Electrochemical Acceleration of Ammonia Synthesis on Fe-Based

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Electrochemical Acceleration of Ammonia Synthesis on Fe-based AlkaliPromoted Electrocatalyst with Proton Conducting Solid Electrolyte Fumihiko Kosaka, Takehisa Nakamura, Akio Oikawa, and Junichiro Otomo ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b02469 • Publication Date (Web): 02 Oct 2017 Downloaded from http://pubs.acs.org on October 2, 2017

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ACS Sustainable Chemistry & Engineering

Electrochemical Acceleration of Ammonia Synthesis on Fe-based Alkali-Promoted Electrocatalyst with Proton Conducting Solid Electrolyte Fumihiko Kosaka, Takehisa Nakamura, Akio Oikawa and Junichiro Otomo＊＊

Corresponding Author: E-mail: [email protected]

Department of Environment Systems, Graduate School of Frontier Sciences, The University of Tokyo, 5-1-5 Kashiwanoha, Kashiwa, Chiba 277-8563, Japan

KEYWORDS ammonia synthesis, proton conductor, electrochemical promotion, energy storage, energy carrier

ACS Paragon Plus Environment

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ABSTRACT

To accelerate ammonia synthesis, the effect of the electrode potential on the kinetics of ammonia synthesis was investigated with a proton-conducting solid electrolyte, BaCe0.9Y0.1O3 (BCY), at temperatures between 500–650 °C. Ammonia synthesis was conducted using a double chamber electrochemical setup with an electrolyte-supported Pt|BCY|K,Al-modified Fe-BCY cell. Although slow ammonia formation kinetics by cathodic polarization was observed when pure N2 was supplied to the cathode side, obvious acceleration of the ammonia formation rate by cathodic polarization was observed following addition of 15% H2 to the cathode side. The ammonia formation rate increased more than 20 times at -1.5 V relative to that at the open circuit voltage, which was not observed by anodic polarization. Notably, the acceleration at cathodic potential was observed over 610 °C. These results indicate that the enhancement of ammonia formation occurs because of promotion of nitrogen dissociation by cathodic polarization and a change in the transport properties of the BCY electrolyte. The acceleration mechanism was discussed based on kinetic measurements and the dependence of the reaction kinetics on temperature and partial pressure.


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INTRODUCTION Ammonia has attracted much interest as an energy carrier for hydrogen source to establish a low carbon society. Ammonia has a high energy density and is easily liquefied at relatively low pressure. Therefore, the present infrastructure can be employed for transporting ammonia as a fuel. Traditionally, ammonia has been produced via the Haber-Bosch process with hydrogen formed from fossil fuels using Fe-based catalysts.1 However, using ammonia as an effective and efficient energy carrier requires a decrease in energy consumption and a reduction in the cost of ammonia production. In this regard, various ammonia formation methods have recently been reported. Therefore, although the Haber-Bosch process is a wellestablished process, many reports concerning novel ammonia production catalysts and novel ammonia production systems such as electrochemical processes have emerged.2–24 Normally, the rate-determining step of ammonia formation on heterogeneous catalysts is dissociation of the nitrogen-nitrogen triple bond. For this reason, accelerating this dissociation is crucial for efficient ammonia production. In the 1970s, Aika et al. reported that Cs-modified Ru-based catalysts have high activity for ammonia synthesis.25–27 They concluded that Cs causes back-donation of electrons to nitrogen molecules, resulting in a promotion of the nitrogen dissociation reaction (i.e., the rate-determining step). Recently, some groups reported high ammonia formation activity of novel catalysts including electride and hydride.20–24 Kitano et al. reported a large ammonia formation rate on a Ru-loaded 12CaO-7Al2O3 electride.20–22 They suggested that back-donation of electrons from the electride to the anti-bonding orbital of nitrogen accelerated the formation of ammonia. Wang et al. reported that LiH addition to transition metals such as Fe and Co accelerated the ammonia formation rate due to the strong reducing property of LiH.23,24 These results suggest that back-donation of electrons and the hydride ion acting as a reducing agent have an important role to accelerate the nitrogen dissociation reaction for efficient ammonia


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production. However, the reported materials require further improvement (in terms of stability and suitably active surface areas) before their practical use can be realized. So far, several research groups and our group carried out electrochemical ammonia synthesis using proton-conducting solid electrolytes such as BaCe0.9Y0.1O3 (BCY). 2,7,13,14,16,28–34

However, the ammonia formation rate was still low for practical applications.

Employing Ru as an electrode catalyst may be one of the solutions to improve electrochemical ammonia formation rate. Ru is well known as a good catalyst for the ammonia synthesis at relatively low temperatures because Ru shows high activity for the dissociation of N-N triple bond which is the rate-determining step for the catalytic ammonia synthesis.6 However, the electrochemical ammonia formation using Ru-based cathode was low,14,34 which is because hydrogen poisoning easily occurs for Ru-based catalyst and especially for a case of proton-conducting electrochemical ammonia production. Therefore, we focused on Fe in this study because of low binding energy with hydrogen and high binding energy with nitrogen.35 In this study, we focused on the electrochemical promotion of ammonia synthesis. Enhancement of the catalytic reaction by applying an electrode potential has been called electrochemical promotion of catalysis (EPOC) or non-faradaic electrochemical modification of catalytic activity (NEMCA).36–41 For both EPOC and NEMCA, change in the catalytic rate, ∆r (= r-r0, where r and r0 are reaction rate with and without applied voltage, respectively), is positive and the reaction rate enhancement, ρ (= r/r0), is over unity. The Faradaic efficiency, Λ (= ∆r/(i/nF), where i, n and F are current, electron transfer number, and Faraday constant, respectively), is over 100% for NEMCA as the term “non-faradaic” indicates, which means that the amount of reaction products is obtained more than the total input current. The enhancement has mainly been reported for the oxidation of some fuels such as CH4 and C2H4 using an oxide ionic conducting electrolyte, YSZ. In addition,


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enhancement of the ammonia formation rate using a proton conducting solid electrolyte has also been reported by Yiokari et al.,42 Ouzounidou et al.,13 and recently, Vasileiou et al.28,30,33 Yiokari et al. reported electrochemical promotion of an ammonia synthesis on an industrial iron catalyst at high pressure (50 atm).42 In the previous study, the maximum reaction rate enhancement, ρ, of 14 was observed. On the other hand, Ouzounidou et al.13 and Vasileiou et al.28,30,33 reported the electrochemical promotion of an ammonia synthesis at atmospheric pressure. They used an industrial Fe catalyst or a Ni-BaZr0.7Ce0.2Y0.1O2.9 (BZCY) cermet as cathodes and observed an enhancement of the ammonia formation rate with cathodic polarization. However, the maximum enhancement of the ammonia formation rate at atmospheric pressure by applying cathodic potential was small (2-3 times acceleration). In this study, we controlled the microstructure of a Fe-BCY cathode by the infiltration method and used potassium as a promoter of electron back-donation to the nitrogen molecules during application of cathodic electrode potential. In the previous studies,13,42 Fe industrial catalyst was loaded on proton-conducting solid electrolyte disks by pasting the catalyst. Therefore, contact area (i.e., interfacial area) between the catalyst and the electrolyte was small. On the other hand, Fe was loaded in a porous BCY electrode by the infiltration method in this study. Appropriate contact at the interface between Fe and BCY was obtained. Therefore, higher reaction rate enhancement, ρ, and higher ammonia formation rate per Fe loading amount is expected in this study. In this study, we found that the ammonia formation rate with a K, Al-modified Fe-BCY cathode increased more than 20 times by applying the cathodic electrode potential, which is higher than those reported previously.13,28,42,30,33 The acceleration mechanism was discussed based on the kinetic measurements and the dependence of the reaction kinetics on temperature and the partial pressures of hydrogen and nitrogen.


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EXPERIMENTAL SECTION Material synthesis, cell fabrication and characterization BaCe0.9Y0.1O3 (BCY) as a proton conducting solid electrolyte was synthesized by the coprecipitation method. A 0.1 M cation solution of Ba(NO3)2, Ce(NO3)3 (Kanto Chemical Co., Inc., Japan) and Y(NO3)3 (Junsei Chemical Co., Ltd., Japan) was added to the 0.1M (NH4)2(COO)2 (Kanto Chemical Co. Inc., Japan). The precipitate was filtrated and the obtained powder was precalcined at 800 °C, and finally calcined at 1200 °C in air. The K, Almodified Fe-BCY cathode was synthesized by the infiltration method. Nitrate solutions were infiltrated into a porous BCY electrode. The porous BCY electrode on a pelletized BCY electrolyte was prepared by the doctor-blade method. First, the pelletized BCY was prepared using a uniaxial press and cold isostatic press (CIP) and calcination at 1600°C. Second, the BCY powder that was dissolved in a slurry, which was a mixture of α-terpineol (solvent), ethyl cellulose (binder), sorbitan sesquioleate (dispersant), dibutyl phthalate (plasticizer) and poly methyl methacrylate resin (pore-formation), was pasted onto the sintered BCY electrolyte and calcined at 1300 °C. Counter and reference electrodes were attached using a Pt paste (Tanaka Kikinzoku Kogyo K. K., Japan) and calcined in air at 900 °C, as shown in Figure 1a. Finally, K, Al, and Fe were infiltrated into the porous BCY cathode. The weight ratios of K, Al, and Fe in the cathode were 2 wt%, 2 wt%, and 10 wt%, respectively. The desired amounts of Fe(NO3)3 and Al(NO3)3 (Wako Pure Chemical Industries, Ltd., Japan) were dissolved in distilled water and dropped into the porous BCY cathode under vacuum. The obtained sample was dried at 90 °C and the nitrate mixture was dropped again until the desired amounts of Fe and Al were attained. The obtained sample was calcined at 700 °C in air. Then, KNO3 (Kanto Chemical Co., Inc., Japan) was infiltrated on the iron surface in the cathode with under vacuum. The sample was finally calcined at 700 °C in air. The area of the working and counter electrodes was 0.39 cm2. The BCY powder and the obtained cathode


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were characterized by X-ray diffraction (XRD, SmartLab, RIGAKU, Japan) and scanning electron microscopy (SEM, JSM-5600, JEOL, Japan).

Electrochemical ammonia synthesis The experimental setup for the electrochemical synthesis of ammonia is shown in Figure 1b. A single cell was set between quartz tubes in a tube furnace. Pyrex glass rings were then used to seal the tubes at 900 °C. After sealing, infiltrated iron was reduced in a 3%H2/Ar atmosphere at 900 °C. Then, the temperature was lowered and electrochemical measurements were conducted at 500-650 °C. A gas mixture of x% H2/100-x% N2 was fed to the working electrode (WE) and a gas mixture of 2%H2O/20%H2/78%Ar was fed to the counter electrode (CE). The electrochemical measurements were performed using an Autolab PGSTAT128N (Metrohm Autolab B.V., Netherlands). The ammonia formation rate was examined under steady-state conditions at several electrode potentials using HPLC (EXTREMA, Jasco, Japan) with an electrical conductivity detector (CD-200, Shodex, Japan). (a)

(b)

Electrolyte (BCY) WE: cathode (K, Al-modified Fe-BCY) CE: anode (Pt)

H2 N2 Ar

Au wire WE

9.5 mm

Glass seal

RE: Pt

17 mm

CE

RE

WE CE

Au mesh

RE

WE 10~50 µm Electrolyte (BCY) 0.7~0.9 mm

H2 H2O Ar

Figure 1. Schematic images of (a) single cell and (b) experimental setup for the electrochemical measurement and analysis of the reaction products for the ammonia synthesis.


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RESULTS AND DICUSSION Characterization of the Fe-BCY cathode Figure 2 shows SEM images of the K, Al-modified Fe-BCY cathode before the ammonia synthesis measurements. A secondary electron image (SEI) and backscattered electron image (BEI) of the same area are shown in Fig. 2(a) and Fig. 2(b), respectively. In the BEI image, the areas of BCY are brighter in color than those of iron. A comparison between the SEI and BEI images indicates that Fe dispersed and formed on the BCY surface inside the porous electrode. Figure 3 shows EDX-mapping of Fe, Ba and Ce in the cathode after the measurement. As seen, Ba and Ce are present in almost identical area, whereas Fe is present in areas different than that of Ba and Ce. The results also indicate that Fe was deposited on the BCY surface in the porous electrode. Existence of K in the cathode after calcination was also confirmed from the EDX spectrum. However, diffusion of iron into the BCY electrolyte was also observed after the ammonia synthesis measurement. The electrode thickness in this study was 20-40 µm as shown in Figure 2 and 3. Since the thinner catalyst layer is favored for electrochemical promotion, higher ammonia synthesis rate enhancement, ρ, will be obtained with a thinner electrode


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(a)

20 µm

(b)

Figure 2. SEM images of the K, Al-modified Fe-infiltrated BCY cathode after calcination in 3%H2 at 900 °C. (a) Secondary electron image (SEI) and (b) Backscattered electron image (BEI).

(a) SEI

(c) Ba

10 µm (b) Fe

(d) Ce

Figure 3. SEM images of the K, Al-modified Fe-infiltrated BCY cathode after ammonia synthesis. (a) SEI, (b)-(d) corresponding EDX mapping; (b) Fe, (c) Ba, and (d) Ce.


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Figure 4 shows XRD patterns of the K, Al-modified Fe-BCY cathode before and after calcination in a reducing atmosphere, (i.e., after calcination in air at 700 °C and in 3%H2 at 900℃, respectively). Peaks derived from the BCY perovskite lattice were mainly observed for both samples. Peaks of iron oxide were not observed in the sample before reduction, which likely occurred because of formation of an amorphous iron oxide phase due to the relatively low calcination temperature of 700 °C. For the cathode after reduction by a heat treatment at 900 °C in 3%H2, a metallic iron phase and impurity phases of CeO2 were observed. It should be noted that Klinsrisuk et al. reported a 25 wt% Fe-infiltrated BCY cathode and peaks of impurity phases were smaller than those of iron oxide.32

Figure 4. XRD patterns of the K, Al-modified Fe-infiltrated BCY cathode. (a) after calcination in air at 700 °C, (b) after calcination in 3%H2 at 900 °C.

Electrochemical promotion of Ammonia synthesis Electrochemical measurements were performed with a Pt|BCY|K, Al-modified Fe-BCY cell. Gas leakage was not observed, which was confirmed by open circuit voltage and cyclic voltammetry with cathodic and anodic polarizations. It should be noted that we observed that degradations of cell performances in terms of current density and ammonia formation rate


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occurred during the measurements for a week. However, the degradations during the measurements in the figures were adequately small to discuss the relevant data. Figure 5 shows the electrochemical ammonia formation rate with and without H2 supply to the cathode side at different potentials at 650 °C. In the cathodic polarization region, protons were pumped to the cathode side from the CE side (Eq. 1). Then, resultant ammonia formation from nitrogen, proton and electron (Eq. 2) and hydrogen formation by recombination of hydrogen atoms (Eq. 3) occur as follows: Anode: H2 → 2H+ + 2e-

(1)

Cathode: N2 + 6H+ + 6e- → 2NH3

(2)

2H+ + 2e- → H2

(3)

A very small ammonia formation rate (5.5×10-12–2.4×10-11 mol s-1 cm-2) was observed without H2 supply to the cathode side, as shown in Fig. 5(a); that is, electrochemical ammonia formation in the presence of pure nitrogen and using pumped protons (Eq. 2) was slow. In contrast, when hydrogen was added to the cathode (Fig. 5(b)), that is, when catalytic ammonia formation occurred at the open circuit voltage (OCV), the ammonia formation rate was clearly accelerated by the cathodic polarization. Although the ammonia formation rate at


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(a)

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OCV in pure N2

(b)

OCV in 85%N2 /15%H2

Figure 5. Effect of potentials on the ammonia formation rates with the K, Al-modified Feinfiltrated BCY cathode at 650 °C. (a) without H2 addition to the cathode side (working electrode: 100%N2, counter electrode: 2% H2O/20% H2/78% Ar) and (b) with H2 addition to the cathode side (working electrode: 15% H2/85% N2, counter electrode: 2% H2O/20% H2/78% Ar).

the OCV was low (2.8×10-11 mol s-1 cm-2), indicating that the pure catalytic activity of the cathode was low, the ammonia formation rate increased exponentially, and more than 20 times from OCV to -1.5V (6.7×10-10 mol s-1 cm-2 at -1.5 V), in the cathodic polarization region. The rate acceleration observed in this study is higher than those measured previously.13,28,30,33,42 Although the maximum ammonia formation rate per electrode area in this study (6.7×10-10 mol s-1 cm-2 at -1.5 V) was lower than those in refs. 13, 28, 30, and 33 (e.g., 2.7×10-9 mol s-1 cm-2 in ref. 13 and 4.1×10-9 mol s-1 cm-2 in ref. 28) and the value was


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converted to 260 µmol h-1 g-1 (equal to 6.7×10-10 mol s-1 cm-2), the ammonia formation rate per active catalyst weight in this study was much higher than those in the refs (e.g., 7.3×10-7 mol s-1 g-1 in this study and 7.3×10-8 mol s-1 g-1 in the ref. 13). The relatively high ammonia formation rate per Fe-loading amount may be caused by the difference in the microstructures of the cathodes. The network of the proton-conducting solid electrolyte was not fully constructed in the cathode in the previous studies, while a 3D network of BCY and Fe was formed in the cathode in this study. The relatively high ammonia formation rate per Feloading amount suggests that proton-conducting network and the appropriate contact of Fe and BCY can play important roles for the acceleration of the reaction rate with cathodic polarization.

H2 in cathode

i (mA cm-2) H2 by proton pumping

Figure 6 Hydrogen concentration and current density vs. potential with the K, Al-modified Fe-infiltrated BCY cathode at 650 °C with H2 addition to the cathode side (working electrode: 15% H2/85% N2, counter electrode: 2% H2O/20% H2/78% Ar).

Figure 6 shows hydrogen concentration, assuming 100% Faraday efficiency for hydrogen production (Eq. 3). The concentration of hydrogen by proton pumping from the anode side was less than 2%, and the total concentration of hydrogen in the cathode side was between 15-17%. Consequently, the increase of the ammonia formation rate cannot be explained by the sequential ammonia formation between hydrogen generated via proton pumping (Eq. 3) and nitrogen, because the amount of pumped hydrogen (less than 2%) was


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much smaller than that of hydrogen added directly to the cathode side (15%). In addition, an increase in the ammonia formation rate was not observed following anodic polarization. Overall, these results clearly indicate that the cathodic electrode potential has a profound effect on accelerating ammonia formation. Figure 7 shows the Faraday efficiency for ammonia formation, as defined by Eq. 4,

=

,

(4)

(a)

OCV in pure N2

(b)

OCV in 85%N2 /15%H2

Figure 7. Faraday efficiency for the ammonia formation and current densities with K, Almodified Fe-infiltrated BCY cathode at different potentials at 650 °C. (a) without H2 addition to the cathode side (working electrode: 100% N2, counter electrode: 2% H2O/20% H2/78% Ar). (b) with H2 addition to the cathode side (working electrode: 15% H2/85% N2, counter electrode: 2% H2O/20% H2/78% Ar).


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where , , i, n, and F are ammonia formation rate at each potential, ammonia formation rate at OCV, current density, electron transfer number, and Faraday constant, respectively. Although the Faraday efficiency remained low, the Faraday efficiency for ammonia formation improved following the addition of H2 to the cathode side; specifically, it was 2 orders of magnitude higher than that without H2 addition at -1.5 V. To investigate the acceleration mechanism of ammonia formation, the effect of the temperature on the acceleration was investigated. Figure 8 shows the ammonia formation rate versus potential at different temperatures between 500 and 650 °C. At the temperatures below 570 °C, the ammonia formation rate was low and acceleration of the rate was not observed. However, rapid acceleration of the ammonia formation rate under -1 V was observed over 610 °C. Figure 9 shows the corresponding Arrhenius plots of the ammonia formation rate at the potentials of -1, -1.5, and -2 V. At temperatures below 570 °C, no obvious relation between temperature and the ammonia formation rate was observed, which is because the rate below 570 °C was too small to be detected. Thus, the apparent activation energies were estimated from the temperatures between 570 and 650 °C to be approximately 200-300 kJ mol-1 for -2, -1.5, and -1 V. These values are much higher than those for typical catalytic ammonia formation with Fe and Ru-based catalysts (70-120 kJ mol-1).25,43,44 The ammonia formation rate clearly increased at temperatures above 610 °C. Around the temperature, transport properties of BCY change. The transport number of H+ decreases, whereas that of O2- increases around over 600 °C.45 Although oxygen and water were not supplied to the cathode side, a small amount of oxygen may exist on Fe catalyst as a contaminant, and water can also be formed from oxygen and hydrogen. Therefore, oxygen at the surface of Fe and poisoning by H2O should be considered. The oxygen species can be removed and the valence of surface iron can be reduced by cathodic polarization, resulting in the activation of the Fe-


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Figure 8. Effect of potentials on ammonia formation rates with the K, Al-modified Feinfiltrated BCY cathode at different temperatures between 500-650 °C. 15% H2 was added to the cathode side (working electrode: 15% H2/85% N2, counter electrode: 2% H2O/20% H2/78% Ar).

○ □

-2 V -1.5 V -1 V

Figure 9. Arrhenius plots of the ammonia formation rate with the K, Al-modified Feinfiltrated BCY cathode at temperatures between 500-650 °C. 15% H2 was added to the cathode side (working electrode: 15% H2/85% N2, counter electrode: 2% H2O/20% H2/78% Ar).

based cathode and the acceleration of the ammonia formation rate by cathodic polarization. That may be because the ammonia formation rate was accelerated only at the high temperatures over 600 °C and the apparent activation energy was higher than those in the previous studies. However, if only the removal of oxygen from the cathode side and the surface activation of iron by reduction with cathode polarization were the main reasons, the acceleration of the ammonia formation rate should be maintained at a low applied potential


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after high polarization. Nevertheless, the ammonia formation rate was decreased by low cathodic polarization. Therefore, not only the effect of BCY transport property, other acceleration mechanism should also be discussed. To examine the effect of K on the acceleration of the ammonia formation rate by cathodic polarization, we conducted an additional ammonia synthesis measurement on a Fe-BCY cathode without K addition as shown in Figure 10. Higher ammonia formation rate and acceleration of the rate with cathodic polarization were observed for K-added Fe-BCY cathode. This experimental result also suggests that activation of N2 molecule may occur with an increase in cathodic polarization due to the electron back-donation via added K. Valov et al. reported the electrochemical activation of N2 molecule on Ir/YSZ.46 They performed insitu XPS analysis at 450 °C with applied voltage and found that N2 molecule can be activated at the three phase boundary of Ir/YSZ under E = −1.25 V. This result clearly suggests that N2 molecule on the surface can be activated by cathodic polarization. In our study, we used a Kmodified Fe-based cathode, and the K addition may improve the electron back-donation because of low work function of K.

Figure 10 Effect of K on the ammonia formation rates with the Fe-infiltrated BCY cathode at 700 °C with H2 addition to the cathode side (working electrode: 10% H2/90% N2, counter electrode: 2% H2O/20% H2/78% Ar).


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Finally, ammonia formation measurements were conducted at different partial pressures of hydrogen (0.045–0.15 atm) and nitrogen (0.15–0.85 atm) in the cathode side. For these experiments, the potential was fixed at -1.5 V. Figure 11 shows the ammonia formation rate at -1.5 V with different hydrogen and nitrogen partial pressures. The nitrogen partial pressure was fixed at 0.85 atm and the hydrogen partial pressure was fixed at 0.15 atm for changes in the partial pressure of hydrogen and nitrogen, respectively (i.e., Ar was added to dilute hydrogen and nitrogen). Since the ammonia formation rate increased monotonously with an increase in the partial pressure of hydrogen and nitrogen, almost the same ammonia formation rate will be obtained at the stoichiometric ration of / = 3. The reaction

Figure 11. Ammonia formation rates vs. (a) hydrogen and (b) nitrogen partial pressures ( = 0.04-0.15 atm, = 0.15-0.85 atm). The measurement temperature was 650 °C and the potential was -1.5 V.


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orders for hydrogen, α, and nitrogen, β, defined in Eq. 5 were 0.61 ± 0.09 and 0.52 ± 0.23, respectively. =

(5)

Since hydrogen poisoning did not occur as found with Ru-based catalysts,47 higher ammonia formation rates will be achieved in a high-pressured reactor. A previous report indicated that the values of α and β were 1.41 and 0.96 for the industrial Fe-based ammonia synthesis catalyst.43 However, in this work, the concentration of N on the surface may be relatively high due to the acceleration of N2 dissociation, resulting in the reaction order of 0.52. As for the reaction order of H2, the value was very small compared with that of conventional Febased industrial catalysts. This result suggests that protons pumped from CE to WE (i.e., anode side to cathode side) may contribute to the formation of ammonia. Also, the effect of hydrogen pressure was weakened because protons pumped from CE to WE raise the concentration of H adatoms on Fe catalyst. In summary, these results may suggest that the cathodic polarization accelerates nitrogen dissociation and therefore improves the ammonia formation rate. In this study, the ammonia formation rate was accelerated more than 20 times by the cathodic electrode potential, which is higher than that reported previously.13,28,30,33,42 Usually, the rate-determining step of ammonia formation is nitrogen dissociation (Eq. 6). N2 + 2* → 2N*

(6)

In this regards, the results presented herein suggest that the cathodic electrode potential may accelerate nitrogen dissociation. The ammonia formation rate was clearly accelerated by cathodic polarization, while an increase in the ammonia formation rate was not observed following anodic polarization. The tendency corresponds to the “type b” electrophilic reaction in the previous report.48 To dissociate the nitrogen bonding, back-donation of electrons is effective, which agrees with the observation for chemical promotion and


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electrochemical promotion of ammonia synthesis. According to the cathodic polarization, electrons may be effectively donated to the nitrogen molecules due to potassium addition into the Fe-BCY cathode because potassium has a low work function. Back donation of electrons to the anti-bonding orbital of nitrogen molecule may occur easily, which would accelerate the ammonia formation rate. Furthermore, nitrogen dissociation may be accelerated by directly supplying protons to adsorbed nitrogen to form adsorbed N2H (Eq. 7), which decreases the activation energy required for nitrogen dissociation (Eq. 8).35 N2 + H+ + e- + * → N2H*

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N2H* + * → NH* + N*

(8)

The dependence of the acceleration on the reaction temperature suggests that the transport properties (i.e., protonic and oxide ionic conductivities) of the BCY electrolyte may also have an important role on the acceleration of ammonia formation rate. Although the acceleration was observed at relatively high temperatures above 610 °C in this study, that was because the ammonia formation rate without the acceleration (i.e., at OCV) was low due to the small loading amount of Fe (0.92 mg cm-2). In addition, optimizations of a cathode structure and alkali-promotion can contribute to further acceleration of the ammonia formation rate, resulting in reduction of temperature at which the acceleration occurs. Overall, these results can contribute to further improvements of ammonia synthesis and reactions controlled by electrode potentials.

CONCLUSIONS Ammonia synthesis was conducted in a double chamber electrochemical setup using an electrolyte-supported Pt|BCY|K, Al-modified Fe-BCY cell at 500–650 °C. Electrochemical synthesis of ammonia was performed with and without H2 supply to the cathode side. A small amount of ammonia formation was observed without H2 supply (5.5×10-12–2.4×10-11 mol s-1


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cm-2), whereas clear acceleration of the ammonia formation rate was observed with 15% H2 to the cathode side. The ammonia formation rate increased exponentially in the cathodic polarization region; specifically, it increased more than 20 times from OCV to -1.5 V (2.8×10-11 mol s-1 cm-2 at OCV; 6.7×10-10 mol s-1 cm-2 at -1.5 V), which is a higher acceleration than that reported previously. Notably, the increase in the rate was not observed in the anodic polarization. The increase of ammonia cannot be explained by the sequential ammonia formation involving nitrogen and hydrogen generated via proton pumping, since the this amount of hydrogen was much smaller than the hydrogen added directly to the cathode side. The result clearly indicates that the cathodic polarization accelerated ammonia formation. Kinetic measurements of the temperature and partial pressure dependence were also performed. The results suggest that cathodic polarization may accelerate the nitrogen dissociation reaction and that the transport properties such as protonic and oxide ionic conductivities of the BCY electrolyte may affect on the ammonia synthesis kinetics. Overall, these results suggest a method to produce ammonia efficiently and may likely contribute to further improvements of ammonia synthesis kinetics.

AUTHOR INFORMATION Corresponding Author Junichiro Otomo, Tel: +81-4-7136-4714, E-mail: [email protected] Author Contributions The paper was written through contributions of all authors. All authors have given approval to the final version of the paper. Funding Sources


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This work was supported by CREST, Japan Science and Technology Agency (JPMJCR1441). Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was supported by CREST, Japan Science and Technology Agency (JPMJCR1441). The authors also thank the Materials Design and Characterization Laboratory, Institute for Solid State Physics, The University of Tokyo for use of SEM and XRD facilities.

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SYNOPSIS

Cathodic electrode potential accelerated ammonia synthesis on a K and Al-modified Fe-BCY cathode and the acceleration mechanism was discussed based on kinetic analysis

TOC/Abstract Graphic


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Electrochemical Acceleration of Ammonia Synthesis on Fe-Based

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