Enhancing ORR Efficiency: Graphene-Porphyrin Metal Organic Framework Composite as Catalyst

The crucial role of cathodic oxygen reduction reaction is an electrochemical energy conversion in fuel cells. Direct Methanol fuel cell (DMFC) is a consist of three main components : Pt-Ru anode, a Pt cathode and a proton exchange membrane (PME). DMFC works by oxidizing an aqueous solution of methanol to CO2 and reducing oxygen to water. The ORR cathode requires an efficient catalyst due to its the kinetics is very slow. Platinum-based is considered the most efficient catalyst for ORR. However, the main disadvantage of pt for used as a catalyst is the high cost.

Alternative composite to Pt-based electrode in a fuel cell for ORR in alkaline media is graphene-porphyrin metal organic framework (MOF).

The important points:

Reduced GO (r-GO) sheets were used that are contain pyridineligands on either side which work as supported to link metalloporphyrin nodes. Also, the addition of r-GO in MOF increase the electrocatalytic active of the iron porphyrin and facilitate ORR by 4-electrons reaction. In addition, it is minimized of methanol crossover reaction via the inactivity of the hybrid MOF to methanol oxidation.

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The electrochemical subject/ technique that covered in this paper:

1. Using Cyclic voltammetry (CV) to examine the electrocatalytic activity of materials.
2.  The optical absorption, vibrational bands and crystal structure of these composites ware carried out to see if effects, because of chemical hybridization as well as charge transfer between the different components can be noticed. In this case, GO has absorption peak at 286 nm due to the characteristic π-plasmon absorption. On the other hand, G-dye that contains conjugated network has adsorption at 319 that considered red-shift.
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3. Fourier transform infrared spectroscopy (FTIR) was utilized for a distinguish functional groups in the starting material and different hybrids. For example, the GO contains carboxylic groups which appear at 1734 cm-1.
4. The electronic interaction of G-dye sheets and porphyrin-MOF was tested via fluorescence spectra. The (Fe-P)n MOF has a strong fluorescence peak at 575 nm which detect there is a powerful interaction between the excited state of TCPP and r-GO in the hybrid. Nevertheless, the fluorescence of excited TCPP decreases due to energy transfer to the r-GO.
5. Using X-ray photoelectron spectroscopy (XPS) to prove changing in the chemical environment of iron and nitrogen. (Fe -P)n MOF has one peak reference to the pyrrole nitrogen. Nevertheless, (G-dye 10 wt %-FeP)n (red line) has two peaks resulting from pyrrole groups in the porphyrin and pyridine groups from the dye.
6. The phase and structure of the synthesized products was tested by powder x-ray diffraction. The lattice deformation of MOF increase with the increase in content of G-dye from 5 to 50 wt% in the composite.
7. Scanning electron microscope was utilized to identify the shape of (Fe-P)n. When increasing the amount of G-dye added, the shape of (Fe-P)n crystal changes from plate shape to rod shape.
8. Using chronoamperometry to test the durability of (G-dye 50 wt % -FeP)n MOF as ORR catalyst for cathode against Pt nanoparticles loaded glassy carbon electrode and Ni foam.
9. The reaction kinetics was studied by rotating-disk voltammetry. The voltammetric shows that the current density increase with increase the rotation rate from 250 t0 2500 rpm.
10. Rotating-disk electrode (RDE) was used in order to study the structure-property correlation of (G-dye 50 wt % -FeP)n MOF composite at the ORR electrochemical process.

The difficulties and challenges in the research conducted in the paper:

The Randles-Sevcik equation was used for evaluating the electroactive surface area.

ip = 2.99 × 105 nACD1/2 v1/2

According to this equation they found that, (G-dye 50% -FeP)n has a larger electroactive surface area than that of bare GC electrode. This is due to the combination of G-dye increases the electroactive surface area of the electrode and promotes the charge transfer kinetics.

To prove the (G-dye 50 wt % -FeP)n is a good catalyst for reduction reaction using cyclic voltammetry (CV). It shows that the reduction peak of (G-dye 50 wt % - FeP)n MOF move to more positive potential than GO that facilitate the reduction reaction. However, the oxidation peak of GO move to more negative potential than (G-dye 50 wt % -FeP)n, which is considered a good catalyst for the oxidation reaction.

The impact of methanol transit was used as evidence to assess the suitableness of (G-dye 50 wt % - FeP)n MOF as electrocatalyst for cathode ORR. The crossover of methanol in DMFC from anode to cathode was caused loss of equilibrium electrode potential. Also, A catalyst was poisoned when the methanol is oxidized, and therefore a good catalyst should be inactive to methanol oxidized. Thus, The electrocatalytic activity of both (G-dye 50 wt % - FeP)n MOF and pt-catalyst loaded GC electrode for electrooxidation of methanol were tested. They note that, there is no clear response when use (G-dye 50 wt % - FeP)n MOF. But there is a strong response noticed for pt catalyst in O2-saturated 0.1 M KOH solution with 3M methanol. Consequently, (G-dye 50 wt % - FeP)n MOF has a high selectivity for ORR with reduced effect of methanol crossover.

The performance of these catalysts (Fe-P)n MOF and (G-dye 5,10,25 and 50 wt % -FeP)n MOF, that coated on GC electrode, for ORR were investigated. The potential of reduction for ORR is shifted to more positive potential when the amount of G-day increases in MOF compound as shown in figure 4a.

The performance of (G-dye 50 wt % - FeP)n MOF in ORR was compared with GO and exfoliated graphite. The electrocatalytically activity of GO is more active than Graphene, due to the overpotential of GO for ORR is more positive and the increase in the current density. However, the overpotential of (G-dye 50 wt % - FeP)n MOF modified cathode for ORR is shifted to most positive by 120 mv. Also, the current density of MOF- modified cathode is the highest between three samples.

The electron transfer number for ORR is different an among 2 to 4 when utilizing (Fe-P)n MOF or GO as cathode, in this case depends on overpotential. While, the electron transfer number is 4 in the (G-dye 50 wt % -FeP)n MOF electrode, which independent of potential.

Rotating-disk electrode was filled with various catalysts each time. It was noticed that the (G-dye 50 wt % -FeP)n MOF is first potential for ORR to be reached. Also, this electrode has higher current densities than the rest. This indicates that the combination of G-day and MOF, instead of physical mixing of G-dye and MOF, gives better electrocatalytical behavior.

The durability of (G-dye 50 wt % -FeP)n is superior to that of Pt and Ni catalyst. This due to the current- time chronoampermetric response of (G-dye 50 wt % -FeP)n shows a very slow decrease in its initial current density than the Pt nanoparticles and Ni foam cathode.

Interesting point about the reported research:

A graphenemetalloporphyrinMOF was synthesized by:

Firstly, r-Go sheet reacted with 4-(4-aminostyryl) pyridine to yield G-dye. Secondly, 5,10,15,20-tetrakis(4-carboxyl)-21H,23H-porphyrinTCPP reacted with Fe ions to form (Fe-P)n MOF. Finally, both G-dye and (Fe-P)n MOF reacted to produce (G-dye-FeP)n as active catalyst.

Using of G-dye when synthesis of (G-dye-FeP)n that works as supported to link metalloporphyrin nodes.

The pyridinium moieties in MOF work for increasing the electrocatalytic active of the iron porphyrin and facilitate ORR by 4-electrons reaction. In addition, it is minimized of methanol crossover reaction via the inactivity of the hybrid MOF to methanol oxidation.

The electron transfer process is the quasi-reversibility because of the formal potential is nearly steady, but ΔEp (Epa - Epc) values increase with increasing scan rate.

Figure 9. Cyclic voltammograms of GO on GC electrode in 10 mM Fe(CN)6 3-/4- / 1 M KCl at various scan rates from 80 mV/s to 270 mV/s.

The kinetic parameters were analyzed on the basis of the Koutecky- Levich equations:

1/(J ) = 1/J_L + 1/J_K = 1/(Bw^(1/2) ) + 1/J_K

B = 0.62 nFC0 (D0)2/3 ν-1/6

Jk = nFkC0

Koutecky- Levich plots were utilized at different electrode potential which appear a good linearity. They are considered as first order reaction kinetics. According to these plots, the number of electrons transferred (n) and Jk obtained from the slope and intercept as shown in figure 10.

Figure 10. Koutecky−Levich plots at different electrode potentials of (G-dye 50 wt % -FeP)n MOF at different electrode potentials.

Oxygen reduction reaction (ORR) can occur in two pathways either by the direct 4-electron pathway where the reduction of O2 into a water or 2-electron pathway where O2 is reduced to hydrogen peroxide (H2O2). The 4-electron direct pathway is preferred in fuel cell processes.

Graphene has an important role to enhance the adsorption surface area can be seen obviously from the volume of adsorbed nitrogen increase with increasing quantity of G-dye in the composite.