Molecular engineering of Ni–/Co–porphyrin multilayers on reduced graphene oxide sheets as bifunctional catalysts for oxygen evolution and oxygen reduction reactions

Ni– and Co–porphyrin multilayers on reduced graphene oxide (rGO) sheets are reported as novel bifunctional catalysts for the oxygen evolution reaction (OER) and the oxygen reduction reaction (ORR).


S5
The energy dispersive X-ray spectroscopy (EDX) analysis proves the homogenous distribution of Ni, Co, N and C elements in as-prepared composites. As shown in Figure S5, there is only slight change in the XPS spectra of Co 2p, Ni 2p and O 1s before and after reduction, revealing the chemical robustness of the (Ni 2+ /THPP/Co 2+ /THPP) n multilayer films. Binding Energy (eV) Norm. Intensity Figure S6. High resolution N 1s spectrum of free THPP molecules. rGO/(Ni 2+ /THPP/Co 2+ /THPP) 12 at potential of 1.55V (vs. RHE) from 10 5 Hz -1 Hz. Figure S8. Armstrong-Henderson equivalent circuit model for oxygen evolution reaction.
In Figure S8, R Ω represents the uncompensated solution resistance (the electrolyte resistance) and C dl models the double layer capacitance associated with the catalyst/electrolyte interface.
R P denotes the OER charge transfer resistance. R S and C ф are the equivalent resistance and capacitance, respectively, associated with adsorption of intermediate.

Note that CPE (constant phase element) is introduced to replace the pure capacitance in order
to fix the frequency dispersion in the capacitive response of the electrochemical system that will cause a deviation from ideal capacitive behaviour.
Where C α=1 is the value of the capacitance without frequency dispersion and α represents the deviation from the ideal behaviour (α ≤ 1 for a physically reasonable situation and being 1 for the perfect capacitors).   As displayed in Figure S12, rGO/(Ni 2+ /THPP/Co 2+ /THPP) 8 has the identical OER catalytic activity compared with rGO/(Ni 2+ /THPP) 4 (Co 2+ /THPP) 4 , disclosing that the adsorption sequence of the transition metal ions in the multilayers is not the key factor to determine the electrocatalytic activity. rGO/(Ni 2+ /THPP/Co 2+ /THPP) 12 at 0.7 (vs. RHE) from 10 5 HZ -10Hz. S15 Figure S14. Equivalent circuit model for the oxygen reduction reaction.
In Figure S14, R Ω represents the uncompensated solution resistance (the electrolyte resistance). R 1 and C 1 are the resistance and capacitance between the electrode and electrolyte.
R 2 is the charge transfer resistance during the ORR process and C 2 denotes the double layer capacitance formed during the reaction. Note that CPE is also introduced to replace the pure capacitance to fix the frequency dispersion.    Table S1. Fitting results evaluated from the equivalent circuits of rGO/(Ni 2+ /THPP/Co 2+ /THPP) n (n = 2, 4 ,6, 8, 10 and 12) for OER.
Note that the fitted results listed in Table S1 are based on the equivalent circuits in Figure S13 and S14. Note that the fitted results listed in Table S2 are based on the equivalent circuits in Figure S7 and S8.