Electrochemical Synthesis of NH3 at Low Temperature and

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Electrochemical Synthesis of NH3 at Low Temperature and Atmospheric Pressure Using a #-Fe2O3 Catalyst Jimin Kong, Ahyoun Lim, Chang Won Yoon, Jong Hyun Jang, Hyung Chul Ham, Jong Hee Han, Suk Woo Nam, Dokyoon Kim, Yung-Eun Sung, Jungkyu Choi, and Hyun S Park ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b02890 • Publication Date (Web): 16 Oct 2017 Downloaded from http://pubs.acs.org on October 22, 2017

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ACS Sustainable Chemistry & Engineering

Electrochemical Synthesis of NH3 at Low Temperature and Atmospheric Pressure Using a γ-Fe2O3 Catalyst Jimin Konga,b,1, Ahyoun Lima,c,1, Changwon Yoona, Jong Hyun Janga, Hyung Chul Hama, Jonghee Hana, Sukwoo Nama, Dokyoon Kimd, Yung-Eun Sung c,d***, Jungkyu Choi b,**, Hyun S. Parka,*

a Fuel Cell Research Center, Korea Institute of Science and Technology (KIST), Seoul 02792, Republic of Korea b Department of Chemical and Biological Engineering, Korea University, Seongbuk-gu, Seoul 02841, Republic of Korea c School of Chemical and Biological Engineering, Seoul National University, Seoul 08826, Republic of Korea d

1

Center for Nanoparticle Research, Institute for Basic Science (IBS), Seoul 08826, Republic of Korea

These authors equally contributed to this work.

* Corresponding author. Tel.: +82-2-958-5250; Fax: +82-2-958-5199; E-mail: [email protected] **Corresponding author. Tel.: +82-2-3290-4854; Fax: +82-2-3290-4854; E-mail: [email protected] ***Corresponding author. Tel.: +82-2-880-1889; fax: +82-2-888-1604; E-mail: [email protected]

1

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Abstract The electrochemical synthesis of NH3 by the nitrogen reduction reaction (NRR) at low temperature (< 65 °C) and atmospheric pressure using a nanosized γ-Fe2O3 electrocatalyst was demonstrated. The activity and selectivity of the catalyst was investigated both in 0.1 M KOH electrolyte and when incorporated into an anion-exchange membrane electrode assembly (MEA). In a half-reaction experiment conducted in KOH electrolyte, the γ-Fe2O3 electrode presented a faradaic efficiency of 1.9% and a weight-normalized activity of 12.5 nmol h-1 mg-1 at 0.0 VRHE. However, the selectivity towards N2 reduction decreased at more negative potentials owing to the competing proton reduction reaction. When the γ-Fe2O3 nanoparticles were coated onto porous carbon paper to form an electrode for an MEA, their weight-normalized activity for N2 reduction was found to increase dramatically to 55.9 nmol h-1 mg-1. However, the weight- and area-normalized N2 reduction activities of the γ-Fe2O3 decreased progressively from 35.9 to 14.8 nmol h-1 mg-1 and from 0.105 to 0.043 nmol h-1 cm-2act, respectively, during a 25 h MEA durability test. In summary, a study of the fundamental behavior and catalytic activity of γ-Fe2O3 nanoparticles in the electrochemical synthesis of NH3 under low temperature and pressure is presented.

Keywords: Electrochemical ammonia synthesis, Electrocatalyst, Iron oxide, Membrane electrode assembly 2


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Introduction NH3 is one of the most important and abundantly produced chemicals in the world, with its annual global production being approximately 150 million metric tons.1 It is mainly used as a raw material in the production of fertilizers, with nearly 80% of synthetic NH3 being consumed by the agricultural industry.2 Furthermore, NH3 has recently emerged as a promising hydrogen energy vector for use in hydrogen-based economies.3 It has a high hydrogen content (17.8 wt%), a high energy density (4.3 kWh h-1), and can be efficiently liquefied under atmospheric pressure at -33 °C, which is a significantly higher liquefaction temperature than that of hydrogen, i.e., -253 °C. Thus, the more convenient and economic storage and transportation of liquid NH3 as well as its high energy capacity make NH3 an attractive hydrogen energy carrier. More than 90% of synthetic NH3 is produced by the Haber-Bosch process, which is performed at high temperatures (400–500 °C) and pressures (150–200 atm).4 This heterogeneous chemical reaction, presented in Eq. 1, employs N2 and H2 as reactants and is performed over Fe or Ru catalysts, which afford typical conversion yields of approximately 20%.5 N2 + 3 H2 → 2 NH3

(1)

(∆Gf = -16.4 kJ mol-1, E0cell = 0.057 V) However, due to its harsh reaction conditions and the extensive gas separation processes required to prepare the reactants, the energy consumption of the Haber-Bosch process accounts for approximately 2% of worldwide energy use, i.e., 34 GJ tonNH3-1.3, 5 Besides this extensive energy consumption, the highly purified H2 gas required for the 3



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process is usually produced by natural gas steam reforming in which a significant amount of CO2 is produced, i.e., approximately 1% of the total global CO2 emission (or approximately 2 tonCO2 tonNH3-1).6 Consequently, the development of alternative lower-energy methods for the production of NH3, ideally at low temperature and pressure, is imperative.5, 6, 7 The electrochemical synthesis of NH3 using renewable-energy-driven electricity is regarded as an epoch-making environmentally friendly and sustainable approach to NH3 production.5, 6, 7, 8, 9, 10 By conducting the hydrogenation of N2 at a lower temperature and pressure with the help of controlled electrochemical potentials, the energy consumption and CO2 emissions associated with NH3 production can be significantly reduced, with a possible energy cost lower than 10 kWh kgNH3-1 at 100% coulombic efficiency.3 However, the efficient electrochemical synthesis of NH3 under mild conditions has not yet been achieved, mainly due to the slow kinetics of the nitrogen reduction reaction (NRR) and the lack of an active catalyst capable of dissociating the strong N2 triple bond.3, 11, 12, 13, 14 In order to increase reaction kinetics, electrochemical ammonia synthesis was also studied at higher temperature.15, 16 Various electrolytes, i.e. molten salt, composite membrane, ceramic and ceramic oxide, were employed to conduct proton or oxygen ion in the high temperature system, and the NH3 production rate was still low in the range of 10-9 - 10-13 mol cm-2 s-1.9 As a means of dissociating the strong N≡N bond, which has a dissociation energy (D0) of 942 kJ mol-1,17 the electrocatalytic NRR has been investigated using metal,11, 12, 13,17, 18, 19, 20

and metal oxide19, 21 electrocatalysts in aqueous,14, 19, 20, 22 non-aqueous,23 and polymer

electrolyte3, 11, 12, 13, 24, 25, 26, 27 systems. For example, in 2000, Kordali et al. studied the NRR in a three-electrode system with Ru electrode using water as the proton source.11 In their study, the reduction was performed in an aqueous electrolyte at temperatures ranging from 20 to 90 °C under a pressure of 1 atm, resulting in a maximum NH3 formation rate (rNH3) of 2.12 4


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× 10-11 mol s-1 cmgeo-2 at -0.96 VAg/AgCl and 90 °C. The maximum reaction selectivity in terms of faradaic efficiency (FE) for NH3 formation was 0.92% at 90 °C.11 Furthermore, Shi et al. also reported rNH3 and FE values of 3.5 × 10-10 mol-1 s-1 mg-1cat and 8.11%, respectively, using a Au/TiO2 catalyst in a 0.1 M aqueous KOH solution.17 For the NRR in aqueous solutions, the presence of protons is essential for the generation of NH3. However, the proton reduction reaction, or hydrogen evolution reaction (HER), compete with the NRR, reducing its efficiency. Consequently, NRRs in non-aqueous ethylenediamine electrolytes, where proton-containing chemicals such as water23 or reducing agents such as tetraisopropoxide,18 are added intentionally to allow nitrogen hydrogenation, have also been studied. MEA devices to which humidified N2 gas is supplied as a nitrogen source have also been used for the NRR. N2 reduction at the cathode and water oxidation at the anode produce NH3 and O2, respectively, using anion-3 or proton-exchange3, 12, 13 membrane electrolytes, as shown in the schematic in Fig. 1. For example, an electrolyzer based on a perfluorosulfonated proton exchange membrane (PEM) and a Pt cathode has been reported to afford an NH3 production rate of 1.2 × 10-9 mol s-1 cmgeo-2 at 1.6 Vcell,12 demonstrating that the NRR can be performed in the presence of O2 despite the oxygen reduction reaction competing favorably at the Pt cathode surface. Furthermore, Renner et al. reported that a high FE of 41% was achieved using an Fe catalyst and an anion-exchange membrane (AEM)-based MEA operated at 50 °C and atmospheric pressure.3 In addition to the substantial kinetic barrier to the NRR, the competing proton reduction reaction and HER at the cathode decrease the efficiency and selectivity of electrochemical NH3 synthesis. The standard electrochemical potential (E0) of the proton 5



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reduction reaction is close to that of the NRR (Eqs. 2 and 3), and many catalysts kinetically favor the HER over the NRR, as revealed by Montoya et al.7 However, the kinetics of the HER depend significantly on pH, and it has been reported that, in basic electrolyte solutions, the HER current can be more than 2 orders of magnitude lower than that in acidic solutions.7, 28

However, the pH dependency of NRR kinetics is not fully investigated yet. Herein, the

basic condition is firstly selected to suppress HER, which is the main competitive reaction of NRR, and the pH dependency of electrochemical NH3 synthesis should be further studied.3 2H+ + 2e- → H2

(2)

(E0 = 0 V vs. SHE) N2 + 6 H2O+ + 6 e- → 2 NH3 + 6 OH-

(3)

(E0 = 0.057 V vs. SHE) Recently, Fe2O3 has been utilized as an active catalyst material in molten-hydroxide water-electrolysis cells for NH3 synthesis.29, 30, 31 Licht et al. demonstrated that nanosized Fe2O3 suspended in a sodium hydroxide/potassium hydroxide solution catalyzes the reaction of N2 and H2 to form NH3 at 200 °C and 25 bar.29, 30 NH3 was chemically synthesized using Fe2O3 nanoparticles as the redox shuttle in the electrolyte, with the reaction of N2, H2O, and Fe to form NH3 and Fe2O3 being followed by the electrochemical reduction of Fe2O3 back to Fe at the nickel cathode (i.e., the electrochemical-catalytic mechanism).30 In addition, the direct electrochemical NRR mechanism at the Fe2O3 cathode, in which N2 is associatively adsorbed on the Fe2O3 (0001) surface and subsequently protonated to generate NH3, has recently been elucidated using first-principles density functional theory calculations.31 However, to the best of our knowledge, no experimental studies on the direct electrochemical synthesis of NH3 using Fe2O3 electrocatalysts have been reported. 6


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Herein, we report the direct electrochemical NRR at an γ-Fe2O3 cathode in both basic aqueous and gaseous conditions at low temperature (< 65 °C) and 1 atm. The NH3 production rate and coulombic efficiency towards the NRR half-reaction at the γ-Fe2O3 electrode was first studied in an aqueous solution at room temperature. Following this fundamental investigation of the NRR in a liquid environment, a porous carbon electrode coated with γFe2O3 nanoparticles was prepared for application to the electrochemical synthesis of NH3 in an AEM-based MEA at 65 °C and 1 atm (Fig. 1). In the device experiments, gaseous N2 and an alkaline aqueous solution were supplied to the membrane reactor as the reactants for NH3 synthesis. A gas-phase reactor was required to facilitate the mass transport of N2 to the catalyst surface, whereas the limited amount of N2 dissolved in aqueous solutions, i.e., approximately 0.7 mM in water under standard conditions, possibly imposes a theoretical maximum performance on the NRR under aqueous conditions. Finally, the production rates and reaction selectivities for the direct electrochemical NH3 synthesis over the γ-Fe2O3 electrode in liquid and membrane electrolytes were compared.

Experimental Half-Cell Electrochemistry γ-Fe2O3 nanoparticles (< 50 nm, Sigma Aldrich), IrO2 (99.99%, Alfa Aesar), and Pt/C (46.5 wt%, Tanaka) were purchased and used without further treatment. The catalyst ink was prepared by mixing 5 mg of catalyst powder and 5 µL of Nafion solution (5 wt% in lower aliphatic alcohols, Sigma Aldrich) in 500 µL isopropyl alcohol (99.5%, Honeywell International Inc.) For the preparation of the Pt/C electrode, 5 µL of the catalyst ink was pipetted onto a glass carbon (GC) disk (5 mm diameter, Pine Instrument Co.) and dried at room temperature. For the γ-Fe2O3 and IrO2 electrode preparations, the catalyst 7



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ink was sprayed onto a F-doped SnO2 (FTO) electrode (2.5 cm × 2.5 cm, surface resistivity ~7 Ω sq-1, Sigma Aldrich). The amount of catalyst to be coated onto the electrode was 3.5 mg cm-2. The FTO electrode was washed with ethanol and deionized water before use. Then, the electrode was thermally treated at 350 °C under air (Fe2O3) or Ar (IrO2) for 5 min after ramping the temperature from room temperature to 350 °C at a rate of 1 °C min-1. The electrochemical experiments were performed at 298 K in either N2- or Ar-saturated aqueous 0.1M HClO4 or 0.1M KOH electrolyte solutions using a Ag/AgCl electrode in saturated KCl and a graphite rod as the reference and a counter electrodes, respectively. However, all the electrode potentials reported were first converted to those vs. a reversible hydrogen electrode (RHE). Cyclic voltammetry (CV) and chronoamperometry (CA) measurements were performed using a potentiostat (Metrohm Autolab PGSTAT302N and GPES software). For the rotating disk electrode (RDE) measurements, cyclic voltammograms were recorded using a rotating GC electrode with a rotating speed of 1600 rpm controlled by an RDE system (Pine Instrument Co.) Membrane Electrode Electrochemistry The γ-Fe2O3 and IrO2 nanoparticles used in the half-cell measurements were incorporated into an MEA. Anion-exchange membranes (FAA-3, Fumatech) were used as ion-conducting membrane electrolytes. Catalyst inks were prepared by mixing a catalyst powder, a polytetrafluoroethylene solution (60 wt% dispersion in H2O, Sigma Aldrich), and isopropyl alcohol in deionized water (18 MΩ). This suspension was sprayed onto 2.5 cm × 2.5 cm squares of carbon paper (TGP-H-120, Toray) and titanium paper (250 µm thickness) for the cathode and anode, respectively, followed by ultrasonication for 1 h. The amounts of catalyst loaded were 1 and 2 mg cm-2 for the cathode and anode, respectively. The catalyst-coated substrates (CCSs) were thermally treated at 350 °C under air or Ar for Fe2O3 and IrO2, respectively, for 5 min, after ramping the temperature from room 8


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temperature to 350 °C at a rate of 1 °C min-1. The MEA was assembled by placing a cathode and anode CCS on opposite sides of an anion-exchange membrane electrolyte, and this MEA was secured between a graphite and a titanium current collector for the cathode and anode, respectively, to construct a single-cell device. The current collectors has a serpentine flow channel to facilitate N2 and water supply to the electrodes, and a probe wire was used to connect the electrodes to a potentiostat (SI 1287, Solartron). For NRR measurements, humidified N2 (relative humidity 100%) and a 0.5 M aqueous KOH solution were fed to the cathode and anode at flow rates of 200 sccm and 1 mL min-1, respectively. The temperature of the device and the reactants was maintained at 338 K. The cathodic gas flow was changed from humidified N2 to humidified Ar (at the same feed rate) when investigating the proton reduction activity at the cathode. CV was performed using applied voltages ranging from 1.1 to 1.8 Vcell at a scan rate of 20 mV s-1. CA was also conducted at different voltages for 1 h. Extended-duration CA measurements and device stability tests were conducted at 1.7 Vcell for 25 h. During the CA measurements and stability tests, the cathode outlet gas was purged with 100 mL of 10 mM H2SO4 solution to trap the synthesized NH3 for further analysis. In the extended-duration experiments, the trap solution was changed to a fresh one several times, and the concentration of NH3 in the spent trap solution was measured using UV-Vis spectroscopy, as described below. NH3 Detection The amounts of NH3 trapped in the acid solutions were determined by indophenol and Nessler methods. For the indophenol method, 1 mL of an aqueous 0.64 M C6H5OH, 0.38 M NaOH, and 1.3 mM C5FeN6Na2O solution was mixed with 1 mL of 55 mM NaOCl and 0.75 M NaOH prior to NH3 detection. Then, 1 mL of either a standard NH3 solution (a known quantity of NH4Cl in an aqueous 10 mM H2SO4 solution) or the NH3containing acid trap solution was added to the indophenol solution after dilution with aqueous 9



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0.1 M KOH. The dissolved NH4+ ions were quantified by assessing the absorbance at 633 nm using UV-Vis spectroscopy. The measurements were calibrated by subtracting the background absorbance measured at 875 nm. In the Nessler method, the acid-trapped-NH3 solution or the standard NH4OH solution was diluted in aqueous 90 mM K2HgI4 (Nessler reagent, Sigma Aldrich) and 0.1 M KOH before spectroscopic analysis. The absorbance measured at approximately 375 nm (corrected using the background at 700 nm) was used to determine the NH4+ concentration. For both the indophenol and Nesseler NH3 detection methods, standard calibration curves were obtained using known amounts of NH4Cl or NH4OH, respectively. Characterization Powder X-ray diffraction (PXRD) patterns of the catalysts were obtained using a MiniFlex 2 X-ray diffractometer (Rigaku) with CuKα radiation (λ = 0.154 nm) operated at 30 kV and 15 mA. The diffraction patterns were collected in the 2θ range 20–80° at a scan rate of 2° min-1. The microstructures of the catalysts were investigated using scanning electron microscopy (SEM, Nova) and transmission electron microscopy (TEM, FEI Talos F200X). X-ray photoelectron spectroscopy (XPS, Thermo Scientific K-Alpha) using a monochromatized Al Kα X-ray source was employed to investigate the catalyst surfaces. The surface areas of the catalysts were determined by Brunauer-Emmett-Teller (BET) measurements (ASAP2000 analyzer, Micromerigtics). Before BET measurements, the nanoparticles were dried at 70 °C for 2 h and then kept at 200 °C in air overnight. UV-Vis absorption spectra were obtained using a Cary UV-Visible 100 Spectrophotometer (Agilent).

Results and discussion Physical characterization The electrochemical activity of γ-Fe2O3 nanoparticles toward the 10


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NRR was first investigated in a 0.5 M KOH aqueous solution. The PXRD patterns of the catalyst nanoparticles agree well with those of standard γ-Fe2O3 (JCPDS #00-039-1346), and the peaks at 30.2, 35.6, and 62.9° are indexed to the (220), (311), and (440) lattices of γFe2O3, respectively (Fig. 2a). In the XPS analysis, Fe 2p3/2 and Fe 2p1/2 peaks are observed at electron binding energies of 710.9 and 724.5 eV, respectively, which are also consistent with γ-Fe2O3 (Fig. 2b).32 In the O 1s spectrum of γ-Fe2O3, the Fe-O and O-H peaks are positioned at 530.1 and 531.2 eV, respectively, as presented in Fig. 2c. The Fe 2p3/2 and Fe 2p1/2 satellite peaks are observed at 718.8 and 732 eV, respectively.33 It is well known that Fe 2p satellite peaks are not presented by Fe3O4.33 Thus, the PXRD and XPS results confirm that the catalyst has the crystal structure of γ-Fe2O3. The crystals shown in the TEM image have sizes distributed in the range ~40–70 nm (Fig. 2d). In order to assess the catalytic activity of γ-Fe2O3, the nanoparticles were layered on an FTO substrate using an ionomeric binder. The nanoparticles were uniformly sprayed on the conducting electrode, and micro- and macropores with diameters ranging from tens- to hundreds-of-nanometers are seen in the SEM image (Fig. 2e). The porous structure of the electrode is critical to its large surface area and facilitates electronic, ionic, and mass transportation through the catalyst layer. In this study, the BET surface area of particulate γFe2O3, i.e., 34.14 m2 g-1, was used to estimate the maximum active area of the porous electrode given the amount of catalyst coated onto the electrode as measured during the deposition. Half-Cell Electrochemistry The NRR activity of the γ-Fe2O3 electrode was first investigated in N2-saturaed aqueous solution (0.1 M KOH) using a conventional three-electrode configuration. The basic electrolyte was later assayed to detect the NH3 generated from the NRR. In Fig. 3a, the current-potential relationship of the γ-Fe2O3 is shown by the CV traces 11



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obtained in a N2- or Ar-saturated basic solution (pH 13). The reduction current of the γ-Fe2O3 is significantly lower than that of the Pt/C electrode, which is the best-known HER catalyst. The current densities measured at -0.3 VRHE are approximately -0.3 and -29 mA cm-2 for γFe2O3 and Pt/C, respectively, in a 0.1 M KOH solution (Figs. 3a and S1). These results indicate that the γ-Fe2O3 electrode is not particularly active for the proton reduction reaction. However, the CV trace of the γ-Fe2O3 obtained in the presence of dissolved N2 shows a slightly increased current in the potential range 0.1– -0.5 VRHE compared with that obtained using the Ar-saturated electrolyte solution. The largest current difference, ∆jN2-Ar, is 83 ± 5 µA cm-2 at around -0.3 VRHE (blue trace in Fig. 3a), and this difference is lower at more positive or negative potentials. The absolute value of ∆jN2-Ar and its error was affected by the different catalyst loading and active surface area of the electrode, and the average error of ∆jN2-Ar was approximately 6% at given electrodes. Similar CV characteristics were also reported by Shi et al. for a Au/TiO2 electrode in N2- and Ar-saturated solutions.14 In their study, the Au/TiO2 electrode showed an approximately 1 mA cm-2 larger current at -0.4 VRHE in a N2-saturated 0.1 M HCl solution than that in an Ar-saturated acidic solution.14 In the present study, the total reduction current using the N2-saturated electrolyte (jtotal,N2) is -0.37 mA cm-2 at -0.3 VRHE, and ∆jN2-Ar is approximately 25% of jtotal,N2 (or -94 µA cm-2) at -0.3 VRHE. The ratio of ∆jN2-Ar to jtotal,N2 indicates the possible FE towards the NRR to generate NH3 and probable other NRR products, such as N2H2, N2H4, and NH2OH. Nonetheless, the amount of NH3 produced at the γ-Fe2O3 electrode during constant potential electrolysis (as quantified by colorimetry) was significantly lower than that predicted from CV measurements. Furthermore, no N2H4 was produced at the γ-Fe2O3 electrode, as determined by colorimetric measurement (data not shown). Figure 3b shows the current at the γ-Fe2O3 electrode measured in constant potential 12


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electrolysis experiments at applied potentials ranging from 0.1 to -0.6 VRHE. The average reduction current during the 1 h measurement is lower than -0.50 mA cm-2 for potentials more positive than -0.3 VRHE, and it increases significantly to -3.8 mA cm-2 at -0.4 VRHE, mainly due to the proton reduction reaction (see below). For each electrolysis experiment conducted, a fresh electrolyte solution (0.1 M KOH) was prepared and assayed after the experiment by both Nessler and indophenol methods to quantify the amount of NH3 dissolved in the 0.1 M KOH solution, as described in the experimental section (Figs. 3c and 3d, Tables S1, S2, and S3). The reactor and electrodes were also cleaned with copious amounts of deionized water before the next electrolysis experiment to remove any NH3 adsorbed on the equipment. The FE values, NH3 production currents (jNH3), and NH3 production rates (rNH3) as determined by the indophenol and Nessler’s methods are summarized in Fig. 3e, Table 1, S1, S2, and S3, respectively. Fig. 3e displays the average F.E.s and rNH3 from repeated experiments using the data shown in Table S1 and S2. The results obtained from these two methods agree within 10%, and the lower values obtained from the indophenol method are reported in the main text. The electrochemical production of NH3 at the γ-Fe2O3 electrode is observed at all potentials more negative than 0.0 VRHE, whereas no NH3 is detected at 0.1 VRHE (Fig. S3). In order to determine whether the NH3 detected is produced by the electrochemical reduction of N2 at the γ-Fe2O3 electrode, NH3 detection was also performed under Ar at different potentials and no meaningful NH3 was detected without the N2 feed (Fig. S4). The N2 reduction reactions were also conducted at different inactive catalysts from which no NH3 was detected (data not shown here). The control experiments using Ar or inactive catalysts under N2 atmosphere indicate the produced NH3 is from NRR at the γ-Fe2O3 electrode. The maximum FE is 1.9% with an NH3 production rate of 43 nmol h-1 cm-2geo (or 0.036 nmol h-1 cm-2act) at 0.0 VRHE (Table 1, data taken from a sample with lowest activities.) The NH3 13



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production rate increases slightly to 58 nmol h-1 cm-2geo at -0.4 VRHE, but the FE decreases to 0.12% with the initiation of the HER at negative potentials. The FE obtained for the production of NH3 at the γ-Fe2O3 electrode is comparable to those reported using Fe, Ni, and Au catalysts. Recently, Shi et al. reported a NH3 formation FE of 8% using a Au/TiO2 electrode in a 0.1 M HCl aqueous solution,14 and Bao et al. obtained an FE of 4% at Au nanorods in a 0.1 M KOH aqueous solution.22 Fe and Ni were also reported to be active electrocatalysts for NH3 synthesis using membrane electrodes, with FEs of 3.1 and 2.4%, respectively.3 The mass activity of the γ-Fe2O3 in the present study is up to 0.21 µgNH3 h-1 mg1

cat,

which is lower than those of smaller Au/TiO2 nanospheres (approximately 20 µgNH3 h-1

mg-1cat) and tetrahexahedral Au nanorod catalysts (1.6 µgNH3 h-1 mg-1cat).14,

22

The mass

activity of nanocrystals is significantly affected by their surface area and size, and the γFe2O3 nanoparticles in this study have significantly larger particles sizes, i.e., 40–70 nm, with an average BET area of 34.14 m2 g-1, as compared to previously investigated Au/TiO2 nanospheres (4 nm) or Au nanorods (10 nm), possibly resulting in the reduced mass activity toward NH3 formation. In order to increase the catalytic efficiency of the γ-Fe2O3, the chemical utilization of the electrocatalyst should be increased by reducing the particle size and improving the material engineering. However, the activity of the γ-Fe2O3 normalized by the geometric area, 43.7 nmol h-1 cm-2geo, is three-times that reported for Fe catalysts, i.e., 14 nmol h-1 cm-2geo.3 The geometric activity is also greatly affected by the amount of catalyst loaded onto the electrode, and a comparison of the intrinsic activities of different catalysts using the current normalized by their actual catalytic areas is still needed (Table 1 and S3). The study of a Fe catalyst by Renner et al. was conducted in a practical AEM electrolyzer to demonstrate the electrochemical synthesis of NH3. The half-cell measurement results in this study imply that the γ-Fe2O3 is a promising catalyst for electrochemical NH3 synthesis with 14


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an activity comparable to or surpassing those previously reported. Accordingly, to further assess the NH3 production activity in a practical reactor design, electrochemical NH3 generation over γ-Fe2O3 was performed using a membrane electrode under a gaseous N2 flow, as discussed below. Membrane electrode electrochemistry Gaseous N2 reduction at the γ-Fe2O3 membrane electrode was conducted in a two-electrode system. In order to measure the NRR current under N2 gas flow, an MEA containing an anion-exchange membrane electrolyte was fabricated by using the γ-Fe2O3 and IrO2 catalysts layered on carbon paper and titanium paper substrates as the cathode and anode, respectively. Figure 4a show the current and cell voltage relationship obtained by CV under a humidified N2 gas flow to the cathode and a 0.5 M KOH solution feed to the anode with flow rates of 200 and 1 mL min-1, respectively, at 338 K. In the CV traces, the on-set potential of the reduction current appears at approximately 1.5 Vcell. In principle, the potential and current relationship detected in the two-electrode device can be explained by the half reactions measured separately for the anode and cathode in a threeelectrode configuration. Figure S5 shows the reduction and oxidation CV traces measured at the γ-Fe2O3 and IrO2 electrodes, respectively, in a 0.1 M KOH solution. As the on-set potentials in the half-reactions are approximately -0.4 and 1.5 VRHE for reduction at the γFe2O3 electrode and oxidation at the IrO2 electrode, respectively, the open circuit cell voltage (OCV) of the MEA is expected to be around 1.9 VCell. However, the actual OCV shown in Fig. 4a is approximately 1.5 V, implying that the cathode pH of the MEA surface, which is probably in the pH range 6–7, is more acidic than that in the 0.5 M KOH solution, reducing the OCV by approximately 400 mV. As discussed previously, pH significantly affects the reaction selectivity toward the NRR or HER, with facile HER kinetics manifesting under more acidic conditions. 15



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In Fig. 4b, the current flows at the MEA in constant voltage electrolysis at various device voltages are displayed. During the CA measurements, the current gradually increases for voltages higher than 1.5 V, which is the on-set voltage of large reduction currents in the CV traces. A current increase in the repeated CVs or CAs is observed both under N2 and Ar atmospheres, mainly due to facilitated reaction kinetics of water splitting. Increased membrane conductivity of the AEM in prolonged electrolysis was also observed by electrochemical impedance spectroscopy (data not shown). During the electrolysis, the outlet gas was passed through 100 mL of 10 mM H2SO4 to trap the NH3 contained in the gas flow. Aliquots of the trap solution were then analyzed by indophenol and Nesseler’s methods to obtain the NH3 concentrations in the solutions (Figs. 4c, 4d, and S6). The FEs and reaction rates for NH3 synthesis at the γ-Fe2O3 in MEA were calculated as shown in Fig. 4e, Table 2 and S4, where the colorimetric data indicate that NH3 is produced by the MEA electrolysis. NH3 was produced by the gaseous NRR at the γ-Fe2O3 nanoparticle membrane electrode at cell voltages larger than 1.5 Vcell (Fig. 4e). Both the reaction activity and selectivity for the NH3 generation abruptly increase from 0.003 to 0.044 % and from 0. 691 to 55.9 nmol h-1 cm-2geo, respectively, as the voltage increases from 1.5 to 1.6 Vcell. However, the maximum FE of the MEA, i.e., 0.044 %, is significantly lower than that measured in the basic electrolyte, i.e., 1.96 %. In other words, owing to the significantly larger (up to forty times) HER current at the porous membrane electrode than that at the disk electrode, the formation selectivity of NH3 becomes insignificant in the gaseous reactor. However, the estimated values of the mass- and area-normalized activities of the γ-Fe2O3 on the MEA, i.e., up to 55.9 nmol h-1 mg-1 and 0.164 nmol h-1 cm-2act, respectively, are approximately three-times larger than those measured at the disk electrode, i.e., up to 16.8 nmol h-1 mg-1 and 0.049 nmol h-1 cm-2act, respectively. These increased activities indicate the improved utilization of the 16


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nanoparticulate electrocatalysts for the NRR at the porous carbon paper electrode. Given that highly selective and active NRR catalysts are being actively developed, MEA reactors will eventually be required for the practical deployment of electrochemical devices for NH3 synthesis to facilitate the mass transport of N2 and to maximize the efficient usage of catalysts. Nonetheless, the HER kinetics are significantly increased in the MEA, resulting in the decreased FE for NH3 production. The enhanced HER is be probably due to the more acidic surface of the membrane electrode, i.e., probably in the pH range 6–7, compared with that of the disk electrode in the 0.1 M KOH aqueous solution, as illustrated in Fig. 4a. The surface pH of the membrane electrode needs to be controlled to enhance the FE of NH3 formation in the gaseous NRR device. Furthermore, it was found that the NH3 produced at the electrode is largely adsorbed on the membrane. After constant voltage electrolysis, the membrane was separated from the MEA and washed with deionized water to remove any NH3 adsorbed on or impregnated into the electrolyte membrane during the NH3 synthesis. The collected washing solution was then examined by the Nesseler’s method, and a significant amount of NH3 was detected (Fig. S7). All these results indicate that the NH3 formation activity of γ-Fe2O3 in the MEA was clearly underestimated considering the adsorbed NH3 on the reactor surfaces, e.g., the membranes, flow channels, and gas tubing. The trapping efficiency for NH3 is under investigation in order to more accurately calculate the NRR activity in MEA measurements. Finally, the stability and long-term activity of γ-Fe2O3 were investigated for NH3 production in the MEA (Fig. 5). During 25 h of constant voltage electrolysis at 1.7 Vcell, several aliquots of the trap solution were taken and examined by colorimetric assay (Figs. 5a and 5b). The concentration of NH3 in the trap solution increases during the electrolysis, as 17



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shown in Fig. 5c. However, FE, geometric- and actual area-normalized NH3 formation rates of the γ-Fe2O3 gradually decrease over 25 h from 10.6 x 10-3 to 1.4 x 10-3 %, from 35.9 to 14.8 nmol h-1 cm-2geo, and from 0.105 to 0.043 nmol h-1 cm-2act, respectively (Table 3 and S3). However, the HER activity of the γ-Fe2O3 increases during electrolysis, and the current measured at the MEA also increases from approximately 50 to 120 mA cm-2 throughout the measurements. The underlying mechanism of the catalytic deactivation for NH3 production is still unclear and under investigation. A possible reason for the degradation of γ-Fe2O3 would be the electrochemical reduction of metal oxide in the presence of the NH3 and H2 produced at negative potentials. Otherwise, the reduction of the catalytic activity might simply be caused by the reduced NH3 trapping efficiency during electrolysis, as discussed above. In order to clarify the reasons behind the reduction in the NH3 formation rate of the γ-Fe2O3 MEA, surface characterization of the γ-Fe2O3 after use is in progress, along with optimization of the experimental setup to increase the trapping efficiency.

Conclusions γ-Fe2O3 nanoparticles were proposed as an active catalyst for the direct electrochemical synthesis of NH3 at low temperature under atmospheric pressure. The activity and selectivity of the γ-Fe2O3 catalyst toward NH3 formation was investigated using a N2-saturated basic aqueous solution and in a membrane electrode reactor using an AEM. In the 0.1 M KOH electrolyte solution, the highest FE and NH3 formation rate were 1.96% and 58.9 nmol h-1 cm-2geo (or 0.049 nmol h-1 cm-2act) at 0.0 and -0.4 VRHE, respectively. The FE decreased significantly owing to the significant HER current flow at potentials more negative than -0.4 VRHE. The electrochemical NH3 production was further studied under gaseous N2 atmosphere 18


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using an AEM-based membrane electrode device. The mass- and area-normalized activities of γ-Fe2O3 on a membrane electrode were improved approximately threefold to 55.9 nmol h-1 mg-1 and 0.164 nmol h-1 cm-2act, respectively, from 16.8 nmol h-1 mg-1 and 0.049 nmol h-1 cm2

act

obtained at a disk electrode in a liquid electrolyte. The activity enhancement in the MEA

might be due to the increased catalytic utilization of the γ-Fe2O3 nanoparticles layered on the porous carbon paper of the MEA. However, the reaction selectivity, or FE, toward NH3 formation decreased significantly from 1.9% in the 0.1 M KOH aqueous solution to 0.044% in the MEA, probably due to the lower pH of the membrane electrode than that of the basic aqueous solution. The efficiency and catalytic activity of the γ-Fe2O3 catalyst in the MEA were significantly underestimated due to the NH3 adsorbed on the electrolyte membrane and reactor surfaces. As a result, the FE and activities of the γ-Fe2O3 for NH3 generation gradually decreased from 10.6 x 10-3 to 1.4 x 10-3%, from 35.9 to 14.8 nmol h-1 cm-2geo, and from 0.105 to 0.043 nmol h-1 cm-2act, respectively, during the 25 h MEA durability test. In order to realize practical and cost-effective electrochemical NH3 synthesis, investigation into the mechanism behind the degradation of the catalytic activity of γ-Fe2O3 toward the NRR is required. Many other factors, such as reaction temperature and acidity, should also be studied for the improved NRR. In the present work, the fundamental behavior and catalytic activity of γ-Fe2O3 nanoparticles were presented in the hope of guiding further development of practically efficient and robust electrocatalysts for electrochemical NH3 synthesis under low temperature and pressure.

ASSOCIATED CONTENT Supporting Information 19



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UV-Vis colorimetry, CV results, and additional Tables. The Supporting Information is available free of charge on the ACS Publications web site at http://pubs.acs.org.

ACKNOLWLEDGEMENT This research was supported by a grant from the National Research Foundation of Korea (2016M3D1A1021142) funded by the Ministry of Science, ICT & Future Planning of Korea, and the Korea Institute of Science and Technology (KIST) through the institutional project.

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Synopsis : Electrochemical synthesis of ammonia with γ-Fe2O3 nanoparticle catalysts was investigated under low temperature and pressure condition.

23



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Figure 1. (a) Schematic of the electrochemical synthesis of NH3 in an anion-exchange-membranebased electrolyzer. (b) Photographic image of the actual device.


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Figure 2. XRD patterns (a), XPS spectrum for Fe 2p (b) and O 1s (c), TEM (d), and SEM images (e) of the γ-Fe2O3 nanoparticles.



(a)

(b)

0.5

0 0.0 -0.5 -1.0

Ar N2

-1.5 -0.6

-0.4

(c)

0.0

0.2

6

4

-3

2

0 400

(e)

500

600

Wavelength (nm)

-1

-0.4

-10

-0.6

0

0 10 20 30 40 50 60

10

20

700

30

40

50

60

Time (min) 12 9 6 3 0 -3 -6 500

600

700

800

Wavelength (nm)

4

0.6

3 0.4 2 0.2

1

0.0

0 0.1

-0.2

-8

5

-2

rNH3 (µg h cm )

0.8

0.0

-6

(d)

-2

Absorbance(x10 , arb. unit)

-0.2

Potential (V vs. RHE)

-4

-12

Absorbance (x10 , arb. unit)

j (mA cmgeo

-2

-2

j (mA cmgeo )

)

-2

F.E. (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 26 of 32

0.0 -0.1 -0.2 -0.3 -0.4 -0.5 -0.6

Potential (V vs. RHE)

Figure 3. (a) Cyclic voltammograms for the γ-Fe2O3 cathode in N2-saturated (blue solid line) or Ar-saturated (yellow dashed line) 0.1 M KOH aqueous solution. Scan rate was 5 mV-1 s-1. (b) CA results for the Fe2O3 electrode measured in N2-saturated 0.1 M KOH aqueous solution at 298 K. Applied potentials were 0.1 (gray), 0.0 (red), -0.1 (blue), -0.2 (cyan), -0.3 (magenta), -0.4 (yellow), -0.5 (navy), and -0.6 VRHE (wine). Inset shows the enlarged CA traces at potentials more positive than -0.3 VRHE. UV-Vis spectra obtained using Nessler’s (c) and indophenol (d) methods to detect NH3 synthesized during CA at different potentials. The color legend displaying the applied potentials is identical to that in (b) with the addition of a black line for the standard solution in the absence of NH3. (e) NH3 formation reaction rate (left y-axis) and faradaic efficiency (right y-axis) determined by CAs.


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Figure 4. (a) Cyclic voltammograms from a membrane electrode device incorporating γ-Fe2O3 and IrO2 into the cathode and anode, respectively. H2O-saturated N2 and 0.5 M aqueous KOH were supplied to the cathode and anode, respectively, at flow rates of 200 sccm and 1 mL min-1, respectively. Inset shows the CA measurements at the beginning and end of repeated measurements. (b) CA results for the membrane electrode measured at cell voltages of 1.3 (red), 1.4 (blue), 1.5 (cyan), 1.6 (magenta), 1.7 (yellow), 1.8 (navy), and 1.9 Vcell (wine). The gas and liquid used was identical to that used in (a). UV-Vis spectra obtained using the Nessler (c) and indophenol (d) methods to detect NH3 synthesized during CA at different cell voltages. The black line represents a standard solution in the absence of NH3. The color legend for the applied potentials is identical with that in (b). (e) NH3 formation reaction rate (left y-axis) and faradaic efficiency (right y-axis) determined from the CA measurements of the device.



(b)

6

Absorbance (x10 , arb.unit)

4 2 0 -2 400

500

600

700

20

0 -5 500

600

700

800

40 35

15

30 10

25 20

5

0

5

10

15

20

-2

15 0

5

-1

Concentration (µM)

10

Wavelength (nm)

Wavelength (nm)

(c)

15

-3

-2

Absorbance (x10 , arb. unit)

(a)

rNH3 (nmol h cmgeo )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 28 of 32

25

Time (h)

Figure 5. UV-Vis spectrum using Nessler’s (a) and Indophenol (b) method to detect NH3 synthesized during CA for different electrolysis durations. Measurement duration of CA was 1 (red), 2 (blue), 5 (cyan), 7 (magenta), and 10 h (yellow). The black line represents a standard solution in the absence of NH3. (c) The concentration of NH3 in the trap solution (yellow) and reaction rate at a particular time (blue) for extended-duration electrochemical NH3 production. The extended-duration NH3 synthesis was conducted at 338 K using a membrane electrode device containing γ-Fe2O3 and IrO2 in the cathode and anode, respectively, at an applied cell voltage of 1.7 Vcell. H2O-saturated N2 and 0.5 M aqueous KOH was supplied to the cathode and anode, respectively, at flow rates of 200 sccm and 1 mL min-1, respectively.


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Table 1. Mass activities, FEs, partial currents, and geometric- and area-normalized reaction rates for electrochemical NH3 synthesis at a γ-Fe2O3 electrode. The results were obtained in N2-saturated 0.1 M KOH solution after 1 h of CA at different applied voltages. Values were obtained using the indophenol method taken from a sample with lowest activities.

Potential

Mass activity

F.E.

j NH3

rNH3

rNH3

(VRHE)

(nmol mg-1 h- 1)

(%)

(µA cm-2)

0.0

12.50

1.96

3.518

43.75

0.036

-0.1

10.87

1.19

3.059

38.04

0.031

-0.2

11.82

0.92

3.326

41.37

0.035

-0.3

4.48

0.34

1.262

15.69

0.013

-0.4

16.85

0.12

4.741

58.97

0.049

-0.5

10.05

0.09

2.829

35.19

0.029

-0.6

13.18

0.09

3.709

46.13

0.039

(nmol h-1cm-2geo) (nmol h-1cm-2act)



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Table 2. Mass activities, FEs, partial currents, and geometric- and area-normalized reaction rates for electrochemical NH3 synthesis at a γ-Fe2O3 electrode in a membrane electrode device. Values were obtained using the indophenol method.

Cell voltage

Mass activity

F.E.

j NH3

rNH3

rNH3

(Vcell)

(nmol mg-1h- 1)

(%)

(µA cm-2)

1.5

0.69

0.003

0.056

0.691

0.002

1.6

55.96

0.044

4.500

55.96

0.164

1.7

46.98

0.011

3.778

46.98

0.138

1.8

41.45

0.004

3.333

41.45

0.121

1.9

20.73

0.001

1.667

20.73

0.061


* Electrochemical NH3 synthesis was conducted for 1 h at 338 K using a membrane electrode

device containing γ-Fe2O3 and IrO2 in the cathode and anode, respectively, at each cell voltage. H2O-saturated N2 and 0.5 M aqueous KOH was supplied to the cathode and anode, respectively, at flow rates of 200 sccm and 1 mL min-1, respectively.


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Table 3. Mass activities, FEs, partial currents, and geometric- and area-normalized reaction rates upon performing extended-duration electrochemical NH3 synthesis at a γ-Fe2O3 electrode in a membrane electrode device. Values were obtained using the indophenol method.

Time

Mass activity

F.E.

j NH3

(h)

(nmol mg-1h- 1)

(x10-3%)

(µA cm-2)

1

35.86

10.6

2.883

35.858

0.105

3

34.75

6.5

2.794

34.745

0.102

8

21.03

2.8

1.691

21.026

0.062

15

17.81

1.9

1.432

17.814

0.052

25

14.82

1.4

1.191

14.818

0.043

rNH3

rNH3


* Extended-duration NH3 synthesis was conducted at 338 K using a membrane electrode device containing γ-Fe2O3 and IrO2 in the cathode and anode, respectively, with an applied cell voltage of 1.7 Vcell. H2O-saturated N2 and 0.5 M aqueous KOH was supplied to the cathode and anode, respectively, at flow rates of 200 sccm and 1 mL min-1, respectively.



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Synopsis : Electrochemical synthesis of NH3 at γ-Fe2O3 nanoparticle catalysts is investigated at low temperature and atmospheric pressure.


Electrochemical Synthesis of NH3 at Low Temperature and

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