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Fully Synthetic Approach towards Transition MetalNitrogen-Carbon Oxygen Reduction Electrocatalysts Rohan Gokhale, Surendra Thapa, Kateryna Artyushkova, Ramesh Giri, and Plamen Atanassov ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b00537 • Publication Date (Web): 17 Jul 2018 Downloaded from http://pubs.acs.org on July 18, 2018

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Fully Synthetic Approach towards Transition Metal-Nitrogen-Carbon Oxygen Reduction Electrocatalysts Rohan Gokhale,† Surendra Thapa,§ Kateryna Artyushkova,† Ramesh Giri,§,* and Plamen Atanassov†,* † §

Center for Micro-Engineered Materials (CMEM), University of New Mexico, Albuquerque, NM 87106, USA. Department of Chemistry, University of New Mexico, Albuquerque, NM, 87106, USA.

ABSTRACT: We report a non-pyrolytic chemical synthesis of model iron-nitrogen-carbon electrocatalysts for oxygen reduction reaction (ORR) to elucidate on the role of Fe-N centers in the catalysis mechanism. The graphene supported, and unsupported catalysts were analyzed in detail by X-ray spectroscopy techniques. The electrochemical analysis was performed by linear sweep voltammetry and square wave voltammetry in 0.5 M H2SO4 and 0.1 M KOH electrolytes. In this article, with the use of model catalysts, we bring forth and confirm the difference in the specific role of Fe-N active sites towards ORR in the acidic and alkaline environment.

Platinum group metal free (PGM-free) iron-nitrogen-carbon (FeN-C) catalysts are some of the best-known substitutes for platinum-based cathode catalysts for oxygen reduction reaction (ORR) in proton exchange membrane and alkaline fuel cells. Extensive research has provided a detailed understanding of platinum-based catalysts, which is not the case for Fe-N-C catalysts. Fundamental understanding of the structure-activity relationship of Fe-N-C materials is one of the most important challenges. Hightemperature pyrolytic methods conventionally used for the synthesis of these materials introduces a high degree of structural heterogeneity and disorder, complicating precise determination of the nature of active sites in the oxygen reduction reaction. Numerous efforts have been directed towards elucidating the nature of active sites, mechanistic understanding of oxygen reduction,1-5 and addressing controversies regarding the precise role of iron and nitrogen dopants in the carbon matrix on the O2 reduction pathways. Most prominently Artyushkova et al.,6 Zitolo et al.,7 and Ramaswamy et al.8 have provided detailed insights into the role of nitrogen coordinated to iron (Fe-Nx) sites using X-ray spectroscopic techniques. There is still uncertainty because of limitations of spectroscopic tools in distinguishing between a multitude of Fe-Nx types possible and the inhomogeneity in the chemistry of the pyrolytically derived materials.9 The study of physical and electrochemical properties of systematically designed model catalysts that possess an accurately known homogeneous structure has gained attention recently. Guo et al. created HOPG structures with specific chemistries of nitrogen doping (e.g., graphitic, pyridinic, etc.) by an ion beam etching method.10 These model catalysts enabled the group to confirm specific importance of different N sites, particularly the role of nitrogen coordinated to iron in both acidic and alkaline media in comparison to other nitrogen chemistries. Liang Shi Li’s group has also established mechanistic aspects of ORR using electrochemical methods applied to a well-

defined nano-graphene structure.11-12 In addition to general exploration of model material systems, non-pyrolytic chemical synthesis of molecular catalysts with desired structure has also been investigated previously to some extent. Klaus Mullen’s group reported the synthesis of triangular trinuclear MN4 complexes for oxygen reduction catalytic activity.13 In this work, we establish a novel molecular synthesis strategy to generate model Fe-N-C catalysts that mimic the microstructure of traditional pyrolyzed Fe-N-C materials. This strategy provides a new non-pyrolytic chemical route to the synthesis of Fe-N-C catalysts enabling better control over the final structure. The model catalysts synthesized in this work are based on arylated arenes. Arylated arenes are large molecular clusters formed by binding of aromatic moieties. These molecules possess strong π-π interaction with graphene-based support. To imitate the mesomeric Fe-N4 centers of the Fe-N-C catalysts in which four pyridinic nitrogens are bound to iron symmetrically, we have synthesized arene with pyridinic nitrogen and complexed it with Fe2+. These molecules were also supported onto 3D graphene nanosheets. Metal-free NC and Fe-N-C model catalysts were characterized by spectroscopic methods and tested electrochemically in both acid and alkaline electrolytes. The strikingly different role of transition metal in mechanism of oxygen reduction acidic and alkaline media is established from this study. This work opens up new avenues to a fully synthetic approach for generating Fe-N-C catalysts by nonpyrolysis routes. Synthesis of the pyridinic arene and complexation was performed based on literature14-15 and depicted in Scheme 1. The compound 6 described in Scheme 1 is termed as PyrN-Ar for simplicity in this article. An Fe-coordinated compound Fe-PyrN-Ar (Fig S1) was also obtained for further study. These catalysts were heterogenized on 3D graphene nanosheets (GNS). The heterogenized catalysts on GNS were then termed as PyrN-Ar-GNS and FePyrN-Ar -GNS. Physical and electrochemical analysis of all the catalysts was performed. Additional synthesis and characterization details and description is available in the supporting information.

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Scheme 1. Synthesis of the molecular model catalyst PyrNAr

Table 1. Elemental composition of model catalysts (% is atomic percentage) Samples

C 1s %

N 1s %

O 1s %

PyrN-Ar

90.7

4.2

5.1

Fe 2p %

PyrN-Ar-GNS

84.9

1.9

13.2

Fe-PyrN-Ar

85.6

3.1

11.0

0.7

Fe-PyrN-ArGNS

83.3

2.0

14.5

0.6

limited signal. Large amounts of surface oxides present in GNS cause increase in the detected oxygen in supported PyrN-Ar-GNS sample. Unsupported Fe-PyrN-Ar contains a high amount of surface carbon oxides as well. A small decrease in nitrogen and increase in oxygen surface concentration is observed when FePyrN-Ar is dispersed on GNS support (Fe-PyrN-Ar-GNS). The chemistry of nitrogen can be studied in detail by high resolution XPS (Figure 1 and Table 2). Very importantly, we were able to achieve a high purity of PyrN-Ar material with 76.7 rel % of nitrogen present as pyridinic. Some edge amine groups are detected as well. A small amount of peak due to N-H comes from some of the pyridinic nitrogen that is hydrogenated. When Fe is coordinated with the PyrN-Ar (Fe-PyrN-Ar), the pyridinic N content decreases stoichiometrically to 47.0 at% as approximately 30% of pyridinic nitrogen is liganded with iron and contributes to the peak due to Nx-Fe.16-17 20% of nitrogen is also present as hydrogenated and protonated. This distribution of nitrogen chemistries is very different from usually observed in the materials obtained via pyrolytic routes.6 In the latter, we usually observe a much smaller amount of pyridinic nitrogen (20-25%) and Nx-Fe (10-15%) with much larger contribution from hydrogenated nitrogens on the order of 30-40 %. Even though the material synthesized via proposed chemical route does not result in the single homogeneous type of nitrogen coordinated to iron, it is much purer than those obtained via pyrolytic routes.

Table 2. Atomic% distribution of nitrogen species in the nitrogen content

Samples

N pyrid %

N amin e%

NxFe %

N-H %

N gr/ N+ %

N O %

NO % Figure 1. High resolution N 1s spectra fitted with individual peaks.

PyrNAr

76.6

11.1

0.0

7.7

0.0

4.7

0.0

PyrNAr-GNS

6.4

40.3

0.0

42.3

10.8

0.1

0.0

FePyrNAr

47.0

0.0

27.6

12.1

13.3

0.0

0.0

FePyrNAr-GNS

13.2

0.0

24.3

32.1

16.4

4.6

9.2

The elemental and structural composition of the model catalysts was studied using X-ray photoelectron spectroscopy (XPS). Table 1 shows the elemental composition of the catalysts. The synthesized PyrN-Ar comprises mainly of carbon with the highest amount of nitrogen detected. When the catalyst PyrN-Ar was supported on GNS (PyrN-Ar-GNS), the overall nitrogen content decreases due to graphene support contributing to the surface

When the PyrN-Ar and Fe-PyrN-Ar catalysts are dispersed on the GNS support, a decrease in the pyridinic N and increase in the hydrogenated N concentration is observed. Surface oxide groups present at the surface of GNS support such as -OH, -COOH can provide a source of protons for protonation and hydrogenation of pyridinic nitrogen to a large extent. The degree of hydrogenation is much larger for metal-free analog PyrN-Ar as a much larger amount of pyridinic nitrogens is available for protonation than in metal containing Fe-PyrN-Ar material. Analysis of the carbon chemistry (Figure S2 and Table S1) shows high aromatic carbon content of both chemically synthesized PyrN-Ar and Fe-PyrN-Ar catalysts (~77.4 and 76.7 at% respectively) Heterogenization of these materials with GNS (PyrN-ArGNS and Fe-PyrN-Ar-GNS) causes an increased content in the disordered carbons and carbon oxides (C-C, C-C=O, C-O, etc.) which in all probability come mainly from the graphene-based support. Finally, analysis of iron content (Figure S3 and Table S2) reveals a high amount of oxide content (~97 at%) in Fe-PyrN-Ar. The supported catalyst Fe-PyrN-Ar-GNS contains a relatively higher amount of Fe-Nx and metallic Fe than unsupported (~13.8 and 6.9


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at% respectively). Not all the iron from Fe salt precursor added during the synthesis is liganded to nitrogen-carbon network with excess contributing as oxidized form of Fe. Square wave voltammetry results in N2 saturated 0.5 M H2SO4 (Fig 2) shows the several redox transitions in the catalysts. The Fe-PyrN-Ar-GNS shows the characteristic Fe2+/3+ redox peak (~0.615 V vs RHE). The Fe2+/3+ redox peak is not observed on the molecular catalyst (Fe-PyrN-Ar on GCE). The unsupported catalyst contained much more of oxides and fewer exposed atomically dispersed Nx-Fe centers. The Fe2+/3+ redox peak may be coming

PyrN-Ar on GCE Fe-PyrN-Ar on GCE PyrN-Ar-GNS Fe-PyrN-Ar-GNS

5 4 3

PyrN-Ar on GCE Fe-PyrN-Ar on GCE PyrN-Ar-GNS

0.012

PyrN-Ar on GCE Fe-PyrN-Ar on GCE

0.008

0 -1 -2 -3

Current (mA)

0.1

1

Current (mA)

Current (mA)

from the latter and not the nanoparticles of iron covered in the oxide. Linear sweep voltammetry (LSV) study in O2 saturated 0.5 M H2SO4 (Fig 3a), demonstrates the oxygen reduction activity of the model catalysts in an acidic electrolyte. No ORR activity is observed on the molecular catalysts loaded directly on the glassy carbon electrode (PyrN-Ar on GCE and Fe-PyrN-Ar on GCE). This can be attributed to absence (in PyrN-Ar) or limited availability (in Fe-PyrN-Ar) of atomically dispersed iron coordinated to nitrogen. Moreover, the absence of electronically conducting framework in these materials inhibits the electrocatalytic reaction.

0.2

2

0.0 -0.1

0.004 0.000 -0.004 -0.008

-4

-0.2

(a)

-5 0.0

0.2

0.4

0.6

0.8

(c)

(b) -0.012

1.0

0.0

0.2

Potential (V vs RHE)

0.4

0.6

0.8

1.0

0.0


0.2

0.4

0.6

0.8

1.0


Figure 2. Square wave voltammetry (SWV) results of the model catalysts in N2 saturated 0.5 M H2SO4 (a) SWV results on all catalysts (bc) magnified images 80 PyrN-Ar on GCE Fe-PyrN-Ar on GCE PyrN-Ar-GNS Fe-PyrN-Ar-GNS

0

0.00

100


-0.75

40 20


(b)

-2

-3


-4

(c)

(a) -1.00 300

400

500

600

700

Potential (mV vs RHE)

800

900

0 300

400

500

600

700


40

-

60

HO2 (%)

-2

j (mA. cm )

H2O2 (%)

-2

-0.50

60

-1

80

-0.25

j (mA. cm )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


100

200

300

400

500


600

20

(d)

0 0

100

200

300

400

500

600


Figure 3. (a) LSV studies of model catalysts in O2 saturated 0.5 M H2SO4 (b) % H2O2 generated on the ring in 0.5 M H2SO4 (c) LSV studies of in O2 saturated 0.1 M KOH (d) %HO2- generated on the ring in 0.1 M KOH. The ORR of the graphene supported Fe-PyrN-Ar-GNS shows an onset potential ~0.68 V vs RHE. The ORR peak overlaps perfectly with the potential of Fe2+/3+ transition which was seen previously in the SWV results (Fig 2). This indicates that the oxidation of Fe2+ to Fe3+ takes place simultaneously with reduction of oxygen and confirms the active role of Fe containing sites in the ORR catalysis. The major factor contributing to the difference between the performance of Fe-PyrN-Ar-GNS and PyrN-Ar-GNS is, undoubtedly absence of Fe-Nx centers in the metal-free material. The peroxide generated by the first 2-electron oxygen reduction step is described in Figure 3b. Metal-free PyrN-Ar on glassy carbon and PyrN-Ar-GNS shows the highest amount of peroxide generated followed by Fe-PyrN-Ar on glassy carbon and FePyrN-Ar-GNS which shows the least amount of peroxide. This confirms the role of Fe-Nx atomically coordinated active sites in the direct 4-electron reduction of O2 and prevalence of the 2electron hydrogen peroxide production in ORR on metal free nitrogen in carbon. Square wave voltammetry results in N2 saturated 0.1 M electrolyte KOH (Fig S4) does not exhibit a characteristic Fe2+/3+ transition peak which is observed in the acidic electrolyte. This could be due to stabilization of a redox state by coordination of OHions.8

Moreover, the ORR observed in the LSV data (Fig 3c) in O2 saturated 0.1 M KOH shows a different trend of performance as compared to the acidic electrolyte. In acidic electrolyte, catalyst without Fe did not show any ORR activity, while in the alkaline medium the difference in activity between metal free PyrN-Ar-GNS and metal containing Fe-PyrN-Ar-GNS is much smaller than the difference in acid medium. Thus the SWV and LSV result in alkaline electrolyte point to a different structural effect on oxygen reduction in comparison to effects on ORR in the acidic medium due to variations in the mechanism of catalysis. Results on these model catalysts confirm some of our earlier published literature where we have claimed that the Fe-N atomic centers in carbon have a limited role as active oxygen reduction catalytic sites in alkaline medium.18 Both metal-free and metal-containing molecular catalysts supported on glassy carbon electrode exhibit negligible activity. These findings show that a contribution of surface area and surface defects of the catalysts is much more important in alkaline media than the presence of iron atomically coordinated to nitrogen. The peroxide production trend, however, is similar to the case in an acidic medium wherein the Fe-PyrN-Ar-GNS exhibits lowest average yield (Figure 3d). In summary, we synthesized model catalyst based on arylated arene with specific coordination of iron to pyridinic nitrogen and nanosheets as a support material. The surface chemistry of catalysts was studied by X-ray photoelectron spectroscopy and their activity towards electrocatalytic oxygen reduction was studied by


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square wave voltammetry and linear sweep voltammetry techniques. Based on the behavior of model catalysts we show that the activity of transition metal-nitrogen-carbon catalysts in acidic electrolytes is highly dependent on the nature of the active sites. Fe coordinated to pyridinic nitrogen contributes to the highest activity because of their participation in the direct 4-electron reduction of O2. Also, the presence of conducting carbon support is critical for efficient electron transfer. In alkaline electrolyte, the structural requirements are drastically different. The efficiency of ORR is a result of cumulative effect of a total number of dopants and surface area of material, while the presence of iron coordinated to nitrogen has a lower effect on activity. The presence of the conducting carbon framework is very important. In summary, this work establishes a novel fully synthetic route to create the model active Fe-N-C oxygen reduction catalysts by synthesis of arylated arene molecules.

ASSOCIATED CONTENT Supporting Information Synthesis and additional physical and electrochemical details. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author [email protected]; [email protected]

Notes The authors declare no competing financial interests.

ACKNOWLEDGMENT We thank the Center for MicroEngineered Materials (CMEM) at the University of New Mexico (UNM) for financial support.

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