Abstract
The present invention relates to palladium-platinum system consisting of palladium layer covered with a platinum overlayer consisting of 1 to 10 platinum monolayers deposited on palladium for use as hydrogen storage. Such system can be used in fuel cells, hydride batteries and supercapacitors. A method for increasing hydrogen absorption kinetics of hydrogen absorption/desorption process is also disclosed.
Claims
1. A method of reversible absorption and desorption of hydrogen comprising the following steps: a) providing a palladium layer having a thickness between 50-1000 nm to obtain a palladium surface; b) covering the palladium surface of the palladium layer provided in step a) with a platinum overlayer consisting of 1 to 10 platinum monolayers to cover the palladium layer, thus obtaining a Pd/Pt system of a reversible and stable hydrogen storage material having increased absorption kinetics; c) absorbing hydrogen in the reversible and stable hydrogen storage material obtained in step (b); and d) desorbing hydrogen from the reversible and stable hydrogen storage material obtained in step (b).
2. The method of reversible absorption and desorption of hydrogen according to claim 1, wherein step (c) and (d) are performed repeatedly.
3. The method of claim 2, further comprising before step a) a step of providing a palladium layer in which said palladium layer is deposited on a solid substrate.
4. The method of claim 3, wherein the platinum overlayer is a two-dimensional platinum overlayer consisting of 1 to 3 platinum monolayers.
5. The method of claim 4, wherein the platinum overlayer is a two-dimensional platinum overlayer consisting of at most two platinum monolayers.
6. The method of claim 5, wherein the platinum overlayer is a two-dimensional platinum overlayer consisting of a single platinum monolayer
7. The method according to claim 3, wherein the substrate is a metallic substrate.
8. The method according to claim 7, wherein the substrate is gold substrate.
9. The method according to claim 3, wherein the substrate is AB5.
10. The method according to claim 2, wherein the reversible absorption and desorption of hydrogen is in fuel cells or hydride batteries or supercapacitors.
11. A method of increasing hydrogen absorption/desorption kinetics of a hydrogen storage material comprising palladium, wherein the method comprise the following steps: a) providing a palladium layer having a thickness between 50-1000 nm to obtain a palladium surface; and b) covering the palladium surface of the palladium layer of step a) with a platinum overlayer consisting of 1 to 10 platinum monolayers to cover the palladium layer, thus obtaining a Pd/Pt system of reversible and stable hydrogen storage material having increased absorption kinetics in comparison to the palladium surface.
12. The method of claim 11, further comprising before step a) a step of providing a palladium layer in which said palladium layer is deposited on a solid substrate.
13. The method of claim 12, wherein the platinum overlayer is a two-dimensional platinum overlayer consisting of 1 to 3 platinum monolayers.
14. The method of claim 13, wherein the platinum overlayer is a two-dimensional platinum overlayer consisting of at most two platinum monolayers.
15. The method of claim 14, wherein the platinum overlayer is a two-dimensional platinum overlayer consisting of a single platinum monolayer.
16. The method according to claim 12, wherein the substrate is a metallic substrate.
18. The method according to claim 16, wherein metallic substrate is gold substrate.
19. The method according to claim 12, wherein the substrate is AB5.
20. The method according to claim 11, wherein the increase kinetics in hydrogen absorption/desorption of hydrogen is in fuel cells or hydride batteries or supercapacitors.
Description
BRIEF DESCRIPTION OF FIGURES
[0029] FIG. 1 presents cyclic voltammogram recorded for 1000 nm Pd in pure 0.1 M HClO.sub.4 (solid line) and in the presence of 1 mM CuSO.sub.4 (dotted line), scan rate 20 mV×s.sup.−1.
[0030] FIG. 2 presents cyclic voltammograms recorded for Pd 500 nm electrode and Pd 500 nm electrode with different platinum coverages (i.e. the Pd/Pt system of the invention) in pure 0.1 M HClO.sub.4, scan rate 100 mV×s.sup.−1.
[0031] FIG. 3A presents cyclic voltammograms for 500 nm Pd before and after Pt deposition, v=20 mV×s.sup.−1
[0032] FIG. 3B presents cyclic voltammetry currents expressed as pseudocapacitances j/v for 1000 nm Pd electrode, scan rates 1.0, 0.5 and 0.2 mV.Math.s.sup.−1
[0033] FIG. 4A presents cyclic voltammograms for Pd electrodes recorded for 500 nm Pd before and after Pt deposition, v=5 mV.Math.s.sup.−1;
[0034] FIG. 4B presents cyclic voltammograms for Pd electrodes recorded for 500 nm Pd before and after Pt deposition v=20 mV.Math.s.sup.−1.
[0035] FIG. 5A presents potentiostatic absorption currents for 1000 nm palladium FIG. 5B presents potentiostatic absorption currents for 1000 nm palladium covered by platinum monolayer 1 ML-Pt@Pd.
[0036] FIG. 6 presents linear dependence of the initial absorption currents for 1 ML Pt@1000 nm Pd in function of the electrode potential. The slope dI/dE=7.40 Ω.Math.cm.sup.2 is in agreement with the solution resistance determined in EIS measurements (6.5 Ω.Math.cm.sup.2).
[0037] FIGS. 7A and 7B present desorption currents registered after electrode saturation at different potentials (indicated in the Figure) for 1000 nm palladium layer (FIG. 7A) and the same electrode covered by platinum monolayer (FIG. 7B).
[0038] FIG. 8A presents chronoamperometric experiments at E=0.040 V vs. RHE performed for different times, t.sub.abs (indicated in the Figure), the inset shows related current desorption curves.
[0039] FIG. 8B presents hydrogen concentration in 1000 nm Pd and 1 ML Pt/Pd expressed as molar ratio n.sub.H/n.sub.Pd in function of equilibration time t.sub.abs.
[0040] FIG. 9 Cyclic voltammograms for Pd electrodes recorded for 1000 nm Pd covered by two Pt monolayers in 3M H.sub.2SO.sub.4, v=100 mV.Math.s.sup.−1, R.sub.f=4
[0041] FIG. 10A presents hydrogen absorption isotherms in pure Pd and 1 ML Pt/Pd.
[0042] FIG. 10B presents hysteresis in the hydrogen absorption/desorption recorded for 1000 nm Pd, v=100 mV.Math.s.sup.−1;
[0043] FIG. 10C presents cyclic voltammograms for Pd electrodes recorded for 500 nm Pd before and after Pt deposition, v=100 mV.Math.s.sup.−1.
[0044] FIGS. 11A and 11B present impedance spectra at E=0.170V vs. RHE for pure Pd (FIG. 11A) and 1 ML-Pt/Pd (FIG. 11B); continuous lines represent the fitting results: T.sub.dI=20.4±0.7 μFs.sup.φ−1cm.sup.−2, φ.sub.dI=0.943±0.002, R.sub.ct=84.3±0.7 Ωcm.sup.2, T.sub.HUPD=3.70±0.02 mFs.sup.φ−1cm.sup.−2, φ.sub.HUPD=0.924±0.002 (FIG. 11A); T.sub.CPE=2.58±0.02 mFs.sup.φ−1cm.sup.−2, φ.sub.CPE=0.93±0.01, W−R=5.4±0.1 Ωcm.sup.2, W−T=(5.7±0. 1)×10.sup.−2 s. W−φ=0.5 (FIG. 11B).
[0045] FIG. 12 presents cyclic voltammograms for AB5 (MmNi4.1Al0.2Mn0.4Co0.45) electrodes recorded before and after 1 ML-Pt/1 μm-Pd deposition, v=5 mV.Math.s.sup.−1.
EXPERIMENTAL
[0046] All palladium films of thickness between 50-1000 nm were deposited on a polycrystalline Au electrode according to under potential deposition (UPD) procedure described by Szpak et al. (S. Szpak, P. A. Mosierboss, S. R. Scharber and J. J. Smith, Journal of Electroanalytical Chemistry, 1992, 337, 147-163). Measurements were carried out in 0.1M HClO.sub.4 at 25±0.01° C. in two compartment electrochemical cell. A counter electrode was a large Pd sheet (˜10 cm.sup.2), the reference electrode was Hg|Hg.sub.2SO.sub.4|0.1M H.sub.2SO.sub.4 with the potential of 0.721V vs. RHE. All potentials are reported against the RHE. Pt overlayers were obtained by Cu displacement method as described in details by S. Brankovic et al. (S. Brankovic, J. Wang and R. Adžić, Surface Science, 2001, 474, L173-L179). Pd electrode covered with Cu monolayer deposited from 0.1M H.sub.2SO.sub.4+1 mM CuSO.sub.4 was transferred to 0.01M K.sub.2PtCl.sub.6 solution without exposure to air. Pt overlayers ranging from 1 to 10 ML were obtained by repeating this procedure. Spontaneous deposition (without Cu-UPD layers) was also performed. For this purpose Pd sample was immersed into 0.1M K.sub.2PtCl.sub.4 solution. Next, the electrode was thoroughly rinsed with deionized water and polarized cathodically in acidic environment. N 6.7 Ar gas (Air Products, BIP-PLUS) was used for solution deaeration. All glassware was cleaned with sulfochromic acid and subsequently with concentrated sulfuric acid and with deionised water in a final step.
[0047] Cyclic voltammograms for palladium limited volume electrode (1000 nm in thickness) recorded in a perchloric acid solution are presented on FIG. 1. The system's purity is confirmed by both the presence of the adsorption peaks at 0.27 V and by high symmetry of currents in the double layer region. The oxide formation/reduction region is typical for palladium electrode in acidic environments (H. Duncan and A. Lasia, Electrochimica Acta, 2008, 53, 6845-6850; C. Gabrielli, P. P. Grand, A. Lasia and H. Perrot, Journal of the Electrochemical Society, 2004, 151, A1937-A1942; A. Czerwinski, I. Kiersztyn, M. Grden and J. Czapla, Journal of Electroanalytical Chemistry, 1999, 471, 190-195; M. Lukaszewski, M. Grden and A. Czerwinski, Journal of New Materials for Electrochemical Systems, 2006, 9, 409-417). The hydrogen region is asymmetrical due to kinetic hindrance, which is particularly visible when the cathodic limit is set to more negative potentials where the beta phase is formed (the dashed curve on FIG. 2). Initially, the influence of platinum on hydrogen sorption was studied for Pt/Pd layers obtained by spontaneous deposition (SD). Pt surface coverage was estimated from the position of the Pd—Pt oxides reduction peak as described in by Grden et al (M. Grden, A. Piascik, Z. Koczorowski and A. Czerwinski, Journal of Electroanalytical Chemistry, 2002, 532, 35-42). After a single SD run, the surface Pt coverage was established to be about 15% of fully packed Pt monolayer. Such surface coverages are in reasonable agreement with results obtained for Pt SD on gold electrodes (Bakos, S. Szabó and T. Pajkossy, Journal of Solid State Electrochemistry, 2011, 15, 2453-2459; S. Kim, C. Jung, J. Kim, C. K. Rhee, S.-M. Choi and T.-H. Lim, Langmuir, 2010, 26, 4497-4505). Cyclic voltammetry curves recorded for 500 nm palladium before and after platinization by SD method are shown on FIG. 2. Much higher current densities related to hydrogen electrosorption were observed for Pt covered electrodes. Further increase in the hydrogen sorption kinetics was achieved by subsequent platinum SD runs. Special attention has been focused on eliminating the oxidation of hydrogen dissolved in the solution, the working electrode was washed with the stream of argon continuously passing through the solution near the electrode. For platinum covered electrodes, the oxide reduction peak is shifted toward more positive potentials to the same position as for the platinum electrode. The oxidation/reduction charges before and after platinization were virtually identical (see inset on FIG. 3A). These results point out the high platinum surface coverage and the lack of the real surface area expansion after first Pt monolayer deposition. These observations imply that the hydrogen insertion occurs through the Pt layer rather than by hydrogen spill-over mechanism.
[0048] FIG. 3A shows comparison between cyclic voltammograms recorded at 20 mV×s.sup.−1 for pure 500 nm Pd electrode and the same electrode covered by 1 ML Pt deposit obtained by Cu-UPD method. It can be observed that the currents related to hydrogen absorption/desorption are significantly higher for electrodes with platinum layer obtained by Cu galvanic displacement. This observation holds with respect to both pure palladium (FIG. 3A) and platinized Pd samples obtained by SD (FIG. 2). The maximum desorption current for 1 ML-Pt/Pd is constant for scan rates higher than 20 mV×s.sup.−1, and approaches 30 mA×cm.sup.−2 (see FIG. 4). This is most likely related to a to p phase transition kinetics. Special attention has been focused on eliminating the oxidation of hydrogen dissolved in the solution. The working electrode was washed with the stream of argon continuously passing through the solution near the electrode. For platinum covered electrodes, the oxide reduction peak is shifted toward more positive potentials to the same position as observed for the platinum electrode (data not shown). The oxidation/reduction charges before and after platinization were virtually identical (see inset in FIG. 3A). These results confirm the high platinum surface coverage and lack of the real surface area expansion after deposition of the first Pt monolayer. These observations imply that the hydrogen insertion occurs through the Pt layer rather than by hydrogen spill-over mechanism.
[0049] Moreover, in comparison to pure palladium, markedly enhanced hydrogen absorption kinetics has been observed for up to 10 ML of Pt (thicker deposit were not prepared). However, higher oxide charges Q.sub.MO observed for these layers indicate a significant real surface expansion after Pt deposition. This effect suggests that after deposition of 2-nd layer further Pt deposition is not epitaxial but rather Pt islands grow in a 3D fashion. The highest absorption/desorption kinetics was observed for 3 ML-Pt/Pd and the roughness factor for this layer was estimated to be R.sub.f˜2. In the alpha phase the cyclic voltammetry currents are symmetrical for platinized Pd electrodes. For low scan rates, currents expressed as pseudocapacitances (j/v) perfectly overlap even for relatively thick films (FIG. 3B). This clearly indicates that Pd—H system attains equilibrium at potentiodynamic conditions. It is important to stress that the hydrogen evolution reaction does not take place in this potential region (down to +0.1 mV). This implies that unusually fast hydrogen sorption kinetics is not the result of the hydrogen evolution at Pt surface. The chronoamperometric courses recorded during electrode equilibration at different potentials for both pure Pd and Pt covered electrodes are shown on FIG. 5. Depending on the electrode potential, after hydrogen saturation, the current tends either to zero or to a constant value, related to hydrogen evolution reaction. Much faster hydrogen insertion can be observed for the electrode covered with Pt monolayer. In this case, the hydrogen absorption is also related to much higher current densities.
[0050] Interestingly, the limiting currents of hydrogen absorption in platinum covered electrodes are in a linear dependence with the equilibration potential (FIG. 6). The slope of this dependence dE/dI=7.4 Ω×cm.sup.−2 coincides with the solution resistance R.sub.s=6.5 Ω×cm.sup.−2, determined in Electrochemical Impedance Spectroscopy (EIS) measurements. This result indicates that the hydrogen absorption current is limited by solution conductivity. This result indicates that the hydrogen absorption current is limited by solution conductivity in 0.1M H.sub.2SO.sub.4. In order to confirm this finding we have recorded cyclic voltammograms in 3 M sulphuric acid (FIG. 9). Indeed, in this solution hydrogen insertion is extremely increased, so that a distinct cathodic peak is formed due to hydrogen absorption for 1000 nm thick Pd electrode. This result indicates that hydrogen absorption is faster than hydrogen evolution at Pt covered Pd electrodes. After electrode equilibration, the electrode potential was scanned to 0.5 V vs. RHE at 10 mV×s.sup.1 and held at this potential until the current approaches zero. The results of such hydrogen desorption in function of equilibrium potential are shown on FIG. 7 for both pure Pd and Pt covered electrodes. Again, hydrogen desorption is markedly faster for the latter. It is interesting to compare conditioning times, t.sub.abs, needed to fully saturate the Pd layer with hydrogen for both pure Pd and platinized electrodes. It is well known that the largest times, t.sub.abs, are needed at potentials close to the phase transition region, at which the kinetics is limited by both surface and bulk phenomena (slow a/s phase transition, W. S. Zhang and X. W. Zhang, Journal of Electroanalytical Chemistry, 1998, 445, 55-62; W. S. Zhang, X. W. Zhang and X. G. Zhao, Journal of Electroanalytical Chemistry, 1998, 458, 107-112).
[0051] On FIG. 8A the results of chronoamperometric experiments performed at E=0.04 V vs. RHE (phase transition region) for different times t.sub.abs are shown. It is important to note that all curves recorded for shorter times perfectly overlap with the longest one, i.e. recorded for t.sub.abs=1500 s. This points out the unusual stability of the Pt monolayer on Pd substrate. The inset to FIG. 8A shows hydrogen desorption curves, recorded after saturation at given t.sub.abs. By integration of desorption currents, one may obtain the charge related to hydrogen absorption and therefore the hydrogen concentration within Pd layer (F. Vigier, R. Jurczakowski and A. Lasia, Journal of Electroanalytical Chemistry, 2006, 588, 32-43; M. Slojewski, J. Kowalska and R. Jurczakowski, Journal of Physical Chemistry C, 2009, 113, 3707-3712).
[0052] On FIG. 8B the hydrogen concentration in function of the conditioning time, t.sub.abs, at potential within the phase transition region, E.sub.abs=0.04V, is presented. It can be seen that the same amounts of hydrogen are inserted at times, t.sub.abs, shorter nearly by two orders of magnitude for platinized samples. For other potentials, i.e. before and after phase transition region, we have observed similar enhancement in the hydrogen insertion rate (10-80×) for Pt covered electrodes. However, at significantly shorter time scales (see FIGS. 5 and 6).
[0053] The hydrogen absorption isotherms for pure and platinized palladium layers, determined by this approach are displayed on FIG. 10A, where low concentration range is plotted in the log scale. Both isotherms, i.e. for pure and platinized Pd largely overlap. Only the sharper phase transition region and slightly larger hydrogen concentrations in the beta phase can be observed for 1 ML Pt-covered Pd electrode. These differences can be explained by kinetic hindrance for Pd pure electrode. At the FIG. 8B for pure Pd a slight but distinct increase in hydrogen concentration between 8 and 16 hours of hydrogen saturation at potential 40 mV vs. RHE is shown. The full saturation is observed after about 50 h of conditioning time.
[0054] The hysteresis in the hydrogen absorption/desorption was recorded for 1000 nm Pd (results are shown on FIG. 10B). Both, the hysteresis width and its position on the potential scale, are in perfect agreement with hysteresis determined in electrochemical conditions for pure Pd (L Birry and A. Lasia, Electrochimica Acta, 2006, 51, 3356-3364).
[0055] On FIG. 10C comparison between cyclic voltammograms recorded at 100 mV×s.sup.1 for 500 nm Pd electrode with and without 1 ML Pt deposit is shown. For platinized sample, voltammograms were recorded before (five first cycles are shown—solid line) and after (dotted line) series of electrochemical measurements consisting on multiple hydrogen saturation/desorption runs recorded for 30 different potentials ranging from 0.37 down to 0 V. Between measurements the sample was cycled in potential region −0.01÷1.40V (over 100 cycles from hydrogen evolution to full oxides coverage). After this quite severe electrochemical treatment the Pt surface coverage is diminished only by circa 15%, and still surface coverage of 0.85 ML-Pt/Pd can be deduced from the position of the surface oxides reduction peak. This result demonstrates the apparent stability of the Pt monolayer against the hydrogen induced lattice migration (HILM) phenomena.
[0056] It is particularly interesting to compare results of electrochemical impedance spectroscopy for bare and platinized Pd films. Typical Nyquist plots are shown on FIG. 11. For palladium electrode, the coupling between double layer capacity and the resistance of the charge transfer results in a semicircle followed by a capacitive line at low frequencies, related to hydrogen electrosorption pseudocapacitance. The spectrum can be fitted to the equivalent circuit shown in inset (FIG. 11A).
[0057] For platinized electrodes the resistance of the charge transfer is absent, similarly as for pure platinum in the classical EIS measurements (B. Losiewicz, R. Jurczakowski and A. Lasia, Electrochimica Acta, 2012, 80, 292-301). Moreover, hydrogen absorption becomes diffusion controlled since a well-developed Warburg impedance is observed on the Nyquist plot. The fitting of the spectrum for 1 ML-Pt/Pd 1000 nm is shown on FIG. 11B and the equivalent circuit is shown above. In this circuit CPE represents both capacity of the double layer and electrosorption pseudocapacitance. Wo stands for Warburg impedance for hydrogen finite-length diffusion in reflective boundary. Diffusion coefficient calculated from Warburg impedance amounts to 1.8×10−7 cm.sup.2×s.sup.−1 (at 0.170 V vs. RHE), thus approaching that reported for bulk palladium, and is nearly two orders of magnitude higher than diffusion coefficients reported to date for thin palladium films, that is 1.3×10−9 cm.sup.2×s.sup.−1 (H. Hagi, Mater T Jim, 1990, 31, 954-958). The diffusion coefficients measured by present inventors for 1 ML-Pt/Pd 200 and 500 nm are 3.5×10.sup.−8 cm.sup.2×s.sup.−1 and 1×10−7 cm.sup.2×s.sup.−1 respectively. The apparent diffusion coefficients reported by Y. Li for 135 nm Pd is 2.95×10.sup.−10 cm.sup.2×s.sup.−1. This result shows that apparent diffusion coefficients of hydrogen reported for thin palladium layers are severely underestimated as a result of the slow reaction at the interface (Y. Li and Y. T. Cheng, Int. J. Hydrogen Energy, 1996, 21, 281-291).
[0058] In another embodiment of the present invention the gold substrate is replaced with inexpensive hydrogen absorbing material, namely nickel alloy storage material AB.sub.5 (MmNi.sub.4.1Al.sub.0.2Mn.sub.0.4Co.sub.0.45). FIG. 12 shows comparison between cyclic voltammograms recorded at 5 mV.Math.s.sup.−1 for AB.sub.5 electrode and the same electrode covered by 1 μm Pd obtained by electroless deposition described in [M. Slojewski et al. Phys. Chem. C, 2009, 113 (9), pp 3707-3712] and 1 ML Pt deposit obtained by spontaneous deposition. It can be observed that the currents related to hydrogen absorption/desorption are significantly higher for electrodes modified with palladium-platinum system of the invention (i.e. palladium with single Pt monolayer).
CONCLUSIONS
[0059] The present inventors have investigated the hydrogen permeability through platinum monolayers deposited on palladium. The obtained results show that two-dimensional Pt deposits (1-3 monolayer in thickness) not only enable hydrogen permeation but also ultimately accelerate the hydrogen charging and discharging processes. The latter process becomes bulk diffusion limited. Moreover, diffusion coefficients of hydrogen determined for thin Pd films (200-1000 nm) covered by Pt monolayers are two orders of magnitude higher than those previously reported and determined for pure Pd thin films. An astonishing stability of platinum overlayers on Pd, after more than a hundred hydrogen absorption/desorption cycles confirms practical applications of the invention.
[0060] Pd/Pt system according to the present invention exhibits enormous enhancement in the hydrogen electrosorption kinetics and can be obtained by depositing of 1-10 platinum monolayers, preferably 1-3 monolayers, at the electrode surface. Comparable enhancement in the electrosorption kinetics cannot be achieved by any of presently known methods. At the same time, superior hydrogen capacity of palladium as well as its bulk properties remain unaffected by deposited platinum.
[0061] The approach provides insight into the process of hydrogen transport in metals-hydrogen systems. Impedance spectroscopy studies have revealed that hydrogen absorption is a diffusion controlled process at thin platinized electrodes (200-1000 nm). Pd/Pt system according to the present invention can find numerous applications in the heterogeneous catalysis, hydrogenation reactions, and hydrogen purification. Another set of industrial applications of the present invention is related to hydrogen storage or sensing systems, where fast and selective reaction in the presence of hydrogen is required.
