R. Jiang, D.T. Tran, J.P. McClure, D. Chu
US Army Research Laboratory,
Keywords: bioinspired material, mesoporous carbon, oxygen reduction, catalyst
Summary:Metalloproteins are a class of bio materials, which contain metal ion cofactors embedded within a larger protein molecule. Metalloproteins are readily available and provide an opportunity to use these low cost biomaterials for synthesis of high performing catalysts. For example, Hemoglobin plays an important role as an oxygen carrier, and has been previously used as a low cost material to make electrocatalysts for fuel cells . In a previous study, we utilized heat-treated hemin supported on high surface area carbon to synthesize highly active electrocatalysts for oxygen reduction [2-3]. It is known that high surface area carbon support must be added in the heat-treated hemin to increase the electric conductivity. However, the addition of large quantities of carbon support dilutes the active sites in hemin and lowers the activity and stability. In the present research we synthesized electrocatalytically active mesoporous carbon for oxygen reduction without the pre-addition of a carbon support. Here, the electrocatalytically active mesoporous carbon is generated from hemin at high temperature with well-ordered hexagonal mesoporous silica structures  as a template, instead of using an inert carbon additive. The unique features of the nano and meso pores in mesoporous carbon provide a unique host for the iron-centered active sites. Different heat-treatments were performed ranging from 600 to 900°C. In addition, we compare various treatment methods for removing the silica phase used to derive the mesoporous carbonaceous material. This includes a conventional method of removing silica with HF, as well as a more desirable and environmental friendly method. The catalytically active mesoporous carbon shows higher electronic conductivity, oxygen reduction activity and chronoamperometry stability in comparison to the heat-treated hemin supported on commercially available high surface area carbon materials. The activity of the catalytically active mesoporous carbon was characterized with rotating disk electrode (RDE). The half-wave potential was 0.76 V (vs. RHE) and the limiting current was 0.56 mA cm-2 at 1600 rpm rotation rate and 0.6V (vs. RHE) for oxygen reduction reaction (ORR) in O2-saturated 0.5M H2SO4 electrolyte. The rotating-ring disk electrode (RRDE) experiment reveal that the catalytically active mesoporous carbon catalyzes oxygen 4-electron reduction to generate H2O. In addition, the chronoamperometric experiments show that the retention of catalytic current density was 78% after 20 hours of testing at 0.6V (vs RHE) at 900 rpm rotation rate in O2-saturated 0.5M H2SO4. The catalytic performance in alkaline electrolyte (0.1M KOH) will also be compared with the low pH media. In addition, we discuss the BET surface area and pore size distribution, as well as nano-structuring based on transmission electron microscopy (TEM). References: (1) Jun Maruyama and Ikuo Abe, Chem. Mater. 2006, 18, 1303-131. (2) Rongzhong Jiang, Dat T. Tran, Joshua P. McClure, and Deryn Chu, Electrochemistry Communications 2012, 19, 73–76. (3) Rongzhong Jiang, Dat T. Tran, Joshua P. McClure, and Deryn Chu, Electrochimica Acta 2012, 75, 185–190. (4) Dongyuan Zhao, et al, Science 1998, 279, 548-522.