Efficiency and stability of transition metal electrocatalysts for the hydrogen evolution reaction using ionic liquids as electrolytes

Different electrodes (nickel, molybdenum, and iron alloys containing chromium, manganese, and nickel) were tested as cathodes for the hydrogen evolution reaction (HER) using 1-n-butyl-3-methylimidazolium tetrafluoroborate (BMI.BF₄) ionic liquid (IL) as the electrolyte. HERs were conducted at room temperature, at a cathodic potential of −1.7 V (PtQRE) using 10 vol.% aqueous BMI.BF₄ solutions. Reactions performed in a thermostated Hoffman cell gave current densities between 14.6 and 77.5 mA cm⁻² and efficiencies in the range 97.0–99.2%. Mo electrocatalysts in IL have been shown to be better than Pt, contrary to the classic behavior observed in an aqueous KOH medium. The electrochemical properties of molybdenum, as well as its resistance to corrosion (studied by Tafel plots and observed using SEM) indicate the potential use of this material as a cathode in an IL medium, which can lead to many attractive technological applications.     

Anodic deposition of porous RuO2 on stainless steel for supercapacitor studies at high current densities

Ruthenium dioxide is deposited on stainless steel (SS) substrate by galvanostatic oxidation of Ru³⁺. At high current densities employed for this purpose, there is oxidation of water to oxygen, which occurs in parallel with Ru³⁺ oxidation. The oxygen evolution consumes a major portion of the charge. The oxygen evolution generates a high porosity to RuO₂ films, which is evident from scanning electron microscopy studies. RuO₂ is identified by X-ray photoelectron spectroscopy. Cyclic voltammetry and galvanostatic charge–discharge cycling studies indicate that RuO₂/SS electrodes possess good capacitance properties. Specific capacitance of 276 F g⁻¹ is obtained at current densities as high as 20 mA cm⁻² (13.33 A g⁻¹). Porous nature of RuO₂ facilitates passing of high currents during charge–discharge cycling. RuO₂/SS electrodes are thus useful for high power supercapacitor applications.     

Carbon supported ruthenium chalcogenide as cathode catalyst in a microfluidic formic acid fuel cell

This work reports the electrochemical measurements of 20 wt.% Ru_xSe_y/C for oxygen reduction reaction (ORR) in presence of different concentration of HCOOH and its use as cathode catalyst in a microfluidic formic acid fuel cell (μFAFC). The results were compared to those obtained with commercial Pt/C. Half-cell electrochemical measurements showed that the chalcogenide catalyst has a high tolerance and selectivity towards ORR in electrolytes containing up to 0.1 M HCOOH. The depolarization effect was higher on Pt/C than on Ru_xSe_y/C by a factor of ca. 23. Both catalysts were evaluated as cathode of a μFAFC operating with different concentrations of HCOOH. When 0.5 M HCOOH was used, maximum current densities of 11.44 mA cm⁻² and 4.44 mA cm⁻² were obtained when the cathode was Ru_xSe_y/C and Pt/C, respectively. At 0.5 M HCOOH, the peak power density of the μFAFC was similar for both catalysts, ca. 1.9 mW cm⁻². At 5 M HCOOH the power density of the μFAFC using Ru_xSe_y, was 9.3 times higher than the obtained with Pt/C.     

Hydrogen production with nickel powder cathode catalysts in microbial electrolysis cells

Although platinum is commonly used as catalyst on the cathode in microbial electrolysis cells (MEC), non-precious metal alternatives are needed to reduce costs. Cathodes were constructed using a nickel powder (0.5–1 μm) and their performance was compared to conventional electrodes containing Pt (0.002 μm) in MECs and electrochemical tests. The MEC performance in terms of coulombic efficiency, cathodic, hydrogen and energy recoveries were similar using Ni or Pt cathodes, although the maximum hydrogen production rate (Q) was slightly lower for Ni (Q = 1.2–1.3 m³ H₂/m³/d; 0.6 V applied) than Pt (1.6 m³ H₂/m³/d). Nickel dissolution was minimized by replacing medium in the reactor under anoxic conditions. The stability of the Ni particles was confirmed by examining the cathodes after 12 MEC cycles using scanning electron microscopy and linear sweep voltammetry. Analysis of the anodic communities in these reactors revealed dominant populations of Geobacter sulfurreduces and Pelobacter propionicus. These results demonstrate that nickel powder can be used as a viable alternative to Pt in MECs, allowing large scale production of cathodes with similar performance to systems that use precious metal catalysts.     

Evaluation of stainless steel cathodes and a bicarbonate buffer for hydrogen production in microbial electrolysis cells using a new method for measuring gas production

Microbial electrolysis cells (MECs) are often examined for hydrogen production using non-sustainable phosphate buffered solutions (PBS), although carbonate buffers have been shown to work in other bioelectrochemical systems with a platinum (Pt) catalyst. Stainless steel (SS) has been shown to be an effective catalyst for hydrogen evolution in MECs, but it has not been tested with carbonate buffers. We evaluated the combined using of SS cathodes and a bicarbonate buffer (BBS) at the applied voltages of 0.5, 0.7 and 0.9 V using a new inexpensive method for measuring gas production called the gas bag method (GBM). This method achieved an average error of only 5.0% based on adding known volumes of gas to the bag. Using the GBM, hydrogen production with SS and a BBS was 26.6 ± 1.8 mL which compared well to 26.4 ± 2.8 mL using Pt and BBS, and 26.8 ± 2.5 mL with a Pt cathode and PBS. Electrical energy efficiency was highest with a SS cathode and BBS at 159 ± 17%, compared to 126 ± 14% for the Pt cathode and BBS, and 134 ± 17% for a Pt cathode and PBS. The main disadvantage of the SS was a lower gas production rate of 1.1 ± 0.3 m³ H₂-m⁻³ d⁻¹ with BBS and 1.2 ± 0.3 m³ H₂-m⁻³ d⁻¹ with PBS, compared to 1.7 ± 0.4 m³ H₂-m⁻³ d⁻¹ with Pt and PBS. These results show that the GBM is an effective new method for measuring gas production of anaerobic gas production processes, and that SS and bicarbonate buffers can be used to effectively produce hydrogen in MECs.     

New nano-structured and interactive supported composite electrocatalysts for hydrogen evolution with partially replaced platinum loading

This work is concerned with preparation and characterization of nano-structured composite electrocatalytic material for hydrogen evolution based on CoPt hyper d-metallic phase and anatase (TiO₂) hypo d-phase, both deposited on multiwalled carbon nanotubes (MWCNTs) as a carbon substrate. The main goal is partially or completely to replace Pt as the electrocatalytic material. Four electrocatalytic systems were prepared with common composition 10% Me + 18% TiO₂ + MWCNTs, where Me = Co, CoPt (4:1, wt. ratio), CoPt (1:1, wt. ratio) and Pt. The structural changes and their influence on electrocatalytic activity were studied by means of XRD, TEM, SEM and FTIR. The electrocatalytic activity was assessed in aqueous alkaline and polymer acidic electrolytes by means of steady-state galvanostatic method. It was found that Co strongly affects the platinum particle size. The addition of Co reduces platinum particle's size from 11 nm (in pure Pt metallic system) to 4 nm (in both systems 4:1 and 1:1), i.e. almost by 3 times. The corresponding increase of the surface area and the number of the active catalytic centres improves the efficiency, despite the fact that the amount of used platinum was decreased up to 5 times. The catalyst based on CoPt (1:1) performed the best, while the activity of the pure platinum and CoPt (4:1) systems were very close. Generally, the studied electrocatalysts have shown good and stable performances for hydrogen evolution in PEM electrochemical cell. The influence of the hydrogen electrodes under investigation on the water electrolysis efficiency at current density of 0.3 A cm⁻² was assessed, using previous data oxygen evolution on IrO_x electrode. Related to the performances of commercial Pt (ELAT) electrode, when hydrogen electrodes with the prepared mixed electrocatalysts were used, the water electrolysis efficiency was only 5% lower for CoPt (1:1), nearly 10% lower for CoPt (4:1) and 13% lower in the case of pure Co-based electrocatalyst.     

Microstructural characterization and electrochemical properties of Ba0.5Sr0.5Co0.8Fe0.2O3−δ and its application for anode of SOEC

Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3−δ (BSCF) was synthesized successfully by a novel citric acid–nitrate combustion method and employed as the anode of solid oxide electrolysis cells (SOEC) for hydrogen production for the first time in this paper. The crystal structure, chemical composition and electrochemical properties of BSCF were investigated in detail. The results showed that BSCF is in good stoichiometry of Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3−σ formation. ASR of BSCF/YSZ is only 0.077 Ω cm² at 850 °C, remarkably lower than the commonly used oxygen materials LSM as well as the current focus materials LSC and LSCF. Also, BSCF electrode exhibited much better performance than LSM under both SOEC and SOFC operating modes. The hydrogen production rate of BSCF/YSZ/Ni-YSZ can be up to 147.2 mL cm⁻² h⁻¹, about three times higher than that of LSM/YSZ/Ni-YSZ, which indicates that BSCF could be a very promising candidate for the practical application of SOEC technology.     

Preparation of the composite LaNi3.7Al1.3/Ni–S–Co alloy film and its HER activity in alkaline medium

The composite LaNi_3.7Al_1.3/Ni–S–Co alloy film was prepared by molten salt electrolysis and aquatic electrodeposition orderly. With Na₃AlF₆–La₂O₃–Al₂O₃ (91:8:1) system as molten salt electrolyte, the LaNi_3.7Al_1.3 alloy film was obtained by galvanostatic electrolysis at 100 mA cm⁻². The results showed that the La³⁺ and Al³⁺ ions could be co-reduced on the nickel cathode to form LaNi_3.7Al_1.3 film, i.e. La³⁺ + 1.3Al³⁺ + 6.9e + 3.7Ni = LaNi_3.7Al_1.3 at c.a. −0.5 V, which is much lower than that of the theoretical decomposition potential of lanthanum and aluminum. With high HER activity, the composite LaNi_3.7Al_1.3/Ni–S–Co film (η₁₅₀ = 65 mV, 353 K) could absorb large amount of H atoms, which would be oxidized and therefore effectively avoid the dissolution of the Ni–S–Co film under the state of open-circuit and consequently prolong the lifetime of the cathode.     

Enthalpy change (ΔH) and entropy change (ΔS) measurement of CeMn1−xAl1−xNi2x (x=0.00, 0.25, 0.50 and 0.75) hydrides by electrochemical P–C–T curve

The thermodynamic properties of CeMn_1−xAl_1−xNi_2x (x=0.00, 0.25, 0.50 and 0.75) hydrides have been investigated in this paper. With increasing Ni substitution content, the hydrogen concentration (H/M) in CeMn_1−xAl_1−xNi_2x (x=0.00, 0.25, 0.50 and 0.75) hydride increases from 0.129 wt% for x=0.00 to 0.421 wt% for x=0.75 at 293 K. The pressure–concentration isotherm (P–C–T) curves show that no hydrogen equilibrium pressure plateau has been observed for CeMnAl hydride while the slope of the plateau become flatter and longer with increasing Ni content. Meanwhile, the enthalpy change (ΔH⁰) and the entropy change (ΔS⁰) of the hydrides for dehydrogenization shift from −67.44 kJ mol⁻¹ (x=0.00) to 21.16 kJ mol⁻¹ (x=0.75) and from −0.24 kJ mol⁻¹ K⁻¹ (x=0) to −0.03 kJ mol⁻¹ K⁻¹ (x=0.75), respectively. With increasing Ni content, both ΔH⁰ and ΔS⁰ for dehydrogenization shift to the positive direction and make alloy hydrides more stable and hydrogen desorption much easier.     

Efficient photocatalytic hydrogen generation by Pt modified TiO2 nanotubes fabricated by rapid breakdown anodization

Photo-assisted hydrogen generation studies of platinum loaded titanium (IV) oxide nanotubes suspended in ethanol–water mixture were carried out at room temperature. The TiO₂ nanotubes synthesized by rapid breakdown anodization technique were loaded with Pt nanoparticles by chemical reduction of aqueous chloroplatinic acid solution using sodium borohydride. The chemisorption (active) surface area of the synthesized nanocomposites for hydrogen was measured by pulse chemisorption method using temperature programmed desorption reduction oxidation equipment and found to decrease with increase in platinum loading in the range 1–10 wt%. The platinum supported nanotube composites were characterized for phase and morphology by XRD, TEM and SEM. The hydrogen generated by the photocatalytic reduction of water from water–ethanol mixture at different wavelengths of incident light, using the Pt-TiO₂ nanocomposite photocatalyst, was determined by using a proton exchange membrane based hydrogen meter. The highest hydrogen generation efficiency was observed at 1–2.5 wt% of Pt loading. The maximum photocatalytic hydrogen generation of 0.03 mol/h/g of Pt-TiO₂ was observed with a 64 W UV light source (λ = 254 nm). The photoluminescence property of the Pt loaded TiO₂ has been correlated with the hydrogen generation efficiency and the reaction mechanism briefly discussed.     

Electrochemical properties of magnesium-based hydrogen storage alloys improved by transition metal boride and silicide additives

Transition metal borides and silicides prepared by mechanical alloying (MA) and chemical reduction methods (CR) were introduced to improve the corrosion resistance of magnesium-based hydrogen storage alloys. The additive of FeB prepared by MA can remarkably enhance the discharge capacity and cycling stability which has initial discharge capacity of 355.9 mA h g⁻¹ and keeps 224 mA h g⁻¹ after 100 cycles, and the exchange density I₀ of MgNi–NiB(CR) electrodes is 344.80 mA g⁻¹ but MgNi is only 67.6 mA g⁻¹ which leads to the better rate capability of the composite alloys. The results of SEM characterization, cyclic charge–discharge tests, potentiodynamic polarization, linear polarization and AC impedance experiment show that the corrosion inhibition property of MgNi in alkaline is improved by transition metal boride and silicide additives.     

Low-temperature protonic ceramic membrane fuel cells (PCMFCs) with SrCo0.9Sb0.1O3−δ cubic perovskite cathode

The SrCo_0.9Sb_0.1O_3−δ (SCS) composite oxide with cubic perovskite structure was synthesized by a modified Pechini method and examined as a novel cathode for protonic ceramic membrane fuel cells (PCMFCs). At 700 °C and under open-circuit condition, symmetrical SCS cathode on BaZr_0.1Ce_0.7Y_0.2O_3−δ (BZCY7) electrolyte showed low polarization resistances (R_p) of 0.22 Ωcm² in air. A laboratory-sized tri-layer cell of NiO–BZCY7/BZCY7/SCS was operated from 500 to 700 °C with humidified hydrogen (∼3% H₂O) as fuel and the static air as oxidant. A high open-circuit potential of 1.004 V, a maximum power density of 259 mW cm⁻², and a low polarization resistance of the electrodes of 0.14 Ωcm² was achieved at 700 °C.     

Electrochemical behavior of Li3−xM′xV2−yM″y(PO4)3 (M′ = K, M″ = Sc, Mg + Ti)/C composite cathode material for lithium-ion batteries

Composites of monoclinic Li_3−xM′_xV_2−yM″_2y(PO₄)₃ (M′ = K, M″ = Sc, Mg + Ti) with carbon were synthesized by solid-state reaction using oxalic acid or 6% H₂/Ar gas mixture as reducing agents at sintering temperature of 850 °C. The samples were characterized by X-ray diffraction (XRD), voltammetry and electrochemical galvanostatic cycling. The capacity of Li₃V₂(PO₄)₃ synthesized using hydrogen as the reducing agent was 127 mA h g⁻¹ and decreased to 120 mA h g⁻¹ after 20 charge-discharge cycles. The substitution of lithium and vanadium for other ions did not result in the improvement of the electrochemical characteristics of the samples.     

Oxidation of methanol on perovskite-type La2-xSrxNiO4 (0 ≤ x ≤ 1) film electrodes modified by dispersed nickel in 1 M KOH

Finely-dispersed nickel particles are electrodeposited on high surface-area perovskite-type La_2-xSr_xNiO₄ (0 ≤ x ≤ 1) electrodes for possible use in a direct methanol fuel cell (DMFC). The study is conducted by cyclic voltammetry, chronoamperometry, impedance spectroscopy and anodic Tafel polarization techniques. The results show that the apparent electrocatalytic activities of the modified oxide electrodes are much higher than those of unmodified electrodes under similar experimental conditions; the observed activity is the greatest with the modified La_1.5Sr_0.5NiO₄ electrode. At 0.550 V (vs. Hg|HgO) in 1 M KOH + 1 M CH₃OH at 25 °C, the latter electrode delivers a current density of over 200 mA cm⁻², whereas other electrodes of the series produce relatively low values (65–117 mA cm⁻²). To our knowledge, such high methanol oxidation current densities have not been reported on any other non-platinum electrode in alkaline solution. Further, the modified electrodes are not poisoned by methanol oxidation intermediates/products.     

Study of molybdenum electrodes for hydrogen evolution reaction

The molybdenum electrode, Mo, has been investigated for hydrogen production via water electrolysis in 10 vol.% aqueous solutions of 1-butyl-3-methylimidazolium tetrafluoroborate (BMI·BF₄) using electrochemical impedance spectroscopy (EIS). The EIS measurements show that the Mo system has much higher interfacial capacitance, and correspondently the electrical double layer formed on this electrode is thicker than those formed on nickel or platinum. The positive displacement of potential of zero charge (PZC) values indicates the specific adsorption of the imidazolium cation on the Mo surface. This study provides an elegant explanation for the better performance of Mo electrodes in the hydrogen evolution reaction (HER): the BMI cation acts as an intermediate for the proton transfer from water to the electrode surface, thereby decreasing the overpotential of HER. This model explains the synergism between Mo and the BMI cation in the HER process.     

La1−xSrxCoO3 (x = 0.1-0.5) as the cathode catalyst for a direct borohydride fuel cell

Direct borohydride fuel cells (DBFCs), with a series of perovskite-type oxides La_1−xSr_xCoO₃ (x = 0.1-0.5) as the cathode catalysts and a hydrogen storage alloy as the anode catalyst, are studied in this paper. The structures of the perovskite-type catalysts are mainly La_1−xSr_xCoO₃ (_x = 0.1-0.5) oxides phases. However, with the increase of strontium content, the intensities of the X-ray diffraction peaks of the impure phases La₂Sr₂O₅ and SrLaCoO₄ are gradually enhanced. Without using any precious metals or expensive ion exchange membranes, a maximum current density of 275 mA cm⁻² and a power density of 109 mW cm⁻² are obtained with the Sr content of x = 0.2 at 60 °C for this novel type of fuel cell.     

Preparation and electrochemical properties of composite LaNix/Ni–S–Co alloy film

The composite LaNi_x/Ni–S–Co film with considerable stability and high HER activity (η₁₅₀ = 70 mV, 353 K) was obtained by molten salt electrolysis combined with aquatic electrodeposition. LaNi_x film was prepared by galvanostatic electrolysis at 100 mA cm⁻² under 1273 K. The results showed that the La³⁺ ions could be reduced on the nickel cathode and the LaNi_x film could form, i.e. La³⁺ + 3e + xNi = LaNi_x (x = 5 or 3) at ca. −0.6 V, which is much lower than that of the decomposition potential of lanthanum, due to the strong depolarization effect of nickel. Furthermore, compared with the traditional amorphous Ni–S film, the composite LaNi_x/Ni–S–Co film could absorb large amount of H atoms, which would be oxidized and avoid the dissolution of the Ni–S–Co film under the state of open-circuit effectively and increase the HER activity.     

Electrochemical corrosion of platinum electrode in concentrated sulfuric acid

We report on the electrochemical corrosion of a Pt electrode in strong sulfuric acid. The electrochemical measurements were conducted using a Pt-flag working electrode, Ag/Ag₂SO₄ reference electrode and Pt counter electrode at 25 °C. The measured cyclic voltammograms significantly changed in the H₂SO₄ concentration range of 0.5–18 mol dm⁻³, especially from 14 to 18 mol dm⁻³. After successive potential sweeps for 15 h in 16 mol dm⁻³ H₂SO₄, a weight loss of the Pt-flag electrode was realized. In contrast, a controlled potential electrolysis by cathodic polarization caused a weight gain, which was attributed to sulfur deposition by the H₂SO₄ reduction. The subsequent anodic polarization produced corrosion of the deposited sulfur. Consequently, the alternating polarization generated platinum corrosion, resulted in the production of platinum and sulfur composite particulates in the solution.     

La0.75Sr0.25Cr0.5Mn0.5O3 as hydrogen electrode for solid oxide electrolysis cells

La_0.75Sr_0.25Cr_0.5Mn_0.5O₃ (LSCM) has been applied as hydrogen electrode (cathode) material in solid oxide electrolysis cells operating with different steam concentrations (20, 40, 60 and 80 vol.% absolute humidity (AH)) using 40 sccm H₂ carrier gas at 800, 850 and 900 °C, respectively. Impedance spectra and voltage-current curves were measured as a function of cell electrolysis current density and steam concentration to characterize the cell performance. The cell resistance decreased with the increase in electrolysis current density while increased with the increase in steam concentration under the same electrolysis current density. At 1.6 V applied electrolysis voltage, the maximum consumed current density increased from 431 mA cm⁻² for 20 vol.% AH to 593 mA cm⁻² for 80 vol.% AH at 850 °C. Polarization and impedance spectra experiments revealed that LSCM-YSZ hydrogen electrode played a major role in the electrolysis reaction.     

Optimisation of the Solid Oxide Fuel Cell (SOFC) cathode material Ca3Co4O9−δ

This paper focuses on the electrochemical potentialities of the 2D misfit compound Ca₃Co₄O_9−δ, so far mainly investigated for its thermoelectric properties. Its expansion coefficient (TEC = (9-10) × 10⁻⁶ °C⁻¹) and its chemical stability are compatible with standard CGO IT-electrolyte and the first optimisation steps of the deposited cathode have been performed with the aim to minimise the ASR and increase the cell durability. Particular attention has been paid on the effect of thickness and microstructure for pure and composite cathodes. The electrode reaction was performed on symmetrical cells. The preliminary results presented here show that the composite (70 wt.% Ca₃Co₄O_9−δ-30 wt.% CGO) gives the lowest ASR values compare to single-phased electrodes. Strikingly, the ASR values increase for thinner deposited layers. The effect of various current collectors (gold grid vs. platinum paste) has been also checked.