Hydrogen production from crude glycerol in an alkaline microbial electrolysis cell

Crude glycerol is an undesired by-product of biodiesel production with a low commercial value (i.e. a ton of biodiesel results in around 110 kg of crude glycerol) and, thus, glycerol needs valorization. In particular, there is a need of providing a benefit to alkaline wastewaters from biodiesel production with excess of glycerol. Bioelectrochemical systems (BES) are an emerging technique to recover the energy contained in a substrate either as electricity or as other added-value products such as hydrogen. Moreover, promising results have been reported with alkaline BES showing higher current intensities than neutral pH conditions. This study is the first experimental evaluation of alkaline bioelectrochemical production of hydrogen from real crude glycerol as sole carbon source. The results show that alkaline glycerol degradation is feasible under both microbial fuel cell mode (2 mA, 71.4 A/m³ and 55% of CE) and microbial electrolysis mode (maximum of 0.46 L_H2/L/d and 85% of r_CAT). The values obtained are promising since they are in the range of those obtained with other simpler carbon sources such acetate. A complex consortium involving fermentative bacteria (such as Enterococcaceae), alkaline exoelectrogens (such as Geoalkalibacter) and homoacetogens (such as Acetobacterium) was naturally developed in the anode of the MEC.     

MnO2 modified multi-walled carbon nanotubes supported Pd nanoparticles for methanol electro-oxidation in alkaline media

A novel method to prepare MnO₂ modified multi-walled carbon nanotubes (MnO₂/MWCNTs) supported Pd (Pd-MnO₂/MWCNTs) electrocatalyst is reported. The morphology, component and crystallinity of the catalyst were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy-dispersive X-ray spectroscopy (EDS), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS), respectively. The activity of Pd-MnO₂/MWCNTs was tested using methanol electro-oxidation in alkaline media. The results showed that the Pd-MnO₂/MWCNTs exhibited higher electrocatalytic activity and stability than Pd/MWCNTs and Pd/Vulcan (Pd/commercial Vulcan XC-72 carbon black).     

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.     

Continuous sorption-enhanced steam reforming of glycerol to high-purity hydrogen production

In this study, the continuous sorption-enhanced steam reforming of glycerol to high-purity hydrogen production by a simultaneous flow concept of catalyst and sorbent for reaction and regeneration using two moving-bed reactors has been evaluated experimentally. A Ni-based catalyst (NiO/NiAl₂O₄) and a lime sorbent (CaO) were used for glycerol steam reforming with and without in-situ CO₂ removal at 500 °C and 600 °C. The simultaneous regeneration of catalyst and sorbent was carried out with the mixture gas of N₂ and steam at 900 °C. The product gases were measured by a GC gas analyzer. It is obvious that the amounts of CO₂, CO and CH₄ were reduced in the sorption-enhanced steam reforming of glycerol, and the H₂ concentration is greatly increased in the pre-CO₂ breakthrough periods within 10 min both 500 °C and 600 °C. The extended time of operation for high-purity hydrogen production and CO₂ capture was obtained by the continuous sorption-enhanced steam reforming of glycerol. High-purity H₂ products of 93.9% and 96.1% were produced at 500 °C and 600 °C and very small amounts of CO₂, CH₄ and CO were formed. The decay in activity during the continuous reaction-regeneration of catalyst and sorbent was not observed.     

Hydrogen production in steam reforming of glycerol by conventional and membrane reactors

The aim of this study is to produce hydrogen through the glycerol steam reforming process. The reaction is carried out in a traditional reactor and an electrolessly plated Pd/Ag alloy membrane reactor, with varying reaction temperature, weight hourly specific velocity (WHSV) and water glycerol molar ratio (WGMR). The non-catalytic test was also employed for comparative purposes. The results show that the reaction is highly depending on temperature, and the maximum glycerol conversion achieved to 96.24% at 800 °C with a hydrogen yield of 5.82 mol-H₂/mol-C₃H₈O₃. It also found that the Pd/Ag membrane can effectively separate hydrogen from the reaction side and subsequently enhance the reaction rate in the membrane reactor. TGA measurements were employed to quantify the amounts of deposited carbon and the results also confirmed that the CeO₂ modified catalyst can improve the carbon resistance as well as activity and stability.     

Glycerol oxidation in a microfluidic fuel cell using Pd/C and Pd/MWCNT anodes electrodes

Pd/C and Pd/MWCNT based electro-catalysts were prepared by impregnation and used as anodes for glycerol electro-oxidation in a microfluidic fuel cell. Average particle size and lattice parameters of the catalysts were determined by X-ray diffraction, resulting in 7.5 and 3.5 nm for Pd/C and Pd/MWCNT respectively. The electro-catalytic activity of Pd/C and Pd/MWCNT was investigated in 0.1 M glycerol. The results obtained by electrochemical studies in half cell configuration showed that the onset potential for glycerol oxidation on Pd/MWCNT was characterized by a negative shift ca. 40 mV compared to Pd/C. The maximum power density obtained was 0.51 and 0.7 mW cm⁻² for Pd/C and Pd/MWCNT respectively. These results are comparable with those obtained for a microfluidic fuel cell that uses glucose as fuel. The results of this work not only show that glycerol can be used as fuel in a microfluidic fuel cell but also its performance is similar to that obtained with others fuels.     

Electrocatalysis of carbon black- or chitosan-functionalized activated carbon nanotubes-supported Pd with a small amount of La2O3 towards methanol oxidation in alkaline media

The Pd/C catalysts with and without a small amount of La₂O₃ were synthesized by a simple reduction reaction with sodium borohydride in aqueous solution. The structure and morphology of these catalysts were characterized by X-ray diffraction, energy dispersive X-ray spectroscopy and transmission electron microscopy. The electrocatalytic performance of these catalysts for methanol oxidation in alkaline media was investigated using cyclic voltammetry, chronoamperometry and CO stripping experiments. The results show that the Pd–La₂O₃/C catalyst has a higher catalytic activity than the Pd/C catalyst, but the effect of La₂O₃ cannot be explained by a bi-functional mechanism. X-Ray photoelectron spectroscopy analyses suggest that the higher content of metallic Pd caused by the addition of La₂O₃ contributes to the better catalytic activity of Pd–La₂O₃/C. Based on the good electrocatalytic performance of Pd–La₂O₃/C, the Pd–La₂O₃ catalyst supported on chitosan (CS)-functionalized activated carbon nanotubes was prepared, and it exhibited a better catalytic activity. The improvement is attributed to the good dispersion status of metal particles and the further increase of metallic Pd due to the presence of CS.     

Effect of N2O-mediated calcination on nickel species and the catalytic activity of nickel catalysts supported on γ-Al2O3 in the steam reforming of glycerol

The steam reforming of glycerol over supported nickel catalysts is a promising and cost-effective method for producing hydrogen. The activity of nickel catalysts supported on γ-Al₂O₃ is low, primarily due to the formation of inactive nickel species during high temperature calcination in air. In order to address this problem, a Ni/γ-Al₂O₃ catalyst was prepared by calcination at 700 °C in a nitrous oxide (N₂O) environment. The N₂O calcined catalyst showed an enhanced activity for the steam reforming of glycerol. A variety of characterization techniques (XRD, TPR, XPS and H₂ Chemisorption) confirmed that the high temperature N₂O calcination resulted in a significant decrease in the levels of nickel aluminate. The N₂O calcination also led to an enhancement in the amount of NiO as well as nickel ions present on the surface of the catalyst. Interestingly, compared to an air calcined catalyst, the N₂O calcined catalyst contained larger nickel particles after reduction but the N₂O calcined catalyst had a much larger nickel surface area and dispersion, which resulted in higher glycerol conversion and hydrogen yield.     

Photoelectrochemical performance of graphene-modified TiO2 photoanodes in the presence of glycerol as a hole scavenger

Graphene-modified TiO₂ (G-TiO₂) photoanode films were successfully prepared by a simple, versatile, and low-cost spray pyrolysis deposition method. The effects of graphene incorporation on the relevant properties of TiO₂ films were investigated by means of XRD, SEM, UV–vis absorbance spectroscopy, and photoelectrochemistry-related measurements. Bias-dependant efficiency calculated from linear-sweep voltammograms shows that the presence of graphene within the film networks, despite its low content, could promote a substantial improvement in maximum photoconversion efficiency from 0.39% (at −0.27 V vs HgO|Hg) to 0.65% (at −0.35 V vs HgO|Hg). This improvement is attributable to the enhancement of the electron-transferring ability upon the insertion of graphene, as confirmed by transient photocurrent analysis and Electrochemical Impedance Spectroscopy (EIS). The effectiveness of the photoelectrochemical cell employing G-TiO₂ as an excellent photoanode was further examined by running it in the presence of glycerol as a hole scavenger. Glycerol plays an important role as an effective sink for the photogenerated holes so that the surface charge recombination can be significantly suppressed and, subsequently, the photocurrent is enhanced. The additional photocurrent due to glycerol introduction into the cell depends upon the initial concentration of glycerol according to a model resembling a Langmuir–Hinshelwood isotherm. Based upon the results of the present study, further improvements in terms of graphene content, surface morphology modification or the use of other organic wastes as hole scavengers may be important for future investigation.     

Glycerol electrooxidation on highly active Pd supported carbide/C aerogel composites catalysts

The nanosized carbide supported on carbon aerogel composites have been synthesized by polycondensation of resorcinol and formaldehyde (RF) method in the presence of sodium tungstate and sodium molybdate. The materials are characterized by X-ray diffraction, transmission electron microscopy, energy dispersive X-ray spectroscopy (EDS), and cyclic voltammetry. The Pd nanoparticles supported on binary-carbide and carbon aerogel composites (Pd@WC-Mo₂C/C) for glycerol oxidation are investigated for the first time. The Pd@WC-Mo₂C/C as electrocatalyst shows a superior activity toward the glycerol oxidation in terms of the peak current density, which is almost two times higher than that of Pd/C and show better poison-resistant ability. The binary transition-metal carbide will be the potential catalyst support for the direct alcohol fuel cells.     

Photocatalytic hydrogen evolution from aqueous solutions of glycerol under visible light irradiation

Multiphase photocatalysts Pt/Cd_1−xZn_xS/ZnO/Zn(OH)₂ and single-phase photocatalysts Pt/Cd_1−xZn_xS were prepared by a two-step technique. The photocatalysts were characterized by a wide range of experimental techniques: X-ray diffraction (XRD), high-resolution transmission electron microscopy (HRTEM) combined with energy-dispersive X-ray (EDX) spectroscopy, low-temperature N₂ adsorption/desorption, and UV/VIS spectroscopy. The photocatalytic activity was tested in a batch reactor in the reaction of hydrogen evolution from aqueous solutions of glycerol under visible light irradiation (λ > 420 nm). The highest achieved photocatalytic activity was 449 μmol H₂ per gram of photocatalyst per hour; the highest quantum efficiency was 9.6% (λ > 420 nm). The activity of the multiphase catalysts was shown to exceed that of the single-phase catalysts by a factor of 2.1, likely because of the heterojunctions between sulfides, oxides and hydroxides.     

Modification of palladium surfaces by bismuth adatoms or clusters: Effect on electrochemical activity and selectivity towards polyol electrooxidation

The syntheses of Bi-modified Pd catalysts with a controlled size distribution are presented as well as the characterizations of their structures and of their surfaces. Effects of the modification either of non-supported Pd nanospheres by spontaneous deposition of Bi or of carbon-supported Pd-based nanomaterials by decoration with bismuth clusters on the electrocatalytic activity towards glycerol electrooxidation were evaluated and compared in alkaline medium. The method of bismuth deposition has a dramatic effect on the activity of the palladium based catalysts: spontaneous deposition of Bi on non-supported Pd nanoparticles leads to relatively low activity enhancement, whereas decoration of carbon-supported Pd nanoparticles by Bi₂O₃ and Bi(OH)₃ clusters leads to very high activity increase at low overpotentials. In situ infrared spectroscopy indicated that the modification of Pd by Bi did not affect the selectivity of glycerol oxidation, whereas in the case of Pt containing catalyst, a dramatic change in selectivity occurred at low potentials.     

Escherichia coli hydrogenase 4 (hyf) and hydrogenase 2 (hyb) contribution in H2 production during mixed carbon (glucose and glycerol) fermentation at pH 7.5 and pH 5.5

Molecular hydrogen (H₂) production by Escherichia coli was studied during mixed carbon sources (glucose and glycerol) fermentation at pH 7.5 and pH 5.5. H₂ production rate (V_H2) by bacterial cells grown on mixed carbon was assayed with either adding glucose (glucose assay) or glycerol (glycerol assay) and compared with the cells grown on sole carbon (glucose or glycerol only) and appropriately assayed. Wild type cells grown on mixed carbon, in the assays with adding glucose, produced H₂ at pH 7.5 with the same level as in the cells grown on glucose only. At pH 7.5 V_H2 in fhlA single and fhlA hyfG double mutants decreased ∼6.5 and ∼7.9 fold, respectively. In wild type cells grown on mixed carbon V_H2 at pH 5.5 was lowered ∼2 fold, compared to the cells grown on glucose only. But in hyfG and hybC single mutants V_H2 was decreased ∼2 and ∼1.6 fold, respectively. However, at pH 7.5, in the assays with glycerol, V_H2 was low, when compared to the cells grown on glycerol only. At pH 5.5 in the assays with glycerol V_H2 was absent. Moreover, V_H2 in wild type cells was inhibited by 0.3 mM N,N^′-dicyclohexylcarbodiimide (DCCD), an inhibitor of the F₀F₁-ATPase, in a pH dependent manner. At pH 7.5 in wild type cells V_H2 was decreased ∼3 fold but at pH 5.5 the inhibition was ∼1.7 fold. At both pHs in fhlA mutant V_H2 was totally inhibited by DCCD. Taken together, the results obtained indicate that at pH 7.5, in the presence of glucose, glycerol can also be fermented. They point out that Hyd-4 mainly and Hyd-2 to some extent contribute in H₂ production by E. coli during mixed carbon fermentation at pH 5.5 whereas Hyd-1 is only responsible for H₂ oxidation.     

Carbon-supported PdSn–SnO2 catalyst for ethanol electro-oxidation in alkaline media

Carbon-supported PdSn–SnO₂ with high electrical catalytic activity for ethanol oxidation in alkaline solution was synthesized using an impregnation reduction method. XRD analysis of the as-prepared PdSn–SnO₂/C revealed that the Pd diffraction peaks shifted to lower 2θ values with respect to the corresponding peaks of the Pd/C catalyst, indicating that Sn doping could shrink the Pd crystalline lattice. XPS measurements confirmed the existence of Sn and SnO₂ in the PdSn–SnO₂/C catalysts. The prepared PdSn–SnO₂/C catalysts presented a remarkably higher electrocatalytic activity than that of the Pd–Sn/C and Pd/C catalysts. This was mainly because the easy adsorption-dissociation of OH_ads over the SnO₂ surface changed the electronic effect and accelerated the adsorption of ethanol on the surface of Pd, thus enhancing the overall ethanol oxidation kinetics and contributing to a significant improvement in the catalytic activity.     

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.     

Catalytic performance of Pd–Ni bimetallic catalyst for glycerol hydrogenolysis

Catalytic conversion of glycerol from biodiesel production to value-added chemicals and fuels is actually of great interest for industrial chemical research. Bimetallic catalysts are confirmed superior to monometallic catalysts in terms of catalytic activity and selectivity for glycerol hydrogenolysis. Accordingly, a series of Pd–M (M = Fe, Co, Ni, Cu, Zn) bimetallic catalysts were prepared in this work via coprecipitation to investigate the promoting effect of Pd. The relationship between the catalytic performance and metal-support interaction was also discussed. Through the catalyst screening, Pd–Ni bimetallic catalyst exhibited moderate activity and the highest selectivity towards ethylene glycol. At 493 K and hydrogen pressure of 6.0 MPa, the glycerol conversion and selectivity of ethylene glycol reached 89% and 22% respectively. XRD and TEM patterns showed that the Pd nanoparticles with an average size of ∼4 nm were uniformly dispersed in the supports. H₂-TPR revealed that the reduction temperatures of metal oxides were significantly decreased by the introduction of Pd component. XPS curves indicated that unique performance of the Pd–Ni bimetallic catalyst might be attributed to the formation of Pd–Ni alloy. And the required metal-support interaction was assumed responsible for the cleavage of C–C bond and generation of ethylene glycol. In the end, hydrogenolysis reactions for the main products of glycerol conversion were carried out over Pd–Ni catalyst to explore the possible reaction pathways for glycerol hydrogenolysis.     

Mechanism of glycerol oxidation reaction on silver-modified palladium electrode in alkaline medium

The silver-modified Pd (Ag/Pd) electrode was prepared by underpotential deposition of Cu adatoms, followed by galvanic displacement with Ag atoms to investigate the effect of silver modification on the catalysis and mechanism for glycerol oxidation reaction (GOR) of Pd. Cyclic voltammograms in alkaline glycerol solution exhibited that the Ag/Pd electrode had ca. 100 mV less positive onset potential of the GOR current density and higher peak current density than the Pd electrode, indicating that the GOR activity of Pd was enhanced by surface modification with Ag atoms. In potentiostatic electrolysis at ?0.1 V vs. Hg/HgO, GOR mainly began with the oxidation of a terminal OH group for both electrodes, and glycerate and glycolate were major products. The glycolate production was more active for the Ag/Pd electrode. The in situ infrared reflectance-absorption spectra (IRAS) exhibited that GOR products with one or two carbons were detected at lower potentials for the Ag/Pd electrode, indicating that the CC bond cleavage was facilitated by the Ag modification.     

Supported gold nanoparticles as anode catalyst for anion-exchange membrane-direct glycerol fuel cell (AEM-DGFC)

The carbon supported Au nanoparticles (Au-NPs) catalyst with a small average size (3.5 nm) and narrow size distribution (2–6 nm) was synthesized by a solution phase-based nanocapsule method. The reactivity of glycerol oxidation on Au/C is much higher than that of methanol and ethylene glycol oxidations in alkaline electrolyte. The anion-exchange membrane-direct glycerol fuel cell (AEM-DGFC) with the Au/C anode catalyst and a Fe-based cathode catalyst shows high performances with both high-purity glycerol and crude glycerol fuel: the open circuit voltages (OCVs) are 0.67 and 0.66 V, and peak power densities are 57.9 and 30.7 mW cm⁻² at 80 °C, respectively. Fed with crude glycerol, the Au/C anode catalyst-based AEM-DGFC also demonstrates high performance stability at 80 °C. The product analysis shows that the electrooxidation of glycerol on the Au/C anode catalyst in AEM-DGFCs favors production of deeper-oxidized chemicals: tartronic acid, mesoxalic acid and oxalic acid, which leads to higher fuel cell's Faradic efficiency.     

One-step CO assisted synthesis of hierarchical porous PdRuCu nanosheets as advanced bifunctional catalysts for hydrogen evolution and glycerol oxidation

Pd-based catalysts have received wide attention due to their outstanding anti-CO poisoning property, whereas the structural instability limits their application. The hierarchical porous PdRuCu nanosheets (HP PdRuCu NSs) with large electrochemically active surface area, abundant active sites, and stable structures are synthesized through continuous access to CO bubbles. HP PdRuCu NSs exhibit excellent hydrogen evolution reaction (HER) catalytic activity with an ultralow overpotential of 25 mV at 10 mA cm^?2 and a Tafel slope of 87.5 mV dec^?1 in alkaline·media. Meanwhile, the peak mass activity and specific activity of HP PdRuCu NSs for glycerol oxidation reaction (GOR) are 1083 mA mg^?1_Pd and 38.8 A m^?2, respectively, superior to that of PdRu nanosheets (PdRu NSs), Pd nanosheets (Pd NSs), and commercial Pd black. The introduction of Ru and Cu atoms facilitates the C–C bond cleavage and the complete oxidation of glycerol to CO₂, as well as the accelerated oxidation/removal of the poisonous CO_ads in between.     

Novel Pd/β-MnO2 nanotubes composites as catalysts for methanol oxidation in alkaline solution

In this work, we demonstrated a completely new, simple and effective strategy for preparing catalysts by using β-MnO₂ nanotubes as the supporting materials, and the Pd nanoparticles were coated onto β-MnO₂ nanotubes through a simple reductive process firstly. The as-prepared materials were characterized by field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), X-ray diffraction (XRD) and electrochemical measurements. The results indicated that the Pd nanoparticles were homogeneously dispersed and well separated from one another on the β-MnO₂ nanotubes surfaces, which makes it have a potential application in catalysts. In this study, we mainly tested the electrochemical performance of Pd/β-MnO₂ for methanol oxidation in alkaline solution. Further research to optimize the synthesis condition, particularly to develop β-MnO₂ nanotubes as supporting materials of other noble metal catalysts is currently in progress.