Effect of nano-MgO template and S doping on ORR activity

In order to probe possible effects of the microstructure on the catalytic activity of the material, the catalyst Fe-N/C was synthesised in the presence of MgO nanoparticles to obtain catalyst MgO APS 1. The nanoparticles are believed to act a template and were added to the precursor mixture during the polymerisation step with ammonium persulfate (APS) (see 3.2.1). During the polymerisation, the naoparticles act as a template and upon pyrolysis can have beneficial influence on the microstructure. After the heat treatment step, the template was removed during the acid leaching step in 0.5 M H2SO4. A second

heat treatment was performed on this catalyst to obtain MgO APS 2. Additionally to probing the effect of the microstrucuture on the ORR activity, the influence of sulphur doping has been investigated. This has been done by changing the oxidant from sulphur containing APS to H2O2, to obtain MgO H2O2 1. MgO H2O2 1was then acid leached

in 0.5 M H2SO4to remove the template. As second heat treatment of MgO H2O2 1gave

MgO H2O2 2. A totally sulphur free catalyst MgO H2O2 2 HClhas been synthesised

by swapping the acid in the leaching step from 0.5 M H2SO4 to 0.5 M HCl.

Table 3.6: External, microporous and total surface area (SA) of templated catalysts,

as determined by nitrogen adsorption analysis.

Catalyst External SA [m2_g-1_{] Microporous SA [m}2_g-1_{] Total SA [m}2_g-1_]

Fe-N/C 38.0 418 457

MgO H2O2 1 505 450 955.0

MgO H2O2 2 570 532 1102

MgO APS 1 505 606 1113

MgO APS 2 570 679 1249

Table 3.6 shows the surface area contribution for the templated and the untemplated ma- terial. Interestingly, although the total surface area more than doubles for the templated (∼1000 m2g-1) versus non-templated material (< 500 m2 g-1), the increase in microporous surface area is moderate. All templated catalysts show a large external surface area of > 500 m2 g-1. It might be concluded that the addition of the template with nanoparticle sizes in the range of ∼50 nm facilitates a significant increase in total BET surface area with the majority of this increase as external surface area.

0.0 0.2 0.4 0.6 0.8 1.0 -6 -5 -4 -3 -2 -1 0 j / mA cm -2 E / V vs RHE Fe-N/C Fe-N/C-MgO Pt/C 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1 MgO_APS_1 MgO_APS_2 MgO_H2O2_1 MgO_H2O2_2 MgO_H₂O₂_2_HCl Normalized Current Density / a. u. E / V vs RHE 0

a)

b)

Figure 3.11: (a) Steady-state RDE measurements of M-N/C catalyst with and without

nano-MgO template in 0.5 M H₂SO₄, catalyst loading: non-precious metal catalysts 750

μg cm-2_{, Pt/C 60μg}

Ptcm-2, rotating speed: 1600 rpm, 30 s hold, 30 mV step potential,

O₂-saturated at 1 atm. (b) Normalised RDE measurements in 0.5 M H₂SO₄ of different

nano-MgO templated catalysts, 5 mV s-1, background corrected cathodic cycle, catalyst

loading: 750 μg cm-2, O₂-saturated at 1 atm

The ORR activity of templated compared to non-templated catalyst and the reference Pt/C catalyst in 0.5 M H2SO4 is shown in Figure 3.11 (a). It can be seen that there

is no significant increase in activity in the kinetic region. Only at higher currents, the templated material seems to perform ”better”. Better in this sense means a higher current at higher potential. Moreover, it can be seen that the limiting current achieved by the templated material surpasses the theoretical limiting current under these conditions (∼5.2 mA cm-2)6. This phenomenon can be explained by inhomogeneities in the catalyst layer of the templated material. If the catalyst layer contains features which can penetrate the diffusion layer (∼10 μm under the utilised conditions), a higher apparent surface area will be accessible to the reaction and hence a higher limiting current is the result. Since the nanoparticles used to synthesise the templated catalysts are in the 50 nm range, it might be possible that the catalyst layer contains features of this length scale which might penetrate the diffusion layer and lead to the observed increase in limiting current. The absence of a higher activity in the templated catalyst, while showing a more than 2 fold increase in total BET surface area is striking. The fact that the microporous surface areas of the templated and non-templated catalyst are similar, supports the hypothesis of an active site which is hosted in the micropores.

The effect of sulphur doping on the ORR activity in 0.5 M H2SO4 is presented in Fig-

ure 3.11 (b). The different limiting currents have been accounted for by rationing the

6_{As calculated from the Levich equation (Equation 2.3) for oxygen saturated 0.5 M H}

2SO4at ambient

measured currents to the respective limiting currents to obtain the ”Normalised Current” for all measurements. It can be seen that there is no significant difference in the kinetic current (i. e. 5% of limiting current as indicated in Figure 3.11 (b)) in catalysts in the presence or absence of sulphur during the heat treatment. For both materials a second heat treatment improves the activity and increases the microporous surface area. The reason for the slightly lower activity of the catalyst treated with HCl instead of H2SO4

might be that in the presence of chloride catalytically active Fe ions are leached more effectively from the material.

To summarise: The effect of surface area and microstructure was investigated by using

MgO nanopowder as templating agent. It was found that the total surface area does not seem to correlate with the electrochemical activity. A correlation with the microporous surface area might be present, supporting the hypothesis of active sites hosted in microp- ores. [4] The effect of aromatic sulphur on the ORR activity in acid was investigated. No significant effect was found.