An electrochemical study of a PEM stack for water electrolysis

https://doi.org/10.1016/j.ijhydene.2011.06.019Get rights and content

Abstract

The electrochemical properties of a proton exchange membrane (PEM) stack electrolyzer (9 cells of 100 cm2 geometrical area) were investigated at different temperatures. An amount of H2 of ∼270 l h−1 was produced at 60 A (600 mA cm−2) and 70 °C under 876 W of applied electrical power. The corresponding specific energy consumed in the process was 3.24 Wh·l−1H2. The Faradic and electrical efficiencies were determined. Overall stack efficiencies of 73% and 85%, at 60 A and 70 °C, with respect to the low and high heating value of hydrogen, respectively, were obtained. These results confirmed the successful scale-up of a previous lab-scale device.

Highlights

► The development of a 9-cell PEM stack electrolyzer was carried out. ► The stack electrochemical properties were investigated at different temperatures. ► Overall stack efficiencies of 73% and 85%, at 60 A and 70 °C were obtained.

Introduction

Nowdays water electrolysis is one of the most promising methods for hydrogen production. Electrolysis technologies are developed and successfully integrated into renewable and hydrogen energy based system [1], [2]. In particular, highly effective environmentally-friendly PEM water electrolyzers are successfully integrated with plants producing electric power using renewable energy sources [3], [4], [5], [6], [7], [8], [9], [10], [11], [12]. For example, electrolytic grade hydrogen (and oxygen) can be produced by using the surplus of electric current from photovoltaic panels, wind turbines or other renewable energy sources. In this regard, water electrolysis represents the most important process to obtain hydrogen without any emission of air pollutants or greenhouse gases [13], [14].

Three distinct water electrolyzer technologies currently exist. Two types are well proven for providing electrolytic hydrogen: alkaline and PEM electrolysers. In general, the PEM electrolyzer offers several advantages compared with alkaline electrolyzer: much higher current densities; smaller mass – volume characteristics; high degree of gas purity; increased level of safety (no circulation of caustic electrolyte); possibility of combining fuel cell and electrolyzer modes. A third type, usually termed as solid-oxide electrolyser (SOE), is currently under development and may be viable in the longer term. There are considerable differences between these electrolyser technologies, but in each case the net process is the same (i.e. dc electricity + 2H2O ↔ 2H2 + O2) and by definition a water electrolyser always produces hydrogen at twice the volumetric rate as oxygen.

Practical SOE operate at high temperature (800 °C). They are properly designed to operate in combination with nuclear plants or other technologies where there is an extra amount of heat at low cost. This is necessary to sustain high temperature operation of an endothermic process like water splitting. Low temperature electrolyzers are more reliable for application with renewable sources like wind or solar energy. In contrast to alkaline electrolyzers, a key aspect of PEM electrolyzers is that the electrolyte is a solid proton conducting polymer [15], [16]. The polymer also behaves as a virtually gas-impermeable membrane, maintaining gas separation between electrode components. As a consequence, the electrolyzer unit is of relatively simple construction and it is potentially highly safe and characterized by a rapid response (due to the absence of a liquid electrolyte with its associated inertia) [3], [4], [5], [17], [18], [19], [20], [21].

In safety-critical applications, PEM electrolyzer reliability has been extremely high, with units achieving 100,000 h operation without failure [22].

The main commercial market for a PEM electrolyzer, besides military and space applications [23], is that of laboratory gas generator (e.g. for gas chromatography and hydrogen for fuel cell test rigs). Several further applications have been engineered, (including oxygen bottle refilling for emergency respiration on aircraft, and on-board rocket fuel generation in space from electrolytic hydrogen and oxygen) but none has yet achieved a significant market [24], [25]. The International Space Station [26] is the most prominent example of PEM electrolyzer technology, where it is employed for generating oxygen.

Presently, PEM electrolyzers are actively developed in several research programs [2], [3], [4], [19], [20], [27], [28], [29], [30], [31].

A PEM electrolyzer based on a nanosized IrO2 anode catalyst was already developed by our group [32], [33], [34]. IrO2 is the most used as anode catalyst, it exhibits high corrosion resistance compared to other catalytic active metal oxides (e.g. RuO2). The irreversible O2 evolution occurring at IrO2 electrocatalysts was largely studied in the literature [9], [35], [36], [37]. A squared geometry was used for the stack instead of the conventional circular design which has been widely adopted for PEM electrolyzers [19], [20], [31]. The squared design may take advantage of the developmental activities carried out on a similar architecture for PEM fuel cells. However, it may suffer more than the circular design in terms of high pressure operation due to the different sealing approach. However, most of the attention in the previous works [32], [33] was addressed to the assessment in a short stack (three cells) of the anode catalyst produced by a novel synthesis route and the optimization of the stack architecture. Both aspects have relevant influence in determining the electrolyzer performance. Electrochemical diagnostics carried out on the short stack indicated that ohmic contact resistance issues between bipolar plates and electrode backing layer affected significantly the stack behaviour.

In this work, we have made a 3-fold scale-up of the stack from 3 to 9 cells and improved stack assembling to reduce the specific power consumption. The increase of stack size was primarily carried out to evaluate scalability of the squared geometry design. Moreover, the experience acquired with a short stack, previously developed, formed the basis to improve stack components especially diffusion layers, electrodes, etc. This allowed to reduce the contact resistance for this stack geometry. The electrochemical experiments were carried out at different temperatures and atmospheric pressure. A Ti-based backing layer was used as both anode gas diffusion layer and current collector on the basis of its resilience to electrochemical corrosion in the operating potential window of interest for practical PEM electrolyzers. The progress achieved in terms of electrical and Faradic efficiency has been elucidated.

Section snippets

Stack components and assembling procedure

A sulfite-complex route, described in detail in Ref. [38], was used to prepare the anodic IrO2 electrocatalyst. A Nafion 115 (Ion Power) membrane was utilized as the solid polymer electrolyte. The IrO2 catalyst was directly deposited onto one side of the Nafion 115 by a spray coating technique (Fig. 1a). The anode catalyst loading was 2.5 mg cm−2. Ti grids (Franco Corradi, Italy) were used as backing layers (Fig. 1b). A commercial 30% Pt/Vulcan XC-72 (ETEK, PEMEAS, Boston, USA) was used as the

Voltage–current measurements

Fig. 2a shows the PEM stack electrolyzer polarization curves at different temperatures under atmospheric pressure. The electro-catalytic activity increased as a function of temperature, mainly in the low temperature range. This is due to the activation behaviour of the irreversible O2 evolution process occurring at the IrO2 electrocatalyst. The best performance was obtained at 70 °C. We have preferred to avoid operation at higher temperatures to avoid dehydration effects of the Nafion membrane

Conclusions

The development of a 9-cell PEM stack electrolyzer was carried out with regard to MEA manufacturing (100 cm2 geometrical area) and stack assembling. The stack electrochemical properties were investigated at different temperatures and the current distribution at different potentials was analyzed for each cell. The results indicated that the central cells and those close to the water inlet/outlet showed the best performance in terms of ohmic and diffusion characteristics. The amount of H2

Acknowledgment

The authors acknowledge Tozzi Renewable Energy S.p.A. for the financial support.

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