# ZIF-8 derived iron-, sulphur-, and nitrogen-doped
 catalysts for anionexchange membrane fuel cell
 application

Iris Palm, Roberta Sibul, Elo Kibena-Põldsepp, Marek Mooste, Jaana Lilloja,
Maike Käärik, Jekaterina Kozlova, Arvo Kikas, Alexey Treshchalov, Jaan
Leis, et al.

► To cite this version:

Iris Palm, Roberta Sibul, Elo Kibena-Põldsepp, Marek Mooste, Jaana Lilloja, et al. ZIF-8 derived iron-, sulphur-, and nitrogen-doped catalysts for anionexchange membrane fuel cell application. Renewable Energy, 2024, 228, pp.120613. 10.1016/j.renene.2024.120613. hal-04740666

HAL Id: hal-04740666

https://hal.science/hal-04740666v1

Submitted on 16 Oct 2024

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Distributed under a Creative Commons Attribution 4.0 International License
ZIF-8 derived iron-, sulphur-, and nitrogen-doped catalysts for anion-exchange membrane fuel cell application

Iris Palm<sup>a</sup>, Roberta Sibul<sup>a</sup>, Elo Kibena-Pöldsepp<sup>a</sup>, Marek Mooste<sup>a</sup>, Jaana Lilloja<sup>a</sup>, Maike Käärik<sup>a</sup>, Jekaterina Kozlova<sup>b</sup>, Arvo Kikas<sup>b</sup>, Alexey Treshchalov<sup>b</sup>, Jaan Leis<sup>a</sup>, Vambola Kisand<sup>b</sup>, Aile Tamm<sup>b</sup>, Nicolas Bibent<sup>c</sup>, Frédéric Jaouen<sup>c</sup>, Steven Holdcroft<sup>d</sup>, Kaido Tammeveski<sup>a,∗</sup>

**Abstract**

This study presents the preparation of tridoped Fe, N, S carbon-based catalysts using ZIF-8 as the main carbon source and iron(II)acetate, 1,10-phenanthroline, ammonia gas and thiourea as precursors for Fe, N and S elements. Thiourea was mixed via planetary ball milling with a Fe-N-C catalyst prepared via inert gas pyrolysis, and different pyrolysis conditions were applied thereafter. The main goal of the work is to synthesise active non-precious metal electrocatalysts for the oxygen reduction reaction (ORR). High electrocatalytic ORR activity in alkaline media was observed for the two materials obtained after thiourea addition and subsequent pyrolysis, while the catalyst resulting from the simple addition of thiourea was less active. Physico-chemical characterisation revealed successful S and N doping for all three catalyst materials with enhanced porosity for the two materials that were subjected to additional pyrolysis after the thiourea addition. The sulphur doping was highest for the FeNSC3 material that was subjected to a second pyrolysis in N<sub>2</sub> and a third pyrolysis in NH<sub>3</sub>, compared to the FeNSC2 material that was subjected to a single pyrolysis in NH<sub>3</sub> flow after thiourea addition. The ORR activity of the FeNSC2 catalyst with half-wave potential of 0.88 V vs. RHE was slightly higher than that of FeNSC3, and the former was selected for evaluation as a cathode catalyst in an anion-exchange membrane fuel cell (AEMFC).

<footnote><sup>a</sup>Institute of Chemistry, University of Tartu, Ravila 14a, 50411 Tartu, Estonia</footnote>

<footnote><sup>b</sup>Institute of Physics, University of Tartu, W. Ostwald Str. 1, 50411 Tartu, Estonia</footnote>

<footnote><sup>c</sup>ICGM, Univ. Montpellier, CNRS, ENSCM, Montpellier, France</footnote>

<footnote><sup>d</sup>Department of Chemistry, Simon Fraser University, 8888 University Drive, Burnaby, BC, V5A 1S6, Canada</footnote>

<footnote>*Corresponding author. Tel.: +372 7375168. E-mail: kaido.tammeveski@ut.ee (K. Tammeveski).</footnote>
**Keywords:** anion-exchange membrane fuel cell, ZIF-8, oxygen reduction reaction, electrocatalysis, non-precious metal catalyst, M-N-C catalyst

# 1. Introduction

The growing population and therefore increasing energy consumption alongside with global warming has led to searching and developing of more environmentally friendly energy sources for both stationary and transportation applications. One of the promising solutions for this problem is using hydrogen as a fuel for producing electricity. In order to convert the chemical energy of hydrogen into electrical energy, fuel cells are needed [1–3]. Fuel cell are clean and sustainable energy conversion devices attracting increasing attention in the transition to hydrogen economy [4]. Green hydrogen is produced by water electrolysis using renewable energy sources (solar, wind, hydropower) and is used then as a fuel in a fuel cell. There are two types of low temperature fuel cells: proton-exchange membrane fuel cells (PEMFC) and anion-exchange membrane fuel cells (AEMFC). While PEMFC is a mature technology that has been employed in several sectors, recent progress in the field of AEMFC development has made these devices very promising for various applications [5]. The main advantage of AEMFCs is the possibility of using Pt-free cathode catalysts due to less harsh conditions. In both devices, the two main reactions taking place are hydrogen oxidation reaction at the anode and oxygen reduction reaction (ORR) at the cathode [6]. The four-electron ORR is sluggish in both AEM and PEM environments and makes it one of the main obstacles in fuel cell technology [6,7]. Pt or Pt-alloy based materials are widely used as electrocatalysts for low-temperature fuel cells to catalyse the ORR [8–10]. Yet, recent progress in non-precious metal catalysts brings hope that such materials might replace precious metal-containing catalysts, the latter having the drawbacks of being expensive and with limited resource [11–14].
Non-precious metal ORR catalysts are typically divided into two main groups: (i) metal-free and (ii) transition metal-containing nanocarbons. Carbon materials can be activated for ORR electrocatalysis by doping it with different heteroatoms, such as nitrogen, sulphur, boron or phosphorus, or a combination of these dopants [15–21]. Such materials have limited ORR activity in acid medium but can reach significant ORR activity in alkaline medium. The second group of materials features a non-precious transition metal, most often iron or cobalt, embedded in a carbon matrix in combination with nitrogen groups acting as ligands of the metal cations [13,22–29]. Beside stabilizing the metal cations through strong metal-N coordination, it is known that nitrogen doping also leads to a synergistic effect between carbon and nitrogen atom, the nitrogen’s lone electron pairs being able to form a delocalised conjugated system $sp^2$ hybridised carbon frameworks. This improves greatly the reactivity and ORR performance of the material [20,21,30,31]. In case of co-doping carbon with sulphur and nitrogen, there has also been observed an increase in the ORR activity due to increased number of active sites and synergetic effect between the two dopants [17,20,21,32–34]. Using iron alongside with nitrogen for doping graphene-based materials has been widely studied [24,25,35,36]. These materials are highly conductive, relatively chemically stable and can be tailored to have high surface areas. The nature of the transition metal in these catalyst materials plays a great role in the formation of active sites for ORR [37–43]. The general idea is that the increase in the ORR electrocatalytic activity can be assigned to either $Fe-N_x$ coordination sites or to iron carbide or metallic iron particles, which are embedded in nitrogen-doped graphitic layers [44–47].

Metal organic frameworks (MOFs), a class of crystalline materials constructed by the coordination of metal ions or clusters and organic bridging ligands in three dimensional space, have been used as sacrificial materials since 2011 to prepare via high-temperature pyrolysis highly active precious-metal-free ORR electrocatalysts [48–53]. Some MOFs have high
porosity and high specific surface area (up to 6200 m² g⁻¹) compared to other porous materials [54–57], and can lead to carbon-based materials also with high porosity and high pore connectivity, after pyrolysis. Zeolitic imidazolate frameworks (ZIFs) is a sub-class of MOFs, which possess the topology of zeolites but are composed of imidazolate groups coordinating either Zn(II) or Co(II) [58]. One of the well-known ZIFs is ZIF-8 which is composed of Zn(MeIm)₂ primary units (Zn ions and 2-methylimidazolate linkers) and has a sodalite-type structure [59]. ZIF-8 has hitherto been the main MOF used to form highly active ORR catalysts via pyrolysis, due to the intrinsic properties of ZIF-8 like high surface area, high nitrogen content and high microporosity combined with facile synthesis as well as controllable ZIF-8 crystal size [50,54,60–63].

Dominant research activity in the field of ORR electrocatalysis has focused on the preparation of non-precious metal catalysts and methodological evaluation of the electrocatalytic properties of these catalysts in the rotating disc electrode (RDE) setup. Much less research effort has been put on evaluating such catalysts in fuel cell environment, using these electrocatalysts as cathode materials. PEM fuel cell testing of precious metal free cathode catalysts is becoming more popular, eased by the availability of highly conductive and robust commercial PEM membranes and ionomers. However, the ORR on precious-metal-free catalysts has often faster kinetics in alkaline conditions compared to acidic media, so there has been a shift towards the research in AEM fuel cells since the advent of AEM with promising conductivity [64].

Although numerous syntheses have been reported using ZIF-8 as a sacrificial MOF for the preparation of carbon nanomaterials co-doped with a transition metal (most often Fe or Co) and a non-metal heterogeneous element (most often N, but also, P, S), however the majority of the catalysts that have been investigated in fuel cell environment are the Fe-N-C and Co-N-C materials, and the overwhelming majority of fuel cell studies involving such materials have
been carried out in PEMFC environment [26,54,62,63,65]. In particular, the study of materials
with co-doping of N and S hetero-elements has not yet been reported in fuel cell environment.
In this work, tri-doped Fe, N and S carbon-based catalysts were synthesised using ZIF-8 as
the main carbon precursor, and applying a facile low energy ball-milling method to introduce
the sulphur dopant, namely thiourea. This was optionally followed by one or two high-
temperature pyrolysis, in order to covalently introduce S in the carbon matrix as well as to re-
open pores blocked by thiourea addition. The obtained materials were electrochemically
characterised using the RRDE method followed by testing in AEM fuel cell configuration.
The physico-chemical characterisation of the catalysts was performed using scanning electron
microscopy (SEM) to describe the morphology of the catalysts, X-ray photoelectron
spectroscopy (XPS) to determine the elemental composition of the surface of the catalysts, X-
ray diffraction (XRD) to identify the crystalline phases present, and N₂ adsorption/desorption
analysis for determining the surface area and porosity of the catalysts.

**2. Material and methods**

**2.1 Synthesis procedure of the ZIF-8 based Fe,S,N-doped catalysts**

To synthesise the catalysts, ZIF-8 (Basolite Z1200, Sigma-Aldrich) was used as the main
carbon source, iron(II)acetate (Fe(OAc)₂, ≥99.99%, Sigma-Aldrich) as an iron source, 1,10-
phenanthroline (phen, ≥99.99%, Sigma-Aldrich) as nitrogen source and thiourea (≥99.99%,
Sigma-Aldrich) as a dual sulphur and nitrogen precursor. ZIF-8, Fe(OAc)₂ and phen were first
weighed (see Table 1), mixed via ball-milling (FRITSCH Pulverisette 7 Premium) in dry
conditions using 5 mm size ZrO₂ beads and a jar with ZrO₂ lining, at 400 rpm for 4 × 30 min
(with 5 min break between each of the four cycles). Subsequently, the obtained material was
ramp heated from room temperature to 1050 °C in N₂ atmosphere with a ramp rate of 5 °C min⁻¹, held at that temperature for 1 h. Afterwards, thiourea was added to the pyrolysed
material and mixed with the same ball-miller but applying a low rotation speed of 100 rpm for 10 min (see Scheme 1). The prepared catalyst is designated as FeNSC1. To potentially improve the ORR activity, an ammonia treatment was applied to FeNSC1, and the obtained catalyst material was flash pyrolyzed at 950 °C in flowing NH₃ atmosphere for 5 min. The resulting catalyst is designated as FeNSC2 (Scheme 1). A third synthesis was also conducted, sharing the same initial steps as for FeNSC1 but differing in the final pyrolysis step after thiourea addition. Namely, after ball milling of the pyrolyzed material with thiourea, the material was flash pyrolyzed at 800 °C in N₂ atmosphere for 20 min directly followed by ammonia treatment for 5 min at 950 °C (see Scheme 1). The resulting catalyst is designated as FeNSC3. The key idea for performing first a pyrolysis at 800° C in N₂ after thiourea addition is to covalently integrate the sulphur in the carbon matrix, before applying the more aggressive NH₃ pyrolysis.

<table><caption>**Table 1.** Synthesis conditions for the N, S co-doped FeNSC1 catalyst</caption><thead><tr><th>Catalyst</th><th>ZIF-8<br>(mg)</th><th>phen<br>(mg)</th><th>Fe(OAc)<sub>2</sub><br>(mg)</th><th>Thiourea<br>(mg)</th></tr></thead><tbody><tr><td>FeNSC1</td><td>800</td><td>100</td><td>15.7</td><td>¼ of the mass of material remaining<br>after the pyrolysis at 1050 °C</td></tr></tbody></table>

<figure><img src="image_2.png" /><figcaption>**Scheme 1.** Scheme of the synthesis approaches.</figcaption></figure>
## 2.2 Physico-chemical characterisation

Helios Nanolab 600 (Thermo Fisher Scientific (former FEI) scanning electron microscope (SEM) was used to investigate the morphology of the prepared catalysts. The elemental composition was investigated with XPS analysis using an electron energy analyzer SCIENTA SES-100 and Mg K<sub>a</sub> radiation (1253.6 eV) from nonmonochromatic twin anode X-ray tube Thermo XR3E2. The XPS data analysis involved removal of X-ray satellites, peak fitting (using the Gauss-Lorentz hybrid functions and blend of linear and Shirley type backgrounds) and calculation of atomic concentrations. N<sub>2</sub> adsorption/desorption isotherms of the catalysts were recorded at 77 K using a NovaTouch LX2 analyser (Quantachorme). The specific surface area (S<sub>DFT</sub>) and pore size distribution (PSD) and the volume of micropores (V<sub>micro</sub>) of the obtained catalyst materials were calculated by the Quantachrome software, using the quenched solid density functional theory (QSDFT) method (with the kernel for N<sub>2</sub> at 77 K on carbon, slit pore, equilibrium model, desorption data). The specific surface area (SSA<sub>BET</sub>) was calculated using the Brunauer-Emmett-Teller theory in the pressure range up to P/P<sub>0</sub>=0.2. The total pore volume (V<sub>tot</sub>) was measured at P/P<sub>0</sub>=0.97. Raman spectroscopy was used to characterise the carbon matrix in these catalyst materials. All samples were suspended in water and drop-coated on silicon substrates. Micro-Raman spectra were recorded in the back-scattering geometry on an inVia Renishaw spectrometer in conjunction with a confocal microscope (Leica Microsystems CMS GmbH), 50X objective and an argon ion laser operated at 514.5 nm. Low incident laser power density at the sample prevented excessive sample heating and/or decomposition, while allowing getting averaged information over ~200 µm<sup>2</sup> area in a single exposure.

X-ray diffraction (XRD) patterns were acquired with a X'Pert MPD instrument equipped with a θ- θ goniometer, in Bragg Brentano geometry and using the Cu Kα<sub>1</sub> radiation. The instrument was also equipped with a sample holder of spinner type and with a linear detector
X'Celerator. Diffractograms were recorded in the 2θ interval from 5 to 90° with a step of 0.0167°, during a total of 4 h for each spectrum. For powder XRD measurements on the different materials, either a polyimide film was placed on top of the powder and the powder placed on a glass support, or no cover film was used but a PTFE support was used. The XRD signal of the supports alone and of the polyimide cover film alone were also acquired.

## 2.3 Rotating ring-disk electrode measurements

The ORR activity of the catalyst materials was evaluated by the RRDE method where glassy carbon disk (GC, geometric area of 0.164 cm²) with Pt ring (collection efficiency of 0.22) was used as a working electrode, saturated calomel electrode (SCE) as a reference electrode and platinum wire as a counter electrode. All the electrochemical potentials are reported with respect to the reversible hydrogen electrode (RHE). Potential values are converted from versus SCE to versus RHE in 0.1 M KOH using the equation: $E_{RHE} = E_{SCE} + 0.241 + 0.059 \times pH = E_{SCE} + 1.008$ V. The potential was controlled by Autolab potentiostat/galvanostat PGSTAT30 (Metrohm Autolab, The Netherlands) using General Purpose Electrochemical System (GPES) software. All the measurements were carried out in 0.1 M KOH electrolyte at the electrode rotation rate (ω) of 1600 rpm and at the potential scan rate (ν) of 10 mV s⁻¹. The data acquired in O₂-saturated electrolyte was corrected for the data acquired in N₂-saturated electrolyte, to obtain the Faradaic polarisation curves. The number of electrons (n) transferred per O₂ molecule was determined from the RRDE data using Eq. (1)

$$n = \frac{4I_d}{I_d + \frac{l_t}{N}} \qquad (1)$$

and the percentage yield of HO₂⁻ formation (%HO₂⁻) was calculated via the Eq. (2):

$$\%\text{HO}_2^- = \frac{2I_d}{I_d + \frac{l_t}{N}} \times 100\% \qquad (2)$$
where $I_d$ is the disc current, $I_r$ is the ring current, and $N$ is the collection efficiency of the Pt ring.

To prepare the FeNSC catalyst inks for coating the GC electrode, 4 mg of catalyst material was dispersed in a mixture of 990 µL of 2-propanol and 40 µL of Nafion solution (5 wt % of Nafion in solution) and sonicated for 1 h. The obtained catalyst ink was drop cast onto a polished GC surface to reach a loading of 0.2 mg cm⁻². For comparison, the GC electrode was modified with a Pt/C catalyst (20% wt%, E-TEK) using the same procedure as for the FeNSC catalysts, also resulting in a Pt/C loading of 0.2 mg cm⁻².

The electrochemical stability of the FeNSC catalysts was tested in O₂-saturated 0.1 M KOH solution by applying 10000 potential cycles between 0.6 and 1.0 V vs. RHE at 200 mV s⁻¹.

## 2.4 Anion-exchange membrane fuel cell (AEMFC) tests

Fuel cell measurements were carried out using Greenlight Fuel Cell Test Station (G40 Fuel Cell System, Hydrogenics, Canada). To prepare the cathode, 12.5 mg of the catalyst material (FeNSC2) was dispersed in a solution, which contained 1340 µL methanol (99.5%, Fisher Scientific), 391 µL Milli-Q water and 130 µL 3 wt.% ionomer solution prepared by dissolving methylated poly[2,2'-(2,2",4,4",6,6"-hexamethyl-p-terphenyl-3,3"-diyl)-5,5'-bibenzimidazole] membrane (iodide form [66]) in methanol. For preparing the anode electrode for the fuel cell, 6.7 mg of Pt-Ru/C catalyst (50:25:25, Alfa Aesar) was dispersed in a solution of 689 µL methanol, 196 µL Milli-Q water and 49 µL 3 wt.% ionomer solution in methanol (Aemion+ powder (AP2-INN8-00-X) (Ionomr Innovations, Inc., Canada). The obtained suspensions were sonicated for 1.5 h and pipetted onto the gas diffusion layer (GDL, Sigracet 39 BB) giving a final loading of 2 mg cm⁻² for cathode and 0.8 mgPt-Ru cm⁻² for anode. Before testing, the electrodes were soaked in 3 M KOH for one day and the AEM for four days. The 3 M KOH solution for ionic exchange in AEM was changed after every 24 h. The prepared anode
and cathode were pressed together into a 5 cm² test cell (Fuel Cell Technologies Inc., Albuquerque, USA) along with a reinforced AEM (Aemion+ AF2-HLE8-10-X, 10 µm from Ionomr Innovations Inc., Canada) with silicone gaskets at a torque of 9 Nm. The single-cell tests were carried out at 65 °C with 75% relative humidity, O₂ and H₂ flow rates were both 0.05 NLPM and the backpressure was 200 kPa on each side.

# 3. Results and discussion

## 3.1 Physico-chemical characterisation of the FeNSC catalysts

To examine the surface morphology of the FeNSC materials scanning electron microscope was used and the SEM micrographs are shown in Fig. 1. As can be seen, all prepared materials have a rough morphology that is composed of irregularly shaped particles ranging in size from a few tens to hundreds of nanometres. The particles are interconnected, providing a porous framework that covers the electrode's surface rather uniformly. According to the N₂ physisorption studies which were conducted to analyse the specific surface area (SSA) and porosity parameters of the obtained FeNSC catalysts confirm that all the materials have highly porous structure (see Table 2).
<figure><img src="image_3.png" /><figcaption>**Fig. 1.** SEM images of (a,d) FeNSC1, (b,e) FeNSC2 and (c,f) FeNSC3 materials.</figcaption></figure>

As can be seen from the inset of Fig. 2, the shape of N₂ physisorption isotherms are similar for all three catalysts and refers to a mix of type I and type II. At the isotherms of all materials, the presence of a weak H4 type hysteresis loop according to IUPAC can be observed [67]. These results show that all catalysts consist of micro-and mesopores. This is also confirmed by the results of pore size distribution (PSD, Fig. 2). Two distinct peaks were detected at 0.8 nm and 3.5 nm for all three materials, corresponding to the hierarchical (micro- and mesopores) porous structure. However, the PSD shows a more intense signal for the micropores centred at 0.8 nm, which indicates that the surface area of the materials is mostly existing in micropores rather than mesopores (see Fig. 2), also reflected in the values of SSA*micro*(dft) and SSA*meso*(dft), as seen in Table 2. The difference in the PSD is mostly seen in the pore size region from 1 to 2 nm, in line with the reported effect that NH₃ pyrolysis generally leads to the formation of micropores by preferential etching of the amorphous carbon phase.
<figure><img src="image_4.png" /><figcaption>**Fig. 2.** Pore size distribution of FeNSC materials. The inset exhibits N₂ adsorption-desorption isotherms for all the prepared catalysts.</figcaption></figure>

The highest SSA$_{BET}$ was determined for FeNSC2 (1260 m² g⁻¹) followed by FeNSC3 (992 m² g⁻¹), whereas FeNSC1 compared to the other two catalysts exhibited significantly lower SSA with the value of 717 m² g⁻¹. Higher SSA of FeNSC2 and FeNSC3 vs. FeNSC1 is related to the pyrolysis steps applied after the thiourea addition by ball-milling, and in particular the NH₃ activation step applied to these two materials. These pyrolysis steps after thiourea addition by ball milling can re-open pores that were filled with thiourea in FeNSC1 or create new pores in the carbon matrix existing in FeNSC1. It was reported that NH₃ pyrolysis can react preferentially with the amorphous carbon phase, gasifying it and resulting in the formation of micropores in particular between the more resistant graphitic crystallites, and thus leading to increased pore volume and surface area [68–70]. Larger surface is significant in the electrochemical reaction for faster electron transfer and mass transport [71,72]. The highest pore volumes (V$_{tot}$ and V$_{micro}$) were obtained for FeNSC2, while the porosity was considerably lower for the other two catalysts (see Table 2).

<caption>**Table 2.** Textural properties of FeNSC catalysts determined by the N₂ physisorption method.</caption>
<table><thead><tr><th>Catalyst</th><th>SSA<sub>BET</sub><br>(m<sup>2</sup> g<sup>-1</sup>)</th><th>SSA<sub>dft</sub><br>(m<sup>2</sup> g<sup>-1</sup>)</th><th>SSA<sub>micro(dft)</sub><br>(m<sup>2</sup> g<sup>-1</sup>)</th><th>SSA<sub>meso(dft)</sub><br>(m<sup>2</sup> g<sup>-1</sup>)</th><th>V<sub>tot</sub><br>(cm<sup>3</sup> g<sup>-1</sup>)</th><th>V<sub>micro(dft)</sub><br>(cm<sup>3</sup> g<sup>-1</sup>)</th></tr></thead><tbody><tr><td>FeNSC1</td><td>717</td><td>767</td><td>692</td><td>75</td><td>0.52</td><td>0.27</td></tr><tr><td>FeNSC2</td><td>1260</td><td>1122</td><td>1032</td><td>90</td><td>0.78</td><td>0.49</td></tr><tr><td>FeNSC3</td><td>992</td><td>942</td><td>881</td><td>61</td><td>0.61</td><td>0.39</td></tr></tbody></table>

X-ray photoelectron spectroscopy analysis was carried out to investigate surface composition of the catalyst materials. The XPS analysis revealed that the introduction of iron as well as nitrogen and sulphur moieties into the FeNSC catalysts was successful. The XPS survey spectra distinguished five peaks at 285, 535.5, 711, 400 and 167 eV, which were attributed to the binding energies of C1s, O1s, Fe2p, N1s and S2p, respectively (see Fig. 3a,c,e).

For sulphur, relatively low contents are measured for all three samples, and in particular for FeNSC1 (see Table 3). This is counterintuitive as thiourea was milled with the pyrolyzed material resulting from ZIF-8/phen/FeOAc2 pyrolysis, with 20 wt% thiourea added relative to the pyrolyzed material. Thus, the expected bulk S content for FeNSC1 would be ca. 8 at.%, while only 0.01 at.% S is detected. This shows that most of the sulphur is not detected by XPS and this can be interpreted as the packing of thiourea in micropores of the pyrolyzed materials, preventing the S-atoms from thiourea molecules located deep in the micropores to be detected by XPS. This hypothesis is perfectly in line with increased amounts of S detected by XPS in FeNSC2 and FeNSC3, increasing to 0.1 and 0.5 at.%, respectively. For FeNSC3, a first pyrolysis at 800 °C in inert gas was applied after thiourea addition and a final pyrolysis in NH₃ made at 950 °C. This results in a significantly higher S content in the final catalyst, compared to the direct application of an NH₃ pyrolysis after thiourea addition (FeNSC2 synthesis). For the FeNSC3 catalyst, the most intense sulphur-related signal was detected at the binding energies of ~164 and ~165 eV, which correspond to S2p<sub>3/2</sub> and S2p<sub>1/2</sub> spin-orbit split components, respectively (Fig. 3e inset). Based on the literature, these peaks belong to thiophene-S (C-S-C) bond. In the case of FeNSC2 (Fig. 3c inset), this sulphur species is absent, and instead there are peaks at ~168 and ~169 eV that can be assigned to oxidised-S
groups ($SO_x$) [73–76]. Oxidised S-groups are present in low amount in FeNSC3 and totally absent in FeNSC1.

The nitrogen content in the three catalyst materials was significantly higher compared to Fe and S, in the range of 6-7 at.% (Table 3). The XPS spectra in the N1s region was deconvoluted into different peaks that identify different N groups (Fig. 3b,d,f). For all three materials, the most prominent ones were pyridinic-N (398.5 eV) and pyrrolic-N (400.6 eV), while peak intensities for imines (397.7 eV), graphitic-N (402 eV) and Fe-Nx (399.5 eV) were lower and comparable among the materials. The Fe-Nₓ signal is of particular interest for ORR catalysis and FeNSC2 had the highest absolute content of Fe-Nₓ followed by FeNSC1, while FeNSC3 has significantly lower absolute content of Fe-Nₓ signal (Table 3). These active centres and different N species play an important role in enhanced ORR performance, as discussed in the next subsection.

The Fe content measured by XPS was the same for all three materials, at 0.3 at.%. This is in line with the Fe content expected, since the Fe content in the ZIF-8/phen mix before pyrolysis is 1.7 wt.%, and ZIF-8 loses typically 66% of its mass during its first pyrolysis at 1050 °C in inert gas. The O content is in the range of 3.9-4.5 at.% for the three materials, in line with previous reports and assigned to adventitious oxygen, and/or $O_2$ adsorbed on Fe-Nₓ sites following exposure of the catalysts to the ambient atmosphere [77].
<figure><img src="image_5.png" /></figure>

<figure><img src="image_6.png" /></figure>

<figure><img src="image_7.png" /></figure>

<figure><img src="image_8.png" /></figure>

<figure><img src="image_9.png" /></figure>

<figure><img src="image_10.png" /><figcaption>**Fig 3.** XPS wide scan and detailed N1s spectra of (a,b) FeNSC1, (c,d) FeNSC2 and (e,f) FeNSC3, respectively. Inset to (c) and (e) shows the detailed spectra in the S2p region.</figcaption></figure>

<table><caption>**Table 3.** Surface elemental composition (at.%) of FeNSC materials measured by XPS, and the absolute content of N subdivided into different N species.</caption><thead><tr><th></th><th>FeNSC1</th><th>FeNSC2</th><th>FeNSC3</th></tr></thead><tbody><tr><td>C</td><td>89.5</td><td>88.8</td><td>88.7</td></tr><tr><td>O</td><td>4.2</td><td>3.9</td><td>4.5</td></tr><tr><td>N</td><td>6</td><td>6.9</td><td>6.0</td></tr><tr><td>S</td><td>0.01</td><td>0.1</td><td>0.5</td></tr><tr><td>Fe</td><td>0.3</td><td>0.3</td><td>0.3</td></tr><tr><td>pyridinic-N</td><td>1.9</td><td>2.2</td><td>1.9</td></tr><tr><td>pyrrolic-N</td><td>1.6</td><td>1.7</td><td>1.7</td></tr><tr><td>imines</td><td>0.9</td><td>0.9</td><td>0.8</td></tr><tr><td>graphitic-N</td><td>0.6</td><td>0.8</td><td>0.7</td></tr><tr><td>Fe-N<sub>x</sub></td><td>0.5</td><td>0.6</td><td>0.3</td></tr><tr><td>other N moieties (N-O, N-H etc.)</td><td>0.5</td><td>0.7</td><td>0.6</td></tr></tbody></table>
The three catalysts were investigated with XRD. For interpreting the diffractograms, it is important to recall that FeNSC1 was measured with a polyimide film placed on top of the powder and the powder placed on a glass support, while FeNSC2 and FeNSC3 diffractograms were acquired without any cover film but using a PTFE support, as detailed in the experimental methods. The reference diffraction patterns for the polyimide film, the glass support and the PTFE support are shown in Fig. 4c,e,f, respectively, while the spectra measured for FeNSC1, FeNSC2 and FeNSC3 are shown in Fig. 4a,b,d.

For FeNSC1, the broad peak centred at ca 20.8° and the shoulder at 25.9° can be assigned to the polyimide film. Then, the well-defined XRD peaks at 30.1, 35.07, 50.2, 60.0 and 62.7° can be assigned to tetragonal ZrO₂, matching in positions and in their relative intensities to reference XRD patterns of this phase (JCPDS file 01-079-1769). ZrO₂ impurities must have been introduced during the first ball-milling process at 400 rpm, since the lining of the jar and the balls used to mill are made of ZrO₂. Such ZrO₂ fragments introduced during milling would then have been subjected to the same pyrolysis treatment applied to prepare the catalysts. Literature has shown that tetragonal ZrO₂ is expected after high temperature treatment, since it is the thermodynamically stable zirconium oxide phase at high temperature [78]. The presence of tetragonal ZrO₂ here is also in line with our previous publication on the preparation of Fe-N-C materials from carbide derived carbons via milling in ZrO₂-lined jar and with ZrO₂ balls, where the main signal was from tetragonal ZrO₂ and a minor signal from hexagonal ZrO₂ [23]. Next, the broad peak centred at 43.5° can be assigned to the (101) diffraction line of graphite, and indicates the amorphous nature of the carbon matrix in FeNSC1.

All the well-defined peaks observed in the XRD pattern of FeNSC2 can be assigned to the PTFE support used for acquisition of that diffractogram, except the peaks located at 30.28° and 44.93°. The former can be assigned to the most intense diffraction peak of tetragonal
ZrO₂, while secondary diffraction peaks of this phase are not detected, suggesting a lower amount of ZrO₂ in FeNSC2 than in FeNSC1. The peak at 44.93° can be assigned to the main diffraction peak of α-Fe. The secondary diffraction peaks of α-Fe are also not visible in the XRD pattern, which is typical in FeNC materials due to low absolute Fe amount, with nanometric metallic Fe particle size. Moreover, α-Fe only accounts for part of the iron, the remainder being present as atomically dispersed Fe-Nₓ sites. Last, the broad peak centred at ca 42° and extending from 35-47° may be due to the amorphous carbon phase in FeNSC2, and/or may be due to the PTFE support since the latter also shows a broad background in that angle region (see Fig. 4f). All the well-defined peaks observed in the XRD pattern of FeNSC3 can be assigned to the PTFE support used for acquisition of that diffractogram. As for FeNSC2, the broad peak centred at ca 42° and extending from 35-47° may be due to the amorphous carbon phase in FeNSC3, and/or may be due to the PTFE support.

Overall, the absence of peaks related to iron-based crystalline phases in FeNSC1 and FeNSC3 suggests that all or the vast majority of iron is present as atomically dispersed iron sites in these materials, while FeNSC2 contains some metallic α-Fe. The presence of some α-Fe in FeNSC2 but absence in FeNSC3 may be assigned to the synthesis path, FeNSC2 being prepared by applying a pyrolysis in flowing ammonia at 950 °C to the milled thiourea and FeNSC1 mix, corresponding to highly reducing conditions (see the synthesis Scheme). In contrast, FeNSC1 was only pyrolyzed in N₂ gas, while FeNSC3 is derived from the milled thiourea and FeNSC1 mix by applying first a pyrolysis at 800 °C in N₂ gas, followed by another pyrolysis in flowing ammonia at 950 °C. The combination of thiourea and NH₃ during a pyrolysis stage therefore seems to explain the presence of some α-Fe in FeNSC2.
<figure><img src="image_11.png" /><figcaption>**Fig. 4.** XRD patterns of the catalysts and films or supports used to acquire the data. (a) FeNSC1, (b) FeNSC2, (c) polyimide film, (d) FeNSC3, (e) glass support and (f) PTFE film.</figcaption></figure>

The first-order Raman spectra of the catalysts normalized to the intensity of the G band are presented in Fig. 5. The spectra were fitted following the four-peak model where G peak (~1590 cm⁻¹) corresponds to the stretching vibrations of the sp² carbon atoms in the ideal graphitic lattice; D1 (~1350 cm⁻¹) to defect-activated breathing mode of aromatic rings; D3
(~1500 cm⁻¹) to amorphous carbon and D4 (~1150 cm⁻¹) to disordered graphitic lattice [79].

A baseline correction was used to compensate for the small fluorescence background and
multiple-peak fitting procedure was applied using the PeakAnalyser software in OriginPro 9.

The intensities of *I*<sub>D1</sub> and *I*<sub>G</sub> (integrated areas under the bands) were extracted from the fitting
of the curves by Voigt functions. In the curve-fitting procedure, all parameters (peak position,
height, and width) were adjusted to attain the smallest value of the chi-square.

<figure><img src="image_12.png" /><figcaption>**Fig. 5.** Raman spectra of the FeNSC catalyst samples.</figcaption></figure>

The *I*<sub>D1</sub>/*I*<sub>G</sub> ratios of FeNSC1, FeNSC2, and FeNSC3 catalysts are 3.7, 3.0, and 3.1 respectively. High value of this parameter indicates a significant presence of disorder and defects in the graphitic structure, meaning all prepared materials are quite amorphous, but among those FeNSC1 is slightly more disordered than FeNSC2 and FeNSC3. This can be explained by the preferential attack by NH<sub>3</sub> of the disordered carbon phase present initially in FeNSC1.

**3.2 Oxygen reduction reaction studies on the FeNSC catalysts**
The electroreduction of oxygen on FeNSC catalysts was investigated by the RRDE technique in O₂-saturated 0.1 M KOH electrolyte solution at 1600 rpm. In Fig. 6a, the RRDE polarisation curves of the prepared catalysts are compared with a commercial Pt/C catalyst. It can be seen that the electrochemical performance of FeNSC2 and FeNSC3 catalysts are similar. Although, FeNSC2 shows a slightly higher ORR performance than FeNSC3, half-wave potential (E₁/₂) being 0.88 and 0.87 V, respectively. Interestingly, both catalysts slightly surpass the ORR activity of the Pt/C catalyst (E₁/₂ = 0.84 V). The poorest ORR performance was observed for FeNSC1 catalyst (E₁/₂ = 0.76 V). Higher electrocatalytic activity on FeNSC2 and FeNSC3 might be related to the materials synthesis, where NH₃ heat-treatment was employed while FeNSC1 was synthesized only via a single pyrolysis under N₂ flow. The addition of thiourea by ball-milling without subsequent pyrolysis may also have blocked some pores in FeNSC1, decreasing the O₂ and/or electrolyte accessibility to some Fe-Nₓ active sites.

<figure><img src="image_13.png" /></figure>

<figure><img src="image_14.png" /></figure>

<figure><img src="image_15.png" /></figure>

<figure><img src="image_16.png" /></figure>
**Fig 6.** RRDE results for ORR on FeNSC and Pt/C (20 wt.%) catalysts: (a) disk current densities, (b) ring currents, (c) $HO_2^-$ yield and (d) dependence of *n* on potential in $O_2$-saturated 0.1 M KOH solution at $\omega = 1600$ rpm and $v = 10$ mV s$^{-1}$. FeNSC catalyst loading: 0.2 mg cm$^{-2}$.

To collect more information on the ORR pathway, the data of the $HO_2^-$ yield and the number of electrons transferred per $O_2$ molecule were obtained and the results are presented in Fig. 6c,d. The selectivity towards $HO_2^-$ formation was quite low in a wide potential range for FeNSC2 and FeNSC3 electrocatalysts. The percentage of $HO_2^-$ formation stayed close to zero in the case of FeNSC2 and FeNSC3 at high potentials ($E > 0.2$ V). At low potential ($E < 0.2$ V) the yield of $HO_2^-$ increased to 10% and 20% for FeNSC2 and FeNSC3, respectively (see Fig. 6c). In the same potential range, FeNSC1 catalyst exhibited significantly higher yield of $HO_2^-$, up to 40% (Fig. 6c). The *n* value during the ORR process is close to four, only at more negative potentials the value decreased somewhat on FeNSC2 and FeNSC3 catalysts. This is in correlation with the percentage yield of $HO_2^-$ formation. In the case of FeNSC1, the *n* value is between 3 and 4 (Fig. 6d). This indicates that the ORR process might occur via $2e^-$ and $4e^-$ pathway, where hydroperoxide anions are formed as intermediate and further reduced to $OH^-$.

The short-term stability of two best-performing catalyst materials was evaluated in 0.1 M KOH solution by measuring ORR polarisation curves at 1600 rpm before and after applying 10,000 potential cycles from 1 to 0.6 V at 200 mV s$^{-1}$. Both FeNSC2 and FeNSC3 catalysts displayed excellent stability (Fig. 7), as the polarisation curve remained virtually the same, the onset potentials did not change and only a minor shift of 4 mV in half-wave potential was observed with FeNSC3 material. Such impressive stability combined with good electrocatalytic activity shows potential applicability of these materials, especially FeNSC2 catalyst.
<figure><img src="image_17.png" /><figcaption>**Figure 7.** ORR polarisation curves of (a) FeNSC2 and (b) FeNSC3 catalyst in O₂-saturated 0.1 M KOH solution before and after 10,000 potential cycles (ω = 1600 rpm, v = 10 mV s⁻¹).</figcaption></figure>

The excellent electrocatalytic performance of FeNSC2 benefits from the following reasons.
First, porous structure and high surface area of the catalyst. The higher SSA and microporous structure can lead to the formation of more ORR-active sites [11,20,80]. Second, the XPS analysis detected that, the most dominant nitrogen species in all the catalysts was pyridinic-N, which can increase the ORR onset potential of the catalyst [81]. Also, it has been proposed that pyridinic-N is one of the ORR-active sites and it is responsible for the reduction of HO₂⁻ to OH⁻ [81,82]. Other nitrogen moieties (confirmed by the XPS), like pyrrolic-N and graphitic-N are believed to improve the ORR performance as well [83,84]. Moreover, it is claimed that graphitic-N can increase the diffusion-limited current density and is responsible for the formation of HO₂⁻ [82]. Third, the presence of S in the catalyst. The synergetic effect between N and S can enhance electrocatalytic activity and favour a 4e⁻ pathway for the ORR [34,34,85]. Dual doping with heteroatoms also allows larger asymmetrical spin and higher charge density resulting larger number of active sites for the ORR to take place [34]. Finally, the presence of Fe-Nₓ active centres, especially in Fe-pyrrole-N₄ formation, on catalysts are known as highly active sites for ORR in alkaline and acidic media [86]. According to the literature, sulphur doping can lead to the structural change in carbon matrix and electronic
structure by delocalising electrons from surrounding carbon atoms, thereby better interaction
between Fe-Nₓ centres is achieved and the ORR activity of the catalyst is improved [81,87].

Different studies on Fe,N,S-doped carbon-based electrocatalysts can be found in the literature.

For example, Li et al. synthesised Fe(Fc)-N/S-C catalyst, which exhibited a comparable
electrocatalytic activity (E₁/₂ = 0.87 V) to the catalysts prepared in this work [88]. They
explained the improved activity with the formation of Fe-N or Fe-S sites. Tan et al. doped
carbon nanotubes with Fe, N and S (Fe/N/S-PCNT) and achieved competitive ORR
electrocatalytic performance with E₁/₂ of 0.84 V, which is associated with the Fe-N-C and C-
S-C active sites, high specific surface area and good hierarchical porous structure as the
authors had pointed out [89]. The excellent ORR performance was confirmed by Jia et al.
with half-wave potential on Fe-NSC catalyst of 0.92 V and the n value was calculated to be
~4 in the studied potential range [90]. The authors claimed that the great activity was
influenced by the S doping-induced charge enrichment in N and Fe for optimal O₂ binding
and fast electron transfer. Considering all this, we can assume that doping with heteroatoms
(N and S) and iron is important to obtain an active catalyst for the ORR.

**3.3 Anion-exchange membrane fuel cell test with FeNSC catalyst**

Since the FeNSC2 showed the best ORR activity using the RRDE method, the catalyst was
further tested as potential cathode catalyst in AEMFC configuration. Commercial Pt-Ru/C
material was used for the anode and Aemion+ (AF2-HLE8-10-X, 10 µm) was applied as an
AEM. Fig. 8 presents the AEMFC testing result. The FeNSC2 catalyst shows a moderate peak
power density (Pmax) of 162 mW cm⁻², with the operating conditions and materials used.
<figure><img src="image_18.png" /><figcaption>**Fig. 8.** Polarisation and power density curves for H₂/O₂ single-cell AEMFC using FeNSC2 as a cathode catalyst (2 mg cm⁻²) along with Aemion+ (10 µm) AEM. Pt–Ru/C (0.8 mgPtRu cm⁻²) was used as the anode catalyst. T = 65 °C. Flow rate for both O₂ and H₂ gases was 0.05 L min⁻¹.</figcaption></figure>

No reports on the AEMFC performance of Fe, N and S triple-doped catalysts can be found. Therefore, it is difficult to compare the obtained results of AEMFC testing with the literature. Although there are numerous studies which have focused on catalysts co-doped with nitrogen and a transition metal, especially with an iron (Table 4). It can be seen that some catalysts have shown excellent performance compared to the results achieved in this work [91,92]. However, it should be noted that the used AEM is different. A similar AEMFC result (Pmax = 220 mW cm⁻²) was obtained by Lilloja et al. [25], who investigated iron and nitrogen-doped mesoporous carbon. The authors justified a lower Pmax value with a thicker cathode catalyst layer and therefore, water management at high current densities might be more complicated [25]. In addition, the material’s high microporosity and low mesoporosity may lead to poorer AEMFC performance. It is claimed that mesopores benefit in facilitating the transport of O₂ [11,16,80]. However, additional work needs to be done to optimise the operating conditions (temperature, humidity, pressure, flow rate of gases, backpressure, ionomer-to-catalyst ratio, water transport in the catalyst layer, etc.) to improve the AEM fuel cell performance.
<table><caption>**Table 4.** Comparison of H₂/O₂ AEMFC performances using iron-based cathode catalysts.</caption><thead><tr><th>Cathode catalyst</th><th>Cathode loading (mg cm<sup>-2</sup>)</th><th>Anode loading (mg<sub>PtRu</sub> cm<sup>-2</sup>)</th><th>Membrane</th><th>T (°C)</th><th>P<sub>max</sub> (mW cm<sup>-2</sup>)</th><th>j<sub>0.75 V*</sub>, (mA cm<sup>-2</sup>)</th><th>Ref.</th></tr></thead><tbody><tr><td>FeNCS2</td><td>2</td><td>0.8</td><td>Aemion+ (10 µm)</td><td>65</td><td>162</td><td>110</td><td>This work</td></tr><tr><td>Fe-N-Gra/CNT</td><td>1-1.25</td><td>0.72</td><td>Aemion+ (10 µm)</td><td>70</td><td>364</td><td>200</td><td>[93]</td></tr><tr><td>Fe/IL-PAN-A1000</td><td>2</td><td>0.6</td><td>HMT-PMBI</td><td>60</td><td>289</td><td>150</td><td>[94]</td></tr><tr><td>FeCoNC</td><td>2</td><td>0.6</td><td>HMT-PMBI</td><td>60</td><td>415</td><td>300</td><td>[95]</td></tr><tr><td>Fe-N-C</td><td>2</td><td>0.6</td><td>HMT-PMBI</td><td>60</td><td>220</td><td>260</td><td>[25]</td></tr><tr><td>Fe-N-Gra</td><td>2</td><td>0.6</td><td>HMT-PMBI</td><td>60</td><td>243</td><td>200</td><td>[36]</td></tr><tr><td>FeN-SiCDC-0.5-400-wet-PVP</td><td>2</td><td>0.8</td><td>HMT-PMBI</td><td>60</td><td>356</td><td>390</td><td>[96]</td></tr><tr><td>Fe-N-C-1000</td><td>4.5</td><td>0.5 Pt</td><td>FAA-3-50</td><td>60</td><td>149</td><td>100</td><td>[97]</td></tr><tr><td>Fe/pNC@HCF-1000</td><td>4</td><td>0.4 PtRu/C</td><td>FAA-3</td><td>60</td><td>343</td><td>100</td><td>[98]</td></tr><tr><td>Fe/N/CNT</td><td>2</td><td>0.4</td><td>aQAPS-S<sub>8</sub></td><td>60</td><td>490</td><td>150</td><td>[99]</td></tr><tr><td>Fe<sub>0.5</sub>-NH<sub>3</sub></td><td>1.5</td><td>0.9</td><td>ETFE</td><td>60</td><td>1100</td><td>500</td><td>[91]</td></tr><tr><td>FeNx-CNTs</td><td>4</td><td>0.3</td><td>PVA-0.8PQVBC40%</td><td>60</td><td>1015</td><td>500</td><td>[92]</td></tr></tbody></table>

## 4. Conclusions

FeNSC catalysts for the ORR were synthesised using a simple ball-milling method, high-temperature pyrolysis and the two catalysts prepared were treated with NH₃. ZIF-8 was doped with iron, nitrogen and sulphur using iron(ii)acetate, 1,10-phenanthroline and thiourea as doping agents, respectively. The morphology of the catalysts was rather similar as revealed by SEM images. According to the N₂ physisorption analysis, all resulting catalysts exhibited high surface area, especially FeNSC2 and hierarchical porous structure (micro- and mesopores). Although, the prepared materials contained a greater amount of micropores. The XPS analysis detected very high nitrogen concentration in the catalysts, whereas the content of the other two elements (Fe, S) was relatively low in the resulting materials. Moreover, electrochemical studies carried out by the RRDE method displayed excellent ORR activity in alkaline media with a half-wave potential of 0.88 and 0.87 V for FeNSC2 and FeNSC3, respectively. In addition, the RRDE data showed low peroxide yield and the *n* value in the wide potential range was close to four for the FeNSC2 and FeNSC3 catalysts. In addition, these materials

<footnote>*Estimated from the Figures if value was not given at that potential</footnote>
possessed excellent stability in alkaline media as the electrocatalytic activity almost did not change after 10,000 repetitive potential cycles. The AEMFC test conducted using FeNSC2 as a cathode catalyst exhibited moderate fuel cell performance with peak power density of 162 mW cm⁻².

### Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

### Data availability

Data will be made available on request.

### Acknowledgements

This study was financially supported by the Estonian Research Council (grants PRG723, PRG4 and PRG1509) and EU through the European Regional Development Fund (TK141 "Advanced materials and high-technology devices for energy recuperation systems", TK134 "Emerging orders in quantum and nanomaterials" and TK143 "Molecular Cell Engineering"). We thank Bernard Fraisse from ICGM for help with the XRD measurements.

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