Invited: Bio-Inspired Electrocatalysts for Electrochemical Processes

Abstract

Enzymes can be highly efficient and selective catalysts for important electrochemical processes such as Oxygen Reduction Reaction (ORR), Hydrogen Oxidation Reaction (HOR) or oxidation of alcohols incluting polyvalent alcohols (polyols) and complex organic acids such as malic or oxalic acid for example. At the same time enzymes (due to their size) have low active site density (one or two active sites per molecule is a norm), they have short operational life spans, and are limited to a relatively narrow pH and temperature operating conditions. We have made efforts to synthesize transition metal-nitrogen-carbon catalysts that resemble Mn-containing active site for oxalate oxidation. This materials were made by pyrolysis and then evalauted with multple spectroscopic techniques and modeled by DFT. We have found that the atomivally dispersed transitin metal coordinates with 3 N moeities in the carboanteous backbone structire of the ctalysts. This resemled closely the structure of the active site of the enzyme oxalate oxidase [1]. This paper will present an overview of our bio-mimietic strtaegies and will include also new results on ORR bio-inspired catalysts. In an effort to capitalize on the excellent selectivity and inexpensive materials content of enzymes while improving site density, stability, and durability we used “Materials data-mining” approach in order to identify copper-based mixed metal oxides that closely match the geometric and electronic structure of copper-center enzymes for catalysis of oxygen reduction reaction ORR. We have imposed the requirements on the conformation of the tri-Cu oxygen binding centers (T2/T3) from the native 3D structures of multi-copper oxidases and relaxed the requirement of the singular Cu site for the primary electron donation (the substrate oxidation active site T1). After screening geological and materials databases we have identified a list of mixed metal oxides that have the motive present as a part (segment) of their elementary cell. The Ln2CuO4 group materials, where Ln is the lanthanide series, most closely matches the geometric requirements of an ORR enzyme and can be tuned for electronic and bonding configuration while also offering the electronic conductivity and stability needed for an efficient ORR catalyst. A range of pure Copper Lanthanide Oxides was made using La/Pr/Nd and copper-like metals doped into the copper positions. These Ln-mixed oxide catalysts were synthesized, characterized, the tested for ORR and OER using RDE in alkaline environment. Catalysts were characterized for surface area, composition, crystal structure, nano-structure, and surface chemical state using XRD, BET, SEM/EDS HRTEM/STEM/EDS, and XPS. We will report on the meso and nano-scale phase purity of these Copper Lanthanides. The materials made were then tested as ORR catalysts. It was found that the La and Pr -based materials performed best. Doping of the Lanthanide and Copper components changes the ORR performance. For nickel doping, the performance of mixed Cu/Ni-lanthanum oxide showed a linear activity trend where Nickel reduced activity. The reactivity trends observed demonstrate a successful synthesis of copper-based mixed oxides which structurally mimic enzymes active for oxygen reduction reaction, and that these bio-mimetic oxides are catalytically active. Further, this work demonstrates a successful “materials genomics” approach to creating biomimetic electrocatalysts. [1] I. Matanovic, S. Babanova, A. Perry, A. Serov, K. Artyushkova and P. Atanassov, Bio-inspired Design of Electrocatalysts for Oxalate Oxidation: a Combined Experimental and Computational Study of Mn-N-C Catalyst, Phys. Chem. Chem. Phys., 17 (2015) 13235-13244