Experimental studies of natural convective mass transfer in a water-splitting system

https://doi.org/10.1016/j.ijhydene.2019.04.043Get rights and content

Highlights

  • Natural convection plays dominant role in oxygen mass transport in a water-splitting reactor.

  • Effective design of reactor may delay the oxygen bubble formation.

  • Planar laser induced fluorescence and particle image velocimetry are important tools for electrochemical systems diagnostics.

Abstract

The present research investigates the mass transfer processes at the electrode-electrolyte interface of a water-splitting, electrochemical cell using particle image velocimetry (PIV) and planar laser-induced fluorescence (PLIF). In a water-splitting device, mass transfer mechanisms usually involve simultaneous convection, diffusion, and migration. Mass transfer rate, at the electrode/electrolyte interface, depends on various factors, including electrode orientation, current density, type of redox agents and the products. The parameters considered here are cell voltage, the orientation of electrode, and the effect of concentration gradient induced by the reactants depletion and product formation at the interface on the mass transfer rate. The present study captures the instantaneous velocity and concentration fields using PIV and PLIF techniques. Conducting the experiments over various current densities and electrode-orientations, present study observes that the reactant depletion and product formation at the anode interface induces buoyancy which in turn causes natural convection even at low current densities. By utilizing the effect of orientation and the natural convection induced by the reactants and products, on the mass transfer rate, the limiting current density can be enhanced, and the supersaturation of products can be prevented at the interface.

Graphical abstract

Mass transfer processes at the electrode/electrolyte interface in a water splitting system.

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Introduction

To achieve sustainable development goals, while meeting the ever-increasing energy demands, it is imperative to develop highly efficient renewable energy systems [1], [2], [3]. Among the renewable energy systems, solar energy has the potential to satiate the demands. Photo-electrochemical (PEC) water splitting is one of the promising technique that converts solar energy into hydrogen [4]. Research on photoelectrochemical water splitting systems is multipronged one. The current research trend on PEC water splitting systems largely focuses on developing efficient semiconductor photoelectrodes [5], [6], [7]. Since the prevailing materials do not meet commercialization efficiency targets of PEC water splitting at large scale. To commercialize the PEC device, both device design and, photoelectrode material research should go hand in hand. But so far, scanty amount of research has been spent on developing effective photo electrochemical devices [8]. Rudimentary and laboratory scale PEC water splitting devices/cells has been built, just to test the developed semiconductor photoelectrodes [9]. To see the light of the day, PEC water splitting research should focus on both electrode and device development simultaneously. Effective PEC water splitting device design should meet the following criteria's: (a) Maximum light absorption; (b) Effective removal and collection/separation of products from the electrode/electrolyte interface; (c) Minimum overpotential losses [10]. Among the criteria's, removal and collection of products are essential, since it has far-reaching consequences on the overall efficiency of PEC device.

Present work focuses on effective removal of products from the electrode/electrolyte interface to avoid its supersaturation, recombination and further reaction with counter electrode products. Further in a PEC water splitting systems, supersaturation of products (H2 & O2) at the interface leads to bubble formation which hinders the photons falling on the photoelectrode surface as well as reducing the active electrode area [11] which will be discussed later in detail. To address the issues mentioned above, understanding the mass transfer process occurring at the electrode/electrolyte interface is essential. Mass transfer in electrochemical systems is accomplished by convection, diffusion, and migration [12]. Convection takes place either due to natural buoyancy and/or by externally forced convection, diffusion occurs due to concentration gradients of products and reactants between the bulk and electrode-electrolyte interface [13], [14]. While migration of ions occurs due to the potential gradients between the electrodes. These processes concomitantly proceed in continuous flow type electrochemical systems.

In-tank type electrochemical systems with quiescent electrolyte conditions, diffusion, and natural convection are predominant. Initial studies reported that natural convection dominates only in highly concentrated electrolytes and at large time scales, but later studies proved that even at less than 10 mM concentrations, convection plays an important role [15]. Natural convection near the electrode/electrolyte interface is caused by desorption/absorption of redox agents and products which in turn induces buoyancy force [16]. The orientation of electrodes aligned with the direction of gravitational acceleration and the redox agents decides which type of mass transfer process predominates [17]. Electrochemical system considered in this work focuses on understanding the different mass transfer processes that get prevailed because of the orientation of electrodes and the redox agents.

The present experimental study focuses on alkaline water splitting electrochemical cell whose reactions are appended below:Anode2OH12O2+H2O+2eCathode2H2O+2eH2+2OH

In order to mimic the PEC water splitting system as well as bubble-free operations, the electrochemical cell is operated at low current density conditions. As seen from the above electrochemical reaction, hydrogen and oxygen are main products which are mostly in gaseous bubble form and are convected by its buoyancy force at high current densities [18]. Effects of the bubble on electrochemical systems parameters such as mass transfer rate, over potential, limiting current density, etc., has been studied in detail [19], [20]. Preceding the formation of bubbles, the products appear in the dissolved form. Once the products reach the limit of supersaturation, as the reaction proceeds, heterogeneous nucleation sets in Refs. [21], [22]. These bubbles are an anomaly, and it has its own relative merits and demerits of its presence. In the specific application of PEC water splitting systems, presence of bubbles at the electrode surface reduces the active electrodes area and reflects the incoming photons [23]. Thus, understanding the mass transfer of dissolved gas helps in mitigating the effects of bubbles at the interface.

When the electrochemical reaction proceeds, at the electrode/electrolyte interface multiple mass transfer processes take place simultaneously as mentioned earlier. The reactants are depleted, and the products are formed at the interface which engenders concentration gradient [24]. The reactants tend to move from the bulk to the interface and the products from the interface to bulk. Concurrently change in the concentration of reactant and products at the interface changes the thermophysical properties of the electrolyte, especially density which induces buoyant forces based on the orientation with the gravitational acceleration [25]. Migration of ions can be nullified by adding a large amount of secondary electrolyte [15].

The region of interest in the present study is the anode side where OH ions get depleted, and O2 molecules are produced. Change in concentration of OH ions and dissolved oxygen gas molecules at the interface develops stable/unstable stratification based on the orientation of electrode/electrolyte interface with respect to gravitational direction. The objective of this work is to analyze the mass transfer processes of dissolved oxygen from the interface to the bulk. With that understanding, the formation of bubbles can be prevented by restraining the supersaturation of products at the interface.

A large gamut of literature available on electrochemical mass transfer mainly focuses on ionic transfer in the electrolyte to ion-selective surfaces [26], [27], [28]. Ions transfer due to natural convection is well studied since it is analogical to natural convective heat transfer [29]. From a mathematical point of view, mass transfer at the electrode/electrolyte interface can be analogized with convective heat transfer. The analogy has the limitation, since in electrochemical systems if the products appear in the dissolved form, two superimposed mass transfer takes place simultaneously, which are interconnected in nature [30]. The more equivalence to mass transfer in electrochemical systems is double-diffusive convection studies. Because the gradients induced by the products as well as the reactants engenders the mass transfer at the electrode/electrolyte interface. This analogy also gets violated if the reactants have multiple ions concentration with varying characteristics [25]. In the present study, the electrochemical reaction is assumed to be isothermal and the density variations are due to the concentration difference of products and reactants. The reactants and products have different diffusivities, which engenders different density variations. In the case of supporting electrolyte, different type of ions comes into play and each gradient of ions influences the buoyancy force differently. It has been reported that the use of secondary electrolyte (H2SO4) in copper electrolysis dampens the natural convection induced by the Cu2+ ions [31]. The general double-diffusive mass transfer equation assuming Boussinesq approximation is as follows.

The variation of density is given by,ρ=ρ0[βR(CRC0)+βP(CPC0)]ρ=ρ(CR,CP)

Volumetric mass expansion coefficient is represented as,βI=(ρC)TρI=R,P

The subscription ‘R’ and ‘P’ represents the reactants and products, and ‘0’ represents initial concentration. The volumetric mass expansion coefficient for different species are reported and can be readily calculated [32]. Depending upon the values of β, buoyancy force will have a differential effect on the flow field inside the boundary layer. The density variations caused by the products and reactants might be synergistic in certain cases and might raise to high magnitude buoyancy force (either positive or negative based on gravitational orientation). If both are opposite in nature, none of them dominates over each other leads to neutral buoyancy. From the literature, it is obvious that the majority of work is mainly focused on ion and multiple ions (supporting electrolyte) induced gradients [33]. In case of products appears in the dissolved gaseous form, induces a different kind of gradients along with the ion-induced one. Since the diffusion coefficients of ions and dissolved gases are very distinctive in nature. During the initial periods, depending upon on the type dissolved gases produced at the interface and its concentration gradient induces the density gradient in the electrolyte, it may be positively buoyant or negatively buoyant depending on the molecular weight of the gas. Eventually due to low diffusivity of gases, supersaturation sets in, which in turn, induces nucleation of bubbles that add to the buoyancy force. Hence, it is necessary to study mass transfer phenomena at the interface to address the bubble anomaly. The ensuing discussion focuses on various experimental studies that are reported on the electrode/electrolyte mass transfer processes.

Optical techniques are employed to visualize the mass transfer processes at the electrode/electrolyte interface. In the beginning, the schlieren technique has been widely used to visualize the mass transfer from different geometric electrode surfaces [29]. However, most of the mass transfer studies are on copper electrolysis because of its analogy with heat transfer processes. Natural convection induced by the copper ions depletion at the cathode surfaces induces density changes which in turn produces buoyancy force. To understand the effect of concentrations on the natural convection lot of numerical simulations has been done using different techniques [34], [35], [36]. Various experimental techniques have been adopted like schlieren [26], interferometer [16] and particle image velocimetry [24], [37] to visualize and quantify the concentration of ions [38] and velocity fields near the electrode interface. Most of the research focused on the mass transfer of charged species in the electrochemical system, and the scanty amount has been spent on understanding how dissolved gases, formed at the interface undergoes diffusion and convection to the bulk.

Above mentioned techniques give qualitative and quantitative information about the mass transfer processes at the interface. To get the quantitative information about the concentration of different products and reactants, interferometry may not serve the purpose if the rate of density variation is small and when multiple products are involved. To quantify the dissolved gas concentration and its flow, it is necessary to adopt a technique which can robustly (in-situ and non-contact type) measure many parallel processes that are in progress. The techniques adopted in this study are particle image velocimetry (PIV) and planar laser-induced fluorescence (PLIF) for velocity and concentration of dissolved gases in the electrolyte, respectively. The ensuing discussion will emphasis on the experimental setup and the calibration techniques adopted.

Section snippets

Experimental setup

The schematic diagram of the complete experimental setup is shown in Fig. 1. To study the mass transfer process at the electrolyte/electrode interface a transparent Perspex rectangular cavity has been made with two lateral sides are of electrodes (Stainless steel). The size of the cavity is 20  mm × 20  mm x 10 mm and the electrode size is of 20  mm × 10 mm as side walls. Small inlet and outlet are kept at the back side to fill the cavity with 1 M KOH electrolyte. Copper tapes connect the

Calibration and image processing

Ruthenium complex has been employed as a fluorescent agent. Its absorption spectrum matches with the Nd-Yag green laser emission spectrum. The excited light reemits to fall back to a fundamental state has a wavelength longer than the absorbed one and is known as fluorescence. Fluorescence can be inhibited by using quenching agents. In this case, our gaseous products (dissolved oxygen) acts as effective quenching through which the concentration of products can be determined.

To quantitatively

Results and discussions

The results and discussions are divided into two parts. First, we discuss the instantaneous concentration field that arises during the electrochemical reaction at different current densities. The second part focuses on the instantaneous velocity fields and the change in current densities due to the induced motion of electrolytes.

Conclusions

Present research uses the planar laser-induced fluorescence and particle image velocimetry techniques to investigate the mass transfer processes near the electrode-electrolyte interface in an electrochemical cell. Experiments captured the influences of electrode orientation, reactants, products and current density on the mass transfer at the electrode-electrolyte interface. The conclusions from the present studies are as follows:

  • 1.

    Using an in-situ measurement technique PLIF, a transient variation

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