Effect of rare earth (RE – La,Pr, Nd) metal-doped ceria nanoparticles on catalytic hydrogen iodide decomposition for hydrogen production

In this work, high surface area rare earth (RE = La, Pr, and Nd) metal-doped ceria (CeO₂) nanocatalysts have been synthesized by the citric-aided sol-gel method for hydrogen-iodide decomposition in thermochemical water-splitting sulfur-iodine (SI) cycle for hydrogen production. This sol-gel method allows the insertion of rare earth metal M³⁺ ions into the CeO₂ material. Incorporation of rare earth metals created a different synergistic effect between RE and Ce components such as increase of oxygen mobility, oxygen vacancy, and thermal stability of the CeO₂ material. These doped-CeO₂ materials were characterized by various physicochemical techniques, namely, XRD, BET, ICP-AES, TEM, TGA, and RAMAN spectroscopy. XRD and TEM studies revealed 5–10 nm particles of the RE-CeO₂ material. Shifting of peaks and increase in lattice parameter values confirmed the formation of Ce-RE solid solutions (XRD and Raman). Incorporation of dopants resulted in an increase in the specific surface area (BET), thermal stability (TGA), and oxygen vacancy concentration (Raman). Among different dopants, CeO₂-L (La-doped CeO₂) material exhibits the highest specific surface area, thermal stability, and oxygen vacancy concentration, and smallest crystallite size. The catalytic activity of doped-CeO₂ materials is explored for hydrogen-iodide decomposition. The order of catalytic activity is as follows: CeO₂ < CeO₂-N (N = Nd) < CeO₂-P (P = Pr) < CeO₂-L (L = La). CeO₂-L shows higher catalytic activity and stability in comparison to the pure CeO₂ material. It also showed an excellent time-on-stream stability for 35 h. The apparent activation energy of CeO₂-L, CeO₂-P, and CeO₂-N was found to be 48.9, 54.8, and 61.4 kJ mol^?1, respectively. The effect of iodine on hydrogen iodide conversion was also studied over a CeO₂-L catalyst. Thus, RE-doped CeO₂ catalysts show a lot of potential of generating hydrogen from hydrogen iodide in the SI cycle.     

Cycling stability of RNi5 (R = La,La+Ce) hydrides during the operation of metal hydride hydrogen compressor

This work presents results of the experimental studies (XRD, SEM, PCT) of hydride forming intermetallides used in the first (LaNi₅) and the second (La_0.5Ce_0.5Ni₅) stages of industrial-scale metal hydride hydrogen compressor providing H₂ compression from 3.5 to 150 atm with the productivity about 10 Nm³/h. During the operation, both materials underwent 18,180 hydrogenation/dehydrogenation (h/d) cycles which included H₂ absorption at the pressure of 3.5 atm (LaNi₅) and 35–38 atm (La_0.5Ce_0.5Ni₅) at T = 15–20 °C followed by H₂ desorption at the pressure of 35–38 atm (LaNi₅) and 150 atm (La_0.5Ce_0.5Ni₅) at T = 150–160 °C. It was found that the observed ～30% drop of the productivity of the compressor by the end of its operation is associated with a degradation of the first stage hydride material (LaNi₅) under conditions specified above. The cycling resulted in the appearance of Ni and LaH_2+x phases in addition to the parent intermetallide. In turn, the cycled LaNi₅ exhibited more than 20% lower hydrogen storage capacity than the alloy at the beginning of the cycling; the cycling was also found to result in a noticeable sloping of initially flat plateau. Conversely, the degradation effects in La_0.5Ce_0.5Ni₅ were found to be much less pronounced, in spite of the higher operating H₂ pressures. The observed effect was associated with the decrease of thermodynamic driving force (TDF) of AB₅ disproportionation in H₂ when substituting La with Ce.     

Effect of element substitution and surface treatment on low temperature properties of AB3.42-type La–Y–Ni based hydrogen storage alloy

The low-temperature performance (LTP) of AB_3.42-type La–Y–Ni hydrogen storage alloy was studied by methods of element substitution and surface treatment. The effect of Mn-additive on LTP of La_1·3Ce_0·5Y_4·2Ni_19.5-xMn_xAl (x = 0, 0.2, 0.5) was systematically investigated. Electrochemical studies showed that Mn-additive deteriorated the LTP of the alloy by reducing platform pressure, deteriorating kinetic performance and forming more oxides on the alloy surface. RE-substitution and hot alkali-ultrasonic treatment of La_1.3RE_0.5Y_4·2Ni_19·5Al (RE = Ce, Sm, Nd) alloys were applied to further optimize the LTP. The maximum discharge capacity and capacity retention at the 100th cycle of La_1·3Ce_0·5Y_4·2Ni_19·5Al alloy were 252.1 mA h/g and 87.1% at 243 K, respectively. Furthermore, the LTP of RE-substitution alloys at 243 K was conspicuously improved by surface treatment, which were raised from 214.7 mA h/g to 301.1 mA h/g by Sm-substitute, from 220.9 mA h/g to 303.9 mA h/g by Nd-substitute and from 252.1 mA h/g to 254.8 mA h/g by Ce-substitute.     

Effect of graphite (GR) content on electrochemical hydrogen storage performances of nanocrystalline and amorphous La9Ce1Mg80Ni5–Ni–GR composites synthesized by mechanical milling

The Mg–Ni-based alloy La₉Ce₁Mg₈₀Ni₅ was fabricated by a vacuum induction furnace with high purity helium gas. The surface modification of the as-cast alloys was operated by mechanical coating Ni and graphite (GR). The composites La₉Ce₁Mg₈₀Ni₅-200 wt% Ni-x wt.% GR (x = 0–4) with nanocrystalline and amorphous structures were synthesized by mechanical milling. Adding appropriate GR brings on the enhancement of ball-milling efficiency and inhibits the agglomeration of alloy powders. Furthermore, the discharge capacity of the composites obtains the maximal values with an optimized GR percentage. Increasing GR content from 0 to 4, the capacity retention rate at 20th cycle (S₂₀ = C₂₀/C_max) of the 20 h milled composite improves from 72.2% to 74.3% and that of the 80 h milled specimen changes from 54.4% to 56.9%. Electrochemical tests indicate that with the optimization of GR percentage, the composites can get the best electrochemical kinetic property, such as the highest HRD value, the highest hydrogen diffusion coefficient and the lowest charge transfer resistance.     

Phase formation of Ce5Co19-type super-stacking structure and its effect on electrochemical and hydrogen storage properties of La0.60M0.20Mg0.20Ni3.80 (M = La,Pr, Nd,Gd) compounds

In order to investigate the formation mechanism of Ce₅Co₁₉-type super-stacking structure phase, La_0.60M_0.20Mg_0.20Ni_3.80 (M = La, Pr, Nd, Gd) compounds are synthesized by powder sintering method. Rietveld refinements of X-ray diffraction patterns find that La_0.80Mg_0.20Ni_3.80 compound has a single Pr₅Co₁₉-type structure. The Ce₅Co₁₉-type phase appears and increases with the decrease of atomic radius of M, until the La_0.60Gd_0.20Mg_0.20Ni_3.80 compound shows a Ce₅Co₁₉-type single phase structure. The cycling stability and high rate dischargeability (HRD) of the alloy electrodes both improve with the increase of Ce₅Co₁₉-type phase. The capacity retention of La_0.60Gd_0.20Mg_0.20Ni_3.80 compound at the 100th cycle is high to 93.6% and the HRD reaches 66.9% at a discharge current density of 1500 mA g^?1. Moreover after 50 charge/discharge cycles, the Ce₅Co₁₉-type particle retains an intact crystal structure while severe amorphization occurs to Pr₅Co₁₉-type particle as shown in graphical abstract. The cohesive energy obtained from the First-principle calculations is analyzed combined with the experimental results. It is found that the La_0.60Gd_0.20Mg_0.20Ni_3.80 compound with Ce₅Co₁₉-type single phase structure has the highest cohesive energy indicating a more stable structure. This work provides new insights into the superior composition-structure design of LaMgNi system hydrogen storage alloys that may improve the cycling stability.     

Comment on: “Influence of the calcination temperature on the structure and reducibility of nanoceria obtained from crystalline Ce(OH)CO3 precursor,” by Poggio et al., Int J Hydrogen Energy 2011;36:15899–15905

In this comment I point out that the electronic structure of CeO₂ and Ce₂O₃ oxides and the physical principles which govern their photoemission should be properly taken into account to peak fit their XPS Ce3d spectra.