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2021B1738

BL14B2XANES Studies on Fe-N-C/Ti3C2Tx MXene as Electrocatalyst for OxygenReduction Reaction for Proton Exchange Membrane Fuel Cell
Wai Yin Wong, a Junji Inukai, a,b Takeshi Watanabe caUniversiti Kebangsaan Malaysia, b University of Yamanashi, c JASRI

Abstract
We found that Fe-N-C catalyst supported on two-dimensional Ti3C2Tx (MXene, T = -O, -OH, or -F)
showed a high activity towards the oxygen reduction reaction. This catalyst contains no precious metals suchas Pt and is expected as a next-generation catalyst for fuel cells. By fitting the XANES supectra using thoseof Fe and Fe oxides, the oxidation states of Fe in Fe-N-C/MXene prepared under different conditions wereestimated.
Keywords: Fe-N-C/MXene, Fuel Cell Catalyst, Oxidation States, XANES
Research Background
The wide commercialization of proton exchange membrane fuel cell (PEMFC) as clean energy generation
technology has been hindered by the high components cost due to the use of noble Pt catalyst. Recent studies
have shown that the abundantly available and low-cost noble-free catalyst of Fe-N-C possess promisingoxygen reduction activity, hence has emerged as the potential candidate as alternative catalyst [1]. Yet, its
performance in actual PEMFC operation has not achieved the desired target to-date.
The two main challenges in this type of catalysts are the lower intrinsic oxygen reduction activity and theblockage of water on the active sites of the hydrophilic Fe-N-C catalyst that results in rapid performance
deterioration. This project is aimed to introduce a novel two-dimensional Ti3C2Tx (MXene, T = -O, -OH, or-F) materials as the support for Fe-N-C, replacing the convention carbon-based support such as graphene.
Despite that XPS can reveal the chemical bonding states, such as Fe–N, Fe2+ and Fe3+, the low attenuation
length and large cross-section of Fe 2p electron with low Fe composition could lead to lower accuracy on themeasurement. In addition, XPS is unable to inform of the local atomic coordination structures of the catalysts
which can be in different coordination numbers, such as Fe-N2 and Fe-N4 moieties, or other possible metalmetal coordination could reveal more information on the active sites of the catalyst. Hence, employing Xray absorption near edge structure XANES is a favorable method that can reveal the abovementioned
information to provide more insight on the identification of the active sites, thus enable us to further tailor
the molecular structure for higher reactivity.
This work is expected to unleash the potential of the novel Fe-N-C/porous MXene as the promising andstable noble-free catalyst for PEMFC. This finding would benefit the fuel cell community in the search formore active and stable Pt-free catalysts to be used in proton exchange membrane fuel cells for clean energy
generation using hydrogen fuel, as one promising method in realizing carbon neutral community and moresustainable living.
Experimental
Samples used are as follows:
1. Fe-N-C (FeNC_No1)
2. MXene (reference sample, not used for XANES)
3. Fe-N-C/MXene (4:1)
3 samples with different synthesis conditions, namely FeNC_No2, FeNC_No3, and FeNC_No4.
4. Fe-N-C/MXene (1:1) (FeNC_No5)
All samples are in powder form. The elemental compositions of the samples were determined via EnergyDispersive X-ray (EDX) analysis and X-ray Photoelectron Spectroscopy (XPS) and their estimated masscompositions are tabulated in Table 1.

XANES Measurements
To identify the coordination states of Fe in each sample, fluorescence XAFS spectra of Fe K-edge wereobtained at the beamline BL14B2 of SPring-8 at room temperature in air on all samples contained in plasticbags.
Results
Fig. 1 shows the XANES spectra of Fe foil, FeO, Fe2O3 (γ), and Fe-N-C/MXene (FeNC_No4). Comparing
also the data of the four samples, it was confirmed that the oxidation states of Fe in all Fe-N-C/MXenesamples were between 2 and 3.
The XANES data of Fe-N-C/MXene samples were fit will with the linear combination of those of Fe metal(Fe foil) and Fe2O3 (γ) with small R-factors between 0.002 and 0.005 [2,3]. As an example, Fig. 2 shows thefitting result of the sprctrum of FeNC_No1.

Fig. 1 XANES spectra of Fe (1), FeO (2), Fe2O3 (3), and Fe-N-C/MXene (FeNC_No4) (4). The
onsets were enlarged and shown on the right.

Fig. 2 XANES data of Fe-N-C/MXene (FeNC_No4) (1), Fe (2), and Fe2O3 (3). The linear combination ofthe curves of Fe (2) and Fe2O3 (3) is shown as (4), where the averaged oxidation number was 2.32.
From the linear combination of the curves of Fe and Fe2O4, the averaged oxidation numbers were obtained
for the five samples.
No.1: 1.99
No.2: 2.18
No.3: 2.05
No.4: 2.32
No.5: 2.34
Summary and Future Perspective
The XANES data of Fe-N-C/MXene samples were successfully obtained under the ambient conditions. The
averaged oxidation numbers were estimated. The in-situ measurements are planned in the future.
References[1] W. Sudarsono, W. Y. Wong et al., Intern. J. Hydro. Energy, 45, 23253 (2020).
[2] V. Lamalingam et al., Adv. Mater. 31, 1903841 (2019).
[3] X. Liang et al., Chem. Mater. 33, 5542 (2021).