Hydrogen production from cellulose in a two-stage process combining fermentation and electrohydrogenesis

A two-stage dark-fermentation and electrohydrogenesis process was used to convert the recalcitrant lignocellulosic materials into hydrogen gas at high yields and rates. Fermentation using Clostridium thermocellum produced 1.67 mol H₂/mol-glucose at a rate of 0.25 L H₂/L-d with a corn stover lignocellulose feed, and 1.64 mol H₂/mol-glucose and 1.65 L H₂/L-d with a cellobiose feed. The lignocelluose and cellobiose fermentation effluent consisted primarily of: acetic, lactic, succinic, and formic acids and ethanol. An additional 800 ± 290 mL H₂/g-COD was produced from a synthetic effluent with a wastewater inoculum (fermentation effluent inoculum; FEI) by electrohydrogensis using microbial electrolysis cells (MECs). Hydrogen yields were increased to 980 ± 110 mL H₂/g-COD with the synthetic effluent by combining in the inoculum samples from multiple microbial fuel cells (MFCs) each pre-acclimated to a single substrate (single substrate inocula; SSI). Hydrogen yields and production rates with SSI and the actual fermentation effluents were 980 ± 110 mL/g-COD and 1.11 ± 0.13 L/L-d (synthetic); 900 ± 140 mL/g-COD and 0.96 ± 0.16 L/L-d (cellobiose); and 750 ± 180 mL/g-COD and 1.00 ± 0.19 L/L-d (lignocellulose). A maximum hydrogen production rate of 1.11 ± 0.13 L H₂/L reactor/d was produced with synthetic effluent. Energy efficiencies based on electricity needed for the MEC using SSI were 270 ± 20% for the synthetic effluent, 230 ± 50% for lignocellulose effluent and 220 ± 30% for the cellobiose effluent. COD removals were ∼90% for the synthetic effluents, and 70–85% based on VFA removal (65% COD removal) with the cellobiose and lignocellulose effluent. The overall hydrogen yield was 9.95 mol-H₂/mol-glucose for the cellobiose. These results show that pre-acclimation of MFCs to single substrates improves performance with a complex mixture of substrates, and that high hydrogen yields and gas production rates can be achieved using a two-stage fermentation and MEC process.     

High-yield biohydrogen production from biodiesel manufacturing waste by Thermotoga neapolitana

Efficient conversion of glycerol waste from biodiesel manufacturing processes into biohydrogen by the hyperthermophilic eubacterium Thermotoga neapolitana DSM 4359 was investigated. Biohydrogen production by T. neapolitana was examined using the batch cultivation mode in culture medium containing pure glycerol or glycerol waste as the sole substrate. Pre-treated glycerol waste showed higher hydrogen (H₂) production than untreated waste. Nitrogen (N₂) sparging and pH control were successfully implemented to maintain the culture pH and to reduce H₂ partial pressure in the headspace for optimal growth rate and to enhance hydrogen production from the glycerol waste. It was found that hydrogen production increased from 1.24 ± 0.06 to 1.98 ± 0.1 mol-H₂ mol⁻¹ glycerol_consumed by optimising N₂ sparging and pH control. We observed that in medium containing 0.05 M HEPES, with three cycles of N₂ sparging, the H₂ yield increased to 2.73 ± 0.14 mol-H₂ mol⁻¹ glycerol_consumed, which was 2.22-fold higher than the non-N₂ sparged H₂ yield (1.23 ± 0.06 mol-H₂ mol⁻¹ glycerol_consumed).     

A pilot-scale high-rate biohydrogen production system with mixed microflora

A pilot-scale high-rate dark fermentative hydrogen production plant has been established in the campus of Feng Chia University to develop biohydrogen production pilot-plant technology. This pilot-plant system is composed of two feedstock storage tanks (0.75 m³ each), a nutrient storage tank (0.75 m³), a mixing tank (0.6 m³), an agitated granular sludge bed fermentor (working volume 0.4 m³), a gas-liquid-solid separator (0.4 m³) and a control panel. The seed mixed microflora was obtained from a lab-scale agitated granular sludge bed bioreactor. This pilot-scale fermentor was operated for 67 days at 35 °C, an organic loading rate (OLR) of 40-240 kg COD/m³/d, and the influent sucrose concentration of 20 and 40 kg COD/m³. Both biogas and hydrogen production rates increased with increasing OLR. However, the biomass concentration (volatile suspended solids, VSS) only increased with an increasing OLR at an OLR range of 40-120 kg COD/m³/d, whereas it decreased when OLR was too high (i.e., 240 kg COD/m³/d). The biogas consisted mainly of H₂ and CO₂ with a H₂ content range of 23.2-37.8%. At an OLR of 240 kg COD/m³/d, the hydrogen content in biogas reached its maximum value of 37% with a hydrogen production rate (HPR) of 15.59 m³/m³/d and a hydrogen yield of 1.04 mol H₂/mol sucrose. This HPR value is much higher than 5.26 m³/m³/d (fermented molasses substrate) and 1.56 m³/m³/d (glucose substrate) reported by other pilot-scale systems. Moreover, HPR was also greatly affected by pH. At an optimal pH of 5.5, the bacterial community became simple, while the efficient hydrogen producer Clostridium pasteurianum was dominant. The factors of energy output compared with the energy input (E_f) ranged from 13.65 to 28.68 on biohydrogen, which is higher than the E_f value on corn ethanol, biodiesel and sugarcane ethanol but in the similar range of cellulosic ethanol.     

Bioenergy production from glycerol in hydrogen producing bioreactors (HPBs) and microbial fuel cells (MFCs)

The supply of glycerol has increased substantially in recent years as a by-product of biodiesel production. To explore the value of glycerol for further application, the conversion of glycerol to bioenergy (hydrogen and electricity) was investigated using Hydrogen Producing Bioreactors (HPBs) and Microbial Fuel Cells (MFCs). Pure-glycerol and the glycerol from biodiesel waste stream were compared as the substrates for bioenergy production. In terms of hydrogen production, the yields of hydrogen and 1,3-propanediol at a pure-glycerol concentration of 3 g/L were 0.20 mol/mol glycerol and 0.46 mol/glycerol, respectively. With glucose as the co-metabolism substrate at the ratio of 3:1 (glycerol:glucose), the yields of hydrogen and 1,3-propanediol from glycerol significantly increased to 0.37 mol/mol glycerol and 0.65 mol/glycerol, respectively. The glycerol from biodiesel waste stream had good hydrogen yields (0.17-0.18 mol H₂/mole glycerol), which was comparable with the pure-glycerol. In terms of power generation in MFCs, pure-glycerol was examined at concentrations of 0.5-5 g/L with the highest power density of 4579 mW/m³ obtained at a concentration of 2 g/L. The power densities from the biodiesel waste glycerol were 1614-2324 mW/m³, which were likely caused by the adverse effects of impurities on electrode materials. An economic analysis indicates that with the annual waste stream of 70 million gallons of glycerol, the expected values generated from HPBs and MFCs were $311 and $98 million, respectively.     

Cloning and knockout of formate hydrogen lyase and H2-uptake hydrogenase genes in Enterobacter aerogenes for enhanced hydrogen production

A 5431-bp DNA fragment partially encoding the formate hydrogen lyase (FHL) gene cluster hycABCDE was isolated and identified from Enterobacter aerogenes IAM1183 chromosomal DNA. All the five putative gene products showed a high degree of homology to the reported bacterial FHL proteins. The gene hycA, encoding the FHL repressor protein, and hybO, encoding the small subunit of the uptake hydrogenase, were targeted for genetic knockout for improving the hydrogen production. The pYM-Red recombination system was adopted to form insertional mutations in the E. aerogenes genome, thereby creating mutant strains of IAM1183-A (△hycA), IAM1183-O (△hybO), and IAM1183-AO (△hycA/△hybO double knockout). The hydrogen production experiments with these mutants showed that the maximum specific hydrogen productivities of IAM1183-A, IAM1183-O, and IAM1183-AO were 2879.466 ± 38.59, 2747.203 ± 13.25 and 3372.019 ± 4.39 (ml h⁻¹ g⁻¹dry cell weight), respectively, higher than that of the wild strain (2321.861 ± 15.34 ml h⁻¹ g⁻¹dry cell weight). The total H₂ yields by the three mutants IAM1183-A, IAM1183-O and IAM1183-AO were 0.73, 0.78, and 0.83 mol-H₂/mol glucose, respectively, while the wild-type IAM1183 was only 0.65 mol-H₂/mol glucose. The metabolites of the mutants including acetate, ethanol, 2,3-butanediol and succinate were all increased compared with that of the wild type, implying the changed metabolic flux by the mutation. In the fermentor cultivation with IAM1183△hycA/△hybO, the total hydrogen volume after 16 h cultivation reached 4.4 L, while that for the wild type was only 2.9 L.     

Continuous biohydrogen production from liquid swine manure supplemented with glucose using an anaerobic sequencing batch reactor

Liquid swine manure supplemented with glucose (10 g/L) was used as substrate for hydrogen production using an anaerobic sequencing batch reactor at 37 ± 1 °C and pH 5.0 under different hydraulic retention times (HRTs). Decreasing HRT from 24 to 8 h caused an increasing hydrogen production rate from 0.05 to 0.15 L/h/L. Production rates of both total biogas and hydrogen were linearly correlated to HRT with R² being 0.993 and 0.997, respectively. The hydrogen yield ranged between 1.18 and 1.63 mol-H₂/mol glucose and the 12 h HRT was preferred for high production rate and efficient yield. For all the five HRTs examined, the glucose utilization efficiency was over 98%. The biogas mainly consisted of carbon dioxide and hydrogen (up to 43%) with no methane detected throughout the experiment. Ethanol and organic acids were the major aqueous metabolites produced during fermentation, with acetic acid accounting for 56–58%. The hydrogen yield was found to be related to the acetate/butyrate ratio.     

Hydrogen production from cassava wastewater using an anaerobic sequencing batch reactor: Effects of operational parameters,COD:N ratio,and organic acid composition

In this work, hydrogen production from cassava wastewater using anaerobic sequencing batch reactors (ASBR) was investigated to determine the optimum number of cycles per day, chemical oxygen demand (COD) loading rate, and COD:N ratio. The system operated at a COD loading rate of 30 kg/m³d and 6 cycles per day provided maximum hydrogen production performance in terms of specific hydrogen production rate (SHPR) (388 ml H₂/g VSS d or 3800 ml H₂/l d) and hydrogen yield (186 ml H₂/g COD removed). The effect of nitrogen supplementation was also studied by adding NH₄HCO₃ into the system at the COD:N ratios of 100:2.2, 100:3.3, and 100:4.4 under the COD loading rate of 30 kg/m³d and 6 cycles per day. The maximum SHPR and hydrogen yield of 524 ml H₂/g VSS d (5680 ml H₂/l d) and 438 ml H₂/g COD removed, respectively, were obtained at the stoichiometric COD:N ratio of 100:2.2. An excess nitrogen was found to promote the productions of higher organic acids and ethanol, resulting in lowering hydrogen production efficiency.     

Dark fermentative hydrogen production with crude glycerol from biodiesel industry using indigenous hydrogen-producing bacteria

Glycerol is an inevitable by-product from biodiesel synthesis process and could be a promising feedstock for fermentative hydrogen production. In this study, the feasibility of using crude glycerol from biodiesel industry for biohydrogen production was evaluated using seven isolated hydrogen-producing bacterial strains (Clostridium butyricum, Clostridium pasteurianum, and Klebsiella sp.). Among the strains examined, C. pasteurianum CH4 exhibited the best biohydrogen-producing performance under the optimal conditions of: temperature, 35 °C; initial pH, 7.0; agitation rate, 200 rpm; glycerol concentration, 10 g/l. When using pure glycerol as carbon source for continuous hydrogen fermentation, the average H₂ production rate and H₂ yield were 103.1 ± 8.1 ml/h/l and 0.50 ± 0.02 mol H₂/mol glycerol, respectively. In contrast, when using crude glycerol as the carbon source, the H₂ production rate and H₂ yield was improved to 166.0 ± 8.7 ml/h/l and 0.77 ± 0.05 mol H₂/mol glycerol, respectively. This work demonstrated the high potential of using biodiesel by-product, glycerol, for cost-effective biohydrogen production.     

Hydrogen production for PEM fuel cell by gas phase reforming of glycerol as byproduct of bio-diesel. The use of a Pd-Ag membrane reactor at middle reaction temperature

Glycerol as a byproduct of biodiesel production represents a renewable energy source. In particular, glycerol can be used in the field of hydrogen production via gas phase reforming for proton exchange membrane fuel cell (PEMFC) applications. In this work, glycerol steam reforming (GSR) reaction was investigated using a dense palladium-silver membrane reactor (MR) in order to produce pure (or at least CO-free) hydrogen, using 0.5 wt% Ru/Al₂O₃ as reforming catalyst. The experiments are performed at 400 °C, water to glycerol molar feed ratio 6:1, reaction pressure ranging from 1 to 5 bar and weight hourly space velocity (WHSV) from 0.1 to 1.0 h⁻¹. Moreover, a comparative study is given between the Pd-Ag MR and a traditional reactor (TR) working at the same MR operating conditions. The effect of the WHSV and reaction pressure on the performances of both the reactors in terms of glycerol conversion and hydrogen yield is also analyzed. The MR exhibits higher conversion than the TR (∼60% as best value for the MR against ∼40% for the TR, at WHSV = 0.1 h⁻¹ and 5 bar), and high CO-free hydrogen recovery (around 60% at WHSV = 0.1 h⁻¹ and 5 bar). During reaction, carbon coke is formed limiting the performances of the reactors and inhibiting, in particular, the hydrogen permeation through the membrane with a consequent reduction of hydrogen recovery in the permeate side.     

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.     

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.     

Bioconversion of crude glycerol from biodiesel production to hydrogen

This study evaluates the potential of bioconversion of crude glycerol, discharged from biodiesel production plant, to hydrogen (H₂) by an enriched microbial community. Microbial community was enriched from activated sludge in a medium amended with 2.5 g/L of crude glycerol. Optimal cultivation parameters for H₂ production such as initial pH, cultivation temperature and substrate concentration were investigated. H₂ yields from raw glycerol at optimal conditions (pH 6.5; 40 °C and 1 g/L raw glycerol) were 1.1 ± 0.1 mol-H₂/mol-glycerol_consumed. H₂ production was associated with acetate-butyrate type fermentation, along with ethanol as one of the end products. Kinetic experiments on H₂ production from pure and crude glycerol indicated the absence of any inhibitory effects from the impurities present in crude glycerol. The community analysis revealed that the enriched microbial consortium was dominated mainly by Clostridium species.     

Manipulating the hydrogen production from acetate in a microbial electrolysis cell-microbial fuel cell-coupled system

A biological hydrogen-producing system is configured through coupling an electricity-assisting microbial fuel cell (MFC) with a hydrogen-producing microbial electrolysis cell (MEC). The advantage of this biocatalyzed system is the in-situ utilization of the electric energy generated by an MFC for hydrogen production in an MEC without external power supply. In this study, it is demonstrated that the hydrogen production in such an MEC-MFC-coupled system can be manipulated through adjusting the power input on the MEC. The power input of the MEC is regulated by applying different loading resistors connected into the circuit in series. When the loading resistance changes from 10 Ω to 10 kΩ, the circuit current and volumetric hydrogen production rate varies in a range of 78 ± 12 to 9 ± 0 mA m⁻² and 2.9 ± 0.2 to 0.2 ± 0.0 mL L⁻¹ d⁻¹, respectively. The hydrogen recovery (RH₂), Coulombic efficiency (CE), and hydrogen yield (YH₂) decrease with the increase in loading resistance. Thereafter, in order to add power supply for hydrogen production in the MEC, additional one or two MFCs are introduced into this coupled system. When the MFCs are connected in series, the hydrogen production is significantly enhanced. In comparison, the parallel connection slightly reduces the hydrogen production. Connecting several MFCs in series is able to effectively increase power supply for hydrogen production, and has a potential to be used as a strategy to enhance hydrogen production in the MEC-MFC-coupled system from wastes.     

Hydrothermally stabilized Fe(III) doped titania active under visible light for water splitting reaction

Fe³⁺ doped TiO₂ photocatalysts were prepared by hydrothermal treatment for the photocatalytic water splitting to produce stoichiometric hydrogen and oxygen under visible light irradiation. It was found that hydrothermal treatment at 110 °C for 10 h was essential for the synthesis of highly stabilized Fe³⁺ doped TiO₂ photocatalysts. The synthesized photocatalysts were characterized by field emission scanning electron microscopy (FE-SEM), X-ray diffraction (XRD), ultraviolet–visible diffuse reflectance spectroscopy (UV–vis DRS) and BET surface area techniques. The doping of highly stabilized Fe³⁺ in the titania matrix leads to significant red shift of optical response towards visible light owing to the reduced band gap energy. Optimum amount of Fe³⁺ doped TiO₂, 1.0 wt% Fe/TiO₂, showed drastically improved hydrogen production performance of 12.5 μmol-H₂/h in aqueous methanol and 1.8 μmol-H₂/h in pure water, respectively. This Fe/TiO₂ photocatalyst was stable for 36 h without significant deactivation in the water splitting reaction.     

A bench scale study of fermentative hydrogen and methane production from food waste in integrated two-stage process

A two-stage fermentation process combining hydrogen and methane production for the treatment of food waste was investigated in this paper. In hydrogen fermentation reactor, the indigenous mixed microbial cultures contained in food waste were used for hydrogen production. No foreign inoculum was used in the hydrogen fermentation stage, the traditional heat treatment of inoculum was not applied either in this bench scale experiment. The effects of the stepwise increased organic loading rate (OLR) and solid retention time (SRT) on integrated two-stage process were investigated. At steady state, the optimal OLR and SRT for the integrated two-stage process were found to be 22.65 kg VS/m³ d (160 h) for hydrogen fermentation reactor and 4.61 (26.67 d) for methane fermentation reactor, respectively. Under the optimum conditions, the maximum yields of hydrogen (0.065 m³ H₂/kg VS) and methane (0.546 m³ CH₄/kg VS) were achieved with the hydrogen and methane contents ranging from 29.42 to 30.86%, 64.33 to 71.48%, respectively. Biodegradability analysis showed that 5.78% of the influent COD was converted to the hydrogen in H₂-SCRD and 82.18% of the influent COD was converted to the methane in CH₄-SCSTR under the optimum conditions.     

Production of hydrogen by the electrochemical reforming of glycerol–water solutions in a PEM electrolysis cell

An alternative method for producing hydrogen from renewable resources is proposed. Electrochemical reforming of glycerol solution in a proton exchange membrane (PEM) electrolysis cell is reported. The anode catalyst was composed of Pt on Ru–Ir oxide with a catalyst loading of 3 mg cm⁻² on Nafion. Part of the energy carried by the produced hydrogen is supplied by the glycerol (82%) and the remaining part of the energy originates from the electrical energy (18%) with an energy efficiency of conversion of glycerol to hydrogen of around 44%. The electrical energy consumption of this process is 1.1 kW h m⁻³ H₂. Compared to water electrolysis in the same cell, this is an electrical energy saving of 2.1 kW h N m⁻³ H₂ (a 66% reduction). Production rates are high compared with comparable sized microbial cells but low compared with conventional PEM water electrolysis cells.     

A pilot-scale study of biohydrogen production from distillery effluent using defined bacterial co-culture

We evaluated the feasibility of improving the scale of hydrogen (H₂) production from sugar cane distillery effluent using co-cultures of Citrobacter freundii 01, Enterobacter aerogenes E10 and Rhodopseudomonas palustris P2 at 100 m³ scale. The culture conditions at 100 ml and 2 L scales were optimized in minimal medium and we observed that the co-culture of the above three strains enhanced H₂ productivity significantly. Results at the 100 m³ scale revealed a maximum of 21.38 kg of H₂, corresponding to 10692.6 mol, which was obtained through batch method at 40 h from reducing sugar (3862.3 mol) as glucose. The average yield of H₂ was 2.76 mol mol⁻¹ glucose, and the rate of H₂ production was estimated as 0.53 kg/100 m³/h. Our results demonstrate the utility of distillery effluent as a source of clean alternative energy and provide insights into treatment for industrial exploitation.     

Fermentative hydrogen production from starch using natural mixed cultures

Batch and continuous tests were conducted to evaluate fermentative hydrogen production from starch (at a concentration of chemical oxygen demand (COD) 20 g/L) at 35 °C by a natural mixed culture of paper mill wastewater treatment sludge. The optimal initial cultivation pH (tested range 5–7) and substrate concentration (tested range 5–60-gCOD/L) were evaluated by batch reactors while the effects of hydraulic retention time (HRT) on hydrogen production, as expressed by hydrogen yield (HY) and hydrogen production rate (HPR), were evaluated by continuous tests. The experimental results indicate that the initial cultivation pH markedly affected HY, maximum HPR, liquid fermentation product concentration and distribution, butyrate/acetate concentration ratio and metabolic pathway. The optimal initial cultivation pH was 5.5 with peak values of HY 1.1 mol-H₂/mol-hexose maximum HPR 10.4 mmol-H₂/L/h and butyrate concentration 7700 mg-COD/L. In continuous hydrogen fermentation, the optimal HRT was 4 h with peak HY of 1.5 mol-H₂/mol-hexose, peak HPR of 450 mmol-H₂/L/d and lowest butyrate concentration of 3000 mg-COD/L. The HPR obtained was 280% higher than reported values. A shift in dominant hydrogen-producing microbial population along with HRT variation was observed with Clostridium butyricum, C. pasteurianum, Klebshilla pneumoniae, Streptococcus sp., and Pseudomonas sp. being present at efficient hydrogen production at the HRTs of 4–6 h. Strategies based on the experimental results for optimal hydrogen production from starch are proposed.     

Hydrogen production in microbial electrolysis cells: Choice of catholyte

A catholyte is a key factor to hydrogen production in microbial electrolysis cells (MECs). Among the four groups of catholytes investigated in this study, a 100 mM phosphate buffer solution (PBS) resulted in the highest hydrogen production rate of 0.237 ± 0.031 m³H₂/m³/d, followed by 0.171 ± 0.012 m³H₂/m³/d with a 134 mM NaCl solution and 0.171 ± 0.004 m³H₂/m³/d with the acidified water adjusted with sulfuric acid. The MEC with all catholytes achieved good organic removal efficiency, but the removal rate varied following the trend of the hydrogen production rate. The reuse of the catholyte for an extended period led to a decreasing hydrogen production rate, affected by the elevated pH. The cost of both the acidified water and the NaCl solution was much lower than the PBS, and therefore, they could be a better choice as an MEC catholyte with further consideration of cost reduction and chemical reuse/disposal.     

Effect of nickel loading on hydrogen production and chemical oxygen demand (COD) destruction from glucose oxidation and gasification in supercritical water

Gasification and partial oxidation of 0.25 molar glucose solution was conducted over different metallic nickel (Ni) loadings (7.5, 11, and 18 wt%) on different catalyst supports (θ-Al₂O₃ and γ-Al₂O₃) in supercritical water. Experiments were carried out at three different temperatures (T) of 400, 450, and 500 °C at constant pressure of 28 MPa and a 30 min reaction time (t). For comparison, some experiments were conducted using high loading commercial catalyst (65 wt% Ni on Silica–alumina). Hydrogen peroxide (H₂O₂) was used as a source of oxygen in the partial oxidation experiments. Oxygen to carbon molar ratios (MR) of 0.5–0.9 were examined to increase the hydrogen production via carbon monoxide (CO) production. Results showed that in the absence of the catalyst, the optimum molar ratio was 0.8 i.e. 80% of the amount of oxygen required for complete oxidation of glucose. At a molar ratio of 0.8, the hydrogen yield was 0.3 mol/mol, as compared to 0.2 mol/mol glucose at molar ratio of 0.5 and 0.9. This optimized oxygen dose was adopted as a base line for catalysts evaluation. The main gaseous products were carbon dioxide (CO₂), carbon monoxide (CO), hydrogen (H₂), and methane (CH₄). Results also showed that the presence of Ni increased the total gas yield increased in the 7.5–18 wt Ni/Al₂O₃ catalyst. An increase in MR from 0.55 to 0.8 increased the of carbon dioxide and hydrogen yields from 1.8 to 3.8 mol/mol glucose and from 0.9 to 1.1 mol/mol. The carbon monoxide and methane yields remain constant at 2 and 0.5 mol/mol glucose, respectively. The introduction of hydrogen peroxide (H₂O₂) prior to the feed injection inhibited the catalyst activity and did not increase the hydrogen yield whereas the introduction of H₂O₂ after 15 min of reaction time increased the hydrogen yield from 0.62 mol/mol to 1.5 mol/mol. This study showed that approximately the same hydrogen yield can be obtained from the synthesized low nickel alumina loading (18 wt%) catalyst as with the 65 wt% nickel on silica–alumina loading commercial catalyst. The highest H₂ yield of 1.5 mol/mol glucose was obtained with commercial Ni/silica–alumina with a BET surface area of 190 m²/g compared to 1.2 mol/mol with the synthesized Ni/θ alumina with a BET surface area of 46 m²/g.