Although many electrolyzers are based on an alkaline (KOH) electrolyte, another option is to use a proton exchange membrane (PEM) as the electrolyte. In PEM electrolysis, water is supplied to the anode and is split into oxygen, protons and electrons by applying a DC voltage. Protons pass through the polymer electrolyte membrane and combine with electrons at the cathode to form hydrogen; thus oxygen is produced at the anode, and hydrogen is produced at the cathode as illustrated in a schematic diagram in FIG. 1. It is important that the hydrogen and oxygen, which evolve at the surfaces of the respective electrodes, are kept separate and do not mix.
The electrolysis process is essentially the reverse of the process in a PEM fuel cell. A PEM electrolyzer cell can be very similar in structure to a PEM fuel cell, with a polymer membrane sandwiched between a pair of porous electrodes and flow field plates. FIG. 2A shows a simplified diagram of an electrolyzer unit cell, and FIG. 2B shows a simplified diagram of a fuel cell unit cell. The materials used in a PEM electrolyzer are generally different because the carbon materials commonly used as catalyst supports, gas diffusion layers and flow field plates in fuel cells cannot be used on the oxygen side of a PEM electrolyzer due to corrosion. Metallic components (for example, tantalum, niobium, titanium, or stainless steel plated with such metals) are often used instead for porous layers and flow field plates in PEM electrolyzers. The catalyst is typically platinum or a platinum alloy, and is designed to operate in the presence of liquid water.
Multiple electrolyzer cells can be connected either in series or in parallel (to get the desired output at a reasonable stack voltage) to form an electrolyzer stack. In addition to one or more electrolyzer stacks comprising end plates, bus plates and manifolds, and other system components, an electrolyzer system will typically comprise a power supply, a voltage regulator, water purification and supply equipment including a circulation pump, water-gas separators for hydrogen and optionally oxygen, a thermal management system, controls and instrumentation, and equipment for storage and subsequent dispensing of the product gas(es).
A fuel cell system can be combined with an electrolyzer system, so that a renewable energy source can be used to power an electrolyzer to generate hydrogen and oxygen which can be stored, and then subsequently used as reactants for a fuel cell to produce electric power. Such a combined electrolyzer/fuel cell system is illustrated in FIG. 3A. Efforts are presently underway to develop a unitized stack that could serve as both fuel cell and electrolyzer. Such a device has been referred to as a “reversible fuel cell” or a “unitized regenerative fuel cell” (URFC). A PEM URFC stack delivers power when operated as a fuel cell using hydrogen as the fuel, and either air or oxygen as the oxidant, and generates hydrogen and oxygen when operated as an electrolysis cell. A URFC system is illustrated in FIG. 3B.
Design of the individual cells and cell components for a URFC should address the distinctly different operating conditions occurring during each mode of operation. For example, the oxygen/air electrode potential is quite different in one mode versus the other. In the exothermic fuel cell mode, humidified, gaseous reactants are generally required along with rapid removal of the heat and water produced, while in the electrolysis mode, liquid water is required as the reactant at one electrode, with rapid removal of the product oxygen at the anode and hydrogen at the cathode. The balance of plant supporting the PEM URFC is designed to handle product water in the fuel cell mode, maintain the thermal balance within the fuel cell (cooling plates are typically used to remove excess heat when the fuel cell is producing power), deliver clean reactants, and produce regulated power. Balance of plant issues for URFC include design of the thermal management system (because operation in the electrolysis mode is slightly endothermic), and collection of the product hydrogen and optionally oxygen.
In a PEM electrolyzer the issues associated with liquid reactant supply and gaseous product removal are somewhat different to those in a PEM fuel cell, where hydrogen and a gaseous oxidant (for example, air) are typically supplied to the anode and cathode respectively, and water is produced at the cathode. In PEM fuel cells the gaseous reactants are generally supplied to the electrodes via channels formed in the flow field plates. A typical reactant fluid flow field plate has at least one channel through which a reactant stream flows. The fluid flow field is typically integrated with the separator plate by locating a plurality of open-faced channels on one or both faces of the separator plate. The open-faced channels face an electrode, where the reactants are electrochemically converted. In a single cell arrangement, separator plates are provided on each of the anode and cathode sides. In a stack, bipolar plates are generally used between adjacent cells; these bipolar plates generally have flow fields on both sides of the plate. The plates act as current collectors and provide structural support for the electrodes.
The flow field used at both the anode and the cathode can have an important influence on fuel cell performance, and much work has been done on the optimization of flow field designs for PEM fuel cells. Conventionally the reactant flow channels in fuel cell flow fields have a constant cross-section along their length. However, U.S. Pat. No. 6,686,082 (which is hereby incorporated by reference herein in its entirety) describes fuel cell embodiments in which the fuel flow channels have a cross-sectional area that decreases linearly in the flow direction. For fuel cells operating on air as the oxidant, as the air flows along the cathode flow channel(s), the oxygen content in the air stream tends to be depleted and the air pressure tends to drop, resulting in reduced performance in the fuel cell. U.S. Pat. No. 7,838,169 (which is hereby incorporated by reference herein in its entirety) describes improved cathode flow field channels that can be used to achieve substantially constant oxygen availability along the channel.
Less work appears to have been done on studying the effect of flow field design on the performance of PEM electrolyzers, although it has been reported that electrolyzer operation is generally less sensitive to changes in flow field design than fuel cell operation.
It has been reported (Hwang, C. M., et al. Abst. #1405 Honolulu PRiME 2012, The Electrochemical Society) that in a PEM URFC, a preferred flow field design for operation in the fuel cell mode does not work so well in electrolysis mode, particularly at higher current densities (where the rate of hydrogen and oxygen production is greater). The study notes that serpentine flow fields are popular for PEM fuel cells because gas flow in a serpentine flow field has a higher velocity and greater shear force providing efficient removal of product water in the channels. Contrary to this, in electrolysis mode the longer serpentine flow field channels can be disadvantageous because product gases (hydrogen and oxygen) can tend to accumulate in the channels and hinder the supply of water to the electrode, and limit the rate of electrochemical oxidation of reactant water.
Although flow fields that are preferred for fuel cells are not necessarily the same as those that are preferred for electrolyzers, the Applicants have discovered that flow fields where the channel cross-sectional area varies along the length of the channel, particularly at the oxygen electrode, can offer advantages in electrolyzers, as well as in URFCs that can operate in both fuel cell and electrolyzer modes.