Design and analysis of a membraneless divergent electrode flow through electrolyser for hydrogen production

Abstract

Cost-effective renewable hydrogen production has been elusive to date, preventing the acceleration of water electrolysis technologies into the industrial market. The vast majority of hydrogen produced globally is derived from methods such as methane steam reforming, and hence renewable sources of hydrogen remain non-competitive with hydrogen derived from fossil fuel sources. A novel process whereby the manipulation of flowing electrolytic solution in opposing directions, through porous metallic electrodes, provides the means to create gaseous separation of constituent gases produced on the surface of the electrodes. The electrolyser configuration required to achieve this, is simplistic and cost-effective, providing reliable, efficient and, durable operation. Mass transfer limitations, and a reduced ionic resistance are characteristic of the Divergent Electrode-Flow-Through (DEFTTM) membraneless alkaline water electrolysis system. Current commercial water electrolysis technologies are connected by a number of issues, which drive up the cost to produce hydrogen and limit their long term reliability. Alkaline Water Electrolysis (AWE) represents the most mature and widely utilised electrolysis technology. It is nevertheless limited by an operating current density threshold, which, if superseded, will be plagued by enhanced bubble resistances and cross gas contamination. Their alkaline environment allows them to be constructed out of non-noble cost-effective materials, however, are large in scale due to their low power densities, leading to inflated capital expenditures. Proton Exchange Membrane (PEM) electrolysers attain greater current densities in reference to AWE systems, and are hence more compact in scale. Their membranes, however, lack reliability and are constructed from scarce and expensive materials. The costs associated with these systems make them non-competitive with large AWE systems. Research involved in this field is concerned with incremental improvements in performance and reliability while indirectly reducing the cost of components, however, restricted or limited improvements are not enough to drive accelerated renewable hydrogen production. A number of membraneless electrolysis concepts exist, however, the fundamental difference between flow along and flow through operational principles provides unique advantages to the DEFTTM alkaline electrolysis solution. Initial investigations into proof of concept have revealed a system capable of efficient stack performance and generation of high purity product gases. This led to the development of a reliable, scalable DEFTTM stack design and practical plant configuration, to provide synchronisation of all essential components that will serve to provide the formula for the continued commercial development of the technology. The complete plastic design successfully demonstrated optimal performance of an enhanced PGM based catalyst comprising of aluminium, nickel, and platinum in mass ratio 0.36:0.55:0.09 respectively as the anode and pure platinum as the cathode. This catalyst demonstrated 0.477 A.cm-2 at 1.77 VDC and 2.62 A.cm-2 at 2.5 VDC, at an operating temperature of 70°C, electrode gap of 2.5 mm, and flow velocity of 0.075 m.s-1. The concept optimisation plant demonstrated optimal gas purities of 98.98 vol% H2 and 97.6 vol% O2 at a flow velocity of 0.075 m.s-1, above the flammability limit of hydrogen gas. After the initial successful demonstration of the technology, scalable designs were fabricated and tested. The Multiple Circular Electrode (MCE) electrolysis stack utilised a common pressurised chamber, housing a number of small circular electrodes, with each polarity discharging the biphase product into its own gas collection chamber. The MCE stack showed an improvement in performance reference to the concept optimisation stack due to the additional exposed surface area incorporated into the design, however, lacked severely in providing adequate product gas quality. The Horizontal Filter Press (HFP) concept utilised elongated electrodes that were fed from a slotted manifold, and exhibited an improvement in gas qualities. Gas qualities remained within the flammability limit and the stack ascertained a reduction in performance due to the lack of balanced flow and uniform current density distributions. A culmination of the DEFTTM concept optimisation test plant, MCE stack, and HFP stack resulted in the optimised design of the technology utilising the compact benefits of a filter press configuration, along with the individual supply of power and fluid to circular electrodes, in order to provide superior balance in flow and current density. The design incorporated a number of flexibilities, one of which was an adjustable electrode gap. Results from the DEFTTM concept optimisation test rig and MCFPE (Mono circular filter press electrode) stack revealed an electrode gap of 2.5 mm to be optimal. An improvement in gas purity was yielded with this stack noting a hydrogen and oxygen gas purity of 99.81 vol% and 99.5 vol% respectively, at a temperature of 50°C, nominal current density of 3.5 A.cm-2, electrode gap of 2.5 mm, and flow velocity of 0.075 m.s-1. A large improvement in cell operating performance was yielded with the increase in available geometric surface area utilising a filtration mesh and metal foam. Compared to the DEFTTM concept optimisation, by doubling the 80 μm mesh yielded an improved performance across the cell potential range for the MCFPE stack. This does, however, have a significant reduction in the pore diameter, and hence restriction in gas and liquid flow. By utilising a larger mesh pore diameter of 200 μm incorporating two layers, enhanced performance greater than that of a single layer of 80 μm mesh along with hydrogen gas purities close to 100 vol%. The MCFPE stack demonstrates ideal performance with overall plant efficiencies not yet comparable with commercial systems, however, with limited additional design optimisation and additional research and development, would enable the DEFTTM technology to compare favourably with existing systems. Development with regard to the balance of plant has yielded an effective solution to the unique problem of the rapid liberation of micro-bubbles from a flowing solution. A working stack and gas/liquid separation solution therefore completes the basic building blocks for the DEFTTM technology. A techno-economic study with current and future cost predictions, with the assumption of operating the technology from solar power, has shown that the current selling cost of hydrogen amounts to 9.85 USD / kg H2 utilising an optimal nickel catalyst. With additional plant and stack optimisations, and the appropriate configuration allowing for lower parasitic loads, the current form of the technology would be capable of achieving a selling cost of 5.37 USD / kg H2. Current forecourt electrolysers operate in the region of 4.15 USD / kg H2, with current indications pointing towards the DEFTTM operating principle surpassing this cost benchmark, close to that of methods associated with producing hydrogen from fossil fuel based sources. The DEFTTM membraneless operating principle is therefore a unique means of performing water electrolysis, without the need of a membrane, to provide separation of product gases. This enhances the power density potential and yields a design with fewer components constructed out of inexpensive materials. The technology demonstrates significant potential to make cost-effective renewable hydrogen a possibility.