Solid Acid Electrochemical Cell for the Production of Hydrogen from Ammonia

Abstract

•Superprotonic CsH2PO4 enables electrochemical cell operation at 250°C•Ammonia decomposition catalyst integrated with hydrogen electrooxidation catalyst•Ammonia converted to hydrogen with 100% faradic efficiency•Hydrogen production rate of 1.5 mol per gram catalyst per hour at 0.4 V bias Ammonia has received increasing attention in recent years as a possible energy carrier, in particular, as a carrier of hydrogen for use in fuel cells. The traditional approach of thermal decomposition suffers from high concentrations of residual ammonia, which poison the catalysts in polymer electrolyte membrane fuel cells, whereas newer strategies based on electrochemical decomposition in aqueous solution operate at high overpotentials, implying low efficiency. Our approach integrates a thermal decomposition catalyst (Cs-promoted Ru on carbon nanotubes) with an all-solid-state electrochemical conversion cell (based on the proton-conducting electrolyte, CsH2PO4) in a device that is operable at 250°C. The resulting polarization curves indicate high current density at a modest voltage (far beyond what can be attained from alkali electrolyte cells), as well as catalyst utilization efficiency that far exceeds traditional thermal decomposition. Production of high-purity hydrogen by thermal-electrochemical decomposition of ammonia at an intermediate temperature of 250°C is demonstrated. The process is enabled by use of a solid-acid-based electrochemical cell (SAEC) in combination with a bilayered anode, comprising a thermal-cracking catalyst layer and a hydrogen electrooxidation catalyst layer. Cs-promoted Ru on carbon nanotubes (Ru/CNT) serves as the thermal decomposition catalyst, and Pt on carbon black mixed with CsH2PO4 is used to catalyze hydrogen electrooxidation. Cells were operated at 250°C with humidified dilute ammonia supplied to the anode and humidified hydrogen supplied to the counter electrode. A current density of 435 mA/cm2 was achieved at a potential of 0.4 V and ammonia flow rate of 30 sccm. With a demonstrated faradic efficiency for hydrogen production of 100%, the process yields hydrogen at a rate of 1.48 molH2/gcath. Production of high-purity hydrogen by thermal-electrochemical decomposition of ammonia at an intermediate temperature of 250°C is demonstrated. The process is enabled by use of a solid-acid-based electrochemical cell (SAEC) in combination with a bilayered anode, comprising a thermal-cracking catalyst layer and a hydrogen electrooxidation catalyst layer. Cs-promoted Ru on carbon nanotubes (Ru/CNT) serves as the thermal decomposition catalyst, and Pt on carbon black mixed with CsH2PO4 is used to catalyze hydrogen electrooxidation. Cells were operated at 250°C with humidified dilute ammonia supplied to the anode and humidified hydrogen supplied to the counter electrode. A current density of 435 mA/cm2 was achieved at a potential of 0.4 V and ammonia flow rate of 30 sccm. With a demonstrated faradic efficiency for hydrogen production of 100%, the process yields hydrogen at a rate of 1.48 molH2/gcath. Hydrogen has been proposed as an energy carrier in a sustainable energy future. When coupled with fuel cells, the energy content is converted on demand and with high efficiency into useful work, with water being the only emission generated at the point of use.1Edwards P.P. Kuznetsov V.L. David W.I.F. Brandon N.P. Hydrogen and fuel cells: Towards a sustainable energy future.Energy Policy. 2008; 36: 4356-4362Crossref Scopus (626) Google Scholar However, hydrogen has a low volumetric energy density, a low flash-point, and lacks a wide infrastructure for its storage and transport.2Markert F. Marangon A. Carcassi M. Duijm N.J. Risk and sustainability analysis of complex hydrogen infrastructures.Int. J. Hydr. 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A mini-review on ammonia decomposition catalysts for on-site generation of hydrogen for fuel cell applications.Appl. Cat. A. 2004; 277: 1-9Crossref Scopus (480) Google Scholar As the electrochemical component, we employ a cell based on the proton-conducting electrolyte, cesium dihydrogen phosphate (CDP), a solid acid compound that has long been exploited in intermediate temperature solid acid fuel cells (SAFCs)22Chisholm C.R.I. Boysen D.A. Papandrew A.B. Zecevic S. Cha S. Sasaki K. Varga A. Giapis K.P. Haile S.M. From laboratory breakthrough to technological realization: the development path for solid acid fuel cells.Electrochem Soc Int. 2009; 18: 53-59Google Scholar, 23Haile S.M. Chisholm C.R.I. Sasaki K. Boysen D.A. Uda T. Solid acid proton conductors: from laboratory curiosities to fuel cell electrolytes.Faraday Discuss. 2007; 134: 17-39Crossref PubMed Google Scholar, 24Uda T. Boysen D.A. Chisholm C.R.I. Haile S.M. Alcohol fuel cells at optimal temperatures.Electrochem. 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Intermediate temperature proton-conducting membrane electrolytes for fuel cells.WIREs Energy Environ. 2014; 3: 24-41Crossref Scopus (58) Google Scholar and is non-reactive with NH3 (Figure S1). The electrocatalyst is Pt, which has demonstrated high tolerance to fuel impurities at the operation temperature of 250°C (for example, up to 20% CO22Chisholm C.R.I. Boysen D.A. Papandrew A.B. Zecevic S. Cha S. Sasaki K. Varga A. Giapis K.P. Haile S.M. From laboratory breakthrough to technological realization: the development path for solid acid fuel cells.Electrochem Soc Int. 2009; 18: 53-59Google Scholar), suggesting electrochemical functionality even in the presence of residual NH3. By integrating the thermal decomposition with electrochemical removal of hydrogen from the reaction zone, we aim to overcome thermodynamic limitations otherwise imposed by product accumulation. The overall configuration of the ammonia decomposition cells is presented in Figure 1. The internal thermal-cracking catalyst layer (TCL), Figures S2–S4, is placed adjacent to the hydrogen oxidation electrocatalyst layer (EL), which is, in turn, adjacent to the CDP electrolyte. The entire structure, which includes a hydrogen evolution electrocatalyst layer at the counter electrode, is placed between stainless steel mesh current collectors. The configuration is analogous to that used in direct methanol SAFCs24Uda T. Boysen D.A. Chisholm C.R.I. Haile S.M. Alcohol fuel cells at optimal temperatures.Electrochem. Solid-State Lett. 2006; 9: A261-A264Crossref Scopus (57) Google Scholar and other fuel cells operated on complex (non-hydrogen) fuels.30Zhan Z.L. Barnett S.A. An octane-fueled solid oxide fuel cell.Science. 2005; 308: 844-847Crossref PubMed Scopus (471) Google Scholar Using three distinct cells to assess reproducibility, we first checked for leakage through the electrolyte membrane by measuring the open circuit voltage (OCV) with dilute H2 supplied to the working electrode. The recorded voltages of 72, 73, and 73 mV were consistent with the value of 73 mV implied by the Nernst equation:EN=12RTln(pH2CEpH2WE)(Equation 2) where R is the universal gas constant, T is temperature, and pH2WE and pH2CE are the respective hydrogen partial pressures at the working and counter electrodes. The agreement between the Nernst and measured values demonstrates not only the absence of gas leaks but also the high ionic transference number of CDP. We then assessed the electrochemical characteristics under open circuit conditions by impedance spectroscopy, Figure S5. The measured ohmic losses of 0.24–0.26 Ωcm2 were comparable with the expected value of 0.25 Ωcm2 for the 50 μm-thick electrolyte with conductivity of 2.0 × 10−2 S/cm at 250°C.23Haile S.M. Chisholm C.R.I. Sasaki K. Boysen D.A. Uda T. Solid acid proton conductors: from laboratory curiosities to fuel cell electrolytes.Faraday Discuss. 2007; 134: 17-39Crossref PubMed Google Scholar,31Baranov A.I. Khiznichenko V.P. Sandler V.A. Shuvalov L.A. Frequency dielectric dispersion in the ferroelectric and superionic phases of CsH2PO4.Ferroelectrics. 1988; 81: 183-186Crossref Scopus (85) Google Scholar,32Otomo J. Minagawa N. Wen C.J. Eguchi K. Takahashi H. Protonic conduction of CsH2PO4 and its composite with silica in dry and humid atmospheres.Solid State Ionics. 2003; 156: 357-369Crossref Scopus (192) Google Scholar Polarization curves obtained under ammonia flow revealed excellent activity for ammonia decomposition, Figure 2A, as well as excellent cell-to-cell reproducibility, Table 1. The coincidence of the curves for the low NH3 condition, measured before and after exposure to high NH3, indicated good stability. A slight loss in performance, amounting to a 5%–6% decline in current density, could be due to the migration of the Cs promotor away from optimal sites in the Ru-Cs/CNT catalyst. Significantly, as expected for a solid-state electrolyte displaying pure protonic conduction, the faradic efficiency for hydrogen production was 100%, Figure 2B, and the generated hydrogen was free of impurities, Figure S6. Moreover, the data immediately revealed that substantially higher current densities were obtained upon supplying humidified NH3 than dilute, humidified H2, implying that electrochemical splitting of H2O, which is in any case thermodynamically unfavorable, does not contribute to the observed currents. Accordingly, in Figure 2C the implied ammonia-to-hydrogen conversion rates are shown, which were computed from the ammonia supply rates in conjunction with the 100% faradic efficiency observation. The possibility of reaction of NH3 with H2O during the ammonia oxidation reaction to form oxidized nitrogen (which would not impact the current efficiency for hydrogen production or hydrogen purity but would nevertheless be detrimental) was eliminated by chemical analysis of the anode side exhaust gas, Figure S7.Table 1Summary of Electrical Characteristics of Three Independent Cells for Ammonia-to-Hydrogen Electrochemical ConversionAt 0.4 pNH3Current Density (mA/cm2) at Voltage IndicatedAt 0.6 pNH3Current Density (mA/cm2) at Voltage IndicatedCell No.OCV (V)0.15 V0.3 VOCV (V)0.15 V0.3 V178126280631733652781372996716335637814229969173367Ave78 ± 1135 ± 8293 ± 1168 ± 4170 ± 6363 ± 6 Open table in a new tab The voltages obtained under open circuit conditions were 78 ± 1 and 68 ± 4 mV (as averaged across the three cells), for the respective ammonia partial pressures of 0.4 and 0.6 atm. Inverting the Nernst relationship, these voltages imply hydrogen partial pressures at the working electrode of 0.019 and 0.033 atm, respectively. From this, we computed respective chemical ammonia-to-hydrogen conversion rates of 3.4% ± 0.1% and 3.5% ± 0.7% at the two ammonia concentrations. These values are generally in line with the results reported by Hill et al. (2%–10% conversion) for similar catalyst materials, with the precise value depending on Cs and Ru loadings.17Hill A.K. Torrente-Murciano L. In-situ H2 production via low temperature decomposition of ammonia: insights into the role of cesium as a promoter.Int. J. Hydr. Energy. 2014; 39: 7646-7654Crossref Scopus (66) Google Scholar Even for identical loadings, differences are expected due to differences in precursor types, deposition methods, CNT source, and gas flow conditions. The general agreement between the results suggests that poisoning of Ru by H2O, present in high concentration in this experiment and absent in Hill’s work,17Hill A.K. Torrente-Murciano L. In-situ H2 production via low temperature decomposition of ammonia: insights into the role of cesium as a promoter.Int. J. Hydr. Energy. 2014; 39: 7646-7654Crossref Scopus (66) Google Scholar does not substantially interfere with ammonia decomposition at 250°C. Away from open circuit conditions, the current rises under both ammonia and dilute hydrogen with a relatively low overall cell resistance, indicating rather moderate and similar overpotentials. Because the hydrogen partial pressures are similar between the three conditions (pH2 = 0.024, 0.019, and 0.033 atm, respectively, in dilute hydrogen and at OCV in dilute and concentrated ammonia), the similarities in IV characteristics indicate that poisoning of the Pt electrocatalyst by unreacted NH3 is negligible. This is further corroborated by the impedance results, which indicate similar electrochemical reaction resistance for the supply of dilute H2 and of NH3 under OCV conditions, Figure S5. These resistance values range from 0.16 to 0.19 Ωcm.2 With increasing current and voltage, the IV curves deviated from linearity and from one another. The concavity of the curves is suggestive of mass transport limitations rather than Bulter-Volmer reaction kinetic limitations. In the case of dilute hydrogen, the IV curve plateaus relatively sharply at a current density corresponding to ∼90% of the limiting value. This is consistent with the supposition that depletion of hydrogen is responsible for the declining rate of increase in cell current density and that H2O electrolysis does not occur under these conditions. Under ammonia, the IV curves followed a much more gradual change in slope. Moreover, the maximum current densities achieved (382 and 480 mA/cm2 at low and high pNH3, respectively) at the maximum measurement voltage of 0.47 V were only 12% and 10% of the respective limiting values. Based on the absence of any evidence of Pt poisoning, we conclude that the overall conversion process is limited by the characteristics of the TCL in the ammonia decomposition process. It is of some note that at these maximum conditions the hydrogen partial pressure at the cathode is estimated to increase from 0.62 to 0.65 atm, adding slightly to the measured voltage. A further striking feature of the polarization curves, already alluded to above, is the substantially higher current densities achieved using 0.6 rather than 0.4 atm pNH3. This behavior, which is consistent with electrochemical oxidation of ammonia being the source of the current, suggests that reactant depletion in the TCL can be compensated, at least in part, by increasing the reactant (NH3) concentration. The resulting increase in current density, and hence the hydrogen production rate, was, however, accompanied by a decrease in conversion efficiency, Figure 2C. Thus, an increase in ammonia concentration did not result in a proportional increase in the availability of hydrogen. This observation points toward possible improvements by increasing the catalyst architecture to facilitate the removal of product N2, or by increasing the thickness of the TCL to increase the residence time of NH3 in the reaction zone, so long as this occurs without increasing the mass diffusion resistance. Beyond such design considerations, dramatic improvements may be possible by leveraging recent advances in TCL catalyst development.18Hill A.K. Torrente-Murciano L. Low temperature H2 production from ammonia using ruthenium-based catalysts: synergetic effect of promoter and support.Appl. Cat. B. 2015; 172–173: 129-135Crossref Scopus (90) Google Scholar Even in the absence of these steps, the polarization characteristics obtained here indicate performance characteristics that are far superior to those reported for alkali electrolysis cells in which large overpotentials (∼0.4 V beyond the open circuit condition) must be overcome before non-negligible current flows.15Modisha P. Bessarabov D. Electrocatalytic process for ammonia electrolysis: a remediation technique with hydrogen co-generation.Int. J. Electrochem. Sci. 2016; 11: 6627-6635Crossref Scopus (5) Google Scholar,16Vitse F. Cooper M. Botte G.G. On the use of ammonia electrolysis for hydrogen production.J. Power Sources. 2005; 142: 18-26Crossref Scopus (209) Google Scholar In the absence of the TCL, Figure 2D, the 20% Pt-C/CDP electrocatalyst displayed a relatively high OCV ∼365 mV under pNH3 of 0.4 atm, indicating negligible thermal ammonia decomposition (< 1 × 10−5 % conversion). The resulting currents under voltage bias were ∼1% of the values obtained from the cells incorporating explicit thermal-cracking layers. From this, it can be concluded that, in the cells with distinct cracking and electrochemical catalyst layers, the Pt serves only to oxidize the hydrogen, at least up to 0.36 V, with the Ru-based catalyst accounting for almost the entirety of the NH3 dissociation. On the other hand, in the absence of the Pt-based electrocatalyst, the cell with only the Ru/CNT + CDP dual-function catalyst layer displayed surprisingly poor IV curves, Figure 2D. When 0.4 atm pNH3 was supplied, the OCV was 170 mV (equivalent to 0.05% conversion), in contrast to 78 mV recorded from the bilayer-electrode system. The omission of the CsNO3 promotor in this catalyst is likely the cause, as Ru is active for hydrogen electrooxidation under low current conditions (Figure S8). The result indicates that CDP is ineffective as a promotor, presumably because the phosphate anion is retained in the material at the temperatures of interest, preventing conversion to CsOH. It is shown here that, by integrating electrochemical product removal with thermal decomposition of ammonia, it is possible to generate hydrogen at a substantially higher rate than by thermal decomposition alone. To put these results into context, the hydrogen production rates achieved here were compared, on a catalyst-mass normalized basis, to those from conventional thermal-cracking experiments reported in the literature, Figure 3.18Hill A.K. Torrente-Murciano L. Low temperature H2 production from ammonia using ruthenium-based catalysts: synergetic effect of promoter and support.Appl. Cat. B. 2015; 172–173: 129-135Crossref Scopus (90) Google Scholar,19Li J.P. Wang W.Y. Chen W.X. Gong Q.M. Luo J. Lin R.Q. Xin H.L. Zhang H. Wang D.S. 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To fully account for the catalysts used in the present work, the catalyst in both the TCL and complete electrochemical cell were included in the normalization. From this representation, it is evident that the application of moderate bias (0.4 V) results in a catalyst-mass normalized hydrogen production rate that matches the results obtained from thermal decomposition at a much higher temperature of 350°C to 500°C. Furthermore, the evolved hydrogen is free of residual ammonia and the configuration is amenable to electrochemical compression of hydrogen, or to operation of a direct ammonia fuel cell without the risk of generating NOx. A detailed efficiency analysis, which would require design of a system to minimize thermal inputs, is beyond the scope of this work, but a simple consideration of the minimum energy inputs relative to the energy content of the output hydrogen suggests high efficiencies are possible, Figure S9. In summary, the hybrid thermal-electrochemical approach demonstrated here, which integrates a solid-state proton conductor with an advanced thermal-cracking catalyst, shows great promise for ammonia-to-hydrogen or even ammonia-to-electricity conversion on demand.