Methylamine Treated Mn3O4 Nanoparticles for Efficient Water Oxidation Catalyst at Neutral Condition

Abstract

Developing alternatives to replace fossil fuels is an important issue for maintaining sustainable energy system in the future. Hydrogen fuel is an attractive alternative owing to its high energy density and environmentally friendly by-products. Electrical water splitting powered by a renewable energy source is a promising way to produce hydrogen fuel. However, anodic water oxidation is considered to be a bottleneck due to its high overpotential. Therefore, designing efficient catalyst for water splitting is regarded as an important issue. Previously, Ir and Ru-based catalysts have been shown to exhibit remarkable catalytic activity toward oxygen evolving reaction (OER) under acidic condition. Despite their high activity, it is difficult to use noble-metal-based catalysts for commercial applications because of the scarcity of precious metals and their costs. Therefore, non-precious transition metal oxide electrocatalysts were investigated for water oxidation reaction such as cobalt oxides, nickel oxides, iron oxides and manganese oxides.1,2 Their activities were only retained under basic conditions and advancement in electrocatalysts that show moderate activity at neutral pH has been relatively insufficient. Nature has highly efficient OER catalysts in photosystem II which is Mn4CaO5 cluster also called as oxygen evolving complex. Inspired by unique structure and catalytic mechanism of this cluster, Mn-based catalysts have been actively studied. Among various manganese oxide, nanoparticle system showed superior catalytic activity due to its totally different reaction mechanism compared to the bulk counterparts.3,4 However, after synthesis of manganese oxide nanoparticles, long alkyl organic ligands are attached on the surface of nanoparticles interrupting the approach of water molecules, which inhibits water oxidation. Previously, hydrophobic organic ligands were removed by thermal decomposition with sufficient annealing process. Alternative way to remove alkyl ligands is replacing it with different ligands through surface ligand treatment such as BF4 -, PF6 -, OH-, thiol, SCN- and PbCl3 -. Surface ligand exchange is beneficial in that it does not need high temperature annealing and possibility of fine surface control. We have successfully synthesized 6 nm Mn3O4 nanoparticles (NPs) and used for further surface treatments. Mn3O4 NPs were OH ligand exchanged as a first step and followed by methylamine treatment in aqueous solution. Identical TEM images and XRD patterns showed that the size of NPs was unchanged and the phase of NPs was maintained as Mn3O4. Electrokinetic study of the OER was analyzed by cyclic voltammetry (CV) at pH 7 and compared between OH ligand exchanged Mn3O4 NPs and further methylamine treated Mn3O4 NPs. Methylamine treatment reduced the overpotential for OER by nearly 100 mV at the current density 5 mA cm-2. Tafel slopes was also decreased after methylamine treatment to 83.1 mV dec-1. To understand the phenomenon of increased catalytic activity after methylamine treatment, further analysis of the surface of NPs were conducted using spectroscopic measurement. In the XPS O 1s spectra, surface hydroxyl group (Mn-OH) was increased up to 57.3 % after OH ligand exchange. Also, FTIR spectra showed increased ratio of Mn-OH to lattice oxygen (Mn-O-Mn) for OH ligand exchanged Mn3O4 NPs compared to Mn3O4 NPs which indicates successful OH ligand exchange on the surface of Mn3O4 NPs. After methylamine treatment of OH ligand exchanged Mn3O4 NPs, methylamine was not attached to the surface of NPs. XPS N 1s spectra did not show characteristic methylamine peak, FTIR did not show C-N vibration mode and elemental analysis indicates that N does not exist in the NPs. To figure out the role of methylamine on the surface of OH ligand exchanged Mn3O4 NPs, titration and zeta potential was measured. After methylamine treatment surface was more deprotonated. Deprotonation resulted in loss of positively charged proton which induced negative charge on the surface of Mn3O4 NPs and showed negative zeta potential. On the basis of nonexistence of methylamine on the surface and surface deprotonation after methylamine treatment, we hypothesized that methylamine work as Brønsted-Lowry base. Methylamine was replaced by ethylamine, ethylenediamine, dimethylamine, trimethylamine and ammonia water, then after reaction with different amines, overpotential was decreased for all cases. Interestingly, amines with larger pK a showed larger decrease in overpotential. Based on these results, we first proposed correlation between the surface charge and oxygen evolving activity. Deprotonating the surface of the catalyst through simple acid-base reaction induced a change in the surface state followed by catalytic activity improvement. This understanding can be applied for developing efficient nano-catalysts by deprotonating the nanoparticle surface to induce surface state change. [1] J. Am. Chem. Soc., 2014, 136, 4201-4211 [2] J. Am. Chem. Soc., 2014, 136, 7435-7443 [3] Sci. Rep., 2015, 5, 10279 [4] J. Am. Chem. Soc., 2017, 139, 2277-2285 Figure 1