Assessment of a Sustainable Electrochemical Ammonia Production

Research Article pubs.acs.org/journal/ascecg

Assessment of a Sustainable Electrochemical Ammonia Production System Using Photoelectrochemically Produced Hydrogen under Concentrated Sunlight Yusuf Bicer* and Ibrahim Dincer Clean Energy Research Laboratory, Faculty of Engineering and Applied Science, University of Ontario Institute of Technology, 2000 Simcoe Street North, Oshawa, Ontario L1H 7K4, Canada ABSTRACT: Intensive fossil fuel usage in ammonia production is considered nonsustainable; hence, alternative ammonia synthesis options are under investigation. In this study, a comprehensive study on environmental impact assessment is performed to investigate the electrochemical synthesis of ammonia at ambient pressure using photoelectrochemically produced hydrogen under concentrated solar light. The photoelectrochemical reactor consists of a membrane electrode assembly with a copper oxide semiconductor on a stainless steel cathode plate. The electrolyte for ammonia synthesis is molten salt containing a eutectic mixture of NaOH and KOH. The electrodes and wires are made of nickel. The life cycle assessment of the concentrated light photoelectrochemical hydrogen production is initially performed and integrated to molten-salt-based electrochemical ammonia synthesis. The material and energy requirements of the life cycle assessment are taken from the experimental data. The results imply that electrochemical ammonia synthesis driven by solar energy can significantly reduce the total environmental impact, corresponding to about 50% of the current steam methane reforming based ammonia production. KEYWORDS: Hydrogen, Electrochemical synthesis, Life cycle assessment, Environmental impact, Energy storage, Haber−Bosch process

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INTRODUCTION

There are multiple pathways under investigation for ammonia synthesis besides the mostly used Haber−Bosch process. Since one of the major problems in the Haber−Bosch process is the high operating pressure and temperature levels, the developing techniques propose low-temperature and low-pressure electrochemical ammonia synthesis. In the electrolytic routes, the required hydrogen can be sourced from natural gas, similar to the Haber−Bosch process, or electrolysis of water, or even decomposition of an organic liquid, such as ethanol and methanol. The greenness level of each method depends on the energy resources. When hydrogen is produced from water electrolysis using a renewable energy source, such as wind or solar, greenhouse gas (GHG) emissions could significantly be reduced during the ammonia synthesis process. Water can also be directly utilized as a source of hydrogen inside the electrolytic cell through its reaction in the electrochemical process. There are four main types of electrolytes1 commonly researched for ammonia production, and they are classified in Figure 1. One of the major developments is the utilization of proton exchange membranes and specific combinations of anode and cathode materials for enhancement of operating conditions. In this respect, a brief review of the electrochemical ammonia synthesis methods is presented herein. Xu et al.2 investigated electrochemical ammonia production under atmospheric pressure levels and lower temperatures, where they have used © 2017 American Chemical Society

Figure 1. Main electrochemical ammonia synthesis electrolyte types.

SmFeCuNi (SFCN) materials for the cathode side and nickeldoped samarium-doped cerium oxide (Ni-SDC) for the anode side. Nafion membrane is used as the electrolyte, where the Received: May 24, 2017 Revised: July 6, 2017 Published: July 26, 2017 8035

DOI: 10.1021/acssuschemeng.7b01638 ACS Sustainable Chem. Eng. 2017, 5, 8035−8043

Research Article

ACS Sustainable Chemistry & Engineering

700 °C with Ru-based catalyst, the conversion rates are observed to be lower compared to nitrogen or steam because of the low conductivity of the working electrode.14 The literature survey shows that the operating temperature and pressure are two critical parameters affecting the overall performance of the ammonia production that can be reduced using electrochemical pathways. Furthermore, the flexibility of using renewable energy sources for distributed ammonia production is one of the key advantages over the Haber−Bosch process, besides the reduced environmental impact. Typically, the electrochemical routes investigated so far require operation at much lower pressures than those used in the Haber−Bosch process, with operating temperatures from near room temperature for liquid and polymer electrolyte systems to between 400 and 800 °C for other electrolytic routes. The low-temperature operation has the potential to decrease material and operating costs and increase the lifetime of the electrochemical reactor by providing high ammonia production rates and high current efficiencies. However, one of the significant advantages of ammonia production by mediumtemperature electrolytic routes, such as use of molten salt, is that such systems can be integrated with excess heat from renewable or nuclear power plants, thus reducing the overall energy input, especially if water is used as the hydrogen source. The literature mainly focused on the formation rates and conversion efficiencies of the electrochemical ammonia synthesis options, whereas the environmental impacts have not yet been assessed. We have previously performed various life cycle assessment studies for Haber−Bosch ammonia production methods using conventional and renewable resources.15−17 In contrast, in this study, we present the life cycle assessment of electrochemical ammonia synthesis that is integrated to photoelectrochemical hydrogen production system. Solar-energy-based hydrogen and ammonia production arises as one of the most sustainable solutions to today’s critical energy, environmental, and sustainability issues. Since solar energy cannot be directly stored or continuously supplied, it is required to convert solar energy to a storable type of energy. Hydrogen and ammonia are significant candidates as a sustainable energy carrier. Specifically, for solar energy storage applications, H2 can act as a short-term storage medium, whereas NH3 can serve as a long-term storage medium that reduces the storage losses significantly. Alternative ammonia production methods are being investigated in which there are fewer environmental emissions and less energy consumption. Electrochemical ammonia synthesis is currently under deep investigation as an alternative pathway. Using renewable energy resources to drive the electrochemical NH3 synthesis, the carbon footprint of the current fossil-fuel-based NH 3 production industry can be lowered significantly. Electrochemical NH3 synthesis routes offer higher integrability to stand alone and distributed NH3 production, which is a carbonfree fuel for various sectors. Assisting electrochemical processes with solar energy will create an environmentally friendly method. The absence of practical solar-energy-based integrated hydrogen and ammonia production systems that are environmentally benign, low cost, efficient, and safe is one of the main complications for the transition to a solar-energy-based economy. The main objective of this study is to investigate the environmental impacts of an electrochemical ammonia synthesis process combining a photoelectrochemical hydrogen production system under concentrated and split solar spectra to

current collector is made of silver−platinum paste. Ammonia with the maximum formation rates of 1.13 × 10−8 mol·cm−2·s−1 was synthesized at 80 °C, corresponding to a current efficiency of about 90%. This study implied that the electrochemical reactions can also be carried out at lower temperatures when suitable membranes are used. When hydrogen is used as a reactant in ammonia synthesis, the source plays an important role in the life cycle. Ammonia is mainly produced from fossil fuels (natural gas and coal). Lan et al.3 reported an artificial ammonia synthesis bypassing N2 separation and H2 production phases. A maximum ammonia production rate of 1.14 × 10−9 mol·cm−2·s−1 was realized when a voltage of 1.6 V was applied. They implied that, in the future, other low-cost ammonia synthesis catalysts, such as Co3Mo3N and Ni2Mo3N41, could be used to exchange Pt for selective ammonia synthesis. Besides solid electrolytes, molten salt electrolytes at elevated temperatures are also preferred in electrochemical ammonia synthesis. Serizawa et al.4 reported conversion rates as high as 70% of Li3N into NH3 using a molten LiCl−KCl−CsCl electrolyte at temperatures between 360 and 390 °C. These conversion rates have been achieved despite the side reactions where some portion of NH3 was dissolved in the form of imide (NH2−) and amide (NH2−) anions, resulting in a lower NH3 yield. The required temperature levels are still high in this method. Licht et al.5 obtained about 35% Faradaic efficiency when they supplied water and air to produce ammonia in molten salt electrolyte. In their further research, Li and Licht6 reported that at 200 mA/cm2 over 90% of applied current generated H2, rather than NH3, since they used water as hydrogen source. In this case, hydrogen was cogenerated but required higher potentials because of the necessary watersplitting voltage. Kim et al.7,8 also performed electrochemical synthesis of ammonia in molten LiCl−KCl−CsCl electrolyte using a mixture of the catalysts nano-Fe2O3 and CoFe2O4. Their maximum formation rate was 3 × 10−10 mol·cm−2·s−1, where they used water as hydrogen source and nitrogen for the reaction. Kyriakou et al.9 recently reported an extensive literature review on low-, medium-, and high-temperature electrochemical NH3 synthesis routes showing that the synthesis rates can reach up to 3.3 × 10−8 mol·cm−2·s−1. Shipman and Symes10 presented the recent developments in electrochemical NH3 production options. They categorized the sources of proton as water, hydrogen, and sacrificial proton donors and concluded that the techniques keeping the temperatures in the range of 100 and 300 °C, such as molten salt, may well be demonstrated to be the most efficient. A number of different systems, based either on proton or mixed proton/oxygen ion conducting solid electrolytes, are undergoing research and development for application in electrochemical ammonia synthesis. The key elements of the solid-state electrochemical system are two porous electrodes divided by a compact solid electrolyte, which permits ion transport of either protons or oxide ions and acts as a barrier to gas diffusion.11,12 Solid-state proton conductors (SSPC) denote a class of ionic solid electrolytes that have the ability to transfer hydrogen ions (H+).13 Along with cathode and ammonia synthesis catalyst, the proton-conducting ceramic membrane is the most important component in these systems. These membranes are required to have substantial proton conductivity at temperatures above 400 °C.1 In a study where researchers used a proton-conducting solid electrolyte at 450− 8036


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Figure 2. Schematic diagram of the experimental setup used for life cycle assessment study.

In this section, we present the materials and energy requirements required for the experimental system. The ammonia electrosynthesis chamber comprises a nickel mesh cathode and a nickel mesh anode immersed in molten hydroxide electrolyte containing 10 g of a suspension of the nano-Fe3O4 contained in an alumina crucible that is sealed to allow a gas inlet at the cathode and a gas outlet from the exit tubes. The reactants, H2 and N2, are bubbled through the mesh over the anode and cathode, respectively. Although there is a stainless steel separator between the anode and cathode sides, the exiting gases can also contain unreacted gases. The combined gas products (H2, N2, and NH3) exit through two exit tubes in the chamber head space. The product gases are bubbled in a dilute H2SO4 solution for NH3 capture. In this respect, first the photoelectrochemical hydrogen production process is built in the life cycle assessment (LCA) software. The material list required for the photoelectrochemical (PEC) reactor design is listed in Table 1. The PEC hydrogen generation system in this work involves mainly a photoelectrochemical cell with a membrane electrode assembly, photovoltaic (PV) module, light source (concentrated sunlight), electricity supply (from photovoltaic module), and optical tools, such as a Fresnel lens and spectrum-splitting mirrors. The PEC system is an integration of a solar

increase the solar spectrum utilization and to make the ammonia production process greener.

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SYSTEM DESCRIPTION In this study, photoelectrochemically generated H2 is directly used in the electrochemical formation of NH3. The electrochemical synthesis of ammonia using photoelectrochemically produced H2 in ambient molten salt including nano-Fe3O4 catalyst is assessed in terms of environmental impact, where N2 is supplied from an air separation plant. The complete schematic diagram of the system is shown in Figure 2. Nitrogen gas sent through the porous nickel cathode is reduced to nitride according to the following equation: N2 + 6e− → 2N3 −

(1)

3−

As it becomes N , the nitride moves to the other electrode, where H2 is being supplied. Hydrogen ions combine with nitrogen ions and form NH3 at the anode electrode, expressed as follows: 2N3 − + 3H 2 → 2NH3 + 6e−

(2)

The overall reaction is

3H 2 + N2 → 2NH3

(3)

The pure alkali hydroxides NaOH and KOH each melt only at temperatures above 300 °C. The individual melting temperatures of NaOH and KOH are 318 and 406 °C, respectively. The NaOH−KOH eutectic is of particular attention and melts at 170 °C. Ammonia synthesis rates increase when the molten hydroxide (NaOH−KOH) electrolyte is mixed with high surface area Fe3O4 to provide iron as a reactive surface and when nitrogen and hydrogen are present in the reactor. Electricity is supplied between two nickel anode and cathode electrodes in the molten salt medium. The mixture is prepared in the beginning by simply adding NaOH and KOH pellets in the reactor. The reactor body is heated up using heating tape positioned around the alumina crucible. After the salts melt, nano-Fe3O4 is added to the electrolyte and the mixture is then stirred.

Table 1. Type and Quantity of the Materials Used in the PEC Reactor

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material

value

unit

high-density polyethylene (HDPE) stainless steel electrodes Nafion membrane copper oxide platinum black washers bolt/nuts acrylic or polycarbonate reactor window plastic piping rubber gasket

2 2 930 2.7 2.7 0.125 0.125 0.4 100 0.2

kg kg cm2 g g kg kg kg g kg


Research Article


the second step, the electrochemical ammonia synthesis process is simulated as shown in Figure 4.

concentrator and photovoltaic (PV) and PEC cells for both hydrogen and electricity production, where the electricity produced by the PV module is used in the PEC cell. The upper wavelength spectrum (above 700 nm) is used for photovoltaics and the lower wavelength spectrum (below 700 nm) is used for PEC cells to increase the solar energy utilization. The cathode plate of the PEC reactor has a copper oxide photosensitive coating, enhancing the hydrogen evolution, as photocathode. This system has been previously tested and reported in our former studies.18,19 The amounts of listed materials are employed in the LCA software. The boundary of the LCA study for PEC hydrogen production is shown in Figure 3. The main energy and material

Figure 4. Boundaries of the conducted LCA for the electrochemical ammonia synthesis process.

The ammonia production reactor consists of the following materials listed in Table 4. The quantities are entered in the LCA software. Table 4. Quantities of the Materials Used in the Ammonia Reactor Figure 3. Boundaries of the conducted LCA for PEC hydrogen production.

material nickel wiring nickel electrodes reactor casing alumina crucible (Al2O3) reactor lids (stainless steel 316 alloy) washers bolt nuts reactor tubes (ceramic, round, single-bore tubes; alumina 99.8%) piping (plastic)

requirements of this PEC hydrogen production system are summarized in Table 2. The required electricity for the electrochemical ammonia and photoelectrochemical hydrogen system is supplied from photovoltaic cells. Table 2. Main Energy and Material Flows in the PEC Hydrogen Production System parameter

value

unit

hydrogen (product) electricity, production from photovoltaic water, deionized solar energy

4.669 × 10−6 0.698 4.2021 × 10−5 9.418 948 327

g J g J

value

unit

2 200 500 2 0.125 0.125 50

m cm2 mL kg kg kg g

2

m

Since the system uses concentrated light with a set of other structures, such as Fresnel lens, support mechanism, dielectric mirrors, and other equipment, the complete setup is also considered in the LCA analysis as listed in Table 3. This step includes only hydrogen production from the PEC system. In this table, the ammonia reactor is not taken into account, since it will be already included in the ammonia production step. In

Nitrogen production is the cryogenic air separation plant considering the plant construction, operation, and maintenance, where the inventory data are taken from the SimaPro database.20 The electricity and material inputs required for the life cycle assessment are derived from the experimental results. The main energy and material requirements of this electrochemical ammonia synthesis which uses PEC hydrogen are summarized in Table 5. Here, the amounts of catalysts and electrolyte are calculated on the basis of service time, since they are not in fact consumed in the reaction. Furthermore, the heat is not taken as input in the LCA analysis, since it is assumed that the synthesis reaction occurs at a constant set temperature from any excess heat source.

Table 3. Materials and Quantities Used in the Integrated System for Concentrated Light PEC Hydrogen Production

Table 5. Main Energy and Material Flows in Electrochemical Ammonia Synthesis Using PEC Hydrogen

material

value

unit

parameter

value

unit

plastic pipes for gases photovoltaic cell, multi-Si Fresnel lens (polycarbonate) dielectric mirrors (six in total) (borosilicate glass) PEC hydrogen production reactor support structure (wood) support mechanism (Al alloy)

300 625 1 0.5 1 50 000 2

g cm2 kg kg item cm3 kg

ammonia (product) hydrogen from PEC integrated system nitrogen, gas, at plant, US grid iron oxide (catalyst) sodium hydroxide (electrolyte) potassium hydroxide (electrolyte) electricity production from PV

1.875 0.331 1.543 2.7778 × 10−9 2.7778 × 10−9 2.7778 × 10−9 105

mg mg mg g g g J

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ACS Sustainable Chemistry & Engineering LCA is an instrument that helps engineers, scientists, and policy-makers to assess and compare energy and material use, emissions and wastes, and environmental influences for various products or processes. The following are some of the assumptions made for the LCA analysis: (1) The nitrogen is considered as gas from a cryogenic air separation unit. (2) The inputs used in calculations are feedstock, energy or electricity, and emissions. (3) The processes for ammonia production contains production of hydrogen and nitrogen separately. (4) The stoichiometric mass balance is used to identify the amount of hydrogen and nitrogen required for unit ammonia production considering multiple passes of the unreacted gases. (5) The functional unit is 1 kg of ammonia production and the process is from cradle to gate. The CML 2001 method is a technique developed by a group of scientists under the lead of the Center of Environmental Science (CML) of Leiden University counting a set of impact categories and characterization methods for the impact assessment step in 2001.21 The CML 2001 method presents the results based on environmental impact categories such as global warming, human toxicity, and abiotic depletion. In this method, the results are mostly presented per equivalent substance amount, such as kg CO2 equivalent and kg SO2 equivalent. The following environmental impact categories are considered for the assessment. (1) Depletion of abiotic resources: Abiotic resources are natural resources, including energy resources, such as natural gas and crude oil, which are considered as nonliving. The unit is expressed as kg Sb (antimony) equiv. (2) Human toxicity: It is computed by the effects of toxic substances for a 500-year time horizon. 1,4dichlorobenzene equivalents/kg emissions is used to express each toxic substance. (3) Global warming: This category is related to greenhouse gas emissions and climate change. The unit kg CO2 equiv is used to express the global warming potential for a time horizon of 500 years (GWP500).

Table 7. Shares of Different Subprocesses in Abiotic Depletion Category for Concentrated Light PEC-Based Electrochemical Ammonia Synthesis inflows

flow

unit

total electricity, production photovoltaic, multi-Si hydrogen, PEC integrated system, concentrated light nitrogen, gas, at plant potassium hydroxide sodium hydroxide iron oxide

100 51.7 24.5 23.8 2.56 × 10−4 1.41 × 10−4 1.83 × 10−5

% % % % % % %

Table 8. Shares of Different Subprocesses in the Global Warming Category for Concentrated Light PEC-Based Electrochemical Ammonia Synthesis inflows

flow

unit

total electricity, production photovoltaic, multi-Si hydrogen, PEC integrated system, concentrated light nitrogen, gas, at plant potassium hydroxide iron oxide sodium hydroxide

100 51.9 24.5 23.6 2.48 × 10−4 1.43 × 10−4 1.43 × 10−4

% % % % % % %

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RESULTS AND DISCUSSION The LCA results are obtained using SimaPro LCA software. PEC-based electrochemical ammonia production, which is the Table 6. Shares of Different Subprocesses in Human Toxicity Category for Concentrated Light PEC-Based Electrochemical Ammonia Synthesis inflows

flow

unit

total electricity, production photovoltaic, multi-Si hydrogen, PEC integrated system, concentrated light nitrogen, gas, at plant potassium hydroxide sodium hydroxide iron oxide

100 63.2 29.9 6.9 1.02 × 10−4 6.96 × 10−5 7.06 × 10−9

% % % % % % %

Figure 5. Share of toxic substances for concentrated light PEC-based electrochemical ammonia synthesis.

in detail to reveal the contribution of various subprocesses. When reporting the contribution of different processes to the overall impact category, a 1% cutoff is applied. There are mainly three processes in the PEC-based ammonia synthesis, namely, hydrogen production from a photoelectrochemical reactor, nitrogen production from an air separation, and electricity production from PV cells to energize the process. The main contributor in all categories is the electricity production from PV cell, as shown in Tables 6−8. Approximately 7% of total human toxicity is caused by the nitrogen production process, whereas about 30% is due to hydrogen production from the PEC system, as listed in Table 6.

method experimentally tested, uses photovoltaic cells for the electricity requirements of the system. PV electrolysis for ammonia production has higher environmental effects mainly because of the production phase of the PV cells and the aluminum support mechanism. Therefore, for PEC-based electrochemical ammonia synthesis option, the environmental effects are higher than some renewable routes, such as hydropower, municipal waste, and wind. The LCA results obtained for the PEC-based electrochemical ammonia production method using concentrated light are given 8039


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Figure 6. Contribution of various subprocesses to human toxicity potential of concentrated light PEC-based electrochemical ammonia synthesis.

Figure 9. Share of greenhouse gas emissions for concentrated light PEC-based electrochemical ammonia synthesis.

support structures, as shown in Figure 6. Polycyclic aromatic hydrocarbons are released due to nitrogen production from the air separation plant since an electricity grid mix is used. The total human toxicity value of the system is found to be 0.949 kg 1,4-DB equiv per kg of ammonia. Table 7 shows the shares of main processes contributing to abiotic depletion potential. Almost half of the total abiotic depletion is because of PV electricity production, whereas about 25% is due to hydrogen production from the PEC reactor. The molten salt electrolyte and reaction catalyst have reasonably small shares in total impact. Furthermore, as shown in Figure 7, coal and natural gas are two main substances depleted in this method due to the high electricity consumption in the factories manufacturing the PV cell and the aluminum needed for the support mechanism. Crude oil and brown coal have shares of 14% and 7%, respectively, as shown in Figure 8. The total abiotic depletion potential of the system is calculated to be 0.008 22 kg Sb equiv per kg of ammonia. The global warming potential of PV electricity production is responsible for almost 50% of the total GHG emissions, where almost 76% of PV electricity is because of the PV cell production process in the factory. The shares of the main processes for global warming potential are tabulated in Table 8. The PEC electrochemical route has considerably lower impact than any other fossil-fuel-based ammonia production options.

Figure 7. Share of depleting abiotic sources for concentrated light PEC-based electrochemical ammonia synthesis.

Figure 8. Contribution of various subprocesses to abiotic depletion potential of concentrated light PEC-based electrochemical ammonia synthesis.

The electricity production from PV is mainly responsible for the remaining part. There are numerous substances causing toxicity for human health, such as arsenic and nickel, as shown in Figure 5. Arsenic and polycyclic aromatic hydrocarbons are the two fundamental toxic substances (about 64% in total) released to the environment in this process. These are mainly caused by copper and aluminum production processes for PV and the 8040


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Figure 10. Contribution of various subprocesses to the global warming potential of concentrated light PEC-based electrochemical ammonia synthesis.

Table 9. Uncertainty Analyses Results of Concentrated Light PEC-Based Electrochemical Ammonia Production impact category

unit

mean

median

standard deviation

% coefficient of variation

standard error of mean

abiotic depletion global warming 500a human toxicity 500a

kg Sb equiv kg CO2 equiv kg 1,4-DB equiv

0.0082 1.09 0.949

0.007 46 1.07 0.884

0.003 61 0.187 0.302

43.90 17.10 31.80

0.007 73 0.003 01 0.005 6

Figure 11. Probability distribution of the global warming potential for concentrated light PEC-based electrochemical ammonia production.

Figure 13. Probability distribution of the abiotic depletion potential for concentrated light PEC-based electrochemical ammonia production.

Figure 12. Probability distribution of the human toxicity potential for concentrated light PEC-based electrochemical ammonia production.

Figure 14. Uncertainty ranges of the selected impact categories for concentrated light PEC-based electrochemical ammonia production.

Overall, concentrated light PEC-based electrochemical synthesis yields about 1.09 kg CO2 equiv/kg ammonia. The global warming category includes all greenhouse gas emissions; however, CO2 is the main gas emitted to the environment, corresponding to about 93% of the total in this method, as shown in Figure 9. Sulfur hexafluoride (3%) and

methane (2%) are the other gases contributing to total GHG emission. Sulfur hexafluoride emission is mainly due to magnesium production in the plant required for PV cell production. As shown in Figure 10, electricity production in a cogeneration plant and hard coal burned in a power plant are mainly because of the silicon production required for PV cells. 8041


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ACS Sustainable Chemistry & Engineering LCA Uncertainty Analyses Results. Defining the uncertainties within the LCA study confirms the accuracy of the results. The uncertainty analyses are performed with SimaPro software using the Monte Carlo technique. The presented results here are for the concentrated light PEC-based electrochemical ammonia production method using the experimental system defined in the system description section. The confidence interval for the results is 95%. The number of runs performed for the results is 3224. The uncertainty analyses results are shown in Table 9 for the selected environmental impact categories. The mean of the global warming value is 1.09 kg CO2 equiv and the standard error of the mean is 0.003 01 kg CO2 equiv, corresponding to a 17.1% coefficient of variation, which is the lowest among other categories. The highest coefficient of variance is found to be 43.9% for the abiotic depletion category. The probability distributions of the selected environmental impact categories are shown in Figures 11−13. Figure 14 shows the comparison of uncertainty ranges for different selected categories. This method is still in the early investigation phase, resulting in less reliable data for the LCA inventory step, especially for the energy requirement. However, the results of this study imply a good indicator regarding the reduction of the total environmental impact in comparison with that of the conventional Haber−Bosch process. Taking into account the uncertainties of the LCA results for PEC-based electrochemical ammonia production method, this process can be more environmentally benign than other renewable routes. As the average GHG emission from commercial ammonia plants range between 2 and 2.5 kg CO2 equiv per kg ammonia, PEC-based electrochemical ammonia production can reduce the GHG emissions down to about 1 kg CO2 equiv. The main reason for having higher environmental effects in the PEC electrochemical route than some renewable routes is that this process still consumes higher energy because of operating conditions and the specific materials used. Because, the pressure is the ambient pressure, the conversion rate is lower. However, the technology and materials are improving quite quickly, which will decrease the energy required for electrochemical ammonia synthesis and eventually yield less environmental impact.

which are considerably lower than those values of the conventional Haber−Bosch process.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Yusuf Bicer: 0000-0003-4753-7764 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge the support provided by the Natural Sciences and Engineering Research Council of Canada.

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NOMENCLATURE CML Center of Environmental Science of Leiden University GHG greenhouse gas HDPE high-density polyethylene LCA life cycle assessment PEC photoelectrochemical PV photovoltaic SDC samarium-doped cerium oxide SFCN SmFeCuNi SSPC solid-state proton conductors

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REFERENCES

(1) Giddey, S.; Badwal, S. P. S.; Kulkarni, A. Review of electrochemical ammonia production technologies and materials. Int. J. Hydrogen Energy 2013, 38, 14576−14594. (2) Xu, G.; Liu, R.; Wang, J. Electrochemical synthesis of ammonia using a cell with a Nafion membrane and SmFe0.7Cu0.3−x Ni x O3 (x = 0−0.3) cathode at atmospheric pressure and lower temperature. Sci. China, Ser. B: Chem. 2009, 52, 1171−1175. (3) Lan, R.; Irvine, J. T. S.; Tao, S. Synthesis of ammonia directly from air and water at ambient temperature and pressure. Sci. Rep. 2013, 3, 1145. (4) Serizawa, N.; et al. Dissolution Behavior of Ammonia Electrosynthesized in Molten LiCl−KCl−CsCl System. J. Electrochem. Soc. 2012, 159, E87−E91. (5) Licht, S.; et al. Ammonia synthesis by N2 and steam electrolysis in molten hydroxide suspensions of nanoscale Fe2O3. Science (Washington, DC, U. S.) 2014, 345, 637−640. (6) 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, 10042−10044. (7) Kim, K.; Yoo, C.-Y.; Kim, J.-N.; Yoon, H. C.; Han, J.-I. Electrochemical synthesis of ammonia from water and nitrogen catalyzed by nano-Fe2O3 and CoFe2O4 suspended in a molten LiClKCl-CsCl electrolyte. Korean J. Chem. Eng. 2016, 33, 1777−1780. (8) Kim, K.; Kim, J.-N.; Yoon, H. C.; Han, J.-I. Effect of electrode material on the electrochemical reduction of nitrogen in a molten LiCl−KCl−CsCl system. Int. J. Hydrogen Energy 2015, 40, 5578−5582. (9) Kyriakou, V.; Garagounis, I.; Vasileiou, E.; Vourros, A.; Stoukides, M. Progress in the Electrochemical Synthesis of Ammonia. Catal. Today 2017, 286, 2. (10) Shipman, M. A.; Symes, M. D. Recent progress towards the electrosynthesis of ammonia from sustainable resources. Catal. Today 2017, 286, 57. (11) Di, J.; et al. Samarium doped ceria−(Li/Na)2CO3 composite electrolyte and its electrochemical properties in low temperature solid oxide fuel cell. J. Power Sources 2010, 195, 4695−4699.

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CONCLUSIONS Solar energy is abundant and clean and can be used in ammonia synthesis applications. The electrochemical ammonia synthesis brings the flexibility of utilizing renewable electricity, which is an alternative to conventional energy-intensive centralized ammonia production plants. This study performs a life cycle assessment for a molten-salt-based electrochemical ammonia production system where the required hydrogen is sourced from a concentrated-light-based photoelectrochemical process. Most of the inventory data are taken from the conducted experimental results, in which ammonia is electrochemically generated at ambient pressure using H2 and N2 in a molten hydroxide medium with nano-Fe3O4 catalyst. The results show that 63.2% of human toxicity, 51.7% of abiotic depletion, and 52% of the global warming potential of the system is caused by the electricity production from photovoltaics. The total abiotic depletion potential of the system is found to be 0.00822 kg Sb equiv/kg ammonia, the global warming potential is calculated as 1.09 kg CO2 equiv/kg ammonia, and the human toxicity potential is obtained as 0.949 kg 1,4-DB equiv/kg ammonia, 8042


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ACS Sustainable Chemistry & Engineering (12) Amar, I. A.; Lan, R.; Petit, C. T. G.; Arrighi, V.; Tao, S. Electrochemical synthesis of ammonia based on a carbonate-oxide composite electrolyte. Solid State Ionics 2011, 182, 133−138. (13) Norby, T. Solid-state protonic conductors: principles, properties, progress and prospects. Solid State Ionics 1999, 125, 1−11. (14) Skodra, A.; Ouzounidou, M.; Stoukides, M. NH3 decomposition in a single-chamber proton conducting cell. Solid State Ionics 2006, 177, 2217−2220. (15) Bicer, Y.; Dincer, I.; Zamfirescu, C.; Vezina, G.; Raso, F. Comparative life cycle assessment of various ammonia production methods. J. Cleaner Prod. 2016, 135, 1379−1395. (16) Bicer, Y.; Dincer, I. Life cycle assessment of nuclear-based hydrogen and ammonia production options: A comparative evaluation. Int. J. Hydrogen Energy 2017, DOI: 10.1016/j.ijhydene.2017.02.002. (17) Bicer, Y.; Dincer, I.; Vezina, G.; Raso, F. Impact Assessment and Environmental Evaluation of Various Ammonia Production Processes. Environ. Manage. 2017, 59, 842−855. (18) Bicer, Y.; Dincer, I. Performance assessment of electrochemical ammonia synthesis using photoelectrochemically produced hydrogen. Int. J. Energy Res. 2017, DOI: 10.1002/er.3756. (19) Bicer, Y.; Dincer, I. Chem. Eng. Process. 2017, 117, 141. (20) Ecoinvent v3|High-Quality LCI Database Integrated in SimaPro, 2017. Available at https://simapro.com/databases/ecoinvent/. (accessed March 8, 2017). (21) SimaPro Life Cycle Analysis, version 7.2; 2017.

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Assessment of a Sustainable Electrochemical Ammonia Production

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