||37|||
|||Bis(thiosemicarbazone)||−1.7 V vs. Fc/Fc+||CH3CN||73|||
|c||Diamine-tripyridine||−0.90 V vs. Fc+/0||acidic-H2O||1.4x 104|||
|d||TMPA||−1.81 V vs. SCE||CH3CN/H2O||6180|||
|||Cl-TMPA||−1.72 V vs. SCE||CH3CN/H2O||10,014|||
|||Bis(benzenedithiolate)||−2.25 V vs. SCE||CH3CN||0|||
|||−1.64 V vs. SCE||CH3CN||6190|||
|||−2.03 V vs. SCE||CH3CN||900|||
|||2-Mercaptophenolate||−1.62 V vs. SCE||CH3CN||5600|||
2.1 Cobalt diimine-dioxime complexes for HER
V. Artero and coworker synthesized cobalt diimine-dioxime complexes as molecular catalysts for H2 evolution [69, 70]. This synthesized ligand (N2, N2−propanediylbis-butan-2-imine-3-oxime) has emerged many years ago through Schiff base condensation of butanedione-monoximeon diamine compounds but not widely used for HER. Cobalt diimine-dioxime catalysts are active for H2 evolution in aqueous solution, both after immobilization on electrode materials and in light-driven homogeneous conditions. The electrocatalytic activity of complexes (
2.2 Electrocatalytic cycle for H2 evolution
In the electrocatalytic cycle, V. Artero and coworker observed that in acetonitrile medium, halide ligands are banished with reduced oxidation state from CoII to CoI. Upon reduction, the coordination in number decreases from six in CoII state to five in CoI state; this characteristic was supported by DFT calculations . In the catalytic cycle (Figure 6), the first step is the transfer of electron and proton by proton-coupled electron transfer (PCET) process. PCET is a chemical reaction that involves the transfer of electron and proton in which the oxidation number changes by CoII to CoI. In the second steps, further electron and proton transfer takes place by PCET process, and the oxidation number changes from CoI to CoII. In the last step, H2 is produced in dihydrogen bond through an intramolecular mechanism. The authors also confirmed cobalt diimine-dioxime catalysts
Similarly, cobalt bis(iminopyridine) complex
2.3 ZnII and CuII complexes for HER
Grapperhaus et al.  recently reported two homogeneous electrocatalysts for H2 production. They derived bis(thiosemicarbazones) ligand from 1,2-diones, considered as a kind of multitalented redox non-innocent system. Tetra-coordinated N2S2 is able to bind with low-valent transition metals centered and formed to stable neutral complexes (
Professor Wang and group proposed  a significant homogeneous mononuclear copper electrocatalyst for H2 production attributed to diamine-tripyridine ligand; complex
According to the author’s studies on the mechanism of this process, the controlled potential electrolysis of complex
Moreover, Wang et al.  fabricated and examined two Cu complexes with TMPA = tris(2-pyridyl)methylamine and Cl-TMPA 1-(6-chloropyridin-2-yl)methyl-
Based on the control potential electrolysis experimental data, the authors proposed photocatalytic hydrogen evolution mechanism. In the first step, excited PS system takes out one electron from TEA and donates to CuII center of complex
2.4 Ni electrocatalysts for HER
Professor Richard Eisenberg and coworker  synthesized a sequence of nickel bis(chelate) complexes; all complexes attained square planar geometry and examined photocatalytic as well as electrocatalytic behavior for hydrogen evolution. Fluorescein (Fl) as the photosensitizer along with triethanolamine (TEOA) as the sacrificial electron donor was used in water under basic medium (pH = 9.8). Bis(chelate) complexes (
3. Concluding remarks and future scope
Here we present a recent development of molecular catalysts toward clean and renewable fuels using earth-abundant metals. We have highlighted a series of Co-, Ni-, Cu-, Zn-based complexes for HER. We have recapitulated the fundamental principles of hydrogen and oxygen evolution reactions with molecular complexes. The designing and fabrication of the molecular complexes with redox-active ligands have been discussed in details; HER activity of the complexes strongly dependent on redox-active ligands as well the central metal ions are discussed in detail. A mechanistic approach and transfer of electron and proton during the homogeneous electrocatalyst and photocatalysts cycle are given in point. Although reasonable progress has been made in the development of metal complexes based electrocatalysts and chromospheres for photocatalytic hydrogen production, still several issues exist which need further improvement: (i) some photocatalytic systems suffer from low activities and short life times which is manifested in the instability of catalytic systems and so concern on the systems with modest water splitting activity and poor stability of the complexes. (ii) most of the complexes are not soluble in water leading to the use of organic solvent or mixture of organic- water solvent. From the future prospective, it is required to develop redox-active ligands with substituted functional group to increase the solubility of complexes in water. More experimental, spectroscopic, magnetic, and theoretical investigations is still needed to be carried out in order to understand the ligand- and metal-centered electron transfer processes. (iii) In addition, the overpotential requirements for most of the organic ligands are still very high, a chelating ligands giving much lower thermodynamic potentials and much smaller oxidation potential that should be utilized in future. (iv) In the regard of future growth in this field, with the need to design molecular complexes that can be immobilized on the surface of the electrode, for this purpose addition of suitable functional group in the ligand is necessity. These complexes can also be supported by the development of surface of the solid photocatalyst, like TiO2, BiVO4, etc. to demonstrate efficient photoelectrochemical cell.
The authors acknowledges the helps received from Ms. Priyadarshini Sahu and Mr. Abhineet Verma during the manuscript preparation.