Hydrogen generation from ammonia electrolysis on bifunctional platinum nanocubes electrocatalysts

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Hydrogen generation from ammonia electrolysis on bifunctional platinum nanocubes electrocatalysts Hui-Ying Sun , Guang-Rui Xu , Fu-Min Li , Qing-Ling Hong , Pu-Jun Jin , Pei Chen , Yu Chen PII: DOI: Reference:

S2095-4956(20)30057-7 https://doi.org/10.1016/j.jechem.2020.01.035 JECHEM 1088

To appear in:

Journal of Energy Chemistry

Received date: Revised date: Accepted date:

16 January 2020 21 January 2020 30 January 2020

Please cite this article as: Hui-Ying Sun , Guang-Rui Xu , Fu-Min Li , Qing-Ling Hong , Pu-Jun Jin , Pei Chen , Yu Chen , Hydrogen generation from ammonia electrolysis on bifunctional platinum nanocubes electrocatalysts, Journal of Energy Chemistry (2020), doi: https://doi.org/10.1016/j.jechem.2020.01.035

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Highlights 

Pt nanocubes (Pt-NCs) with 4.5 nm size are achieved by 2-methylimidazole assisted hydrothermal synthesis.



Pt-NCs display outstanding electroactivity for ammonia oxidation and hydrogen evolution reactions.



A symmetric Pt-NCs||Pt-NCs ammonia electrolyzer based on bifunctional Pt-NCs electrocatalyst is constructed.



The ammonia electrolyzer only requires 0.68 V electrolysis voltage for hydrogen generation.



The ammonia electrolyzer has excellent reversible switch capability for anodic/cathodic reactions.

Hydrogen generation from ammonia electrolysis on bifunctional platinum nanocubes electrocatalysts Hui-Ying Suna,1, Guang-Rui Xua,1, Fu-Min Lib, Qing-Ling Hongb, Pu-Jun Jina,*, Pei Chena, Yu Chena,* a Key Laboratory of Macromolecular Science of Shaanxi Province, Key Laboratory of Applied Surface and Colloid Chemistry (Ministry of Education), Shaanxi Key Laboratory for Advanced Energy Devices, School of Materials Science and Engineering, Shaanxi Normal University, Xi’an 710062, Shaanxi, China b School of Chemistry and Chemical Engineering, Shaanxi Normal University, Xi’an 710062, Shaanxi, China *

Corresponding authors

E-mail addresses: [email protected] (Y. Chen), [email protected] (P. Jin).

1

These authors contributed equally to this work.

Abstract The ammonia electrolysis is a highly efficient and energy-saving method for ultra-pure hydrogen generation, which highly relies on electrocatalytic performance of electrocatalysts. In this work, high-quality platinum (Pt) nanocubes (Pt-NCs) with 4.5 nm size are achieved by facile hydrothermal synthesis. The physical morphology and structure of Pt-NCs are exhaustively characterized, revealing that Pt-NCs with special {100} facets have excellent uniformity, good dispersity and high crystallinity. Meanwhile, the electrocatalytic performance of Pt-NCs for ammonia electrolysis are carefully investigated in alkaline solutions, which display outstanding electroactivity and stability for both ammonia electrooxidation reaction (AEOR) and hydrogen evolution reaction (HER) in KOH solution. Furthermore, a symmetric Pt-NCs||Pt-NCs ammonia electrolyzer based on bifunctional Pt-NCs electrocatalyst is constructed, which only requires 0.68 V electrolysis voltage for hydrogen generation. Additionally, the symmetric Pt-NCs||Pt-NCs ammonia electrolyzer has excellent reversible switch capability for AEOR at anode and HER at cathode, showing outstanding alternating operation ability for ammonia electrolysis. Keywords: Ammonia electrolysis; Water electrolysis; Ammonia oxidation reaction; Hydrogen evolution reaction; Platinum nanocubes

1. Introduction Anodic ammonia electrooxidation reaction [AEOR, 6OH−(aq) + 2NH3 (aq) → 6H2O + N2(g) + 6e−, φ0= −0.77 V] in alkaline media is an important electrochemical half-reaction for industrial wastewater treatment for ammonia removal, direct ammonia fuel cells and ammonia electrolysis [1–4]. Among them, ammonia electrolysis is attracting increased attention, which is considered as an environmentally friendly and efficient ultra-pure hydrogen (H2) generation method [5–9]. The ammonia electrolysis in alkaline media is constructed of AEOR at anode and hydrogen evolution reaction [HER, 6H2O+ 6e− →6OH− + 3H2(g), φ0= −0.83 V] at the cathode, which requires much less theoretical electrolytic voltage than water electrolysis (0.06 V vs. 1.223 V) [10]. Compared to electrochemical reforming of alcohol molecules to ultra-pure H2 generation, no COx release during ammonia electrolysis, which provides a COx-free ultra-pure H2 generation and avoids greenhouse effect ultimately [11]. Although various nanomaterials (such as Pt, Rh, Ru, Pd, Au, Ir, Cu, Ag, Ni, and Co) have been found to have certain activity for AEOR in alkaline media, Pt is still most effective electrocatalyst for AEOR in alkaline media to date [2,12]. Meanwhile, Pt is also the most active and durable electrocatalyst for HER in alkaline media [13–15]. Unfortunately, the high cost and scarcity of Pt metal hinder its wide applications in various electrocatalysis-related energy conversion devices. Thus, it is extremely important to improve the utilization and electrocatalytic activity of Pt metal. Since both AEOR and HER are typical structure-sensitive reactions, the control of morphology and/or surface crystalline orientation is an efficient strategy for electroactivity enhancement. Among various Pt nanostructures with different morphologies, such as nanochains [16], nanowires [17], tetrahedral [18], cuboctahedra [19], nanorods [20], nanodendrites [21], nanosheets [22], and nanocubes [23–25]. Pt nanocubes (Pt-NCs) with {100} facets are of especial interest for both AEOR and HER. Specifically, HER activity on Pt{100} facets is much higher than that on Pt{111} facets in alkaline media due to preferentially competitive H adsorption and optimal binding energy of Pt-Had [24]. Meanwhile, Pt-NCs have extreme chemical stability under electrochemical conditions, which can effectively enhance their electrocatalytic durability [23,26]. Additionally, AEOR on Pt{100} facets has lower onset oxidation potential over on Pt{111} and on Pt{110} facets [27]. As a result, Pt-NCs with {100}

facets are more electroactive than conventional Pt nanoparticles with polycrystalline property [28– 30], which originates from the kinetically facile dimerization of *N with *N/*NH species on 4-fold hollow sites on Pt{110} facets [27]. In this work, high-quality Pt-NCs with 4.5 nm size were achieved by facile hydrothermal synthesis, which simultaneously revealed the outstanding electroactivity for both AEOR and HER in alkaline media. To the best of our knowledge, a symmetric ammonia electrolyzer based on bifunctional Pt-NCs electrocatalyst was firstly explored in alkaline media. Only 0.68 V electrolysis voltage was required for H2 generation, which was much lower than that of water electrolysis (ca. 1.8 V). 2. Experimental 2.1. The synthesis of Pt-NCs Firstly, 7.6 mg of platinum(II) acetylacetonate, 15.9 mg of 2-methylimidazole, and 50 mg polyvinyl pyrrolidone (PVP) were added into 12 mL of a mixed solution containing 6 mL of dimethylformamide and 6 mL of diethylene glycol under continuous stirring for half an hour at room temperature. Then, 20 mg of ascorbic acid was added into the above solution under continuous stirring. Then, the solution was transferred to a 20 mL Teflon-lined stainless-steel autoclave and heated at 180 °C for 5 h. After reaction, the black samples were separated by centrifugation, completely washed by acetic acid solution for several times, and lastly dried at 60 °C for 8 h in a vacuum dryer. 2.2. Electrochemical measurements In a standard three-electrode system, cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), linear sweep voltammetry (LSV), and chronoamperometry were measured on CHI 660D electrochemical workstation at room temperature. A graphite rod serves as the counter electrode. A saturated calomel electrode (SCE) serves as the reference electrode. An electrocatalyst-modified glassy carbon serves as working electrode. The measured potentials were presented with reference to reversible hydrogen electrode (RHE) according to the following equation: ERHE=ESCE+0.0591pH+0.242. All polarization curves were measured in an N2-saturated 1 M KOH solution at 5 mV s−1 with a 95% iR drop correction. The catalyst ink-transfer method

was used to prepare the working electrode. The electrocatalyst ink was got ready by dispersing 10 mg of obtained electrocatalysts in 5.0 mL of deionized water. Then 4 μL of the electrocatalyst ink was carefully loaded onto the polished glassy carbon electrode surface and dried for half an hour at room temperature. Lastly, 4 μL of Nafion solution (0.05 wt%) was coated on the electrocatalyst-modified electrode surface and then dried at the same conditions. The metal loading density on working electrode was 0.11 mg cm−2. 2.3. Physical characterization Transmission

electron

microscopy

(TEM),

high-resolution

TEM

(HRTEM),

high-angle-annular-dark-field scanning transmission electron microscopy (HAADF-STEM), selected area electron diffraction (SAED), and energy-dispersive X-ray (EDX) maps were investigated the physical structure and crystal structure of samples by utilizing a TECNAI G2 F20 instrument. Scanning electron microscopy (SEM) and EDX analysis were captured on a SU-8020 instrument. X-ray diffraction (XRD) was utilized to analyze the crystal phases of the samples, which was performed on a DX-2700 powder X-ray diffractometer. The surface chemical composition and chemical state of sample were measured by X-ray photoelectron spectroscopy (XPS). 3. Results and discussion 3.1. Characterization of Pt-NCs Pt-NCs were easily achieved by facile hydrothermal synthesis, in which platinum(II) acetylacetonate, ascorbic acid, 2-methylimidazole, and a mixed solution of dimethylformamide and diethylene glycol were used as PtII precursor, reductant, facet-selective agent, and mixed-solvent, respectively (see Experimental for details). EDX and XRD were performed to investigate the physical structure and composition of products. EDX spectrum displays that Pt element is dominant species (Fig. S1). XRD pattern shows distinct diffraction peaks at 39.87°, 46.22°, 67.40°, and 81.45° (Fig. 1a), which perfectly match the standard diffraction data of face-centred cubic (fcc) Pt crystal (JCPDS no. 04-0802). Based on Scherrer formula, the mean diameter of Pt-NCs is estimated to be 3.8 nm. XPS spectrum shows that the binding energies of Pt4f7/2 and Pt4f5/2 locate at 71.25 and 74.65 eV, respectively (Fig. 1b), which is consistent with the

standard data of Pt crystal (71.20 and 74.53 eV). Pt 4f XPS signal was further deconvoluted into Pt0 and PtII species by fitting. Based on relative peak areas, the ratio of Pt0 species is estimated to be ca. 89.9%, revealing the successful reduction of platinum(II) acetylacetonate.

Fig. 1. (a) XRD pattern and (b) high resolution Pt 4f XPS spectrum of Pt-NCs. TEM measurement was carried out to visualize the morphology of products (Figs. 2a, b, and S2). The monodisperse Pt nanocrystals with cubic shape are dominant products (91.0% cubes, 7.0% cuboids, and 2% irregular morphology). The corresponding size histogram reveals that the average size is ca. 4.5 nm (Fig. 2c). HRTEM image further verifies the lattice fringes with a lattice spacing of 0.196 nm (Fig. 2d), corresponding to the Pt{100} facets. The lattice fringes of Pt{100} facets parallel the edges of nanocube, indicating that epitaxial growth at <100> direction. Additionally, the fast Fourier transform (FFT) pattern exhibits the spots with 4-fold rotational symmetry (Fig. 2e), confirming Pt-NCs Pt{100} facets, again.

(a)

(b)

20 nm 20 nm nm 20 20 nm

5 nm

(c) (d)

(e) 0.196 nm

2 nm

2 nm

Fig. 2. (a) TEM image, (b) magnified TEM image, (c) size distribution histogram, (d) HRTEM image, and (e) FFT pattern of Pt-NCs. 3.2. Formation mechanism of Pt-NCs To clarify the formation mechanism of Pt-NCs, further controlled experiments were performed. In the absence of PVP, the aggregated nanocrystals are obtained, which in turn suggests that PVP efficiently serves as the stabilizing agent (Fig. 3a). In the absence of 2-methylimidazole, irregular nanocrystals are obtained (Fig. 3b). The previous works have demonstrated that 2-methylimidazole ligands can interact with Pt2+ ions to generate methylimidazole-PtII complex [31,32] which decreases reduction rate of PtII precursor and results in the kinetically controlled growth. Additionally, N atom at 2-methylimidazole has strong affinity with Pt surface. Thus, 2-methylimidazole effectively acts as facet-selective agent due to the specific adsorption of N atom at Pt{100} facets, resulting in the formation of Pt-NCs. In the absence of ascorbic acid, Pt nanodendrites are obtained (Fig. 3c), which in turn indicates that ascorbic acid also plays an important role in the formation of Pt-NCs. After increasing concentration of ascorbic acid, Pt nanocuboids are obtained (Fig. 3d), indicating the reduction rate of platinum(II) acetylacetonate is also important for Pt-NCs formation. Taken altogether, homogeneous and stable PtII precursor, suitable facet-selective guide agent and low reduction rate

are pivotal factors for the formation of nanoscale cubes.

(a)

(b)

100 nm

2020 nmnm

(c)

(d)

20 nm

20 nm

Fig. 3. TEM images of obtained Pt nanocrystals in the absence of (a) PVP, (b) 2-methylimidazole and (c) ascorbic acid, respectively. (d) TEM images of obtained Pt nanocrystals after increasing concentration of ascorbic acid. 3.3. AEOR activity of Pt-NCs The electrochemical properties of Pt-NCs and commercial Pt nanoparticles from Johnson Matthey Corporation (Pt-NPs-JM) were evaluated firstly by CV technique in HClO4 solution using a three-electrode workstation (Fig. 4a). The peak in the range of 0.05 to 0.15 V for Pt-NCs and Pt-NPs-JM relates to H-adsorption at Pt{111} and/or Pt{100} facets whereas the peak in the range of 0.20 to 0.30 V originates from H-adsorption at Pt{100} facets [33,34]. Compared to commercial Pt-NPs-JM with polycrystalline property, Pt-NCs display the characteristic H-adsorption peak on Pt {100} facets at 0.25 V, verifying Pt-NCs are mainly enclosed by Pt {100} facets [30,35]. By accumulating H-adsorption charge [36–38], the electrochemically active surface areas (ECSAs) of Pt-NCs and Pt-NPs-JM are measured to be 26.23 m2 g−1 and 18.96 m2 g−1, respectively. TEM image shows that Pt-NPs-JM severely aggregate (Fig. S3), resulting in a small ECSA. The mass electroactivities of Pt-NCs and Pt-NPs-JM for AEOR were investigated by CV in 1 M KOH + 0.1 M NH4OH solution (Fig. 4b). The AEOR peak current of Pt-NCs is 135.25 mA mg–1, which is 1.95 times bigger than Pt-NPs-JM (69.23 mA mg−1). Meanwhile, AEOR peak

potential at Pt-NCs negatively shift ca. 33 mV in comparison to Pt-NPs-JM. The higher AEOR peak current and lower onset potential indicate that Pt-NCs exhibit higher AEOR electroactivity relative to Pt-NPs-JM. According to AEOR peak current and/or onset potential, Pt-NCs also outperforms previously reported Pt-based electrocatalysts under the same measurement conditions (Table 1), further confirming high AEOR electroactivity of Pt-NCs. Table 1. The peak potential and peak current of AEOR at various Pt-based electrocatalysts in alkaline solution. Catalysts

Electrolyte

CNH3·H2O

Scan rate

Peak potential

Peak current

(M)

(mV s–1)

of AEOR

of

Ref. (year)

(V vs. RHE)

(mA mg–1)

0.660

135.25

This work

AEOR

Pt-NCs

1 M KOH

0.1

5

Pt nanosheets

1 M KOH

0.1

10

70

2013 [39]

Pt-decorated

1 M KOH

0.1

10

0.690

75

2016 [40]

Pt/NiO

1 M KOH

0.2

50

0.710

21.8

2016 [41]

Pt-NiO/C

1 M KOH

0.1

50

0.773

86.9

2017 [42]

Pt/NG(1-1)

1 M KOH

0.1

10

0.680

55.6

2016 [43]

Pt

1 M KOH

0.1

10

0.698

100

2017 [44]

PtIr/C

1 M KOH

0.1

20

0.650

46

2018 [45]

Cu–Pt

1 M KOH

0.1

2

0.824

2.5

2019 [46]

0.718

flower-like Ni particles

nanoparticle/ITO

nanoparticles

To understand the AEOR mass activity enhancement, the intrinsic AEOR activities of Pt-NCs and Pt-NPs-JM were evaluated by CV, in which the current densities of AEOR at Pt-NCs and Pt-NPs-JM were normalized to their ECSA values, respectively (Fig. 4c). As observed, AEOR peak current density at Pt-NCs is still bigger than that at Pt-NPs-JM, revealing improved intrinsic activity. Compared to Pt{111} and Pt{110} facets, Pt{100} facets can restrain the generation of poisoning Nad intermediates more effectively, which is responsible for the intrinsic activity enhancement of Pt-NCs [28,35,47,48]. Thus, high ECSA and intrinsic activity of Pt-NCs contribute to their mass activity enhancement. The durability of Pt-NCs and Pt-NPs-JM for AEOR were assessed by chronoamperometry tests (Fig. 4d). Within 6000 s, Pt-NCs reveal a higher current and slower current attenuation compared to Pt-NPs-JM, suggesting better activity and

stability. Since the morphology of Pt-NCs remains constant after chronoamperometry tests, the notable durability enhancement can be ascribed to the less poisoning Nad intermediates at Pt {100} facets because of its low adsorption energy at Pt {100} facets [35].

(a)

(b) ∆E=33 mV

(c)

(d)

Fig. 4. (a) CV curves of Pt-NCs and Pt-NPs-JM in 1M HClO4 solution at 5 mV s−1. (b) Mass-Pt normalized and (c) ESCA-Pt normalized CV curves of Pt-NCs and Pt-NPs-JM in 1 M KOH solution containing 0.1 M NH4OH at 5 mV s−1. (d) Chronoamperometry curves of Pt-NCs and Pt-NPs-JM in 1 M KOH solution containing 0.1 M NH4OH at 0.6 V potential. 3.4. HER activity of Pt-NCs The electroactivity of Pt-NCs and Pt-NPs-JM for HER were investigated by LSV (Fig. 5a). The geometrical current density was calculated according to geometrical area of the working electrode. The potential of HER to achieve a 10 mA cm−2 current density was defined as η10-HER. The η10-HER values at Pt-NCs and Pt-NPs-JM are 45 and 65 mV, respectively. Such η10-HER value at Pt-NCs is comparable for most reported η10-HER values of Pt-based nanomaterials in alkaline media (Table 2). Besides η10-HER, Tafel slope is also used as an important parameter for evaluating HER activity,

which can effectively reflect the HER kinetics [49]. Tafel slope of HER at Pt-NCs is close to that at Pt-NPs-JM (40 mV dec−1 vs. 36 mV dec−1), revealing similar reaction kinetics (Fig. 5b). EIS at Pt-NCs and Pt-NPs-JM were carried out to analyze the charge transfer capability of HER (Fig. 5c). The charge transfer resistance of HER at Pt-NCs is measured to be 14.4 Ω, much smaller than that (Rct=30.0 Ω) at Pt-NPs-JM, suggesting improved charge-transfer rate. So far, all electrochemical results reveal that Pt-NCs have higher intrinsic activity for HER compared to Pt-NPs-JM, originating from preferential H adsorption and optimal binding energy of Pt-Had at Pt{100} [24]. Table 2. HER activity of various Pt-based electrocatalysts in alkaline media. Catalysts

Electrolyte

Sweep rate −1

η

(mV s )

(mV)

10- HER

Ref. (year)

Pt-NCs

1 M KOH

5

45

This work

Pt2Ni3 NWs-S/C

1 M KOH

10

50

2016 [50]

NiFe LDH-Pt-ht/CC

1 M KOH

5

101

2017 [14]

Pt/Ni@NGNTs

1 M KOH

10

50

2017 [51]

Ni3N/Pt nanosheets

1 M KOH

5

50

2017 [52]

CoMoP@C

1 M KOH

5

81

2017 [53]

Pd−Pt-S

1 M KOH

5

70

2017 [54]

PtCo/C

0.1 M KOH

100

50

2017 [55]

Pt–Ni branched nanocages

0.1 M KOH

10

105

2018 [56]

Ni3[Fe(CN)6]2/Pt

1 M KOH

2

165

2018 [57]

Pt1@Fe-N-C

1 M KOH

5

110

2018 [58]

PtNi–Ni NA/CC

0.1 M KOH

5

51

2018 [59]

Mo2C@NC@Pt

1 M KOH

5

47

2019 [60]

Pt85Mo15–S

0.1 M NaOH

10

210

2019 [61]

Additionally, the electrocatalytic stabilities of Pt-NCs and Pt-NPs-JM for HER were measured by chronoamperometry tests at 0.1 V vs. RHE (Fig. 5d). After 5 h, HER current density at Pt-NCs maintains stable whereas the HER current density at Pt-NPs-JM undergoes a palpable attenuation, indicating that Pt-NCs has better electrocatalytic stability for HER than Pt-NPs-JM. Indeed, the HER polarization curve of Pt-NCs is very close to the initial curve after the chronoamperometry test (Fig. S4), also confirming excellent durability of Pt-NCs for HER. To further understand the improved stability for HER, the self-stabilities of Pt-NCs and Pt-NPs-JM were evaluated by accelerated durability test, which was completed by repeating CV at 50 mV s−1 in the range of 0.6 to 1.2 V. After 2000 scan cycles, Pt-NCs maintain 82.3% of the initial ECSA (Fig. S5a), whereas

Pt-NPs-JM maintains 62.5% of the initial ECSA (Fig. S5b). The high crystallinity degree and abundant Pt {100} facets with low energy can effectively inhibit the electrochemical dissolution/corrosion of Pt, which contributes to enhanced electrochemical self-stability of Pt-NCs[62].

(a)

(b)

(c)

(d)

Fig. 5. (a) HER polarization curves at Pt-NCs and Pt-NPs-JM in 1.0 M KOH solution at 5 mV s−1. (b) Tafel plots and (c) Nyquist plots at −0.05 V potential of Pt-NCs and Pt-NPs-JM. (d) Chronoamperometric curves of Pt-NCs and Pt-NPs-JM at −0.05 V potential. 3.5. Ammonia electrolysis in two-electrode system Since Pt-NCs have excellent electrocatalytic performance for both HER and AEOR, Pt-NCs can serve as a bifunctional electrocatalyst for ammonia electrolysis under alkaline condition in two-electrode system. For comparison, a two-electrode water electrolyze cell is constructed by using Pt-NPs-JM and commercial RuO2 nanoparticles (RuO2-NPs) as cathode and anode for the water splitting, respectively. Herein, RuO2-NPs is selected as anode electrocatalyst because Ru-based NPs generally reveal the high electroactivity for oxygen evolution reaction (OER). As observed, Pt-NPs-JM||RuO2-NPs water electrolyzer requires overall voltage of 1.65 V to acquire

geometrical current density of 5 mA cm−2, whereas Pt-NCs||Pt-NCs and Pt-NPs-JM||Pt-NPs-JM ammonia electrolyzers only require overall voltage of 0.68 and 0.72 V to require the same current density in the presence of 0.1 M NH4OH, respectively (Fig. 6a). Obviously, ammonia electrolysis can provide higher energy conversion efficiency for H2 production than conventional water electrolysis.

Meanwhile,

Pt-NCs||Pt-NCs

ammonia

electrolyzer

outperforms

Pt-NPs-JM||Pt-NPs-JM ammonia electrolyzer for ammonia electrolysis in alkaline media, which is ascribed to the excellent HER and AEOR activities of Pt-NCs. During ammonia electrolysis in alkaline media, no CO2 or O2 generate at electrode, which avoids the possibility of H2/O2 explosion and KOH-consumption by CO2 issues. Unfortunately, the maximum current on Pt-NCs||Pt NCs ammonia electrolyzer is much lower than that Pt-NPs-JM||RuO2 NPs water electrolyzer, which can be ascribed to lower reaction kinetics of AEOR relative to OER. Thus, elevating AEOR reaction kinetics may be a focus of ammonia electrolyzer in the future works. For the water and other small molecules electrolysis, the bifunctionality of electrocatalysts (i.e., identical electrocatalyst at cathode and anode) can endow symmetric electrolyzer with reversible switch capability for anodic and cathodic reactions, which can effectively extend the service life of electrolyzer [63–66]. The alternating ability of Pt-NCs||Pt-NCs ammonia electrolyzer was explored by switching periodically anode and cathode in the course of chronoamperometry measurement (Fig. 6b). After switching 10 times, the current density of Pt-NCs||Pt-NCs ammonia electrolyzer keeps constant, demonstrating an excellent alternating operation ability for ammonia electrolysis. Furthermore, TEM image shows that Pt-NCs still maintain their cubic morphology after switching 10 times (Fig. S6), confirming that Pt-NCs are highly stable, again.

(a)

(b)

Fig. 6. (a) Polarization curves of Pt-NPs-JM||RuO2-NPs water electrolyzer, Pt-NCs||Pt-NCs and

Pt-NPs-JM||Pt-NPs-JM ammonia electrolyzers in 1 M KOH solution at 5 mV s−1. (b) chronoamperometric curves of symmetric Pt-NCs||Pt-NCs ammonia electrolyzer in 1 M KOH solution containing 0.1 M NH4OH in periodically switching anode and cathode. Applied voltage: 0.8 V; Switching time interval: 1000 s. 4. Conclusions Herein, we successfully prepared Pt-NCs enclosed Pt{100} facets by simple hydrothermal method with the assistance of 2-methylimidazole molecule. The high crystallinity degree and dominant Pt{100} facets impart Pt-NCs with ultrahigh AEOR and HER activity and stability, which significantly outperform the electroactivity of Pt-NPs-JM for AEOR and HER. As a bifunctional electrocatalyst, Pt-NCs are interested in ammonia electrolysis in alkaline media in two-electrode system. The constructed Pt-NCs||Pt-NCs ammonia electrolyzer merely requires 0.68 V voltage for H2 generation, which is much lower than electrolysis voltage of Pt-NPs-JM||RuO2-NPs water electrolyzer for H2 generation. Obviously, the ammonia electrolysis is an energy-saving method than water electrolysis for H2 generation. Meanwhile, the symmetric Pt-NCs||Pt-NCs ammonia electrolyzer has outstanding alternating operation ability for AEOR and HER, which may effectively extend the service life of ammonia electrolyzer.

Acknowledgments This research was sponsored by the Fundamental Research Funds for the Central Universities (GK201901002 and GK201902014), and the National Natural Science Foundation of China (21875133 and 51873100), and the 111 Project (B14041).

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Graphical abstract The symmetric Pt-NCs||Pt-NCs ammonia electrolyzer based on bifunctional Pt-NCs electrocatalyst only requires 0.68 V electrolysis voltage for hydrogen generation, which also showing outstanding alternating operation ability for ammonia electrolysis. 0.68 V

H2 O

5 nm

H2

NH3

N2