Assessing scalability and economic viability of a membraneless divergent electrode flow-through electrolyser through design modifications

Abstract

The alkaline Divergent-Electrode-Flow-Through (DEFT™) electrolyser represents the core novelty of the presented technology in that it utilises the flow of electrolyte entering an electrode gap with the subsequent diversion of fluid through porous electrodes to maintain separation of the produced hydrogen and oxygen gases. Inventiveness is introduced by the absence of a conventional ion-exchange membrane or diaphragm, and consequently, none of the limitations associated with this component are present. The core advantages of the technology are defined by (i) A simplified reactor design with fewer components reducing capital, operation and maintenance costs, (ii) Temperature limitations are defined only by the materials of construction, with potential to operate at intermediate temperatures (150 – 300°C), (iii) Greater operating current densities thresholds in contrast to conventional alkaline systems with the ability to easily accept a fluctuating load, ideally from a renewable energy source, and (iv) Reduced cell resistance through effective bubble management and removal of membrane/diaphragm resistances. Early-stage prototype developments showed high current density thresholds and impressive current-voltage (IV) data with 1.36 A.cm-2 achieved at 2.5 VDC [1]. The technology, however, relied on high electrolytic solution throughput with associated flow velocities of 0.075 m.s-1 to maintain high purity hydrogen gas production (>99 vol%). This brought about two notable drawbacks as the technology exhibited low system efficiencies (<40 HHV%) due to high parasitic loads and was limited in scaling ability (electrodes size of 30 mm in diameter). The purpose of this study was to address these issues through iterative design modifications. This included the commissioning of a first phase prototype for investigation followed by the design, build and commissioning of a commercial representative 3 – 5 kg/day H2 demonstration plant. The objective was to obtain representative performance and economic data such that the economic viability of the technology could be assessed through comparisons to existing electrolytic hydrogen generation solutions. The study culminated into four major design modifications to the original DEFT™ prototype design, these include: 1. The inclusion of a non-conductive mesh (nylon filtration material), which served the purpose of (i) increasing the flux distribution across larger electrode surfaces by providing a source of backpressure and amplifying flow velocities (at set flowrates) into the electrode chamber, and (ii) providing resistance to mass transfer for gas-crossover of larger gas bubbles (with high buoyancy forces). 2. Modifying the electrode to an annulus design such that a flow path can be defined between the outer and inner diameter while still allowing the electrode to be scalable. 3. Modifying the electrolyte injection and bi-phase solution collection manifolds such that backflow and mixing in the electrolyser stack are minimised. 4. Modifying the electrolyser stack to a bipolar configuration allowing for a compact and scalable design that operate with lower amperage power conditioning equipment.