Fabrication of Platinum-Decorated NiCo-Layered Double Hydroxide Nanoflowers for Electrocatalytic Ammonia Oxidation Reaction

Abstract

The complete anodic oxidation of ammonia is an important part of direct ammonia fuel cells. Fabricating a high-performance electrocatalyst for ammonia oxidation reaction is meaningful for developing a direct ammonia fuel cell. Herein, we designed one platinum-decorated NiCo-layered double hydroxide nanoflower on Ni foam (Pt-NiCo-LDH-Ni foam) and measured the electrocatalytic performance via the cyclic voltammetry (CV) technique. The experimental results demonstrated that the optimized Pt-NiCo-LDH-Ni foam showed great electrocatalytic performance, with a low overpotential with a value of −0.573 V, a high current density of 17.75 mA cm⁻² for the ammonia oxidation reaction, and good stability.

1. Introduction

Owing to increasing energy demands and consequent environmental pollution, fossil fuels do not meet the requirements of energy and environmental protection [1]. Therefore, recently, developing clean energy sources has been given more attention [2]. Ammonia exhibits two obvious advantages [3]. First, ammonia can be used as green fuel for direct ammonia fuel cells. As the anodic reaction, the thermodynamically required potential of the electrochemical ammonia oxidation reaction is 0.06 V, which is lower than that of the overpotential of water splitting (1.23 V). In addition, the final production of electrochemical ammonia oxidation reactions is N₂ and H₂O, achieving zero carbon emissions. Because of the above advantages, ammonia has received more attention for direct ammonia fuel cells.

However, the ammonia oxidation reaction is a multistep process, and the intermediates have also shown poisoning toward the electrocatalysts of the ammonia oxidation reaction process. Therefore, designing high-performance electrocatalysts to meet the requirements is important. Today, the electrocatalysts for ammonia oxidation reactions have been reported and researched including noble metals, metal oxides, layered double hydroxides, and so forth. Noble metals with high electrocatalytic performances, including Pt [4], Ir [5], and noble metal-based composites [6,7,8], have been reported. For example, Su et al. successfully designed carbon paper-supported Pt with various morphologies via the electrodepositing process under different electrodeposition potentials. The measured results proved that the cauliflower-like Pt catalyst exhibited better catalytic activity and durability 4.7 times than that of commercial Pt black [9]. Kim and their group reported on PtZn alloy nanoparticles that were prepared via the one-pot polyol process. The electrocatalytic performances of PtZn alloy nanoparticles were discussed via four different ammonia solutions, and synthesized PtZn electrocatalysts exhibited enhanced electrocatalytic performances in terms of the current density by 2.5–3.3, 2.3–2.7, 2.2–2.8, and 3.9–4.8 fold for NH₄OH, NH₄Cl, NH₄NO₃, and (NH₄)₂SO₄, respectively, compared with Pt/C [8]. In addition, layered double hydroxides have also been used as electrocatalysts for ammonia oxidation reactions owing to the great advantages of their unique three-dimensional structure, good conductivity, high surface area, and low cost [10,11,12]. Using a facile hydrothermal coupled with electroreduction method, Xu et al. successfully constructed amorphous NiFe-layered double hydroxide nanosheets modified with boron nanoclusters (B-NiFe-LDH/NF), and this B-NiFe-LDH/NF showed the great electrocatalytic performance for the ammonia oxidation reaction [13]. Nickel–copper-layered double hydroxide has also been used as a catalyst for the electrochemical oxidation reaction of ammonia. Mao et al. prepared one NiCu-layered double hydroxide (NiCu-LDH) on nickel foam for ammonia oxidation via a facile hydrothermal reaction. Compared with Ni(OH)₂/NF, the obtained Ni_0.9Cu_0.1-LDH/NF exhibited smaller and denser nanosheets, providing more electrochemical active areas and active sites. The electrochemical measurements indicated that Ni_0.9Cu_0.1-LDH/NF exhibited great electrocatalytic activities [14]. However, it was difficult to find related reports for ammonia oxidation reactions based on Pt and layered double hydroxide composites that acted as electrocatalysts.

In this work, to enhance the kinetic process of ammonia oxidation reaction, we first grew NiCo-layered double hydroxide nanoflowers on Ni foam and then electrodeposited platinum to decorate NiCo-layered double hydroxide nanoflowers (Pt-NiCo-LDH-Ni foam). The optimized Pt-NiCo-LDH-Ni foam was characterized and measured by transmission electron microscopy, scanning electron microscopy, X-ray photoelectron spectroscopy, and an electrochemical station. In addition, the catalytic process for ammonia oxidation reaction is discussed.

2. Results and Discussion

To prove the synthesis of NiCo-LDH-Ni foam nanoflowers and Pt-NiCo-LDH-Ni foam, SEM, TEM, XRD, and XPS were used to characterize the morphology and composition. Figure 1a shows the SEM image of the NiCo-LDH-Ni foam. Many nanowires could be found, and these nanowires were assembled into nanoflowers. After depositing Pt, these nanowires gathered further, as shown in Figure 1b. The HRTEM of Pt-NiCo-LDH-Ni foam is shown in Figure 1c; the obvious lattice stripe with a distance of 0.227 nm was found to belong to the crystal plane (111) of Pt [15], indicating that Pt nanoparticles were loaded on the NiCo-LDH nanoflowers. Figure 1d shows the XRD pattern of the Pt-NiCo-LDH-Ni foam; three strong diffraction peaks belonging to the Ni foam could be found, and three diffraction peaks, which were located at 20.89, 34.11, and 38.32, could be ascribed to the crystal plane (006), (009), and (015) of NiCo-LDH, indicating the formation of NiCo-LDH. However, no obvious diffraction peaks that could be attributed to Pt nanoparticles could be found. A possible reason is that the diffraction peaks were covered because of the strong diffraction peaks of the Ni foam.

To further analyze the composition of the Pt-NiCo-LDH-Ni foam, XPS measurement was conducted, and the XPS results are shown in Figure 2. As shown in Figure 2a, four fitted peaks could be found. Two peaks at binding energies of 855.99 eV and 873.67 eV belonged to Ni²⁺ 2p_3/2 and Ni²⁺ 2p_1/2. In addition, two peaks at 861.70 eV and 879.71 eV were the satellite peaks of Ni 2p_3/2 and Ni 2p_1/2 [16]. The XPS spectrum of the Co element is given in Figure 2b. The fitted peaks, which were located at 781.28 eV and 797.04 eV, could be attributed to Co²⁺ 2p_3/2 and Co²⁺ 2p_1/2, and the two satellite peaks could be detected at binding energies of 786.18 eV and 803.29 eV. In addition, one fitted peak at 774.74 eV could be ascribed to Co^δ+(δ = 0~2) [16]. As shown in Figure 2c, the XPS spectrum of the O element could be fitted to one peak, which was located at 531.71 eV and could be attributed to OH^- [16]. The fitted results of the XPS spectra of the Ni, Co, and O elements further proved the formation of NiCo-LDH. Finally, the XPS spectrum of the Pt element is shown in Figure 2d. Two peaks at 70.85 eV and 73.14 eV could be attributed to Pt 4f_7/2 and Pt 4f_5/2 [17], indicating that Pt was zero-valent. In addition, the fitted peak at 67.96 eV could be ascribed to Ni 3p. The above results indicate that Pt nanoparticles were deposited on NiCo-LDH.

To obtain the best electrocatalyst for ammonia oxidation reaction, the deposition cycles of the Pt nanoparticles were controlled. Figure 3 shows the CV curves of the Pt-NiCo-LDH-Ni foam-1, Pt-NiCo-LDH-Ni foam-2, Pt-NiCo-LDH-Ni foam-3, and Pt-NiCo-LDH-Ni foam-4 electrodes in 1 M KOH with the presence and absence of 0.1 M NH₄Cl. As shown in Figure 3a, the CV curves of the Pt-NiCo-LDH-Ni foam-1 electrode in 1 M KOH with the presence and absence of 0.1 M NH₄Cl exhibited obvious changes. A clear oxidation peak could be found at a potential of about −0.2 V in 1 M KOH with the presence of 0.1 M NH₄Cl, indicating that the Pt-NiCo-LDH-Ni foam-1 electrode showed electrocatalytic activity for the ammonia oxidation reaction. Figure 3b–d show the CV curves of the Pt-NiCo-LDH-Ni foam-2, Pt-NiCo-LDH-Ni foam-3, and Pt-NiCo-LDH-Ni foam-4 electrodes in 1 M KOH with the presence and absence of 0.1 M NH₄Cl, and similar oxidation peaks at about −0.2 V could be found. Differently, with the increasing deposition cycles from 5 to 15 cycles, on one hand, the onset potentials gradually shifted in a negative direction, and the onset potential of the Pt-NiCo-LDH-Ni foam-3 electrode achieved −0.573 V. On the other hand, the oxidation peak current densities increased from 5.51 to 17.75 mA cm⁻² (Figure 3e,f). However, when the deposition cycles achieved 20 cycles, the Pt-NiCo-LDH-Ni foam-4 electrode was etched during the deposition process, resulting in a larger onset potential and smaller oxidation peak current density than that of the Pt-NiCo-LDH-Ni foam-3 electrode. The above results proved that when the deposition cycle was 15, the Pt-NiCo-LDH-Ni foam-3 electrode showed the best electrocatalytic performance.

In comparison, the electrochemical activity of the NiCo-LDH-Ni foam and the Pt-Ni foam for the catalytic ammonia oxidation reaction was also measured, as shown in Figure 4. As shown in Figure 4a, the CV curves of the NiCo-LDH-Ni foam have clear current changes after being added into 0.1 M NH₄Cl; this obvious current change could be attributed to the electrocatalytic ability of NiCo-LDH according to Equations (1)–(4). Differently, the Pt-Ni foam electrode exhibited an obvious oxidation peak, and the oxidation peak current achieved 13.85 mA cm⁻² after adding 0.1 M NH₄Cl (Figure 4b) according to Equations (5)–(9). The above results indicate that Pt was the main factor in enhancing the electrocatalytic performances and that NiCo-LDH nanoflowers provided a large surface for the deposition of Pt nanoparticles, resulting in Pt-NiCo-LDH-Ni foam-3 electrodes with better electrocatalytic performances than those of Pt-Ni foam.

$$\mathrm{C}\mathrm{o}{\left(\mathrm{O}\mathrm{H}\right)}_2+{\mathrm{O}\mathrm{H}}^{-}-{\mathrm{e}}^{-}\rightleftharpoons \mathrm{C}\mathrm{o}\mathrm{O}\mathrm{O}\mathrm{H}+{\mathrm{H}}_2\mathrm{O}$$

(1)

$$\mathrm{N}\mathrm{i}{\left(\mathrm{O}\mathrm{H}\right)}_2+{\mathrm{O}\mathrm{H}}^{-}-{\mathrm{e}}^{-}\rightleftharpoons \mathrm{N}\mathrm{i}\mathrm{O}\mathrm{O}\mathrm{H}+{\mathrm{H}}_2\mathrm{O}$$

(2)

$$2\mathrm{N}\mathrm{i}\mathrm{O}\mathrm{O}\mathrm{H}+2{\mathrm{N}\mathrm{H}}_3\to 2\mathrm{N}\mathrm{i}\mathrm{O}\mathrm{O}\mathrm{H}{\left({\mathrm{N}\mathrm{H}}_3\right)}_{\mathrm{a}\mathrm{d}\mathrm{s}}\to 2\mathrm{N}\mathrm{i}\mathrm{O}\mathrm{O}\mathrm{H}+{\mathrm{N}}_2+3{\mathrm{H}}^{+}+3{\mathrm{e}}^{-}$$

(3)

$$2\mathrm{C}\mathrm{o}\mathrm{O}\mathrm{O}\mathrm{H}+2{\mathrm{N}\mathrm{H}}_3\to 2\mathrm{C}\mathrm{o}\mathrm{O}\mathrm{O}\mathrm{H}{\left({\mathrm{N}\mathrm{H}}_3\right)}_{\mathrm{a}\mathrm{d}\mathrm{s}}\to 2\mathrm{C}\mathrm{o}\mathrm{O}\mathrm{O}\mathrm{H}+{\mathrm{N}}_2+3{\mathrm{H}}^{+}+3{\mathrm{e}}^{-}$$

(4)

$${\mathrm{N}\mathrm{H}}_3\left(\mathrm{a}\mathrm{q}\right)\to {\mathrm{N}\mathrm{H}}_{3,\mathrm{a}\mathrm{d}\mathrm{s}}$$

(5)

$${\mathrm{N}\mathrm{H}}_{3,\mathrm{a}\mathrm{d}\mathrm{s}}\to {\mathrm{N}\mathrm{H}}_{2,\mathrm{a}\mathrm{d}\mathrm{s}}+{\mathrm{H}}^{+}+{\mathrm{e}}^{-}$$

(6)

$${\mathrm{N}\mathrm{H}}_{2,\mathrm{a}\mathrm{d}\mathrm{s}}\to {\mathrm{N}\mathrm{H}}_{\mathrm{a}\mathrm{d}\mathrm{s}}+{\mathrm{H}}^{+}+{\mathrm{e}}^{-}$$

(7)

$${\mathrm{N}\mathrm{H}}_{\mathrm{x},\mathrm{a}\mathrm{d}\mathrm{s}}+{\mathrm{N}\mathrm{H}}_{\mathrm{y},\mathrm{a}\mathrm{d}\mathrm{s}}\to {\mathrm{N}}_2{\mathrm{H}}_{\mathrm{x}+\mathrm{y},\mathrm{a}\mathrm{d}\mathrm{s}}$$

(8)

$${\mathrm{N}}_2{\mathrm{H}}_{\mathrm{x}+\mathrm{y},\mathrm{a}\mathrm{d}\mathrm{s}}\to {\mathrm{N}}_{2,\mathrm{g}}+\left(\mathrm{x}+\mathrm{y}\right){\mathrm{H}}^{+}+\left(\mathrm{x}+\mathrm{y}\right){\mathrm{e}}^{-}$$

(9)

$$\mathrm{w}\mathrm{h}\mathrm{e}\mathrm{r}\mathrm{e}\mathrm{x}=1\mathrm{o}\mathrm{r}2,\mathrm{y}=1\mathrm{o}\mathrm{r}2$$

To analyze the kinetics of ammonia oxidation reaction, the scan rates were controlled, and the CV curves are shown in Figure 5a; with increased scan rates, the oxidation peak current densities increased. In addition, the oxidation peak potentials showed slight shifts to positive potentials. Figure 5b shows a fitted curve of the current densities and scan rates. The fitted curve shows a good linear relationship with the linear regression values (R²) of 0.999, indicating that the electrochemical oxidation of ammonia on the Pt-NiCo-LDH-Ni foam-3 electrode is dominated by a surface-controlled process. In addition, when the scan rate increased, the oxidation peak potential shifted slightly in the positive direction. There is also a good linear relationship between the oxidation peak potentials and scanning rates (R² = 0.997) (Figure 5c), indicating an irreversible process of the ammonia oxidation reaction on the Pt-NiCo-LDH-Ni foam-3 electrode. The oxidation peak potential (E_pa) and the logarithm of the scan rate (ln ν) (ν ≥ 0.2 V s⁻¹) exhibited a good linear relationship (Figure 5d). The equation can be fitted by Laviron’s theoretical model, as shown in Equation (10), and the number of electrons ($n$) involved in the rate-determining step of ammonia oxidation reaction on the Pt-NiCo-LDH-Ni foam-3 electrode can be calculated [18].

$$E_{Pa}=E^{\theta }+\left({\displaystyle \frac{RT}{\alpha nF}}\right)\mathrm{l}\mathrm{n}\left({\displaystyle \frac{RT{k}^{\theta }}{\alpha nF}}\right)+\left({\displaystyle \frac{RT}{\alpha nF}}\right)\ln \nu$$

(10)

The parameters in Equation (1) are given in the supporting information. The $\alpha n$ value was determined by the slope of E_pa versus ln ν as 0.552. Meanwhile, the value of $\alpha$ can be computed by Equation (11):

$$E_{Pa}-E_{P/2}=1.857\left({\displaystyle \frac{RT}{\alpha F}}\right)$$

(11)

In Equation (11), $E_{P∕2}$ is the potential of a half peak. $\alpha$ was calculated to be 0.530, and then the number of transfer electrons in the rate-determining step of the ammonia oxidation reaction was determined to be 1.042 $\approx$ 1.

Figure 6 shows the stability measurements of the Pt-NiCo-LDH-Ni foam-3 electrode and the Pt-Ni foam electrode for 5000 s. The Pt-NiCo-LDH-Ni foam-3 electrode showed better stability than the Pt-Ni foam electrode. In addition,

Table 1. The ammonia oxidation reaction test conditions and the activity of reported catalysts.

Electrode	Onset Potential /V	Current Density /mA cm−2	Electrolyte	Stability	Refs
PtNC/C	0.480 VRHE	-	0.1 M NH3 + 1.0 M KOH	at 0.600 VRHE for 1800 s	[19]
Pt cubesH-S	0.450 VRHE	5.240 at 0.700 VRHE	0.5 M NH4OH + 0.5 M KOH	at −0.220 VHg/HgO for 1000 s	[20]
er-GOx/Pt hydrid materials	−0.300 VAg/AgCl	0.255 at −0.075 VAg/AgCl	1.0 M NH4OH	-	[21]
PtIr/C	0.470 VRHE	-	1.0 M NH3	-	[22]
Pt90Ru10/C	-	0.920 at −0.210 VHg/HgO	1.0 M NH4OH + 1.0 M KOH	at −0.250 VHgHgO for 2.0 h	[23]
Pt0.2Ir0.8	−0.600 VAgAgCl	0.470 at −0.350 VAg/AgCl	1.0 M NH3 + 1.0 M KOH	−0.500 VAgAgCl for 900 s	[24]
Pt6Ru-NCs	0.500 VRHE	-	1.0 M NH3 + 1.0 M KOH	at −0.670 VRHE for 7000 s	[25]
Pt-NiCo-LDH-Ni foam-3	−0.573 VHg/HgO	17.75 at −0.197 VHg/HgO	0.1 M NH4Cl + 1.0 M KOH	at −0.197 VHg/HgO for 5000 s	This work

shows the ammonia oxidation reaction test condition and the activity of reported catalysts, indicating that the introduction of NiCo-LDH nanoflowers enhanced the dispersibility of Pt and further enhanced the electrochemical performance of the Pt-NiCo-LDH-Ni foam-3 electrode.

3. Experimental

The Synthesis of Pt-NiCo-LDH-Ni Foam

Ni foam with a size of 1 cm × 2.5 cm was washed with acetone, HCl (1 M), and deionized water and dried before use. Next, 2.4 mM NiCl₂·6H₂O and 0.8 mM CoCl₂·6H₂O were used to prepare the mixed solution with the volume of 50 mL deionized water. A total of 0.3 g urea was added to the above solution and stirred for 1 h. The obtained suspension and the Ni foam slice were moved into Teflon-lined stainless autoclaves to be further heated at 120 °C for 12 h. NiCo-LDH could grow on the surface of Ni foam during the above process. For the synthesis of Pt-NiCo-LDH, a three-electrode system was used with the NiCo-LDH-Ni foam as the working electrode, a Pt sheet as the counter electrode, and the Saturated Calomel Electrode (SCE) as the reference electrode in a mixed solution (30 mL) including 0.25 M H₂SO₄ and 1.25 mM H₂PtCl₆·6H₂O. The potential was from −0.3 V to 0.4 V, and the scan rate was 0.25 mV s⁻¹. The obtained samples were named Pt-NiCo-LDH-Ni foam-1, Pt-NiCo-LDH-Ni foam-2, Pt-NiCo-LDH-Ni foam-3, and Pt-NiCo-LDH-Ni foam-4 with 5, 10, 15, and 20 deposition cycles. In addition, when there were 15 deposition cycles, the Ni foam acted as the working electrode, and the obtained sample was named as Pt-Ni foam.

4. Conclusions

In this work, we successfully fabricated a platinum-decorated NiCo-layered double hydroxide on Ni foam via a hydrothermal reaction and an electrodeposition technique. Under optimal preparation conditions, Pt-NiCo-LDH-Ni foam showed the best electrocatalytic performance, with a low overpotential and a high oxidation peak current density. The great catalytic performances could be attributed to (1) the good electrocatalytic activity of the Pt- and NiCo-layered double hydroxide for the ammonia oxidation reaction, (2) the NiCo-layered double hydroxide nanoflowers providing large surface for depositing Pt, and (3) a good synergistic effect between the Pt- and NiCo-layered double hydroxide nanoflowers.