Lewis F. A.,
School of Chemistry, The Queen's University of Belfast, BT9 5AG, Northern Ireland, U.K.
Hydrogen Energy Progress and Transfer Processes of Purified
Hydrogen
ECOLOGICAL IMPORTANCE OF HYDROGEN
Worldwide interest continues in consideration of hydrogen as an economically important central source of energy. This interest in hydrogen has been paralleled by Series of World Hydrogen Energy related reviews and allied conferences [1 — 10]. Taken collectively, such meetings initially have tended to be centred for subject matter on the earlier investigated areas of interest in hydride range extensions and hydrogen embrittlement problems [1 — 5]. Areas of interest gradually have developed in cases of hydrogen storage [6] and co-utilisations of hydrogen purification membranes [7,8]. Additional information contained in reports corresponding to references [1 — 8] has generally included main details of determinations of phase relationships and results of calculations of complementary thermodynamic functions usually including extension to hydrogen isotope differences.
Also importantly contained in such reports [1 — 8] has been information on hydrogen content dependent changes of measurements of physical properties including magnetic susceptibility and electrical resistivity - studied within wide ranges of temperature and extensive variations of alloy compositions.
Over more current times there have been significant shifts of research and publication background interests towards topics of ecological significance including areas of progress in hydrogen study as a centre of increasing importance involving a wide variety of interrelated subject areas. Such hydrogen research directed subject areas have included recently reviewed studies of hydrogen involvements in biological environments [11], water splitting reactions [12], thermochemical processes [13] and developments of fuel cells [14]. Further, in relation to the retained elastic character of transition metals in interactions with hydrogen, there has been significant latter increased research activity in associated mechanically destructive and energetically advantageous aspects [15].
TRANSITION METAL - HYDROGEN SYSTEMS
Research concerning utilisations of palladium and palladium alloys as membranes for hydrogen permeation and purification have been subjects of continuing academic and technological interest. From both standpoints, certain membranes of the palladium - platinum alloy series have been subjects of several studies in recent years. In particular, a body of results obtained with Pd81Pt19, and Pd77Ag23 and
Pd membranes, has also drawn attention to the developments of lattice strain gradients in conjunction with the hydrogen concentration gradients, in regard to estimations of hydrogen diffusion coefficients, DH and dependencies of DH on the hydrogen contents of the membranes.
GORSKY EFFECTS
In regard to general principles of diffusion processes, attention has seemed drawn by studies of Gorsky [15,16] to needs for attention to be given to possibilities of simultaneous lattice expansive strain influences arising from the diffusing species themselves. In particular reference to developments in the knowledge of permeation processes with elastic media, studies of hydrogen permeation within membranes of palladium and selected alloys of palladium with platinum, silver and cerium, have provided convincing demonstration of effects of combinations of concentration and strain gradient operative influences.
UPHILL HYDROGEN DIFFUSION EFFECTS
In circumstances of initial «Uphill» hydrogen gas pressure change observations [21,22] hydrogen diffusion membranes were of Pd alloy composition in the form of tubes with a closed rounded lower end and with the upper end alternatively connected to vacuum or to internal hydrogen gas phase pressure measurement gauges or, eventually, other facilities [17,18].
In many studies, it has been a thermodynamic requirement, that external and internal tube membrane surfaces are coated with a «hydrogen chemical potential transfer medium» such as palladium black, for catalytically effective equilibration of surface processes [17—24] involving
H (ads) + H (ads) « H2
(1)
Outline descriptions of experimental observations and subsequent interpretations related to «Uphill Effects» have been broadly characterised and figuratively illustrated in Fig. 1 and 2.
BREAKTHROUGH TIMES
In Fig. 1 examples of initial values of quasi-static hydrogen gas pressures pA and pB represented as initially
present within tubular membranes of Pd81Pt19 [17 - 22, 31, 32], Pd [23, 24] and Pd73Ag27 [25 - 29] - where pA and pB are equivalent through p-c(n) - T relationships [17, 18, 21, 22] to closely similar values of membrane wall hydrogen contents, nA~NB where n = H/M (atomic ratio).
In Fig. 1 subsequent time dependent changes of hydrogen gas pressures within tubular membranes have been produced by alternative means of increases of the hydrogen contents of outer surfaces, either: (i) by cathodisa-tion [17-31] at times indicated by pA, or, (ii) by external hydrogen gas pressure [17-20,22,25-32] at times corresponding to pB.
H+
Time
Fig. 1. Time dependent changes of hydrogen pressures within tubular membranes, following increases of hydrogen contents of external surfaces - either by electrolysis (lower curve) or by external hydrogen gas pressure (upper curve) from closely similar initial quasi static hydrogen pressures pA and pB respectively.
In explanations [21,22] there has been very substantial evidence [17-19,23-26,30-32], that significant variations in breakthrough time-dependencies on nA or nB associated with Gorsky Effect migrations of hydrogen in-terstitials induced by elastic lattice strain gradients produced in the course of the lattice expansive processes involved [17-20]. Theoretical calculations have been subjects of recent correlations with appropriate experimental data [33-37].
PERMEATION PROCESS FEATURES
In the case of the upper curve in Fig. 1 originating at pB, the first initial increase of pressure associated [25,26,32] with initial migration of hydrogen interstitials towards the inner membrane wall surface, produced by mechanically generated lattice strain have internal pressures for both curves, over DpB2 and DpA respectively, in terms of initial conclusions [17-19,21] of uphill diffusion of hydrogen from inner surface regions towards the lattice expanding hydrogen entry regions.
Fig. 2. Representations of directions of hydrogen transfers during hydrogen permeation stages corresponding to the lower curve in Figure 1.
HYDROGEN MIGRATION EQUILIBRIA: DIFFUSION PARAMETERS
In Fig. 2, the «Uphill» transfer of hydrogen interstitials from near the inner surface towards outer surface regions near initial hydrogen entry of highly surface active catalytic conditions [17-20] by entry through the inner surface of hydrogen atoms from hydrogen molecules present in the interior tube volume.
EFFECTS ON ESTIMATIONS OF DIFFUSION
PARAMETERS
Comparisons of the two curves in Fig. 1 show extrapolation differences between the breakthrough times tL(A) and tL(B). The value of tL(B) from the upper curve is clearly lower than the value of tL(A) — which is characteristic of comparisons of results over wide ranges of initial hydrogen contents, n, in studies with Pd81Pt19Hn [30,31] and Pd73Ag27Hn [25] systems. Such differences between breakthrough times have importance in estimations of hydrogen diffusion coefficients DH in membranes of thickness L and temperature, T, through appropriate relationships of a generalised form such as [17,18, 20-26,30-32].
Dh =
Jl.
6Tt
(2)
Theoretical calculations of relative contributions of concentration and strain gradient components to diffusion flux JH(n) have been subjects of recent correlations with appropriate experimental data [33-37].
CONSOLIDATIONS AND EXTENSIONS OF STRESS/
STRAIN GRADIENT RELATED HYDROGEN
PERMEATION STUDIES
Additional experimental factors which modify or conceal underlying control have included complexities of membrane geometry and variations of surface catalytic activities [17,18] sometimes combined with important modifications of structural uniformity in courses of permeation [23,24].
Further studies of palladium [38] and series of Pd-Ag alloy [39] membranes and Pd81Pt19 membranes in the form of tubes [32,40,41] have experimental material suitabilities over wide ranges of hydrogen contents (n) valuably relat-
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6
ed to convertible possibilities between hydrogen gas pressure, p, and electrode potential, E, for appropriate uses in relation to p(E) - c(n) - T relationships over a wide range of temperature T — in conditions of high and well sustained surface catalytic activities such as can be provided by finely divided «black» surfaces of platinum, palladium or Pd-Pt co-depositions [42].
Comprehensive series of measurements over ranges of Pd-Ag [20,39] and Pd-Ce [43] alloy compositions have been obtained by both galvanostatic and potentiostatic techniques with bielectrode membranes.
Evidence of retained involvement of strain gradient contributions in steady state permeation conditions has been provided by observations of enhancements of permeation flux on interruption of flux input supply [44-46].
Explanations of strain gradient permeation and involvement in hydrogen diffusion explanations in terms of Uphill operations of inconsistencies of directions of dependencies of hydrogen Diffusion Coefficiencies DH on hydrogen contents n have been advanced in relationships for the Pd-H System [47] and Pd77Ag23Hn system [48,49].
REFERENCES
[1] Smith, D.P. Hydrogen in Metals, University Press, Chicago, 1948.
[2] Flanagan, T. B. Engelhard Technical Bulletin (Thomas Graham Commemorative Issue), 1966, vol.17, p.9.
[3] Lewis, F.A. The Palladium Hydrogen System, Academic: London; Platinium Met. Rev., 1970, vol. 26, pp. 20, 70, 121.
[4] Lewis
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