Ammonia for hydrogen storage; A review of catalytic ammonia decomposition and hydrogen separation and purification

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 4 ( 2 0 1 9 ) 3 5 8 0 e3 5 9 3

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Review Article

Ammonia for hydrogen storage; A review of catalytic ammonia decomposition and hydrogen separation and purification Krystina E. Lamb a,*, Michael D. Dolan a, Danielle F. Kennedy b a

CSIRO, Energy, Queensland Centre for Advanced Technologies, 1 Technology Court, Pullenvale, QLD 4069, Australia b CSIRO, Manufacturing, Private Bag 10, Clayton South, VIC 3169, Australia

article info

abstract

Article history:

Ammonia is of interest as a hydrogen storage and transport medium because it enables

Received 29 March 2018

liquid-phase hydrogen storage under mild conditions. Although ammonia can be used

Received in revised form

directly for energy applications, its use in conventional fuel cell electric vehicles necessi-

19 November 2018

tates decomposition into nitrogen and hydrogen, and the purification of the hydrogen to

Accepted 1 December 2018

the composition required for commercial proton exchange membrane fuel cells. This

Available online 11 January 2019

article provides a review of the material and process considerations for catalytic ammonia decomposition and shows that Ru-based catalysts on conductive support materials are

Keywords:

active at < 500
C, but further understanding around lifetimes and deactivation conditions

Ammonia decomposition

is required. This review then explores materials and technologies for hydrogen purification

Catalysis

from decomposed ammonia gas streams, and our experiments show that defect-free

Hydrogen separation

dense-metal membranes are uninhibited by ammonia and can achieve the required

Membrane

product purity. © 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

Contents Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3581 Overview of ammonia for hydrogen storage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3581 Key challenges . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3583 Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3583 Review method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3583 Calculations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3583 Membrane testing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3583 Review of concepts; ammonia synthesis and decomposition fundamentals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3583 Current knowledge . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3584 Primary catalyst component . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3584 * Corresponding author. E-mail address: [email protected] (K.E. Lamb). https://doi.org/10.1016/j.ijhydene.2018.12.024 0360-3199/© 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

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Trends in promoter activity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3585 Trends in support materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3585 Catalyst deactivation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3585 Summary of literature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3585 Considerations for future work . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3588 Testing vs. operating conditions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3588 Suggested testing conditions for commercial applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3588 Catalyst and catalytic process optimization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3589 Purification and separation of product gases from ammonia decomposition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3589 Ammonia absorption . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3589 Absorption processes for separating ammonia, nitrogen, and hydrogen . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3589 Hydrogen permeable membranes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3590 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3590 Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3591 Acronyms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3591 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3591

Introduction Ammonia (NH3) is an excellent candidate for hydrogen (H2) storage and transport as it enables liquid-phase storage under mild conditions at higher volumetric hydrogen density than liquid H2. Because NH3 is liquid at lower pressures and higher temperature than H2, liquefaction is less energy intensive, and the storage and transport vessels are smaller and lighter. Around 150 Mt of NH3 is produced a year and transported around the world, using existing marine, road, rail, and pipeline networks. While NH3 can be used directly in combustion engines or high-temperature electrochemical systems, it cannot be used in low-temperature polymer-based proton exchange membrane (PEM) fuel cells (FCs) like those found in commercially available vehicles and electricity generators. Its use as a medium for transporting H2 for PEM FCs necessitates decomposition into nitrogen (N2) and H2, and separation and purification of H2 to remove contaminants that can damage FC components or cause difficulty with compression. There are five steps in the process for producing energy from H2 from stored NH3, which are outlined in Fig. 1. Technologies for the storage and transport of NH3 are commercial, as are those for the storage, compression and use of H2, however, the technologies and processes for NH3 decomposition and H2 separation and purification for this application are less developed. This is the reason for this review, which focuses on NH3 decomposition via thermo-catalysis, and H2 separation and purification using absorption, adsorption and membrane technologies. This review article provides an overview of NH3 for H2 storage, introduces the fundamental concepts around NH3 catalysis, and shows that there are opportunities to investigate catalyst properties that are relevant to commercialisation such as lifetime, non-ideal condition behavior, and deactivation behaviours. This article then presents a brief review of separation and purification technologies and shows that using metal membranes enables separation and purification of H2

from decomposed NH3 gas streams that meet the composition requirements for PEM FCs in one step.

Overview of ammonia for hydrogen storage Hydrogen energy technologies are rapidly becoming comparable in cost and performance to battery and hydrocarbon fuelled technologies, and have the potential to disrupt the energy markets [1]. However, the uptake of these technologies has been relatively slow, likely due to the high up-front costs of the new infrastructure for accessing H2 supplies, storing H2 in the medium and long-term, and transporting H2 longdistances. The main arguments for the development and use of H2 energy technologies are for the storage of renewable energy, for the long-distance transport of renewable energy, and for contributing to de-carbonising the transport or manufacturing industries. H2 has several advantages for these applications. Firstly, it can draw on the existing infrastructure for transporting gases and liquids around the world, though this would require adjustments for large quantities of H2 [2]. Secondly, the economics of buying and selling H2 fits well within the current paradigm of commodity trading [3]. Thirdly, it can provide more secure baseload power at the scale required by storing excess renewable energy as H2. Finally, the multiple sources of H2, from biomass to electrolysis using renewable energy to coal gasification, mean that the market is likely to be very open and competitive. These advantages contributed to the aim of the Government of Japan to be the world's first H2-based energy economy [4], with the intention to showcase the technologies at the 2020 Olympics. The Government of Japan is providing financial support for businesses to develop these technologies for commercial production. However, one of the challenges for the use of H2 energy technologies is in obtaining sufficient and reliable supplies of H2, which would be exacerbated by a significant push for that H2 to be from renewable sources. Part of this challenge is the inherently inefficient and challenging process of storing bulk H2 as a liquid, requiring insulated

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Fig. 1 e The five steps in the process for producing energy from H2 from stored NH3, and the commercial availability of the technologies.

vessels which significantly decrease the actual energy density. There is also a significant energy penalty due to its low vaporization temperature of 253
C [5] and boil-off resulting in increasing loss with increasing storage time [2]. These inefficiencies and losses are significant issues for distributing renewable H2, where the places it can be produced most efficiently are often geographically mismatched to where it will be used. This dislocation is highlighted by the distance between the energy markets in Japan and the abundant solar energy potential for producing H2 for energy storage in Australia's Pilbara region [6]. To reduce these inefficiencies and loses in transport and storage, H2 can be stored chemically in solid hydrides (chemisorption), hydrocarbons (solid, liquid or gas), or carbonfree NH3. Table 1 shows the relative power and volume densities of different H-containing chemicals. NH3 can be liquefied easily at 33
C at 1 bara or 20
C at 7 bara, its production is relatively efficient at around 70% (7.9 MWh to produce 5.6 MWh of chemical energy) [7], and the world already produces, transports and uses ~ 150 Mt a year [8], with installed capacity soon reaching 250 Mt [9]. Both NH3 and LNG have high storage densities of H2, and both are routinely transported around the world. The advantages of using NH3 for H2 storage are the absence of carbon, high production efficiency, and potential for renewable production using current facilities with different feedstocks. The absence of carbon in NH3 eliminates one of the pollutive aspects of energy production and reduces the potential for damage to polymer-based PEM FCs from CO or CO2 poisoning. As NH3 can also damage PEM FC components [10], separation and purification of the H2 for use in these systems is still of paramount importance. LNG is mostly methane (CH4), which must be decomposed into either CO2þ2H2 or Cþ2H2, creating by-products that should be managed sustainably. While CH4 and other hydrocarbons can have renewable sources, the authors direct the reader to a

recent article exploring the factors influencing biomass availability for energy [12]. H2 production from non-renewable sources is well established, though into the future H2 from renewable sources will likely become more common, see Refs. [13,14] for reviews on renewable H2 production. Regardless of the source of the H2 feedstock, after the NH3 is manufactured and transported to or near the point of use, it requires decomposing into N2 and H2 then separation and purification of the product H2 to meet the composition requirements set out by the International Organization for Standardization (ISO) for fuel cells; ISO 14687e2:2012 and 14687e3:2014 [10,15]. The lack of byproducts and renewable potential of NH3 are the reasons for the interest in using NH3 for H2 storage, for energy applications and the reasons for this review article exploring the challenges for H2 to be commercially produced from stored NH3 for use in diverse energy applications.

Table 1 e Power densities per kilogram and liter of various H2 carriers in liquid form. The boiling point at standard pressure of each is in brackets. Data sourced from Ref. [11]. Fuel (boiling point) H2 (-253
C) Methanol Ethanol NH3 (-33
C) Diesel LPG (propane) (-42
C) LNG (methane) (-160
C)

Power density (kWh kg1) (liq)

Density (g l1)

Energy density (kWh l1) (liq)

33.3 5.6 7.6 5.2 12.4 14.8

71 791 780 674 745 560

2.4 4.4 5.9 3.5 9.3 8.3

12.5e14.0

450

5.6e6.3

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Key challenges The key challenges in using NH3 as a H2 storage intermediate are in making these two processes, the decomposition of NH3 and subsequent separation and purification of the H2 product, energy efficient, reliable and scalable. To determine what the barriers are to the commercialisation of these processes, this article explores the state of the literature, and starts with an overview of NH3 decomposition via thermo-catalysts, then discusses H2 separating and purification technologies. The NH3 decomposition section contains a discussion of the underlying kinetics of the reversible NH3 synthesis/decomposition process over heterogeneous thermo-catalysts. Some of the issues inherent in comparing catalysts are discussed, then the literature is reviewed and summarised in a table. Some engineering considerations are presented, as well as some suggestions for future catalyst researchers in order to make direct comparisons of catalysts easier and enable a broader audience to engage with this area of research. The H2 separation section consists of a discussion of the various technologies available and commercialisation considerations, and we present the results of a small study of our metal membrane separation technology that produces high purity H2 in a single step from decomposed NH3.

Methods Review method A systematic approach and narrative style were used for this review, with keyword searches using Scopus and Google scholar conducted in 2018. Keyword searches used were; ammonia decomposition, hydrogen ammonia, ammonia hydrogen fuel cell, ammonia decomposition catalyst, hydrogen separation ammonia, and membrane reactor hydrogen ammonia. For the introduction of fundamental concepts and understandings around catalyst and catalyst design, only seminal papers have been included. For the comparison of NH3 decomposition catalysts, catalysts were included if they were tested under 100% NH3 gas conditions (though a couple of exceptions were made for novel materials), in the range of 600e900 K, at 1 atm (or assumed to be when no pressure was quoted), and where turn-overfrequency (TOF) in s1 or conversion in % were quoted. Review articles were excluded, except where new data was presented. All articles included were published in English. The bulk of literature on NH3 decomposition focuses on ruthenium (Ru), which has experimentally and theoretically been identified as the most active single-metal catalyst, hence the majority of the literature cited here is on Ru-based catalysts.

Calculations The reversible reaction of NH3 synthesis and decomposition follows the equation; 1 3 NH3 # N2 þ H2 (1) 2 2 Catalyst activities in this paper are quoted in three ways; mol NH3 m2 hr1, TOF in s1, and conversion in %. Mol NH3

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m2 hr1 is calculated from the total surface area of the catalyst, usually measured using adsorption techniques and a BET or similar calculation method, and the moles of NH3 converted after passing through or over the catalyst (Eq. (2)). TOF is calculated by measuring the number of active sites on a material using a chemisorption process, making some assumptions about those active sites, and the number of NH3 molecules decomposed over time is divided by the number of active sites to give a number of interactions per units of time (Eq. (3)). Conversion is the NH3 outlet concentration divided by the inlet concentration multiplied by 100% (Eq. (4)). Catalyst activity m2 ¼

mol NH3 decomposed g1 catalyst m2 g1 catalyst (2)

TOF ¼

number of NH3 molecules decomposed g1 catalyst s1 number of active sites g1 catalyst (3)

Conversion ¼

NH3 outlet concentration  100% NH3 inlet concentration

(4)

Membrane testing One H2-selective membrane was prepared by coating a 99.9% vanadium tube with inner and outer palladium catalyst layers of 0.5 mm. Membrane dimensions were 70 mm long, 9.52 mm outer diameter and 250 mm wall thickness. The manufacturing and sealing processes were reported previously [16]. NH3, N2, and H2 gas flows were controlled using Bronkhorst IN-FLOW analog mass flow controllers (MFC), calibrated using a 30e3000 mln min1 volumetric flow meter (Mesa Labs DryCal Defender 510®). The H2 permeate gas was measured using a Mesa Labs DryCal Defender 510®. The H2 flux (mln cm2 min1) was measured at a fixed membrane temperature of 300
C, and at several simulated decomposed NH3 feeds.

Review of concepts; ammonia synthesis and decomposition fundamentals The patent that Fritz Haber submitted for NH3 synthesis in 1908 is the foundation of the Haber-Bosch process of today [17]. In this process, NH3 is produced catalytically in a reversible reaction, Eq. (1) (See Refs. [18,19] for equilibrium constants of NH3 under various temperatures and pressures), and until recently, NH3 decomposition was primarily of academic interest, hence, much of the research on NH3 decomposition has focused on iterative development of ‘highly active’ catalysts and novel materials [3,20,21]. Fundamental studies have shown that the same principles and kinetics apply to both synthesis and decomposition, where the overall DH for the conversion of the N2 and H2 gases into NH3 is 46 kJ mol1. This DH is the total energy change and doesn't represent the actual multi-step process of the catalytic reaction. It was found experimentally that NH3 synthesis and decomposition rates (in most heterogeneous catalysts) follow the Sabatier rule [22], which states that the chemisorption energy of the precursors and products onto the catalyst surface must be neither too strong, nor too weak, but within a

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‘just right’ range. Principally, this rule shows that the activity of a catalyst is a compromise between sufficient adsorption energy to ensure some surface coverage of the reactants, which is also low enough to leave free sites for reactions to occur [23]. While the Sabatier rule applies to heterogeneous catalysts, the fact that they are heterogeneous makes measuring the fundamental activity and comparing these catalysts difficult, as each material can have very different numbers and types of active sites. The TOF of a catalyst is underpinned by an understanding of the surface processes and energies by determining which part of the material is active and is defined as the number of interactions per unit of time for each active site (Eq. (3)). Presenting data in this way should enable the elimination of factors such as catalyst loading, surface area, and flow rate on the final values, enabling comparison between different catalysts [24,25] . As the activity of a heterogeneous catalyst is principally due to interactions with specific sites on the catalyst that have adsorption energies in the ‘just right’ range, if these chemisorption energies are plotted against the TOF, there is a volcano-shaped relationship [25]. The calculation of these chemisorption energies are possible due to the linear relationship between the N2 activation energy and the binding energy of atomic N to the surface, known as BrønstedeEvansePolanyi relationship [26,27]. Recently, computational chemistry studies confirmed this linear relationship on a range of transition metals for NH3 synthesis [23]. Because NH3 decomposition has been found to follow the same relationship, catalyst activity or TOF can be predicted based on the N2 adsorption/desorption energy of active sites [21,28]. Studies have shown that in high NH3 conditions, Ru has the highest activity for NH3 decomposition of the single-metal catalysts. There is some concern in the literature that Ru-based catalysts are too expensive for commercial applications and research efforts have been to reduce or eliminate the Ru in a catalyst while maximising the number and activity of the relevant active sites. This can be done by modifying the primary component of the catalyst and using promoter and support materials to increase activity and the number of active sites. The next section outlines the current literature and discusses trends in primary components, promoters and support materials, as well as conflicting results, gaps in understanding and some considerations for future research.

Current knowledge While a broad range of materials has been tested for NH3 decomposition under various conditions, this review is limited to gas phase decomposition, mainly under 100% NH3 testing conditions. The literature is in agreement that Ru is the most active single-metal NH3 decomposition catalyst, though the exact effectiveness of other metals is not as apparent. Pure Ru [29], VC [30], MoC [31], TiOxNy [32], Ni-Pt supported on Al2O3, NiMoNx on a-Al2O3 [33], and supported Ru [34e41], Rh, Ir, Pt, Pd [41], Ni [35,42e45], and Fe [46e49] metals on various substrates including carbon nanotubes (CNT), activated carbon (AC), Al2O3, MgO, ZrO2, SiO2 and TiO2 have been shown to

be active for NH3 decomposition [50]. LiNHx and NaNHx are also active for NH3 decomposition [51e53].

Primary catalyst component A study that examined the TOF of 13 different single-metal catalysts found that the activity varied in the order of Ru > Ni > Rh > Co > Ir > Fe y Pt > Cr > Pd > Cu y Te, Se, Pb, supported on Al2O3 [54]. A later study reported the order of activity as Ru > Rh y Ni > Pt y Pd > Fe supported on CNT [41]. Another study of various metals, using a mixed gas in a ratio of 60:20:20 of H2:N2:NH3, determined the order of activity to be Ru > Co > Ni > Fe > Cu [21]. Here we note that order of Pt, Pd, Ni, Co, and Fe are different between these studies, and this may be due in part to the variation in NH3 concentration and different support materials used which can have a significant effect on the TOF of metals [55,56]. Some alternative materials to Ru-based catalysts have been tested; for example, experiments showed that VC [30] and MoC materials [31] were active for NH3 decomposition. It has also been suggested that surface modifications, the addition of promoters, and alloying techniques can be used to enhance inactive metals into the optimal range of N2 adsorption energy for NH3 decomposition at the required conditions [57]. A linear combination of adsorption energies has been demonstrated to be a good approximation for the catalytic activity for bimetallic compounds [21], and this has led to the development of bimetallic catalysts, such as Ni-Pt [58] and NiMo [33]. In addition, MoNx/a-Al2O3 and NiMoNx/aAl2O3 were found to have NH3 conversation rates of 75.2 and 79.8% of the thermodynamic equilibrium at 600
C at a gas hourly space velocity (GHSV) of 3600 hr1 [33], compared to the commercial NiO/MgO catalyst which converted 69.9% of the NH3 at the same temperature at half of the previous GHSV [33]. These catalysts were nitrided in NH3 under high temperature, which means the catalyst was self-activating. The effect of support material in this study was found to be negligible. The linear combination approach to identify potential bimetallic materials has been used to develop both NH3 synthesis and decomposition catalysts when the compounds formed a homogenous alloy or an advantageous structure. Using the linear combination approach, Co3Mo3N was predicted to be active for NH3 decomposition and was found to be highly active in conditions of 20% NH3 in a 3:1 ratio of H2:N2 [21]. While these studies were able to use the simple linear combination approach, using first principles to calculate the adsorption energies of more complex structures such as surface or subsurface monolayers of various metals showed that the surface activity could be tuned to the ideal N* binding energy for the required application [58]. Despite the uncertainty around the exact order of activity of Pt, Pd, Ni, Co and Fe, all studies have shown that Ru is the most active single-metal catalyst. Studies of bimetallic materials also show promising results and have the potential to be as active as or more so than Ru-based materials. However, bimetallic catalyst design is complicated by the different properties that materials have, which may not always have linear or predictable responses. Catalysts designed in this rational approach show promise, however, as the area is still

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 4 ( 2 0 1 9 ) 3 5 8 0 e3 5 9 3

new, few studies show the stability or repeatability of the catalysts between different labs and production techniques.

Trends in promoter activity For Ru supported on high surface area CNTs, promoting compounds increase the activity in the order of KeRu > NaeRu > LieRu > CeeRu > BaeRu > LaeRu > CaeRu > Ru [59]. Further, various K-based compounds were active in the order of KNO3 > KOH > K2CO3 > KF > KCl > KBr > K2SO4 > K3PO4 [59]. Another study showed the most active Ru-based catalyst was promoted with Ba on high surface area Al2O3 and was able to produce sufficient H2 for a 1 W fuel cell in a reactor volume of only 20 mL at 365
C [60]. These results have been explained using an electronic promotion theory [61], where the catalytic activity of Ru is increased because the promotor donated electrons to the support material surface, which enabled charge balancing of the intermediary steps of NH3 decomposition [41]. A subsequent DFT study reinforced the theory that conductive support materials and promoters with low ionization energy enabled intermediate step stabilization [58]. Another group studied Cs and K as Ru-promoters and found Cs to be a more active promoter, most likely due to it having lower ionization energy and electron affinity than K [62]. However, a different study showed that the activity of Ru on Al2O3 could be enhanced significantly through a promotion with LiOH, where KOH and CsOH were less active then LiOH at a ratio of MOH/Al of 1.7, where M ¼ Li, K, or Cs [63]. The authors found the formation of LiAlO2 at moderately low temperatures (200
C), and as another study found that NaNHx and LiNHx were active for NH3 decomposition, with the activity decreasing down the periodic table [51], and it is possible that the unusual results from study [63] may have been due to conversion of the LiAlO2 to make AlOxLiNHy complexes in situ. From this mixture of results, we can see that the activity of a promoted catalyst is not as simple as volcano curves and linear combinations, and are difficult to predict, with differences in the activities between the studies.

Trends in support materials As mentioned in the previous section, the support material can affect the TOF. It seems that electronically conductive support materials with basic surface groups are most active for Ru-based catalysts [43,44,50,60,61,64,65]. There are several options to consider in electronically conductive support, though CNTs have been shown to have the highest activity [41], however, at the cost of ~ USD$100,000 kg1 (Sigma Aldrich, 2018), commercialisation opportunities are limited. Also, the high activity of Ru on CNT was attributed to the purity of the support materials [59], which may limit manufacturing techniques. Due to their range of applications, there is much interest in the development of cheaper, high surface area materials. These include templated SiO2 [35], porous Al2O3 [63], and mesoporous carbon [50]. Prices for these materials range from USD$25,000 to USD$144 kg1, with materials of higher surface area often being higher in cost (Sigma Aldrich, 2018). These high costs are because higher surface area materials usually require more complex processing, though organic materials can offer cheaper ways to produce

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high surface area materials, such as AC, though these are not usually as high in purity as that required for CNT support materials. Catalyst support materials that are conductive with high surface area that are cost-effective may still need to be developed for the scaling of on-demand H2 production from stored NH3. However, for commercial applications, it may not be necessary that the catalyst activity is maximised. It may be more important that it is active enough while being stable over an extended period to reduce maintenance costs of field equipment. The exact definition of ‘active enough’ is a worthy pursuit, as the cost-benefit analysis of these materials for this purpose has been lacking, which creates uncertainty for what is required for the emerging H2 energy industry. The US DoE's development of H2 storage targets created certainty when developing H2 storage materials, and defined the ‘good enough’ materials and technologies to enable commercialisation. By developing a similar target for NH3 decomposition catalyst for various applications, through a cost-benefit analysis of temperatures, pressures, activity, and catalyst materials, researchers would be able to identify the ‘good enough’ materials to commercialize for use in decomposing NH3 for H2 production.

Catalyst deactivation Ru-based catalysts are the focus of much research and likely closest to commercialisation, and study of the deactivation and stability of catalysts is important, however, there is little knowledge of this in the literature. There are several reports of Ru-based catalyst deactivation mechanisms and solutions which focusing on Fischer Tropsch-type hydrocarbon synthesis as these are commercial processes using Ru-based catalysts, and the deactivation mechanisms reported include; RuOx and RuCxOy formation [66], Ru sintering, and diffusion of Ru into the support [67e70]. Attempts to prevent RuOx and RuCxOy catalyst deactivation have been made by using Fe2O3 as the support material, and it was hypothesised that the Ru formed Fe-Ru complexes which preventing Ru volatilization [71]. The problem of Ru sintering and diffusion are more significant at higher temperatures but still occur at lower temperatures [67e70]. Further studies that explore the potential for decomposition catalyst deactivation are needed in order to understand long-term operation strategies including shut-down and startup procedures for on-demand H2 production from stored NH3.

Summary of literature As discussed previously, TOF is a way to directly compare the activity of catalysts, however, the variations in the conditions of each experiment in the literature mean that the results are not always directly comparable. For this reason, Table 2 lists of catalysts tested in similar conditions, with at least two measures of activity to enable comparison across studies. It can be seen that, even in literature selected for maximum consistency, there are still significant differences between studies making it challenging to compare catalysts. To illustrate this Table 2 shows Ru on Al2O3 catalysts, two of which use 0.5 wt% and one 5 wt%, all at different temperatures. Another example

Catalytic species

Promoter

Wt.% promoter

Support

0.5 1 0.5 1 1 1 1 1 0.5 1 1 1 1 100

Al2O3 Al2O3 Al2O3 Al2O3 Al2O3 Al2O3 Al2O3 Al2O3 Al2O3 Al2O3 Al2O3 Al2O3 Al2O3

10 10 10 10 10 10 8 8

SiO2 SiO2 SiO2 SiO2 SiO2 SiO2 Al2O3 Al2O3

12 7.2 & 12 6 5 5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5

a-Al2O3 a-Al2O3 MgO CNT CNT CNT CNT CNT CNT CNT CNT CNT CNT CNT CNT CNT CNT

KNO3 KOH K2CO3 KF KCl KBr K2SO4 K3PO4

0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2

KNO3 KOH

0.2 0.2

Pressure (Bara)

Temperature (K)

Gas composition

1 1 1 1 1 1 1 1 1 1 16 16

853 853 853 853 853 853 853 853 853 853 853 853 853 843 800 800 843 873 673 873 673 873 673 823 873

NH3 NH3 NH3 NH3 NH3 NH3 NH3 NH3 NH3 NH3 NH3 NH3 NH3 NH3 NH3 NH3 NH3 NH3 NH3 NH3 NH3 NH3 NH3 N2, CO2, CO, H2, NH3 N2, CO2, CO, H2, NH3

873 873 973 923 923 723 723 723 723 723 723 723 723 723 673 673 673

NH3 NH3 NH3 NH3 NH3 NH3 NH3 NH3 NH3 NH3 NH3 NH3 NH3 NH3 NH3 NH3 NH3

1 1 1

1 1 1 1 1 1 1 1 1 1 1 1

Rate of reaction (mol NH3 m2 hr1)

1.4e-4* 1.2e-5* 2.4e-4* 1.9e-4*

TOF (s1) 6.85 4.21 2.26 1.33 7.9e-1 3.3e-1 2.2e-2 2.2e-2 1.9e-2 1.3e-2 5.6e-3 4.4e-3 2.4e-3 12.9 0.18 28.3 1.77* 0.12* 11.92* 0.46* 44.72* 6.64*

2.3e-4* 7.2e-4* 1.7e-2* 1.58e-2* 2.0e-2* 5.4 5.4 7.6e-3* 1.7e-2* 1.6e-2* 1.6e-2* 9.1e-3* 7.0e-3* 4.1e-3* 2.1e-4* 2.2e-4* 1.3e-3* 1.0e-3* 9.4e-3*

Conversion Source (%)

56 3.9 36.4 1.4 97 14.3 17 53

[54] [54] [54] [54] [54] [54] [54] [54] [54] [54] [54] [54] [54] [30] [30] [30] [30] [40] [40] [40] [40] [40] [40] [72] [72]

75.2 69.9 89.2 25 50 48 85 82 81 45 35 19 1 1 8 50 48

[33] [33] [33] [46] [46] [59] [59] [59] [59] [59] [59] [59] [59] [59] [59] [59] [59]

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 4 ( 2 0 1 9 ) 3 5 8 0 e3 5 9 3

Ru Ni Rh Co Ir Fe Pt Cr Pd Cu Te Se Pb VC (VaC-7) VN Mo2N Mo2C Ir Ir Ni Ni Ru Ru Ni Ni (membrane reactor) MoNx NiMoNx NiO CoFe CoFe (enclosed) Ru Ru Ru Ru Ru Ru Ru Ru Ru Ru Ru Ru

Wt.% catalytic species

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Table 2 e Summary of catalyst for NH3 decomposition tested in similar conditions (~1 bar), similar temperatures and mostly under pure NH3. Some of the conversion % have been calculated from the data provided in the source paper, and represent kinetic studies where the conversion was intentionally small. Additionally, where the pressure is not quoted, it was not given by the source. * Calculated by the authors based on information provided in the referenced article.

0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5

K2CO3 KF KCl KBr K2SO4 K3PO4

0.2 0.2 0.2 0.2 0.2 0.2

LiOH 100 100 5 66 7 7 7 7 5.2 100 5.4 4.6 5 8

Cs Cs Cs

20 20 50

CNT CNT CNT CNT CNT CNT MgO TiO2 g-Al2O3 ZrO2 AC ZrO2-BD K-CNT K-ZrO2-BD CNT CNT CNT CNT CNT CNT K-ZrO2-CP K-ZrO2-KOH K-ZrO2-NH4OH K-ZrO2-KOH K-ZrO2-KOH K-ZrO2-NH4OH g-Al2O3 g-Al2O3

Al2O3 SiO2/Al2O3 CNT CNT CNT CNT MgAl2O5 MgAl2O5 MgAl2O5 MgAl2O5 MgAl2O5

1 1 1 1 1 1 1 1 1 1 1 1 1 1

1 1 1 1 1 1 1 1 1 1 1 1 1 1

673 673 673 673 673 673 673 673 673 673 673 673 673 673 673 673 673 673 673 673 673 673 673 673 673 673 673 673 723 723 723 723 600 600 600 600 773 773 773 773 773 773

NH3 NH3 NH3 NH3 NH3 NH3 NH3 NH3 NH3 NH3 NH3 NH3 NH3 NH3 NH3 NH3 NH3 NH3 NH3 NH3 NH3 NH3 NH3 NH3 NH3 NH3 NH3 NH3 NH3 NH3 NH3 NH3 NH3, He NH3, He NH3, He NH3, He H2, N2, NH3 H2, N2, NH3 H2, N2, NH3 H2, N2, NH3 H2, N2, NH3 H2, N2, NH3

9.4e-3* 4.5e-3* 3.6e-3* 1.9e-3* 2.1e-5* 2.2e-5* 2.5e-2* 8.7e-2* 1.3e-4* 2.1e-3* 1.2e-2* 3.6e-4* 3.2e-3* 3.5e-3* 2.0e-3* 1.2e-4* 2.1e-4* 5.6e-5* 7.3e-5* 4.1e-1* 1.6e-2* 1.4e-2* 1.3e-2* 7.5e-3* 7.0e-3*

1.2e-7 7.1e-7 25.2e-7 42.7e-7 1.7e-1 1.5e-1 1.1e-1 1.0e-1 1.2e-1 3.7e-1

48 22 18 9 0.1 0.1 1.3* 1.0* 2.9e-1* 7.8e-1* 8.6e-1* 2.5e-1* 1.3* 1.4* 1.5* 1.5e-1* 1.5e-1* 3.3e-2* 2.2e-2* 4.1e-2* 3.7 4.9 4.1 4.7 2.4 2.3 3.0e-1 2.0 6.6 6.6 334 9.3 3.0e-6 3.8e-6 3.5e-6 3.2e-6

90.7 54.9 53.7 34.0

[59] [59] [59] [59] [59] [59] [50] [50] [50] [50] [50] [50] [50] [50] [50] [50] [50] [50] [50] [50] [36] [36] [36] [36] [36] [36] [63] [63] [52] [52] [52] [52] [62] [62] [62] [62] [21] [21] [21] [21] [21] [21]

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 4 ( 2 0 1 9 ) 3 5 8 0 e3 5 9 3

Ru Ru Ru Ru Ru Ru Ru Ru Ru Ru Ru Ru Ru Ru Ru Ni Rh Pd Fe Pt Ru Ru Ru K-Ru Ru (RuCl3) Ru (RuCl3) Ru Ru LiNHx NaNH2 Ru Ni Ru Ru Ru Ru Co Co3Mo3N Cu Fe Ni Ru

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is where 0.5 wt% Ru on CNT is presented with activities 1.3e-3 and 2.0e-3 mol NH3 m2 hr1 at the same temperature in different studies. Though Al2O3 is a conventional support material for many catalysts, several studies have shown different TOFs at similar conditions for Ru-based materials, making the exact activity of these materials uncertain. It is likely that these differences can be attributed to minor differences in the material used in each study, but it is subideal that most studies do not provide enough information to eliminate these issues. The non-standard presentation of data across the catalyst literature reduces comparability across studies, which makes it difficult for industry and new academics and researchers to understand these results. The focus on novel results and materials ignores industries needs for an understanding of repeatability, reliability and deactivation conditions. Perhaps because the inter-study comparability is low, many studies now compare multiple catalysts, several of which are outlined in Table 2. In these studies, the trends discussed above are reflected, and Ru-containing catalysts are consistently the most active. However, it can be seen that many materials may be more active than Ru catalysts, such as the VC catalyst [30], which has a TOF of double that of Ru in one study [54] but barely 1/20th when compared to Ru in another study [52].

Considerations for future work Some of the differences highlighted in this review could be due to differences in evaluation conditions, such as NH3 flow rates, concentrations, and temperatures. The development of standard operating conditions for characterizing the activity of various catalysts is paramount to developing optimal catalyst formulations for H2 production from stored NH3. For real-world operations, it is likely that a graded-bed system will eventually be used, hence knowledge of catalyst behavior and activity for a range of conditions would be ideal. Some ways to address these issues are to; develop a set of standard conditions for catalyst tests, selecting a repeatable and reliable material to use as a standard, and optimising the process of catalyst development on the micro through to macro stages.

Testing vs. operating conditions The volcano curve for NH3 decomposition shifts with concentration, which results in quite different optimal N2 adsorption energies [21]. For example, under 99% NH3, the optimal energy is ~ 0.5 eV, while at 20% is ~0.8 eV [21]. While this may not seem like a large difference, the TOF maximum in 99% feed is near five times greater than 20% NH3 feed, hence the decomposition of NH3 on Ru will proceed five times faster in 99% NH3 feed. Previous researchers and research groups have identified this issue, and some have taken steps to move experiments closer to ‘real’ operating conditions [44,73]. Though these authors stated what the conditions they used were, they did not specify or focus on what they should be for distributed, on-demand H2 production from stored NH3. The issue of optimising the catalyst to the variety of reactor conditions can be mitigated by the use of graded bed

reactors, where the catalyst bed consists of multiple layers such that the catalyst is ideal for the conditions at that point in the reactor [74]. For this approach, the efficiencies of the catalyst need to be tested at a variety of feed conditions, and we make suggestions below for the testing conditions of NH3 decomposition catalysts to enabling easier comparison of materials.

Suggested testing conditions for commercial applications The testing conditions for commercial applications of NH3 decomposition catalysts may not be the ideal conditions for any given catalysts, but it is still crucial that catalysts be tested in similar conditions to enable comparison across studies and laboratories. Increasing the pressure shifts the equilibrium unfavorably towards NH3, while increasing temperature shifts it towards N2þH2. Higher pressures and temperatures require more engineering and safety, hence reducing both of these conditions is optimal. For ease of operation and reducing costs, the operating temperature of the catalyst should be minimized, and this requirement balanced against the conversion. Ideally, the catalyst should operate well between the ranges of 1e10 bara depending on downstream processing. As such, catalysts proposed for this purpose should be tested at several pressures in this range; we suggest 1, 3, 5, 8 and 10 bara (assuming the NH3 supply can reach these pressures). Three to five data points should be collected in order to identify nonlinear trends in activity, and this range is suggested because the vapor pressure of NH3 from a liquid storage tank is typically between 6 and 10 bara depending on storage temperature. While a heating blanket can be used to increase this pressure, it may not be practical given that NH3 is favored in the equilibrium at higher pressures. The temperature range tested should ideally be between 250 and 500
C. However, the range should include the minimum (~0%) and maximum (~100%) of equilibrium conversion for the conditions. Additional testing of the catalyst in simulated, partially decomposed gases, for example in 50% NH3, 37.5% H2 and 12.5% N2, would also be useful for determining the behavior of the catalyst within a graded reactor bed. The GHSV is also a significant factor in industrial-scale testing, as the flowrate in practice is often significantly lower than the kinetic testing conditions. However, if there are no mass and heat transfer limitations, and the catalyst's rate of conversion is reported, then the flow rate only influences % conversion. For experimental procedures for measuring rates of catalytic reactions without artefacts such as temperature gradients, see Refs. [24,75,76]. Experimenters consistently using these recommended testing conditions, quoting the catalyst activity in mol NH3 m2 hr1 versus Weight/Flow (g catalyst hr g1 NH3 mol1) as well as in TOF (s1), would enable industry and researchers to more easily compare catalyst efficiencies and costs, and understand what further optimization is required. Another approach to address this problem is to use a standard catalyst as a comparison. As Ru catalysts are the highest activity, we suggest it should be Ru-based, on an active support such as MgO. This kind of material could be well characterized and repeatably produced in multiple particle sizes and dispersions that would quickly enable

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 4 ( 2 0 1 9 ) 3 5 8 0 e3 5 9 3

comparison between materials using any of the methods discussed here.

Catalyst and catalytic process optimization There are several stages of optimization for any catalytic process; broadly classified as the micro (<2 nm), meso (2e50 nm) and macro-scales (>50 nm). A significant focus of catalyst development is on micro-scale optimization, which generally means maximising the TOF of the material, which is arguably the most crucial part of the system. However, many of the gains in activity and efficiency of a material can be lost by inefficient meso (particle size or surface) or macro (reactor) designs [77]. For this reason, the micro, meso and macroscales of optimization must be considered together for the serious development of industrially significant NH3 decomposition catalysts, and the reader is referred to a guide proposed by Bartholomew and Hecker recommending that the catalyst requirements are included in the reactor design and that all parts are optimized together [78]. Not all information and optimizations will need to be done from first principles, as there is already literature to draw on. As NH3 decomposition catalysts become more common due to optimization and industrialization of materials and reactors, it may be found that deactivation of the catalyst is a limitation to the long-term operation of on-demand H2 production from stored NH3. There are many more materials to be tested for NH3 decomposition, and it is essential for future research to use rational and efficient methods for the further development of this field. We direct the reader to Refs. [73,77,79e81] for reviews and information on experimental design.

Purification and separation of product gases from ammonia decomposition The extent of the purification of the H2 produced from decomposing stored NH3 required depends on the anticipated use, and hence must comply with relevant standards. There are standards now available for the composition requirements for H2 for various energy applications, for example, ISO 14687e2:2012 for FCEVs and ISO 14687e3:2014 for stationary PEM FCs. For stationary PEM FCs, N2 concentration is not of great concern and can constitute up to 50% of the feed gas by volume, but the NH3 concentration must be less than 100 parts per billion by volume (ppbv) [15]. For FCEVs, the H2 gas must be purified to 99.97%, with less than 100 part per million by volume (ppmv) of N2, and NH3 must still be less than 100 ppbv [10]. Also, in both standards sulfur and carbon compound concentrations must be less than 2 and 0.2 parts ppmv, respectively. While decomposed NH3 gas streams should not contain sulfur and carbon compounds, if the NH3 is manufactured from natural gas or biomass, then it is possible that these contaminations may be present and removal would be required. There are several methods for gas cleaning and separation; absorption and adsorption processes for NH3, adsorption processes for separating NH3, N2 and H2, and H2-permeable membranes.

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Ammonia absorption For gas-phase removal of small concentrations of NH3, the gas can be reacted by passing through liquids or solid-packed beds, and various reactions for NH3 are well known. Solid alkali-chloride salts react to form metal-NH3-chloride compounds; an example is MgCl2 which absorbs 1.07 g NH3 g1 (forming Mg(NH3)6Cl2) [82]. Phosphoric acid (H3PO4) has also been used to adsorb gaseous NH3, at 0.53 g NH3 g1 [83]. Silica in various forms (beads, gels, molecular sieves) will adsorb various quantities of NH3 depending on the surface acidity, area and water content [84]. Recently, oxyfluorinated activated carbon fibers were proposed as an NH3 scrubbing material [85], and 0.1 g of the best performing material was found to be able to remove 1013 ppm of NH3 in argon at 10 ml min1 for ~4.5 h, which resulted in an exceptional adsorption capacity of ~2.7 g NH3 g1. Further study is required to determine the cost, cyclability or disposal requirements of the material to determine if this would be practical. Silica gels, molecular sieves (zeolites), metal-NH3-chlorides, and H3PO4 can be renewed multiple time by heating to above the desorption temperature, and the NH3 can be retrieved. NH3 also dissolves in water at STP to ~3.1 g NH3 g1, which can be further increased by the addition of an acid such as H2SO4 to form (NH4)2SO4. The use of each of these reactions would depend on the application. To scrub residual NH3 directly from a catalyst system, an interchangeable or high volume scrubber would be required, even when very high decomposition efficiencies are consistently achieved. When demonstrating H2 production from lithium imide to power a small proton exchange membrane fuel cell, Hunter and colleagues used anhydrous MgCl2 citing ease of use and safety considerations for the choice [53]. These absorption methods do not remove N2, nor any unknown contaminants, which will be required if the H2 produced is to be compressed for vehicular use. Hence an absorption or separation processes may be more applicable.

Absorption processes for separating ammonia, nitrogen, and hydrogen Of the absorption processes, pressure swing adsorption (PSA) is one of the most well developed, with commercial NH3/N2/H2 separating units available. PSA was initially developed by Skarstrom in the 1950s [86] and is a process where the feed gases are passed through a selectively absorbent material bed at high pressure, and at some point, the system pressure is reversed, backflushing the unwanted compounds [87,88]. Various materials can be used for this process; activated carbon, zeolites, and silica compounds are used in several patented and commercially available units [89]. Due to the technological readiness of this process, off the shelf units that can provide the required purity H2 are available. The main problem with this technique is that it is a batch process and multiple units are used to ensure a continuous supply. For the distributed and on-demand production of H2 from stored NH3, the cost of these units likely to be quite a high portion of the total budget, and a portion of the pure gas must be used to backflush the system to maintain purity which reducing efficiency [88].

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300 °C, Pfeed = 5.0 bara, Ppermeate= 1.0 bara, 250 μm thick

80

-2

-1

H2 flux (ml cm min )

70 60 50

100%

75%

40

50%

30

25%

20

10%

10 0 0

10

20

30

40

50

60

70

80

90

time (hours) Fig. 2 e H2 flux through a 250 mm Pd coated V membrane in a simulated decomposed NH3 feed of various conversion rates. Green represents H2þN2 only where N2 replaces NH3, while blue represents H2þN2þNH3. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)

Hydrogen permeable membranes Another method of purifying gases is through the use of H2 permeable membranes that do not require backflushing. This method is advantageous as it is a single step that removes most, if not all, potential contaminants. Currently, there are three types of membranes for H2 separation; polymeric, porous ceramic, and metal (porous and solid) [90]. Polymeric membranes use differences in solution diffusion rates of molecules through rubbery and glassy polymers to separate mixtures, where the diffusion rate of any particular molecule is a combination of partial pressure, size, and interaction with the membrane [91]. This technique is limited by a trade-off between selectivity and permeability; where selectivity increases, permeability decreases [92]. Many of these polymers are patented and include polysulfones, silicone rubbers, poly (vinyl chloride), natural rubbers, polycarbonates, and polystyrenes. The weaknesses with these compounds are; they can be damaged by contaminants, they must operate in a specific temperature range, and have limitations in selectivity and permeability. Metal and ceramic membranes have been used for H2 separation and purification due to their high physical and thermal stability as well as high H2 permeability [72,93e95]. Palladium (Pd) alloy membranes are used commercially for producing ultrapure H2 because Pd has catalytically active surfaces for H2 dissociation and has relatively high hydrogen permeability. However, the cost of Pd is a significant limiting factor in their widespread adoption for separating H2 from mixed gases in a variety of applications [96]. For this reason, various other alloy membrane materials have been investigated such as Ni alloys [96], Ni-Nb-Zr [97], ceramics [94], porous stainless steel [98], and Pd coated V, Nb, and Ta [99], with varying results. Metals such as V, Nb and Ta are permeable to H, but must be coated with Pd due to the formation of stable surface oxides that prevent H2 dissociation at the surface [100]. Recent works on thin Pd membranes [95],

and our work on Pd-coated V materials has shown that N2 and NH3 do not inhibit H2 flux beyond reducing the partial pressure of H2. Our work on a Pd-coated V membrane has shown excellent stability and H2 flux in NH3 containing mixed gases as well as excellent mechanical strength, and these properties make this type of membrane well suited for use in producing high purity H2 from decomposed NH3. Fig. 2 shows the H2 flux of one such membrane at various simulated NH3 decomposition rates, using a system design previously reported [16]. Variations in the H2 flux are due to pressure and temperature drift with changes in the environment (day/night and weather) over the timescale of the test. These membranes have the potential for future commercialisation, as they are cheaper than current Pd based membranes, and also scalable into the future. Our recent demonstration of this technology by providing H2 from decomposed NH3 for FCEVs proves that this technology can meet the ISO standards [101] though the quality assurance of gas produced using any separation technique is another challenge to be addressed before this becomes standard practice. Because all systems have the potential for failure, gas quality monitoring and the capability to rectify any issues in operation would be required for commercial and industrial operation of any of the systems presented here.

Conclusions Renewable H2 energy technologies have the potential to provide emissions-free power and disrupt the current energy market. One of the critical problems is distributing H2 cheaply, safely and efficiently, and NH3 is an ideal H2 storage intermediate chemical due to advantages in energy densities, longterm storage, ease of manufacture, and potential renewable future. This review has identified where more research can address critical gaps in knowledge, and made recommendations for further testing and data presentation methods to

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 4 ( 2 0 1 9 ) 3 5 8 0 e3 5 9 3

enable more robust comparisons between catalysts and studies. We also reviewed, discussed and compared technologies for H2 separation and purification and identified that further research could be beneficial for the application of membrane-based technologies to this area. The critical areas that future research could focus on for thermo-catalytic NH3 decomposition are; lifetimes and deactivation conditions of Ru-based catalysts, lower temperature and higher pressure catalysis, catalyst behavior under non-ideal conditions such as with contamination, costbenefit analyses of NH3 decomposition catalysts to determine the optimal activity and costs, testing catalysts under a standard set of conditions, the development of a standard catalyst for NH3 decomposition, and micro- and macrooptimization of highly active catalysts. Some future research directions for H2 separation and purification are; identify lifetime limitations on H2 permeable membranes, further work on absorption processes to ensure outflows from this process are within environmental and human safety standards, and online gas quality analysis for ondemand H2 production. For renewable H2 to be transported via NH3 and produced on-demand, further consideration of the economics and physical limitations of current catalysts are required. Given the high activities of several Ru-based catalysts demonstrated, as well as the suitability of emerging membranebased H2 separating and purification technologies that are scalable, we believe that current technologies can be used, though improvement to efficiencies and cost are likely to be made into the future.

Acknowledgments This research is supported by the Science and Industry Endowment Fund.

Acronyms AC Bara CNT FC g GHSV hr kWh ln Liq LPG LNG mol Mt mln MWh PEM FC PSA TOF US DoE

Activated carbon Absolute pressure in bar Carbon nanotubes Fuel cell Gram Gas hourly space velocity (hr1) Hours Kilowatt hours Litre at standard temperature and pressure Liquid Liquefied petroleum gas Liquid natural gas Mole Megaton Millilitre at standard temperature and pressure Megawatt hours Proton exchange membrane fuel cell Pressure swing adsorption Turn-over-frequency (s1) United States Department of Energy

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