Electrolysis of supercritical aqueous solutions at temperatures up to 800 K and pressures up to 400 MPa

Abstract

A study on the electrolysis of sub- and supercritical aqueous solutions of sodium hydroxide is presented here. Current density–potential curves are recorded at temperatures up to about 800 K and pressures up to 400 MPa. The molar concentration of the solutions, as referred to T=298 K, is varied from $\text{c=(0.01}\text{to}\text{1}\text{)}\text{mol}\text{·}\text{dm}{}^{\mathrm{-3}}$. The autoclave is described in detail. Several electrode materials are examined. The highest reproducibility is obtained with gold electrodes. Some results are also given for platinum, silver and nickel electrodes. Below T=473 K the current–potential curves show the familiar transition from a range of very low currents to an ohmic behaviour with steeply increasing currents above the decomposition potential. The decomposition potential decreases with increasing temperature. Above T=473 K the current–potential curves no longer reflect distinct transitions from low-current to high-current behaviour, and at T=773 K, almost linear current–potential curves are observed over the entire voltage range. Up to current densities of $\text{50}\text{mA}\text{·}\text{cm}{}^{\mathrm{-2}}$, the pressure dependence of the current density at a given potential is marginal. At higher current densities, up to $\text{35}\text{A}\text{·}\text{cm}{}^{\mathrm{-2}}$, a substantial pressure effect is observed.

Introduction

Dense and supercritical fluids have properties which differentiate them from the solid, liquid and gaseous states of matter and constitute almost a fourth state of aggregation. Thermodynamic and kinetic properties can be varied continuously over wide ranges from liquid-like to gas-like states. Transport coefficients are high, even at liquid-like densities. And, particularly interesting, there are many binary and multi-component systems which show complete miscibility of their highly polar with nonpolar constituents. Many such examples are binary aqueous systems. They are characterised by “critical curves”, which begin at the critical point of pure water at T=647.3 K and 22.1 MPa, and extend uninterrupted or interrupted to the critical point of the nonpolar component [1], [2].

A few experimental examples of binary aqueous systems are shown in figure 1 which depicts critical lines of aqueous solutions of carbon dioxide [3], [4], hydrogen [5], oxygen [6], and ethane [7]. To the left of these critical curves, the low-temperature side, is the two-phase region. To the right, at higher temperatures, exists the homogeneous one-phase supercritical area. The figure also shows the initial part of the critical curve of sodium chloride which, from the critical point of water, extends to higher temperatures and pressures [8], [9]. One may expect that the critical curve of sodium hydroxide shows a similar behaviour. Some knowledge of these critical curves is necessary in order to operate the electrolysis cells discussed below.

We study here the electrolysis of dilute aqueous solutions of NaOH at temperatures up to about 800 K and at high enough pressures to achieve liquid-like densities. Knowledge of the electrolytic conductance of such systems is an important prerequisite for the design of electrolysis cells. The isothermal pressure dependence of the electrolytic conductance has been studied in several instances, see for example the early work on the conductance of dilute solutions of KCl, HCl and KOH up to T=1023 K and nearly 300 MPa [10], [11], [12]. At room temperature and a density near $\text{1}\text{g}\text{·}\text{cm}{}^{\mathrm{-3}}$, a pressure increase to 400 MPa causes a decrease of the molar conductance by about 8%. At these conditions the dielectric constant of water is high and the salts are highly dissociated. The conductance behaviour then reflects mobility effects, in accordance with the decrease of the diffusion coefficient (D) [13], [14] and the increase of the viscosity (η) of water [15], as approximately reflected by the Stokes-Einstein law [16]

$$\text{D=}\text{kT}\text{6πηr}\text{,}$$

where k stands for the Boltzmann constant and r for the hydrodynamic radius of the considered ion. For NaOH the extra mobility contribution of OH⁻ should be considered [17].

In contrast, at higher temperatures the pressure dependence of the molar conductance of aqueous solutions is generally positive. At high temperatures and low pressures the dielectric constant of water is low, and the salts are only partially dissociated. An increase in density causes an increase in the dielectric constant [18], [19] and hence, higher ionisation of the solute. For example, at T=673 K a pressure of 400 MPa leads to a water density of about $\text{0.9}\text{g}\text{·}\text{cm}{}^{\mathrm{-3}}$ [20] and a dielectric constant of about 24 [18], [19]. At 100 MPa and T=673 K the density is only $\text{0.69}\text{g}\text{·}\text{cm}{}^{\mathrm{-3}}$ and the dielectric constant is 12.5, which is, however, still sufficient for partial ionisation of salts with univalent ions.

The discussion of the performance of electrolysis in wide ranges of temperature and pressure requires the determination of current–potential curves and of decomposition potentials. Already in 1904, Th. Wulf reported on the pressure dependence of the decomposition potential for H₂ and O₂ of aqueous solutions with platinum electrodes up to T=338 K and 100 MPa [21]. He found that the decomposition potential varied only slightly and within the uncertainty ranges. In 1928 Tamman and Jenckel (1928) investigated the effect of pressure on current-voltage curves upto 300 MPa at room temperature [22]. They found that at constant potential the pressure caused the electric current density to grow. Lange showed in 1933 that at higher, but subcritical temperatures, higher pressures decreased the overpotential at an electrode [23]. The application of high pressures in these measurements required, however, comparatively low temperatures, because of the limited mechanical strength of the autoclave materials. Modern materials and techniques make possible the simultaneous application of high pressures and high supercritical temperatures.

Since the 1980s, there has been considerable interest in electrochemistry in supercritical fluids. However, this interest has been mainly limited to solvents such as CO₂ with critical points at comparatively low temperatures and pressures [24]. A few voltametric measurements for supercritical aqueous solutions have been reported by Bard and coworkers [25], [26]. However, in these studies only moderate pressures, typically of about 30 MPa, were applied. No attempts were made to investigate the pressure dependence of the voltametric properties in detail, or to generate supercritical states of liquid-like density.

Besides an interest in the scientific character of current–potential curves and the contributions of the overpotential, there is a possible practical interest. Decomposition under pressure can produce pressurised hydrogen and oxygen gases directly without later compression. High-pressure electrolysis and fuel cells could perhaps be combined to achieve energy storage. At a later stage, electrolysis of binary water-salt systems could be extended to homogeneous ternary systems with hydrocarbons or other organic compounds. Hydrogenation and other chemical processes might be carried out using a supercritical electrolytic aqueous phase cell. In supercritical water oxidation processes [27] the generation of oxygen could be achieved by electrolysis [28].