Advanced alkaline water electrolysis using inorganic membrane electrolyte (I.M.E.) technology

The Belgian Hydrogen Research Programme, aiming at the development of a new concept in advanced alkaline water electrolysis has resulted in demonstration projects with prototype electrolyser units.The new concept, named I.M.E. technology (inorganic membrane electrolyte) produces hydrogen under pressure (0.5–4.0 MPa) at current densities up to 12500 Am^?2 and temperatures not exceeding 120°C.Electrocatalysts based on non-noble metals are deposited on perforated nickel plates. These electrodes are pressed against an inorganic ion-exchanger membrane based on polyantimonic acid.Circular cells (surface area up to 0.2 m²) are assembled in a filter-press type manner.Prototype electrolysers, having hydrogen production rate from 2.5 Nm³h^?1 to 25 Nm³h^?1 have been constructed and tested.Multicell performances (up to 60 unit cells) at 2000 Am^?2 are 1.6 V at 90°C and even 1.5 V at 120°C.At a 5 times higher current density (10000 Am^?2) cell voltages only increase with 0.3 V at 90°C and 0.2 V at 120°C.Since 100% Coulombic efficiency has been measured in these 60 unit cell stacks the electrical power consumption (at 2000 Am^?2) per normal cubic meter of hydrogen produced is 3.81 kWh at 90°C and 3.65 kWh at 120°C.A techno-economic analysis was performed to quantify the cost of hydrogen as a function of design parameters such as module cost and module performance capability.     

Diaphragms obtained by radiochemical grafting of PTFE

Diaphragms for alkaline water electrolysis are prepared by radiochemical grafting of PTFE fabric with styrene, which is later on sulfonated, or with acrylic acid. The diaphragms obtained are mechanically resistant to potash at temperatures up to 200°C, but show some degrafting, which limits the lifetime. The sulfonated styrene group has been found to be more stable in electrolysis than the acrylic acid. In both cases, the incorporation of a cross-linking agent like divinyl benzene improves the lifetime of the diaphragms. Electrolysis during 500 hours at 120°C and 10 kAm² could be performed.     

Syngas production through CH4-assisted co-electrolysis of H2O and CO2 in La0.8Sr0.2Cr0.5Fe0.5O3-δ-Zr0.84Y0.16O2-δ electrode-supported solid oxide electrolysis cells

High temperature co-electrolysis of H₂O/CO₂ allows for clean production of syngas using renewable energy, and the novel fuel-assisted electrolysis can effectively reduce consumption of electricity. Here, we report on symmetric cells YSZ-LSCrF | YSZ | YSZ-LSCrF, impregnated with Ni-SDC catalysts, for CH₄-assisted co-electrolysis of H₂O/CO₂. The required voltages to achieve an electrolysis current density of ?400 mA·cm^?2 at 850 °C are 1.0 V for the conventional co-electrolysis and 0.3 V for the CH₄-assisted co-electrolysis, indicative of a 70% reduction in the electricity consumption. For an inlet of H₂O/CO₂ (50/50 vol), syngas with a H₂:CO ratio of ≈2 can be always produced from the cathode under different current densities. In contrast, the anode effluent strongly depends upon the electrolysis current density and the operating temperature, with syngas favorably produced under moderate current densities at higher temperatures. It is demonstrated that syngas with a H₂:CO ratio of ≈2 can be produced from the anode at a formation rate of 6.5·mL min^?1·cm^?2 when operated at 850 °C with an electrolysis current density of ?450 mA·cm^?2.     

Effect of flow-field pattern and flow configuration on the performance of a polymer-electrolyte-membrane water electrolyzer at high temperature

This study aimed to optimize the flow-field pattern and flow configuration of a polymer-electrolyte-membrane water electrolyzer, with a particular focus on high-temperature operation up to 120 °C. Three types of flow-field pattern (serpentine, parallel, and cascade) were tested in both the anode and cathode sides of a water electrolyzer cell, and the current-voltage characteristics and high-frequency resistance were measured to examine which overpotential components are impacted by the flow-field pattern. The experimental results revealed that the cathode flow-field pattern only affects the ohmic overpotential, while the anode flow-field pattern significantly affects the overpotential related to liquid water shortage at catalyst layer, and the flow configuration (counter- and co-flow) does not affect the electrolysis performance. Finally, under operating conditions of 120 °C and 0.3 MPa, we found that the optimized cell configuration consisted of cascade and serpentine flow-field patterns in the anode and cathode, respectively; this configuration produced the minimum electrolysis voltage of 1.69 V at 2 A/cm².     

Kinetic and electrochemical analyses of a CuCI/HCl electrolyzer

An electrochemical analysis is carried out from a kinetic electrochemistry perspective of a CuCl/HCl electrolysis cell, within the CuCl thermochemical water splitting process for hydrogen production. The anolyte is a solution of 2 mol L^?1 CuCl(aq) and 10 mol L^?1 HCl(aq) while the catholyte solution is 11 mol L^?1 HCl(aq) . The cell current density of 0.5 A cm^?2 and voltage of 0.7 V are the desired working conditions for a CuCl/HCl electrolyzer. The current density of 0.5 A cm^?2 is assumed to occur at a 5% anolyte conversion degree. At 25°C , the activation overpotential of the anode half‐reaction is found to be 53 mV for a current density of 0.5 A cm^?2 while the activation overpotential of the cathode half‐reaction for the same condition is 87 mV. An increase in working temperature decreases the overpotential of the anode half‐reaction and increases the cathode half‐reaction activation overpotential. The ohmic overpotential of the cell membrane is almost 1000 times smaller than that of the activation overpotentials of the electrode half‐reactions for the same temperature and current density. A higher working temperature results in a lower membrane ohmic overpotential. The required voltage to trigger electrolysis for a current density of 0.5 A cm^?2 is found to be 0.53 V at 25°C and 0.59 V at 80°C and a higher temperature results in a higher electrochemical efficiency. The cell electrochemical efficiency increases linearly with working temperature while the voltage efficiency peaks at 75% at 60°C .     

Effects of porous support microstructure enabled by the carbon microsphere pore former on the performance of proton-conducting reversible solid oxide cells

Proton-conducting reversible solid oxide cells (PC-RSOCs) have attracted extensive attention due to their high efficiencies as energy conversion devices. Generally, the performance of the cell is affected to a certain extent by the microstructure of the electrodes, which is closely related to the gas diffusion and surface reaction processes. Herein, different contents of the carbon microspheres (CMSs) are used as the pore formers to control the microstructure of the hydrogen electrode. Experimental results reveal that the porosity, line shrinkage, and thermal expansion coefficient of the hydrogen electrode support simultaneously increase with the CMS content. The support with 30 wt% CMS presents high porosity (39.27 vol%) with uniform-size pores. Subsequently, the corresponding single cells were fabricated successfully, particularly, the cell with 30 wt% CMS exhibiting the best electrochemical performance in both fuel cell (0.46 W cm^?2 at 700 °C) and electrolysis cell (1.41 A cm^?2 at 1.3 V and 700 °C) operational modes. Further results demonstrated the highest performance was attributed primarily to the maximal three-phase boundary length, which mainly originates from the high porosity and unique microstructure of the hydrogen electrode.     

Improvements of water electrolysis in alkaline media at intermediate temperatures

Studies were carried out on the electrochemical splitting of hydrogen from water in an aqueous KOH solution at 110–140°C and in molten hydroxides at 300–400°C with the aim of increasing the energy efficiency of hydrogen production. The investigations on the electrolysis in an aqueous KOH solution were concentrated on developing metal oxide diaphragms and improved activated Ni electrodes. At 140°C and a pressure of 8 bar, a cell voltage of 1.55 V was obtained at a current density of 500 mA cm^?2. In molten NaOH at 350°C, a cell voltage of 1.3 V was achieved at a current density of 500 mA cm^?2. However, current yields were only ca 90%, due to side reactions producing peroxides. The formation of peroxides is significantly reduced in a LiOH/NaOH melt. Current yields of 100% have now been obtained at a cell voltage of 1.45 V and current density 500 mA cm^?2. The hydrogen and oxygen formed are separated by a Ni-diaphragm which is cathodically protected to eliminate corrosion.     

Developing an Empirical Formula for the Assessment of the Char Yield of Pyrolyzed Lignocellulosic Materials

Abstract

A new empirical formula, based on the porosity, the botanical composition and the ash content of the precursor, is proposed for the assessment of the char yield, Y (wt%), of lignocellulosic materials pyrolyzed in a 600°C–900°C temperature range. The following equation is proposed:

Y (wt%) = (0.8815 ? 2.281/? + 61.44/?²)

· {[L(0.59 ? 2.74 × 10^?4 (t°C ? 600) + 0.22C]

+ A ? 0.1 E [1 + 2 × 10^?3 (600 ? t°C)]}

where ?, L, C, A, and E are parameters of the precursor, namely: porosity (%) and percentage (wt%) of lignin, cellulose, ash, and extractives, the t temperature, being in the Celsius grade.

The values calculated by the formula fit satisfactorily with the experimental results, the gap being less than 0.5%.     

Long-term stability of Ni–Sn porous metals for cathode current collector in solid oxide fuel cells

Ni–Sn porous metals with different concentrations of Sn were prepared as potential current collectors for solid oxide fuel cells (SOFCs). The weight increase of these species was evaluated after heat-treatment under elevated temperatures in air for thousands of hours to evaluate the long-term oxidation resistance. Ni–Sn porous metals with 5–14 wt% of Sn exhibited excellent oxidation resistance at 600 °C, although oxidation became significant above 700 °C. Intermetallic Ni₃Sn was formed at 600 °C due to phase transformation of the initially solid solutions of Sn in Ni in the porous metals. For the porous metal with 10 wt% of Sn, the oxidation rate constant at 600 °C in air was estimated to be 8.5 × 10^?14 g² cm^?4 s^?1 and the electrical resistivity at 600 °C was almost constant at approximately 0.02 Ω cm² up to an elapsed time of 1000 h. In addition, the gas diffusibility and the power-collecting ability of the porous metal were equivalent to those of a platinum mesh when applied in the cathode current collector of a SOFC operated at 600 °C. Ni–Sn porous metals with adequate contents of Sn are believed to be promising cathode current collector materials for SOFCs for operation at temperatures below 600 °C.     

Developments on IME-alkaline water electrolysis

A research programme aiming at the development of a new advanced concept in alkaline water electrolysis has been demonstrated at S.C.K.-C.E.N. under the auspices of the Commission of the European Communities. The first R&D task was the development of an alkali-compatible ion exchange membrane as a replacement of the chrysotile asbestos diaphragm. After a screening test, polyantimonic acid manufactured in thin sheets was shown to display the required ion conduction in alkaline solution.Using polysulfone as an organic binder, the sheets withstand 120°C without deterioration. Several membrane characteristics such as ionic conductance, membrane potential and Hittorf transference numbers were measured in different experimental set-ups. The temperature dependence of the membrane conductance exhibits an exponential decay when working in an alkaline solution (? 1 N KOH). Membrane resistance decreases from a 1.0-0.8 Δ cm² range at 25°C to a 0.25-0.15 Δ cm² range at 120°C. Gas tightness and mechanical stability were demonstrated for 1000 h of continuous operation up till now.The electrodes under investigation were mainly composed of perforated nickel plates, catalytically activated using a thermal decomposition technique. Performances up to 120°C in 50 wt % KOH for 2000 h operation were investigated for Ni, NiCo₂O₄, NiCoO₂ and La_xCoO₃ as the anode electrocatalyst. As a result of these investigations, the spinel type NiCo₂O₄ showed the best performance under the testing conditions. At the cathode, NiB, NiS_x and NiCo₂S₄ were investigated up to 120°C as the hydrogen evolution electrocatalyst.A demonstration unit of a 1 kW electrolyser has been built in order to experiment on the newly introduced components (membranes, electrodes, gaskets, etc.). It consists of a 14-cell filter press unit, each cell of 40 cm². The loop built around the stack allows an upscaling up to 10 kW.     

Investigation of montmorillonite as carrier for OER

The aim of this work is to investigate the natural mineral Montmorillonite (MMT) as catalytic support and to assess the efficiency of the composite MMT-supported Ir toward OER in acidic electrochemical water splitting. MMT is a phyllosilicate layered clay with 2:1 type sheet structure with high cation exchange capacity, high surface area and low cost. Three different catalyst with iridium loadings of 10, 20, and 30 wt% Ir supported on MMT are synthesized. Their phase identification, crystallite size, elemental analysis, and thermal stability are studied by means of XRD, HRTEM, EDX, and TGA, respectively. The catalytic performance is examined in 0.5 M H₂SO₄ and in electrolysis cell with proton conductive polymer membrane (PEMEC). The results obtained prove that montmorillonite is a promising alternative of the conventional carbon supports with the advantage of being both easily available and cost favourable. Ir/MMT loaded with 30 wt% Ir is the best performed catalyst. In PEMEC operated at 80 °C the catalyst loading of 0.5 mg_Ir cm^?2 ensures intensive and sustainable oxygen evolution with current density reaching 200 mA cm^?2 already at 1.6 V.     

Hydrogen production by high temperature,high pressure water electrolysis,results of test plant operation

A high temperature, high pressure water electrolyzer has been developed. A test plant was built with a hydrogen production capacity of 4 m³ h^?1, operating at 120°C and 20 kg cm^?2 G. The development of the test plant was reported previously. The test plant was operated for 22 months under various operating conditions and with various combinations of electrodes. Of the electrodes, a high surface area nickel electrode showed the best results, its cell voltage being 1.64 V at 102°C and 400 mA cm^?2. PTFE impregnated with a Ti compound was used as the diaphragm material. The operating pressure did not show much effect on cell voltage.     

Alkaline inorganic-membrane-electrolyte (IME) water electrolysis

An R & D programme based on the development and parametric testing of water electrolysis cells for hydrogen production has been directed towards the development of electrolytic systems based on the use of inorganic ion exchange membranes.Attention was focused on the use of polyantimonic acid-teflon bound membranes which can be produced in thin sheets of 250 μm thickness. By attaching on both sides of the membrane catalytically active electrodes an electrolysis unit cell is obtained.At 10 kAm^?2 and 85 C, cell voltages of 1.8 V have been measured in a reproducible way using these heterogeneous inorganic ion exchange membranes.A technological demonstration set-up has been initiated involving the construction of a 1 kW filter press type electrolyzer.     

Bio-oil Production from an Oilseed By-product: Fixed-bed Pyrolysis of Olive Cake

Abstract

A study of pyrolysis of olive cake at the temperature range from 400°C to 700°C has been carried out. The experiments were performed in a laboratory scale tubular reactor under nitrogen atmosphere. The yields of derived gases, liquids, and char were determined in relation to pyrolysis temperature and sweeping gas flow rates, at heating rates of about 300°C min^?1. As the pyrolysis temperature was increased, the percentage mass of char decreased whilst gas product increased. The oil products increased to a maximum value of ～39.4 wt% of dry ash free biomass at a pyrolysis temperature of about 550°C in a nitrogen atmosphere with flow rate of 100 mL min^?1 and with a heating rate of 300°C min^?1. Results showed that the bio-oil obtained under the optimum conditions is a useful substitute for fossil fuels or chemicals.     

Development and performances of a 0.5 kW high-pressure alkaline water electrolyser

The purpose of this research paper is to describe the characteristics and electrochemical performances of a pressurized alkaline water electrolysis short stack (5-cells, 0.5 kW) operated at 80 °C, from atmospheric pressure up to 100 bars. Expanded grids of metallic nickel covered with specific porous catalytic structures have been used as working electrodes. A polysulfone-based diaphragm with a high ionic conductivity has been specifically designed for operation in pressurized alkaline water electrolysis cells. I–V polarization curves have been recorded at current densities up to 1000 mA/cm², at temperatures up to 80 °C and under pressures up to 100 bars. The water electrolysis efficiency of this short-stack has been determined. A specific energy consumption of ca. 4.4–4.5 kWh/Nm³ has been obtained in the high current density range. Durability tests have been performed on the short stack over 1000 h. A limited degradation rate <5 μV/h has been recorded over that period of test.     

Electrocatalytic properties of Ti2Ni/Ni-Mo composite electrodes for hydrogen evolution reaction

A composite electrode as hydrogen cathodes composed of Ti₂Ni hydrogen absorbing alloys and a Ni-Mo electrocatalyst was prepared for alkaline water electrolysis. The electrocatalytic properties of hydrogen evolution reaction (HER) are carried out in a 30 wt% KOH solution at 70 °C. The surface morphology and chemical composition of the cathode were also examined. The experimental results show that the composite cathode has a low hydrogen overpotential (ca. 60 mV at 70 °C in 30 wt% KOH) and excellent stability under conditions of continuous electrolysis and intermittent electrolysis with power interruption shutdown. The stability mechanism of the cathode against intermittent electrolysis is discussed.     

The effect of sintering aids on BaCe0·7Zr0·1Y0.1Yb0.1O3-δ as the electrolyte of proton-conducting solid oxide electrolysis cells

Proton-conducting solid oxide electrolysis cells (H-SOECs) are attracting attentions of researchers due to their unique advantages. The proton-conducting material BaCe_0·7Zr_0·1Y_0.1Yb_0.1O_3-δ (BZCYYb) has both the advantages of barium ceria-based and barium zirconate-based materials. BZCYYb material usually is synthesized by solid-state reaction (SSR) method, hence the densification of this electrolyte material is the key to restrict the application of H-SOECs. In this paper, effects of adding 1 wt% of different sintering aids (NiO, CuO, ZnO) to BZCYYb on the grain size and the conductivity are investigated. After adding 1 wt% of NiO and CuO sintering aids, BZCYYb electrolyte achieves ideal density. The electrical conductivity of four samples (BZCYYb without adding sintering aid, BZCYYb with 1 wt% NiO, CuO, ZnO, respectively) is tested under different steam concentrations of air, nitrogen, hydrogen, and nitrogen-hydrogen mixture. The conductivity increased after the sintering aid is added. With the increase of steam concentration, the conductivity decreased slightly and then increased due to electron holes under oxygen atmosphere with steam at high temperature. In other atmospheres, the conductivity increases with the steam concentration. In the atmosphere of 20 vol% H₂O-Air and H₂, the conductivity of the BZCYYb samples with 1 wt% CuO is about 1.087 × 10^?2 S cm^?1 and 9.02 × 10^?3 S cm^?1 at 650 °C, and the conductivity of the BZCYYb samples with 1 wt% NiO is 1.277 × 10^?2 S cm^?1 and 8.24 × 10^?3 S cm^?1, respectively. However, compared with NiO, CuO has advantages in promoting hydration reaction and proton conduction of BZCYYb electrolyte.     

Performance evaluation of a 4-stack solid oxide module in electrolysis mode

To support the current trend of testing bigger reversible Solid Oxide Cell (rSOC) modules, CEA has built the 120 kW_DC Multistack platform. It was used to test SOLIDpower recently developed-Large Stack Module (LSM) in electrolysis mode.Results show high thermal performance of the LSM, with homogeneous temperature distribution and losses in the kilowatt range above 700 °C. A performance map was recorded between 712 and 744 °C over 22.4-to-29.6 kg h^?1 steam flowrates using a fast control strategy to avoid endothermic operation. A peak power of 74 kW_DC was converted into more than 50 kg day^?1 of H₂ (35.5 kWh_DC kgH₂^?1). In addition, fuel utilization of more than 90% and steam conversion above 80% were demonstrated at the module level. In the end, the modular design of the LSM seems well suited for system scale up, paving the way for mutualization of auxiliaries and CAPEX reduction.     

Reversible operation performance of microtubular solid oxide cells with a nickelate-based oxygen electrode

This paper describes the reversible operation of a highly efficient microtubular solid oxide cell (SOC) with a nickelate-based oxygen electrode. The fuel cell was composed of a microtubular support of nickel and yttria stabilized zirconia (Ni-YSZ), an YSZ dense electrolyte, and a double oxygen electrode formed by a first composite layer of praseodymium nickelate (PNO) and gadolinium-doped ceria (CGO) and a second one of PNO. A good performance of the cell was obtained at temperatures up to 800 °C for both fuel cell (SOFC) and electrolysis (SOEC) operation modes, specially promising in electrolysis mode. The current density in SOEC mode at 800 °C is about −980 mA cm⁻² at 1.2V with 50% steam. Current density versus voltage curves (j-V) present a linear behavior in the electrolysis mode, with a specific cell area resistance (ASR) of 0.32 Ω cm⁻². Durability experiments were carried out switching the voltage from 0.7V to 1.2V. No apparent degradation was observed in fuel cell mode and SOEC mode up to a period of about 100 h. However, after this period especially in electrolysis mode there is an accumulated degradation associated to nickel coarsening, as confirmed by SEM and EIS experiments. Those results confirm that nickelate based oxygen electrodes are excellent candidates for reversible SOCs.     

Hydrogen evolution in microbial electrolysis cells treating landfill leachate: Dynamics of anodic biofilm

This study investigates the potential opportunities of hydrogen evolution treating landfill leachate in a set of two microbial electrolysis cells (MEC-1 and 2) under 30 °C and 17 ± 3 °C temperatures, respectively. The system achieved a projected current density of 1000–1200 mA m^?2 (MEC-1) and 530–755 mA m^?2 (MEC-2) coupled with low cost hydrogen production rate of 0.148 L La^?1 d^?1 (MEC-1) and 0.04 L La^?1 d^?1 (MEC-2) at an applied voltage of 1.0 V. Current generation led to a maximum COD oxidation of 73 ± 8% (MEC-1) and 65 ± 7% (MEC-2) with ≥100% energy recovery. The system also exhibited a high hydrogen recovery (66–95%), pure hydrogen yield (98%) and tremendous working stability during two months of operation. Electroactive microbes such as Pseudomonadaceae, Geobacteraceae and Comamonadaceae were found in anodophilic biofim, along with Rhodospirillaceae and Rhodocyclaceae, which could be involved in hydrogen production. These results demonstrated an energy-efficient approach for hydrogen production coupled with pollutants removal.