Carbon-free Energy Carrier Research Articles

Thermochemical and Techno-Economic Evaluation of Ammonia Synthesis Via Lithium-Molten Salt Processes

Ammonia is an attractive carbon-free energy carrier as it is easily liquified, transported and stored at a lower energy penalty than other non-carbon containing fuels. Anhydrous ammonia production occurs today in large Haber-Bosch (H-B) plants fueled from natural gas. Industrial (H-B) plant techno-economics are poorly suited for remote small-scale ammonia production from renewable energy. Highly efficient ammonia synthesis in small-scale H-B systems does not exist, thus stimulating research towards a low temperature and pressure processes. The thermodynamics of low temperature ammonia synthesis are quite favorable however, the high activation energy for nitrogen reduction requires efficient catalysts and high temperatures. Electrochemistry offers a means to activate the process at lower temperatures however this has proven to be very challenging [1]. Direct electrochemical protonation of nitrogen at low temperatures has produced rates on the order of 10-9 to 10-10 moles/cm2-sec, many orders of magnitude too low for industrial production [2]. Nitrogen reduction on lithium metal occurs spontaneously; hence molten lithium salt processes may prove to be attractive for ammonia generation [3,4,5]. We have evaluated the thermodynamics and techno-economics of intermediate temperature NH3 synthesis processes, based on the spontaneous electro-reduction of nitrogen in lithium ion systems where the potential for lithium is close to metal deposition and the subsequent formation of lithium nitride. The advantage of the molten salt lithium processes is the reaction is autocatalytic however the direct protonation of Li3N with hydrogen yields lithium amide. Protonation with water does yield ammonia however the reaction is very exothermic. The electrolysis of the LiOH product is also a major obstacle as the reaction between LiOH and Li produces Li2O[6]. To overcome this difficulty, we have also evaluated alternative iodide and bromide cycles were lithium electrowinning is known to be viable. The techno-economics of the processes were also considered in a preliminary study.

Recent progress in heterogeneous ammonia synthesis

Ammonia is feeding nearly half the world population and also holds the promise as a carbon-free energy carrier to store and transport renewable energy. However, the catalytic synthesis of ammonia from N2 and H2 is currently realized industrially under harsh conditions (10−25 MPa, 350−550°C), consuming 1%−2% of the power produced globally. This fossil fuel-based process also results in the emissions of 670 million tonnes of CO2. The development of more sustainable and environmentally benign processes is highly desirable. To this end, heterogeneous catalysts that allow ammonia synthesis under lower temperature and pressure, solid materials for chemical looping ammonia synthesis, as well as electron-, photon- and plasma-driven routes have been actively explored in recent years. In this review article, some recent progress on these topics are summarized and discussed. Major effort has been devoted to searching for more active heterogeneous catalysts under milder conditions. The tailoring of particle size and shape of transition metals as well as the control of metal-support and -promoter interactions are key factors for activity enhancement. Surface science and theoretical results reveal that the scaling relations among the adsorption energies of reacting species place limitations on the activity enhancement of a catalytic process. To develop more active catalysts, the scaling relations should be circumvented or broken. Some strategies including the introduction of a non-transition metal site, or construction of surface cluster site have been shown to be effective approaches. Chemical-looping ammonia synthesis may provide an alternative approach, which enables ammonia production under ambient pressure by decoupling a catalytic process into several separated reactions. This process circumvents the scaling relations and the competitive adsorption of N2 and H2 or H2O on metal catalysts. Challenges arise from tuning the thermodynamic and kinetic properties of all of the separated steps by selecting materials containing H, N, O, etc. The electron-, photon- and plasma-driven routes for ammonia synthesis have been received increasing attention recently. The poor selectivity and pretty low ammonia production rates are the two key challenges in electro- or photon-chemical processes. The ammonia synthesis rates should be carefully measured and quantified by combining multiple methods. The strategy for designing catalysts with high efficiency may also be focused on the modification of catalyst surface structural and electronic properties to circumvent the scaling relations. Through these efforts, processes that are relied on renewable energy, which are quite different from the conventional Haber-Bosch process, may come to the fore in the future.

Heterogeneous electrocatalysts design for nitrogen reduction reaction under ambient conditions

Ammonia, as an important carbon-free energy carrier and also an important chemical for producing fertilisers, is mainly synthesized by a traditional Haber–Bosch process with high energy consumption and large amounts of greenhouse gas emissions. Recently, electrocatalytic nitrogen reduction reaction (NRR) has attracted worldwide research attentions as a promising route for achieving green and sustainable ammonia synthesis at ambient conditions. Although exciting advances have been made in the NRR field, the development of electrochemical nitrogen-to-ammonia conversion is still challenging because of the low ammonia yield and unsatisfactory Faradaic efficiency mainly deriving from the poor catalytic activity of catalysts. Herein, various catalyst design strategies for increasing the exposed active sites or altering the electronic structure aiming at improving the apparent activity or intrinsic activity are summarized in this review article. On the basis of effective design strategies, a range of recently reported NRR electrocatalysts, including noble metal-based materials, non-noble metal-based materials, single-metal-atom catalysts, and metal-free materials, are summarized, and the mechanisms of tuning the catalytic activity by applying the design strategies are emphasized based on the combination of theoretical calculations and experimental investigations. It is anticipated that the established correlation between physicochemical properties of catalysts and NRR performance can provide guidance for designing heterogeneous NRR electrocatalysts with high activity, good selectivity, and high stability.

Catalytic hydrogen evolution reaction on "metal-free" graphene: key role of metallic impurities.

Molecular hydrogen is widely considered as a potential carbon-free and renewable energy carrier for a sustainable energy strategy. The electrocatalytic hydrogen evolution reaction is a major pathway to obtain ultra-pure hydrogen, and it vastly relies on the development of inexpensive electrocatalysts. In recent years, graphene has been proposed as a "metal-free" catalyst for the hydrogen evolution reaction. Here, we showed that metallic impurities are introduced during the synthesis process and these impurities act as the catalyst for the hydrogen evolution reaction.

Production of ammonia via a chemical looping process based on metal imides as nitrogen carriers

Ammonia is a promising carbon-free energy carrier, but is currently synthesized industrially under harsh conditions. Synthesizing ammonia using lower temperatures and pressures could therefore improve its prospects as a chemical means to store and transport energy. Here we report that alkali and alkaline earth metal imides function as nitrogen carriers that mediate ammonia production via a two-step chemical looping process operating under mild conditions. Nitrogen is first fixed through the reduction of N2 by alkali or alkaline earth metal hydrides to form imides and, subsequently, the imides are hydrogenated to produce NH3 and regenerate the metal hydrides. The oxidation state of hydrogen therefore switches between −1 (hydride), 0 (H2) and +1 (imide and NH3). Late 3d metals accelerate the reaction rates of both steps. The chemical loop mediated by BaNH and catalysed by Ni produces NH3 at 100 °C and atmospheric pressure. Reducing the severity of the conditions required to synthesize ammonia would increase the viability of its use as a carbon-free energy carrier. Here the authors use metal imides to mediate ammonia production via a two-step chemical looping process that operates under mild conditions.

Ammonia capture by water splitting and hollow fiber extraction

An efficient method for capturing ammonia from wastewater will effectively solve the ammonia nitrogen pollution in hydrosphere, and the captured ammonia can be utilized potentially as carbon-free energy carriers instead of hydrogen. While the ammonia removal from wastewater has been previously demonstrated, many challenges remain, including indirect ammonia removal, chemicals consumption and secondary pollution. Here, we propose a chemical-free method for ammonia capture by a hybrid system consisting of bipolar membrane electrodialysis (BMED) and hollow fiber membrane contactor (HFMC). It is achieved by taking advantage of the water splitting in BMED as a sustainable “chemical supplier” for hollow fiber extraction. Results indicate that a normalized current density of ∼0.29 mA cm−2 (mg/min)−1 should be applied to BMED unit; the minimum energy consumption is 63.59 kJ/(mol NH4+-N) for treating the wastewater with NH4+-N concentration of 200 mg/L; and the ammonia capture ratio can be reached at 65.2%, which can be increased further to >98% through increasing the stages of HFMC modules.

Acceleration of hydrogen absorption by palladium through surface alloying with gold

Enhancement of hydrogen (H) absorption kinetics improves the performance of hydrogen-purifying membranes and hydrogen-storage materials, which is necessary for utilizing hydrogen as a carbon-free energy carrier. Pd-Au alloys are known to show higher hydrogen solubility than pure Pd. However, the effect of Au on the hydrogen penetration from the surface into the subsurface region has not been clarified so far. Here, we investigate the hydrogen absorption at Pd-Au surface alloys on Pd(110) by means of thermal desorption spectroscopy (TDS) and hydrogen depth profiling with nuclear reaction analysis (NRA). We demonstrate that alloying the Pd(110) surface with submonolayer amounts of Au dramatically accelerates the hydrogen absorption. The degree of acceleration shows a volcano-shaped form against Au coverage. This kinetic enhancement is explained by a reduced penetration barrier mainly caused by a destabilization of chemisorbed surface hydrogen, which is supported by density-functional-theory (DFT) calculations. The destabilization of chemisorbed surface hydrogen is attributed to the change of the surface electronic states as observed by angle-resolved photoemission spectroscopy (ARPES). If generalized, these discoveries may lead to improving and controlling the hydrogen transport across the surfaces of hydrogen-absorbing materials.

(Invited) Electrochemical Synthesis of Ammonia Using Ion Conducting Membranes

Ammonia offers potential as a carbon-free energy carrier with a high hydrogen content and if fully oxidized, the products nitrogen and water are environmentally benign. It also serves as the major feedstock for fertilizer production and a building block for nitrogen containing chemicals and polymers. Ammonia is readily liquefied and conveniently stored in low cost steel tanks. It may function as a high energy density storage medium for hydrogen produced by renewable energy derived electricity. Ammonia synthesis is currently carried out in a few very large Haber-Bosch plants, mostly fueled from natural gas. The current large-scale Haber-Bosch (H-B) technology needs to run at constant inputs of energy and reactants. The economics dictate very large process reactors and separation systems, consequently the technology does not downscale well to small sized plants, nor does it work with interruptible energy supplies. Even though the ammonia synthesis efficiency can be considered low at ~70%, these efficiencies are only obtained in large-scale integrated facilities. Highly efficient ammonia synthesis in small-scale H-B systems does not exist, thus stimulating research towards a low temperature and pressure processes. Electrosynthetic pathways for NH3 production theoretically offer higher efficiency and scalability but, the challenges are quite foreboding as the dinitrogen triple bound is extremely stable and most known nitrogen dissociation catalysts are deactivated by oxygen or water [1]. We are currently engaged in a DOE ARPA-e funded program to develop electrochemical methods for ammonia synthesis. We are developing and evaluating intermediate NH3 synthesis processes, one based on the spontaneous electro-reduction of nitrogen in lithium ion systems where the potential for lithium is close to metal deposition and the subsequent formation of lithium nitride. The other utilizes proton conducting, anhydrous metal pyrophosphate/polymer composite membranes for protonation of nitrogen [2]. The advantage of the lithium process is the reaction is autocatalytic, however the reaction of Li3N with water to form NH3 and LiOH is exothermic, with an amount of enthalpy of -581.62 kJ mol-1 and thus presents challenges for the design of an energy efficient process. The direct protonation process is thermodynamically favorable at modest potentials near hydrogen evolution potentials, however it requires suitable electrocatalysts that dissociate dinitrogen but inhibit the parasitic hydrogen evolution reaction (HER). Using proprietary Li ion solid electrolytes, we have demonstrated lithium nitride formation at high coulombic efficiency and ammonia evolution upon hydrolysis. We have also demonstrated intermittent ammonia production at >8% efficiency using Pt catalysts, however the coulombic efficiency is limited by HER. We are exploring the use of carbon supported Ru, MoxC and Ru/MoxC catalysts to limit current efficiency losses due to HER.

Techno-economic assessment of membrane-assisted gas switching reforming for pure H2 production with CO2 capture

Hydrogen has significant potential as a future carbon-free energy carrier. Given that the vast majority of the current global hydrogen is produced from fossil fuels, CO2 capture and storage (CCS) emerges as a promising strategy for delivering clean hydrogen on a global scale. The primary challenge with CCS is the large energy penalty imposed, but this problem is circumvented by the membrane-assisted gas switching reforming (MA-GSR) technology evaluated in this work. Combined reactor and process simulations showed that the MA-GSR process achieves a similar equivalent H2 production efficiency as conventional steam methane reforming without CO2 capture. The results of the process simulations also show that the new MA-GSR concept can achieve a higher H2 production efficiency than the conventional plant when integrated with CO2 capture (20% higher). However, a drawback of the MA-GSR concept is the low utilization of the membranes in the system, requiring a large membrane surface area for a given rate of hydrogen production. This makes the economics of the concept highly sensitive to the cost and durability of the H2 perm-selective membranes. In the base-case, the cost of hydrogen (0.274 €/Nm3H2) was lower than the conventional process with CO2 capture (0.282 €/Nm3H2), with a cost of CO2 avoidance of 85.14 €/tCO2 instead of 97.06 €/tCO2 for the conventional technology. Future technology improvements leading to a threefold reduction in the cost/permeability ratio of H2 perm-selective membranes can allow the MA-GSR technology to produce hydrogen at a similar cost as conventional steam methane reforming without CO2 capture.

Characterization of Electrocatalytic Water Splitting and CO2 Reduction Reactions Using In Situ/Operando Raman Spectroscopy

The electrocatalytic reduction of CO2 and water splitting have received significant attention because recycling CO2 into fuels and chemical feedstock is a crucial step to close the anthropogenic carbon circle, whereas splitting water produces H2 gas, which is a valuable carbon-free energy carrier. The clear identification of the catalytic-active sites and elucidation of the reaction mechanisms in these systems remain a grand challenge. It requires simultaneous characterizations of the catalysts under actual reaction conditions. Raman spectroscopy is among the handful of techniques that are suitable for the in situ/operando investigations of heterogeneous catalytic systems. This Perspective will highlight primarily recent works on the application of Raman spectroscopy in unraveling the structural changes of catalysts, their possible active sites, and the intermediates formed during water electrolysis and CO2 electroreduction. Results from complementary techniques such as X-ray absorption spectroscopy, amon...

Recyclable ammonia uptake of a MIL series of metal-organic frameworks with high structural stability

Metal-organic frameworks (MOFs) have been rapidly developed in the fields of gas adsorption and storage, but are unsatisfactory in NH3 adsorption due to their unstable structures. Considering NH3 is both a toxic gas and the only carbon-free chemical energy carrier, MOFs exhibit great prospects for the adsorption and storage of NH3 due to their advantages of structural diversity, modifiable structures and high surface area. In this work, we have researched, in detail, the structural characteristics, stability and NH3 adsorption properties of MIL-53, NH2-MIL-53, MIL-100 and MIL-101. In addition, the NH3 adsorption sites have been investigated using Grand canonical Monte Carlo simulations. The results showed that MIL-100 and MIL-101 have a large NH3 uptake of 8 mmol/g and 10 mmol/g, respectively, and that the modified amino functional groups improve the NH3 adsorption capacity in NH2-MIL-53. More importantly, the four MIL materials have reusable NH3 uptake and excellent NH3 (and NH3/H2O) stability, and are promising materials for NH3 adsorption.

Development of Ammonia-Fueled SOFC

Ammonia has been studied as a carbon-free energy carrier. In this work, performances of 200W-class solid oxide fuel cell (SOFC) stack using ammonia instead of hydrogen are evaluated to investigate whether ammonia can be a substitute for hydrogen. This stack consists of some constituents being resistance to ammonia at operating temperature of 700-800 oC. In addition, ammonia is supplied directly into an anode chamber without using a reformer. The ammonia-fueled SOFC stack achieves almost same performances as that of the hydrogen-fueled one.

Antenna-Protected Metal–Organic Squares for Water/Ammonia Uptake with Excellent Stability and Regenerability

Ammonia is a hazardous gas and the only carbon-free chemical energy carrier that can be largely adsorbed on metal–organic frameworks (MOFs). However, because of the destructive effect of H2O/NH3 on the metal nodes, most MOFs cannot be applied in ammonia capture and uptake. Herein, three Co-4,5-imidazoledicarboxylic series metal–organic squares (MOSs)—Co4(IDC)4(pda)4, Co4(IDC)4(phen)4, and Co4(IDC)4(bpy)4—were synthesized with a special independent square configuration and zeolite-like supramolecular structures, and their structure and H2O/NH3 uptake capacity were investigated. Based on the four antennas-protected squares and porous structures, the three MOSs have excellent H2O/NH3 stability, whose structures were not affected by the ad-desorption of H2O, NH3, or H2O/NH3. The three MOFs have a H2O uptake of 17.63, 8.35, and 7.75 mmol/g, respectively, as well as the facile release and repeatable of high ammonia uptakes of 11.5, 5.2, and 3.8 mmol/g, respectively. In addition, the MOFs have good stability and...

Metal-free catalytic hydrogen production from a polymethylhydrosilane–water mixture

Hydrogen gas is the most promising carbon-free energy carrier although its on-demand generation remains a formidable challenge.

(Invited) Renewable Hydrogen Evolution on Nickel Phosphide Electrocatalysts: A Comparative Study of Efficiency and Tolerance to Corrosion

The intermittent nature of renewable energy sources like photovoltaics and wind mills requires storage of large quantities of energy. Renewable hydrogen produced by electrochemically water splitting into its elements pose an unlimited resource for energy storage. Gaseous hydrogen is not only a carbon-free energy carrier which could replace fossil fuels, but also an important bulk chemical of which ~108 ton/year is produced from natural gas. Platinum and nickel (or alloys thereof) are the electrocatalysts used today for hydrogen evolution in acidic and alkaline solution, respectively. However, the scarcity of platinum and the low efficiency of nickel, have greatly restricted the development of renewable hydrogen from water splitting. Since 2013 the number of reports on the use of transition metal phosphides as a replacement for Pt for hydrogen evolution have virtually exploded. Ni3P and Ni2P were the first phases shown to be highly electrochemically active for hydrogen evolution. Ni2P was unfortunately was shown to deactivate in acid and alkali. We recently reported, that another crystalline phase of the nickel phosphide family, Ni5P4, outlperforms Ni2P and Ni3P and is comparable to platinum on a geometric basis in its electrical efficiency at hydrogen evolution (kinetics and intrinsic overpotential). Moreover, unlike Ni2P this catalyst is corrosion resistant, being stable in both strong acid and alkali during electrocatalysis. Here, we will present our most recent work on multiple nickel phosphides as well a structural basis for the unusually high catalytic activity and corrosion resistance. The work was supported by AFOSR, the NSF and NATCO Pharma. Figure 1

Plasmonic hydrogen sensing with nanostructured metal hydrides.

In this review, we discuss the evolution of localized surface plasmon resonance and surface plasmon resonance hydrogen sensors based on nanostructured metal hydrides, which has accelerated significantly during the past 5 years. We put particular focus on how, conceptually, plasmonic resonances can be used to study metal-hydrogen interactions at the nanoscale, both at the ensemble and at the single-nanoparticle level. Such efforts are motivated by a fundamental interest in understanding the role of nanosizing on metal hydride formation processes in the quest to develop efficient solid-state hydrogen storage materials with fast response times, reasonable thermodynamics, and acceptable long-term stability. Therefore, a brief introduction to the thermodynamics of metal hydride formation is also given. However, plasmonic hydrogen sensors not only are of academic interest as research tool in materials science but also are predicted to find more practical use as all-optical gas detectors in industrial and medical applications, as well as in a future hydrogen economy, where hydrogen is used as a carbon free energy carrier. Therefore, the wide range of different plasmonic hydrogen sensor designs already available is reviewed together with theoretical efforts to understand their fundamentals and optimize their performance in terms of sensitivity. In this context, we also highlight important challenges to be addressed in the future to take plasmonic hydrogen sensors from the laboratory to real applications in devices, including poisoning/deactivation of the active materials, sensor lifetime, and cross-sensitivity toward other gas species.

Cation/anion dependence of metal ammine borohydrides/chlorides studied by ab initio calculations

Ammonia (NH3), which contains about 18mass% of hydrogen, is regarded not only as one of the most promising hydrogen storage materials but also as an efficient carbon-free energy carrier which can be directly used by e.g. high-temperature solid oxide fuel cells. For the development of the NH3-based sustainable society, it is important to improve NH3 storage technologies. In this paper, we have studied the microscopic properties of NH3 absorption in metal borohydrides and metal chlorides by ab initio calculations. We have revealed that there is a systematic cation dependence in the NH3 absorption properties of such materials, and the properties of NH3 absorption are mainly governed by NH3-cation interactions and NH3-anion repulsions.

Solid ammonia as energy carrier: Current status and future prospects

Ammonia is very attractive as an energy carrier for its high energy density and potential of being a ‘zero-carbon' fuel. However, ammonia is toxic, and because of the very high vapor pressure of liquid ammonia (∼8 bar at room temperature), there are safety concerns about storing liquid ammonia for end-user applications. Amminex has developed a method to store ammonia safely as solid metal ammines. The Amminex product, Hydrammine™, is a non-pressurized storage material, and has an energy density similar to that of liquid ammonia (∼110 kg H2/m3). It enables safe use of ammonia as an energy carrier for end-user applications. Amminex has been active in integrating the solid ammonia storage technology with PEMFC and SOFC stacks. This article focuses on the potential of ‘solid' ammonia as a carbon-free energy carrier for mobile and transport applications, system integration (PEMFC and SOFC), and future opportunities.

Carbon-Free Hydrogen and Electricity From Coal: Options for Syngas Cooling in Systems Using a Hydrogen Separation Membrane Reactor

Conversion of coal to carbon-free energy carriers, H2 and electricity, with CO2 capture and storage may have the potential to satisfy at a comparatively low cost much of the energy requirements in a carbon-constrained world. In a set of recent studies, we have assessed the thermodynamic and economic performance of numerous coal-to-H2 plants that employ O2-blown, entrained-flow gasification and sour water-gas shift (WGS) reactors, examining the effects of system pressure, syngas cooling via quench versus heat exchangers, “conventional” H2 separation via pressure swing adsorption versus novel membrane-based approaches, and various gas turbine technologies for generating coproduct electricity. This study focuses on the synergy between H2 separation membrane reactors (HSMRs) and syngas cooling with radiant and convective heat exchangers; such “syngas coolers” invariably boost system efficiency over that obtained with quench-cooled gasification. Conventional H2 separation requires a relatively high steam-to-carbon ratio (S/C) to achieve a high level of H2 production, and thus is well matched to relatively inefficient quench cooling. In contrast, HSMRs shift the WGS equilibrium by continuously extracting reaction product H2, thereby allowing a much lower S/C ratio and consequently a higher degree of heat recovery and (potentially) system efficiency. We first present a parametric analysis illuminating the interaction between the syngas coolers, high temperature WGS reactor, and HSMR. We then compare the performance and cost of six different plant configurations, highlighting (1) the relative merits of the two syngas cooling methods in membrane-based systems, and (2) the comparative performance of conventional versus HSMR-based H2 separation in plants with syngas coolers.

Separation of hydrogen from syngas using a regenerative system

A method of separating hydrogen from syngas for use as a carbon-free energy carrier is reported. The overall process consists of a hydrogen enrichment step and a solids regeneration step in a lab-scale, fixed-bed reactor under atmospheric pressure. During the enrichment step, syngas and steam are passed through iron oxide and calcium oxide beds where multiple reactions occur simultaneously to yield a stream of hydrogen. The syngas and steam feeds are then replaced by air to regenerate the solids, releasing sequestration-ready carbon dioxide. The effects of moderate temperatures (600– 800 ∘ C ), steam concentration, syngas concentration, residence time, and solids composition on hydrogen purity were studied. The information obtained was used to optimize the process and achieve a 99% hydrogen stream. Ten cycles using the same solids were completed with a minimal loss in performance observed.