An Electrochemical Haber-Bosch Process

Article

An Electrochemical Haber-Bosch Process Vasileios Kyriakou, Ioannis Garagounis, Anastasios Vourros, Eirini Vasileiou, Michael Stoukides [email protected]

HIGHLIGHTS Combination of H2 production and purification with NH3 synthesis in a single cell Up to 14% NH3 faradaic efficiency by employing a VN-Fe electrocatalyst Enhanced CH4 conversion to CO2 upon H2 extraction from the reforming chamber An electrochemical Haber-Bosch can produce NH3 with less energy and CO2 emissions

Ammonia is the primary chemical intermediate in the fertilizer industry and an important carbon-free energy carrier. Currently, however, ammonia constitutes the most energy-intensive chemical worldwide. Using the feed gases of the conventional Haber-Bosch process, we combined the main stages of an ammonia plant in a single protonic ceramic membrane reactor. The electrochemical process designed can synthesize ammonia with as little as 50% the CO2 emissions and 25% the energy.

Kyriakou et al., Joule 4, 1–17 January 15, 2020 ª 2019 Elsevier Inc. https://doi.org/10.1016/j.joule.2019.10.006

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Article

An Electrochemical Haber-Bosch Process Vasileios Kyriakou,1,2,4,* Ioannis Garagounis,2,3 Anastasios Vourros,2,3 Eirini Vasileiou,2,3 and Michael Stoukides2,3

SUMMARY

Context & Scale

Ammonia, produced via the Haber-Bosch (HB) process, is globally the leading chemical in energy consumption and carbon dioxide emissions. In ammonia plants, hydrogen is generated by steam-methane reforming (SMR) and watergas shift (WGS) and, subsequently, is purified for the high-pressure ammonia synthesis. Herein, we demonstrate how these steps are integrated into a single BaZrO3-based protonic ceramic membrane reactor (PCMR), operating at atmospheric pressure. Hydrogen generation occurs on a Ni-composite electrode, while VN-Fe is the ammonia synthesis electrocatalyst. Hydrogen extraction from the reforming compartment enhances the thermodynamically limited methane conversions, whereas 5%–14% of the pumped protons are converted to ammonia. An electrochemical HB is designed by combining this PCMR with a protonic ceramic fuel cell to recover electricity and separate nitrogen from ambient air by exploiting by-product hydrogen. This process could potentially require less energy and release less carbon dioxide emissions than its conventional counterpart, holding promise for sustainable decentralized applications.

Ammonia is a key chemical for the fertilizer industry and also a potential clean energy carrier for the future. Ammonia, produced via the Haber-Bosch process, is the most energy-intensive commodity chemical, responsible for 1%–2% of global energy consumption and 1.44% of CO2 emissions. Here, we employ a protonic ceramic membrane reactor to incorporate the essential stages of the conventional plant in a single device, including ammonia synthesis, steam-methane reforming, water-gas shift, and hydrogen purification. This capability of the proposed cell allows notable conversions under milder conditions than a typical ammonia plant. By integrating the proposed reactor with a protonic ceramic fuel cell to exploit the hydrogen residual, we design an electrochemical alternative to conventional HB plants. This new strategy can lead to up to 4 times less energy consumption, with 50% lower emissions than its conventional counterpart, holding promise for sustainable decentralized applications.

INTRODUCTION Ammonia is a key chemical, largely used by the fertilizer industry, with an annual production exceeding 150 million tons worldwide.1–3 Moreover, due to its high gravimetric hydrogen density (17.75 wt %), ammonia is considered a clean energy carrier for chemical energy storage applications.3–7 The Haber-Bosch (HB) process constitutes the dominant route for ammonia production. Gaseous N2 and H2 react at elevated pressures (>100 bar) and temperatures (
500 C) in the presence of an Fe-based catalyst.8 The rate-determining step is the dissociation of dinitrogen to atomically adsorbed N species.9 Although the reaction itself is exothermic, it requires significant energy input. Nowadays, ammonia synthesis consumes 1%–2% of the total energy worldwide.2 The energy is primarily consumed for hydrogen production from the strongly endothermic steam-methane reforming (SMR) at 800 C–1,000 C, CH4 + H2O / CO + 3H2, DH R,298 K = +206.1 kJ$mol1,

(Equation 1)

as well as reactant purification and compression.2,10 The effluents are subsequently fed to a shift reactor with excess steam at 350 C– 550 C, to eliminate carbon monoxide and maximize hydrogen yield through the slightly exothermic water-gas shift (WGS) reaction: CO + H2O / CO2 + H2, DH R,298 K = 41.2 kJ$mol1.

(Equation 2)

Contrastingly, ammonia has been produced in nature for millions of years under ambient conditions. Plants and bacteria utilize atmospheric nitrogen and protons

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to synthesize ammonia over nitrogenase metallo-enzymes.9,11,12 As opposed to industry, enzyme-catalyzed synthesis follows an associative mechanism; the NhN bond is cleaved after dinitrogen partial hydrogenation.13–15 The involvement of protons and electrons in the natural process, in conjunction with the discovery of high-temperature protonic conductors,16 motivated the solid-state electrochemical approach.17 Since then, numerous studies have been reported, and the main challenges were identified in recent reviews.18–23 First, the electrolyte should allow sufficient proton fluxes (>107 mol H+$s1$cm2), and second, the electrocatalyst should exhibit adequate electronic conductivity and catalytic activity toward ammonia synthesis by supressing the side reaction of hydrogen evolution (HER). Recently, tubular BaZrO3-based protonic ceramic cells with low thickness (<30 mm) electrolyte were developed.24 These cells exhibit high protonic conductivities, appreciable mechanical strength, chemical stability, and have thus been successfully used for protonic ceramic membrane reactor (PCMR) applications.25–28 Nevertheless, the main hurdle for commercialization is selecting an effective electrocatalyst for ammonia synthesis. Industrial catalysts, unfortunately, either are poor electronic conductors or favor HER, leading to poor faradaic efficiencies (FE).18–21 Recently, density functional theory (DFT) calculations were employed to identify promising metal mononitrides for ammonia generation through a Mars-van Krevelen mechanism, i.e., protons react with lattice nitrogen, and the lattice vacancy is replenished by dissociated gas-phase nitrogen.29–33 Among the most promising nitrides was VN, whose capability to form ammonia at low applied bias was experimentally verified in acidic solutions.32,34–37 Here, we report on a PCMR that incorporates the essential HB steps; hydrogen production over Ni-composite anode, purification through BaZrO3-based electrolyte, and ammonia synthesis over VN-based cathode. The proposed device exhibits three unique functions. First, ammonia is synthesized at atmospheric pressure by employing protons and electrons. Second, the thermodynamic restrictions of methane conversion at low temperatures (550 C–650 C) are surpassed through hydrogen extraction from the steam reforming chamber. Last, methane is directly converted to CO2 instead of CO due to WGS dominance at these temperatures, thereby maximizing hydrogen yield in a single device and eliminating the need for a shift reactor downstream. An electrochemical alternative to the HB (EHB) is designed by combining this PCMR with a protonic ceramic fuel cell (PCFC) to generate electricity and purify N2 by exploiting the by-product hydrogen. This simple, electrochemically driven route for ammonia synthesis could offer substantially decreased energy consumption and CO2 emissions than its conventional counterpart.

RESULTS AND DISCUSSION Integration of SMR, WGS, and Ammonia Synthesis in a PCMR To combine SMR, WGS, and ammonia synthesis, we employed a PCMR of tubular geometry, consisting of a Ni-BaZr0.7Ce0.2Y0.1O3a (Ni-BZCY72) composite anode support, a
25 mm dense BaZr0.8Ce0.1Y0.1O3a (BZCY81) electrolyte, and a porous VN-Fe cathode (Figures 1 and S1). The microstructure of the cell is illustrated in Figures 1B and S2–S4. The Ni-based cermet exhibits excellent adhesion with the solid electrolyte as well as relatively high porosity (
30%) resulting from the reduction of NiO at 1,000 C. Ni was uniformly distributed across the supporting layer with grain

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1Dutch

Institute for Fundamental Energy Research (DIFFER), De Zaale 20, 5612 AJ Eindhoven, the Netherlands

2Department

of Chemical Engineering, Aristotle University of Thessaloniki, Thessaloniki, Greece

3Chemical

Processes & Energy Resources Institute, CERTH, Thessaloniki 56071, Greece

4Lead

Contact

*Correspondence: [email protected] https://doi.org/10.1016/j.joule.2019.10.006

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Figure 1. Integration of Hydrogen Production and Purification as well as Ammonia Synthesis in the PCMR (A) At the Ni-BZCY72 anode compartment, the CH4 -H 2 O mixture is converted selectively to CO 2 and H +, with the latter being transported to the VN-Fe cathode through the BZCY81 membrane. On VN, lattice N reacts with H+ to form NH 3 , while the N-vacancy is replenished by dissociated N2 from the gas-phase. (B) Scanning electron microscopy (SEM) images of VN-Fe cathode surface, cell cross-section, and Ni-BZCY72 anode surface after exposure to 20% H 2 /N 2 at 650  C for 48 h (Figures S1–S4).

size varying from 2 to 5 mm. The VN-Fe cathode also showed adequate adhesion with the electrolyte and was highly porous (Figures 1B, S2A, and S4A) to facilitate diffusion of the gaseous dinitrogen. The phase purity and crystal structure of the electrodes was investigated by room temperature XRD. The diffractogram of the reduced Ni-BZCY72 anode displayed only the characteristic peaks of the BZCY perovskite and metallic nickel (Figure S5A).22–25 The XRD analysis of the VN-Fe cathodic electrode revealed two separate phases, i.e., those of VN and metallic Fe (Figure S5C). Specifically, the peaks at 37.5 , 44.0 , 64.1 , 76.4 , and 80.5 correspond to the diffraction of the (111), (200), (220), (311), and (222) lattice planes of VN, respectively. The VN diffraction angles were not shifted compared to the Fe-free samples, indicating no change in the VN lattice parameters following electrode fabrication. To verify VN-Fe chemical integrity, we exposed the sample to 20% H2/N2 at 600 C for up to 100 h. The diffractograms of the exposed samples were practically unchanged, implying good stability of the nitride catalyst under high-temperature reducing conditions (Figure S5C). Following fabrication, the cells were mounted in the testing setup, and the temperature was increased slowly to 550 C under a mild reducing environment (1% H2/N2). A methane-steam mixture was introduced over the anode (steam reforming

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Figure 2. Experimental Results of the Integrated SMR, WGS, and Ammonia Synthesis in a Single PCMR Effect of the applied current on (A) CH 4 conversion, (B) CO 2 selectivity, (C) H 2 generated under closed-circuit conditions, (D) NH3 synthesis rate, (E) faradaic efficiency to NH 3 , and (F) developed cell voltage versus open-circuit voltage (OCV). Anode feed: P CH4 = 3 kPa, PH2O = 9 kPa, Ft = 15 sccm; cathode feed: PN2 = 100 kPa, Ft = 100 sccm. Repeated measurements and the standard errors are shown in Figures S9 and S10 and Tables S7 and S8.

compartment), while the cathode was exposed to gaseous nitrogen (ammonia synthesis chamber). The selected H2O/C ratio was 3/1 to avoid carbon deposition and simultaneously keep Ni in its active metallic state.26,28 Figure 2A shows that under open-circuit conditions (I = 0), CH4 conversions between 44% and 61% were obtained, displaying the high catalytic activity of Ni-BZCY72 for both SMR and WGS. Besides the large Ni content (50 wt %) of the electrode, the outstanding performance is partly attributed to the mixed cerium-zirconium oxides contained in the BZCY72. The addition of zirconia and especially ceria, promotes the SMR reaction rate and decreases the susceptibility to coking.38–40 On the Ni anode, H2 is formed through SMR and WGS reactions (Equations 1 and 2) with or without imposing electrical current across the electrolyte. Upon closing the circuit, however, hydrogen is converted into protons, and therefore, the electrochemical reaction at the anode can be written as CH4 + 2H2O / CO2 + 8H+ + 8e.

(Equation 3)

The formed protons at the anode are now able to travel through the proton-conducting membrane, leading to a considerable increase of CH4 conversion from 64% and 86 percentage points at 550 C and 650 C, respectively. Clearly, the electrochemical extraction of hydrogen from the anode chamber pushes the equilibrium of reactions (1) and (2) to the product side, hence allowing enhanced CH4 conversions, which could not be achieved under the same temperatures in a conventional catalytic reformer. Moreover, the majority (>92%) of the reacted methane is converted to CO2 due to WGS dominance at low temperatures (Figure 2B). The H2 removal from the anode chamber, however, leads to even higher selectivities, and

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practically all reacted methane is converted to CO2, thus maximizing the hydrogen yield in a single device (i.e., no need for a shift reactor downstream) as shown in Figure 2C. Concurrently, over the VN-Fe cathode the protons react with lattice nitrogen from VN toward ammonia, while the remained N-vacancy is replenished from gaseous dinitrogen dissociation: 4/3N2 + 8H+ + 8e/ 8/3NH3.

(Equation 4)

Figure 2D shows that the observed ammonia rate increases with current up to a certain value, above which it levels off and even drops slightly. Such behavior has also been reported in previous works.18,21,27,41 The high currents, or equivalently, high protonic fluxes, "poison" the cathode by inhibiting dinitrogen adsorption.18,21 It is possible that a large number of protons is recombined to form molecular H2 on the Fe surface, since metallic Fe is an active catalyst for HER.11 This has a detrimental effect on the faradaic efficiency to NH3 that declines with applied current as depicted in Figure 2E. Despite this, the obtained values above 10% are among the highest reported for electrochemical ammonia synthesis.18,20,21 The maximum ammonia rate reaches up to 68 mmol NH3$h 1$m2 (Figure 2D) corresponding to 5.5% FENH3 (Figure 2E) and 0.63 V (Figure 2F) at 600 C. While the capability of VN to electrocatalytically form NH3 has been demonstrated in acidic solutions under ambient conditions,34–37 the formation rates observed here are higher and with significantly improved FEs, due to the elevated temperatures examined. The rise in operation temperature is known to facilitate the cleavage of dinitrogen triple bond, favoring NH3 synthesis.11,21 As depicted in Figures 2D and 2E, the temperature has a positive effect up to 600 C, above which, both rNH3 and FE decline due to the reverse reaction of ammonia decomposition. Recently, Du et al.42 performed a comprehensive study employing isotope labeling for electrocatalytic synthesis of ammonia over early transition metal nitrides in aqueous systems. The authors reported that no activity was observed for polycrystalline VN in these systems. It was concluded that ammonia was only produced by leeching lattice N from the nitride, while no replenishment from gaseous N2 took place. Even though the system reported by Du et al. is fundamentally different with the one described here (e.g., the use of liquid instead of solid electrolytes that have no NH3 solubility as well as the significantly lower temperatures), we carried out background experiments over VN at 600 C, in which H2/N2 feed was substituted with H2/Ar to investigate the N-vacancy replenishment (Figure S7A). In the presence of 20% H2/N2, the ammonia rate reached steady state after 4–6 h and then it remained practically constant for >60 h of continuous operation. However, by switching to H2/Ar, a sharp peak of ammonia formation was initially observed, followed by a rapid decline toward zero in the next hours. This finding clearly demonstrates that only in the presence of gaseous N2 the catalyst exhibits stable behavior, thus indicating N-vacancies replenishment by N-atoms derived by N2 dissociation over the nitride. To further assess the durability of our PCMR, we performed a 48-h transient experiment with consecutive galvanostatic steps. The feed ratio was kept the same as in Figures 2, and 550 C was selected as the operating temperature. Figure 3 displays the molar rates of ammonia, hydrogen produced under closed circuit, and the residual methane exiting the reactor. Initially, the cell operates as a conventional low-temperature

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Figure 3. Durability Study of the PCMR Ammonia synthesized (A), hydrogen produced under closed-circuit conditions (B), unreacted methane exiting the reactor (C), and developed cell voltage versus OCV during the 48-h galvanostatic transient experiment (D). See also Table S9 for the raw experimental data and Figures S4 and S5 for the physicochemical characterizations of the cell components following the stability study.

catalytic reformer, achieving 44% methane conversion. By applying a current of 20 mA, the residual methane decreases, producing NH3 and H2 of 26 and 220 mmol$h1$m2, respectively. Similarly, by raising the current to 40 and 80 mA, an up to 61% of methane is converted to ammonia and hydrogen of 50 and 898 mmol$h1$m2. By returning to 20 mA, we performed a 24-h stability study, where the molar rates, potential, and faradaic efficiency remained practically unaltered, suggesting negligible degradation of the system during the examined time period. These results imply that the PCMR allows the control of methane conversion and product selectivity, simply by switching the applied current (or voltage). Figures S4 and S5 contain room temperature X-ray diffractograms and elemental maps from EDX analysis of the cell cross-section before (a) and after (b) this test, corroborating the PCMR’s robustness. The Electrochemical Haber-Bosch Plant The overall reaction occurring in the present PCMR is the sum of Equations 3 and 4, and it is the same as in conventional HB plants, albeit, with gaseous H2 as a by-product at the cathode from HER: CH4 + 2H2O + 4/3N2 / CO2 + 8/3NH3, DH R,298 K = 46.7 kJ$mol1.

(Equation 5)

Based on the aforementioned reaction scheme and the experimental findings, we designed an electrochemical alternative to the conventional HB process. A

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Figure 4. Conceptual Design of the Electrochemical HB Process The unit contains two protonic ceramic cells, the PCMR and the PCFC, and three condensers. The electrical power consumed in the PCMR could be covered by renewables and by-product hydrogen oxidation in the PCFC. The PCFC is also used for N 2 purification from ambient air. The detailed flow diagram, along with tables of the mass balances are presented in the Supplemental Information.

simplified flowsheet of ammonia production in the proposed EHB process is illustrated in Figure 4 (Figure S11 is the detailed flow diagram). A steam-methane mixture (H2O/C = 3) and nitrogen is introduced to the PCMR operating at 550 C. As described in the previous section, the PCMR integrates ammonia synthesis (cathode), SMR, and WGS (anode) as well as hydrogen purification (protonic membrane). The effluent of the steam reforming chamber is passed through a condenser, where excess steam is collected and recycled, leaving pure CO2 for capture. The ammonia chamber outlet is a mixture of NH3 and unreacted H2–N2. Ammonia is then separated via a second condenser, and the leftover reactants are fed to a PCFC, operating with excess of H2 at 550 C and 45% efficiency. The PCFC exploits the by-product hydrogen to recover part of the electricity consumed in the PCMR and produce pure N2 from the air after steam condensation. Here, a PCFC was selected over a solid oxide fuel cell (SOFC), since it operates at temperatures similar to our PCMR (500 C–600 C) with high efficiency (>45%) and durability (1.5% degradation after 6,000 h).43–47 On the other hand, SOFCs perform adequately above 700 C, due to the greater activation energy for oxygen ion conduction, compared to protonic, in ceramic oxides.44 Another advantage is that by employing the same electrolyte material for both PCMR and PCFC, the cost of cells fabrication decreases. The results of the energy and mass balance calculations for the EHB plant are summarized in Figure 5. The methods and assumptions used for the calculations are

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Figure 5. Energy Analysis and CO2 Emissions Results of the EHB (A and B) Dependence on the faradaic efficiency to NH 3 of the process energy demands under (A) low (0.3 V) and (B) high (1.2 V) applied voltage in the PCMR. Thermal energy includes the LHV of the total consumed methane and synthesized ammonia in the process. (C) Effect of PCMR operation voltage on electrical energy demands. (D) Total CO 2 emissions for several faradaic efficiencies to NH 3 .

described in detail in the Experimental Procedures and in the Supplemental Information. Figures 5A and 5B show the effect of FENH3 on the energy demands per mol of NH3 synthesized in the PCMR. The energy consumption of an optimum HB plant is also plotted for a direct comparison. At 0.3 V (voltage close to our PCMR operation), FENH3 >35% is required in order for the EHB to consume less energy than a conventional plant (
500 kJ$molNH31).2 However, at high voltages, e.g., 1.2 V, this only occurs when the FENH3 exceeds 75%, thus complicating the applicability of EHB. The critical role of PCMR operating potential is also shown in Figure 5C, where the electrical energy demands are plotted against the voltage for various FEsNH3. Ideally, if the reactor could operate at a <0.2 V applied bias, there could be a net electrical power generation in this process. Another interesting point is that increasing the FE, not only decreases the energy demand but also mitigates the negative effect of voltage (smaller slopes), emphasizing the space for improvements in the cathodic electrode. The linear behavior stems from the fact that the current was kept constant for each FE value, as dictated by reaction stoichiometry (hydrogen mass balance). The CO2 emissions are crucial for a sustainable process due to environmental considerations. Ammonia synthesis plants are the most polluting in the chemical

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Figure 6. Theoretical Energy Requirements of the PCMR, Resulting from the Different Hydrogen Sources (A) Total (DH R ) and (B) electrical (DG R) energy demands as well as the reversible cell potentials (Experimental Procedures, Equation 14) of ammonia synthesis from nitrogen and molecular hydrogen (blue), steam (red), and a methane-steam mixture (magenta).

industry, releasing 1.44% of the global CO2 emissions.2 For this reason, we have calculated the total emitted CO2 for different FEsNH3, and we compare them to the emissions of an industrial plant (Figure 5D). At FEs higher than 35%, the CO2 emissions of the proposed EHB become lower than those of a conventional plant. Moreover, if the FENH3 exceeds 75%, the EHB could theoretically release less than half CO2 emissions (1.45 kgCO2/kgNH3 instead of 2.89 kgCO2/kgNH3) of efficient ammonia plants. CH4–H2O versus H2O as a Hydrogen Source for the EHB The thermodynamic limitations (particularly that of the pressure) of the HB process have forced ammonia manufacturers to large and centralized plants in order to achieve economic viability. Our proposed EHB process is simpler and contains fewer steps than an industrial unit. Hence, it could be potentially applied to remote areas, bringing electrification of the chemical industry closer to realization. A network of small-scale plants coupled to electricity from the available renewable sources of each region could be employed for on-site production of ammonia, substantially decreasing the distribution costs.45 One of the main advantages of the electrification of the chemical industry is to restrain greenhouse gas (GHG) emissions by employing non-carbonaceous sources for electricity, such as the wind and the sun.48 This EHB could be redesigned without methane feedstock, by only introducing steam over the anode, offering an important benefit from an environmental point of view. Nevertheless, this exhibits two important drawbacks; the energy requirements and the cell components’ stability. To highlight the former, Figure 6 depicts the ideal total and electrical energy demands for ammonia synthesis from dinitrogen and (1) molecular H2, (2) H2O, and (3) CH4– H2O, as calculated by the enthalpy and the Gibbs free energy of the PCMR overall reaction. A process with ammonia production from its elements requires the least energy among these three and, hypothetically, could co-generate electricity and ammonia at temperatures below 200 C. On the other hand, when hydrogen is derived from steam, the energy demands dramatically increase by almost 365 kJ$molNH31. By co-feeding methane with steam over the anode, however, electrical work could be recovered by the system from their reaction toward carbon dioxide (DGf < 0), thus lowering the energy demands close to the corresponding values of molecular H2 feed. To translate the electricity-saving into cell potential

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(Figure 6B), the required voltage decreases by
1.0 V. A similar example in literature is the so-called fuel assisted steam electrolysis, where hydrogen is generated from steam at inferior voltages or even spontaneously by sacrificing fuel at the anode.49 Figure 6 describes ideal cases, in which all hydrogen is converted to ammonia. To understand the gain in electricity in an actual process, the amount of hydrogen necessary to produce a specific amount of ammonia should be calculated not only from the reaction stoichiometry but also from the FENH3. If, for instance, the FENH3 is 30%, 10 mol H+ (5 mol H2) should be pumped through the electrolyte to form one mol of NH3. This is equivalent to a current of nearly 965 kA. Therefore, if the applied voltage is 0.3 V, which could be achieved with steam-methane inlet, the electricity demands are 289.5 kJ$molNH31. Likewise, by applying the reversible potential for steam electrolysis, i.e., 1.2 V, this value climbs to 1,158 kJ$molNH31, nearly 900 kJ higher! Even if the electrical power is completely renewable, this value is tremendous and a serious deterrent for practical applications. Even though our process employs methane as one of the hydrogen feedstocks, this does not mean that it could not be environmentally friendly. It is evident from Figure 5D that the CO2 emissions could decrease by up to 60% from an efficient ammonia plant if the electrocatalyst produces ammonia at FEs greater than 35%. Besides, it is not necessary for the methane feedstock to derive from natural gas. The hydrogen production in the EHB could be switched from natural gas to biogas steam reforming50,51 or steam gasification of solid carbonaceous biofuels52 to minimize the total carbon footprint, rendering the present process even more attractive. The second disadvantage of using only steam instead of the methane-steam mixture, is the efficiency and stability of the cell components. The tubular PCMR used in the present studies is based on yttrium doped mixed barium zirconatecerate oxide, which is to the best of our knowledge, the closest thing to scalable intermediate to high temperature (>400 C) protonic conductors.25,28,43–47 This type of cells is supported on a Ni-based cermet to achieve adequate mechanical stability due to the low thickness of the dense electrolyte. To electrolyze steam at these electrodes is challenging due to Ni oxidation at high steam concentrations. The co-feed of hydrogen as a protective agent, similar to solid oxide electrolyzers, would not be a solution since the system would preferentially remove the available molecular hydrogen instead of splitting steam. Hence, a process that includes steam electrolysis would likely need a complete re-invention of the membrane-electrode assembly taking it many steps back from scale up. Besides, even when this issue is overcome, the >1.2 V operation voltages could be another hurdle for the long-term operation (>1,000 h) of the PCMR. The high imposed cell voltages could cause major degradation to the cell components. On the contrary, in our PCMR, the use of a steam-methane mixture at the anode, combined with a nitride catalyst at the cathode allows operation under milder conditions (0.3–0.6 V), theoretically extending the lifetime of the system. Nonetheless, the faradaic efficiency to NH3 of the cathodic electrode remains the main challenge of the present process. In the present study, the obtained FEs of 5%–14% are among the highest reported,18–22 especially if we take into account the absence of molecular hydrogen feed and the relatively low applied voltages. However, the observed values are far from the anticipated for vanadium nitride from DFT calculations.32 One can attribute them to the dominance of ammonia decomposition at elevated temperatures. Here, this effect is diminished by the quick recovery of ammonia from the reactor through relatively high volumetric flow rates.

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In identical reactor designs and in the presence of an active catalyst, the measured decomposition has been quantified below 20% for similar flow rates.17,53 Most likely, the deviation from the anticipated values is due to the mixing of VN with Fe to enhance adhesion with the BZCY81 electrolyte. A fraction of protons react selectively with lattice nitrogen from VN toward ammonia, while the remaining protons undergo charge transfer reactions on Fe and preferentially evolve as H2. To improve the FENH3, different deposition techniques, such as chemical vapor deposition on a porous backbone of BZCY, might be employed to obtain pure layers of the nitride electrocatalyst. Further, the cathode microstructure optimization could maximize the nitride-electrolyte contact length (three-phase boundary), thereby enhancing the sluggish electrocatalytic ammonia synthesis reaction. Conclusions We have demonstrated how hydrogen production and purification as well as ammonia synthesis stages of an HB plant are integrated in a single BaZrO3-based PCMR, operating under atmospheric pressure. The electrocatalyst for SMR and WGS was a Ni-composite electrode, whereas ammonia was produced over VN-Fe at 5%–14% efficiency under low applied potentials (0.3–0.6 V). The hydrogen recovery from the steam reforming chamber enhanced the thermodynamically limited methane conversions by up to 40%, producing almost exclusively CO2. Based on the experimental results a simpler, electrochemically driven alternative to the conventional HB process was designed, wherein the PCMR is combined with a PCFC to exploit the by-product hydrogen to recover electricity and purify reactant nitrogen from ambient air. The energy analysis revealed that at FENH3 exceeding 35% and low applied bias (<0.6 V), the proposed process could consume substantially less energy, releasing up to 60% fewer CO2 emissions than its conventional counterpart. By taking into account that ammonia is globally the leading chemical in energy consumption and GHG emissions, the proposed EHB holds promise for sustainable agricultural applications, especially in remote areas, where distribution costs could be prohibitive.

EXPERIMENTAL PROCEDURES Cell Fabrication The PCMR used (Figures 1 and S1) is based on the proprietary doubly doped barium zirconate tubes developed by CoorsTek Membrane Sciences. The electrolyte is a thin layer (
25 mm) of BaZr0.8Ce0.1Y0.1O3-a (BZCY81) supported on the porous NiBaZr0.7Ce0.2Y0.1O3-a (Ni-BZCY72) anode cermet with a Ni content of ca. 50 wt %. The fabrication, merits, and applications of these tubes have been discussed and presented in detail in previous communications.24–28 In brief, a slip casting slurry containing NiO and BZCY72 at the appropriate amounts was first prepared. After drying, a new slurry containing BZCY81 precursors was spray-coated around the tube. The tube was fired at 1,585 C for 6 h in ambient air. The as-prepared tube was reduced at 1,000 C for 24 h in 5% H2/Ar forming successfully a porous and conductive Ni-BZCY72 electrode-support. The vanadium nitride was prepared by high-temperature ammonolysis of V2O5 (Sigma Aldrich). The nitride was prepared in small batches of 0.5 g in order to achieve adequate and uniform nitridation. For each batch, the oxide was placed in a quartz U-shaped reactor and heated to 900 C, in a tubular geometry oven for 8 h in a pure NH3 flow (100 cm3$min1). The reactor was then cooled to room temperature under the same atmosphere. Before retrieving the final product, the nitride was passivated with 0.1% O2/N2 for 8 h. To fabricate the cathodic electrode, the as-prepared VN powder was mixed with a terpineol-based ink vehicle (Fuel Cell Materials) and applied to the outside surface of the tube with a doctor

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blade in three layers. VN was ball-milled together with Fe powder (Alfa Aesar) at 1/ 1 and 3/1 weight ratio for the first (in contact with the electrolyte) and second layer, respectively, while the top layer consisted only of VN. The main purpose of the Fe addition to the first layers is to improve the adhesion between the nitride and the electrolyte. Fe was chosen as a cheap, non-toxic, and non-critical substance, while it is also a main component of most industrial catalysts for ammonia synthesis. Nevertheless, experiments with the same Fe powder as the working electrode, in the absence of VN, gave comparatively negligible ammonia yields (2 orders of magnitude lower). The total weight of the cathodic electrode was 0.4 g and covered an area of 8 cm2. Characterizations The crystallography of the cell components was examined by room temperature XRD in a Bruker D8 Discover theta-theta diffractometer with a Cu Ka beam operating at 40 kV, 40 mA, and a scan-rate of 0.03 in the 2q range of 20 C–90 C. A JEOL JSM 6300 microscope, equipped with an Oxford Instruments INCA x-sight detector for EDX measurements was used to acquire images of the cell’s cross-section and electrode surfaces. The electrochemical characterizations were performed with a VersaSTAT 4 (Princeton Applied Research) electrochemical workstation combined with the accompanying VersaStudio software. Current-voltage sweeps were performed with a scan-rate of 0.02 V$s1. Potensiostatic impedance measurements were carried out with an amplitude of 30 mV in the frequency range of 1 MHz to 102 Hz. Analysis of Reactant and Product Gases High purity CH4 and N2 (99.999%, Air Liquide Hellas) were mixed with steam by bubbling the CH4–N2 mixture through a temperature-controlled saturator, whereas pure N2 (99.999%, Air Liquide Hellas) was fed to the cathode. The tubing from the saturator to the reactor was heated above 120 C to avoid liquids condensation. The total flow rates were 15 and 100 sccm for anode and cathode chamber, respectively. The analysis of the reactants and products (CH4, CO, CO2, and H2) was performed by online gas chromatography (SHIMADZU GC-2014) and an infra-red spectrometer (Binos 100, Rosemount Analytical). The conversion of CH4 and selectivity to CO2 were calculated as follows:
(Equation 6) XCH4 = ðrCH4;in  rCH4;out Þ rCH4;in
SCO2 = rCO2 ðrCH4;in - rCH4;out Þ:

(Equation 7)

In Equations 6 and 7, rCH4 represents the molar rate of CH4 entering (in) or exiting (out) the reactor and rCO2, the molar rate of CO2 at the exit of the reactor. Ammonia was continuously monitored by cavity ring-down spectroscopy with an EAA24 online laser ring-down analyzer (Los Gatos Research). The analyzer measured ammonia twice per second, logging a value every 10 s in order to minimize noise. The standard deviation at steady-state conditions was <1%. The ammonia concentration was further verified by using the chemical method of indophenol, with the help of a 0.4% boric acid solution. The discrepancy between photometric calculations and ammonia analyzer indication did not exceed 10% in all experiments. The FE to ammonia is the fraction of protons reaching the cathode, which are converted to ammonia: FENH3= (3$rNH3)/(IH+/F),

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(Equation 8)

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where rNH3 is the molar rate of ammonia, IH+ is the current which is carried by protons, and F is Faraday’s constant (96,485 C$mol1). The electrochemical H2 is the extra hydrogen produced under closed-circuit and is calculated by subtracting the open-circuit H2 rate from that achieved under galvanostatic operation. Description of the Proposed EHB In the proposed EHB process (Figure S11), atmospheric air is fed to the cathode of the BZCY-based PCFC stack (R2). This fuel cell serves two main purposes: first, the production of pure nitrogen (the feed for the PCMR cathode) from the air, and second, the production of electricity, to cover part of the requirements of the PCMR (R1). In the PCFC, all the oxygen of the air stream reacts with most of the hydrogen from the recycle stream, to produce a mixture of nitrogen and steam, which passes through separator S3, where H2O is separated from N2. The separated water (4) is combined with the recycled water (23) and then mixed with natural gas (18) to make up the steam reforming feed of the PCMR anode. Condenser S1 produces a stream of pure nitrogen (5), part of which (7), determined by the reaction’s stoichiometry and recycle rate (the latter depends solely on FE for these calculations), is used for ammonia synthesis. For ammonia FEs in R1 up to ca. 85% this produces adequate N2; for higher FEs another N2 source is necessary. This stream (7) is mixed with the hydrogen-depleted stream from the PCFC cathode (15), and the resulting stream is fed to the cathode of the PCMR. R1 is an electrochemical reactor where, in the anode chamber, the steam reforming of natural gas (and the WGS) takes place over the Ni-BZCY electrode at 550 C. The produced hydrogen is pumped through the electrolyte membrane, which shifts the SMR equilibrium toward the products and expedites the spontaneous WGS reaction, transforming carbon monoxide to carbon dioxide and producing further quantities of hydrogen. In this way, methane can be completely converted to carbon dioxide by the end of the PCMR. All of the produced hydrogen is assumed to be pumped to the cathode side through the electrolyte in the form of protons, under the applied voltage. The stream (21) exiting the anode side of R1 (consists of water and carbon dioxide), is driven to condenser S3, where the water is condensed, leaving pure carbon dioxide ready for capture. The hydrogen (protons), which is pumped to the cathode, reacts with the gaseous nitrogen on a highly active electrocatalyst (e.g., nitride, perovskite or cermet) forming ammonia. The produced ammonia is then cooled and separated from the unreacted nitrogen and hydrogen in condenser S2. The unreacted gas mixture (13), which is approximately at the stoichiometric ratio (75 mol% H2-25% N2) is fed to the PCFC, R2. The H2–N2 mixture exiting the PCFC anode chamber is combined with pure nitrogen after condensing steam of the PCFC cathode stream to form the PCMR cathode feed. The sum of the reactions taking place in R1 is slightly endothermic and non-spontaneous, so both heat and electricity for H+ pumping is required. While stoichiometrically, the steam/carbon ratio in the reformer feed should be 2, the mass balances conducted in this analysis use a ratio of 3, which has been proposed in order to avoid carbon deposition and to enhance methane conversion (Le Chatelier’s principle).26,28 On the other hand, the process in the PCFC is both exothermic and spontaneous. The maximum thermodynamic efficiency (DG/DH) of a H2 fuel cell, operating at 550 C is about 83%, with values as high as 65% in actual practice.54

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In the energy balances, a rather moderate efficiency of 45% (hydrogen to electricity) is assumed for an operating voltage of 0.6 V. Residual hydrogen is necessary, if all the cathodic oxygen is going to be converted to steam so that pure nitrogen can be produced. The main assumptions for the calculation of mass and energy balances can be summarized as follows:  Air consists of 80% nitrogen and 20% oxygen  Overall efficiency (hydrogen to electricity) of the PCFC, R2, is 45%  The consumption of methane over the anode of R1 and of oxygen over the cathode of R2 is 100%  All the hydrogen produced in the anode chamber of reactor R1 is transferred to the cathode side (hydrogen separation efficiency is 100%)  The separation efficiency of S1, S2, and S3 is 100%  The ambient temperature is assumed to be 20 C  No ‘‘thermal losses’’ have been estimated for any part of the process  No pressure drop has been calculated, which would necessitate pumps and other mechanical work  Heat demand is calculated as the sum of the heat-duties of the reactors and separators, plus the LHV of consumed CH4 (802 kJ/mol, endothermic),55 minus the LHV of produced NH3 (316 kJ/mol, exothermic).55 Energy and Voltage Calculations To calculate the Gibbs free energy and the enthalpy change of a reaction at non-standard temperature (Figure 6) the integral of the heat capacity change of the reaction, DCp, from T0 = 25 C (standard temperature) to the temperature in question, was used. The enthalpy change is related to temperature via the equation Z DHo = DHo0 + R

DC oP dT; R

(Equation 9)

DC oP dT ; R T

(Equation 10)

T

T0

while for the entropy change, Z DS o = DS o0 + R

T

T0

and since DGo = DHo  TDS o , DGo = DHo0 

 T  o DH0  DGo0 + R T0

Z

T T0

DC oP dT  RT R

Z

T

T0

DC oP dT : R T

(Equation11)

Assuming that the change in Cp with temperature is given by (Appendix C.1 in Smith et al.55) Cp = A + BT + CT 2 + DT 2 ; R

(Equation 12)

then DCp can be calculated by summing the Cp of each chemical, j, participating in the reaction, weighted with the corresponding coefficient, nj, (negative for reactants) to give: DC p = DA + DB,T + DC,T 2 + DD,T 2 ; R P P where DA = j nj A, DB = j nj B, etc.

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(Equation 13)

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Table 1. List of Abbreviations BZCY x(0.9-x)

BaZrxCe0.9-XY0.1O3-a

LHV

lower heating value

DFT

density functional theory

OCV

open-circuit voltage

EHB

electrochemical HaberBosch

PCFC

protonic ceramic fuel cell

FE

Faradaic efficiency

PCMR

protonic ceramic membrane reactor

GHG

greenhouse gases

SMR

steam-methane reforming

HB

Haber-Bosch

SOFC

solid oxide fuel cell

Her

hydrogen evolution reaction

WGS

water-gas shift

Hence, every term in Equations 6 and 7 can be found or calculated from the appropriate reference tables55 and the calculations, albeit lengthy, are easy to carry out in a spreadsheet. More details can be found in the ESI along with the values of A, B, C, and D used (Table S5). To obtain the reversible potential values of Figure 6B the corresponding DGR values are divided by the number of electrons involved in the electrode reaction (n = 3 per NH3 molecule) and Faraday’s constant: E = DGR/(n$F).

(Equation 14)

For list of abbreviations used in this paper, please see Table 1.

DATA AND CODE AVAILABILITY The data that support the plots and findings of this research are available from the corresponding author upon reasonable request.

SUPPLEMENTAL INFORMATION Supplemental Information can be found online at https://doi.org/10.1016/j.joule. 2019.10.006.

ACKNOWLEDGMENTS The authors gratefully acknowledge the support of this research from CoorsTek Membrane Sciences, AS.

AUTHOR CONTRIBUTIONS V.K. conceived the research and designed the experimental studies. V.K., I.G., A.V., and E.V. performed the experiments. I.G. carried out the energy analysis with assistance from V.K. and A.V. M.S. guided and supervised the project. V.K. wrote the manuscript with contributions from I.G. and M.S. All authors discussed the results and commented on the manuscript.

DECLARATION OF INTERESTS The authors declare no competing interests. Received: June 3, 2019 Revised: August 10, 2019 Accepted: October 14, 2019 Published: November 5, 2019

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