Chien-I.
Li
,
Hiroki
Matsuo
and
Junichiro
Otomo
*
Department of Environment Systems, Graduate School of Frontier Sciences, The University of Tokyo, 5-1-5 Kashiwanoha, Kashiwa-shi, Chiba 277-8563, Japan. E-mail: otomo@k.u-tokyo.ac.jp
First published on 3rd November 2020
The electrochemical promotion of ammonia formation on Fe-based electrode catalysts is investigated using proton-conducting-electrolyte-supported cells of H2–Ar, Pt|BaCe0.9Y0.1O3 (BCY)| Fe-based catalysts, H2–N2 at temperatures between 550 °C and 600 °C, and ambient pressure. To clarify the reaction mechanism, the ammonia formation rate is examined using two cathodes: (I) a porous pure Fe electrode with a shorter triple phase boundary (TPB) length and (II) a cermet electrode consisting of Fe–BCY (or W–Fe–BCY) with a longer TPB length. Using the different electrode structures, we investigate the effects of cathodic polarization, hydrogen partial pressure, and electrode materials. The porous pure Fe electrode shows better performance than the Fe–BCY cermet electrode, which suggests that the ammonia formation is accelerated by the electrochemical promotion of catalysis (EPOC) effect on the Fe surface rather than the charge-transfer reaction at the TPB. The electrochemical promotion is governed by a dissociative mechanism, i.e., acceleration of direct N2 bond dissociation with cathodic polarization on the Fe surface, with a smaller contribution by a proton-assisted associative mechanism at the TPB. These findings indicate that the porous pure Fe electrode is more effective for ammonia formation than the (W–)Fe–BCY cermet electrode. Despite the relatively short TPB length, the porous pure Fe cathode achieves a very high ammonia formation rate of 1.4 × 10−8 mol cm−2 s−1 (450 μg h−1 mg−1) under appropriate conditions. This significant result suggests that the effective double layer spreads widely on the Fe electrode surface. Using the identified reaction mechanism, we discuss key processes for improving ammonia formation.
In the electrochemical synthesis of ammonia using a proton-conducting electrolyte membrane, water dissociates to form protons at the anode (eqn (1)), and then the protons pass through the electrolyte membrane toward the cathode and react with nitrogen and electrons to form ammonia (eqn (2)). The overall reaction of ammonia formation is described in eqn (3). Some previous studies replaced H2 for H2O in the anode to simplify the system and to investigate the N2 reduction in the cathode.5,6,8–11,15,16,21–23
Anode:
![]() | (1) |
Cathode:
N2 + 6H+ + 6e− → 2NH3 | (2) |
Overall:
![]() | (3) |
The reaction mechanisms for ammonia electrochemical synthesis can be divided into two mechanisms, as described in eqn (4)–(9). For ammonia formation, N2 and H2 molecules adsorb on the catalyst surface and dissociate to form 2N* and 2H*, and then the adsorbates of H* and N* react to form NH3.27 In general, the reaction of N2 dissociation is considered as the rate-determining step in ammonia formation.28–33 To promote the electrochemical reaction of ammonia formation, appropriate catalysts,34–37 and/or applying a voltage to accelerate the reaction of N2 dissociation11,15,24,36 were proposed. The reaction mechanism in N2 dissociation was the same as that in the Haber–Bosch process with the Fe catalyst, in which the NH3 formation reaction is followed by a dissociative mechanism (eqn. (4)–(7)). The rate-determining step is the dissociation of into 2N* on the catalyst surface (a two-phase boundary) (eqn (5)),28–33 and the dissociated N* then reacts with 3H* to form NH3. On the other hand, an alternative route of N2 association to form NH3 was also proposed.38,39 First principles calculations based on density functional theory (DFT) have proposed that the associative mechanism (eqn. (8) and (9)), in which the adsorbed
reacts with protons and electrons to promote N2 bond cleavage, with subsequent formation of NH3 at the triple phase boundary (TPB) between the electrolyte, electrode, and gas phase (eqn (8)), can occur with cathodic polarization even at ambient temperature.38,39
Dissociative mechanism:
![]() | (4) |
![]() | (5) |
H2 + 2* → 2H* | (6) |
N* + 3H* → NH3 + 4* | (7) |
Associative mechanism:
N2 + 3H+ + 3e− + * → NH3 + N* | (8) |
N* + 3H+ + 3e− → NH3 + * | (9) |
Many researchers have investigated the electrochemical reduction of N2 to NH3 at high temperature (>500 °C) using a variety of catalysts including metals such as Fe,11 Pd,5 Ag,17 Pt,17 and AgPd,6,8–10 as well as cermet electrodes such as Ni–BaCe0.2Zr0.7Y0.1O3,15,16,24 Ni–BaCe0.9Y0.1O3,23 Ru-doped BaCe0.9Y0.1O3,22 Ru-doped La0.3Sr0.6TiO3,22 and K–Al–Fe–BaCe0.9Y0.1O3.21 Both noble metal6–10 and ceramic catalysts result in similar ammonia formation rates of approximately 10−9 mol s−1 cm−2.15,16,24 The atmosphere in the cathode is another important factor affecting the ammonia formation rate. With the supply of pure N2 to the cathode, some previous studies have shown that the mechanism of the electrochemical reduction of N2 is initiated by pumped H+ and dissociated N* reacting with H+ to form NH3.7,16,19 In addition, Kosaka et al. reported that the rate-determining step of N2 dissociation can be accelerated by cathodic polarization using a Ru-based catalyst.22 The hydrogen coverage surface at high applied voltage, however, hindered N2 molecule adsorption and NH3 formation.11,13,18–20,22–24
On the other hand, the ammonia electrochemical synthesis has also been investigated for the supply of a gaseous mixture of H2–N2 to the cathode.11,15,16,21,24 Generally, the ammonia formation rate in H2–N2 is higher than that in pure N2 because H2 in the cathode acts as an additional source of ammonia formation.11,21 In that case, the ammonia formation rate can be enhanced by the electrochemical promotion of catalyst (EPOC) effect (i.e., non-faradaic process), which promotes the electron donation/backdonation reaction by an applied voltage.11
In our previous study, we also reported that the electrolyte-supported cell of H2O–H2–Ar, Pt|BaCe0.9Y0.1O3 (BCY)|Al–K–Fe–BCY, N2 exhibited a low electrochemical ammonia formation rate. Nevertheless, when a gaseous mixture of 15% H2–85% N2 was supplied to the cathode side, there was a significant increase in the ammonia formation rate from 2.8 × 10−11 to 6.7 × 10−10 mol s−1 cm−2, which was observed with cathodic polarization at 650 °C.21 However, for a gaseous mixture of H2–N2 in the cathode, it is unclear whether the electrochemical promotion is caused by a dissociative mechanism (i.e., non-faradaic process without charge–transfer reaction), which accelerates N2 dissociation on the Fe surface (eqn (5)), or a proton-assisted associative mechanism (i.e., faradaic process with charge-transfer reaction), which promotes the charge-transfer reaction of at the TPB (eqn (8)).
In this study, the ammonia formation performance with cathodic polarization was examined using the following configuration of single cells: 3% H2O–20% H2–77% Ar, Pt|BCY| Fe-based catalysts, H2–N2. To investigate the electrochemical promotion of ammonia formation via either the dissociative or associative mechanism, a porous pure Fe cathode with a relatively short TPB length (the relevant reactions are governed by a two-phase boundary, i.e., the Fe surface) and 10 wt% Fe–BCY and 0.5 wt% W–10 wt% Fe–BCY cermet cathodes with a relatively long TPB were used, as shown in Table 1. First, by comparing the 10 wt% Fe–BCY cermet cathode (type A) with the porous pure Fe cathode (type B), the nature of the electrochemical promotion mechanism of ammonia formation was investigated. Second, a modified Fe–BCY cermet electrode, i.e., the same electrode structure as type A but with the addition of W (type A′), was investigated. The exchange current density for H2 evolution, i0,H2, was low, and it could suppress the hydrogen evolution reaction and reduce the current density with cathodic polarization because of the higher adsorption energy of W–H formation relative to that of Fe–H.40,41 Therefore, the effect of a low current density, i.e., low flux of pumped protons from the anode, on the electrochemical promotion of ammonia formation rate was also evaluated by comparing 10 wt% Fe–BCY (type A) and 0.5 wt% W–10% Fe–BCY (type A′: x wt% W–y wt% Fe–BCY cathodes, where x and y represent the weight ratios of W and Fe, indicated by xW–yFe–BCY hereafter).
Cathode | Electrode structure and properties | |
---|---|---|
Type A | Fe–BCY | Cermet electrode with a relatively long TPB length and high i0,H2 |
Type A′ | W–Fe–BCY | Cermet electrode with a relatively long TPB length and low i0,H2 |
Type B | Fe | Porous Fe electrode with a relatively short TPB length and low i0,H2, active site: Fe surface |
![]() | ||
Fig. 1 XRD patterns for the as-prepared samples of 10Fe–BCY (type A), 0.5W–10F–BCY (type A′), and Fe (type B). ◊: Fe, Δ:Fe3O4. BCY reference: PDF#01-070-1429. |
Fig. 2 shows the cross-sectional scanning electron microscopy (SEM) images of the three cathode catalysts. The size of BCY particles was around 300 nm in pure BCY, 10Fe–BCY, and 0.5W–10Fe–BCY electrodes. Although it is difficult to distinguish each position of Fe particles in the 10Fe–BCY cathode from Fig. 2f, the deposition of Fe particles on BZY can be observed from the TEM images (see the next section). In the 0.5W–10Fe–BCY cathode, Fe particles tend to aggregate on the BCY surface. In the porous pure Fe cathode, the size of Fe particles was around 200–400 nm. The thicknesses of the BCY and Fe porous cathodes were approximately 10–15 μm (see Fig. S2 in the ESI†). Fig. 2i–k correspond to the SEM cross-sectional images of the cathodes after the electrochemical measurements. The particle aggregation of around 50, 130, and 150 nm was observed in 10Fe–BCY, 0.5W–10Fe–BCY, and porous pure Fe cathodes, respectively.
For further observation of the cathode structures of 10Fe–BCY and 0.5W–10Fe–BCY, transmission electron microscopy (TEM) was used to examine the detailed particle structures, as shown in Fig. 3. The TEM image of the pure porous BCY cathode showed that the BCY particle size was around 300 nm. After Fe or W–Fe infiltrated into BCY, small particles located on the BCY surface were observed in 10Fe–BCY and 0.5W–10Fe–BCY cathodes. The energy-dispersive X-ray spectroscopy (EDX) mapping of 0.5W–10Fe–BCY showed that the Fe signal around the BCY surface was detected as well as the Ce signal (Fig. 4). Judging from the TEM-EDX, we think that the small particles located on the BCY surface in Fig. 3b are Fe particles. However, because the W amount is too low to detect and the EDX peaks of W (Mα and Mβ edges) and Y (Lα edge) are overlapping, we could not confirm the exact W position.
![]() | ||
Fig. 4 (a) TEM image of 0.5W–10Fe–BCY (type A′) and EDX mapping for the elements (b) Fe, (c) Ce, and (d) W. |
2H+ + 2e− → H2 | (10) |
![]() | (11) |
With W addition to 10Fe–BCY, the type A′ cell 0.5W–10Fe–BCY cathode exhibited a higher ammonia formation rate of 5.7 × 10−10 mol s−1 cm−2 at −1.2 V (137 μg h−1 mg−1 Fe) even at a lower operating temperature (550 °C) than that of the 10Fe–BCY cathode. In addition, current density for the type A′ (0.5W–10Fe–BCY) cathode, i.e., proton flux from the counter electrode (anode) to the working electrode (cathode), was reduced by approximately 40% in comparison with that of the type A (10Fe–BCY) cathode.
As for the exchange current density, the exchange current densities at 600 °C for type A (10Fe–BCY) and type A′ (0.5W–10Fe–BCY) were 0.037 and 0.014 A cm−2, respectively. The reason for the low exchange current densities for 0.5W–10Fe–BCY was due to the suppression of the hydrogen evolution reaction. Although the highest voltage of −1.5 V was applied for both the cathodes at 600 °C, the ammonia formation rates for both the cathodes were mostly the same, whereas 0.5W–10Fe–BCY had a lower current density than that of 10Fe–BCY. Therefore, the results suggest that the influence of applied voltage on ammonia formation is more significant than that of current density.
When using the type B cell with a porous pure Fe cathode, the ammonia formation rate increased with increasing cathodic polarization and reached 1.3 × 10−9 mol s−1 cm−2 (44.33 μg h−1 mg−1 Fe) at 550 °C, which was the best performance of these three cathodes. Because the type B porous pure Fe possessed a shorter TPB length than the type A 10Fe–BCY, a low current density of approximately 0.03 A cm−2 at around −1.2 V was observed. As for the exchange current density, the exchange current density at 600 °C for type B was 0.017 A cm−2, which was also lower than that for type A due to the shorter TPB length in type B.
The current efficiency ηCE (eqn (12)) and the fraction of obtained NH3 concentration to NH3 concentration at equilibrium XEqu (eqn (13)), using the 10Fe–BCY, 0.5W–10Fe–BCY, and porous pure Fe cathodes, were examined, as discussed in Section 3 in the ESI (Fig. S3†).
![]() | (12) |
![]() | (13) |
![]() | (14) |
The electrochemical ammonia synthesis involved the electrochemical synthesis of ammonia and hydrogen evolution reaction in parallel. According to Fig. S3,† the current efficiency for NH3 formation was below 2%, which implied that the hydrogen evolution reaction was more favourable (eqn (10)) in the electrochemical reaction of ammonia synthesis.
Obtained XEqu about 50% (equilibrant to 36.5 ppm NH3) in type B of porous pure Fe was higher than that of 26% in type A or type A′. The best performance for ammonia formation rate was achieved using the type B porous pure Fe cathode, which had a relatively shorter TPB length than that of type A or type A′ cermet electrodes.
![]() | ||
Fig. 6 (a) Ammonia formation rates and (b) current densities obtained using the 10Fe–BCY cathode (type A) at 600 °C at different hydrogen partial pressures (pumped protons from the anode). |
Table S1† shows the observed ammonia partial pressure and theoretical ammonia partial pressures in the cathode at different ratios of H2 to N2. XEqu reached around 40% in 10% H2–90% N2, whereas it decreased to around 30% in 50% H2–50% N2 because the NH3 partial pressure in 50% H2–50% N2 was higher than that in 10% H2–90% N2.
The ammonia formation rate increased by about 220 times to 1.4 × 10−8 mol s−1 cm−2 at around −1.2 V compared with that at the rest potential. This result also confirms the conclusion that the ammonia formation rate has a strongly positive correlation with the H2 partial pressure in the cathode. To the best of our knowledge, this ammonia formation rate of 1.4 × 10−8 mol s−1 cm−2 was quite high compared with the reported values in other existing electrochemical ammonia syntheses under moderate or ambient pressure. The representative previous studies are shown in Fig. 8, and the details are shown in Table S2 in the ESI.†11,16,21,42,43 Furthermore, the ammonia formation rate normalized by weight reached 450 μg h−1 mg−1, which was much higher than those in other previous reports,11,21,24,42,43 because the weight of Fe catalyst of 0.7 mg in this study was much less than that in the previous reports. Notably, the ammonia formation rate of 450 μg mg−1 h−1 at 550 °C and 0.1 MPa is a similar level of performance to those of 250–976 μg mg−1 h−1 in the conventional Haber–Bosch process at 400 °C and 7–10 MPa with Fe-based catalysts.27 This result suggests that the Fe catalyst has significant performance in ammonia formation, and that Fe has the potential to be a cathode catalyst for ammonia electrochemical synthesis. To clarify the promotion of ammonia electrochemical synthesis using the Fe cathode catalyst, the reaction mechanism is discussed in the next section.
![]() | ||
Fig. 8 Ammonia formation rates, rNH3, were normalized by (a) area of electrode and (b) metal catalyst weight. The data in red and black were respectively obtained in H2–N2 and pure N2 atmospheres. |
On the basis of the results, the details of the ammonia formation mechanism are discussed. In our system, both the dissociative mechanism (i.e., non-faradaic process without charge–transfer reaction) and proton-assisted associative mechanism (i.e., faradaic process with charge–transfer reaction) were possible routes for ammonia formation, as described in Table 2.
In the Haber–Bosch process, ammonia formation with an Fe-based catalyst is governed by a dissociative mechanism. The dissociative mechanism for ammonia formation on an iron catalyst has been extensively discussed in previous experimental and theoretical research.45–50 For example, Ertl's group discussed the potential energy for the synthesis of ammonia over a potassium-promoted Fe catalyst based on the dissociative mechanism.50 In addition, density functional theory (DFT) calculations were used to discuss a similar reaction mechanism.45,47 The rate-determining step in the dissociative mechanism was considered to be dissociative chemisorption of N2 on the Fe catalyst.28–30 N2 dissociation proceeded by electron donation from the Fe surface to the N2 orbital, weakening the bonding of N
N and thus promoting N2 cleavage directly (eqn R2 in Table 2). In addition, a previous study showed that alkali metal (K) addition causes electron transfer from the alkali metal to the Fe catalyst, elevating the Fermi level of Fe and then promoting the electron-backdonation reaction into N2
and N2 dissociation.51
However, N2 dissociation cannot proceed easily at ambient temperature because of the insufficient energy to overcome the energy of N2 dissociation. Previous DFT studies have shown that the electrochemical reaction of ammonia formation is dominated by a proton-assisted associative mechanism rather than a dissociative mechanism.39 In the proton-assisted associative mechanism, protonation of adsorbed proceeds to form NH3 and N* without direct N2 cleavage.
Generally, in SOFCs, the relevant charge–transfer reaction proceeds at the TPB. If ammonia formation was governed by the proton-assisted associative mechanism, then the type A (10Fe–BCY) and type A′ (0.5W–10Fe–BCY) cermet electrodes with relatively long TPB lengths should exhibit higher ammonia formation rates than the type B electrode. However, this hypothesis contradicts conclusion 3 stated above. In addition, according to conclusion 4 on the strong correlation between the ammonia formation rate and hydrogen partial pressure in the cathode, the ammonia formation process appears to be governed by the dissociative mechanism (eqn (4)–(7)) rather than by the proton-assisted associative mechanism (eqn. (8) and (9)). In the final section, we discuss the mechanism of enhancement of the N2 dissociation process on the Fe surface in terms of electrochemical promotion of the catalyst surface reaction.
![]() | (15) |
To support this hypothesis of the effective double layer, we conducted electrochemical ammonia synthesis using an yttria-stabilized zirconia (YSZ) electrolyte-based cell, which is an oxide ion conductor. The details of the experiment are described in the ESI (Section 7†). Using the cell composed of 20% H2–80% Ar, Pt|YSZ|10Fe-YSZ, 10% H2–90% N2, we did not observe any electrochemical promotion of the ammonia formation rate (Fig. S6†). This suggests that the formation of the effective double layer with protons in the cathode plays an important role in the promotion of the NH3 formation reaction, as well as the cathodic polarization. Therefore, the structure of the effective double layer is very important and those of type A and type B are discussed in the last section.
To understand the proton diffusion length and the area of the effective double layer, we propose three assumptions. (1) Proton diffusion length on the Fe surface is adequately long. Thus, we can assume that the Fe particles of type A can be covered by protons because the proton can fully diffuse on the Fe particle surface of type A (the size of Fe particles on BCY in type A was around several tens of nanometers) and cover the Fe surface of type A to form an effective double layer (Fig. 9b). Therefore, the area of the effective double layer in type A, , is equal to the effective surface area of Fe particles, SA (eqn (16)), as shown in Fig. 9b.
![]() | (16) |
(2) In type B, the proton diffusion length, h, should be less than or equal to the thickness of the porous pure Fe cathode, H. The proton diffusion length could be represented as eqn (17):
h = H × τ | (17) |
![]() | (18) |
(3) Because the electrochemical reaction of ammonia formation in type A and type B was carried out using Fe-based catalysts and at the same H2 and N2 partial pressures, the reaction rate constants for type A and type B were the same. The relation between the ammonia formation rate and the area of the effective double layer can be simplified as eqn (19):
![]() | (19) |
Supposed that the average values of the sizes of Fe particles in type A and type B were 42 nm and 190 nm, respectively, the corresponding effective proton diffusion length h was around 1.03 μm (Fig. 9c), and the value of τ was about 0.08. Fig. S17† shows the details of the relationship among the effective proton diffusion length h, the size Fe particles in types A and B, and the porosity in type B. Therefore, the effective double layer can be formed on a part of the Fe electrode surface (1.03 μm distance from the BCY electrolyte). The details of the estimation procedure of the effective double layer and the relevant parameters are summarized in the ESI (Section 11†).
In conclusion, the present rough estimation of the effective surface area provides the following views: (I) protons diffusing from the electrolyte can migrate to form an effective double layer (proton diffusion length: submicron order), and the effective double layer in type B (pure Fe) spreads adequately on the Fe surface. (II) N2 dissociation will be enhanced on the effective double layer via electron backdonation with cathodic polarization; thus, the improvement in ammonia formation rate was observed in type B.
In our present study, the high ammonia formation rate in type B will be caused by the relatively large area of the effective double layer. Our findings will aid in designing new reactors for the electrochemical synthesis of ammonia. To further improve the ammonia formation rate, controlling the effective proton diffusion length in metal electrodes, designing relevant electrode structures, and reducing the operating temperature are particularly important. Design and optimization of the new reactor is our next challenge.
At 550 °C and 10% H2–90% N2 in the cathode, type A 10Fe–BCY, which had a relatively long TPB length, showed an ammonia formation rate of 4.2 × 10−10 mol cm−2 s−1, while type A′ 0.5W–10Fe–BCY reduced the current density by 40% in comparison with type A 10Fe–BCY, and it increased the ammonia formation rate to 5.7 × 10−10 mol cm−2 s−1. These results suggest that the reduction of current density and the increase in cathodic polarization can contribute to an improved ammonia formation rate and current efficiency. On the other hand, type B porous pure Fe, which had a relatively short TPB length, exhibited a higher ammonia formation rate of 1.3 × 10−9 mol cm−2 s−1 than those of types A and A′. The high ammonia formation rate was probably achieved by accelerating N2 dissociation on the Fe surface, i.e., dissociative mechanism, rather than that at the TPB via a proton-assisted associated mechanism with the charge-transfer reaction. Furthermore, with an increase in the H2 partial pressure to 50% in the cathode, a significantly high ammonia formation rate, 1.4 × 10−8 mol s−1 cm−2 (450 μg h−1 mg−1) at −1.2 V, was observed, which was one of the best performances in the world. The electrochemical promotion of ammonia formation rate, which is higher by ca. 220 times than that at the rest potential, will be caused by the EPOC effect rather than by the faradaic electrochemical process, i.e., the charge-transfer reaction at the TPB. The results suggest that the formation of an effective double layer on the Fe surface is very important for the ammonia formation process. Through the observations, the present work provides new strategies for designing efficient electrolysis cells for ammonia synthesis.
A porous pure Fe cathode (ca. 0.5 mg) on the BCY electrolyte was prepared by the doctor-blade method. Fe2O3 powder was mixed with a slurry of α-terpineol (solvent) (98% purity; Fujifilm Wako Pure Chemical, Co., Inc., Japan), ethyl cellulose (binder) (48.0–49.5% ethoxy content; Kanto Chemical, Co., Inc., Japan), Nonion OP-83 RAT sorbitan sesquioleate (dispersant) (NOF, Co., Japan), dibutyl phthalate (plasticizer) (99.5% purity; Kanto Chemical, Co., Inc., Japan), and poly(methyl methacrylate) resin (pore formation) (99.9% purity; Tokyo Chemical Industry, Co., Ltd., Japan). The mixed slurry was then pasted onto the BCY electrolyte and calcined at 900 °C in air to obtain a porous pure Fe cathode. A porous BCY (ca. 1 mg) electrode on the BCY electrolyte was fabricated for the porous pure Fe cathode by the same process by using BCY powder.
In this paper, x wt% W–y wt% Fe–BCY cathodes (x and y are the weight ratios of W and Fe) are represented by xW–yFe–BCY. 10Fe–BCY and 0.5W–10Fe–BCY cathodes were fabricated by the impregnation method. Ammonium metatungstate (99.99% purity; Sigma-Aldrich, USA) and Fe(NO3)3·9H2O (99.99% purity; Wako Chemical Co., Inc., Japan) were stoichiometrically dissolved in water. The mixture solution was poured onto the BCY porous cathode, and then annealed at 700 °C to obtain the W–Fe–BCY cathode. The above processes were carried out several times until the amount of Fe and W reached an appropriate weight ratio. The 10Fe–BCY cathode was fabricated using the same process as W–Fe–BCY except that only iron nitrate was used as a precursor and a lower annealing temperature of 500 °C was applied. The Pt counter electrode (CE) and Pt reference electrode (RE) for all pellets were attached on the opposite side of the BCY electrolyte by the doctor-blade method. Finally, the obtained samples were then annealed at 900 °C for 3 h in 3% H2, as shown in Fig. S18.† Fig. S19† shows a schematic image of the three types of cathode structures.
AC impedance spectroscopic measurements from 1 to 106 Hz and potentiostatic measurements were performed using an Autolab PGSTAT128N (Metrohm Autolab B.V., Netherlands). In the electrochemical measurements using the three-electrode method, the electrode potential, E, was defined as follows:
![]() | (20) |
Ammonia formed in the cell was captured by allowing the outlet gas of the cathode side to flow into 0.01 mM H2SO4 solution, which was prepared by mixing ultrapure water (100 ml) (Autopure WT 100 compatible with Milli-Q, Yamato, Scientific Co., Ltd., Japan) and 0.005 M H2SO4 solution (0.1 ml) (Kanto Chemical, Co., Inc., Japan), for 5 min, and then the solution was analyzed by high performance ion chromatography (HPLC) (Extrema, Jasco, Japan).
A hydrogen pumping test was also conducted using a single cell of 20% H2–80% Ar, Pt|BCY| porous pure Fe, Ar. The current efficiency achieved 80–85% because the energy loss was probably caused by the leakage current (Fig. S21†).
The blank test, ammonia deformation reaction test, and reversible test for ammonia electrochemical synthesis are discussed in the ESI (Sections 13–15†), respectively. The stability of porous pure Fe was examined in 10% H2–90% N2 in the cathode. The details are discussed in the ESI (Section 16†).
Footnote |
† Electronic supplementary information (ESI) available. See DOI: 10.1039/d0se01385d |
This journal is © The Royal Society of Chemistry 2021 |