Synthesis and Characterization of Titanium Oxynitride Catalyst via Direct Ammonia Nitridation of Titanium Polyacrylate for Oxygen Reduction Reaction

doi:10.21203/rs.3.rs-4679013/v1

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Synthesis and Characterization of Titanium Oxynitride Catalyst via Direct Ammonia Nitridation of Titanium Polyacrylate for Oxygen Reduction Reaction

https://doi.org/10.21203/rs.3.rs-4679013/v1

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A titanium oxynitride catalyst for the oxygen reduction reaction (ORR) in polymer electrolyte fuel cells was synthesized through the direct ammonia nitridation of titanium complexes. Titanium polyacrylate was employed as the catalyst precursor, and the effect of the calcination temperature between 600 and 1000°C on the catalyst structure was studied. The catalysts were characterized via X-ray diffraction, X-ray absorption near edge structure, transmission electron microscopy, cyclic voltammetry, and powder electrical resistivity measurements. The formation of titanium oxynitride particles and deposited carbon were observed for all the samples; however, significant variations in the catalyst structure and catalytic activity were also observed. With an increase in the calcination temperature, nitridation of titanium oxynitride progressed, and the conductivity of the catalyst powder increased. The highest rest potential and ORR current density were achieved with calcination at 800°C. Importantly, the results suggest that maintaining an optimal nitrogen doping level within the catalyst particles, along with ensuring the formation of electroconductive deposited carbon, is essential for achieving a high ORR current. This work introduces the direct ammonia nitridation of metal complexes as a promising process for designing metal oxynitride catalysts.

ammonia nitridation

non-noble metal catalyst

oxygen reduction reaction

polymer electrolyte fuel cells

Polymer electrolyte fuel cells (PEFCs) are an important power system because of their high efficiency and zero-emission power generation. Despite the expectations for further dissemination of PEFCs, their application remains limited owing to the use of the expensive platinum catalyst (Whiston et al. 2019). Therefore, the development of non-platinum electrocatalysts for PEFCs has been investigated (Nie et al. 2015; Shao et al. 2016; Madheswaran and Jayakumar 2021). As promising alternatives, the group 4 and 5 metal-oxide catalysts have been explored (Ishihara et al. 2008, 2010; Chisaka et al. 2011). They have several advantages, such as high chemical durability and abundant resources. However, metal-oxide catalysts suffer from two main problems. First, metal oxides inherently have low electroconductivity, reducing the current density (Ishihara et al. 2008; Ota et al. 2011). Second, their catalytic activity is lower than that of platinum catalysts (Nie et al. 2015).

Concerning electroconductivity, oxide catalysts require the formation of a pathway for electron conduction, i.e., an electron conduction path connecting the current collector to the active sites on the oxide surface. In conventional platinum catalysts, a carbon support, such as carbon black, is used to form a macro-electron conduction path that connects the current collector to the catalyst surface and increases the specific surface area. However, in metal-oxide catalysts, it is also necessary to form a local micro-electron conduction path connecting the catalyst surface to the active sites (Ishihara et al. 2020). We previously reported that deposited carbon derived from the calcination of oxy-zirconium phthalocyanine effectively formed a micro-electron conduction path, enhancing the oxygen reduction reaction (ORR) activity of zirconium oxide-based catalysts (Ishihara et al. 2019). Deposited carbon near the oxide surface increased the local electroconductivity and increased the surface-area utilization of the oxide particles. This deposited carbon was crucial for promoting the ORR. Furthermore, Takeuchi et al. exploited the deposited carbon to connect the current corrector to the active sites on the oxide surface directly without a carbon support (Takeuchi et al. 2022). Their method, which not only achieved the effective formation of the electron conduction path but also increased the content of the oxide, enhanced the ORR mass activity. Therefore, exploiting the deposited carbon is deemed a promising approach for the fabrication of an electron conduction path between a current collector and active sites on a metal-oxide surface.

With regard to the ORR activity of metal-oxide catalysts, it is reported that oxygen vacancies on the oxide surface function as the active sites (Imai et al. 2010). Doping of other elements into oxides is typically employed to form oxygen vacancies. There are numerous reports of doping with various metals and non-metals (Arashi et al. 2014; Chisaka et al. 2020, 2022)—particularly nitrogen doping (Dam et al. 2012; Chisaka et al. 2015; Kinumoto et al. 2015; Seifitokaldani et al. 2015). Moreover, the degree of nitrogen doping significantly affects the catalytic activity, emphasizing the importance of controlling the amount of nitrogen dopant (Chisaka et al. 2013). Various techniques for nitrogen doping into metal oxides have been reported, e.g., heat treatment of metal oxides in a nitrogen atmosphere, using nitrogen-containing aromatic compounds such as zirconium pyrazine carboxylic acid as precursors, and employing urea in a hydrothermal synthesis (Chisaka et al. 2015, 2016; Takeuchi et al. 2022). Among them, the direct ammonia nitridation method has the advantage of control of the nitrogen doping level over a wide range, from very little doping to nearly complete nitridation, through manipulation of the heat-treatment conditions (Bolokang and Phasha 2010; Zukalova et al. 2010; Liu et al. 2020). Hence, direct ammonia nitridation is favorable for controlling the active-site formation on oxide surfaces.

In this paper, we attempted direct ammonia nitridation of a metal complex precursor (Fig. 1). Our approach would afford both active-site formation on oxide surface by nitrogen doping and electron conduction path formation by precipitating electroconductive deposited carbon. In this research, we focused on the variations in catalyst structure with changes in calcination temperature.

2.1. Preparation of catalyst precursor

The detailed procedure for preparation is shown in S1. As the starting materials, polyacrylic acid 25000 (FUJIFILM Wako Pure Chemical), titanium tetraisopropoxide (Sigma–Aldrich Co. LLC), a sulfonated tetrafluoroethylene solution (Nafion® DE520 CS type, FUJIFILM Wako Pure Chemical Corp.), and iron acetate (Tokyo Chemical Industry Co. Ltd) were used. The materials were weighed with a 14:2:0.02:0.2 molar ratio, added to appropriate solvents, and homogenized together. The solution was dried to obtain titanium polyacrylate as a powder.

2.2. Ammonia nitridation protocol

Titanium polyacrylate (300 mg) was placed on an alumina boat (SSA–H#5, Nikkato), and the boat was laid into a calcination tube. Then, the tube was placed over a furnace and connected to an ammonia gas cylinder. The furnace temperature was increased to the target temperature (600, 700, 800, 900, and 1000°C) at a rate of 200°C h^–1 with a flow of ammonia gas (350 mL min^–1). After the target temperature was reached, the calcination was continued at the same temperature for 3 h. The heating was then stopped, and the sample was allowed to cool to room temperature. One hour after the start of cooling, the calcination tube was purged with nitrogen gas. After cooling, the calcinated sample was ground using a pestle to obtain a fine powder.

2.3. Characterization

Transmission electron microscopy (TEM) images were recorded using a JEOL JEM-2100 electron microscope at an accelerating voltage of 200 kV. For TEM measurements, a sample was dispersed in ultrapure water (≥ 18 MΩ cm, Millipore Direct-Q UV) using ultrasonic waves for 1 min, and few drops of the suspension were cast on a carbon-coated copper grid (Cat. 653, Nisshin-EM).

Powder X-ray diffraction (XRD) analysis was conducted using a Rigaku MiniFlex 600 (40 kV, 15 mA) with Cu-Kα ($\lambda$ = 1.5406 Å) radiation in the continuous-scan mode. A non-reflective silicon board was used as a sample holder. A silicon standard sample was measured to correct for the instrument-derived diffraction broadening. The crystallite size was calculated using Scherrer’s equation:

$$d=\frac{0.9\lambda }{Wcos\theta }, \left(1\right)$$

where d, λ, W, and θ represent the crystallite size (nm), X-ray wavelength (nm), full width at half maximum of the diffraction peak, and diffraction angle, respectively.

Ti K-edge X-ray absorption spectroscopy (XAS) was performed at the BL14B2 beamline of SPring-8 with the transmission method. Details are presented in Section S2 (Supporting Information).

The specific electrical resistance was obtained using a powder electrical resistivity meter (MCP–PD51, Nittoseiko Analytech). The powder sample was packed into a cylindrical holder and pressurized to 60 MPa, and its thickness and resistance were measured. The specific electrical resistance was calculated as follows:

$$\rho =\frac{RA}{l}, \left(2\right)$$

where $\rho$, R, A, and t represent the specific electrical resistance (Ω cm), the electrical resistance (Ω), the sample area (0.196 cm²), and the sample thickness (cm) under the pressure. The mesurement results correspond to the resistivity of the deposited carbon.

2.4. Electrochemical measurements

Electrochemical measurements were conducted in accordance with a published paper (Ishihara et al. 2019). For preparing catalyst ink, the catalyst powder (2.0 mg) was mixed with 1-hexanol (200 $\mu$L) and a 1.0 wt.% Nafion^® solution (10 $\mu$L). The 1.0 wt.% Nafion^® solution was prepared by diluting a 5.0–5.4 wt.% Nafion^® solution with a solution containing ultrapure water and 1-propanol at a 1:1 mass ratio. The catalyst ink was dropped on a polished glassy carbon (GC) rod (diameter of 5.2 mm, Tokai Carbon) and dried. The catalyst loading was set as ~ 0.5 mg_catalyst cm^–2.

A static three-electrode cell in 0.5 mol dm^–3 H₂SO₄ and a potentiostat (SP-50, BioLogic Sciences Instruments) were used for measurements. A reversible hydrogen electrode (RHE) and a GC plate were used as the reference and counter electrodes, respectively. The cell was maintained at 30°C during the measurements. As a pretreatment, the electrode underwent 200 cycles of sweeping in the range of 0.2–1.1 V under an O₂ atmosphere to clean it prior to measurements. Slow scan voltammetry (SSV) was performed at a scan rate of 5 mV s^–1 in the range of 0.05–1.1 V under O₂ and N₂ atmospheres. The current density of the ORR (i_ORR) was calculated by subtracting the current density under the N₂ atmosphere from that under the O₂ atmosphere. The rest potential E_rest under the O₂ atmosphere was determined by keeping the electrode undisturbed for 1 h. Cyclic voltammetry (CV) was performed at a scan rate of 50 mV s^− 1 in the range of 0.05–1.2 V under a N₂ atmosphere.

In this study, E_rest was selected as an index of ORR activity. For the electrochemical evaluation of the electron conduction path formation, the CV current was selected as an index. The current density of the ORR (i_ORR) was considered as a measure of catalyst performance encompassing both the ORR activity and electron conduction path formation.

3.1 Material characterization

The crystal structure of the samples was evaluated via XRD. Figures 2a and 2b show the XRD patterns for the samples calcinated at temperatures ranging from 600 to 1000°C. For the sample calcined at 1000°C, the diffraction peaks were between those of cubic TiN (JCPDS 38–1420) and TiO (JCPDS 8–117), suggesting the formation of a cubic titanium oxynitride phase in this sample. As the calcination temperature decreased, the peaks corresponding to titanium oxynitride broadened and shifted to larger angles. Additionally, a new peak appeared at approximately 2θ = 25.7° at 600 and 700°C, which may have been due to an abundant carbonaceous product from the precursor.

Figure 2c presents the change in the lattice constant of the cubic titanium oxynitride phase. The lattice constants were approximately 0.422 nm for temperatures between 1000 and 800°C. At 700°C, the lattice constant was clearly reduced to 0.418 nm. Referring to the database, the lattice constants of cubic TiO and TiN are 0.417 nm and 0.424 nm, respectively. The lattice-constant variation of the samples indicated the decreasing TiN composition in the titanium oxynitride phase at lower calcination temperatures. The crystalline phase composition of the catalysts was calculated by applying Vegard’s law to TiO and TiN (see Figure S2) (Djaoued et al. 2002).

Figure 2 Crystalline phase states for catalysts calcinated at different temperatures. Powder XRD patterns for catalysts (a), an enlarged view of the XRD pattern around 42.8° (b), the lattice constant (c), and the crystallite size calculated using Scherrer’s equation (d). The two dotted lines in (c) represent the lattice constants of TiN (JCPDS 38–1420) and TiO (JCPDS 8–117), respectively. The black dashed line in (d) represents the curve fittied to the data. The crystallite size and lattice constant were determined using the strongest diffraction peak at approximately 42.8°, which corresponds to the (200) plane of cubic titanium oxynitride.

To investigate the local structure and electronic state of the prepared catalysts, XAS experiments were conducted. Ti K-edge X-ray absorption near edge structure (XANES) of the series of catalysts and references are shown in Figs. 3a and S1. The spectrum of the sample calcined at 1000°C was similar to that of TiN. Meanwhile, the XAS spectra of the catalysts calcinated at lower temperatures ranging from 600 to 900°C did not correspond to any of the reference materials, and a remarkable pre-edge peak was observed at 4966 eV, as shown in Fig. 3b, which was also detected in the XANES spectrum of the titanium polyacrylate precursor. Farge et al. reported that the pre-edge peak increases in intensity and shifts to a lower energy as the coordination number of the titanium compound decreases (Farges et al. 1997). Accordingly, it is assumed that complete thermal decomposition of the precursor did not occur at these temperatures, yielding a mixture of titanium intermediates with a lower coordination number. In addition, this titanium intermediates phase was a non-crystalline phase, as no corresponding diffraction peak is observed in the XRD patterns in Fig. 2(a).

Figure 3 XANES spectra for the catalysts and reference materials (a), enlarged view of the pre-edge region in the XANES spectra (b), and the edge position shift with respect to the calcination temperature (c). All the XANES spectra were normalized. The black dotted line in (b) represents a pre-edge peak position at 600°C. The three dotted lines in (c) represent the edge positions of anatase TiO₂, Ti₄O₇, and TiN respectively.

In this section, we discuss the electroconductivity of the deposited carbon in the samples. Figure 4 illustrates the variation in specific electrical resistivity with respect to the calcination temperature. Increasing the calcination temperature significantly reduced the resistivity, with the value decreasing by five orders of magnitude between 600 and 1000°C. The specific electrical resistivity decreased sharply at 800°C, and in response, the CV current increased rapidly (see Figure S3). The reduction in resistivity at 800°C is attributed to the promotion of deposited-carbon graphitization. Barbela et al. reported that graphitization of organic compounds progressed, yielding deposited carbon, over 750°C (Barbera et al. 2014). The electroconductivity measurements and CV measurements confirmed that an electron conduction path was formed through calcination of titanium polyacrylate.

To assess the change in catalyst morphology, TEM was conducted. Figures 5 and S4 show TEM images of the samples. Titanium compound grains and deposited carbon were observed under all the calcination conditions. As the calcination temperature increased, the particle size increased to several tens of nanometers, and the amount of deposited carbon covering the catalyst particle surface decreased.

3.2 ORR activity

Figure 6 shows the trends of E_rest and i_ORR for each sample. The SSV data are presented in Figure S5. When the calcination temperature decreased from 1000 to 800°C, E_rest increased significantly. The i_ORR exhibited a similar trend to the E_rest. However, the trend differed under calcination at 700°C. The i_ORR exhibited a sharp decrease, whereas the E_rest remained relatively constant.

3.3 Effect of calcination temperature on catalyst structure

Details regarding the catalyst structure for each calcination temperature are presented as Table 1 and Fig. 7. At 1000°C, the crystalline titanium oxynitride phase dominated in most of the catalyst particles. The chemical state of the particles was close to TiN. Although the specific resistance of the deposited carbon was the lowest among all the samples, the CV current was low, indicating that the electron conduction path formation was insufficient. At 900 and 800°C, the chemical state of the crystalline phase was hardly changed from that at 1000°C, whereas the proportion of the non-crystalline oxide-like phase in the catalyst particles was increased, as indicated by the XANES spectra. The resistivity of the deposited carbon was on the order of 10–100 Ω cm, and the CV current was relatively high, suggesting that there was a sufficient electron conduction path. At 700 and 600°C, most of the catalyst particles consisted of non-crystalline oxide-like phase. The resistivity of the deposited carbon was on the order of 10⁴–10⁵ Ω cm, and the CV currents were poor, suggesting that the electron conduction path formation was insufficient.

Chisaka et al. reported that the ORR activity decreased when the crystal structure transformed into cubic TiN because of excess nitrogen doping (Chisaka et al. 2013). In our study, a similar trend was observed. At 900 and 1000°C, the crystal structures were almost cubic TiN, and the E_rest values were poor. Our results suggested that the decline in activity under high calcination temperatures was due to the excess nitrogen doping. Notably, E_rest exhibited a strong correlation with the absorption edge position of the XANES spectra (see Figure S6). The absorption edge position located on the higher-energy side was recognized as the result of slight nitrogen doping in the oxides. While a detailed discussion of the mechanism underlying the activity is beyond the scope of this study, this trend suggests that the slight nitrogen doping of the non-crystalline titanium oxide-like phase may induce active-site formation. Although the samples calcinated at 600 and 700°C had high E_rest values, their i_ORR values were poor. This is attributed to the insufficient electron conduction path formation. While there were catalytic particles capable of functioning as active sites, the poor electrical effective surface area resulted in a low current. According to the findings of this study, forming sufficient electroconductive deposited carbon and maintaining an appropriate nitrogen doping level of the oxynitride phase are important for achieving high E_rest and i_ORR values. In future research, calcination conditions that allow precise manipulation of both the deposited carbon and the catalyst particle structure should be investigated.

Table 1

Material characterization and catalytic activity.
Calcination temperature /°C	Crystallite size /nm	Crystalline TiO_xN_1−x phase composition	Specific resistance /Ωcm	E_rest /V	i_ORR at 0.60 V vs. RHE /mA g^–1
Calcination temperature /°C	Crystallite size /nm	/-	Specific resistance /Ωcm	E_rest /V	i_ORR at 0.60 V vs. RHE /mA g^–1
600	n/a	n/a	34$\times$10⁴	0.865	0.091
700	1.9	TiO_0.85N_0.15	35$\times$10³	0.884	8.9
800	15	TiO_0.35N_0.65	51	0.861	4.9$\times$10²
900	39	TiO_0.33N_0.67	14	0.784	3.5$\times$10²
1000	77	TiO_0.29N_0.71	4.5	0.631	31

We synthesized titanium oxynitride catalysts for ORR via direct ammonia nitridation of titanium polyacrylate. The composition of titanium oxynitride and the formation of deposited carbon serving as the electron conduction path were controlled by the calcination temperature. A higher calcination temperature corresponded to a higher degree of nitridation of the titanium oxynitride phase, as well as higher electroconductivity of the deposited carbon. Thus, moderate nitrogen doping into oxynitrides and sufficient conductivity of deposited carbon were achieved at 800°C, leading to high E_rest and i_ORR values. Our results indicate that the direct ammonia nitridation of the metal complex allows active-site formation and electron conduction path formation for metal oxynitride catalysts.

Cyclic voltammetry

Grassy carbon

ORR

Oxygen reduction reaction

PEFCs

Polymer electrolyte fuel cells

RHE

Reversible hydrogen electrode

SSV

Slow scan voltammetry

TEM

Transmission electron microscopy

XANES

X-ray absorption near edge structure

XAS

X-ray absorption spectroscopy

XRD

X-ray diffraction

Declarations

Competing interests

The authors declare that they have no competing interests.

Ethical approval

Not applicable.

Funding

This work was supported by the Japan Society for the Promotion of Science KAKENHI (Grant No. 23KJ1509 and 23K04912).

Authors’ contributions

Conceptualization: Yushi Tamaki, Seino Satoshi, Akimitsu Ishihara

Acknowledgments

We express our appreciation to Dr. Yuu Takeuchi and Momo Obata for their dedicated support and valuable suggestions. The XAS experiments were performed at BL14B2 of SPring-8 with the approval of the Japan Synchrotron Radiation Research Institute (Proposal No. 2021B1941 and 2022B1890).

Availability of data and materials

The datasets used and/or analyzed during the current study are available from the corresponding author on reasonable request.

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Conducting experiments: Yushi Tamaki, Naoki Shinyoshi, Yuta Uetake, Nagai Takaaki, Ryuji Monden

Data analysis and investigation: Yushi Tamaki

Writing–original draft preparation: Yushi Tamaki, Satoshi Seino

Writing– review and editing: Yushi Tamaki, Naoki Shinyoshi, Satoshi Seino, Yuta Uetake, Nagai Takaaki, Ryuji Monden, Akimitsu Ishihara, Takashi Nakagawa

Funding acquisition: Yushi Tamaki and Satoshi Seino

Resources: Takashi Nakagawa, Akimitsu Ishihara

Supervision: Seino Satoshi, Takashi Nakagawa

Supplementary information

An electronic supplementary material is available.

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Synthesis and Characterization of Titanium Oxynitride Catalyst via Direct Ammonia Nitridation of Titanium Polyacrylate for Oxygen Reduction Reaction

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Abstract

Figures

1. Introduction

2. Materials and methods

2.1. Preparation of catalyst precursor

2.2. Ammonia nitridation protocol

2.3. Characterization

2.4. Electrochemical measurements

3. Results and discussion

3.1 Material characterization

3.2 ORR activity

3.3 Effect of calcination temperature on catalyst structure

4. Conclusions

Abbreviations

Declarations

Declarations

Competing interests

Ethical approval

Funding

Authors’ contributions

Acknowledgments

Availability of data and materials

References

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