- Email: [email protected]
Production of hydrogen by the electrochemical reforming of glycerol–water solutions in a PEM electrolysis cell
international journal of hydrogen energy 33 (2008) 4649–4654
Available at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/he
Production of hydrogen by the electrochemical reforming of glycerol–water solutions in a PEM electrolysis cell A.T. Marshall*, R.G. Haverkamp School of Engineering and Advanced Technology, Massey University, Private Bag 11222, Palmerston North 4442, New Zealand
article info
abstract
Article history:
An alternative method for producing hydrogen from renewable resources is proposed.
Received 24 January 2008
Electrochemical reforming of glycerol solution in a proton exchange membrane (PEM) elec-
Received in revised form
trolysis cell is reported. The anode catalyst was composed of Pt on Ru–Ir oxide with a cata-
2 May 2008
lyst loading of 3 mg cm2 on Nafion. Part of the energy carried by the produced hydrogen is
Accepted 5 May 2008
supplied by the glycerol (82%) and the remaining part of the energy originates from the
Available online 21 August 2008
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 m3 H2.
Keywords:
Compared to water electrolysis in the same cell, this is an electrical energy saving of
PEM electrolysis
2.1 kW h N m3 H2 (a 66% reduction). Production rates are high compared with comparable
Hydrogen production
sized microbial cells but low compared with conventional PEM water electrolysis cells.
Glycerol oxidation
ª 2008 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights
Glycerol reforming
reserved.
1.
Introduction
Civilisation relies on energy. Currently the primary sources of energy are fossil carbon and hydrocarbon, nuclear power, hydroelectric power, biomass, wind, solar and tidal power. Between the primary source and the consumer of the energy there is normally some means to transport the energy and often temporary storage of the energy. The energy may be transported as electricity or in a chemical form – normally hydrocarbon or carbon. Hydrocarbons as an energy carrier are very convenient and have the advantage that they may be produced efficiently from a primary source. However, they suffer from the disadvantage that they usually release carbon dioxide upon use and at the point of use it may be hard to capture this CO2. CO2 is believed by many to result in global warming and there is therefore political pressure to reduce CO2 emissions. If an alternative energy carrier is used, such as hydrogen which produces only water, the need to capture CO2 at the
point of use is eliminated. Hydrogen also has the advantage that there are many ways to produce it, including water electrolysis, hydrocarbon reforming, and biomass conversion by thermochemical and biological processes [1–3]. Hydrogen may be stored, transported and used where required either in fuel cells or by combustion. To produce hydrogen in a way that is considered environmentally benign solar, wind and hydropower sources are preferred. These methods do not use fossil fuels and are considered sustainable. The production of hydrogen from biomass has been proposed as also being a sustainable method to produce hydrogen. This production method has been suggested as an interim method between the current hydrogen production from fossil fuels (which accounts for as much as 97% of production [4]) and hydrogen production by efficient water electrolysis [2]. Hydrogen production from biomass can be considered as renewable, as the released CO2 can be recaptured by living plants to regenerate the required biomass.
* Corresponding author. Tel.: þ64 6 350 4077; fax: þ64 6 350 5604. E-mail address: [email protected] (A.T. Marshall). 0360-3199/$ – see front matter ª 2008 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2008.05.029
4650
international journal of hydrogen energy 33 (2008) 4649–4654
Existing processes to convert biomass to hydrogen largely rely on fermentation or thermochemical processes [3,5], however, we are proposing another option to convert biomass. Using electrochemical reforming it is possible to convert a biologically or synthetically produced organic compound to hydrogen. In this process, the conversion of the organic compound to hydrogen is driven by an electrical potential between two electrodes separated by a proton exchange membrane (PEM) (Fig. 1). Electrochemical reforming may operate in a similar way to a PEM water electrolysis cell. However, instead of just water, the anodic side contains an aqueous solution of the organic compound. The organic compound and water are oxidised to carbon dioxide, hydrogen ions and electrons at the anode. The hydrogen ions travel through the membrane where they recombine with the electrons, which have passed through the external circuit to form pure hydrogen gas at the cathode. The external circuit provides the electrical potential to drive the reaction. Thus the cathode reaction is the same as the PEM water electrolysis. A candidate organic molecule for hydrogen production by electrochemical reforming is glycerol. Glycerol is a byproduct in soap manufacturing and more recently in biodiesel production. Given that the biodiesel production rates are increasing considerably, a glut in the global glycerol market has developed driving down the price of glycerol [6]. In Canada alone, increased biodiesel production is expected to add around 55.4 million litres of glycerol per year by 2010 [7]. Glycerol is also renewable, relatively safe, has low toxicity, is non-flammable and has a high boiling point. Furthermore, unlike other alcohols such as methanol and ethanol, glycerol does not swell the Nafion proton exchange membrane [8]. These factors make glycerol a very suitable compound for hydrogen production by PEM based electrochemical reforming.
Fig. 1 – Schematic of PEM based reforming of biomass and water to produce hydrogen gas. The reactants and products are given for the case of glycerol as the biomass promoting the conversion of water to hydrogen and CO2.
The overall cell reaction for the electrochemical reforming of glycerol, assuming glycerol is oxidised completely to carbon dioxide with the addition of water, is: C3 H8 O3 þ 3H2 O # 3CO2 þ 7H2
(1)
With the anode and cathode reactions given by Eqs. (2) and (3), respectively. C3 H8 O3 þ 3H2 O # 3CO2 þ 14Hþ þ 14e
(2)
14Hþ þ 14e # 7H2
(3)
Part of the energy is supplied by electricity and the remaining part by the chemical energy of glycerol. From thermodynamic data we can calculate that the cell voltage required to drive this reaction (Eq. (1)) is around 0.22 V. This is significantly lower than the electrical requirements of water electrolysis (1.23 V). Therefore, to a first approximation, 18% of the energy carried by the hydrogen produced by this method is provided by electricity and 82% is provided by glycerol. In order to produce hydrogen by electrochemical reforming at a viable rate it is necessary to have fast reaction kinetics. Because the oxidation reaction of glycerol is complex (particularly the cleavage of the C–C bonds) [9], we predict there may be a large overpotential required in order for the reaction to proceed at a reasonable rate. Therefore, choosing a suitable electrocatalyst to minimise this overpotential will be critical. Electrocatalysts for glycerol oxidation are described in the literature. Polyaniline supported Pt, Pt–Pd and Ru doped Pt– Pd nanoparticles were examined for glycerol oxidation in 0.5 mol L1 H2SO4, with the activity towards glycerol oxidation improving by modifying Pt with the other metals [10]. Less electrode poisoning was observed when Ru nanoparticles were added to Pt–Pd bimetallic nanoparticles due to the ability of Ru to initiate dissociative adsorption of H2O, which helps oxidise the adsorbed CO on Pt. Under basic conditions, the addition of CeO2 to Pt/C electrocatalysts increased the activity towards alcohol oxidation (including glycerol) [11]. The mechanism of this improvement was suggested to be similar to the standard concept of Ru addition to Pt, in that the CeO2 enabled OHads groups to be available for CO oxidation at lower potentials. We note that Ru (or more correctly, hydrous RuO2) is a classic additive to Pt to improve the activity of Pt towards methanol oxidation [12], and that addition of IrO2 promotes ethanol oxidation [13]. Therefore, we chose to begin our investigations using RuO2–IrO2–Pt (as the glycerol oxidation electrode). Hydrogen production by electrochemical reforming has been described by other workers. Electrochemically assisted microbial production of hydrogen from acetate has been examined in aqueous electrolyte with the anode and cathode compartments separated by a Nafion membrane [14]. In this system the current efficiency for hydrogen production was 90–100%, with the current density ranging from 1.4 to 7.1 A m2 at cell voltages of 0.45 and 0.85 V, respectively. At the lowest reported cell voltage of 0.25 V (current density of about 0.14 A m2) this equates to hydrogen production power requirements of 0.6 kW h m3 H2 This electrical power requirement is much lower than a very efficient PEM water electrolysis cell (3.75 kW h m3 H2) [1] and less than the
international journal of hydrogen energy 33 (2008) 4649–4654
thermodynamic requirement for water electrolysis of around 2.6 kW h m3 H2. A similar biocatalyzed cell was used by others to convert acetate to hydrogen, although the specific hydrogen production rates were much less (0.1–1 A m2 at 0.25–0.75 V) [15]. The large cathode overpotential (0.28 V at a cell voltage of 0.5 V) was proposed as the main cause for the low hydrogen production rates, however, it is well known that the hydrogen evolution reaction is very reversible so we question this explanation. Current efficiencies for hydrogen evolution of 100% have been achieved in other microbial electrolysis cells operating with either cation or anion exchange membranes [16]. The cathode was a layer of platinum electroplated onto each membrane, although these cells also seemed to suffer from large cathodic overpotentials. The ‘‘on-board’’ conversion of methanol in a PEM electrolysis cell has been examined [17], with a cell voltage of 0.951–1.34 V (0.043– 0.35 A cm2, room temperature) achieved. However, given the suitability of methanol as a fuel in a direct methanol fuel cell, the efficiency would have to be enhanced considerably for such a system to be commercially advantageous. HBr and SO2 electrolyses coupled to high temperature regeneration cycles have been examined as a means of hydrogen production within a PEM based reactor [18]. Thermodynamically a cell voltage of 0.17 and 0.58 V is required for the SO2 and HBr processes, respectively, although the processes required overpotentials of 530 (SO2) and 80 mV (HBr) to drive the reaction at 0.2 A cm2. This paper describes a relatively simple method for producing hydrogen from glycerol using an electrochemical reformer based on a PEM electrolysis cell.
2.
Experimental
The anode electrocatalyst was prepared by mixing equal molar amounts of 0.05 mol L1 RuCl3 (Alfa Aesar, 99.9%) and 0.05 mol L1 IrCl3 (American Elements, 99.9%). This solution was placed in sealed glass tubes and hydrolysed by heating to 150 C for 120 h, after which 1 mol L1 NaOH solution was added (1:4 noble metals/NaOH molar ratio) to complete the hydrolysis process. This procedure was developed based on the finding that hydrothermal preparation of RuO2 reduces the loss of electrochemically active surface area during later annealing stages [19]. The precipitated material was recovered by centrifugation, washed with deionised water several times, and dried in a desiccator. The hydroxide precipitate was then annealed at 400 C for 30 min to produce an oxide. The Pt was applied to the surface of this oxide by suspending the oxide in a mixture consisting of 23 mL deionised water, 2 mL 0.05 mol L1 H2PtCl6 (prepared from E-TEK Inc. Pt black), and 5 mL 0.1 mol L1 sodium citrate (Merck, ACS grade). This was heated to 50 C with constant stirring. After heating, 10 mL 0.1 mol L1 NaBH4 (Ajax Fine Chemicals, >97%) was added to reduce H2PtCl6 with further stirring. The precipitate was recovered by centrifugation, washed with deionised water three times, and dried in air at 105 C for 120 h. The Pt loading on the oxide support was 20 wt%. X-ray diffraction was used to confirm the structure of the prepared anode electrocatalyst. A rutile type oxide with lattice parameters between that of IrO2 and RuO2 was found as well as broad peaks corresponding
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to metallic platinum. Based on Scherrer’s equation, the crystallite size of the Pt was estimated to be 8 nm. To confirm the activity of this electrocatalyst material towards the oxidation of glycerol in acid, we performed cyclic voltammetry of a 1 mol L1 glycerol solution in 0.5 mol L1 H2SO4. For this test a working electrode consisting of a composite layer of 20% Pt on Ru–Ir oxide and 10 wt% Nafion on a glassy carbon rod (diameter 3 mm, polished with 1 mm alumina) was used. This was prepared by placing a 5 mL drop of the same ink as used for the PEM electrodes on the glassy carbon followed by drying under vacuum at room temperature overnight. A standard three-electrode glass cell was used along with a Pt foil counter and Ag/AgCl/KClsat reference electrode. The electrolyte was bubbled with N2 for 30 min prior to the experiments and flowed over the electrolyte during the cyclic voltammetry measurements. A membrane electrode assembly (MEA) was prepared by spraying an ink containing the electrocatalyst and Nafion ionomer in water directly to Nafion 212 membranes (Ion Power Inc.) at 100 C using a handheld airbrush system. The electrode area was 3.14 cm2 and the Nafion content of the electrocatalytic layers was 10 wt%. The total catalyst loading was approximately 3 mg cm2 per electrode. 20 wt% Pt on Ru–Ir oxide (1:1 mole ratio) on Nafion, prepared as above, was used as the anode and 20% Pt on Vulcan XC-72R (E-TEK Inc.) as the cathode. Once sprayed, the MEA was protonated by immersion in 0.5 mol L1 H2SO4 solution for 30 min at 80 C, followed by repeated rinsing in deionised water (Milli-Q, 18.2 MU cm). The MEA was mounted between two gold plated porous titanium sinters (Mott Corp, diameter 25 mm, thickness 3 mm, 50% porosity). Deionised water preheated to 70 C was supplied to the cathode compartment and preheated glycerol solution (BDH, 99.95%) was supplied to the anode, both at 4.2 L h1. Cell polarisation curves were recorded galvanostatically using a Gamry G-300 potentiostat. Electrochemical impedance spectroscopy was conducted at a cell voltage of 0.5 V with a 5 mV amplitude over the frequency range 300 kHz to 1 Hz.
3.
Results and discussion
3.1. Catalytic activity of Ru–Ir oxide–Pt towards oxidation of glycerol in 0.5 mol L1 H2SO4 Cyclic voltammetry of the prepared 20% Pt on Ru–Ir oxide with Nafion on glassy carbon in 0.5 mol L1 H2SO4 without glycerol revealed a large pseudo-capacitive charging background from the oxide support, along with the characteristic peaks associated with platinum (Pt–Hads at 0.1 V and Pt-oxide reduction at 0.55 V) (Fig. 2a). The platinum active area was estimated to be around 30 m2 Pt g1 Pt based on the charge associated with hydrogen underpotential deposition. This compares well to the value calculated based on the crystallite size (35 m2 Pt g1 Pt). When glycerol was added, the voltammograms changed dramatically, as expected, with large anodic currents due to the oxidation of glycerol (Fig. 2b). On the forward scan, oxidation of glycerol commences around 0.5 V, with the current peaking around 0.65 V before decreasing, due to the formation
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international journal of hydrogen energy 33 (2008) 4649–4654
a
b
Fig. 2 – Cyclic voltammogram of 20% Pt on Ru–Ir oxide with 10% Nafion on glassy carbon at 20 mV sL1 in (a) 0.5 mol LL1 H2SO4 and (b) 0.5 mol LL1 H2SO4 with 1 mol LL1 glycerol.
of Pt oxide. On the reverse scan, the Pt oxide is reduced, allowing for the glycerol oxidation to recommence. The overall behaviour of the oxidation is similar to that seen for other alcohols in acid [20]. This differs from other work reported for glycerol oxidation, where only the forward going peak at around 0.6 V is clearly seen [10,11]. At 50 mV s1, the peak current of the forward going scan is approximately the same as that found by others on a platinum mass basis (130– 140 mA mg1 Pt) [11]. The voltammetric behaviour was stable during our measurements (20 cycles).
3.2. PEM electrolysis cell performance for hydrogen production from glycerol The performance of the PEM cell for glycerol reforming to produce hydrogen was assessed by recording polarisation curves. These polarisation curves of the conversion of glycerol solutions to hydrogen in our PEM electrolysis cell reveal that hydrogen can be produced at reasonable rates at cell voltages much lower than water electrolysis (Fig. 3). Cell voltages in the range 0.48–0.7 V were obtained compared with 1.33–1.40 V for water in the same cell.
Fig. 3 – Polarisation curve of the PEM electrolysis cell at 70 8C with (C) 2 mol LL1 and (B) 8.5 mol LL1 glycerol circulating through the anode compartment. For comparison, pure water electrolysis polarisation curves recorded in the same cell exhibited cell voltages from 1.33 to 1.40 V over the same current density range.
Our results also show that the performance of the cell depends on the glycerol concentration, with the cell operating on 2 mol L1 glycerol exhibiting voltages around 20–50 mV less than the cell operating on 8.5 mol L1 glycerol. Further experiments may be useful to determine the optimum operating conditions. Comparing our results with the electrochemically assisted microbial conversion of acetate (which has a lower thermodynamic cell voltage), reveals that at cell voltages around 0.5 V, hydrogen production rates for the PEM system are around 6– 80 times larger than for electrochemically assisted microbial conversion [14–16]. This rate is also greater than some of biological routes to hydrogen (9–250 g H2 day1 m3 reactor) [21– 23]. This indicates that the overall reactor size could be smaller than these other systems for a given hydrogen production rate, thus reducing the cost of the system. Overall, the best performance we achieved equates to 1.1 kW h m3 H2 at production rates of 10 A m2 or 0.37 g H2 h1 m2 (electrode area basis). For comparison purposes to other similar low rate processes like biological H2 production, we estimate that 1 m2 electrode area would equate to a reactor volume of about 0.01 m3. This gives hydrogen production rates of 888 g H2 day1 m3 or about 10 m3 H2 day1 m3 (reactor volume basis). Compared with water electrolysis, in the same cell and at the same H2 production rate, electrochemical glycerol reforming saves around 2.1 kW h m3 H2 of electrical energy. This saving in electrical energy is of course substituted by the chemical energy provided by the glycerol. The energy efficiency of conversion of glycerol to hydrogen is around 44%. Although 82% of the energy carried by the produced hydrogen is supplied by glycerol and 18% of the energy originates from the electrical energy, the reduction in electricity consumption is only 66% not 82%. The difference arises from the overpotentials of the cell which means some of the electrical energy does not contribute to the chemical reaction. Based on the findings of others [16]
international journal of hydrogen energy 33 (2008) 4649–4654
and the fact that there are limited possibilities for side reactions, we believe that the cathode coulombic efficiency will be close to 100%. At this stage, the level of glycerol conversion to CO2 is unknown and the subject of on-going work.
3.3.
Cell stability and possible poisoning
To assess the stability of the electrolysis cell operating under glycerol reforming conditions, the current was fixed at 5 mA cm2 for 30 min (Fig. 4). For the cell operating on 8.5 mol L1 glycerol the cell voltage increased by around 0.23 V over this time and at 2 mol L1 glycerol the increase was 0.06 V over a similar time period. Based on the cyclic voltammetry measurements, we do not believe this instability is caused by corrosion of our anode electrocatalyst, as repeated cycling revealed no changes in the voltammetric currents observed. Corrosion of the cathode electrocatalyst is equally unlikely. One possible explanation for the cell voltage increase, and therefore loss in performance, would be poisoning of the catalyst by glycerol or its oxidation products. It was found that a short reversal in current (1 mA cm2, 30 s) had the effect of reducing the cell voltage by about 0.08 V on returning to the original current (5 mA cm2), a behaviour which seems to point to a poisoning effect. We also confirmed that the hydrogen evolution reaction (HER) is not inhibited by glycerol (in case of glycerol cross-over), by measuring the HER polarisation curve in 0.5 mol L1 H2SO4 with and without 1 mol L1 glycerol on a 99.99% pure platinum wire, with no differences observed. This indicates that the anode may be the cause of the instability, by slowly being poisoned by glycerol or its oxidation products, with these poisons able to be removed upon cathodic polarisation. Further work is needed to confirm this proposition. A different electrocatalyst may be able to be used to reduce this poisoning effect.
3.4.
Cell resistance
We used electrochemical impedance spectroscopy to determine the cell resistance, a key factor in the performance of
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a PEM cell. The resistance (found at high frequency) in the presence of 2 or 8.5 mol L1 glycerol was found to be higher than when only water was circulated through the cell (300 mU cm2 vs 60 mU cm2). Normally this resistance is dominated by the ionic resistance of the membrane, added to the contact resistances between the cell hardware, electrodes and membrane. In alcohol solutions the membrane can swell which may influence the proton conduction and the membrane–electrode contact resistances [24], however, with glycerol Nafion swelling is believed to be minimal [8]. Instead, as the water activity within the membrane is a contributing factor to proton conduction, the decrease in water activity due to the presence of glycerol may reduce conductivity. Further work needs to be conducted to confirm the effect of glycerol on Nafion conductivity. The total cell resistance did not change over the period of the stability test.
4.
Conclusions
A simple PEM electrolysis cell has been used to electrochemically reform glycerol to hydrogen gas at a cell voltage lower than that for water electrolysis. Hydrogen can be produced at around 10 m3 H2 day1 m3 (reactor volume basis) at an energy consumption of 1.1 kW h m3 H2. Compared to water electrolysis in the same cell, a total energy saving of 2.1 kW h N m3 H2 is possible, equating to a 66% reduction in electrical energy consumption. The reaction kinetics are slow and it is likely that poisoning of the anode electrocatalyst occurs over time. By developing better electrocatalysts, a more stable and efficient cell should be possible. The electrochemical reforming of glycerol solutions in a PEM electrolysis cell has promise as a new hydrogen production method.
Acknowledgments The authors would like to acknowledge the financial support from the Foundation for Research, Science and Technology, NZ.
references
Fig. 4 – Stability of the cell voltage at 5 mA cmL2 and 70 8C with 8.5 mol LL1 glycerol circulating through the anode compartment.
[1] Marshall A, Sunde S, Tsypkin M, Tunold R. Performance of a PEM water electrolysis cell using IrxRuyTazO2 electrocatalysts for the oxygen evolution electrode. Int J Hydrogen Energy 2007;32:2320–4. [2] Navarro R, Pen˜na M, Fierro J. Hydrogen production reactions from carbon feedstocks: fossil fuels and biomass. Chem Rev 2007;107:3952–91. [3] Ni M, Leung D, Leung M, Sumathy K. An overview of hydrogen production from biomass. Fuel Process Technol 2006;87:461–72. [4] Koroneos C, Dompros A, Roumbas G, Moussiopoulos N. Life cycle assessment of hydrogen fuel production processes. Int J Hydrogen Energy 2004;29:1443–50. [5] Manish S, Banerjee R. Comparison of biohydrogen production processes. Int J Hydrogen Energy 2008;33:279–86.
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[6] Fernando S, Adhikari S, Kota K, Bandi R. Glycerol based automotive fuels from future biorefineries. Fuel 2007;86: 2806–9. [7] Valliyappan T, Bakhshi N, Dalai A. Pyrolysis of glycerol for the production of hydrogen or syn gas. Bioresour Technol 2008;99:4476–83. [8] Arechederra R, Treu B, Minteer S. Development of glycerol/O2 biofuel cell. J Power Sources 2007;173:156–61. [9] Calegaro M, Suffredini H, Machado S, Avaca L. Preparation, characterization and utilization of a new electrocatalyst for ethanol oxidation obtained by the sol–gel method. J Power Sources 2006;156:300–5. [10] Grace A, Pandian K. Pt, Pt–Pd and Pt–Pd/Ru nanoparticles entrapped polyaniline electrodes – a potent electrocatalyst towards the oxidation of glycerol. Electrochem Commun 2006;8:1340–8. [11] Xu C, Zeng R, Shen P, Wei Z. Synergistic effect of CeO2 modified Pt/C catalysts on the alcohols oxidation. Electrochim Acta 2005;51:1031–5. [12] Rolison D, Hagans P, Swider K, Long J. Role of hydrous ruthenium oxide in Pt–Ru direct methanol fuel cell anode electrocatalysts: the importance of mixed electron/proton conductivity. Langmuir 1999;15:774–9. [13] Suffredini H, Salazar-Banda G, Avaca L. Enhanced ethanol oxidation on PbOx-containing electrode materials for fuel cell applications. J Power Sources 2007;171:355–62. [14] Liu H, Grot S, Logan B. Electrochemically assisted microbial production of hydrogen from acetate. Environ Sci Technol 2005;39:4317–20. [15] Rozendal R, Hamelers H, Euverink G, Metz S, Buisman C. Principle and perspectives of hydrogen production through biocatalyzed electrolysis. Int J Hydrogen Energy 2006;31: 1632–40.
[16] Rozendal R, Hamelers H, Molenkamp R, Buisman C. Performance of single chamber biocatalyzed electrolysis with different types of ion exchange membrane. Water Res 2007;41:1984–94. [17] Take T, Tsurutani K, Umeda M. Hydrogen production by methanol–water solution electrolysis. J Power Sources 2007; 164:9–16. [18] Sivasubramaniana P, Ramasamya R, Freireb F, Hollanda C, Weidner J. Electrochemical hydrogen production from thermochemical cycles using a proton exchange membrane electrolyzer. Int J Hydrogen Energy 2007;32:463–8. [19] Chang K, Hu C, Chou C. Textural and capacitive characteristics of hydrothermally derived RuO2$xH2O nanocrystallites: independent control of crystal size and water content. Chem Mater 2007;19:2112–9. [20] Gu Y, Wong W. Electro-oxidation of methanol on Pt particles dispersed on RuO2 nanorods. J Electrochem Soc 2006;153: A1714–8. [21] Li S, Kuo S, Lin J, Lee Z, Wang Y, Cheng S. Process performance evaluation of intermittent-continuous stirred tank reactor for anaerobic hydrogen fermentation with kitchen waste. Int J Hydrogen Energy 2008;33:1522–31. [22] Nguyen T, Kim J, Kim M, Oh Y, Sim S. Optimization of hydrogen production by hyperthermophilic eubacteria, Thermotoga maritima and Thermotoga neapolitana in batch fermentation. Int J Hydrogen Energy 2008;33:1483–8. [23] Chou C, Wang C, Huang C, Lay J. Pilot study of the influence of stirring and pH on anaerobes converting high-solid organic wastes to hydrogen. Int J Hydrogen Energy 2008;33: 1550–8. [24] Song S, Zhou W, Liang Z, Cai R, Sun G, Xin Q, et al. The effect of methanol and ethanol cross-over on the performance of PtRu/ C-based anode DAFCs. Appl Catal B Environ 2005;55:65–72.
Available at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/he
Production of hydrogen by the electrochemical reforming of glycerol–water solutions in a PEM electrolysis cell A.T. Marshall*, R.G. Haverkamp School of Engineering and Advanced Technology, Massey University, Private Bag 11222, Palmerston North 4442, New Zealand
article info
abstract
Article history:
An alternative method for producing hydrogen from renewable resources is proposed.
Received 24 January 2008
Electrochemical reforming of glycerol solution in a proton exchange membrane (PEM) elec-
Received in revised form
trolysis cell is reported. The anode catalyst was composed of Pt on Ru–Ir oxide with a cata-
2 May 2008
lyst loading of 3 mg cm2 on Nafion. Part of the energy carried by the produced hydrogen is
Accepted 5 May 2008
supplied by the glycerol (82%) and the remaining part of the energy originates from the
Available online 21 August 2008
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 m3 H2.
Keywords:
Compared to water electrolysis in the same cell, this is an electrical energy saving of
PEM electrolysis
2.1 kW h N m3 H2 (a 66% reduction). Production rates are high compared with comparable
Hydrogen production
sized microbial cells but low compared with conventional PEM water electrolysis cells.
Glycerol oxidation
ª 2008 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights
Glycerol reforming
reserved.
1.
Introduction
Civilisation relies on energy. Currently the primary sources of energy are fossil carbon and hydrocarbon, nuclear power, hydroelectric power, biomass, wind, solar and tidal power. Between the primary source and the consumer of the energy there is normally some means to transport the energy and often temporary storage of the energy. The energy may be transported as electricity or in a chemical form – normally hydrocarbon or carbon. Hydrocarbons as an energy carrier are very convenient and have the advantage that they may be produced efficiently from a primary source. However, they suffer from the disadvantage that they usually release carbon dioxide upon use and at the point of use it may be hard to capture this CO2. CO2 is believed by many to result in global warming and there is therefore political pressure to reduce CO2 emissions. If an alternative energy carrier is used, such as hydrogen which produces only water, the need to capture CO2 at the
point of use is eliminated. Hydrogen also has the advantage that there are many ways to produce it, including water electrolysis, hydrocarbon reforming, and biomass conversion by thermochemical and biological processes [1–3]. Hydrogen may be stored, transported and used where required either in fuel cells or by combustion. To produce hydrogen in a way that is considered environmentally benign solar, wind and hydropower sources are preferred. These methods do not use fossil fuels and are considered sustainable. The production of hydrogen from biomass has been proposed as also being a sustainable method to produce hydrogen. This production method has been suggested as an interim method between the current hydrogen production from fossil fuels (which accounts for as much as 97% of production [4]) and hydrogen production by efficient water electrolysis [2]. Hydrogen production from biomass can be considered as renewable, as the released CO2 can be recaptured by living plants to regenerate the required biomass.
* Corresponding author. Tel.: þ64 6 350 4077; fax: þ64 6 350 5604. E-mail address: [email protected] (A.T. Marshall). 0360-3199/$ – see front matter ª 2008 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2008.05.029
4650
international journal of hydrogen energy 33 (2008) 4649–4654
Existing processes to convert biomass to hydrogen largely rely on fermentation or thermochemical processes [3,5], however, we are proposing another option to convert biomass. Using electrochemical reforming it is possible to convert a biologically or synthetically produced organic compound to hydrogen. In this process, the conversion of the organic compound to hydrogen is driven by an electrical potential between two electrodes separated by a proton exchange membrane (PEM) (Fig. 1). Electrochemical reforming may operate in a similar way to a PEM water electrolysis cell. However, instead of just water, the anodic side contains an aqueous solution of the organic compound. The organic compound and water are oxidised to carbon dioxide, hydrogen ions and electrons at the anode. The hydrogen ions travel through the membrane where they recombine with the electrons, which have passed through the external circuit to form pure hydrogen gas at the cathode. The external circuit provides the electrical potential to drive the reaction. Thus the cathode reaction is the same as the PEM water electrolysis. A candidate organic molecule for hydrogen production by electrochemical reforming is glycerol. Glycerol is a byproduct in soap manufacturing and more recently in biodiesel production. Given that the biodiesel production rates are increasing considerably, a glut in the global glycerol market has developed driving down the price of glycerol [6]. In Canada alone, increased biodiesel production is expected to add around 55.4 million litres of glycerol per year by 2010 [7]. Glycerol is also renewable, relatively safe, has low toxicity, is non-flammable and has a high boiling point. Furthermore, unlike other alcohols such as methanol and ethanol, glycerol does not swell the Nafion proton exchange membrane [8]. These factors make glycerol a very suitable compound for hydrogen production by PEM based electrochemical reforming.
Fig. 1 – Schematic of PEM based reforming of biomass and water to produce hydrogen gas. The reactants and products are given for the case of glycerol as the biomass promoting the conversion of water to hydrogen and CO2.
The overall cell reaction for the electrochemical reforming of glycerol, assuming glycerol is oxidised completely to carbon dioxide with the addition of water, is: C3 H8 O3 þ 3H2 O # 3CO2 þ 7H2
(1)
With the anode and cathode reactions given by Eqs. (2) and (3), respectively. C3 H8 O3 þ 3H2 O # 3CO2 þ 14Hþ þ 14e
(2)
14Hþ þ 14e # 7H2
(3)
Part of the energy is supplied by electricity and the remaining part by the chemical energy of glycerol. From thermodynamic data we can calculate that the cell voltage required to drive this reaction (Eq. (1)) is around 0.22 V. This is significantly lower than the electrical requirements of water electrolysis (1.23 V). Therefore, to a first approximation, 18% of the energy carried by the hydrogen produced by this method is provided by electricity and 82% is provided by glycerol. In order to produce hydrogen by electrochemical reforming at a viable rate it is necessary to have fast reaction kinetics. Because the oxidation reaction of glycerol is complex (particularly the cleavage of the C–C bonds) [9], we predict there may be a large overpotential required in order for the reaction to proceed at a reasonable rate. Therefore, choosing a suitable electrocatalyst to minimise this overpotential will be critical. Electrocatalysts for glycerol oxidation are described in the literature. Polyaniline supported Pt, Pt–Pd and Ru doped Pt– Pd nanoparticles were examined for glycerol oxidation in 0.5 mol L1 H2SO4, with the activity towards glycerol oxidation improving by modifying Pt with the other metals [10]. Less electrode poisoning was observed when Ru nanoparticles were added to Pt–Pd bimetallic nanoparticles due to the ability of Ru to initiate dissociative adsorption of H2O, which helps oxidise the adsorbed CO on Pt. Under basic conditions, the addition of CeO2 to Pt/C electrocatalysts increased the activity towards alcohol oxidation (including glycerol) [11]. The mechanism of this improvement was suggested to be similar to the standard concept of Ru addition to Pt, in that the CeO2 enabled OHads groups to be available for CO oxidation at lower potentials. We note that Ru (or more correctly, hydrous RuO2) is a classic additive to Pt to improve the activity of Pt towards methanol oxidation [12], and that addition of IrO2 promotes ethanol oxidation [13]. Therefore, we chose to begin our investigations using RuO2–IrO2–Pt (as the glycerol oxidation electrode). Hydrogen production by electrochemical reforming has been described by other workers. Electrochemically assisted microbial production of hydrogen from acetate has been examined in aqueous electrolyte with the anode and cathode compartments separated by a Nafion membrane [14]. In this system the current efficiency for hydrogen production was 90–100%, with the current density ranging from 1.4 to 7.1 A m2 at cell voltages of 0.45 and 0.85 V, respectively. At the lowest reported cell voltage of 0.25 V (current density of about 0.14 A m2) this equates to hydrogen production power requirements of 0.6 kW h m3 H2 This electrical power requirement is much lower than a very efficient PEM water electrolysis cell (3.75 kW h m3 H2) [1] and less than the
international journal of hydrogen energy 33 (2008) 4649–4654
thermodynamic requirement for water electrolysis of around 2.6 kW h m3 H2. A similar biocatalyzed cell was used by others to convert acetate to hydrogen, although the specific hydrogen production rates were much less (0.1–1 A m2 at 0.25–0.75 V) [15]. The large cathode overpotential (0.28 V at a cell voltage of 0.5 V) was proposed as the main cause for the low hydrogen production rates, however, it is well known that the hydrogen evolution reaction is very reversible so we question this explanation. Current efficiencies for hydrogen evolution of 100% have been achieved in other microbial electrolysis cells operating with either cation or anion exchange membranes [16]. The cathode was a layer of platinum electroplated onto each membrane, although these cells also seemed to suffer from large cathodic overpotentials. The ‘‘on-board’’ conversion of methanol in a PEM electrolysis cell has been examined [17], with a cell voltage of 0.951–1.34 V (0.043– 0.35 A cm2, room temperature) achieved. However, given the suitability of methanol as a fuel in a direct methanol fuel cell, the efficiency would have to be enhanced considerably for such a system to be commercially advantageous. HBr and SO2 electrolyses coupled to high temperature regeneration cycles have been examined as a means of hydrogen production within a PEM based reactor [18]. Thermodynamically a cell voltage of 0.17 and 0.58 V is required for the SO2 and HBr processes, respectively, although the processes required overpotentials of 530 (SO2) and 80 mV (HBr) to drive the reaction at 0.2 A cm2. This paper describes a relatively simple method for producing hydrogen from glycerol using an electrochemical reformer based on a PEM electrolysis cell.
2.
Experimental
The anode electrocatalyst was prepared by mixing equal molar amounts of 0.05 mol L1 RuCl3 (Alfa Aesar, 99.9%) and 0.05 mol L1 IrCl3 (American Elements, 99.9%). This solution was placed in sealed glass tubes and hydrolysed by heating to 150 C for 120 h, after which 1 mol L1 NaOH solution was added (1:4 noble metals/NaOH molar ratio) to complete the hydrolysis process. This procedure was developed based on the finding that hydrothermal preparation of RuO2 reduces the loss of electrochemically active surface area during later annealing stages [19]. The precipitated material was recovered by centrifugation, washed with deionised water several times, and dried in a desiccator. The hydroxide precipitate was then annealed at 400 C for 30 min to produce an oxide. The Pt was applied to the surface of this oxide by suspending the oxide in a mixture consisting of 23 mL deionised water, 2 mL 0.05 mol L1 H2PtCl6 (prepared from E-TEK Inc. Pt black), and 5 mL 0.1 mol L1 sodium citrate (Merck, ACS grade). This was heated to 50 C with constant stirring. After heating, 10 mL 0.1 mol L1 NaBH4 (Ajax Fine Chemicals, >97%) was added to reduce H2PtCl6 with further stirring. The precipitate was recovered by centrifugation, washed with deionised water three times, and dried in air at 105 C for 120 h. The Pt loading on the oxide support was 20 wt%. X-ray diffraction was used to confirm the structure of the prepared anode electrocatalyst. A rutile type oxide with lattice parameters between that of IrO2 and RuO2 was found as well as broad peaks corresponding
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to metallic platinum. Based on Scherrer’s equation, the crystallite size of the Pt was estimated to be 8 nm. To confirm the activity of this electrocatalyst material towards the oxidation of glycerol in acid, we performed cyclic voltammetry of a 1 mol L1 glycerol solution in 0.5 mol L1 H2SO4. For this test a working electrode consisting of a composite layer of 20% Pt on Ru–Ir oxide and 10 wt% Nafion on a glassy carbon rod (diameter 3 mm, polished with 1 mm alumina) was used. This was prepared by placing a 5 mL drop of the same ink as used for the PEM electrodes on the glassy carbon followed by drying under vacuum at room temperature overnight. A standard three-electrode glass cell was used along with a Pt foil counter and Ag/AgCl/KClsat reference electrode. The electrolyte was bubbled with N2 for 30 min prior to the experiments and flowed over the electrolyte during the cyclic voltammetry measurements. A membrane electrode assembly (MEA) was prepared by spraying an ink containing the electrocatalyst and Nafion ionomer in water directly to Nafion 212 membranes (Ion Power Inc.) at 100 C using a handheld airbrush system. The electrode area was 3.14 cm2 and the Nafion content of the electrocatalytic layers was 10 wt%. The total catalyst loading was approximately 3 mg cm2 per electrode. 20 wt% Pt on Ru–Ir oxide (1:1 mole ratio) on Nafion, prepared as above, was used as the anode and 20% Pt on Vulcan XC-72R (E-TEK Inc.) as the cathode. Once sprayed, the MEA was protonated by immersion in 0.5 mol L1 H2SO4 solution for 30 min at 80 C, followed by repeated rinsing in deionised water (Milli-Q, 18.2 MU cm). The MEA was mounted between two gold plated porous titanium sinters (Mott Corp, diameter 25 mm, thickness 3 mm, 50% porosity). Deionised water preheated to 70 C was supplied to the cathode compartment and preheated glycerol solution (BDH, 99.95%) was supplied to the anode, both at 4.2 L h1. Cell polarisation curves were recorded galvanostatically using a Gamry G-300 potentiostat. Electrochemical impedance spectroscopy was conducted at a cell voltage of 0.5 V with a 5 mV amplitude over the frequency range 300 kHz to 1 Hz.
3.
Results and discussion
3.1. Catalytic activity of Ru–Ir oxide–Pt towards oxidation of glycerol in 0.5 mol L1 H2SO4 Cyclic voltammetry of the prepared 20% Pt on Ru–Ir oxide with Nafion on glassy carbon in 0.5 mol L1 H2SO4 without glycerol revealed a large pseudo-capacitive charging background from the oxide support, along with the characteristic peaks associated with platinum (Pt–Hads at 0.1 V and Pt-oxide reduction at 0.55 V) (Fig. 2a). The platinum active area was estimated to be around 30 m2 Pt g1 Pt based on the charge associated with hydrogen underpotential deposition. This compares well to the value calculated based on the crystallite size (35 m2 Pt g1 Pt). When glycerol was added, the voltammograms changed dramatically, as expected, with large anodic currents due to the oxidation of glycerol (Fig. 2b). On the forward scan, oxidation of glycerol commences around 0.5 V, with the current peaking around 0.65 V before decreasing, due to the formation
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a
b
Fig. 2 – Cyclic voltammogram of 20% Pt on Ru–Ir oxide with 10% Nafion on glassy carbon at 20 mV sL1 in (a) 0.5 mol LL1 H2SO4 and (b) 0.5 mol LL1 H2SO4 with 1 mol LL1 glycerol.
of Pt oxide. On the reverse scan, the Pt oxide is reduced, allowing for the glycerol oxidation to recommence. The overall behaviour of the oxidation is similar to that seen for other alcohols in acid [20]. This differs from other work reported for glycerol oxidation, where only the forward going peak at around 0.6 V is clearly seen [10,11]. At 50 mV s1, the peak current of the forward going scan is approximately the same as that found by others on a platinum mass basis (130– 140 mA mg1 Pt) [11]. The voltammetric behaviour was stable during our measurements (20 cycles).
3.2. PEM electrolysis cell performance for hydrogen production from glycerol The performance of the PEM cell for glycerol reforming to produce hydrogen was assessed by recording polarisation curves. These polarisation curves of the conversion of glycerol solutions to hydrogen in our PEM electrolysis cell reveal that hydrogen can be produced at reasonable rates at cell voltages much lower than water electrolysis (Fig. 3). Cell voltages in the range 0.48–0.7 V were obtained compared with 1.33–1.40 V for water in the same cell.
Fig. 3 – Polarisation curve of the PEM electrolysis cell at 70 8C with (C) 2 mol LL1 and (B) 8.5 mol LL1 glycerol circulating through the anode compartment. For comparison, pure water electrolysis polarisation curves recorded in the same cell exhibited cell voltages from 1.33 to 1.40 V over the same current density range.
Our results also show that the performance of the cell depends on the glycerol concentration, with the cell operating on 2 mol L1 glycerol exhibiting voltages around 20–50 mV less than the cell operating on 8.5 mol L1 glycerol. Further experiments may be useful to determine the optimum operating conditions. Comparing our results with the electrochemically assisted microbial conversion of acetate (which has a lower thermodynamic cell voltage), reveals that at cell voltages around 0.5 V, hydrogen production rates for the PEM system are around 6– 80 times larger than for electrochemically assisted microbial conversion [14–16]. This rate is also greater than some of biological routes to hydrogen (9–250 g H2 day1 m3 reactor) [21– 23]. This indicates that the overall reactor size could be smaller than these other systems for a given hydrogen production rate, thus reducing the cost of the system. Overall, the best performance we achieved equates to 1.1 kW h m3 H2 at production rates of 10 A m2 or 0.37 g H2 h1 m2 (electrode area basis). For comparison purposes to other similar low rate processes like biological H2 production, we estimate that 1 m2 electrode area would equate to a reactor volume of about 0.01 m3. This gives hydrogen production rates of 888 g H2 day1 m3 or about 10 m3 H2 day1 m3 (reactor volume basis). Compared with water electrolysis, in the same cell and at the same H2 production rate, electrochemical glycerol reforming saves around 2.1 kW h m3 H2 of electrical energy. This saving in electrical energy is of course substituted by the chemical energy provided by the glycerol. The energy efficiency of conversion of glycerol to hydrogen is around 44%. Although 82% of the energy carried by the produced hydrogen is supplied by glycerol and 18% of the energy originates from the electrical energy, the reduction in electricity consumption is only 66% not 82%. The difference arises from the overpotentials of the cell which means some of the electrical energy does not contribute to the chemical reaction. Based on the findings of others [16]
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and the fact that there are limited possibilities for side reactions, we believe that the cathode coulombic efficiency will be close to 100%. At this stage, the level of glycerol conversion to CO2 is unknown and the subject of on-going work.
3.3.
Cell stability and possible poisoning
To assess the stability of the electrolysis cell operating under glycerol reforming conditions, the current was fixed at 5 mA cm2 for 30 min (Fig. 4). For the cell operating on 8.5 mol L1 glycerol the cell voltage increased by around 0.23 V over this time and at 2 mol L1 glycerol the increase was 0.06 V over a similar time period. Based on the cyclic voltammetry measurements, we do not believe this instability is caused by corrosion of our anode electrocatalyst, as repeated cycling revealed no changes in the voltammetric currents observed. Corrosion of the cathode electrocatalyst is equally unlikely. One possible explanation for the cell voltage increase, and therefore loss in performance, would be poisoning of the catalyst by glycerol or its oxidation products. It was found that a short reversal in current (1 mA cm2, 30 s) had the effect of reducing the cell voltage by about 0.08 V on returning to the original current (5 mA cm2), a behaviour which seems to point to a poisoning effect. We also confirmed that the hydrogen evolution reaction (HER) is not inhibited by glycerol (in case of glycerol cross-over), by measuring the HER polarisation curve in 0.5 mol L1 H2SO4 with and without 1 mol L1 glycerol on a 99.99% pure platinum wire, with no differences observed. This indicates that the anode may be the cause of the instability, by slowly being poisoned by glycerol or its oxidation products, with these poisons able to be removed upon cathodic polarisation. Further work is needed to confirm this proposition. A different electrocatalyst may be able to be used to reduce this poisoning effect.
3.4.
Cell resistance
We used electrochemical impedance spectroscopy to determine the cell resistance, a key factor in the performance of
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a PEM cell. The resistance (found at high frequency) in the presence of 2 or 8.5 mol L1 glycerol was found to be higher than when only water was circulated through the cell (300 mU cm2 vs 60 mU cm2). Normally this resistance is dominated by the ionic resistance of the membrane, added to the contact resistances between the cell hardware, electrodes and membrane. In alcohol solutions the membrane can swell which may influence the proton conduction and the membrane–electrode contact resistances [24], however, with glycerol Nafion swelling is believed to be minimal [8]. Instead, as the water activity within the membrane is a contributing factor to proton conduction, the decrease in water activity due to the presence of glycerol may reduce conductivity. Further work needs to be conducted to confirm the effect of glycerol on Nafion conductivity. The total cell resistance did not change over the period of the stability test.
4.
Conclusions
A simple PEM electrolysis cell has been used to electrochemically reform glycerol to hydrogen gas at a cell voltage lower than that for water electrolysis. Hydrogen can be produced at around 10 m3 H2 day1 m3 (reactor volume basis) at an energy consumption of 1.1 kW h m3 H2. Compared to water electrolysis in the same cell, a total energy saving of 2.1 kW h N m3 H2 is possible, equating to a 66% reduction in electrical energy consumption. The reaction kinetics are slow and it is likely that poisoning of the anode electrocatalyst occurs over time. By developing better electrocatalysts, a more stable and efficient cell should be possible. The electrochemical reforming of glycerol solutions in a PEM electrolysis cell has promise as a new hydrogen production method.
Acknowledgments The authors would like to acknowledge the financial support from the Foundation for Research, Science and Technology, NZ.
references
Fig. 4 – Stability of the cell voltage at 5 mA cmL2 and 70 8C with 8.5 mol LL1 glycerol circulating through the anode compartment.
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