Tunable graphene-based metal nanocomposite materials for electro-catalytic application
Final Report Abstract
Graphene is an excellent support for electrocatalysts due to its high electronic conductivity and low charge transfer energy. Commonly, wet chemical synthesis of metal nanoparticle graphene composite materials relies either on graphene oxide or on surfactant stabilized few layer graphene, involve the mixing of the metal precursor, the respective carbon dispersion, and a reduction agent. The particles are synthesized in solution in the first step and are absorbed on the substrate subsequently resulting in products, where issues like particle size distribution, side reaction with the stabilization agents or functional groups, trapping of residual metal salts are frequently observed. In order to circumvent those, graphenide solutions, generated from dissolving potassium intercalated graphite, have been studied as reduction reagent in nanoparticle synthesis. The advantages can be summarized as followed: (i) graphenide solutions are strong reducing agents and no additional reducing agent is needed, (ii) the redox reaction takes place in close proximity of the carbon lattice and the decoration of the carbon framework thus proceeds efficiently, (iii) the amount of reduction agent can be controlled by the concentration of the respective graphenide solution used, (iv) the by-product of the reaction is only the respective potassium salt, which can be removed easily (v) the reaction occurs at room temperature. This protocol has been applied to various transitions metals salts (Fe, Mn, Cu, Co Ni) but also for noble metals ones (Pt). The resulting metal nanoparticle nano carbon composite materials (M(nP)/nC) exhibit sub 10 nm sized nanoparticles with a narrow size distribution. The particles are metal oxides for the transition metals cases and in the metallic state for the noble metal ones, as to be expected for such small nanoparticles. Additionally, the average nanoparticle size of about 4 nm, 35 and 50 nm could be generated, by exploiting graphenide solution that exhibit a larger graphene size distribution. The synthesized materials of the sized varied Fe(nP)/nC components have been exploited as electrocatalysts for the oxygen evolution (OER), oxygen reduction (ORR). The measured ORR onset potential is + 0.79 V vs RHE for the small sized components, which is 40 mV more positive for the middle one, and 60 mV more positive than the largest counterpart. The electron transfer number, sought after to be close to 4, was 3.6 for the small Fe(nP)/nC component at the potential of 0.45 V, revealing that the ORR generates mainly water. In comparison, the other two components exhibit lower electron transfer numbers of 3.37 and 2.96 respectively. The Cu(nP)/nC material has been studied in the the carbon dioxide reduction reaction (CO2RR) and electrodes prepared from this material hasve shown activity toward CO2 reduction at potentials as low as -0.54 V vs SHE. The product that is primary formed is formate and could be detected at low overpotentials around 270 mV, with the respective yield in the range of a few percent. The yield increased progressively with increased overpotential, reaching maximum values of ca. 50 % at overpotentials ~ 700 mV exceeding literature values for polycrystalline copper or electrochemically activated copper surfaces. The direct reduction of metal salt by graphenide solutions is a direct, reliable and generic pathway to obtain composite materials which are neither limited by the type of metal precursor nor the carbon starting material. The obtained catalysts are efficient in a range of challenging electrochemical reactions such as OER, ORR and CO2RR demonstrating the overall value of the materials and the synthesis protocol.
Publications
- From Food Waste to Efficient Bifunctional Nonprecious Electrocatalyst. Chem. - A Eur. J. 23, 15283–15288 (2017)
Hof, F. et al.
(See online at https://doi.org/10.1002/chem.201704041) - Graphenide Solutions: A Chemical Platform for Nanoparticle- Nanocarbon Composites. Chem. - A Eur. J. 24, 16246–16250 (2018)
Hof, F. & Pénicaud, A.
(See online at https://doi.org/10.1002/chem.201801694)
