A cobalt coordination compound with indole acetic acid for fabrication of a high performance cathode catalyst in fuel cells

Abstract

As a catalyst precursor, a cobalt coordination compound with indole acetic acid (Co–IAA) was synthesized using the hydrothermal method. Pyrolysis significantly improved the catalytic activity of the carbon supported Co–IAA (Co–IAA/BP) toward the oxygen reduction reaction (ORR) due to the formation of nano-Co₃O₄. Both original and pyrolyzed Co–IAA/BP exhibited higher catalytic activity in an alkaline electrolyte than that in an acidic one because dioxygen and water were absorbed at the single electrophilic Co center in the alkaline electrolyte but dioxygen and proton were separately absorbed at the electrophilic Co center and nucleophilic center (O) in the acidic electrolyte. Construction of a coupling site where electrophilic and nucleophilic centers coexisted was the key to enhance the ORR in an acidic electrolyte. The pyrolyzed Co–IAA/BP exhibited excellent performance in the direct borohydride fuel cell, comparable to the commercialized 20 wt% Pt/XC-72. A peak power density as high as 186 mW cm⁻² was achieved at ambient conditions.

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1. Introduction

A fuel cell is a device that converts the chemical energy from a fuel into electricity through a fuel oxidation reaction at the anode and an oxygen reduction reaction (ORR) at the cathode. The performance of a fuel cell highly depends on the catalytic activity of the cathode catalyst. ORR is a condition-sensitive reaction where dioxygen reacts with protons or water when an acidic or alkaline electrolyte is employed. Expensive platinum is the best electrocatalyst for ORR no matter what the electrolyte is, revealing that active Pt sites function not only as electrophilic centers for dioxygen or water adsorption but also as nucleophilic centers for proton adsorption. Nowadays, cheap non-Pt catalysts have attracted much attention owing to their excellent performance comparable to carbon supported Pt catalyst in alkaline electrolyte.¹

Numerous Pt alternative ORR catalysts have been developed based on metal oxides,^2,3 polyoxometalates,^4–6 and metallomacrocyclic compounds^7–9 as well as metal doped polymers.^10,11 M–O and M–N (M = transition metal) bonds function as the catalytic sites for ORR. Catalysts with Co–O^12–17 or Co–N^18–22 bond showed relatively high catalytic activity. It was considered that dioxygen absorbed at the positive charged Co (electrophilic center) like Pt.²³ However, electrophilic proton may absorb at the nucleophilic O or N (nucleophilic center). Construction of ORR catalytic sites with electrophilic and nucleophilic centers (named coupling site for convenience) was thus rather important for fabrication of high performance catalyst. Pyrolysis of coordination compounds or metal-doped polymers was a convenient method to construct such coupling site.^19,20,24 The obtained non-Pt catalysts usually showed high performance in alkaline electrolyte but low one in acidic electrolyte. However, the reasons to account for this were not well understood yet. The extensive changes in composition and structure during pyrolysis led to the difficulty to establish a clear relation between the catalytic activity and the formed catalytic sites.

Indole acetic acid (IAA) is a white crystalline substance, poorly soluble (8 g L⁻¹) in water at room temperature but soluble at higher temperature (say 100 °C). Chemically, IAA is a carboxylic acid in which the carboxyl group is attached through a methylene group to the 3C position of an indole ring consisting of a six-membered benzene ring fused to a five-membered nitrogen-containing pyrrole ring. As a ligand, IAA has three attachable points (one from pyrrolic N and two from carboxylic O) to connect with transition metal ion or proton to form coordination or hydrogen bond. Moreover, IAA has good affinity to carbon owing to the benzene ring. The use of Co coordination compound with IAA may enhance the Co distribution on carbon support particles. Hydrothermal reaction of IAA with Co nitrite is thus applied for synthesis of the catalyst precursor to construct catalytic site in this work. The catalytic cluster formation is investigated through the high resolution transmission electron microscope (HRTEM) observation. The mechanisms of ORR in acidic and alkaline electrolytes are discussed based on material characterizations and electrochemical analyses. The performance of the synthesized catalyst is finally verified in a direct borohydride fuel cell.

2. Method and procedures

2.1. Synthesis of coordination compound and catalyst

Cobalt coordination compound was prepared with a small molar ratio of ligand (IAA) to Co(NO₃)₂ for utilization of the utmost attachment points in ligand. The ligand (1.0 mmol) was added to a Teflon-coated autoclave containing 90 mL of Co(NO₃)₂ (9 mmol) solution. After ultrasonic mixing for 20 min, the autoclave was sealed and then heated at 160 °C for dozens of hours. The coordination compound was obtained through filtrating and washing repeatedly with de-ionized water, and drying at 60 °C under vacuum for 12 hours. When 0.5 g of carbon support material (BP2000) was added into the autoclave in addition, the carbon supported coordination compound was then obtained. The ORR catalyst was prepared via pyrolysis of the carbon supported coordination compound at 900 °C for 2 hours in Ar atmosphere. Cobalt, nitrogen and oxygen contents in the synthesized catalyst were determined by inductively coupled plasma-atomic emission spectroscopy (Varian ICP 725-ES) and oxygen/nitrogen/hydrogen elemental analyzer (LECO ONH836), respectively.

2.2. Physical characterizations

The precursor structure was characterized by X-ray diffraction (XRD) with Cu Kα radiation (λ = 1.5406 Å), (X'Pert PRO PANalytical B.V.), and scanning range from 5° to 60° at a 4° min⁻¹ rate with a 0.2 s residence time. Morphological observations were performed through field emission scanning electron microscopy (FESEM, Zeiss Ultra55) operated at 5 kV, and transmission electron microscope (TEM, Hitachi H-9500) operated at 300 kV. Thermogravimetric analysis (TGA) was carried out in N₂ at a gas flow rate of 50 mL min⁻¹ and a heating rate of 10 °C min⁻¹ by using Pyris 1 TGA (Perkin-Elmer) to obtain the information about the catalyst precursor decomposition during pyrolysis.

The Fourier transform infrared (FTIR) spectra of the synthesized samples in KBr pellets were recorded in the range 400–4000 cm⁻¹ with a 4 cm⁻¹ spectral resolution using a Nicolet-IR560 FTIR spectrometer to understand the coordination situation of –NH and –COOH in the catalyst precursor. ¹H nuclear magnetic resonance (NMR) spectra were measured for solutions of the synthesized complexes and ligands in CD₃COCD₃ at 500 MHz with a DMX-500 spectrometer (Brulcer Co.) to confirm the FTIR results. Chemical shifts are shown in δ values (ppm) with tetramethylsilane (TMS) as the internal reference.

2.3. Electrochemical evaluation

The electrocatalytic activity of the synthesized catalyst was evaluated in a three-electrode system with the CHI 1140A electrochemical workstation (CH Instruments), using a disk electrode as the working electrode. The working electrode was prepared by loading exactly 20 μL of a catalyst ink onto the pretreated glassy carbon (GC) electrode (3 mm in diameter) and then dried at room temperature. 8.0 mg of the catalyst sample, 3 mL ethanol, and 0.2 mL Nafion solution (5 wt%) were ultrasonically mixed to form the homogenous catalyst ink. Calomel electrode in saturated KCl solution (SCE) and Pt-wire electrode were used as the reference and counter electrodes, respectively. A salt bridge was used to connect the reference electrode to the cell. Cyclic voltammograms (CVs) were recorded at 25 °C and a scan rate of 10 mV s⁻¹ in alkaline (1 M NaOH) and acidic (0.5 M H₂SO₄) O₂-saturated solutions, respectively. The potentials were converted to the values vs. reversible hydrogen electrode (RHE) according to the pH value of the applied electrolyte:²⁵

E(V vs. RHE) = E(V vs. SCE) + 0.241 + 0.0591 × pH, at 25 °C (1)

Rotating disk electrode (RDE) (RDE-2, BASi Inc.) test was performed to estimate the apparent kinetically-limiting current density (j_k = I_k/A, I_k: kinetic current, A: area of the electrode) and electron transfer number (n) of the ORR according to the K–L equation:²⁶

where I is the limiting disk current, ω is the angular frequency of rotation, A_e is the electroactive area of the electrode, F is the Faraday constant. The published data of the saturated concentrations (C₀) and diffusion coefficients (D₀) of O₂ in 0.1 M KOH and 0.5 M H₂SO₄ solutions, and the kinematic viscosities (ν) of both solutions were used.^26,27 RDE CVs were recorded in the both O₂-saturated solutions at 25 °C, at 10 mV s⁻¹ scan rate, and 300, 500, 800, 1000, and 1300 rpm rotation rates, respectively. RDE was prepared via pipetting 5 μL of the catalyst ink onto the polished GC electrode and then dried at room temperature.

Test cells with an active area of 6 cm² were assembled to evaluate the polarization stabilities of the synthesized catalyst in a direct methanol fuel cell (DMFC, an acidic fuel cell) and a direct borohydride fuel cell (DBFC, an alkaline fuel cell), respectively. The cathode and anode catalyst inks were prepared by mixing the catalyst powder, Nafion solution (5 wt%), deionized water, and ethanol with a mass ratio of 1 : 7 : 3 : 3. The cathode was prepared by coating 3.0 mg cm⁻² of catalyst onto a piece of hydrophobic carbon cloth and then heated at 130 °C for 2 h. The anodes were prepared by coating the commercial Pt/XC-72 catalyst (28.6 wt% Pt, catalyst loading: 3.0 mg cm⁻²) onto a piece of carbon paper (for DMFC) and Ni foam (for DBFC), respectively. Nafion 112 membrane was used as the electrolyte after sequentially boiled in 3 wt% H₂O₂ solution, de-ionized water, 0.5 M H₂SO₄, and de-ionized water for 30 min. An acidic MeOH solution (1 wt% MeOH in 0.5 M H₂SO₄) and an alkaline borohydride solution (1 wt% NaBH₄ in 1 M NaOH) were used as the fuels to run the DMFC and the DBFC, respectively. A salt bridge was used to connect the reference electrode (SCE) to the fuel tank for cathode polarization measurements. Cell performance of the DBFC was measured at a fuel flow rate of 15 mL min⁻¹ and a dry O₂ flow rate of 150 mL min⁻¹ at 25 °C.

3. Results and discussion

3.1. Characterizations of Co coordination compound

Fig. 1 shows the morphologies and XRD patterns of the samples synthesized through the hydrothermal reaction of ligand with Co nitrate for 12 and 72 hours at 160 °C. Co–IAA spheres and plates with light brown color were obtained after 12 and 72 hours of the hydrothermal treatment, respectively. The obtained Co–IAA was soluble in ethanol and acetone but not in water. Crystallization degree of the Co–IAA was significantly improved when the hydrothermal treatment time increased from 12 up to 72 hours since four new peaks appeared at 18.9°, 20.9°, 22.8°, and 27.7° instead of the original broad peak from 14° to 30.6° in Fig. 1(B). The morphological observations and XRD results indicated that the hydrothermal reaction of IAA with Co nitrate produced a new substance.

Fig. 1 (A) Morphologies of the samples synthesized via the hydrothermal reaction of Co nitrate with IAA (a and b) or indole (c and d) for 12 h (a and c) and 72 h (b and d) at 160 °C, and (B) their XRD patterns.

Oxygen in –COOH of IAA had two unshared pair electrons. When one pair electrons coordinated to Co(II) in the Co–IAA, the other pair electrons may bond to proton through hydrogen bond connecting the polymeric chains of coordination complexes together to form a metal–organic framework (MOF).²⁸ Therefore, the Co–IAA trended to form crystalline substance after the hydrothermal reaction with longer time.

Previous study on the hydrothermal reaction of polypyrrole (PPy) with Co(NO₃)₂ revealed that PPy reacted with Co²⁺ and released the NH proton, which led to the decrease in pH value of the treatment solution through following reaction:¹⁰

The two pyrrolic N atoms in the obtained Co–PPy bonded to the single Co ion.²⁹ However, the solution pH value changed insignificantly after hydrothermal reaction of IAA with Co(NO₃)₂ (Table 1), suggesting that Co(II) coordinated to the pyrrolic N or/and carboxylic O without loss of hydrogen in –NH and –COOH.

Table 1 pH values of the solutions before and after hydrothermal reaction of indole and IAA with Co(NO₃)₂ for 12 hours

Reactants	Chemical structure	Before reaction	After reaction
Indole + Co(NO3)2		4.80	4.73
IAA + Co(NO3)2		3.25	3.18

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In order to understand the Co coordination situation through comparison, Co coordination compound with indole (Co–indole) was also synthesized because indole was a monodentate ligand without carboxyl group, and Co(II) only can coordinate with NH. SEM images and XRD results of the Co–indole were illustrated in Fig. 1. Unlike the Co–IAA, the Co–indole was an amorphous substance with dark brown color. The FTIR spectra indicated that the Co–IAA and Co–indole exhibited the vibration bands of the out-of-plane γ C–H (748–742 cm⁻¹)³⁰ and in-plane δ C=C (1458 cm⁻¹),^30,31 similar to their corresponding ligands (Fig. 2), revealing that the benzene ring was not affected after the hydrothermal reaction. Bands at 3413 and 3425 cm⁻¹ were assigned to the associated NH mode of indole and IAA,^31,32 respectively. The bands of IAA at 1705 cm⁻¹ and 1225 cm⁻¹ were attributed to the C=O and C–O stretching vibrations of the COOH group.³¹ Compared with indole, the Co–indole exhibited a broadened NH stretching vibration band at 3413 cm⁻¹, suggesting that Co coordinated with NH. Similarly, the Co–IAA exhibited the band widening at 3425 cm⁻¹, revealing that Co coordinated with NH in the Co–IAA. Moreover, the overlapped peak around 3277 cm⁻¹ was assigned to the coordinated H₂O, suggesting that the Co ion also coordinated to H₂O together with NH. The cease of vibration bands at 626–615 cm⁻¹ and 522–511 cm⁻¹ revealed that the out-of-plane NH vibration³¹ and γ NH wagging³⁰ were restrained in the Co–indole and Co–IAA. All these evidences indicated Co(II) coordination with NH in both of Co–indole and Co–IAA. The appearance of the new band around 1624 cm⁻¹ (blue shift of C=O), and the weakening and shifting of C–O stretching vibration band from 1225 to 1184 cm⁻¹ indicated that Co(II) was also coordinated with –COOH by two points of attachment in the Co–IAA.

Fig. 2 FTIR spectra of Co–indole and Co–IAA in KBr pellets. The Co–indole and Co–IAA were synthesized by hydrothermal reactions of indole and IAA with Co nitrate at 160 °C for 12 h.

Fig. 3 shows the ¹H NMR spectra of the Co–indole and Co–IAA as well as their corresponding ligands, revealing that all the H in –NH and –COOH remained.³¹ This is coincident with the fact that pH value of the treatment solution changes insignificantly after the hydrothermal reaction of either indole or IAA with Co(NO₃)₂ (Table 1). The Co–indole exhibits a new peak at lower chemical shift than indole (Fig. 3(b)), confirming the Co coordination with –NH. Similarly, the decreases in chemical shift of –NH and –COOH in Co–IAA confirm that both –NH and –COOH are coordinated with Co(II). It is noted that the chemical shift of 10H in the Co–IAA is significantly changed. This result indirectly proves that the chemical situation of –COOH has changed because only –COOH could affect the chemical situation of 10H. 11C and 3C positions are inactive (valence bond saturated) and unable to affect the chemical situation of 10H.

Fig. 3 ¹H NMR spectra of Co–indole and Co–IAA and their corresponding ligands (a) and fine chemical shifts of –NH and –COOH before and after hydrothermal reaction of indole and IAA with Co nitrate for 12 h (b) with TMS as the internal reference. Frequency: 500 MHz, solvent: CD₃COCD₃.

Fig. 4 shows the CVs of the synthesized Co–IAA/BP and Co–indole/BP in alkaline and acidic O₂-saturated solutions. The Co–IAA/BP and Co–indole/BP exhibited the same on-set ORR potential in alkaline electrolyte, suggesting that Co(II) as the electron deficient electrophilic center in both catalysts behaved in the same way: trended to draw the nucleophiles of dioxygen and H₂O. Comparing (a) with (b) of Fig. 4, it was found that both catalysts had higher ORR currents and potentials in alkaline electrolyte than those in acidic one, indicating that both catalysts catalyzed ORR (4) more effectively than ORR (5).

O₂ + 2H₂O + 4e⁻ → 4OH⁻ (4)

O₂ + 4H⁺ + 4e⁻ → 2H₂O (5)

Fig. 4 CVs of the hydrothermally-synthesized Co–indole/BP and Co–IAA/BP in the (a) alkaline (1 M NaOH) and (b) acidic (0.5 M H₂SO₄) O₂-saturated solutions at 25 °C. Scan rate: 10 mV s⁻¹.

It was considered that the electrophilic Co(II) drew the nucleophilic dioxygen²³ but repelled the electrophilic proton. Protons were absorbed at nucleophilic centers such as O in Co–O and N in Co–N bond such that a space steric effect arose from the separated adsorption of dioxygen and proton. This space steric effect was the barrier to hinder the reaction of dioxygen with protons, leading to the large polarization of ORR in acidic electrolyte. The Co–IAA/BP showed higher ORR potential than the Co–indole/BP (Fig. 4(b)), revealing that the extra Co–O bond in the Co–IAA/BP (the part of Co coordination with –COOH) promoted ORR in acidic electrolyte. Moreover, the Co–IAA/BP exhibited higher ORR currents than the Co–indole/BP, revealing that more catalytic sites were created compared with the Co–indole/BP.

3.2. Pyrolyzed Co coordination compound

MOF-based ORR catalysts usually show high catalytic activity in alkaline electrolyte.³³ Pyrolysis can effectively enhance the ORR activity of the MOF derived catalysts in acidic electrolyte due to the formation of catalytic Co–N_x.^34,35 Similar to the nitrogen in Co–N_x, the oxygen in Co–O bond as the nucleophilic center can draw proton to enhance ORR in acidic electrolyte. Co oxides are full of Co–O bond. Co oxides could be obtained via pyrolysis of a cobalt compound but they were easy to agglomerate during pyrolysis.³⁶ Mesoporous Co₃O₄ had excellent ORR activity with small overpotential.³⁷ It was considered that carbonization of ligand in the Co coordination compound could effectively hinder the growth of Co oxides to prevent the agglomeration. The Co–IAA was thus used as the template molecule to distribute cobalt on carbon support.

The TG curve showed a two-stage mass loss when heating the Co–IAA from 50 to 900 °C (Fig. 5). From the differential thermogravimetric (DTG) curve of IAA, peak at 207 °C (started at 166 °C) was assigned to the IAA decomposition. The large DTG peak of the Co–IAA (with 87% of mass loss at 166 °C) was attributed to the decomposition of the coordinated H₂O.³⁸ It was found that the sequent DTG peak temperature of the Co–IAA (270 °C) was higher than that of its ligand, revealing that the molecular structure of Co–IAA was more stable than that of IAA. This result suggested that a MOF structure might be established in the Co–IAA because decomposition of a MOF was more difficult than its ligand.

Fig. 5 TG and DTG curves of (a) Co–IAA, (b) IAA at a heating rate of 10 °C min⁻¹.

Fig. 6 shows the TEM image of the Co–IAA/BP and HRTEM image of the pyrolyzed Co–IAA/BP. The selected area electron diffraction (SEAD) pattern (inset of Fig. 6(a)) indicated that the crystalline Co–IAA was formed on carbon support particles after the hydrothermal reaction of IAA with Co(NO₃)₂ in the carbon suspension. It was expected that both Co–N_x and Co–O_x would be created after pyrolysis of the Co–IAA/BP. However, elemental analyses revealed that nitrogen was not detected in the catalyst. The pyrolyzed Co–IAA/BP had relative higher content of oxygen than the Co–indole/BP, as tabulated in Table 2. The HRTEM observation indicated that nano-crystals were formed on the carbon support particles (Fig. 6(b)). Through Fast Fourier Transformation (FFT) treatment, the crystalline lattice stripes represented the (400) and (311) planes of Co₃O₄ (Fig. 6(c)). Because the electronegativity of oxygen was more negative than that of nitrogen, Co tended to bind with oxygen rather with nitrogen during pyrolysis. The oxygen in Co₃O₄ came from the coordinated H₂O in the Co–IAA (FTIR in Fig. 4 and TGA in Fig. 5). As IAA had higher coordination capability (with three attachable points) than indole (with single attachable point), the pyrolyzed Co–IAA/BP contained more Co than the pyrolyzed Co–indole/BP (Table 2).

Fig. 6 (a) TEM image and SEAD pattern of the Co–IAA/BP, (b) HRTEM image of the pyrolyzed Co–IAA/BP and (c) its corresponding fast Fourier transformation.

Table 2 Cobalt, nitrogen and oxygen contents in the pyrolyzed Co–indole/BP and Co–IAA/BP

Catalyst	C/wt%	Co/wt%	N/wt%	O/wt%	H/wt%
Pyrolyzed Co–indole/BP	98.84	0.12	Null	0.79	0.21
Pyrolyzed Co–IAA/BP	98.08	0.45	Null	1.22	0.23

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Fig. 7(a) shows the RDE CVs of the original and pyrolyzed Co–IAA/BP in the O₂-saturated 0.5 M H₂SO₄ and 0.1 M KOH solutions. Pyrolysis enhanced the catalytic activity of the Co–IAA/BP, leading to the significant increase in ORR current and potential. This result suggested that the catalytic site was increased in number (showing large current) after the pyrolysis, and the formed site was more active (showing small polarization) than the site in the Co–IAA. Regarding the K–L plots in Fig. 7(b), the apparent kinetically-limiting current density and electron transfer number of ORR at the pyrolyzed Co–IAA/BP were calculated and listed in Table 3. The pyrolyzed Co–IAA/BP showed higher apparent kinetically-limiting current density and larger electron transfer number than the Co–IAA/BP. Coincident with the RDE CV results, the chronoamperometric curves of ORR also showed that the pyrolyzed Co–IAA/BP had higher ORR potential in the DBFC (an alkaline fuel cell) than that in the DMFC (an acidic fuel cell), as shown in Fig. 8. Moreover, the pyrolyzed Co–IAA/BP demonstrated good short-term polarization stabilities in both acidic and alkaline electrolytes.

Fig. 7 (a) RDE CVs of the synthesized catalysts in O₂ saturated 0.5 M H₂SO₄ and 0.1 M KOH solutions at a rotation rate of 1300 rpm. Scan rate: 10 mV s⁻¹, temperature: 25 °C. (b) The K–L plots at 0.2 V vs. RHE in 0.5 M H₂SO₄ and 0.4 V vs. RHE in 0.1 M KOH.

Table 3 Apparent kinetically-limiting current density (j_k) and electron transfer number (n) of ORR at the synthesized catalysts in acidic and alkaline electrolyte

Electrolyte	Catalyst	jk/mA cm^−2	n
0.5 M H2SO4	Pyrolyzed Co–IAA/BP	9.4	2.56
0.1 M KOH	Co–IAA/BP	13.3	3.10
	Pyrolyzed Co–IAA/BP	23.8	3.91

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Fig. 8 Polarization stabilities of the pyrolyzed Co–IAA/BP in the DMFC and the DBFC. Both cells were operated at a cathode current of 60 mA, an anolyte flow rate of 5 mL min⁻¹, a dry O₂ flow rate of 50 mL min⁻¹ and 25 °C. Anolyte: 1 wt% MeOH in 0.5 M H₂SO₄ for DMFC, 1 wt% NaBH₄ in 1 M NaOH for DBFC.

Dioxygen absorbed at two active Pt sites in “bridge-cis” interaction such that ORR occurred at Pt via 4e-transfer pathway.³⁹ The pyrolyzed Co–IAA/BP catalyzed ORR with an electron transfer number of 3.91 in alkaline electrolyte but 2.56 in acidic electrolyte. Regarding the atom arrangement of the Co₃O₄ (111) plane,⁴⁰ it was considered that dioxygen probably absorbed at two adjacent Co centers in “bridge-trans” interaction, as shown the speculated ORR models in Fig. 9. The 2e-transfer pathway dominated the ORR on the pyrolyzed Co–IAA/BP in acidic electrolyte. Because H₂O molecules were coordinated to the single Co center together with dioxygen, each oxygen atom of dioxygen (at two adjacent Co centers) easily reacted with two coordinated H₂O when alkaline electrolyte was employed. The 4e-transfer pathway thus dominated the ORR, as shown in Fig. 9(b). Similar to the Co–IAA/BP, the pyrolyzed Co–IAA/BP showed larger ORR polarization in acidic electrolyte than that in alkaline one, which can be attributed to the space steric effect of the Co–O bond.

Fig. 9 Speculated ORR models in (a) acidic and (b) alkaline electrolytes on (111) of Co₃O₄.

The performance of the original and pyrolyzed Co–IAA/BP was then verified in an alkaline fuel cell: direct borohydride fuel cell (DBFC) which uses alkaline borohydride solution as the anode fuel.^41–44 The DBFC using the original Co–IAA/BP showed comparable performance to the cell using the commercial 10 wt% Pt/XC-72 at ambient conditions, as shown in Fig. 10. When the pyrolyzed Co–IAA/BP was employed, the cell showed a peak power density of 186 mW cm⁻² which was comparable to the performance of the cell using 20 wt% Pt/XC-72 (190 mW cm⁻²), confirming that the pyrolyzed Co–IAA/BP was comparable to 20 wt% Pt/XC-72 in performance though it contained low content of cobalt (0.45 wt%, much lower than 20 wt% of Pt in the Pt/XC-72 catalyst).

Fig. 10 Performance comparison of DBFCs using pyrolyzed Co–IAA/BP and commercial Pt/XC-72 as the cathode catalyst at ambient conditions. Catalyst loading in cathode and anode: 3 mg cm⁻², anode catalyst: 28.6 wt% Pt/C, electrolyte: Nafion 112. Fuel: 5 wt% NaBH₄ and 10 wt% NaOH solution at a flow rate of 15 mL min⁻¹. Dry O₂ at 150 mL min⁻¹.

4. Conclusions

Crystalline cobalt coordination compound with indole acetic acid (Co–IAA) was synthesized using the hydrothermal method for fabrication of high performance catalyst. Co–IAA converted to Co₃O₄ after pyrolysis at 900 °C.

The catalytic site composed of an electrophilic center coupled with a nucleophilic center (coupling site) is necessary to catalyze ORR effectively in acidic electrolyte.

The original and pyrolyzed carbon supported Co–IAA (Co–IAA/BP) exhibited higher catalytic activity toward ORR in alkaline electrolyte than that in acidic one because dioxygen and water were absorbed at the electrophilic Co center in alkaline electrolyte whereas dioxygen and proton were separately absorbed at electrophilic Co center and nucleophilic center (O) in acidic electrolyte. The performance of the original and pyrolyzed Co–IAA/BP was comparable to that of 10 and 20 wt% Pt/XC-72 in the direct borohydride fuel cell, respectively.

Acknowledgements

This work is financially supported by the National Natural Science Foundation of China, Grant No. 21476200, 21276229 and 51271164; the Zhejiang Provincial Natural Science Foundation of China, Grant No. Z4110126; and the Fund from Science & Technology ministry of Zhejiang province (2014C31003).

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