Graphene oxide modified non-noble metal electrode for alkaline anion exchange membrane water electrolyzers

This paper reports the performance of a graphene oxide modified non noble metal based electrode in alkaline anion exchange water electrolyzer. The electrolytic cell was fabricated using a polystyrene based anion exchange membrane and a ternary alloy electrode of Ni as cathode and oxidized Ni electrode coated with graphene oxide as anode. The electrochemical activity of the graphene oxide modified electrode was higher than the uncoated electrode. The anion exchange membrane water electrolyzer (AEMWE) with the modified electrode gave 50% higher current density at 30 °C with deionised water compared to that of an uncoated electrode at 2 V. Performance was found to increase with increase in temperature and with the use of alkaline solutions. The results of the solid state water electrolysis cell are promising method of producing low cost hydrogen.     

Separators for alkaline water electrolysis prepared by plasma-initiated grafting of acrylic acid on microporous polypropylene membranes

Highly hydrophilic separators for alkaline water electrolysis were prepared by plasma-initiated grafting of acrylic acid on porous polypropylene (PP) membranes. The membranes were activated in a low-pressure radio-frequency discharge in oxygen and subsequently graft polymerization of acrylic acid was performed in aqueous solution. The membranes were characterized by gravimetric grafting degree (GD), SEM, FTIR, critical wetting surface tension (CWST) test, mechanical strength, and electrolytic conductivity. Moreover, the membranes were applied as separators in alkaline electrolysis cell, and content of hydrogen in the produced oxygen was measured to determine membrane permeability to hydrogen dissolved in the electrolyte. It was observed that increasing GD improves performance of membranes as separators in alkaline electrolysis, although the particular effects on the electrolytic conductivity and hydrogen permeability strongly depend on structure the of initial PP substrate. Ageing test conducted in 30 wt% KOH at 60 °C revealed that although considerable degrafting took place at beginning of the test, the remaining polyacrylic acid provided highly hydrophilic character to membrane for 7000 h of the test.     

Effects of Nafion content in membrane electrode assembly on electrochemical Bunsen reaction in high electrolyte acidity

The electrochemical Bunsen reaction is an alternative way to traditional Bunsen reaction for hydrogen production, which can produce H₂SO₄ and HI in lower iodine and water condition in an electrochemical cell. However, energy consumption cannot be neglected due to its high electrolytic voltage. It is reported that the anode overpotential of electrochemical Bunsen reaction was much higher than other parts of the cell voltage. In this work, membrane electrode assemblies were prepared using a commercial 60 wt% Pt/C catalyst by spray method. The influence of Nafion content on the electrochemical Bunsen reaction was studied at room temperature (298 K) by multiple electrochemical means such as galvanostatic polarization studies, IV measurement and impedance studies. The energy consumed in the electrochemical cell was calculated based on hydrogen production. With the increase of Nafion content (13%–64%), the voltage and energy consumption displayed a V-shape. The experimental results shown that the best performance was achieved with a Nafion content of 37.50% for the voltage was as low as 0.6 V and the corresponding energy consumption was 290.5641 kJ/mol-H₂.     

Energy analysis of an electrolytic-based selective CO oxidation system for on-board pure hydrogen production

Carbon monoxide (CO) in the hydrogen (H₂) stream diminishes the performance of a low temperature polymer electrolyte membrane (PEM) fuel cell significantly. The existing technologies for on-board/site production of pure hydrogen with low CO concentrations operate at high temperatures and pressures and need on-board oxygen or air supply for CO oxidation. This study offers an alternative solution to on-board/site production of hydrogen suitable for low temperature PEM fuel cell applications. A thorough energy analysis and parametric study have been carried out to investigate the effect of different operating parameters on the performance of an electrolytic-based selective CO oxidation system operating at room temperature. Such a system electrochemically removes the CO molecules in a fuel stream where they are adsorbed onto the catalyst surface at the anode side of an electrolyzer, and produces additional hydrogen at the cathode side through an electrochemical water gas shift reaction. This additional hydrogen thus offsets the energy required to drive the electrolysis, making it an attractive solution for on-board/site applications. The breakthrough curves are measured for the adsorber (50 cm² area PEM reactor) fed with different CO concentrations (100, 300 and 500 ppm CO) present in the wet hydrogen stream at different flow rates (40, 80, 120, 160 and 200 ml min⁻¹), under the room temperature and atmospheric pressure. The electrolytic process (regeneration) parameters are optimized with an attempt of completely removing the adsorbed CO molecules. Furthermore, a thermodynamic-electrochemical model is developed to simulate the energy balance of the electrolyzer indicating that this process offsets the actual energy consumption as it produces extra hydrogen. Thermodynamically, removing CO by electricity is more efficient than by heat which leads to high energy efficiencies (>80%). The study has demonstrated that the electrolytic-based selective CO oxidation system is a promising system for on-board/site pure hydrogen production.     

A novel micro protective layer applied on a simplified PEM water electrolyser

This study presents a solid polymer electrolyte (SPE) technique, using a proton exchange membrane (PEM) water electrolyser, to produce high purity hydrogen and oxygen (both 99.99%). A proposed structural design provides comprehensive sealing of the assembly while combining a current collector and a flow field plate into a single component. The titanium porous disc with a highly condensed structure compared to the carbon based gas diffusion layer plays a key role as the support for the proton exchange membrane when producing high-pressure hydrogen. The electrochemical stability is enhanced as well owing to the consolidated component. The titanium porous disc is coated with noble metal catalysts (IrO₂/Ta₂O₅ composition) as a micro protective layer (MPL) to prevent corrosion and oxidation during hydrogen production from water electrolysis. The proposed MPL can effectively transform active oxygen species (such as oxygen atoms and hydroxyl free radicals) into harmless oxygen gas during water electrolysis. In this study, it is found that the electrolytic cell can sustain up to 10 bar and its lifetime is capable of lasting over 600 h continuously while maintaining a voltage of 2.33 to 2.35 V and current density of 1 A cm⁻².     

Performance of an alkaline direct ethanol fuel cell with hydrogen peroxide as oxidant

An alkaline direct ethanol fuel cell (DEFC) with hydrogen peroxide as the oxidant is developed and tested. The present fuel cell consists of a non-platinum anode, an anion exchange membrane, and a non-platinum cathode. It is demonstrated that the peak power density of the fuel cell is 130 mW cm⁻² at 60 °C (160 mW cm⁻² at 80 °C), which is 44% higher than that of the same fuel cell setup but with oxygen as the oxidant. The improved performance as compared with the fuel cell with oxygen as the oxidant is mainly attributed to the superior electrochemical kinetics of the hydrogen peroxide reduction reaction and the reduced ohmic loss associated with the liquid oxidant.     

Optimization of catalyst-coated membranes for enhancing performance in proton exchange membrane electrolyzer cells

To achieve large-scale application of proton exchange membrane electrolyzer cells (PEMECs) for hydrogen production, it is highly desirable to reduce the manufacturing cost while enhancing cell performance. In the PEMPECs, a catalyst-coated membrane (CCM) is the vital component where electrochemical reactions and mass transport mainly occur. The fabrication methods and catalyst layer (CL) structure can significantly affect the cell performance. Herein, for the first time, a comparative study of CCM fabrications with decal transfer and direct spray deposition methods have been conducted by both ex-situ materials characterization and in-situ performance testing in PEMECs. It is found CCMs that are fabricated with a direct spray deposition method display enhanced cell performance compared to CCMs fabricated with a decal transfer method, mainly due to the largely reduced ohmic resistance and improved mass transport. More importantly, cell performance can be greatly enhanced by simply regulating the Nafion ionomer content at the anode CL. The optimal Nafion ionomer content of 10 wt% gives the best cell performance at 80 °C with a low cell voltage of 1.887 V at 2 A cm^?2, outperforming the commercial CCM and most other previous publications. Our study provides a valuable guidance for fabrication and optimization of CCMs with significantly enhanced performance and reduced cost for practical application of the PEMECs.     

Sulphonated (PVDF-co-HFP)-graphene oxide composite polymer electrolyte membrane for HI decomposition by electrolysis in thermochemical iodine-sulphur cycle for hydrogen production

In overall iodine-sulphur (I-S) cycle (Bunsen reaction), HI decomposition is a serious challenge for improvement in H₂ production efficiency. Herein, we are reporting an electrochemical process for HI decomposition and simultaneous H₂ and I₂ production. Commercial Nafion 117 membrane has been generally utilized as a separator, which also showed huge water transport (electro-osmosis), and deterioration in conductivity due to dehydration. We report sulphonated poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-co-HFP) (SCP) and sulphonated graphene oxide (SGO) composite stable and efficient polymer electrolyte membrane (PEM) for HI electrolysis and H₂ production. Different SCP/SGO composite PEMs were prepared and extensively characterized for water content, ion-exchange capacity (IEC), conductivity, and stabilities (mechanical, chemical, and thermal) in comparison with commercial Nafion117 membrane. Most suitable optimized SCP/SGO-30 composite PEM exhibited 6.78 × 10^?2 S cm^?1 conductivity in comparison with 9.60 × 10^?2 S cm^?1 for Nafion® 117. The electro-osmotic flux ofSCP/SGO-30 composite PEM (2.53 × 10^?4 cm s^?1) was also comparatively lower than Nafion® 117 membrane (2.75 × 10^?4 cm s^?1). For HI electrolysis experiments, SCP/SGO-30 composite PEM showed good performance such as 93.4% current efficiency (η), and 0.043 kWh/mol-H₂ power consumption (Ψ). Further, intelligent architecture of SCP/SGO composite PEM, in which hydrophilic SGO was introduced between fluorinated polymer by strong hydrogen bonding, high efficiency and performance make them suitable candidate for electrochemical HI decomposition, and other diversified electrochemical processes.     

Overvoltage reduction in membrane Bunsen reaction for hydrogen production by using a radiation-grafted cation exchange membrane and porous Au anode

An electrochemical membrane Bunsen reaction using a cation exchange membrane (CEM) is a key to achieving iodine-sulfur (IS) thermochemical water splitting for the mass-production of hydrogen. In this study, we prepared a radiation-grafted CEM with a high ion exchange capacity (IEC) and a highly-porous Au-electroplated anode, and then used them for the membrane Bunsen reaction to reduce cell overvoltage. The high ionic content of our CEM led to low resistivity for proton transport, while the high porosity of the electrode led to a large effective surface area for anodic SO₂ oxidation. The cell overvoltage for the membrane Bunsen reaction was significantly reduced to 0.21 V at 200 mA/cm², one-third of that achieved using a commercial CEM and non-porous anode. From the analysis of the current-voltage characteristics, the grafted CEM was demonstrated to play a dominant role in the overvoltage reduction compared to the porous Au anode.     

High performance porous polybenzimidazole membrane for alkaline fuel cells

In this study, a highly ion-conductive and durable porous polymer electrolyte membrane based on ion solvating polybenzimidazole (PBI) was developed for anion exchange membrane fuel cells (AEMFCs). The introduction of porosity can increase the attraction of electrolytic solutions (e.g., potassium hydroxide (KOH)) and ion solvation, which results in the enhancement of PBI's ionic conductivity. The morphology, thermo-physico-chemical properties, ionic conductivity, alkaline stability, and the AEMFC performance of KOH-doped PBI membranes with different porosities were characterized. The ionic conductivity and AEMFC performance of 70 wt.% porous PBI was about 2 times higher than that of the commercially available Fumapem^® FAA. All KOH-doped porous PBI membranes maintained their ionic conductivity after accelerated alkaline stability testing over a period of 14 days, while the commercial FAA degraded just after 3 h. The excellent performance and good durability of KOH-doped porous PBI membrane makes it a promising candidate for AEMFCs.     

Development and preliminary testing of a unitized regenerative fuel cell based on PEM technology

Results related to the development and testing of a unitized regenerative fuel cell (URFC) based on proton-exchange membrane (PEM) technology are reported. A URFC is an electrochemical device which can operate either as an electrolyser for the production of hydrogen and oxygen (water electrolysis mode) or as a H₂/O₂ fuel cell for the production of electricity and heat (fuel cell mode). The URFC stack described in this paper is made of seven electrochemical cells (256 cм² active area each). The nominal electric power consumption in electrolysis mode is of 1.5 kW and the nominal electric power production in fuel cell mode is 0.5 kW. A mean cell voltage of 1.74 V has been measured during water electrolysis at 0.5 A cm⁻² (85% efficiency based on the thermoneutral voltage of the water splitting reaction) and a mean cell voltage of 0.55 V has been measured during fuel cell operation at the same current density (37% electric efficiency based on the thermoneutral voltage). Preliminary stability tests are satisfactory. Further tests are scheduled to assess the potentialities of the stack on the long term.     

High performance direct liquid fuel cells powered by xylose or glucose

In this work, we report the first abiotic, direct liquid fuel cells powered by the monosaccharide xylose using both a fully alkaline fuel cell (with anion exchange membrane) and a split pH fuel cell (with cation exchange membrane). We also report that the same fuel cells can be used with the monosaccharide glucose to produce much higher maximum power density than previously reported for abiotic, direct glucose fuel cells. This first alkaline direct xylose fuel cell (DXFC) produces a maximum power density of 57 mW cm⁻² at optimum conditions, while the split pH DXFC produces a maximum power density of 160 mW cm⁻². Our significantly improved alkaline direct glucose fuel cell (DGFC) produces 90 mW cm⁻² at optimum conditions, while the split pH DGFC produces 189 mW cm⁻². In addition to being high performing, these sugar molecules are naturally abundant, renewable, and known to convert to valuable products such as gluconic acid, glucaric acid, and xylonic acid during electrochemical oxidation. Other fuel cell and electrochemical cell data is also reported herein to understand the role of pH and fuel concentrations on behavior toward the electrochemical oxidation of these sugar molecules in alkaline media.     

Recovery of hydrogen from hydrogen sulfide by indirect electrolysis process

Recovery hydrogen from hydrogen sulfide is an effective way of utilizing exhaust gas. In this paper, removal of hydrogen sulfide by indirect electrochemical process was studied using acidic aqueous solution of Fe³⁺/Fe²⁺ as the electrochemical intermediate. Solid polymer electrolyte was applied to hydrogen production by indirect electrolysis of H₂S, in which the anode was graphite cloth, the cathode was the platinized graphite cloth, and the membrane was proton exchange membrane. The results of electrolysis experiments showed the relationship of current density as a function of electrolytic voltage at constant flow rate of electrolyte, temperature, and electrolyte composition. The effect of the cathode liquid velocity on current density was small. When the flow rate of anode electrolyte was greater than 200 L/hr., the current density tended to be stable. When [Fe³⁺]>0.20 mol/L, the concentrations of Fe²⁺ and Fe³⁺ ions in the anode solution had no significant impact on the current density. The current density gradually increased with temperature. In the electrolytic process of hydrogen production, the Fe²⁺ ions diffused from the anode to the cathode. The amount of diffusing Fe²⁺ ions gradually increased with time. The effect of Fe²⁺ ions diffusion from anode to cathode on hydrogen production was discussed.     

An alkaline direct ethylene glycol fuel cell with an alkali-doped polybenzimidazole membrane

An alkaline direct ethylene glycol fuel cell (DEGFC) with an alkali-doped polybenzimidazole membrane (APM) is developed and tested. It is demonstrated that the use of APMs enables the present fuel cell to operate at high temperatures. The fuel cell results in the peak power densities of 80 mW cm⁻² at 60 °C and 112 mW cm⁻² at 90 °C, respectively. The power output at 60 °C is found to be 67% higher than that by DEGFCs with proton exchange membranes, which is mainly attributed to the superior electrochemical kinetics of both ethylene glycol oxidation and oxygen reduction reactions in alkaline media.     

A small-scale and portable 50 W PEMFC system that automatically generates hydrogen from a mixture of Al,CaO, NaOH and sodium CMC in water without external power supply

The production of hydrogen from water under kinetic control is studied using a hydrophobic pouch filled with a mixture of aluminum, calcium oxide, water-soluble alkaline sodium CMC (Carboxymethylcellulose), and sodium hydroxide particles. NaOH particles easily absorb moisture from the air. Thus, CaO is added to protect NaOH from melting. To control the hydrogen generation rate, the aluminum powder is shaped into the spherical solids (M1) and irregular pellets (M2) using the alkaline sodium CMC. The hydrogen-generating pouch is prepared before hydrogen generation rate test. Results show that the best recipe for the range test is 40 wt% M1, 48 wt% M2, and 12 wt% mixed powder including NaOH, CaO and NaHCO₃, because of its greater stability and high hydrogen concentration. The best ratio of the aluminum powder and alkaline sodium CMC in three tests is 95 wt% to 5 wt%. The reaction of the pouch and water produces an on-board hydrogen supply for a polymer electrolyte membrane fuel cell (PEMFC) that can remain stable for 5 h or more, without requiring the addition of energy. This pouch has been applied in small-scale to large-capacity hydrogen generators for the PEMFC. Furthermore, this pouch has been used successfully to develop a 50 W portable hydrogen PEMFC generator.     

Thermodynamic aspects of urea oxidation reaction in the context of hydrogen production by electrolysis

This work reports the revisited thermodynamic parameters of urea oxidation reaction CO(NH₂)₂+H₂O→N₂+3H₂+CO₂, which is used in the urea-assisted electrolytic cell for the electrochemical production of hydrogen. It has been repeatedly stated in the literature that the open-circuit voltage for this reaction is equal to 0.37 V, which is significantly less than in the case of hydrogen evolution with a coupled oxygen evolution reaction (1.23 V at 298 K). This feature is considered as a very important advantage of urea-assisted systems. Our calculations showed that the value of 0.37 V is erroneous, and the correct value of the open-circuit voltage is ∼0.07 V. The specified thermodynamic parameters confirm the excellent prospects of electrochemical hydrogen production with the coupled reaction of urea oxidation and indicate wide potential opportunities for further reduction of energy consumption in the electrolysis process.     

Electroreduction of oxygen on sputter-deposited Pd nanolayers on multi-walled carbon nanotubes

The electrochemical reduction of oxygen on Pd nanoparticle/multi-walled carbon nanotube (PdNP/MWCNT) catalysts was studied in acid and alkaline solutions using the rotating disk electrode (RDE) method. The PdNP/MWCNT nanocomposites were prepared by sputter-deposition of palladium onto the surface of MWCNTs. The surface morphology of the PdNP/MWCNT composites was characterised by scanning electron microscopy (SEM). The SEM images showed the formation of Pd nanolayers around MWCNTs. The RDE results revealed a high electrocatalytic activity of PdNP/MWCNT catalysts towards the oxygen reduction reaction (ORR) in alkaline media. Two Tafel regions with different slope can be distinguished. At low overpotentials the Tafel slope is close to −60 mV dec⁻¹ and at higher current densities the slope is approximately −100 mV dec⁻¹. The PdNP/MWCNT composite is a promising material to be used as a cathode catalyst for alkaline membrane fuel cells.     

Treatment of low concentration hydrogen by electrochemical pump or proton exchange membrane fuel cell

Proton exchange membrane fuel cell (PEMFC) can produce electricity through electrochemical reaction of hydrogen with oxygen with the use of a membrane and electrode assembly (MEA). In other words, the hydrogen pressure difference between the anode and cathode can produce electricity via an electrochemical process. Conversely, when we supply electricity to MEA from an external power source, we can pump up or separate hydrogen from the low-pressure anode to the high-pressure cathode, according to the principle of “concentration cell”. By the way, PEMFC cannot use the fuel completely, because a cell potential decreases and electrode material may corrode when most of the fuel is consumed. Therefore the fuel released from PEMFC should be treated safely. The depleted hydrogen from PEMFC can be recovered by the electrochemical hydrogen pump, or further can be used as a fuel for the power generation by PEMFC, even though the cell voltage might be low. In this study we preliminarily measured the voltage–current characteristics of hydrogen pump and PEMFC changing the hydrogen concentration from 99.99% to 1%, as another option to platinum catalytic combustion of low concentration hydrogen. Moreover we could successfully treat the low concentration hydrogen by electrochemical pump or PEMFC, for the widely changing hydrogen concentration and mixture flow rate. The gas chromatography confirmed the hydrogen concentration of the treated gas to be 1000 ppm at most.     

Fabrication and characterization of NiCoZn-M (M: Ag, Pd and Pt) electrocatalysts as cathode materials for electrochemical hydrogen production

Ag, Pd and Pt-modified alkaline leached NiCoZn composite coatings were prepared on a copper specimen by electrochemical technique. The chemical composition of layers before and after leaching as well as after noble metal modification was determined by energy dispersive X-ray spectroscopy (EDX). The surface morphologies of the composite coatings were examined with the help of scanning electron microscopy (SEM). The hydrogen evolution activity of the electrodes was studied in 1 M KOH solution. For this purpose, cathodic current-potential curves and electrochemical impedance spectroscopy (EIS) techniques were used. Furthermore, the change of hydrogen evolution activity of the electrodes as a function of operation time in alkaline solution was also investigated. Surface morphologies showed that the composite coatings prepared to have compact and porous surface. EDX analysis confirmed the presence of Ag, Pd and Pt metals over the NiCoZn layer. The co-deposition of nickel, cobalt and zinc on copper surface and subsequently alkaline leaching of zinc rendered cathode material very active in hydrogen evolution. The modification of alkaline leached NiCoZn ternary coating by deposition of small amounts of Ag, Pd and Pt can further enhance the hydrogen evolution performance of this Raney-type electrode when compared to NiCoZn individually. The order of hydrogen evolution activity of catalysts studied is Ni < NiCoZn < NiCoZn-Pd < NiCoZn-Ag < NiCoZn-Pt. The long-term electrolysis tests showed that the Pt-modified electrode has the better time stability than the others. The superiority of Pt-modified catalyst explained by well known intrinsic catalytic activity of Pt.     

Development of crosslinked SEBS-based anion exchange membranes for water electrolysis: Investigation of the crosslinker effect

SEBS (styrene-b-(ethylene-co-butylene)-b-styrene)) is a non-aryl-ether-type tri-block copolymer widely used as an anion exchange membrane (AEM) material due to its excellent alkaline stability and phase separation properties. However, low tensile strength due to the aliphatic chains and the poor physical properties of the SEBS-based membranes limit their practical application for AEM water electrolysis (AEMWE) or AEM fuel cell (AEMFC). In this study, three types of crosslinked AEMs were prepared using bromohexyl pentafluorobenzyl SEBS as a polymer backbone, and three different crosslinkers, dimethyl amine (DMA), tetramethyl diaminohexane (TMHA), and tris(dimethyl aminomethyl) phenol (TDMAP). Once introduced, these crosslinking agents were converted into the corresponding conducting head groups. The thermal, chemical, physical, and electrical properties of the obtained crosslinked membranes were then investigated for use in AEMWE. In particular, the TDMAP-50x-SEBS membrane with 50% degree of crosslinking experienced hydrogen bonding with water and OH⁻ due to the presence of OH groups in the structure of the crosslinking agent (TDMAP). Because of this, the membrane showed an improved morphology and high conductivity (20 °C: 31.8 mS cm⁻¹, 80 °C: 109.9 mS cm⁻¹). In addition, TDMAP induced physical crosslinking by hydrogen bonding between molecules so that the corresponding membrane (TDMAP-50x-SEBS) exhibited high alkaline and oxidative stability and good mechanical properties. This SEBS-based membrane has a tensile strength of 18.0 MPa and Young's modulus of 165.14 MPa. The WE single-cell test (1 M KOH solution at 70 °C) using TDMAP-50x-SEBS also showed a cell performance of 1190 mA cm⁻² at 2.0 V. This is 126% higher than the cell performance measured for FAA-3-50, a commercialized AEM material, under the same conditions.