IACS team develops high-performing bio-inspired electrocatalyst for hydrogen generation in an aqueous medium
11 March 2013
Researchers from the Indian Association for the Cultivation of Science (IACS), an autonomous—and the oldest—research institute in India, have developed a high-performing bio-inspired catalyst (an Fe−Fe hydrogenase mimic immobilized on graphite surfaces) for electrocatalytic hydrogen generation in an aqueous medium.
In a paper published in the journal ACS Catalysis, they report that the catalyst shows a turnover frequency of 6,400 s−1 at −0.5 V and an onset potential of −0.36 V vs NHE (normal hydrogen electrode, an early standard for zero potential). Prolonged electrolysis shows that the catalyst has a turnover number ≫108 and a Faradaic efficiency > 95%. Even at pH 2, more than 400 s−1 is obtained. The catalyst can be immobilized on inexpensive carbon electrodes, such as those used in domestic Zn-carbon dry batteries, to generate H2 from acid aqueous solutions.
An efficient Fe-based molecular electrocatalyst is desirable as iron is the most abundant transition metal in the lithosphere (∼1,000 times more abundant than Co) and ∼2.6 billion tonnes of it is produced annually worldwide which is higher than any other transition metals (copper comes second in the list with an annual production of 16 million tonnes). However, very few Fe-complexes are known to produce H2 via H+ reduction in the aqueous medium.
Hydrogenase (H2ase) enzymes reversibly converts protons to hydrogen with minimum over potential at physiological pHs with turnover frequency (TOF) of 100 to 10,000 mols of H2 per mole of enzyme per second. There are three types of H2ase, namely, binuclear Ni−Fe H2ase, binuclear Fe−Fe H2ase, and mononuclear Fe-only H2ase, which are abundant in nature. The Fe−Fe H2ase mainly reduces protons to H2 whereas the Ni−Fe H2ase is known for H2 oxidation while the Fe-only H2 ase is not catalytic by itself.
...Here, we report, for the first time, heterogeneous electrochemical H2 production by reducing protons in acidic aqueous media by a robust synthetic Fe−Fe H2ase active site mimic having a Fe2S2(CO)6 core (complex A) and a bridging ADT ligand. The catalytic property of this catalyst in aqueous medium is investigated using RDE, RRDE coulometry, and bulk electrolysis technique.
—Dey et al.
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| Video of evolution of H2 gas on the cathode. Dey et al, Supporting info. Click to launch. |
Controlled potential electrolysis experiments at −0.5 V indicated that ∼1.1 × 10−3 moles of H+ was reduced without any decay in the catalytic activity in 8 h. Thus, they estimated the turnover number for the catalyst to be ≫108.
With ∼10−11 moles of complex A producing 12 mL (0.6 × 10−3 moles) of hydrogen over a period of 8 h in 0.5 N H2SO4 at −0.5 V, they estimated the yield of H2 per mole of the catalyst to be ∼4.4 × 104 liter/sec per mole of complex A. This is equivalent to ∼83 L/sec/gram of catalyst. The researchers said that, to the best of their knowledge, this is possibly the highest electrocatalytic H2 generation rate reported so far and is comparable to the highest activity reported for the native enzymes.
| Comparison of H2 Evolving Electrocatalysts (from other work cited in Dey et al.) | ||||||
|---|---|---|---|---|---|---|
| Catalyst | Onset E (V) vs. NHE |
pH | TON | TOF | ||
| Complex A (Dey et al.) | -0.3V | pH 2−1 | ≫108 | 6400 s−1 | ||
| Co pentapyridine | −1.07 V | pH 7 | ∼104 | 0.35 s−1 | ||
| MoS2 catalyst | −0.65 V | pH 3 | ∼103 | 280 s−1 | ||
| Mo Oxo catalyst | −0.93 V | pH 7 | ∼105 | 2.4 s−1 | ||
| Co tetraaza macrocycle | −0.40 V | pH 2.2 | 23 | |||
| Co tetraimine catalyst | −0.40 V | pH 2 | ∼105−6 | |||
| Co bis iminopyridine | −0.90 V | pH 2 | 2.2 h−1 | |||
| Fe2S2Ph(CO)6-SDS surfactant | >−0.7 V | pH 3 | 52 | 2600 s−1 | ||
| Table data from Dey et al. | ||||||
Enhanced solvation of the active species (i.e., the reduced complex A) in an aqueous medium raises the thermodynamic reduction potential which allows H2 production from H2O at reasonable potentials. FTIR data of the catalyst, immobilized on the electrode, before and after the electrocatalytic processes indicates that the complex is quite stable under these conditions. The onset potential, TON, and TOF obtained, when compared to that of other catalysts reported in the literature suggests that this is an excellent electrocatalyst for H2 generation. While some Mo based catalysts show reasonable H2 generation at pH 7, this Fe based catalyst has superior TOF and low overpotential. This catalyst has the highest TON (≫108) and TOF in aqueous medium of all the catalysts reported to date. In addition to that, its reactivity is retained when immobilized on cheap carbon electrodes, which is of significant practical interest.
—Dey et al.
Resources
Subal Dey, Atanu Rana, Somdatta Ghosh Dey, and Abhishek Dey (2013) Electrochemical Hydrogen Production in Acidic Water by an Azadithiolate Bridged Synthetic Hydrogenese Mimic: Role of Aqueous Solvation in Lowering Overpotential. ACS Catalysis 3 (3), 429-436 doi: 10.1021/cs300835a
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