ARTICLE pubs.acs.org/JPCC
Mitigation of Hydrogen Capacity Losses during Pressure Cycling of the Li3NH System by the Addition of Nitrogen Joshua Lamb, Dhanesh Chandra,* and Wen-Ming Chien Chemical and Materials Engineering, 1664 North Virginia Street, MS 388, University of Nevada, Reno, Reno Nevada 89557, United States
erny , and Klaus Yvon Delphine Phanon, Nicolas Penin, Radovan C Laboratoire de Crystallographie, University of Geneva, Quai Ernest-Ansermet 24, CH-1211 Geneve 4, Switzerland
bS Supporting Information ABSTRACT:
We report PCT measurements, reaction paths, and thermodynamic modeling of prolonged pressure cycling of Li3N in mixed hydrogen/nitrogen atmospheres. For 20 mol % N2 in H2 mixtures, we observe a significant enhancement of the reversible capacity (from ∼3 to ∼10 wt % H after 516 cycles at 528 K) as compared with cycling without nitrogen additives. We attribute this enhancement to the reaction of nitrogen with liquid lithium during cycling as the Gibbs free energy of formation of Li3N (ΔGo = 98.7 kJ/mol) is more negative than that of LiH (ΔGo = 50.3 kJ/mol). We propose that the mitigation of hydrogen capacity losses is due to the partial destabilization of the LiH phase that tends to accumulate during cycling and the liquid Li formation.
1. INTRODUCTION The depletion of fossil fuels and the harmful environmental effects of greenhouse gas emissions and other pollutants caused by automobiles have led to a drive to develop alternative fuels capable of reducing pollution and of improving energy efficiency. This triggered intensive research on hydrogen as a renewable fuel because the exhaust gases in hydrogen-powered vehicles mainly contain water vapor. In a classical approach, hydrogen is used as a fuel for internal combustion (IC) engines, but by far a more energy efficient and popular method is the direct conversion of hydrogen into electricity in a proton exchange membrane (PEM) fuel cell. At present, however, both technologies are not yet competitive commercially with gasoline and diesel fuel, which offer both an established infrastructure and a high density, stable energy storage. Development and research into LiNH-based compounds as hydrogen storage media began in 2002 when Chen et al.1,2 reported a total absorption of 11.5 wt % hydrogen by lithium nitride, Li3N. Their studies revealed that Li3N was able to store hydrogen in the lithium amide form following the multistep reaction shown in eq 1 Li3 N + 2H2 ¼ Li2 NH + LiH + H2 ¼ LiNH2 + 2LiH r 2011 American Chemical Society
ð1Þ
Although initial hydriding of this material provides a relatively high hydrogen capacity (∼11 wt % H), its reversible H content is typically only 6 to 7 wt % at 558 K.3 The reduction in overall capacity comes primarily from the irreversibility of the exothermic conversion of Li3N into Li2NH, reported already in 1909 by Dafert and Miklauz4 and shown in eq 2.1,3 Li3 N + H 2 ¼ Li2 NH + LiH ΔH 161 kJ=mol 3 H2
ð2Þ
Because of this irreversibility, the bulk of the reversible capacity of this material comes from the second half of the reaction, detailed in eq 3.1,2 Li2 NH + H2 ¼ LiNH2 + LiHΔH 45 to 51 kJ=mol 3 H2 ð3Þ Efforts to increase the reversible capacity of the Li3N + H system had so far only limited success.1,2,524 The central problem is that Received: March 13, 2011 Revised: June 4, 2011 Published: June 13, 2011 14386
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Figure 1. Pressure composition isotherms (absorption) at 528 K of Li3NH2(N2) systems (a) Li3N hydrided with pure H2 (0th cycle), 1 cycle with pure H2, and 501 cycles with industrial hydrogen. (b) We compare the isotherms of the 345 and 853 pressure cycles (with 100 ppm N2 in H2) with 1 cycle H2 isotherm, showing a trend of increasing capacity with pressure cycling.
Li2NH does not reverse easily to either Li4NH or Li3N at 528 K, leading to ∼6.6 wt % H reversible capacity. Recently, however, Huq et al.25 showed that at ∼593 K the reaction of LiH f Li2NH was possible. Later, Chien et al.26 showed that during repeated cycling in a pure hydrogen atmosphere (under dynamic vacuum) there was a loss of overall nitrogen that degraded significantly the reversible hydrogen capacity of the Li3NH system. This was mainly due to the buildup of the unwanted LiH phase in the system. Because LiH is relatively stable at moderate temperatures (∼4 wt %) the absorption paths are very similar, albeit at slightly higher pressures for the nitrogen containing working gas. In both cases, the material reaches full capacity (∼9.5 wt %) below 1 bar pressure.
Table 1 and as predicted by Kim et al.29 We also calculated the decomposition of NH3 and LiNH2 at different pressures and temperatures show that NH3 decomposes at lower temperature in the regime of 1 and 10 atm pressure. These results are shown in Figure 5; the details of how we derived these plots are shown in the Supporting Information in Figures S1S3 and Table S1. We calculated the gas phase equilibria of N2/H2/NH3 in a similar manner, as suggested by Kim et al.29 for NH3 decomposition and plotted in Supporting Information in Figures S4 and S5 and Table S2. Kim et al.29 also predicted the equilibrium concentration of gaseous and condensed phases in their paper, suggesting that decomposition of LiNH2 produces NH3 gases at temperatures >644 K. They also commented that NH3 is formed because of slow kinetics of formation of N2. Ichikawa et al.13 also found similar results above 625 K, where there was appreciable formation of NH3, but very small amounts of ammonia formed below 570 K. From our calculations, we find that there is very little NH3 gas formed at 528 K (Table 1). We had experimentally deduced that there was loss of nitrogen gas during cycling, as more LiH phase, rather than imideamidetype phases, was produced during cycling. The modeling studies by Kim et al.29 also show nearly the same behavior. To compensate for the loss of N2 in the system, the addition of relatively small amounts of nitrogen gas to hydrogen improved the cycling stability of the Li3NH system. This surprising result may be explained as follows: nitrogen, largely an inert gas, reacts with very few materials but is noted for reaction with solid or liquid lithium. The free energy for the formation of LiH and Li3N from pure lithium can be estimated from eqs 4 and 5, respectively. Formation of LiH (456.7962 K, kcal/mol)30,31 ΔGf o ¼ 22:743 + 3:151 103 T lnðTÞ 3:957
4. DISCUSSION In the Li3NH system, the main problems are: (1) Parasitic LiH phase is formed along with other desirable LiNH phases during hydriding. The LiH phase has ∼12 wt % H trapped in it, and it is difficult to dissociate this phase under normal operating temperatures. Chen et al.1,2 reported Li3N + H2 T Li2NH + LiH (eq 2), followed by Li2NH + H2 = LiNH2 + LiH (eq 3). (2) During the desorption of LiNH2 phase, H2, N2, and also NH3 gases are produced, but the amount of NH3 is low, as shown in
106 T 2 + 170:675T 1 + 2:035 103 T
ð4Þ
Formation of Li3N (453.71086 K, kcal/mol)31,32 ΔGf o ¼ 40:510 + 9:683 103 T lnðTÞ 9:687 106 T 2 + 233:425T 1 24:376 103 T 14388
ð5Þ
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Table 1. Mass Fractions Representing Coordinates Marked in Figure 6, showing progression of hydriding along the line Li3NH from points 1 to 4 in Figure 6, ending in Li2NH + LiH + Gasa coordinates of points on the ternary diagram along Li3NH line
Li2NH
LiH
hyd. gas
NH3 gas
H/(Li + N + H)
Li/(Li + N + H)
N/(Li + N + H)
grams
1
0.10
0.54
0.36
0.75
0.20
0.049
2.062 1008
2
0.20
0.48
0.32
0.67
0.18
0.15
6.300 1008
3 4
0.60 0.90
0.24 0.06
0.16 0.04
0.33 0.08
0.10 0.03
0.57 0.89
2.394 1007 3.738 1007
5
0.97
0.01
0.02
0.01
0.00
0.99
1.683 1003
6
0.15
0.2
0.65
0.22
0.00
0.76
5.200 1002
a
Point 5 shows the composition with 100 ppm molar N2 added to the gas phase. Point 6 is situated on the line progressing to 80 mol % H2 and 20 mol % N2, which occurs at Point 7 (∼23 wt % H2, 77 wt % N2).
Figure 5. Pressure dependent on decomposition temperatures of LiNH2 and NH3 at 1, 10, and 100 atm under gas mixtures (from FACT Sage calculations).
For the temperature at which our PCT data was taken (T = 528 K), these equations yield a free energy of formation of 50.3 kJ/mol for LiH and 98.7 kJ/mol for Li3N. This suggests that Li3N tends to form preferentially over LiH, at least at this temperature. Thus, if pure lithium forms as an intermediary phase during cycling of Li3N in hydrogen, the addition of nitrogen might possibly mitigate the formation of the more stable and unwanted LiH. To see whether the formation of Li during cycling was thermodynamically feasible, we constructed a ternary LiNH phase diagram at 528 K and 2 bar hydrogen pressure by using the CALPHAD approach. Results obtained by using the FactSage software33 are shown in Figure 6. The phase diagram shows reaction paths for Li3N reacting with pure hydrogen, following the indicating arrow as the material hydrogenates. Hydrogenation in gas containing 100 ppm N2 also follows this line closely, with some difference shown at high levels as marked by point 5. The line containing point 6 shows the path for hydrogen containing 20 mol % N2 (point 6). The exact coordinates of the various points are detailed in Table 1. Clearly, at low hydrogen concentrations, that is, in early stages of the hydriding process, the formation of liquid lithium is a thermodynamically favored for all reaction paths. In case Li3N reacts with pure hydrogen (see reaction path 1234 in
Figure 6. LiNH phase diagram at 528 K and 2 bar hydrogen pressure from CALPHAD simulations. Highlighted areas show phase fields containing liquid lithium [Li(liq)]. The two red dashed lines represent the reaction paths for Li3N in pure hydrogen and in hydrogen containing 20 mol % N2; the blue arrows show the direction of absorption along these paths. Point 7 here indicates the hydrogen containing 20 mol % N2 in H2 (converted to mass fractions as 78 wt % N2 in H2).
Figure 6), the following intermediary phases are expected to form as detailed in eq 6: Absorption at 528 K, 2 bar H2 Li3 NðsÞ + H2 ðgÞ f Li3 NðsÞ + Li2 NHðsÞ + LiðliqÞ ðup to 2:5 wt %HÞ f LiH + Li2 NHðsÞ + LiðliqÞ ðup to ∼ 5 wt %HÞ f LiHðsÞ + Li2 NHðsÞ + gas ð > ∼ 5 wt %HÞ ð6Þ Note that in the absence of nitrogen Li(liq) reacts with hydrogen to form LiH, which presumably accumulates during cycling and thus is responsible for the observed capacity loss. If Li3N is exposed to hydrogen containing 100 ppm N2 (see point 5 in Figure 6), the reaction pathway is essentially the same as that in pure hydrogen. However, in this case, nitrogen may react with Li(liq) to form Li3N (rather than LiH) and thus contributes in mitigating capacity losses during cycling. 14389
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The Journal of Physical Chemistry C A closer inspection of the reaction pathways at increasing H content reveals that liquid lithium is present until ∼5 wt % hydrogen absorption in Li3N, corresponding to about half its overall H capacity. Therefore, Li(liq) is able to react with nitrogen during a wide range of H concentration. However, at higher hydrogen content, the phase diagram does not show a formation of pure lithium, likely rendering the nitrogen present inert from that point onward and behaving as if only pure hydrogen was present. This is also supported by the PCT data obtained by using 20 mol % N2 in H2 working gas mixture (Figure 2). Whereas the general shape of the absorption isotherm at low gas pressures (concentrations) differs from that measured in pure hydrogen, the isotherms become very similar above ∼4 to 5 wt % apparent absorption, albeit at a slightly higher pressure in the presence of nitrogen compared with that for pure hydrogen. Cycling using 20 wt % N2 in H2 mixtures has shown even further reductions to capacity loss. A total capacity of 8.2 wt % was observed at 1 bar. This compares to an observed initial capacity of up to 10 to 11 wt % at 1 bar1,13,26 for an overall capacity loss of ∼2 wt % H after 516 cycles. The reaction path is shown along a line starting from the point Li3N to the point 7 [with 0.22 mass fraction H along H-N line in this case (eq 7)] Absorption at 528 K, 2 bar 20 mol % N2 in H2 Li3 NðsÞ + 0:8H2 ðgÞ + 0:2N2 ðgÞ f Li3 NðsÞ + Li2 NHðsÞ + LiðliqÞ ðup to 2:5 wt % HÞ f LiHðsÞ + Li2 NHðsÞ + LiðliqÞ ðup to ∼ 3:5 wt % HÞ f LiHðsÞ + Li2 NHðsÞ + gas ðup to 4 wt % HÞ f Li2 NHðsÞ + LiNH2 ðsÞ + gas ðup to ∼ 7:4 wt % HÞ f LiNH2 + gas ðup to ∼ 10:125 wt % HÞ ð7Þ Both reaction pathways (eqs 6 and 7) illustrated in Figure 6 show the formation of Li2NH and Li early in the hydrogenation reaction. This may be an indication that the capacity improvements may be coming from the direct formation of Li2NH or LiNH2 at higher nitrogen content. At low concentrations, the reactions pass over similar phase fields, but much better mitigation is seen for the 20% N2 in H2 mixtures, as seen in Figure 2. This pathway does, however, have a smaller window where LiH is produced, being present from ∼2.5 to ∼7.4 wt % H, whereas in the case of concentrations close to pure hydrogen gas, LiH is stable at all concentrations above ∼2.5 wt % H. (See also the pathways detailed in eq 6 vs eq 7). The formation of greater quantities of Li2NH and LiNH2, which hydride and dehydride more easily than LiH, respectively, is a possible source of the observed improvement.
5. CONCLUSIONS Cycling lithium nitride with hydrogen containing 100 ppm nitrogen gas showed improvement of hydrogen storage properties over that cycled without the nitrogen addition. Specifically, the reversible capacity more closely approached the theoretical capacity (∼10.5 wt %) and showed improved cycling stability, losing little overall capacity over up to 853 cycles. At 1 bar, a capacity of 6.8 wt % was observed after 853 cycles compared with a capacity of ∼3 wt % observed with industrial hydrogen after 501 cycles. The addition of nitrogen in this case mitigated almost 4 wt % of capacity loss or ∼36% of the total theoretical capacity. We propose that the ability of the nitrogen to have an effect in such small quantities is due to the batch reactor nature of the apparatus used. As the hydrogen
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reacts, the quantity of nitrogen builds until it is much higher than the initial quantity. This can be seen by following the path on the phase diagram as hydrogen is absorbed the amount in the gas is reduced, causing the system to behave as if a higher concentration of nitrogen in the gas phase was used. Cycling with hydrogen containing 20 mol % N2 was able to improve the cycling stability over a large number of cycles and allowed the lithium nitride to maintain a capacity close to its starting capacity. An absorption capacity of ∼10 wt % at 528 K was seen up to at least 500 cycles. It is proposed that based on the data obtained and CALPHAD modeling that the improvement in cycling is due to the formation of pure liquid lithium, which is able to react with nitrogen forming lithiumnitrogen or lithiumnitrogen-hydrogen compounds, specifically Li3N, Li2NH, or LiNH2 depending on the point of the system on the phase diagram. The continuous presence of nitrogen in the system prevents the continuous buildup of LiH, which over time causes a decrease in the overall capacity of the hydride bed.
’ ASSOCIATED CONTENT
bS
Supporting Information. NH3 decomposition reaction data, Gibbs energy plots of NH3 and LiNH2 decomposition reactions, and gas phase equilibria of N2-H2-NH3 mixtures. This material is available free of charge via the Internet at http://pubs. acs.org.
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