Bi-functional Oxygen Electrocatalysts Using Mixed-Metal Tungsten-Nitrides in Alkaline Media

Abstract

Transition metal-tungsten nitrides (CrWN₂, MnWN₂, FeWN₂, Co₃W₃N, Ni₂W₃N) could be synthesized and electrocatalytic properties on electrochemical oxygen reduction and evolution reactions were examined. The oxygen reduction performance was in the order of Co₃W₃N > MnWN₂ > Ni₂W₃N ≫ FeWN₂ ≫ CrWN₂. While the oxygen evolution was in that of Co₃W₃N ≫ CrWN₂ > Ni₂W₃N > MnWN₂ ≫ FeWN₂. Then, the Co₃W₃N gave high bi-functional oxygen electrocatalytic properties. Furthermore, Ni-doped (Co_1−xNi_x)₃W₃N (x = 0–1.0) were prepared and it was found that the (Co_0.6Ni_0.4)₃W₃N gave the highest bi-functional electrocatalytic properties. The cathode performance was achieved with the electrode containing 32 wt% (Co_0.6Ni_0.4)₃W₃N, i.e., the current density as high as 280 mA cm⁻², which was 10 times higher than that of a carbon-only electrode, was obtained at (0.80 V vs. RHE) in 5 mol L⁻¹ KOH at 70 °C. Also, the (Co_0.6Ni_0.4)₃W₃N electrode showed high activity to oxygen evolution as high as 300 mA cm⁻² at (1.60 V vs. RHE). As it was observed the changes in binding energy at O_1s, N_1s, W_4f, Co_2p, and Ni_2p spectra among Co₃W₃N, (Co_0.6Ni_0.4)₃W₃N, and Ni₂W₃N by X-ray photoelectron spectroscopy, the change in electrical properties at the surface of the nitrides should be one of the reasons of the high performance.

1. Introduction

In recent years, metal-air batteries (MAB) have been receiving a great deal of attention. There are several merits in the MAB, i.e., high energy density, low application temperature, and low cost.^1,2 Then, MABs with environmental friendliness expect for clean energy of automobile, motorcycle, drone, etc. These MABs, i.e., Mg-Air, Al-Air, Zn-Air, and Fe-Air batteries can be used as a high–capacity primary battery. However, one of the most important demerits is its reversibility, because most of the conventional MABs could not directly charge due to the low performance of electro-catalytic properties of oxygen evolution when the MAB is charging. There is an idea to use an additional electrode³ for only charge to prevent the damage of oxygen cathodes such as containing platinum based electrocatalysts. The use of Pt-based oxygen electrode is not only increasing the material cost but also poor stability as the Pt metal migrated during charge to anode metal which cause serious self-discharge and reduce battery life.⁴ In order to design high-performance secondary MABs, it needs to develop cost-effective and highly active bi-functional electrocatalysts, which could use both electrochemical oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) for discharge and charge, respectively.⁵

So far, much oxygen electrocatalysts have been studied such as those using perovskite-type oxides,^6–9 pyrochlore-type oxides,^10,11 spinel-type oxides,^12,13 Fe-Ni alloy,¹⁴ metal-phthalocyanines,^15,16 transition metal nitrides,^17–27 and so on. However, they still have some difficulties such as a dual mode performance for ORR/OER and the material stability. Among the bifunctional catalyst’s candidate, we focused on metal-nitrides. The transition metal nitride, i.e., MN (M = transition metal) has a face-centered cubic lattice structure and has an ionic crystal character.²⁸ However, most of transition metal nitrides do not follow the Pauling’s electrostatic valence rule and make interstitial solid solutions like metal-alloys. For example, 25 % and 50 % position are filled with nitrogen atoms in M₄N and M₂N crystals, respectively.^28–30 Also, there are nitrides with non-stoichiometric interstitial solid solutions. These interstitial solid solutions feature of the nitrides give high hardness, high chemical stability, and high electrical/thermal conductivity. Then, the transition metal nitrides have been studied as wide-band gap semiconductors, catalysts for ethanol amination, electrochemical catalysts including proton exchange membrane fuel cells,³¹ direct methanol fuel cells,^32,33 nitrogen reduction reaction,³⁴ metal-air batteries,^30,35 etc. Recently, we have found that the tungsten-based nitrides, such as the Co₃W₃N system, showed rather good bifunctional electrocatalytic properties for ORR as well as OER. Also, the Co₃W₃N system partially substituted at Co-site with Ni element was found to be very promising for improving the electrocatalytic properties.

In this study, synthesis methods of various tungsten-based nitrides were developed and the electrochemical cathodic and anodic oxygen catalytic performances using their nitrides were systematically investigated for application in alkaline electrolyte-type fuel cells (AFC), metal-air batteries, and secondary metal-air batteries. Also, Ni-doping at Co-site of Co₃W₃N system was found to improve the catalytic activities. The crystal structures and energy states of materials in the nitride catalysts were further discussed.

2. Experimental

2.1 Synthesis of metal-tungsten nitrides

Metal-tungsten nitrides (AWN₂: A = Cr, Mn, Fe; Co₃W₃N, Ni₂W₃N) and the (Co_1−xNi_x)₃W₃N (x = 0–1.0) were prepared via wet-chemical routes. Regent grade metal salts of constituent metals (10 mmol), such as (NH₄)₁₀W₁₂O₄₁·5H₂O (Kishida Chemical Co Ltd.), M′(NO₃)₂·6H₂O (M′ = Mn, Co, Ni) (Wako Pure Chemical Co Ltd.), M′′(NO₃)₃·9H₂O (M′′ = Cr, Fe) (Wako Pure Chemical Co Ltd.), were mixed with pure water (300 mL) at room temperature. The solution was evaporated to dryness at 70 °C, and then fully dried at 120 °C for 12 h. The obtained precursors were directly nitridated in 200 mL min⁻¹ NH₃ (> 99.5 %, Mitsui Chemical) flow at 600–900 °C for 9 h (Route A) or precalcined in air at 600–650 °C for 9 h before the same nitridation of Route A (Route B). The obtained metal-nitride powders were characterized by means of X-ray diffraction analysis (XRD, RU-200, Rigaku) using CuK_α1 radiation (0.15405 nm), X-ray photoelectron spectroscopy (XPS, KRATOS: AXIS-NOVA), and BET surface area measurement (SORP-mini II, BEL).

2.2 Electrochemical measurements

A gas-diffusion-type electrodes using PTFE bonded carbon and an electrochemical cell used in the present study were similar to those already reported.^6,36,37 The gas-diffusion-type electrode consists of gas diffusion and reaction layers. The former was based on carbon (60 wt%: AB-7, Denki Kagaku Kogyo K. K.) and Teflon (40 wt%: D-1, Daikin Ind., Ltd.). The latter consisted of a nitride catalyst (32 wt%), carbon (36 wt%: ECP600JD, Lion Corp.), and Teflon (32 wt%: D-1, Daikin Ind., Ltd.). These two layers were hot-pressed on a nickel mesh (100 mesh) current corrector at 60 MPa and 380 °C, then a laminated gas-diffusion-type electrode, 15 mm in diameter and 0.4 mm thick, was obtained. The cathodic and anodic polarization curves of the electrodes were measured with a potentiostat (HAL3001A, Hokuto Denko) with a function generator (HB-305, Hokuto Denko) in 5 mol L⁻¹ KOH aqueous solution at 70 °C using a Pt-plate counter electrode and a reversible hydrogen electrode with 5 mol L⁻¹ KOH (RHE) as the reference electrode. Pure oxygen or dry synthetic air (79 vol% N₂ + 21 vol% O₂) was supplied to the back side of the gas diffusion type electrodes during ORR or OER measurements, respectively. In these all measurements for the electrode performance, I-V measurements were not IR corrected.

3. Results and Discussion

3.1 Preparation of nitride catalysts

XRD patterns of transition metal nitrides prepared are shown in Fig. 1. They were well crystallized and almost single-phase nitrides which were synthesized via Route A for Ni₂W₃N and CrWN₂, via Route B for MnWN₂, FeWN₂, and Co₃W₃N. They were well fitted with powder diffraction files 00-051-0810: CrWN₂, 00-050-0845: MnWN₂, 01-082-1869: FeWN₂, 01-089-1026: Co₃W₃N, and 01-089-1313: Ni₂W₃N, respectively. BET surface areas of the synthesized nitrides were 2.9–9.0 m² g⁻¹, except CrWN₂ which showed as large as 31.2 m² g⁻¹, as also shown in Fig. 1.

Figure 1.

XRD patterns and BET surface areas of the synthesized various W-based nitrides.

As for the (Co_1−xNi_x)₃W₃N (x = 0.1–1.0) system prepared by Route A, the Co₃W₃N phase (η-carbide structur³⁸) was mainly observed for x = 0–0.4 in (Co_1−xNi_x)₃W₃N, while x = 0.5–0.6 in (Co_1−xNi_x)₃W₃N consisted of mixture of Co₃W₃N and Ni₂W₃N, and x = 0.7–1.0 in (Co_1−xNi_x)₃W₃N gave almost Ni₂W₃N phase (β-Mn structure³⁹), as shown in Figs. 2a–2c. It seems come from the easiness of the formation of β-Mn structured Ni₂W₃N as a nitride consist of Ni, W, and N, compared with Ni₃W₃N which has η-carbide structure, when increasing Ni content x in (Co_1−xNi_x)₃W₃N at x > 0.6.

Figure 2.

XRD patterns of synthesized (Co_xNi_1−x)₃W₃N powders, (a) x = 0–0.3, (b) 0.4–0.7, (c) 0.8–1.0.

3.2 Electrochemical activity of metal-nitrides

Figure 3 shows the electrocatalytic activities for ORR of the prepared nitrides using gas-diffusion-type PTFE-bonded carbon electrodes loaded with nitride catalysts under pure oxygen flow in 5 mol L⁻¹ KOH at 70 °C. The cathodic performances of the electrocatalysts of M_xW_yN_z (M = Cr, Mn, Fe, Co, Ni; x = 1–3, y = 1, 2, z = 1, 2) were largely dependent on the M elements. The ORR performance was in the order of Co₃W₃N > MnWN₂ > Ni₂W₃N ≫ FeWN₂ ≫ CrWN₂. Surprisingly, the electrode performances of Co₃W₃N, MnWN₂, and Ni₂W₃N to ORR were as high as more than 400 mA cm⁻², although the simple W₂N catalyst gave 111 mA cm⁻² at 700 mV vs. RHE.

Figure 3.

Performance of oxygen reduction reaction of gas-diffusion-type carbon electrodes loaded with various W-based nitrides in 5 mol L⁻¹ KOH at 70 °C.

Figure 4 shows the OER of the gas-diffusion-type carbon electrodes loaded with Co₃W₃N, MnWN₂, Ni₂W₃N, FeWN₂, and CrWN₂ under dry synthetic air flow in 5 mol L⁻¹ KOH at 70 °C. The OER performance was in the order of Co₃W₃N ≫ CrWN₂ > Ni₂W₃N > MnWN₂ ≫ FeWN₂. Then, it was revealed that the Co₃W₃N gave the highest activities to both ORR and OER among the binary transition metal tungsten nitrides tested. The reason of these performance was not clear now, but crystal structures, the use of cobalt, electric properties of catalysts should give one of the effects.

Figure 4.

Performance of oxygen evolution reaction of gas-diffusion-type carbon electrodes loaded with various W-based nitrides in 5 mol L⁻¹ KOH at 70 °C.

In our previous study, it was found that doping other element into A site of A₃B₃N system gave improvement of ORR, then Ni-doped (Co_1−xNi_x)₃W₃N (x = 0.1–1.0) system was investigated. Figure 5 shows ORR performance of the gas-diffusion-type electrodes loaded with (Co_1−xNi_x)₃W₃N (x = 0–1.0) catalysts, where x = 0.7–1.0 gave Ni₂W₃N crystal phase. The ORR performance was largely dependent on the x value and x = 0.4 in (Co_1−xNi_x)₃W₃N showed the highest ORR performance than none-doped Co₃W₃N. The (Co_0.6Ni_0.4)₃W₃N showed the ORR performance as high as 280 mA cm⁻² at 0.8 V vs. RHE. Figure 6 summarizes the ORR performance at 750 and 800 mV vs. RHE. As the ORR performance of (Co_1−xNi_x)₃W₃N (x = 0.1–0.9) system showed larger value than none-doped Co₃W₃N and Ni₂W₃N nitrides. ORR performance for x = 0.2, 0.3, 0.5 and 0.6 was lower than expected. This should be due to the change in crystal phases among x = 0.1–0.3 and existence of mixed phases of Co₃W₃N and Ni₂W₃N for x = 0.5 and 0.6. Anyway, a synergetic effect between Co and Ni should be occurred in the (Co_1−xNi_x)₃W₃N system.

Figure 5.

Oxygen reduction performance of gas-diffusion-type carbon electrodes loaded with (Co_1−xNi_x)₃W₃N (x = 0–1.0).

Figure 6.

ORR current densities of (Co_1−xNi_x)₃W₃N (x = 0–1.0) at 800 mV and 750 mV vs. RHE.

Figure 7 shows the ORR (a) and OER (b) performances of GDEs loaded with (Co_0.6Ni_0.4)₃W₃N, compared with Co₃W₃N, Ni₂W₃N, and Pt-doped carbon. Although ORR performance of the GDE loaded with (Co_0.6Ni_0.4)₃W₃N was lower than that of Pt-doped electrode in the overpotential of ca. 100 mV, the current density gave more than 280 mA cm⁻² at 800 mV vs. RHE. As for the OER, remarkable performance was obtained for the (Co_0.6Ni_0.4)₃W₃N loaded GDE, the current density as high as 300 mA cm⁻² at 1600 mV vs. RHE was achieved. On the other hand, the OER performance of Pt-loaded carbon was decreased at anodic potential above ca. 1480 mV vs. RHE. This seems to be come from high catalytic property of Pt on carbon oxidation. As the GDEs loaded with binary nitrides of Co₃W₃N or Ni₂W₃N showed lower OER activities than that for (Co_0.6Ni_0.4)₃W₃N, this should be come from the Co and Ni combination in the ternary (Co_1−xNi_x)₃W₃N system, which should be more effective to OER performance, and this is a new finding of the high active bi-functional catalyst.

Figure 7.

Performance of oxygen reduction and oxygen evolution reactions of gas-diffusion-type carbon electrodes loaded with various W-based nitrides and Pt in 5 mol L⁻¹ KOH at 70 °C.

In order to examine the effects of Ni-doping in (Co_1−xNi_x)₃W₃N on the electrocatalytic performance, the XPS spectra of N_1s, O_1s, W_4f, Co_2p, and/or Ni_2p in the nitrides of Co₃W₃N, Ni₂W₃N, and (Co_0.6Ni_0.4)₃W₃N were analyzed.⁴⁰

Figure 8 shows the XPS spectra of N_1s orbitals of the Co₃W₃N, Ni₂W₃N, and (Co_1.6Ni_0.4)₃W₃N system. In N_1s of the nitrides, the binding energy (B.E.) between 396.5 and 399.0 eV seems the bonding of M-N (M = W, Ni, Co), from the low B.E. for W-N, Ni-N, and Co-N, respectively. The B.E. at ca. 400 eV should be NH_x at the surface of the nitrides. The B.E. of M-N was decreased with increasing x in (Co_1−xNi_x)₃W₃N. This shows M-N bonding energy was shifted to lower B.E. with increasing Ni amount. Ni₂W₃N showed lower B.E. at N_1s compared with others, which shows Ni-N and W-N bonding is not strong. As the Co-based nitrides showed strong W-N peaks, W_xN units were formed in these crystals.⁴¹

Figure 8.

XPS spectra of N_1s orbitals of Co₃W₃N, (Co_0.6Ni_0.4)₃W₃N, and Ni₂W₃N.

Figure 9 shows the XPS spectra of O_1s orbitals of the Co₃W₃N, Ni₂W₃N, and (Co_0.6Ni_0.4)₃W₃N system. As the B.E. at ca. 528 eV of lattice oxygen (O_L) was not observed for Co₃W₃N, Ni₂W₃N, and (Co_0.6Ni_0.4)₃W₃N, any oxides were not remained after nitridation among them. The B.E. peaks at 530.5 and 531.5 eV are attributed to adsorbed oxygen (O_ad) and hydroxide (OH_ad), respectively. The peak at 533.0 eV should be adsorbed water (H₂O_ad).

Figure 9.

XPS spectra of O_1s orbitals of Co₃W₃N, (Co_0.6Ni_0.4)₃W₃N, and Ni₂W₃N.

Because the B.E. of O_ad for (Co_0.6Ni_0.4)₃W₃N was smaller than that of Ni₂W₃N, the oxygen adsorption energy on (Co_0.6Ni_0.4)₃W₃N became weaker by combination of Ni and Co. Also it was recognized that the amount of O_ad on (Co_0.6Ni_0.4)₃W₃N was larger than that on Co₃W₃N from intensity. Then, it was found that Ni-doping in (Co_0.6Ni_0.4)₃W₃N largely influenced the intensity and B.E. of the O_ad.

The XPS spectra of the W, Co, and Ni of these nitrides showed unique peaks. Figure 10 shows XPS spectra of W_4f orbitals of these catalysts. Metallic W⁰ was observed for Ni₂W₃N at 31 eV (W_4f7/2) and 33.2 eV (W_4f5/2) orbitals. Strong W⁶⁺ bonds 35 eV (W_4f7/2) and 37.5 eV (W_4f5/2) were observed for (Co_0.6Ni_0.4)₃W₃N and Ni₂W₃N. XPS spectra of Co_2p were shown in Fig. 11a, metallic Co⁰ was observed at 778 eV (Co_2p3/2) for Co₃W₃N and (Co_0.6Ni_0.4)₃W₃N, also Co²⁺ at 781 and 783 eV (Co_2p3/2) was observed both Co₃W₃N and (Co_0.6Ni_0.4)₃W₃N. The Co³⁺ peak at 782 eV shifted rather higher energy for Co₃W₃N than that for (Co_0.6Ni_0.4)₃W₃N. This should be the effect from doped Ni. XPS spectra of Ni_2p were shown in Fig. 11b. Metallic Ni⁰ was observed at 853 eV (Ni_2p3/2) for only Ni₂W₃N. Ni²⁺ at 856 eV (Ni_2p3/2) and 863 eV (Ni_2p3/2sat), Ni³⁺ at 857 eV (Ni_2p3/2) were observed both (Co_0.6Ni_0.4)₃W₃N and Ni₂W₃N. As for Ni₂W₃N, the Ni²⁺ peak at 856 eV was shifted to higher binding energy and had larger Ni²⁺ satellite peak, therefore, Ni₂W₃N should have more Ni²⁺ than and (Co_0.6Ni_0.4)₃W₃N system.

Figure 10.

XPS spectra of W_4f orbitals of (Co_1−xNi_x)₃W₃N (x = 0, 0.4, 1.0).

Figure 11.

XPS spectra of Co_2p orbitals of (Co_1−xNi_x)₃W₃N (x = 0, 0.4) (a) and Ni_2p orbitals of (Co_1−xNi_x)₃W₃N (x = 0.4, 1.0) (b).

In summary of the XPS analyses, the (Co_0.6Ni_0.4)₃W₃N system showed large Metal-N bonds, low energy O_ad bond, the peaks of small-metallic Co, small Ni²⁺, and large Co³⁺. Therefore, the (Co_0.6Ni_0.4)₃W₃N has properties of partially alloy and partially ionic crystal structure. Then this might be one of the reasons of high oxygen electrocatalytic active mechanism. Anyway, further studies should be necessary to clarify the electrocatalytic properties of the (Co_1−xNi_x)₃W₃N system.

4. Conclusion

Various metal-tungsten nitrides (CrWN₂, MnWN₂, FeWN₂, Co₃W₃N, and Ni₂W₃N) were successfully synthesized using the wet chemical routes. The ORR and OER performance of the nitrides was dependent on the binary tungsten system. And the Co₃W₃N showed a high performance both for ORR and OER. Ni-doped Co₃W₃N improved the cathode and anode performance, and the electrode loaded with (Co_0.6Ni_0.4)₃W₃N gave the very high ORR/OER performance of 280 mA cm⁻² at 800 mV and 300 mA cm⁻² at 1600 mV vs. RHE, respectively, in 5 mol L⁻¹ KOH at 70 °C. X-ray photoelectron spectroscopy revealed the changes in binding energy at O_1s, N_1s, W_4f, Co_2p, and Ni_2p spectra among Co₃W₃N, (Co_0.6Ni_0.4)₃W₃N, and Ni₂W₃N, which should show the change in electrical properties, crystal characteristics, and effects of catalytic activities.

Acknowledgments

The authors are grateful to the Center for Instrumental Analysis, Kyushu Institute of Technology, for the XRD, and XPS measurements. The authors also acknowledge Daikin Ind. Ltd., the Lion Corp., and Denki Kagaku Kogyo K.K. for providing the PTFE, Ketjen black, and Denka black, respectively.

CRediT Authorship Contribution Statement

Shotaro Nomoto: Conceptualization (Lead), Data curation (Lead), Formal analysis (Lead), Investigation (Lead), Methodology (Lead), Validation (Lead), Writing – original draft (Supporting)

Hiroki Kitamura: Conceptualization (Equal), Data curation (Lead), Formal analysis (Lead), Investigation (Lead), Methodology (Equal), Validation (Equal)

Satoko Takase: Data curation (Equal), Formal analysis (Equal), Methodology (Equal), Supervision (Supporting), Writing – review & editing (Supporting)

Youichi Shimizu: Conceptualization (Lead), Funding acquisition (Lead), Investigation (Lead), Methodology (Lead), Supervision (Lead), Writing – original draft (Lead), Writing – review & editing (Lead)

Conflict of Interest

The authors declare no conflict of interest in the manuscript.

Footnotes

S. Takase: ECSJ Active Member

Y. Shimizu: ECSJ Fellow

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