Role of Protons in Electrochemical Ammonia Synthesis Using Solid

Research Article pubs.acs.org/journal/ascecg

Role of Protons in Electrochemical Ammonia Synthesis Using SolidState Electrolytes Chung-Yul Yoo,*,†,§ Jong Hyun Park,†,§ Kwiyong Kim,‡ Jong-In Han,‡ Eun-Young Jeong,† Chan-Hee Jeong,† Hyung Chul Yoon,† and Jong-Nam Kim† †

Korea Institute of Energy Research, 152 Gajeong-ro, Yuseong-gu, Daejeon 34129, Republic of Korea Department of Civil and Environmental Engineering, KAIST, 373-1, Guseong-dong, Yuseong-gu, Daejeon 34141, Korea

‡

ABSTRACT: Electrochemical methods of synthesizing ammonia from nitrogen gas have the potential to replace the energyintensive Haber-Bosch process. In doing so, they offer a CO2-free route to the production of the ever-promising energy carrier. In this study, an effort was made to reveal the relationship between proton involvement in the rate-limiting step of the ammonia synthesis reaction and the overall ammonia synthesis rate, particularly for electrolytic cells using solid-state electrolytes, as no such rule based on the measured parameters of the materials has ever been reported. An empirical atomistic expression was derived to explain the observed correlation between the proton conductivity of the solid-state electrolyte and the ammonia formation rate, by considering the proton excorporation and migration enthalpies. This relationship was determined by examining experimental results from the literature that had been obtained using diverse proton-conducting electrolytes. An almost linear energy relationship was demonstrated for state-of-the-art heterogeneous electrocatalysis. KEYWORDS: Electrochemical ammonia synthesis, Proton-conducting electrolyte, Solid-state electrolyte, Heterogeneous electrocatalysis, Energy relationship, Proton conductivity

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INTRODUCTION Extensive efforts have been made to reduce fossil fuel dependency and CO2 emissions, with renewable energy technologies such as photovoltaics and wind power being well poised in this regard.1 Their widespread use will be an absolute necessity in the future; however, despite high levels of technical maturity, they are limited at present due in part to the intermittent and localized nature of solar and wind power. The successful application of these technologies can only result from the development of energy storage technologies that are capable of overcoming this difficulty, such as H2 fuel cells combined with water electrolysis, which still have many obstacles to overcome.1 Recently, ammonia (NH3) has received attention as an alternative energy carrier, as it has a large weight fraction of hydrogen (17.6%) and high energy density comparable even to coal, as well as advantages over hydrogen in terms of storage, liquefaction energy, and ease of transportation.2 In addition, ammonia itself is an important industrial chemical for producing synthetic chemicals. The state-of-the-art technology for NH3 production is the Haber-Bosch catalytic process, which is energy intensive and highly CO2 emissive.3−5 An alternative route is the electrochemical synthesis of ammonia from nitrogen and hydrogen (and/or water), which is at an early research stage.5−9 This could be a sustainable option for converting renewable electricity into ammonia, which, importantly, could be performed under mild conditions.10 An © 2017 American Chemical Society

electrochemical cell comprises three components: an anode, a cathode, and an electrolyte. Under external voltage, oxidation and reduction reactions occur at the anode and cathode, respectively, while ion transport takes place in the electrolyte. To date, both solid-state ammonia synthesis5−7 and molten-salt ammonia synthesis8,11−16 have been investigated. Solid-state ammonia synthesis appears to be more advantageous than the molten salt-based method because state-of-the-art fuel-cell and electrolysis technologies can be employed, and it can be easily scaled by simply stacking electrochemical cells. A number of studies have ensued since Marnellos et al.17 first reported electrochemical ammonia synthesis using SrCe0.95Yb0.05O3‑δ as a proton-conducting oxide electrolyte, mostly based on solid-state systems using various proton- and oxygen ion-conducting electrolytes as well as cathode electrocatalysts for nitrogen dissociation.5−7 Figure 1 presents schematic diagrams of solid-state ammonia synthesis using proton- and oxygen ion-conducting electrolytes. The electrodes and overall reactions for proton-conducting cells can be represented as Received: May 15, 2017 Revised: July 13, 2017 Published: August 5, 2017 7972

DOI: 10.1021/acssuschemeng.7b01515 ACS Sustainable Chem. Eng. 2017, 5, 7972−7978

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ACS Sustainable Chemistry & Engineering

Figure 1. Electrochemical ammonia synthesis from water and nitrogen using proton- and oxygen ion-conducting electrolyte-based electrochemical cells (left and right, respectively).

anode: 3H 2 → 6H+ + 6e− or 3H 2O → 6H+ + 6e− + 1.5O2 +

The objective of this study is therefore to determine an empirical atomistic relationship between the proton conductivity of an electrolyte and the electrochemical ammonia synthesis rate by examining state-of-the-art solid-state ammonia synthesis results obtained using various perovskite- and fluoritetype proton-conducting oxide electrolytes and bimetallic Ag− Pd-based cathode electrocatalysts. The obtained relationship is anticipated as a useful tool for predicting electrochemical ammonia synthesis rates from the proton conductivity of an electrolyte. In addition, potential avenues for further research into solid-state ammonia synthesis are also outlined. At present, energy relationships in heterogeneous electrocatalysis have yet to be reported in the literature, and to the best of our knowledge, this is the first report of an atomistic expression for solid-state ammonia synthesis.

−

cathode: N2 + 6H + 6e → 2NH3 overall: N2 + 3H 2 → 2NH3 or N2 + 3H 2O → 2NH3 + 1.5O2 (1)

whereas those for oxygen ion-conducting electrolyte-based cells are anode: 3O2 − → 1.5O2 + 6e− cathode: N2 + 3H 2O + 6e− → 2NH3 + 3O2 − overall: N2 + 3H 2O → 2NH3 + 1.5O2

(2)

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In the case of the proton-conducting electrolyte-based cell, N2 dissociation/hydrogenation and H2/H2O dissociation occur separately at the cathode and anode, respectively. On the other hand, both the N2 dissociation/hydrogenation and H2O dissociation reactions take place at the cathode in the oxygen ion-conducting electrolyte-based cell. Because of the difficulty of finding adequate cathode electrocatalysts for oxygen ionconducting electrolyte-based cells, the ammonia synthesis rates of such devices are more than 1−2 orders of magnitude lower than those of proton-conducting electrolyte-based cells.6,18,19 In the case of proton-conducting electrolytes, despite many attempts to develop highly conductive electrolytes and active cathode electrocatalysts for satisfactory ammonia formation rates and Faraday efficiencies (the ratio between the actual and theoretical amount of ammonia produced by the electrochemical cell), the performance still suffers from poor rates due to the limited catalytic activity of the cathode electrocatalysts for N2 dissociation and hydrogenation.5−7 In proton-conducting electrolyte-based cells, the most commonly investigated electrolytes are based on various perovskite- and fluorite-type oxides, while bimetallic Ag−Pd is often investigated as a cathode electrocatalyst for N2 dissociation and hydrogenation due to its excellent proton solubility and transport properties.20 Despite these facts, there have been no phenomenological studies showing the relationship between the materials’ parameters and performance, which are required for a quantitative understanding of solid-state ammonia synthesis.

RESULTS AND DISCUSSION Mechanism of Electrochemical Ammonia Synthesis. The mechanistic understanding of electrochemical ammonia synthesis is rather rudimentary. It is generally believed that electrochemical ammonia synthesis from nitrogen comprises a series of steps including nitrogen adsorption, hydrogenation, electron transfer, nitrogen cleavage, and finally ammonia formation. In more detail, there are two possible, but certainly not exclusive, reaction mechanisms that have been suggested: (i) the dissociative and (ii) the associative mechanism.21,22 The reaction schemes of these mechanisms are as follows: (i) Dissociative mechanism: +6H+ + 6e−

+2s

N2,g ⎯⎯→ 2Nad ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯→ 2NH3,ad → 2NH3,g + 2s

(3)

(2) Associative mechanism: +H + + e −

+s

+H + + e −

+H + + e −

N2 → N2,ad ⎯⎯⎯⎯⎯⎯⎯⎯→ N2H ,ad ⎯⎯⎯⎯⎯⎯⎯⎯→ N2H 2,ad ⎯⎯⎯⎯⎯⎯⎯⎯→ N2 +H + + e −

+H + + e −

H3,ad ⎯⎯⎯⎯⎯⎯⎯⎯→ N2H4,ad ⎯⎯⎯⎯⎯⎯⎯⎯→ NH 2,ad + NH3,g +H + + e −

⎯⎯⎯⎯⎯⎯⎯⎯→ NH3,g + s

(4)

where “s” and “ad” denote adsorption sites and adsorbed intermediates, respectively. Both mechanisms include the 7973


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ACS Sustainable Chemistry & Engineering adsorption of nitrogen gas at the cathode surface and subsequent hydrogenation together with electron transfer to form the adsorbed nitrogen intermediates. The associative mechanism is composed of electrochemical reactions alone, whereas the dissociative mechanism also involves a non+2s

electrochemical reaction step (N2,g⎯⎯→2Nad). The dissociation energy of N2,g is 945.41 kJ·mol−1,23 and it is this high energy that makes the associative pathway preferable to the dissociative one in the electrochemical process. In addition, the associative mechanism appears to be controlled by the first hydrogenation reaction plus electron transfer after the N2,g adsorption (N2,ad + H+ + e− → N2Had).24−28 Accordingly, it seems reasonable to describe the electrochemical ammonia formation rate (rNH3) in terms of a single rate-determining step, assuming that the other reaction steps are in a quasi-steady-state and that the external electron supply to the electrochemical cell is sufficient. Then, rNH3 can be expressed as rNH3 ∝ θN2,adθH+, where θN2,ad and θH+ are the surface coverages of N2,ad and H+, respectively, at the electrolyte/cathode interface. In this way, the concentration and mobility of protons in the electrolyte, i.e., the proton conductivity, can be treated as a discrete catalytic descriptor for the electrochemical ammonia formation reaction, while the composition of the cathode electrocatalyst remains the same. Empirical Linear Energy Relationship for Ag−Pd Cathode Electrocatalysts. The nitrogen coverage, θN2,ad, is an important parameter for describing the ammonia synthesis rate and thus must be investigated not only theoretically but also experimentally. However, to date, there have been no experimental reports dealing with this parameter under biased potentials. With this limitation in mind, it is possible to assume a constant nitrogen coverage for a particular electrocatalyst; by doing so and then measuring the proton conductivity of the electrolyte (σH+T), the ammonia formation rate can be found to be proportional to the proton coverage (rNH3 ∝ θH+). This rate expression can be commonly interpreted, assuming (i) identical proton concentration and transport properties at both the surface and in the bulk of the electrolyte; (ii) low coverage of adsorbed or nitrogen intermediate species; and (iii) a single rate-limiting step (first hydrogenation reaction of N2), while all other reaction steps have attained virtual equilibrium; while neglecting (iv) the possible relevance of reaction symmetry parameters analogous to those used in the description of electrochemical electrode kinetics. When the observed rNH3 versus the proton conductivity was plotted (Figure 2), a somewhat surprising linear correlation resulted for Ba(Ce,Zr,AE,RE)O3‑δ29−35 and (La,AE)(Ga,Mg)O3‑δ36−38 perovskite-type and (Ce,RE)O2−δ39,40 and (La,Ca)2(Zr,Ce)2O7−δ41−43 fluorite-type oxide electrolytes (AE = alkaline earth elements and RE = rare-earth elements) when using a fixed bimetallic Ag−Pd cathode electrocatalyst at temperatures below that of thermal ammonia decomposition. It is possible to approximate both rNH3 and σH+T using the power-law relationship, as follows: log rNH3 = a + b log(σH+T )

Figure 2. Proton conductivity dependence of the electrochemical ammonia formation rate for selected proton-conducting Ba(Ce,Zr,AE,RE)O3‑δ29−35 and (La,AE)(Ga,Mg)O3‑δ36−38 perovskitetype and (Ce,RE)O2−δ39,40 and (La,Ca)2(Zr,Ce)2O7−δ41−43 fluoritetype oxide electrolytes (AE = alkaline earth elements and RE = rareearth elements) using bimetallic Ag−Pd cathode electrocatalysts at temperatures of 350−650 °C.

ErNH = bE σ H+T

(6)

3

The ion conductivity can be written as σionT =

(z ion e)2 c ion Dion k

(7)

where zion is the charge of a mobile ion, e is the charge of an electron, cion is the concentration of mobile ions, k is the Boltzmann constant, and Dion is the ion diffusion coefficient. The ion diffusion coefficient, Dion, can be described by Dion = Dion,0e−ΔHmig / kT

(8)

where Dion,0 is the pre-exponential factor, and ΔHmig is the ion migration activation enthalpy. The proton conductivity of the electrolyte for electrochemical ammonia synthesis can be found by combining eqs 7 and (8) to give σH+T =

e 2 c H+ D H+,0e−ΔHmig /kT k

(9)

The protons for electrochemical ammonia synthesis could be generated by proton excorporation from the surface reaction of the hydroxide ions (OH•O) in the first bulk layer of the electrolyte, which are converted to surface protons (H+) and lattice oxygens (OxO). Using Kröger-Vink notation, this reaction K exc

can be written as OH•O←→H+ + OxO. In terms of proton excorporation enthalpy (ΔHexc) and entropy (ΔSexc), the equilibrium constant of the proton excorporation reaction is Kexp =

[H+][OOx] = e−ΔHexc /kT eΔSexc /k [OH•O]

(10)

The effective proton concentration (cH+) can be further expressed by

(5)

where a = −8.66 ± 0.02 and b = 0.25 ± 0.03 for the protonconducting electrolyte, as shown in Figure 2. This equation suggests a linear energy relationship for the apparent activation energies under the assumption of Arrhenius-type behavior:

c H+ = [H+] =

[OH•O] −ΔHexc / kT ΔSexc / k e e [OOx]

(11)

Combining eqs 9 and (11) yields 7974


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ACS Sustainable Chemistry & Engineering ⎛ e 2[OH•O] ΔSexc / k ⎞ −(ΔHexc +ΔHmig)/ kT ⎟e σH+T = ⎜D H+,0 e k[OOx] ⎝ ⎠

with an activation energy of 0.35 ± 0.02 eV, which is in good agreement with the predicted value provided by eq 13. Since only a few attempts have been made to investigate the relationship between the surface and bulk properties for catalytic gas−solid reactions,51−53 it is surprising that a linear energy relationship could be obtained for electrochemical ammonia synthesis by correlating the proton excorporation and migration enthalpies of the electrolyte and the ammonia formation rate. Figure 4 illustrates the log−log plot of the proton conductivities of the electrolytes and the ammonia synthesis

(12)

The parenthetical values in eq 12 can be considered to be approximately constant as [OHO• ], [OxO], DH+,0, and ΔSexc exhibit very weak temperature dependence in the temperature range used for electrochemical ammonia synthesis.44,45 Proton excorporation is the reverse of the proton incorporation reaction, and thus, its enthalpy can be estimated using ΔHexc ≈ −ΔHinc. Enthalpies of proton incorporation and migration for perovskite and fluorite electrolytes taken from both experiments and ab initio/thermodynamic calculations are given in Table 1. Table 1. Experimental and Theoretical Proton Incorporation (ΔHinc) and Migration (ΔHmig) Enthalpies for Perovskiteand Fluorite-Type Oxide Electrolytesa electrolyte composition

ΔHinc (eV)

ΔHmig (eV)

ΔHexc + ΔHmig (eV)

BaCe0.9Y0.1O3‑δ BaZr0.9Y0.1O3‑δ BaCe0.85Gd0.15O3‑δ BaCe0.875Gd0.125O3‑δ Y0.2Ce0.8O2‑δ La1.9Ca0.1CeZrO7‑δ average value standard deviation

−1.68 −0.82 −1.78 −1.34 −0.78 −0.73 −1.19 0.47

0.48 0.44 0.58 0.58 0.78 0.97 0.64 0.20

2.16 1.26 2.36 1.92 1.56 1.7 1.83 0.40

ref 44 44 46 47 48,49 50

Figure 4. Proton conductivity dependence of the electrochemical ammonia formation rate for oxide,17,18,54−56 carbonate-based,57−66 polymer67−70 proton conducting electrolytes and cathode electrocatalysts (Pt, Ru, Pd, Ag, Fe, oxides, and nitrides) other than Ag−Pd.

The proton excorporation enthalpy (ΔHexc) was estimated by ΔHexc ≈ −ΔHinc. a

rates using oxide,17,18,54−56 carbonate-based,57−66 and polymer67−70 proton-conducting electrolytes and cathode electrocatalysts (Pt, Ru, Pd, Ag, Fe, oxides, and nitrides) other than Ag−Pd. It must be noted that a linear correlation between the proton conductivity of the electrolyte and the ammonia formation rate cannot be found in Figure 4 since the different chemical compositions of the cathode electrocatalysts exhibit different nitrogen affinity and coverage values. Therefore, the observed linear correlation (eq 5) only holds for identical cathode electrocatalysts (e.g., Ag−Pd). Challenges and Perspectives for Further Research. To date, no systematic study has been carried out regarding the use of electrocatalysts to boost the electrochemical ammonia synthesis rate. This may have something to do with the rather rudimentary understanding of the relevant electrocatalysts. As is widely perceived in general and strongly demonstrated by Figure 4, rigorous investigation is absolutely needed to develop potent and selective electrocatalysts. The maximum ammonia formation rate using various electrochemical cells is directly proportional to the Faraday efficiency, with a one-to-one correlation (Figure 5). Therefore, it is essential to develop cathode electrocatalysts with superb nitrogen affinities and/or nitrogen dissociation activities to boost the Faraday efficiency and, as a result, the ammonia formation rate. As shown in Figure 5, molten-salt ammonia synthesis exhibits a higher ammonia formation rate and Faraday efficiency than solid-state ammonia synthesis. It has been reported that the electrochemical reaction of N2 to nitride ion (N2 + 6e− → 2N3−) occurs in a molten-salt electrolyte.8,11−16 The formation of N3− facilitates the NH3 formation reaction after hydrogenation. In the case of solid-state ammonia synthesis, the nitrogen reduction potential can be lowered by

The activation energy of rNH3 can be described using the activation energy of σH+T (eq 6, b = 0.25) ErNH = bE σ H+T = 0.25(1.83 ± 0.40eV) = 0.46 ± 0.10eV 3

(13)

using the average values of ΔHexc and ΔHmig from Table 1. Figure 3 shows an Arrhenius plot of rNH3 when using bimetallic Ag−Pd cathodes with various proton-conducting oxide electrolytes. It can be seen that rNH3 is thermally activated

Figure 3. Temperature dependence of the electrochemical ammonia formation rate for selected proton-conducting Ba(Ce,Zr,AE,RE)O3‑δ29−35 and (La,AE)(Ga,Mg)O3‑δ36−38 perovskite-type and (Ce,RE)O2−δ39,40 and (La,Ca)2(Zr,Ce)2O7−δ41−43 fluorite-type oxide electrolytes (AE = alkaline earth elements and RE = rare-earth elements). 7975


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Florida, 2002. http://www.fsec.ucf.edu/en/research/hydrogen/ analysis/documents/FY02_TechnoeconomicPart2.pdf. (3) Hoffman, B. M.; Lukoyanov, D.; Yang, Z.-Y.; Dean, D. R.; Seefeldt, L. C. Mechanism of nitrogen fixation by nitrogenase: The next stage. Chem. Rev. 2014, 114 (8), 4041−4062. (4) Bicer, Y.; Dincer, I.; Vezina, G.; Raso, F. Impact assessment and environmental evaluation of various ammonia production processes. Environ. Manage. 2017, 59 (5), 842−855. (5) Giddey, S.; Badwal, S. P. S.; Kulkarni, A. Review of electrochemical ammonia production technologies and materials. Int. J. Hydrogen Energy 2013, 38 (34), 14576−14594. (6) Amar, I. A.; Lan, R.; Petit, C. T. G.; Tao, S. Solid-state electrochemical synthesis of ammonia: a review. J. Solid State Electrochem. 2011, 15 (9), 1845−1860. (7) Lan, R.; Irvine, J. T. S.; Tao, S. Ammonia and related chemicals as potential indirect hydrogen storage materials. Int. J. Hydrogen Energy 2012, 37 (2), 1482−1494. (8) Murakami, T.; Nishikiori, T.; Nohira, T.; Ito, Y. Electrolytic synthesis of ammonia in molten salts under atmospheric pressure. J. Am. Chem. Soc. 2003, 125 (2), 334−335. (9) van der Ham, C. J. M.; Koper, M. T. M.; Hetterscheid, D. G. H. Challenges in reduction of dinitrogen by proton and electron transfer. Chem. Soc. Rev. 2014, 43 (15), 5183−5191. (10) Jewess, M.; Crabtree, R. H. Electrocatalytic nitrogen fixation for distributed fertilizer production? ACS Sustainable Chem. Eng. 2016, 4 (11), 5855−5858. (11) Murakami, T.; Nishikiori, T.; Nohira, T.; Ito, Y. Investigation of anodic reaction of electrolytic ammonia synthesis in molten salts under atmospheric pressure. J. Electrochem. Soc. 2005, 152 (5), D75−D78. (12) Murakami, T.; Nohira, T.; Araki, Y.; Goto, T.; Hagiwara, R.; Ogata, Y. H. Electrolytic synthesis of ammonia from water and nitrogen under atmospheric pressure using a boron-doped diamond electrode as a nonconsumable anode. Electrochem. Solid-State Lett. 2007, 10 (4), E4−E6. (13) Murakami, T.; Nohira, T.; Goto, T.; Ogata, Y. H.; Ito, Y. Electrolytic ammonia synthesis from water and nitrogen gas in molten salt under atmospheric pressure. Electrochim. Acta 2005, 50 (27), 5423−5426. (14) Murakami, T.; Nohira, T.; Ogata, Y. H.; Ito, Y. Electrochemical synthesis of ammonia and coproduction of metal sulfides from hydrogen sulfide and nitrogen under atmospheric pressure. J. Electrochem. Soc. 2005, 152 (6), D109−D112. (15) Licht, S.; Cui, B.; Wang, B.; Li, F.; Lau, J.; Liu, S. Ammonia synthesis by N2 and steam electrolysis in molten hydroxide suspensions of nanoscale Fe2O3. Science 2014, 345 (6197), 637−640. (16) Li, F. F.; Licht, S. Advances in understanding the mechanism and improved stability of the synthesis of ammonia from air and water in hydroxide suspensions of nanoscale Fe2O3. Inorg. Chem. 2014, 53 (19), 10042−10044. (17) Marnellos, G.; Stoukides, M. Ammonia synthesis at atmospheric pressure. Science 1998, 282 (5386), 98−100. (18) Skodra, A.; Stoukides, M. Electrocatalytic synthesis of ammonia from steam and nitrogen at atmospheric pressure. Solid State Ionics 2009, 180 (23−25), 1332−1336. (19) Jeoung, H.; Kim, J. N.; Yoo, C.-Y.; Joo, J. H.; Yu, J. H.; Song, K. C.; Sharma, M.; Yoon, H. C. Electrochemical synthesis of ammonia from water and nitrogen using a Pt/GDC/Pt cell. Hwahak Konghak 2014, 52 (1), 58−62. (20) Vadrucci, M.; Borgognoni, F.; Moriani, A.; Santucci, A.; Tosti, S. Hydrogen permeation through Pd−Ag membranes: Surface effects and Sieverts’ law. Int. J. Hydrogen Energy 2013, 38 (10), 4144−4152. (21) Howalt, J. G.; Bligaard, T.; Rossmeisl, J.; Vegge, T. DFT based study of transition metal nano-clusters for electrochemical NH3 production. Phys. Chem. Chem. Phys. 2013, 15 (20), 7785−7795. (22) Abghoui, Y.; Garden, A. L.; Howalt, J. G.; Vegge, T.; Skúlason, E. Electroreduction of N2 to ammonia at ambient conditions on mononitrides of Zr, Nb, Cr, and V: A DFT guide for experiments. ACS Catal. 2016, 6 (2), 635−646.

Figure 5. Plot of the maximum ammonia formation rate versus Faraday efficiency for H+ conducting oxide-,17,18,29,30,32,33,36,43,54−56,71,72 O2− ion conducting oxide-,18,19 carbonate-based composite-,57−66 molten salt-,8,11−16 Nafion-,67−70 and anion exchange membrane-based73 electrolytes using various cathode electrocatalysts.

raising the nitrogen affinity of the electrocatalyst, which is enabled by the proper selection of the cathode electrode, as demonstrated by recent density functional theory studies on various metal and metal nitride electrocatalysts.21,22,74−76 It is our expectation that this theoretical approach will aid in the exploration of cathode electrocatalysts for the solid-state synthesis of ammonia. In summary, the observations in this study clearly confirm the importance of protons in electrochemical ammonia synthesis; furthermore, the proton conductivity of the electrolyte (i.e., eq 5) can be employed as a descriptor for predicting the ammonia formation rate when using a bimetallic Ag−Pd cathode electrocatalyst. Further studies with a greater focus on enhancing the nitrogen affinities of cathode electrocatalysts must therefore ensue.

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AUTHOR INFORMATION

Corresponding Author

*Tel: +82 42 8603083. Fax: +82 42 8603133. E-mail: cyoo@ kier.re.kr. ORCID

Chung-Yul Yoo: 0000-0002-9175-2229 Author Contributions §

C.-Y.Y. and J.H.P. contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was conducted under the framework of the Research and Development Program of the Korea Institute of Energy Research (KIER) (B7-2433).

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Role of Protons in Electrochemical Ammonia Synthesis Using Solid

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