manganese-based non-precious metal catalyst for oxygen

Manganese-Based Non-Precious Metal Catalyst for Oxygen Reduction in Acidic Media

Drew C. Higginsa,b, Gang Wub, Hoon Chungb, Ulises Martinezb, Shuguo Mac,

Zhongwei Chena, and Piotr Zelenayb

a Department of Chemical Engineering, Waterloo Institute for Nanotechnology University of Waterloo, Waterloo, Ontario, N2L 3G1, Canada.

b Materials Physics and Applications Division, Los Alamos National Laboratory Los Alamos, New Mexico 87545, USA

c Department of Chemical Engineering, University of South Carolina Columbia, South Carolina 29208, USA

Non-precious metal catalysts (NPMCs) based on manganese (Mn) are prepared by heat treating polyaniline (PANI), manganese acetate and Ketjenblack EC300J (KJ) carbon supports. Using a heat treatment temperature of 950oC, followed by an acid leaching and second heat treatment step, Mn-PANI-KJ catalysts are found to provide onset and half-wave potentials of ca. 0.90 and 0.77 V vs. RHE, respectively, in 0.5 M H2SO4 electrolyte. After 5,000 cycles of electrochemical durability testing, Mn-PANI-KJ demonstrates a half-wave potential loss of only ca. 20 mV, superior to the 80 mV loss for our previously developed iron (Fe)-PANI-KJ catalyst. Increased surface nitrogen concentrations and relative ratios of pyridinic to graphitic nitrogen species were observed at increased Mn-PANI-KJ preparation temperatures, along with the evolution of graphene-like and graphitic nanoshell structures.

Introduction Despite significant progress realized in recent years in the development of polymer electrolyte membrane fuel cells (PEFCs), widespread commercial success of this technology is still hindered by several technical challenges, including cost, performance and durability. The extensive reliance on platinum (Pt) based catalysts to facilitate the anodic hydrogen oxidation reaction (HOR) and cathodic oxygen reduction reaction (ORR) has been a major limiting factor, owing to the high cost of this precious metal and monopolized global distribution (1). Combined with the volatile pricing of this natural resource, reducing or eliminating the Pt dependence of PEFC technologies is required to realize sustainable commercial success. As the ORR reaction kinetics are inherently six order of magnitude slower than the HOR (2), significantly higher Pt loadings at the cathode are required to facilitate the ORR at rates sufficient for practical devices. This has inspired significant research activities directed towards the replacement of conventional Pt catalysts with ORR-active non-precious metal catalysts (NPMCs) (3-9).

Since the 1960s, the majority of NPMC research has focused on the development of transition metal-nitrogen-carbon complexes (M-N-C), with the most active and stable catalysts prepared using high-temperature (i.e. 700-1100oC) heat treatment, and with iron (Fe) or cobalt (Co) employed as the transition metal of choice (10-15). While new

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synthesis strategies have advanced the performance and electrochemical durability of these catalysts to unprecedented levels (12,14), the current capabilities are still not adequate for practical deployment. It is therefore of interest to deviate from conventional approaches to develop and investigate unique NPMC configurations with the potential to overcome remaining challenges.

Manganese (Mn)-based materials have commonly been employed as ORR active catalysts in alkaline media (16-19); however their application as ORR catalysts under acidic conditions, such as those encountered during PEFC operation, remains very limited (20). Additionally, the recent emergence of many supposedly “metal-free” graphene-based catalysts has been reported in the literature, with these materials fabricated using graphene oxide precursors (GO). The GO materials are commonly prepared by the Hummers method, employing potassium permanganate (KMnO4) as a strong oxidizing agent, and inevitably leading to confounding Mn impurities. These impurities have been shown to have a significant effect on the resulting ORR activity in alkaline media (21,22). This work has inspired us to investigate the role of Mn on the structure, properties and performance of NPMCs operating in acidic electrolytes. It has been well established that these properties are directly governed by the selection of transition metal species, and we investigate the effect of using Mn to prepare our previously developed polyaniline (PANI)-based catalyst technologies (14,23,24).

Experimental Methods Catalyst Synthesis

In a typical synthesis, 2 mL of aniline was added to 400 mL of 1.5 M HCl, followed by the addition of 2 g manganese acetate. To polymerize the aniline, 5 g of ammonium persulfate was added, and the mixture stirred for 3 hours. During this time, 0.4 g of functionalized Ketjenblack EC300J (KJ) carbon support was dispersed in distilled deionized water by ultrasonication. The KJ had previously been functionalized by treatment in 70% HNO3 at 70oC for 8 hours. The entire mixture was stirred at room temperature for 48 hours, after which the solvent was evaporated at 80oC. The resulting solid was collected, grinded to a fine powder using a mortar and pestle, and then heat treated under a nitrogen environment at a temperature ranging from 850 to 950oC for 1 hour. The heat-treated materials were then subject to acid leaching in 0.5 M H2SO4 at 80oC for 8 hours, followed by a second heat treatment at 900oC for 3 hours. The resulting Mn-PANI-KJ catalyst was then collected for subsequent characterization and electrochemical performance evaluation. Characterization

Transmission electron microscopy (TEM) images were collected on a FEI Tecnai F30

Analytical TEM system operating at 300 keV. X-ray photoelectron spectroscopy (XPS) was carried out to investigate surface atomic contents and species identification, done using a Thermo Scientific ESCALAB 210 and MICROLAB 310D spectrometer employing Mg Kα radiation.

ECS Transactions, 61 (31) 35-42 (2014)



Electrochemical Evaluation

Catalyst inks for all electrochemical evaluations were prepared by sonicating 10 mg of Mn-PANI-KJ catalyst in 1 mL of isopropanol. The inks were then deposited onto a glassy carbon disk electrode using a micropipette to achieve a loading of 0.6 mg cm-2. ORR activity and four-electron selectivity of the Mn-PANI-KJ catalysts were evaluated using rotating disk electrode (RDE) and rotating ring disk electrode (RRDE) testing, respectively. All measurements were conducted using a CHI electrochemical station and a standard three-electrode electrochemical glass cell. An Hg/HgSO4 electrode and a graphite rod were used as the reference and counter electrodes, respectively. After measurements, all potentials were converted to the RHE scale. ORR polarization curves were obtained at room temperature using steady-state staircase voltammetry testing in oxygen-saturated 0.5 M H2SO4. The electrode was rotated at 900 rpm, and a step size of 30 mV and time period of 30 s step-1 was used. The ring electrode was polarized at 1.2 V vs. RHE for hydrogen peroxide detection, with the method for calculating the selectivity towards the four-electron reduction of oxygen to water described elsewhere (25). Electrochemical durability was investigated by repeatedly cycling the electrode potential between 0.6-1.0 V vs. RHE in oxygen-saturated solution.

Results and Discussion

ORR Performance Evaluation

The ORR performance of Mn-PANI-KJ catalysts prepared using heat-treatment temperature ranging from 850 to 950oC is displayed in Figure 1a. The sample heat treated at 850oC shows an onset potential of ca. 0.84 V vs. RHE, whereas the samples heat-treated at 900 and 950oC display a relatively higher onset potential of ca. 0.89-0.90 V vs. RHE. This near identical onset potential for the samples prepared at 900 and 950oC likely indicates a similar active site structure formed in this catalyst system at temperatures beyond 900 oC. The Mn-PANI-KJ catalyst prepared at 950oC shows the highest ORR performance, including a half-wave potential of 0.77 V vs. RHE, the best ever reported for Mn-based catalysts. Upon comparison with Co-PANI-KJ catalyst (Figure 1b), a significant increase in both onset (70 mV) and half-wave (30 mV) potential is observed. Additionally, Mn-PANI-KJ demonstrates an onset potential similar to that of our previously reported high-activity Fe-PANI-KJ catalyst (14), along with a half-wave potential only ca. 40 mV lower. The H2O2 selectivity of the optimized (950oC treatment) Mn-PANI-KJ catalyst was evaluated by RRDE testing to be ca. 5% over the broad range of electrode potentials investigated. These results indicate that Mn can act as an effective transition metal species for preparing NPMC with good ORR performance in acidic electrolytes. Electrochemical Durability Investigation

The electrochemical durability of Mn-PANI-KJ catalyst prepared at 950oC was

evaluated by repeated electrode potential cycles from 0.6 to 1.0 V vs. RHE under oxygen saturation (Figure 2a). After 5,000 cycles, Mn-PANI-KJ demonstrates a loss in half-wave potential of only ca. 20 mV, along with a small loss in the mass-transport limited current density likely arising from the changes in the electrode microstructure (26). This




is far superior to our previously prepared Fe-PANI-KJ catalysts, that, despite showing higher initial ORR performance, demonstrates a ca. 80 mV loss in half-wave potential after 5,000 cycles (Figure 2b). Additionally, after cycling the Mn-PANI-KJ electrode 15,000 times, the loss of half-wave potential was only ca. 30 mV, indicating the excellent stability capabilities of this newly developed catalyst system.

Figure 1. (a) ORR polarization curves for Mn-PANI-KJ catalysts prepared at different heat-treatment temperatures and (b) Mn-PANI-KJ, Co-PANI-KJ and Fe-PANI-KJ catalysts obtained in oxygen-saturated 0.5 M H2SO4 at 900 rpm. Figure 2. ORR activity of (a) Mn-PANI-KJ and (b) Fe-PANI-KJ catalysts after electrochemical durability investigations involving repeated cycling of the electrode potential between 0.6 and 1.0 V vs. RHE in 0.5 M H2SO4. Electron Microscopy Studies

To evaluate the effect of heat-treatment temperature on the morphology of the

resulting Mn-PANI-KJ catalysts, TEM analysis was carried out with typical images of the catalyst materials provided in Figure 3. At temperatures exceeding 900oC (Figure 3b-d), the evolution of distinct graphene-like structures is observed in the catalysts, including the formation of graphitic nanoshells observed at 950oC (Figure 3d). Although the detailed mechanistic pathway of active site formation is still unclear for NPMCs, a corresponding increase in ORR activity above 900oC is observed along with the formation of these graphene-like and graphitic nanostructures. This may provide more

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edge-plane exposure for oxygen adsorption and subsequent reduction (27-29), and thereby contribute to the higher activity observed by RDE evaluation.

Figure 3. TEM images of Mn-PANI-KJ catalysts prepared at (a) 850, (b) 900 and (c) 950oC. (d) High-resolution TEM image of a graphitic nanoshell in the sample heat-treated at 950oC. X-ray Photoelectron Spectroscopy Investigation

XPS was used to investigate the surface nitrogen concentrations and relative species

concentrations of the developed catalysts. High-resolution N 1s spectra for Mn-PANI-KJ catalysts prepared at varying temperatures along with their evaluated nitrogen content are displayed in Figure 4. Each signal is deconvoluted into three individual peak contributions arising from pyridinic (ca. 398.6 eV), graphitic (ca. 401.1 eV) and oxidized (ca. 403.2 eV) nitrogen species. Increased surface nitrogen concentrations are observed with increasing heat-treatment temperatures, with a maximum of 8.3 at.% of nitrogen found for the catalyst prepared at 950oC. Interestingly, this observation is contradictory to the traditionally lower nitrogen surface concentrations observed at higher temperatures for a wide array of Fe and/or Co based NPMCs (10,12,23,30). This provides further indication that the role of transition metal species during catalyst preparation is unique to the choice of metal type. Additionally, increased pyridinic to graphitic nitrogen peak ratios are observed at higher temperatures, with ratios of 0.82, 0.97 at 0.99 determined for Mn-PANI-KJ catalysts prepared at 850, 900 and 950oC, respectively. This again is in contrast to previous reports on NPMC and nitrogen-doped carbon development, whereby increased graphitic nitrogen contents are commonly observed at elevated temperatures (31-33). This suggests that Mn-based species may provide some sort of stabilizing role




for edge-plane residing pyridinic nitrogen species at elevated temperatures, although this observation requires further investigation.

Figure 4. High resolution N 1s XPS spectra for Mn-PANI-KJ catalysts prepared at 850, 900 and 950oC.

Conclusions

A new class of Mn-based NPMCs was prepared by heat treating a mixture of manganese acetate, polyaniline and a high surface area carbon support. The Mn-PANI-KJ catalysts prepared at a temperature of 950oC provides an ORR onset potential of 0.90 V vs. RHE, and a half-wave potential of ca. 0.77 V vs. RHE in acidic (0.5 M H2SO4) electrolyte. This represents the first report of an Mn-based catalyst with high ORR activity in acidic electrolyte, approaching that of state-of-the-art Fe-based NPMCs. Additionally, after 5,000 cycles of electrochemical durability testing, Mn-PANI-KJ displays a loss in half-wave potential of only ca. 20 mV, a significant improvement over the 80 mV loss demonstrated by Fe-PANI-KJ. The role of the heat-treatment temperature employed during catalyst synthesis was investigated, with the formation of graphene-like structures and graphitic nanoshells observed at a temperature of 900oC and higher.




Higher surface nitrogen concentrations and pyridinic to graphitic nitrogen content ratios are indicated by XPS for samples prepared at increased temperatures, which is opposite the trend reported for conventional Fe and Co-based NPMCs.

Acknowledgments This work was supported by the Natural Science and Engineering Research Council of Canada (NSERC), the U.S. Department of Energy (DOE) through the Fuel Cell Technologies Office, and by Los Alamos National Laboratory through the Laboratory-Directed Research and Development Program.

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manganese-based non-precious metal catalyst for oxygen

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