Nanosphere-structured composites consisting of Cs-substituted phosphotungstates and antimony doped tin oxides as catalyst supports for proton exchange membrane liquid water electrolysis

Proton exchange membrane liquid water electrolyser operated blow 80 °C suffers from insufficient catalyst activity and durability due to the slow oxygen evolution kinetics and poor stability. Aiming at enhancing oxygen electrode kinetics and stability, composite materials consisting of antimony doped tin oxide and Cs-substituted phosphotungstate were synthesized as the support of iridium oxide and possessed functionality of mixed electronic and protonic conductivity. At 80 °C under dry ambient atmosphere, the materials showed an overall conductivity of 0.33 S cm⁻¹. The supported IrO₂ catalysts were characterized in sulfuric acid electrolyte, showing significant enhancement of the oxygen evolution reaction (OER) activity. Electrolyser tests of the catalysts were conducted at 80 °C with a Nafion membrane. At an IrO₂ loading of 0.75 mg cm⁻² and a Pt loading of 0.2 mg cm⁻², the cell performance of a current density of 2 A cm⁻² at 1.66 V was achieved. The cell showed good durability at 35 °C under a current density of 300 mA cm⁻² in a period of 464 h.     

Antimony doped tin oxides and their composites with tin pyrophosphates as catalyst supports for oxygen evolution reaction in proton exchange membrane water electrolysis

Proton exchange membrane water electrolysers operating at typically 80 °C or at further elevated temperatures suffer from insufficient catalyst activity and durability. In this work, antimony doped tin oxide nanoparticles were synthesized and further doped with an inorganic proton conducting phase based on tin pyrophosphates as the catalyst support. The materials showed an overall conductivity of 0.57 S cm⁻¹ at 130 °C under the water vapor atmosphere with a contribution of the proton conduction. Using this composite support, iridium oxide nanoparticle catalysts were prepared and characterized in sulfuric and phosphoric acid electrolytes, showing much enhanced catalytic activity. Electrolyzer tests were conducted at both 80 °C with an Aquivion^™ membrane and at 130 °C with a phosphoric acid doped Aquivion^™ membrane. Significant improvement in the anodic kinetics was achieved on the composite supported catalysts at 130 °C although the electrolyzer cells showed higher ohmic resistance primarily from the membrane and catalyst layer. A durability test of electrolyzer cells was carried out at 130 °C under a current density of 400 mA cm⁻² in a period of up to 760 h, showing rather good stability of the system.     

Mesoporous iridium oxide/Sb-doped SnO2 nanostructured electrodes for polymer electrolyte membrane water electrolysis

In proton exchange membrane (PEM) water electrolysis, iridium oxide (IrO₂) has often been utilized as a main catalyst for the oxygen evolution reaction (OER) as a rate-determining step. In general, the performance of PEM water electrolysis is dominantly affected by the specific surface area and the porous structure of the IrO₂ catalyst. Thus, in this study, IrO₂ and antimony-doped tin oxide (ATO) nanostructures with high specific surface areas were synthesized through the Adams fusion method. The as-prepared samples showed well-defined porous high-crystalline nanostructures. The ATO nanoparticles as a support were surrounded by IrO₂ nanoparticles as a catalyst without serious agglomeration, indicating that the IrO₂ catalyst was uniformly distributed on the ATO support. Compared to pure IrO₂, the IrO₂/ATO mixture electrodes showed superior OER properties because of their increased electrochemical active sites.     

A novel catalyst coated membrane embedded with Cs-substituted phosphotungstates for proton exchange membrane water electrolysis

Catalyst coated membrane (CCM) is the core component of proton exchange membrane (PEM) water electrolysis and the main place for electrochemical reaction and mass transfer. Its properties directly affect the performance of PEM water electrolysis. Aiming at decreasing the polarization loss and the ohmic loss, a novel CCM embedded with Cs_1.5HPA in the skeleton of the Nafion^® ionomer and the Nafion^® membrane was prepared and possessed functionality of improved protonic conductivity. Meanwhile, the Cs_1.5HPA-Nafion ionomer content in the catalyst layers was further optimized. The SEM, EDS and pore volume distribution measurement showed that the Cs_1.5HPA embedded in the CCM without agglomeration and the micropore and mesopore were well distributed in the catalyst layer. Furthermore, CCMs were tested in a PEM water electrolyser at 80 °C, beneficial effects on both the Tafel slope and the iR loss were obtained due to the improved protonic conductivity as well as the appropriate pore structure and increased specific pore volume. The performance of the electrolyser cell was obviously improved with the novel CCM. The highest cell performance of 1.59 V at 2 A cm⁻² was achieved at 80 °C. At 35 °C and 300 mA cm⁻², the cell showed good durability within the test period of up to 570 h.     

Experimental studies on the effects of sheet resistance and wettability of catalyst layer on electro-catalytic activities for oxygen evolution reaction in proton exchange membrane electrolysis cells

As the most important part of electrochemical reaction in proton exchange membrane electrolysis cells (PEMECs) for water splitting, oxygen evolution reaction (OER) occurs at the anode catalyst layer (CL). The distribution of the OER site is affected by many factors, such as properties of CL, operation parameters, procedures, etc. To study the effects of properties of CLs on the distribution of OER site on the CL, and consequently affect the performance of PEMECs, CLs with different sheet resistances are tested under different operation conditions. The phenomena of OER on CLs are captured by a high-speed and micro-scale visualization system in-situ and analysed coupled with electrochemical results. The results show that both sheet resistance and wettability of CLs have significant impact on the distribution of the OER site, which can help optimize the design of membrane electrode assembly and improve the operating parameters for electrochemical devices.     

Proton exchange membrane water electrolysis with short-side-chain Aquivion membrane and IrO2 anode catalyst

A series of three membrane types has been screened for medium temperature solid polymer electrolyte water electrolysis in membrane electrode assemblies coated with 2 mg cm⁻² of iridium oxide as a catalyst for the oxygen evolution reaction, synthesised via a hydrolysis method from the hexachloroiridic acid precursor, and deposited on the membrane either directly by spray deposition or by decal transfer. The short-side-chain perfluorosulfonic acid Aquivion^® ionomer of equivalent weight 870 meq g⁻¹, in membranes of thickness 120 μm, gives higher water electrolysis performance at 120 °C than a composite membrane of Aquivion^® with zirconium phosphate, while a sulfonated ether-linked polybenzimidazole, sulfonated poly-[(1-(4,4′-diphenylether)-5-oxybenzimidazole)-benzimidazole], shows promising performance and no transport limitations up to 2 A cm⁻². The lowest cell voltage was observed at 120 °C for an MEA prepared using spray-coating directly on the Aquivion^® membrane, 1.57 V at 1 A cm⁻².     

Electrochemical- and mechanical stability of catalyst layers in anion exchange membrane water electrolysis

Anion exchange membrane (AEM) water electrolysis is considered a promising solution to future cost reduction of electrochemically produced hydrogen. We present an AEM water electrolyzer with CuCoO_x as the anode catalyst and Aemion as membrane and electrode binder. Full cell experiments in pure water and 0.1 M KOH revealed that the optimum binder content depended on the type of electrolyte employed. Online dissolution measurements suggested that Aemion alone was not sufficient to establish an alkaline environment for thermodynamically stabilizing the synthesized CuCoO_x in a neutral electrolyte feed. A feed of base is thus indispensable to ensure the thermodynamic stability of such non-noble catalyst materials. Particle loss and delamination of the catalyst layer during MEA operation could be reduced by employing a heat treatment step after electrode fabrication. This work summarizes possible degradation pathways for low-cost anodes in AEMWE, and mitigation strategies for enhanced system durability and performance.     

Ternary NiCoFe nanosheets for oxygen evolution in anion exchange membrane water electrolysis

Tuning nickel-based catalyst activity and understanding electrolyte and ionomer interaction for oxygen evolution reaction (OER) is crucial to improve anion exchange membrane (AEM) water electrolyzers. Herein, an investigation of multimetallic Ni_0.6Co_0.2Fe_0.2 OER activity, coupled with in-situ Raman spectroscopy to track dynamic structure changes, was carried out and compared to other Ni catalysts. The effect of KOH concentration, KOH purity, ionomer type, and electrolyte with organic cations was evaluated. The Ni_0.6Co_0.2Fe_0.2 catalyst achieved 10 mA/cm² at 260 mV overpotential with stability over 50 h and 5000 cycles in 1 M KOH. In-situ Raman spectroscopy showed that Ni_0.6Co_0.2Fe_0.2 activity originates from promoting Ni(OH)₂/NiOOH transformation at low potentials compared to bi- and mono-metallic nickel-based catalysts. Fumion anion ionomer in the catalyst inks led to a lower OER activity than catalysts with inks containing Nafion ionomer. The OER activity of Ni_0.6Co_0.2Fe_0.2 is adversely influenced in the presence of fumion anion ionomer and benzyltrimethylammonium hydroxide (BTMAOH) with possible phenyl oxidation under applied high anodic potentials. The alkaline AEM water electrolyzer circulating 1 M KOH electrolyte, with a Pt/C cathode and a Ni_0.6Co_0.2Fe_0.2 anode, achieved 1.5 A/cm² at 2 V.     

Investigation of supported IrO2 as electrocatalyst for the oxygen evolution reaction in proton exchange membrane water electrolyser

Indium tin oxide (ITO) was used as a support for IrO₂ catalyst in the oxygen evolution reaction. IrO₂ nanoparticles were deposited in various loading on commercially available ITO nanoparticle, 17–28 nm in size using the Adam's fusion method. The prepared catalysts were characterised using X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The BET surface area of the support (35 m²/g) was 3 times lower than the unsupported IrO₂ (112.7 m²/g). The surface area and electronic conductivity of the catalysts were predominantly contributed by the IrO₂. The supported catalysts were tested in a membrane electrode assembly (MEA) for electrolyser operation. The 90% IrO₂-ITO gave similar performance (1.74 V@1 A/cm²) to that of the unsupported IrO₂ (1.73 V@1 A/cm²) in the MEA polarisation test at 80 °C with Nafion 115 membrane which was attributed to a better dispersion of the active IrO₂ on the electrochemically inactive ITO support, giving rise to smaller catalyst particle and thereby higher surface area. Large IrO₂ particles on the support significantly reduced the electrode performance. A comparison of TiO₂ and ITO as support material showed that, 60% IrO₂ loading was able to cover the support surface and giving sufficient conductivity to the catalyst.     

Phosphoric acid doped composite proton exchange membrane for hydrogen production in medium-temperature copper chloride electrolysis

A copper chloride (CuCl) electrolyzer that constitutes of composite proton exchange membrane (PEM) that functions at medium-temperature (>100 °C) is beneficial for rapid electrochemical kinetics, and better in handling fuel pollutants. A synthesized polybenzimidazole (PBI) composite membrane from the addition of ZrO₂ followed with phosphoric acid (PA) is suggested to overcome the main issues in CuCl electrolysis, including the copper diffusion and proton conductivity. PBI/ZrP properties improved significantly with enhanced proton conductivity (3 fold of pristine PBI, 50% of Nafion 117), superior thermal stability (>600 °C), good mechanical strength (85.17 MPa), reasonable Cu permeability (7.9 × 10⁻⁷) and high ionic exchange capacity (3.2 × 10⁻³ mol g⁻¹). Hydrogen produced at 0.5 A cm⁻² (115 °C) for PBI/ZrP and Nafion 117 was 3.27 cm³ min⁻¹ and 1.85 cm³ min⁻¹, respectively. The CuCl electrolyzer efficiency was ranging from 91 to 97%, thus proven that the hybrid PBI/ZrP membrane can be a promising and cheaper alternative to Nafion membrane.     

Development of efficient membrane electrode assembly for low cost hydrogen production by anion exchange membrane electrolysis

Electrochemical production of hydrogen from water using anion exchange membranes (AEMs) can be achieved with non-noble catalysts, other than traditional proton exchange membranes that use platinum group metals. Using non-noble metals in the catalyst layer will reduce the capital costs associated with water electrolysis systems. The objectives of this study were to develop an effective membrane electrode assembly (MEA) for AEM electrolysis and to determine the effects of various operating parameters on AEM electrolysis. Here, the MEA consisted of the commercially available A-201 AEM and non-noble transition metal oxides as catalysts. The best electrolysis performance recorded was 500 mA cm^?2 for 1.95 V at 60 °C with 1% K₂CO₃ electrolyte. For the purpose of comparison, we also considered commercially available AEMs for AEM electrolysis: Fumapem^® FAA-3 and Fumapem^® FAA-3-PP-75. The performances achieved with these AEMs were comparable with the performance recorded for the conventional AEM A-201. Overall, our results indicated that AEM electrolysis clearly manifests the feasibility of commercial viability.     

Analytical modelling and experimental validation of proton exchange membrane electrolyser for hydrogen production

Proton Exchange Membrane (PEM) Electrolysers (ELSs) are considered as pollution-free with enhanced efficiency technology. Hydrogen can be easily produced from different resources like biomass, water electrolysis, natural gas, propane, and methanol. Hydrogen generation from water electrolysis, which is the splitting of water molecules into hydrogen and oxygen using electricity, can be beneficial when used in combination with variable Renewable Energy (RE) technologies such as solar and wind. When the electricity used for water electrolysis is produced by a variable RE source, the hydrogen stores the unused energy for a later use and can be considered as a renewable fuel and energy resource for the transport and energy sectors.This paper aims to propose a novel graphical model design for the PEM-ELS for hydrogen production based on the electrochemical, thermodynamical and thermal equations. The model under study is experimentally validated using a small-scale laboratory electrolyser. Simulation results, using Matlab-Simulink?, show an adequate parameter agreement with those found experimentally. Therefore, the impact of the different parameters on the electrolyser dynamic performance is introduced and the relevant analytical-experimental comparison is shown. The temperature effect on the PEM-ELS dynamic behaviour is also discussed.     

Three-dimensional copper cobalt hydroxide electrode for anion exchange membrane water electrolyzer

Anion exchange membrane (AEM) water electrolyzers are promising energy devices for producing low-cost and clean hydrogen using platinum group metals (PGMs). However, AEM water electrolyzers still do not show satisfactory performance due to the sluggish kinetics of the electrodes. In this work, copper cobalt hydroxide (CuCo-hydroxide) nanosheet was synthesized on commercial nickel foam (NF) via electrochemical co-precipitation, and used directly as an oxygen evolution reaction (OER) electrode for an AEM electrolyzer. The interaction between Cu and Co induces a change in the electronic structure of Co(OH)₂ and improves the performance of the OER electrode. In addition, the AEM electrolyzers catalyzed by CuCo(OH)₂ showed high energy conversion efficiency of 73.5%. This work demonstrates that non-PGM based electrodes fabricated using a simple electrochemical co-precipitation apply to AEM electrolyzers for low-cost and clean hydrogen production.     

Mathematical modeling of novel porous transport layer architectures for proton exchange membrane electrolysis cells

Thin foil based porous transport layers (PTLs) that contain highly structured pore arrays have shown promise as anode PTLs in proton exchange membrane electrolysis cells. These novel PTLs, fabricated with advanced manufacturing techniques, produce thin, tunable, multifunctional layers with reduced flow and interfacial resistances and high thermal and electric conductivities. To further optimize their design, it is important to understand their fundamental impact on the transport of protons, electrons, and liquid/vapor mixtures in the electrode. In this work, we develop a two-dimensional multiphysics model to simulate the coupled electrochemistry and multiphase transport in an electrolysis cell operated with the novel PTL architecture. The results show that larger pores improve access of water to the anode catalyst layer, which is beneficial for both the oxygen evolution reaction and membrane hydration. Larger pore sizes also improve oxygen gas transport from the catalyst layer, because generated oxygen gas is forced to travel in-plane through the anode catalyst layer until it reaches a pore opening that is connected to a channel. The discussed results confirm that the proposed thin foil based PTLs are fundamentally different from conventional PTLs, such as felts or layered meshes. The model developed in this work also provides generalizable insight into fundamental PEMEC phenomena, such as the competition between liquid and gas phase transport, membrane hydration and water management, and nonuniform electrochemical reactions, which are processes relevant to all PEMEC designs.     

Nano-crystalline RuxSn1 − xO2 powder catalysts for oxygen evolution reaction in proton exchange membrane water electrolysers

Nano-crystalline powders of Ru_xSn_1 − xO₂ (1.0 ≥ x ≥ 0.2) were prepared as high performance electrocatalysts for oxygen evolution in polymer electrolyte membrane water electrolysers (PEMWE). A modified Adams fusion method was developed to produce these oxides. The Ru_xSn_1 − xO₂ powder catalysts were investigated with XRD, SEM, TEM, CV, and EIS. XRD showed a nano-crystalline rutile structure results over the whole composition range. The particle sizes determined by TEM were between 5 and 20 nm. With an increase in the Sn content in Ru_xSn_1 − xO₂ (x-value was decreased), the catalytic performance increased initially and then decreased dramatically. The catalyst Ru_0.6Sn_0.4O₂ demonstrated the best performance in general, which may be due to its smaller particle size and greater ratio of outer active surface area. Repetitive cyclic voltammograms demonstrated that the Ru_0.6Sn_0.4O₂ catalyst had better stability than pure RuO₂. Both the mass normalized current density and chronocoulometry at 1.4 V indicated that Ru_0.6Sn_0.4O₂ and RuO₂ had better performance at 70 °C than at 25 °C.     

Gold as an efficient hydrogen isotope separation catalyst in proton exchange membrane water electrolysis

The development of proton exchange membrane water electrolysis (PEMWE) offers an updating potential for electrolytic hydrogen isotope separation. However, it has a significantly lower separation factor than the traditional alkaline water electrolysis. In this study, we propose gold as a promising cathodic catalyst for efficient hydrogen isotope separation in PEMWE. Au/C has a protium-to-deuterium (H/D) separation factor of 7.47 in PEMWE, about twice that of Pt/C. In addition, the full cell's electrochemical performance is comparable to that of its Pt/C counterpart. The separation mechanism in PEMWE is explained by the transitional hydrogen evolution reaction mechanism from Heyrovsky to Tafel for Pt and the unchangeable Volmer mechanism for Au. The high separation factor for Au is also calculated by the H/D zero-point vibrational energy difference between transition state and reaction state through a simple density functional theory calculation. This work offers an effective strategy to improve hydrogen isotope separation efficiency in PEMWE.     

Proton exchange membrane water electrolysis system-membrane electrode assembly with additive

In proton exchange membrane water electrolysis system, the performance is highly affected by the anode materials and the operation modes. In addition, high voltages are for higher hydrogen production and also ozone for disinfection. After switching off of the power and restarted, a decrease in electric conductivity may lead to a performance drop in further hydrogen/ozone/generation. In this study, three different additives, A, Z and V are adopted which respectively mixed with the PbO₂ and to become anode catalyst ink. The characteristics of the anode catalysts are determined by interruptive power supply, electrochemical impedance spectroscopy, and cyclic voltammetry tests. The results show that additives A and Z have batter current efficiency than the other groups. Additionally, anode catalyst withadditive V possess the most outstanding durability among all groups.     

Effect of treatment temperature on the performance of RuO2 anode electrocatalyst for high temperature proton exchange membrane water electrolysers

Ruthenium oxide catalysts were prepared by a sol–gel technique and calcined at different temperatures e.g., 400 °C, 500 °C and 600 °C. The catalysts performance for the oxygen evolution reaction was studied using cyclic voltammetry and their performance in a high temperature proton exchange membrane water electrolyser (PEMWE) examined. Physio-chemical characterization was carried out to study the thermal stability, oxygen-metal bond formation, crystallinity phase and crystallite size, particle size and elemental analysis by TGA, FTIR, XRD, TEM and EDX respectively. The electrolyte used for electrochemical characterisation was 1.0 M H₃PO₄ and 0.5 M H₂SO₄. Additionally, the effect of calcination and electrolyte temperature on oxygen evolution reaction of RuO₂ catalysts was studied and the apparent activation energy was determined using chronoamperometry. The prepared RuO₂ were tested as anode catalyst in PEMWE in the temperature range of 120–150 °C using phosphoric acid doped polybenzimidazole membrane electrolyte. The physio-chemical and electrochemical characterization results indicate that RuO₂ calcined at 500 °C gave the best performance with a current density of 0.875 A cm⁻² at 1.8 V in a PEMWE operated at 150 °C.     

IrO2-graphene hybrid as an active oxygen evolution catalyst for water electrolysis

In the present work, graphene supported IrO₂ catalyst (IrO₂/RGO) has been synthesized by hydrothermal method in ethanol/water mixture solvent. X-ray diffraction (XRD) and transmission electron microscopy (TEM) tests reveal that IrO₂ is uniformly supported on RGO surface with ultrafine IrO₂ nanoparticles (ca. 1.7 nm). Linear sweep voltammetry (LSV) tests indicate that the catalytic activity of IrO₂/RGO hybrid towards oxygen evolution reaction (OER) is 2.3 times that of commercial IrO₂. The superior OER activity of IrO₂/RGO hybrid is attributed to the enhanced surface area and the improved electrical conductivity of IrO₂ due to the introduction of graphene support. Lifetime tests demonstrate that IrO₂/RGO hybrid has unexpectedly high OER durability. It also displays an excellent performance in long-time water electrolysis. This may be interpreted in terms of the dispersion retention of IrO₂ nanoparticles on RGO surface, which is caused by the interaction between IrO₂ and Π-electrons of RGO.