Bio-inspired and electrodeposited solid-state catalytic Materials
While allowing for a fine tuning of the catalytic properties through ligand design, molecular approaches that we developed are frequently criticized because of the inherent fragility of the resulting catalysts, when exposed to extreme redox potentials. In a number of cases, it has been clearly established that the true catalytic species is heterogeneous in nature, arising from the transformation of the initial molecular species, which should be considered rather as a pre-catalyst. We have been able to demonstrate that the grafting of molecular catalysts onto a electrode substrate provides them with a largely increased protection against modification and allows preserving their molecular structure. Nevertheless, we found interesting to investigate the preparation of novel solid state electrocatalytic materials, provided that they are based on Earth-abundant elements and obtained through low-cost processes.
Electrodeposition is a technique of choice in this context, that has been recently used by Dan Nocera (MIT) and Holger Dau (Freie Universität Berlin) to develop novel electrocatalytic materials for water oxidation based on cobalt and manganese oxide respectively. In the context of H2 evolution, electrodeposition has allowed the preparation of novel molybdenum sulphide materials as efficient catalysts. Pioneering work has been accomplished in the group of Dr. Xile Hu at EPFL (Lausanne, Switzerland). We also collaborate with Dr. Phong D. Tran and Prof. Jim Barber from the Solar Fuel Lab of Nanyang Technological University in Singapore to develop such novel electrodeposited H2-evolving electrocatalytic materials based on a MoS2 or WS2 frameworks. We recently reported on ternary sulfide copper-molybdenum-sulfide (Cu2MoS4) as a new inorganic solid state electrocatalyst. The structure of this layered material contains a [Cu4S4Mo] building block (Figure 1) that possesses several structural similarities to the active sites of hydrogenase and molybdenum CO-dehydrogenase.
Figure 1: Structure of the Cu2MoS4 materials inspired from the active site of the molybdenum CO-dehydrogenase.
We also found that electro-reductive transformation of molecular cobalt complexes, for example the cobaloxime [Co(dmgBF2)2(H2O)2] or the diimine-dioxime cobalt complex [Co(DO)(DOH)pnCl2] ((DOH)(DOH)pn = N2,N2'-propanediylbis(2,3-butandione 2-imine 3-oxime) into a metallic deposit occurs under aqueous conditions. In this case the hydrolysis of the complex proceeds even at neutral pH (phosphate buffer) and a nano-particulate electrocatalytic material, H2-CoCat, is electrochemically deposited as revealed by the coloration of the transparent FTO electrodes and confirmed by SEM measurements (Figure 2). Electro-reductive decomposition of the molecular compounds initially results in the formation of small nanoparticles with average diameter of 10 nm. Upon prolonged deposition time or when reduction is carried out at a more negative potential, a film of ~2 µm thickness made from larger particles (100 nm) is obtained.
Figure 2: SEM images of electrodes modified by electrolysis at (left) -0.9 V vs Ag/AgCl for 1 h (ITO, 0.1 C.cm--2geometric) and (right) -1.0 V vs Ag/AgCl for 3 h (FTO, 6.5 C.cm-2geometric) vs RHE in aqueous phosphate buffer (KPi, 0.5 M, pH 7) containing Co(DO)(DOH)pnCl2 (0.5 mM).
On the basis of advanced spectroscopic measurements (XPS, EDX, EXAFS and XANES) we could show that H2-CoCat consists of metallic cobalt coated with a cobalt-oxo/hydroxo-phosphate layer in contact with the electrolyte. The organic ligand is not present anymore in the material and, actually, H2-CoCat can be obtained from simple Co(II) salts. H2-CoCat mediates H2 evolution from neutral aqueous buffer at modest overpotentials (onset potential for H2 evolution is 50 mV vs RHE) and proves significantly more active than bulk Co metal or than Co coatings deposited from NH4Cl and LiClO4 solutions. Remarkably, H2-CoCat can be converted upon anodic equilibration into the O2-CoCat cobalt-oxide film described by Kanan and Nocera catalysing O2 evolution. The switch between the two catalytic forms is fully reversible (Figure 3) and corresponds to a local interconversion between two morphologies and compositions at the surface of the electrode. After deposition, the noble-metal-free coating thus functions as a robust, bifunctional and switchable catalyst.
Figure 3: SEM image showing the transient coexistence of the H2-CoCat and O2-CoCat materials at the surface of an electrode after potential switch from reductive to oxidative conditions..