Intermetallics as advanced cathode materials in hydrogen production via electrolysis

Intermetallics phases along Mo–Pt phase diagram have been investigated as cathode materials for the production of hydrogen by electrolysis from water KOH solutions, in an attempt to increase the electrolytic process efficiency. These materials were compared with conventional cathodes (Fe and Ni), often used in the alkaline electrolysis, and also with the intermetallic Ti–Pt. An significant upgrade of the electrolytic efficiency using intermetallics in pure KOH electrolyte was achieved in comparison with conventional cathode materials.     

Improved energy efficiency of the electrolytic evolution of hydrogen—Comparison of conventional and advanced electrode materials

With the idea to improve the efficiency of the electrolytic production of hydrogen by electrolysis, from water KOH solutions, the intermetallic Hf₂Co was investigated as cathode. This cathode was used single or in combination with ionic activators, and compared with several intermetallics previously investigated: Hf₂Fe, TiPt, and PtMo₃. A comparison with conventional cathode, nickel, was also evaluated. An significant upgrade of the electrolytic efficiency using intermetallics was achieved in comparison with conventional cathode materials. The influence of ionic activators on the process efficiency was significant, too.The effects of those cathode materials on the electrolytic evolution of hydrogen were discussed in the context of transition metals features that issue from their electronic configuration.     

Electrocatalytic effects of Mo–Pt intermetallics singly and with ionic activators. Hydrogen production via electrolysis

The intermetallic phases along the Pt–Mo phase diagram, singly or in combination with specific ionic activators, have been investigated as cathode materials for the production of hydrogen by electrolysis from water KOH solutions in an attempt to decrease energy consumption. The influence of ionic activators (activating compounds) on energy consumption was significant. The intermetallic phases, as cathode materials, were activated by the surface deposition of activating compounds from electrolyte. The influence of these cathode materials on the electrolytic evolution of hydrogen was discussed in the context of transition metals features and their electronic configuration.     

Fe–P and Fe–P–Pt co-deposits as hydrogen electrodes in alkaline solution

Fe–P and Fe–P–Pt alloys for use as electrodes for alkaline water electrolysis are prepared by an electroplating technique which employs an acidic complex bath solution. After heat treatment, the plated alloys act as effective electrocatalytic materials by lowering the hydrogen overpotential sufficiently. The improved electrocatalytic activity is due to an increase in effective surface area, a change in surface features upon heat treatment, and the presence of traces of platinum. Electrodes of the plated alloys are stable even in a highly corrosive electrolytic medium (6 M KOH).     

Preparation and characterization of Pt-CeO2 and Pt-Pd electrocatalysts for the oxygen reduction reaction in the absence and presence of methanol in alkaline medium

In this work, we report the synthesis and characterization of unsupported Pt-CeO₂ (1:1 wt. % Pt:CeO₂ ratio) and Pt-Pd (1:1 wt. % Pt:Pd ratio) electrocatalysts as candidate cathodes for alkaline direct methanol fuel cells (A-DMFCs). The catalytic activity of the cathodes for the oxygen reduction reaction (ORR) in the absence and presence of methanol, in KOH as electrolyte, was evaluated at room temperature. The materials were prepared by chemical reduction with NaBH₄, and pyrolysis at 300 and 600 °C under a H₂/N₂ atmosphere. The XRD results indicated the formation of polycrystalline materials with particle sizes ranging from 9 to 19 nm. Analysis by HRTEM showed the formation of nanostructures with lattice fringes corresponding to Pt, Pd (i.e., the Pt-Pd cathode), or CeO₂ (i.e., the Pt-CeO₂ material). The electrochemical characterization in 0.1 mol L⁻¹ KOH showed that the Pt-Pd is highly active for the ORR in alkaline medium, delivering higher onset potential and mass activity than Pt-alone. Meanwhile, the Pt-CeO₂ material showed slightly lower ORR mass activity than Pt. However, in the presence of methanol, the Pt-CeO₂ nanocatalyst demonstrated significantly higher selectivity and tolerance capability to the alcohol than Pt and Pt-Pd.     

Investigation of suitable cathodes for the production of hydrogen gas by electrolysis

The anodic and cathodic behaviours of aluminium, iron, mercury steel (HgSt), chrome-nickel steel (CrNiSt) and platinum have been studied in a 2N NaCl electrolyte (pH = 5) by means of electrolysis. The potentials of the cathodes and the theoretical and experimental discharge potentials of the systems have been determined. The cathodic and anodic overpotentials produced on Al, Fe, HgSt, CrNiSt and Pt were predicted against a platinum anode and a platinum cathode, respectively. The amounts of hydrogen gas produced at different times on the cathodes at a constant potential were measured and the hydrogen yield was calculated. From the data obtained, it is suggested that Pt anode and Al or HgSt cathode, or Al anode and Pt cathode, couples should be employed for the best electrolysis system.     

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.     

High-efficiency integrated multijunction photovoltaic/electrolysis systems for hydrogen production

In this study, two integrated PV/electrolysis systems were studied. An integrated PV/electrolysis device is a device where the PV system and the electrolyser are combined into a single system. In the case studied here, the areas of the PV device and the electrolyzer are identical. Multijunction photovoltaic (PV)/electrolysis configurations were investigated, and a high solar-to-hydrogen conversion efficiency cell demonstrated. The energy diagram of the configuration has been evaluated and exemplified with n/p, n/p-GaInP/GaAs(Pt)/KOH electrolyte and triple-junction p–i–n a-Si(Pt)/KOH electrolyte photovoltaic/electrolysis cells. The efficiency of the cells was determined based on the short-circuit photocurrent. Measurements were obtained both indoors under 100-mW/cm² insolation, and outdoors. For the a-Si system, a 7.8% solar-to-hydrogen conversion efficiency was demonstrated. For the GaAs/GaInP₂ system, the solar-to-hydrogen conversion efficiency was over 16%.     

Catalytic activity and selectivity for the ORR of rapidly synthesized M@Pt (M = Pd,Fe3O4, Ru) core–shell nanostructures

We report the performance of M@Pt (M = Pd, Fe₃O₄, Ru) core–shell nanostructures for the ORR in H₂SO₄. The nanomaterials were rapidly synthesized in a total of 120 s applying magnetic (MS), mechanical (UT) and sonochemical (USS) stirring. Pt-alone nanoparticles were also synthesized by UT and served as a reference to evaluate the performance of the core–shell cathodes. Crystalline features were obtained from the Pd and the Fe₃O₄ cores, while Ru cores apparently having quasi-amorphous characteristics or too small particle sizes were formed. Nevertheless, the three core–shell materials showed crystalline Pt features. From Scherrer analysis, it was determined that core–shell nanostructures with particle sizes from 7.3 to 9.2 nm were formed. The electrochemical characterization confirmed that active core–shell cathodes were obtained, with two of the Pd@Pt materials showing catalytic activities as high as that of the Pt-alone nanoparticles, or even higher in terms of mass activity. XPS analysis indicated that mainly Pt(0) species were formed at the cathodes, along with Pt(II) and Pt(IV). It was found that catalysts with high Pt(IV):Pt(II) showed enhanced catalytic activity for the reduction of oxygen. In the presence of ethanol, the evaluated Pd@Pt cathode showed the higher selectivity towards the ORR compared to the other cathodes.     

Hydrogen-retention capacity,release kinetics and structural stability of some titanium-based alloys

The H₂ storage characteristics of certain Ti-based transition metal alloys, containing Mg, Co, Fe and Ni were investigated by the electrochemical generation of H₂ using alloy specimens as cathodes in an electrochemical cell. Metallographic evaluation of the cathodes indicates that alloy composition, rate of electrolysis, mechanical strength and stability of the alloys are interrelated. These properties may be advantageously regulated by the electrolytic current density which controls the rate of H₂ formation per unit area of the alloy. The results are of potential usefulness for the rational design of H₂ storage devices of a chemo-metallurgical nature.     

Composite cathode based on Ni-loaded La0.75Sr0.25Cr0.5Mn0.5O3−δ for direct steam electrolysis in an oxide-ion-conducting solid oxide electrolyzer

Composite cathode based on La_0.75Sr_0.25Cr_0.5Mn_0.5O_3−δ (LSCM) can be utilized for direct steam electrolysis in an oxide-ion conducting solid oxide electrolyzer; however, the insufficient electro-catalytic activity of LSCM still restricts the electrode performance and the Faraday efficiency. In this work, catalytic-active Ni particles are loaded to cathode via impregnation method. The electrical properties of LSCM are investigated and further correlated to the electrochemical performance of LSCM-SDC cathodes. The AC impedance spectroscopy and current–voltage tests demonstrate that the electrochemical reduction of LSCM cathodes is the main process at low voltages; however, the steam electrolysis is the dominant process at high voltages. The Faraday efficiency with Ni-loaded LSCM was enhanced by 20% in 4.96%H₂/Ar/3%H₂O and 11% in 97%Ar/3%H₂O in contrast to the bare LSCM-SDC cathodes, respectively. The synergetic effect of catalytic-active Ni and redox-stable LSCM contributes to improved performance and the stability of the cathode for direct steam electrolysis.     

A new cathodic electrode deposit with palladium nanoparticles for cost-effective hydrogen production in a microbial electrolysis cell

Microbial electrolysis cell (MEC) provides a sustainable way for hydrogen production from organic matters, but it still suffers from the lack of efficient and cost-effective cathode catalyst. In this work carbon paper coated with Pd nanoparticles was prepared using electrochemical deposition method and used as the cathodic catalyst in an MEC to facilitate hydrogen production. The electrode coated with Pd nanoparticles showed a lower overpotential than the carbon paper cathode coated with Pt black. The coulombic efficiency, cathodic and hydrogen recoveries of the MEC with the Pd nanoparticles as catalyst were slightly higher than those with a Pt cathode, while the Pd loading was one order of magnitude less than Pt. Thus, the catalytic efficiency normalized by mass of the Pd nanoparticles was about fifty times higher than that of the Pt black catalyst. These results demonstrate that utilization of the cathode with Pd nanoparticles could greatly reduce the costs of the cathodic catalysts when maintaining the MEC system performance.     

Non-platinum cathode catalysts for alkaline membrane fuel cells

Hydrogen–oxygen fuel cells using an alkaline anion exchange membrane were prepared and evaluated. Various non-platinum catalyst materials were investigated by fabricating membrane-electrode assemblies (MEAs) using Tokuyama membrane (# A201) and compared with commercial noble metal catalysts. Co and Fe phthalocyanine catalyst materials were synthesized using multi-walled carbon nanotubes (MWCNTs) as support materials. X-ray photoelectron spectroscopic study was conducted in order to examine the surface composition. The electroreduction of oxygen has been investigated on Fe phthalocyanine/MWCNT, Co phthalocyanine/MWCNT and commercial Pt/C catalysts. The oxygen reduction reaction kinetics on these catalyst materials were evaluated using rotating disk electrodes in 0.1 M KOH solution and the current density values were consistently higher for Co phthalocyanine based electrodes compared to Fe phthalocyanine. The fuel cell performance of the MEAs with Co and Fe phthalocyanines and Tanaka Kikinzoku Kogyo Pt/C cathode catalysts were 100, 60 and 120 mW cm⁻² using H₂ and O₂ gases.     

Influence of graphitic materials microstructure in the hydrogen evolution in aqueous solution of tetra-alkylammonium-sulfonic acid ionic liquid

Vitreous carbon (VC), pyrolytic carbon (PC) and moulded graphite (MG) were tested as cathodic materials in hydrogen production by water electrolysis in the presence of the ionic liquid tetrafluoroborate of 3-triethylammonium-propane sulfonic acid (TEA-PS.BF₄). The physical characterization of the carbon materials indicated large differences in the microstructure of VC, PC, and MG and this significantly affected their electrochemical response. The mechanism presented for all the materials studied in the hydrogen evolution reaction (HER) was Volmer–Heyrovsky, where the H₂ desorption at the catalytic surface is the determining step. The MG electrode presented an unfavourable performance due to the formation of nanobubbles that coalesced without H₂ desorption and led to the deactivation of the catalytic sites. This behaviour is attributed to the presence of large and ordered crystallites at the material surface, with a greater number of hydrophobic domains in comparison with VC and PC material surfaces. The VC and PC electrodes presented a higher performance compared to the Pt cathode, showing lower activation energy, higher cathodic exchange current and lower charge transfer resistance. The set of results indicates VC and PC as promising alternative materials to constitute cathodes for the electrolysis of water using TEA-PS.BF₄ aqueous solution as electrolyte.     

Hydrogen production from macroalgae by simultaneous dark fermentation and microbial electrolysis cell with surface-modified stainless steel mesh cathode

An affordable cathode material for microbial electrolysis cells (MECs) was synthesized via surface-modification of stainless steel mesh (SSM) by anodization. The anodization parameters, such as wire mesh size, temperature, applied voltage, operating time, were optimized. The surface-modified SSM (smSSM) exhibited porous surface and higher specific surface area. The as synthesized smSSMs were utilized as freestanding cathodes in a conventional microbial electrolysis cell (MEC) and a simultaneous dark fermentation and MEC process (sDFMEC). The H₂ production in MEC and sDFMEC with smSSM as cathode was approximately 150% higher than that with SSM. The performance of smSSM was 67–75% of that of Pt/C. The sDFMEC with smSSM as cathode was stable for 12 cycles of fed-batch operation in 60 days. Overall, energy conversion from S. japonica by sDFMEC was as high as 23.4%.     

Low overvoltage electrocatalysts for hydrogen evolving electrodes

An electrocatalyst for hydrogen evolving cathodes has been developed for use in alkaline media. The materials employed in manufacturing the coating are relatively cheap transition metals. The results of a variety of performance tests are reported. The electrodes exhibit a low overvoltage for hydrogen evolution (70–90 mV at 70°C and 1A cm^?2 in 5 N KOH), stability to abuse (e.g. current interruption or reversal) and a long cathode life under operating conditions.     

Optimization of process parameters using response surface methodology (RSM) for power generation via electrooxidation of glycerol in T-Shaped air breathing microfluidic fuel cell (MFC)

The present work focuses on the optimization of operating parameters using Box Behnken design (BBD) in RSM to obtain maximum power density from a glycerol based air-breathing T-shaped MFC. The major parameters influencing the experiment for enhancing the cell performance in MFC are glycerol/fuel concentration, anode electrolyte/KOH concentration, anode electrocatalyst loading and cathode electrolyte/KOH concentration. The ambient oxygen is used as the oxidant. The acetylene black carbon (C_AB) supported laboratory synthesized electrocatalyst Pd–Pt (16:4)/C_AB is used as anode electrocatalyst and commercial Pt (40 wt%)/C_HSA as the cathode electrocatalyst. The quadratic model predicts the appropriate operating conditions to achieve highest power density from the laboratory designed T-shaped MFC. The p-value of less than 0.0001 and F-value of greater than 1 i.e., 26.32 indicate that the model is significant. The optimum conditions predicted by the RSM model were glycerol concentration of 1.07 M, anode electrolyte concentration of 1.62 M anode electrocatalyst loading of 1.12 mg/cm² and cathode electrolyte concentration of 0.69 M. The negligible deviation of only 1.07% between actual/experimental power density (2.76 mW/cm²) and predicted power density (2.79 mW/cm²) was recorded. This model reasonably predicts the optimum conditions using Pd–Pt (16:4)/C_AB electrocatalyst to obtain maximum power density from glycerol based MFC.     

Evaluation of low cost cathode materials for treatment of industrial and food processing wastewater using microbial electrolysis cells

Microbial electrolysis cells (MECs) can be used to treat wastewater and produce hydrogen gas, but low cost cathode catalysts are needed to make this approach economical. Molybdenum disulfide (MoS₂) and stainless steel (SS) were evaluated as alternative cathode catalysts to platinum (Pt) in terms of treatment efficiency and energy recovery using actual wastewaters. Two different types of wastewaters were examined, a methanol-rich industrial (IN) wastewater and a food processing (FP) wastewater. The use of the MoS₂ catalyst generally resulted in better performance than the SS cathodes for both wastewaters, although the use of the Pt catalyst provided the best performance in terms of biogas production, current density, and TCOD removal. Overall, the wastewater composition was more of a factor than catalyst type for accomplishing overall treatment. The IN wastewater had higher biogas production rates (0.8–1.8 m³/m³-d), and COD removal rates (1.8–2.8 kg-COD/m³-d) than the FP wastewater. The overall energy recoveries were positive for the IN wastewater (3.1–3.8 kWh/kg-COD removed), while the FP wastewater required a net energy input of −0.7–−1.2 kWh/kg-COD using MoS₂ or Pt cathodes, and −3.1 kWh/kg-COD with SS. These results suggest that MoS₂ is the most suitable alternative to Pt as a cathode catalyst for wastewater treatment using MECs, but that net energy recovery will be highly dependent on the specific wastewater.     

Intermediate temperature steam electrolysis using strontium zirconate-based protonic conductors

Steam electrolysis using SrZr_0.9Y_0.1O_3-α (SZY-91) electrolyte was investigated. Different electrode materials, i.e., porous platinum, Sr_0.5Sm_0.5CoO₃ (SSC-55) for the anode and nickel for the cathode were examined in order to achieve better energy performance. The electrodes had poor electrode activity when platinum was used for both the anode and the cathode, while the SSC-55 anode displayed significantly low overpotentials and the nickel cathode exhibited low overpotentials upon the introduction of an SrCe_0.95Yb_0.05O_3-α (SCYb) interlayer. Moreover, a partial substitution of cerium for zirconium in the strontium zirconate electrolyte, i.e., SrZr_0.5Ce_0.4Y_0.1O_3-α (SZCY-541), was found to be effective for improving the current efficiency of the hydrogen evolution rate. Accordingly, the cell SSC-55|SZCY-541|SCYb|Ni exhibited much higher energy efficiency for the steam electrolysis than the Pt|SZY-91|Pt cell.     

Evaluation of TiO2 as catalyst support in Pt-TiO2/C composite cathodes for the proton exchange membrane fuel cell

Anatase TiO₂ is evaluated as catalyst support material in authentic Pt-TiO₂/C composite gas diffusion electrodes (GDEs), as a different approach in the context of improving the proton exchange membrane fuel cell (PEMFC) cathode stability. A thermal stability study shows high carbon stability as Pt nanoparticles are supported on TiO₂ instead of carbon in the Pt-TiO₂/C composite material, presumably due to a reduced direct contact between Pt and C. The performance of Pt-TiO₂/C cathodes is investigated electrochemically in assembled membrane-electrode assemblies (MEAs) considering the added carbon fraction and Pt concentration deposited on TiO₂. The O₂ reduction current for the Pt-TiO₂ alone is expectedly low due to the low electronic conductivity in bulk TiO₂. However, the Pt-TiO₂/C composite cathodes show enhanced fuel cell cathode performance with growing carbon fraction and increasing Pt concentration deposited on TiO₂. The proposed reasons for these observations are improved macroscopic and local electronic conductivity, respectively. Electron micrographs of fuel cell tested Pt-TiO₂/C composite cathodes illustrate only a minor Pt migration in the Pt-TiO₂/C structure, in which anatase TiO₂ is used as Pt support. On the whole, the study demonstrates a stable Pt-TiO₂/C composite material possessing a performance comparable to conventional Pt–C materials when incorporated in a PEMFC cathode.