Energy & Chemicals from Renewable Resources by ...

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Journal of The Electrochemical Society, 161 (7) D3032-D3043 (2014) 0013-4651/2014/161(7)/D3032/12/$31.00 © The Electrochemical Society

JES FOCUS ISSUE ON ELECTROCHEMICAL PROCESSING AND MATERIALS TAILORING FOR ADVANCED ENERGY TECHNOLOGY

Energy & Chemicals from Renewable Resources by Electrocatalysis M. Bellini,a M. Bevilacqua,a,∗ M. Innocenti,a,b,∗ A. Lavacchi,a,∗ H. A. Miller,a,∗ J. Filippi,a A. Marchionni,a W. Oberhauser,a L. Wang,a,c,∗∗ and F. Vizzaa,z a ICCOM-CNR, Sesto Fiorentino (FI) 50019, Italy b Dipartimento di Chimica, Universit` a di Firenze, Sesto Fiorentino (FI) 50019, Italy c Department of Chemical and Pharmaceutical Sciences, University of Trieste, Trieste

34127, Italy

The selective production of chemicals from renewable resources with contemporaneous release of energy is perhaps one of the most desired targets of sustainable chemistry. Here, we report an overview of our recent research efforts, where we have demonstrated that this can be achieved using renewable alcohols, by means of two electrochemical devices: direct fuel cells and electrolyzers. In either case, an aqueous solution of the fuel in the anode compartment is oxidized on a nanostructured electrocatalyst that promotes selectively the partial oxidation of the anolyte with high stability and fast kinetics. We have found that anode electrocatalysts based on nanosized Pd particles, alone or promoted by Ni-Zn phases as well as by CeO2 or TiO2 , are able to accomplish this goal in alkaline environment when used in conjunction with commercially available cathode electrocatalysts and solid or liquid electrolytes. In an electrolyzer, containing an anode electrocatalyst similar to that employable in a DAFC, the electrolyte may be either an anion exchange-membrane or a solution of an alkali metal hydroxide (NaOH or KOH, for example) and the alcohol is converted to the corresponding alkali metal carboxylate, while hydrogen gas is produced at the cathode upon water reduction. © 2014 The Electrochemical Society. [DOI: 10.1149/2.005407jes] All rights reserved. Manuscript submitted January 23, 2014; revised manuscript received February 26, 2014. Published March 8, 2014. This paper is part of the JES Focus Issue on Electrochemical Processing and Materials Tailoring for Advanced Energy Technology.

DAFCs are attracting increasing interest as power sources for portable applications due to some unquestionable advantages over analogous devices fed with hydrogen.1–3 Alcohols, such as methanol, ethanol, ethylene glycol and glycerol, exhibit high volumetric energy density and their storage and transport are much easier as compared to hydrogen. On the other hand, the oxidation kinetics of any alcohol are much slower and still H2 -fuelled polymer electrolyte fuel cells (PEMFCs) exhibit superior electrical performance as compared to DAFCs with comparable electroactive surface areas. Increasing research efforts are therefore being carried out to design and develop more efficient anode electrocatalysts for DAFCs. The most common DAFC is the direct methanol fuel cell (DMFC) of which there exist commercial devices with power densities spanning from a few watts to 100 W. The large majority of DMFCs, either monoplanar cells for laboratory testing or commercial stacks, operate in acidic media with anode catalysts containing Pt and make use of solid electrolytes constituted by proton exchange membranes of the Nafion family. These DMFCs, however, suffer a number of disadvantages: slow oxidation kinetics, CO poisoning of the Pt-based catalysts, extensive methanol cross-over, degradation of the membrane and corrosion of the carbon materials and cell hardware. As a result, i) the fuel utilization and the cell voltage are lower than expected, ii) high catalyst loadings are required for long lasting applications and iii) the rather toxic methanol could be released into the atmosphere. Overall, these drawbacks are boosting research aimed at using other alcohols as fuels in DAFCs. Indeed, several higher molecular weight alcohols and polyalcohols have high solubility in water, low toxicity, high boiling points, high specific energy and the capacity of some of them to be renewable. Included in this group are ethanol, ethylene glycol (EG), glycerol (G) and 1,2-propanediol. Ethanol can be produced on a large scale from biomass feedstocks originating from agriculture (first-generation bio-ethanol), and forestry and urban residues (second-generation bio-ethanol). Ethylene glycol has a volumetric energy density of 5.9 kWh L−1 and can be produced by heterogeneous hydrogenation of cellulose derivatives.4–6 Glycerol has a volumetric energy density of 6.3 kWh L−1 and is a by-product of biodiesel production. Glycerol is also inexpensive (0.3 US$ kg−1 )

∗ ∗∗ z

Electrochemical Society Active Member. Electrochemical Society Student Member. E-mail: [email protected]

and widely available (2.4 million tons per year). Moreover, the increasing demand for methylesters as fuel additives is expected to increase the G production, thus leading to further cost reductions.7 It is also important to stress that some EG and G oxidation products, such as glycolic acid, oxalic acid, glyceric acid, tartronic acid, mesoxalic acid, and 1,3 dihydro-acetone, currently produced by costly and non-environmentally friendly processes, are valuable compounds that serve as versatile building blocks for the synthesis of a variety of fine chemicals and bio-sources of polymers. Indeed, G oxidation reactions are generally performed with stoichiometric oxidants, such as permanganate, nitric acid or chromic acid, and are characterized by low yields and poor selectivity.8 Fermentation processes exhibit higher oxidation selectivity, but have a low conversion rate. Alternatively, one may apply heterogeneous catalytic processes with molecular oxygen as oxidant, but these processes are generally characterized by harsh conditions and low selectivity, and as a consequence low sustainability.9–13 Glycerol, can be transformed by dehydration/hydrogenation reactions,14 into 1,2-propanediol which is an effective intermediate for the production of lactic acid. Lactic acid can be used as a building block for the development of degradable biopolymers15,16 and currently is used mainly as a fine chemical, with application in cosmetic, textile and food industries. Notable efforts are therefore being carried out to design new catalytic structures for DAFC anodes that do not contain platinum or contain tiny amounts of this rare metal and, most of all, are able to oxidize primary and secondary alcohols with fast kinetics and tolerable deactivation. Within this context, DAFCs operating in alkaline media with solid electrolytes constituted by anion exchange-membranes (AEMs) can provide a number of advantages, especially in view of recent successful developments in the design and production of AEMs. The drawback of traditional alkaline fuel cells that undergo electrolyte carbonation is, in fact, strongly minimized by the use an anion conductive polymeric membrane, while the advantages of operating in alkaline conditions are manifold and include: a) usability of both noble and non-noble metals as electrocatalysts; b) Improved kinetics at both cathode and anode, in particular low anodic over-voltages for alcohol oxidation; c) alcohol cross-over from anode compartment to cathode compartment is reduced by electro-osmotic drag of hydrated hydroxyl ions; d) easier water management as water is formed at the anode side where an aqueous solution already exists, while the electro-osmotic drag transports water away from the cathode preventing its flooding;

Downloaded on 2014-04-27 to IP 193.146.32.73 address. Redistribution subject to ECS terms of use (see ecsdl.org/site/terms_use) unless CC License in place (see abstract).

Journal of The Electrochemical Society, 161 (7) D3032-D3043 (2014)

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Figure 1. Spontaneous deposition of Pd onto Ni-Zn/C.

e) reduced risk of materials corrosion, including catalysts and carbon materials; f) reduced adsorption of spectator ions that might limit electrocatalysis; g) immunity toward peroxide radical attack. In traditional water splitting electrolyzers, hydrogen is generated at the cathode and oxygen at the anode. The standard reaction potential for this process at low temperatures is 1.23 V, meaning that water splitting is a strongly uphill reaction. In practice to get current densities of hydrogen production in the range of 1 A cm−2 the cell potential is usually between 1.6 and 2 V. Considering 1.8 V as a reasonable average, the 68.3% of the energy input of an electrolyzer is consumed by thermodynamics, while kinetic factors account for only 31.7%. Replacing oxygen evolution at the anode with the oxidation of much more readily oxidizable species leads to a significant decrease of the potential required to produce hydrogen. Among them, biomass derivate compounds such as ethanol, EG, G and 1,2 propandiol should be the most promising. By the use of such compounds electrolysis can be performed at potentials much lower than 1 V, leading to electrical power savings as compared to conventional electrolytic water splitting. DAFC Development Pd-(Ni-Zn)/C anode electrocatalysts.— A crucial role in achieving the selective partial oxidation of alcohols to carboxylic compounds, with fast kinetics, is played by the anode electrocatalyst. In general, all Pd-based catalysts in strongly alkaline media (pH > 13) are able to promote the partial oxidation of (poly)alcohols to the corresponding (poly)carboxylates.17–19 We have synthesized particular catalysts that combine high selectivity with high activity and durability. These catalysts consist of original architectures where nanosized Pd is combined with nanosized Ni-Zn alloys.20 The synthetic protocol to these catalysts involves the reaction in water between a Pd(IV) salt and Ni-Zn or Ni-Zn-P alloys supported on a conductive carbon material such as Vulcan XC-72. The process is actually a redox transmetalation reaction (Figure 1) that allows for the obtainment of structurally and morphologically unique materials, denoted as Pd-(Ni-Zn)/C and Pd(Ni-Zn-P)/C (C = Vulcan), where the noble metal Pd is diluted but not alloyed with nickel. The characterization of these materials by HRTEM, EXAFS, XRPD and XANES techniques has shown their surface to contain small (0.5–1 nm), highly dispersed and crystalline aggregates of Pd and Ni clusters as well as single Pd sites. Cyclic voltammetry and chronopotentiometric tests have demonstrated that Pd-(Ni-Zn)/C outperforms any known anode electrocatalyst for the oxidation of alcohols in alkaline environment, especially in terms of onset oxidation potential (as low as −0.6 V vs. Ag/AgCl/KClsat ) and specific current densities which can be higher than 3600 A gPd −1 . A simplified working scheme of the DAFC developed is shown below (Figure 2). The corresponding Membrane Electrode Assembly (MEA) comprises a Pd-(Ni-Zn)/C anode supported on Ni foam, proprietary FeCo/C cathode supported on carbon cloth and a Tokuyama A201 anion-exchange membrane. On the cathode each oxygen molecule in conjunction with two water molecules is reduced to 4 OH− ions by the electrons coming from anode side (Eq. 1). The hydroxyl groups move, through the anion-exchange membrane, to the

anode where they are consumed in the alcohol oxidation reaction. By virtue of the intrinsic chemical properties of these Pd-based anode electrocatalysts, the alcohol oxidation process terminates at the stage of carboxylic acid formation. For instance, ethanol is selectively transformed into acetic acid, which in turn is converted to acetate by the excess of OH− groups in the solution (Eq. 2). The optimum concentrations of ethanol and alkali metal hydroxide that give 100 percent selectivity in acetate are 10 wt% and 2 M, respectively, as indicated by an in situ FTIR spectro-electrochemical investigation.21 O2 + 2 H2 O + 4 e− → 4 OH− C2 H5 OH + 5 OH− → CH3 COO− + 4 H2 O + 4 e−

[1] [2]

Under these conditions, a passive air-breathing direct ethanol fuel cell (DEFC) can deliver power densities as high as 58 mW cm−2 at 22◦ C (Figure 3a), while up to 170 mW cm−2 can be obtained by an active cell at 80◦ C (Figure 3b).22,23 In either case, ethanol is selectively converted to acetate. Galvanostatic experiments at 60◦ C in active cells showed the preservation of selectivity for hundreds of hours with a tolerable polarization increase (less than 15 percent after 300 hours). It is worth recalling two important points: the large majority of acetic acid is industrially produced by the CATIVA or MONSANTO processes24,25 where expensive and limited in nature iridium or rhodium catalysts promote the carbonylation of methanol by CO in the presence of methyl iodide (all reagents derived from fossil fuels); the present DEFCs produce sodium or potassium acetate that are indeed higher value products as compared to acetic acid. The power density released by a device which combines four airbreathing mono-planar (5 cm2 ) DEFCs of the type described above is sufficient to charge the battery of a portable phone (Figure 4). From the values reported in Figure 4, it is apparent that DEFC devices can potentially provide power densities up to 100 W with a relatively low number of monoplanar units.

Figure 2. Simplified functioning scheme of a fuel cell of the DAFC type used for the partial oxidation of alcohols and the concomitant production of electric energy.


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Scheme 1. Proposed mechanism for the selective conversion of ethanol into acetate on Pd-based electrocatalysts in alkaline media (pH > 13).

Figure 3. Polarization and power density curves provided by DEFCs fuelled with a 2 M KOH solution of ethanol (10 wt%): (a) air-breathing system at 20◦ C; (b) active system, 4 mL min−1 EtOH, O2 flow 200 mL min−1 . In either case, the MEA (5 cm−2 ) was composed by a Pd–(Ni–Zn)/C anode (Ni foam), Proprietary Fe–Co cathode (carbon cloth) and Tokuyama A201 membrane. Pd loading 1 mg cm−2 . The inset of b reports the temperature of fuel (left), cell (central) and oxygen gas (right).

Theoretical and experimental studies carried out in our laboratories have rationalized the observed chemoselectivity of ethanol oxidation on Pd-based electrocatalyst promoted by Ni-Zn alloys. In particular, it has been suggested that the combination of Pd and Ni favors the coupling of the acetyl group on the catalyst surface with surface hydroxyl groups (Scheme 1).

Replacing ethanol with EG decreases the cell performance in DEGFCs (Direct Ethylene Glycol Fuel Cell), yet a significant power density of 24 mW cm−2 at 0.17 V is still obtained (Figure 5). On a single fuel load, the DEGFC containing the Pd-(Ni-Zn)/C anode continued to deliver constantly 20 mA cm−2 for 10.2 h, yielding glycolate (55.4%), oxalate (37.6%), and carbonate (7.0%) with a total conversion of 77.1% (Figure 6). Independent experiments show that oxalate is not oxidized to an appreciable extent on Pd-(Ni-Zn)/C, whereas glycolate is readily oxidized to give oxalate and carbonate. Polarization and power density curves obtained with passive DGFCs (Direct Glycerol Fuel Cell), containing a Pd-(Ni-Zn)/C anode, a FeCo/C cathode, and a Tokuyama A201 anion exchange membrane, fueled with 5 wt% G in a 2M KOH solution and at room temperature have also been obtained. In this case, under comparable conditions, the DGFC containing Pd-(Ni-Zn)/C exhibits a power density of 16.5 mW cm−2 (Figure 5). To the best knowledge of the authors this is the most efficient electrocatalyst reported so far for the oxidation of G at room temperature. A galvanostatic experiment performed at 20 mA cm−2 , without refueling, showed that the DGFC released 578 J of energy (Figure 6). The following product distribution was determined by NMR and ionic chromatography (IC): glycerate (27.2%), tartronate (28.1%), glycolate (4.2%), oxalate (14.1%), formate (2.4%), and carbonate (24.0%), with a total conversion of 55.5%. The oxidation of G is more complex than that of EG. Scheme 2 illustrates the possible reaction mechanisms proposed for G oxidation on Pd, Pt and Au based anode electrocatalysts. G is first oxidized to glyceraldehyde, which in turn is quickly oxidized to glycerate in a subsequent two electron transfer step (path a).

Figure 4. Polarization and power density curves of a four air-breathing DEFC device and its use to charge a cellular phone battery. Downloaded on 2014-04-27 to IP 193.146.32.73 address. Redistribution subject to ECS terms of use (see ecsdl.org/site/terms_use) unless CC License in place (see abstract).


Figure 5. Potentiodynamic and power density curves at 25◦ C for passive DEGFCs and DGFC containing the Pd-(Ni-Zn)/C anode electrocatalyst and fueled with 2 M KOH and 5 wt% EG or 5 wt% G solutions. The cathode (Fe-Co/C) was exposed to an oxygen atmosphere. Scan rate: 5 mV s−1 .

Glycerate is further oxidized to tartronate (path a2 ) and, by cleavage of C-C bond, into glycolate and formate (path b1 ). The latter species leads to carbonate and glycolate is oxidized to oxalate (path a4 ).1–3,21,26–28 G is also oxidized directly to tartronate (path b) by chelating adsorption on catalytic surfaces. However, the oxidation of G yields significant amounts of carbonate. Since oxalate is very slowly oxidized on Pdbased electrodes,29–31 CO2 is prevalently a by-product of the oxidation of either glycerate or glycolate (paths a1 and a3 ). Notably, no trace of dihydroxyacetone, hydroxypiruvate, or mesoxalate was detected at any stage of the galvanostatic experiments. This evidence allows us to rule out any oxidation of the secondary alcohol in G (path c). The oxidation of the secondary alcohol of G

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Figure 6. Galvanostatic curves (at 20 mA cm−2 ) for the passive DEGFC and DGFC containing the Pd-(Ni-Zn)/C as anode electrocatalyst and fueled with 2 M KOH 5 wt% EG or 5 wt% G solutions.

has been reported by Koper et al.28 on a Pt electrode, but it was not observed on an Au electrode. Li et al.32–34 have demonstrated that this can occur in DGFCs containing nanostructured anodic catalysts such as Pt/C and Au/C in which both the product selectivity and Faradic efficiency depends on the working voltages of the fuel cell. Moreover, Coutanceau et al.,7 observed the formation of dihydroxyacetone on Pt/C, Pd/C and Au/C, although hydroxypyruvate was only detected on a Au catalyst. In conclusion, the size, dispersion, and morphology of the Pd clusters as well as the presence of single Pd sites may well account for the excellent electrocatalytic performance of Pd-(Ni-Zn)/C which, as a whole, is a better catalyst than Pd/C, where the Pd particles are larger, less dispersed, and quite amorphous. In view of the CV experiments in KOH solution with electrodes coated with only the Ni-Zn/C sup-

Scheme 2. Main oxidation products of glycerol oxidation. Downloaded on 2014-04-27 to IP 193.146.32.73 address. Redistribution subject to ECS terms of use (see ecsdl.org/site/terms_use) unless CC License in place (see abstract).

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port, prior to and after addition of alcohols, any direct role of the Ni support in the ethanol oxidation reaction at the potentials shown by Pd-(Ni-Zn)/C seems highly unlikely. However, one cannot exclude the existence of a co-catalytic effect of the Ni support on the Pd-catalyzed alcohol oxidation reaction. Indeed, several researchers have observed that the presence of Ni has a beneficial effect on the electrooxidation of alcohols by late transition metals in either acidic or alkaline media. As for Pd catalysts, it has been reported that the addition of NiO increases remarkably the activity and stability of carbon-supported Pd nanoparticles for the electrooxidation of ethanol in alkaline media. Although no clear-cut explanation has been offered so far, it is generally believed that a key role should be played by the capacity of Ni and NiO to generate surface NiOH moieties at low potentials. Increasing the amount of OHads on the catalyst surface would actually favor the formation of acetate by coupling with CO(CH3 )ads . In turn, the greater oxophilicity of Ni as compared to Pd and the higher binding affinity of Ni toward the acetate ions may facilitate acetate desorption from the active Pd sites, thus accounting for the higher stability of the Pd-(Ni-Zn)/C catalyst even at high alcohol conversion. Pd–CeO2 /C anode electrocatalysts.— In terms of the energy efficiency of DEFCs one of the main disadvantages of using Pd based electrodes in strongly alkaline media is the incomplete oxidation to acetate. (Eq. 3).1–3,22,26 Accordingly, each ethanol molecule can release a maximum of four electrons, instead of the twelve electrons theoretically obtainable from the total oxidation to carbonate (Eq. 4). C2 H5 OH + 5OH− → CH3 COO− + 4H2 O + 4e−

[3]

C2 H5 OH + 16OH− → 2CO3 −2 + 11H2 O + 12e−

[4]

Conversely, it is worth stressing that the drawback represented by the low faradaic efficiency is largely compensated by the generally fast oxidation kinetics, which leads to high current densities. The selectivity toward the formation of acetate (Eq. 3) on one hand is an advantage if we may consider that acetate has a higher commercial value than ethanol. The recent discovery that the acetate ion can be converted to ethanol in a renewable way by electro-bioreduction,35,36 thus closing the fuel production/transformation cycle, offers a further strong argument in favor of the use of Pd-based electrocatalysts for DEFC anodes. In addition to the low faradaic efficiency, Pd-based electrocatalysts exhibit a second drawback: at anode potential values higher than 0.6 V vs. reversible hydrogen electrode (RHE) in half cells and 0.15 vs. RHE in monoplanar DEFCs, the surface adsorbed Pd-OHads species (Eq. 5), active for the oxidation of ethanol, starts converting into inactive Pd-O37,38 according to Equations 6 and 7. As a result, the number of active sites on the electrode surface decreases, and fuel conversion is slowed down and ultimately inhibited. Pd + OH− → Pd − OHads + e−

[5]

Pd − OHads + OH− → Pd − O + H2 O + e−

[6]

Pd − OHads + Pd − OHads → Pd − O + H2 O

[7]

The extent of Pd oxidation to Pd-O on DEFC anodes has been recently minimized by adding a small amount of NaBH4 into the anolyte compartment as such a reagent is able to reduce back Pd-O to Pd.38 Recently, we proposed an alternative way to enhance the capacity of Pd nanoparticles to oxidize ethanol in alkaline media.39 Our approach focused on the process described in Equation 5 and, in particular, is aimed at anticipating the oxidation of Pd0 to PdI -OHads . We have found that an effective material to accomplish this goal is ceria (CeO2 ). CeO2 is a mixed conductor, showing both electronic and ionic conduction, with many applications in catalysis in conjunction with transition metals.39

We reported the first examples of direct ethanol fuel cells containing anode electrocatalysts made of Pd nanoparticles supported on ceria (Pd–CeO2 /C).38 A comparison with a standard anode electrocatalyst containing Pd nanoparticles (Pd/C) has shown that, at the same metal loading and experimental conditions, the energy efficiency of a DEFC assembled with the Pd–CeO2 /C anode electrocatalyst is twice as much as that supplied by the cells with the Pd/C electrocatalyst. In fact, an efficiency of 6% was found for the DEFC equipped with the Pd/C electrocatalyst and an efficiency of 12.2% for the DEFC containing Pd–CeO2 /C as anode electrocatalyst. A tentative explanation for such a dramatic enhancement of the performance has been proposed through a cyclic voltammetry study that has shown that the mixed carbon-ceria support contributes to the remarkable decrease of the onset oxidation potential of ethanol. It was proposed that ceria promotes the formation at low potentials of species adsorbed on Pd, PdI -OHads , that are responsible for ethanol oxidation. Monoplanar passive and active DEFCs using the Pd-CeO2 /C anode and fed with aqueous solutions of 10 wt% ethanol and 2 M KOH, supplied power densities as high as 66 mW cm−2 at 25◦ C and 140 mW cm−2 at 80◦ C. Pd/(TiO2)-nanotube anode electrocatalysts.— One of the most important issues regarding the anode and cathode electrodes of DAFCs is the high loading of precious metals required to obtain acceptable performance. In order to increase the mass specific activity of nanostructured palladium electrocatalysts we have carried out research aimed at increasing both the electrochemically active surface area (EASA) and the number and nature of active sites on the Pd nanoparticle surface.40 Electrochemical methods have been developed which allow a nanoengineering of the metal particles terminated with high-index facets (HIFs) having a high density of low-coordinated surface atoms. The exploitation of the catalytic properties of the HIFs depends on the availability of methods capable of generating supported NPs with both high-index terminations as well as controlled particle size and metal loading. The contemporaneous control of these parameters is a major challenge for catalytic and electrocatalytic nanotechnology. We reported a novel method for the modification of metal nanoparticles (NPs), denoted as electrochemical milling and faceting (ECMF), by which large Pd NPs (35 nm) with low-index facets supported on titanium dioxide nanotube arrays (TNTA) can be milled into small NPs (7 nm) with both HIFs and a high density of step atoms. By this approach, the catalytic activity of supported Pd NPs was enhanced by an order of magnitude for the electrooxidation of ethanol, and was even three times higher than the highest activity reported so far in the literature. In detail the method consists in first depositing palladium nanoparticles onto a titania nanotube array (TNTA). The choice of the TNTA support was made in view of its robustness to electrochemical treatments as well as the possibility to precisely control the TNTA structure (anatase) and morphology. In particular, nanotubes with a diameter of 80 nm and a length of 2.0 mm have been prepared. The TNTAs were impregnated with palladium chloride which was then reduced with sodium borohydride (Figure 7A). A two-step square wave electrochemical treatment was then applied. In the first “heavy” step a palladium oxidation was applied at 4.55 V (vs. RHE) for 180 s, followed by the reduction of the Pd oxides at −1.95 V (vs. RHE) for 180 s (Fig. 7B). This was followed by a milder treatment with a frequency of 0.025 Hz for 3 h between +3.35 and −0.75 V (vs. RHE) (Fig. 7C). The overall treatment resulted not only in a net reduction in mean particle size to 7 nm particles (milling) but also the formation of HIFs, multiple twins and a high density of step atoms (Faceting). Analysis showed the presence of high index facets {210} and {410} along the 100 direction, and {211} and {311} facets along the 110 direction, respectively (Fig. 8). Cyclic voltammetry using an ethanol containing electrolyte showed a peak current density of 201 mA cm−2 (Figure 9, curve 3), corresponding to a normalized mass-specific activity for Pd of 11167 A g−1 . A value remarkably higher than those reported in the literature



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Figure 7. TNTAs with as-deposited Pd and a) the corresponding SEM image. B) TNTAs with Pd after heavy ECMF and b) the corresponding SEM image. C) TNTAs with Pd after heavy and mild ECFM and c) the corresponding SEM image. The white scale bars in (a–c) are 200 nm. Reprinted from reference 40 with permission from John Wiley and Sons.

determined under comparable conditions (e.g. 3600 A g−1 ).20 The onset potential for the oxidation of ethanol shifted 0.17 V more negative than the potential obtained for the as-deposited sample. This material is currently being employed in direct ethanol fuel cells (DEFCs) as well as for hydrogen production by aqueous ethanol electrolysis.41 An analogous treatment applied directly to a polycrystalline Pd electrode lead to an increase in activity for alcohol electrooxidation which was attributed to a combination of increased electro-active surface area and an increased concentration of low coordination palladium atoms.42 We have shown that the electrochemical milling and faceting is a promising technique which shows several advantages compared to conventional direct current electrodeposition and to the conventional

chemical deposition methods (e.g. impregnation, spontaneous deposition, electroless etc.). Indeed, the key feature of this new technique is that nucleation and growth of the metal particles on the substrate can be controlled separately by varying the potential pulse amplitudes and durations providing control in metal particle size, adhesion with the substrate, uniformity of the distribution and metal particles having high index facets and low coordination surface atoms. This new synthetic method could be extended to a wide range of catalysts to increase their catalytic activity and simultaneously decrease the metal loading. OrganoMetallic fuel cells that convert renewable alcohols into energy and chemicals.— As described above we have demonstrated

Figure 8. TEM image of the Pd-loaded TNTA electrode after heavy and mild ECMF (scale bar = 50 nm). b) Pd nanoparticles found in the electrolyte after heavy and mild ECMF (scale bar = 35 nm). c) HRTEM image (scale bar = 2 nm) and d) atomic models with face assignment of the TNTA-supported Pd nanoparticle along the 100 direction. e) HRTEM image (scale bar = 2 nm) and f) face assignment of the TNTA-supported Pd nanoparticles along the 110 direction. Reprinted from reference 40 with permission from John Wiley and Sons. Downloaded on 2014-04-27 to IP 193.146.32.73 address. Redistribution subject to ECS terms of use (see ecsdl.org/site/terms_use) unless CC License in place (see abstract).

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Figure 9. Cyclic voltammograms of TNTA-Pd in 2 M KOH with 10 wt % EtOH. Scan rate: 50 mV s−1. Curve 1: TNTA-Pd as deposited. Curve 2: TNTA-Pd after heavy ECMF. Curve 3: TNTA-Pd after heavy and mild ECMF. Reprinted from reference 40 with permission from Jon Wiley and Sons.

that industrially relevant feedstocks (such as carboxylic acids) can be obtained using DAFCs. These devices convert the free energy of an alcohol into electrical energy and the alcohol itself is transformed into an oxidation product or products which are different from CO2 . Two types of DAFCs have been developed for this purpose: 1) devices such as those described in section 2, where the anode and cathode are separated by an anion-exchange membrane and are coated with an electrocatalyst, generally a nanostructured noble metal, supported on

a conductive material and 2) enzymatic biofuel cells (EBFC) utilizing oxidation enzymes such as dehydrogenases in conjunction with an electron transfer mediator (Figure 10).43,44 Figure 10a shows a simplified working scheme of a DAFC where alcohols (in this case ethanol) are selectively converted into carboxylates (RCOO− ) where the electrolyte is an anion-exchange membrane. Typical power density curves at 25◦ C for air-breathing cells fueled with aqueous solutions of ethanol, ethylene glycol or glycerol are shown in Figure 10b. Recently, in collaboration with Grützmacher’s research group at ETH Zurich, we have developed another type of fuel cell, known as an OrganoMetallic Fuel Cell (OMFC), operating in alkaline media where the anode catalyst is an organometallic complex.45 In this device, the organometallic complex evolves through fast chemical equilibria to form specific catalysts for ethanol dehydrogenation, aldehyde dehydrogenation and H+ /electron transfer. The idea of using an organometallic complex as an anode was developed after Grützmacher et al.46–48 noted a study in which the organometallic complex [Rh(trop2 N)(PPh3 )] 1 (Scheme 3) was shown to catalyze the oxidation of primary alcohols to give carboxylic acid derivatives in the presence of hydrogen acceptors such as RR’C=O, olefins, or palladium nanoparticles in either homogeneous or heterogeneous phase. The amido complex 1 is simply accessed by deprotonation of the precursor complex [Rh(OTf)(trop2 NH)(PPh3 )] 2 under basic conditions (OTf = trifluoromethanesulfonate). Subsequently, it has been shown that the [Rh(OTf)(trop2 NH)(PPh3 )] complex 2 can be deposited intact onto Vulcan XC-72 (C), a conductive carbon support that is often utilized for the preparation of electrocatalysts. A MEA was fabricated for a fuel cell comprising

Figure 10. a) Working scheme of a DAFC in alkaline environment. b) Power density curves of air-breathing DAFCs fueled with ethanol (10 wt%), ethylene glycol (5 wt%) or glycerol (5 wt%) in 2 M KOH at 22◦ C. Anode: Pd-(CeO2 )/C on Ni mesh; cathode: Fe-Co/C on carbon paper; membrane Tokuyama A201. c) Ethanol/O2 enzymatic biofuel cell, where alcohol dehydrogenase and aldehyde dehydrogenase catalyze a stepwise oxidation of ethanol to acetaldehyde and then to acetate, passing electrons to the anode via the mediator NAD+ /NADH. d) Power density of the cell operating in buffered solution at pH 7.15, containing 1 mM ethanol and 1 mM NADH. Downloaded on 2014-04-27 to IP 193.146.32.73 address. Redistribution subject to ECS terms of use (see ecsdl.org/site/terms_use) unless CC License in place (see abstract).


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Scheme 3. Acceptor-assisted catalytic oxidation of primary alcohols with water to give carboxylates; Reproduced from reference 45 with permission from John Wiley and Sons.

a nickel foam anode coated with the complex deposited onto Vulcan, 2@C (ca. 1 mg cm−2 of rhodium), a carbon-paper cathode coated with a FeCo/C electrocatalyst and a Tokuyama A201 anion-exchange membrane. The anode compartment was filled with 10.5 mL of a water solution of ethanol (10 wt%) and 2 M KOH. Figure 11 (blue trace) shows the polarization and power density curves of this passive cell recorded at 22◦ C. A maximum power density of 7 mW cm−2 was obtained, which is far higher than that of any biofuel cell, yet lower than that observed with a DAFC equipped with palladium-based anodes as described above. The power density supplied by the OMFC increases remarkably by increasing the working temperature of the MEA, in an active cell under control of the oxygen and fuel fluxes. Indeed, 24 mW cm−2 was obtained at 60◦ C with a fuel flow of 4 mL min−1 and an oxygen flow of 0.2 L min−1 (Figure 12, red trace) Such a value is still lower than that obtainable with the best palladium based anode electrocatalysts reported to date (Pd-(CeO2 )/C),38 yet it falls in the upper range of power densities

Figure 11. Polarization and power density curves of OMFCs fueled with 10 wt% ethanol in 2 M KOH (anode: [Rh(OTf)(trop2 NH)(PPh3 )] on Ni mesh; cathode: anode, Fe-Co/C on carbon paper; Tokuyama A201 membrane (airbreathing OMFC at 22◦ C); active OMFC at 60◦ C (fuel flow: 4 mL min−1 ; oxygen flow 0.2 L min−1 , active cell). Reprinted from reference 45 with permission of John Wiley and Sons.

Figure 12. Parallel electrolyzers: (left hand side) 2 M KOH electrolysis at 0.2, 0.5 and 1 A (Pt/C electrodes and Tokuyama A201 membrane); (right hand side) ethanol 10 wt% in 2 M KOH electrolysis at 0.2, 0.5 and 1 A (Pd-(Ni-Zn)/C anodes, Pt/C cathodes and Tokuyama A201 membrane.

produced by the vast majority of DAFCs containing nano-sized noble metal electrocatalysts. The passive OMFC was subjected to galvanostatic experiments at low current intensities. After running for 44.3 h, 14.4 mmol of potassium acetate was produced, which corresponds to 48% conversion of the starting ethanol. Experimental evidence (i.e NMR, XRPD, CV, etc.) led us to propose the mechanism of ethanol oxidation shown in Scheme 4. On the electrode surface, the precursor 2@C is rapidly converted into the hydroxo complex, [RhOHeq (trop2 NH)(PPh3 )]@C (4@C), which is in a rapid equilibrium with the amide [Rh(trop2 N)(PPh3 )]@C (1@C) and water. The amide 1@C dehydrogenates ethanol to acetaldehyde; the aldehyde reacts further with OH− to form the acetate ion and the [RhH(trop2 NH)(PPh3 )]@C (3@C). The latter complex is oxidized at the electrode releasing two H+ (neutralized to give water under basic conditions) and two electrons with regeneration of the amide 1@C. There is some resemblance with an enzymatic biofuel cell, but the main characteristic of this system is that one molecular rhodium complex is capable of evolving through fast chemical equilibria in the course of the catalytic cycle to form a specific catalyst for alcohol dehydrogenation (the amide 1@C), a specific catalyst for aldehyde dehydrogenation (the hydroxo complex 4@C), and a specific catalyst for the H+ / electron transfer (the hydride 3@C). The success of the OMFC is based on a series of different properties all related to the molecular complex architecture and its interaction with the carbon conductive support. It is well known that support morphological features, such as surface area and pore distribution, together with the structural presence of different functional surface groups, can strongly affect the complex deposition morphology on carbon (constituting the complete composition of anode catalyst) during the wet impregnation step. Furthermore a crystalline or amorphous phase of the supported molecular complex directly affects all the catalytic reaction steps occurring on the anode surface. An improvement in stability was obtained by changing the carbon conductive support from Vulcan XC-72 (labeled C) to Ketjenblack ED600 (labeled Ck), characterized by surface areas of 254 and 1400 m2 g−1 and pore volumes of about 170 mL/100 g and 500 mL/


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Scheme 4. Proposed mechanism for the ethanol electrooxidation reactions occurring on the anode of the OMFC.

100 g, respectively. Under the same experimental conditions, two OMFC devices using the two different anode catalyst compositions, i.e. [Rh(trop2 N)(PPh3 )OTf]@C (2@C) vs. [Rh(trop2 N)(PPh3 )OTf]@Ck (2@Ck), have shown a fundamental difference: the OMFC based on 2@Ck greatly improved the efficiency of the cell in galvanostatic experiments.49 In preliminary experiments, we have tested other renewable alcohols, and the results obtained with glycerol, ethylene glycol, 1,2propandiol, 1,3 propandiol, 1,4 butanediol and glucose are very encouraging. Well established methods of organometallic synthesis, that lead to well-defined molecular metal complexes, offer enormous advantages in the rational design and optimization of fuel cell catalysts, including a reduced metal loading due to the fact that all metal sites in the catalyst are active. The possibilities and range of applications of OMFC technology may be very large in view of the fact that these molecular metal complexes can be easily embedded in a huge variety of nanosized conductive supports of great relevance in; drug delivery, (electro)catalysis and photocatalysis. Such materials include, functionalized fullerenes, carbon nanotubes, nanofibers and other nanosized matrices, for example titania nanotubes. Consequently, the tuning of well defined molecular architectures and combination with a matching support material, will in the future allow for further development of the selective oxidation of polyalcohols into novel chemicals, under waste-free conditions.

Hydrogen and Chemicals by Electrolysis of Aqueous Solutions of Renewable Alcohols State of the art of hydrogen production by water electrolysis.— Current thermochemical technologies produce hydrogen at a price of 3–5 € kg−1 , mostly due to the high temperature and pressure required and the cost of separation and purification. In addition, environmentally unfriendly by-products, such as NOx e COx are produced. The

DOE, like similar European organizations, has set a goal for the cost of the production of 1 Kg of hydrogen at ca 1.5 €.50 The electrochemical reduction of water represents a significant alternative to the production of hydrogen from fossil fuels and the only route that permits the use of renewable energy sources (photovoltaic, wind, bio-mass, geothermal etc).51,52 It also allows the production of pure hydrogen (up to 99.999%). Water electrolysis or water splitting into hydrogen and oxygen occurs in an electrolyzer (Eq. 8). This is the opposite reaction of what happens in a H2 /O2 fuel cell. 2H2 O + energy → 2H2 + O2

[8]

The electrolysis of water is a well known and consolidated process. However, electrolysis accounts for only a fraction of world hydrogen production, mostly due to a high electrical energy consumption. The efficiency of an electrolyzer is directly related to the electrode materials, in particular of the catalysts deposited onto the surface of the electrode whose role is that of reducing the activation energy for both the anode (oxygen production) and cathode (hydrogen production) reactions. Therefore, a crucial role for the improvement of the effective energetic efficiency of an electrolyzer is played by the electrode materials, given that they determine both the energy consumption (for a given reaction rate) and the maximum reaction rate in the cell. There exist three distinct electrolyzer technologies for hydrogen production: two of which are well established: alkaline electrolyzers (AE), solid polymeric membrane electrolyzers (PME) and solid oxide electrolyzers (SOE). The SOE technology is at present experimental and is designed for use in large reactors at high temperatures, while PME devices are more adapted for the production of hydrogen in small devices, also being able to be used in portable devices at temperatures below 100◦ C. The main difference between AE and PME technologies is due to the fact that the electrolyte in the former is an alkaline solution, generally KOH (25–30 wt%), while in PME devices the electrolyte consists of an ion-exchange polymeric membrane through which pass either the H+ or OH− ions depending upon the type of ionomer used, moving from the anode to the cathode (H+ ) or from



Scheme 5. Chemicals obtained by electrolysis of a 2 M KOH solution containing either; ethanol, G, EG or 1,2-propandiol in a PME electrolyzer with a MEA consisting of Pd-(Ni-Zn)/C on Ni mesh anode, (E-TEK) Pt/C cathode on carbon paper and Tokuyama A201 membrane (5 cm2 active area).

the cathode to the anode (OH− ). The solid membrane is also used to separate the gases that develop at the two electrodes. The advantages of the PME electrolyzers are many: no moving parts, very low volume of corrosive liquid or none, high current densities, production of gas at high pressures, rapid response to applied current and minimal carbonatation of the electrolyte from atmospheric CO2 (with anion exchange membranes). Production of hydrogen and chemicals from renewable alcohols by electrolyzers containing Pd-based anode electrocatalysts.— The standard reaction potential for water electrolysis is 1.23 V, meaning that water splitting is a strongly uphill reaction. In practice to get current densities in the range of 1 A cm−2 the cell potential usually ranges between 1.6 and 2 V.53 Considering 1.8 V as a reasonable average, we conclude that 68.3% of the energy input is consumed by thermodynamics, while kinetic factors account for only 31.7%. Replacing anodic oxygen evolution with the oxidation of much more readily oxidizable species leads to a significant reduction of the potential required to produce hydrogen. Following this strategy compounds such as ammonia,54,55 methanol,56 ethanol,57 and urea58 have been recently tested. As discussed previously, DAFCs are able to promote the selective oxidation of alcohols to the corresponding alkali metal carboxylates in alkaline media. This prompted us to investigate replacing the oxygen

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evolution reaction with the ethanol electrooxidation reaction in a PEM electrolyzer bearing a Pd-(Ni-Zn)/C anode, a commercial (E-TEK) Pt/C cathode on carbon paper and a Tokuyama A201 membrane.41 As shown in Figure 12, electric currents with intensities of 0.2, 0.5 and 1 A were applied to two parallel electrolyzers. On the left hand side, 2 M KOH solutions were introduced into both the anode and cathode compartment. As a result, hydrogen and oxygen were produced at effective potentials of 1.58, 1.89 and 2.44 V, respectively. By simply replacing the 2M KOH solution with a solution of ethanol (10 wt%) in 2 M KOH in the anode compartment of the electrolyzers on the right hand side, hydrogen was evolved at much lower effective potentials: 0.63, 0.75 and 0.85 V, respectively. More importantly, no oxygen evolution occurred as only potassium acetate was produced at the anode. The fact that no oxygen is evolved at the anode side is highly intriguing: a membrane acting as gas separator is unnecessary and simple alkaline electrolyzers containing a concentrated solution of KOH as electrolyte can be equally used for the selective production of hydrogen and alkali metal carboxylate from aqueous solutions of renewable alcohols. The electrolysis of an ethylene glycol solution in 2M KOH at a constant current of 1 A produces hydrogen at the cathode and a 1:1 mixture of potassium glycolate and potassium oxalate at an effective potential of 0.87 V (Scheme 5, Table I). The product distribution obtained by the electrolysis of a 2 M KOH solution of glycerol (10 wt%) at 0.1 A for 15 hours in a PME device with a MEA identical to that utilized for ethanol (vide infra) was much more complex. In fact, besides the expected partial oxidation products glycolate and tartronate, various compounds derived from C-C bond cleavage; glycolate, oxalate, formate and carbonate, are also obtained (Scheme 5). Similar product distributions have been observed in direct glycerol fuel cells equipped with Pd-based anodes described in section 2. When 1,2-propandiol was used in a PEM electrolyzer a highly selective oxidation to potassium lactate (90%), potassium acetate (5%) and CO3 2− (5%) were was obtained (Scheme 5).59 A simplified working scheme of the aqueous ethanol PEM electrolyzer developed by us is shown in Figure 13.60 Table I shows that the energy cost per kilogram of hydrogen is much lower than in classical water electrolysis. In a traditional alkaline electrolytic device, a difference in potential between the electrodes splits water into hydrogen and oxygen according to the anodic and cathodic half reactions 9 and 10: 4OH− → O2 + 2H2 O + 4 e−

anode

Ea ◦ = 0.40 V

[9]

4H2 O+4e− → 2H2 +4 OH−

cathode

Ec ◦ = −0.83 V

[10]

4H2 O → O2 + 2 H2 + O2

overall

E◦ = 1.23 V

[11]

Ethanol-water electrolyzers function in alkaline conditions using Pd-(NiZn)/C anodes with the half cell reaction 12 (the cathodic

Table I. Electrolysis of aqueous solutions of ethanol, ethylene glycol, glycerol and 1,2-propandiol. Substrate (10 wt%) in KOH 2 M

Effective cell potential (@ 160 mA cm−2 )a

H2 O Ethanol Ethylene glycol Glycerol

2.44 V 0.85 V 0.87 V 0.87 V

1,2-propandiol

0.67 V

Productsb H2 /O2 H2 / acetate H2 / glycolate, oxalate (1:1) H2 / glycerate+tartronate (>70%), glycolate, oxalate, formate, carbonate H2 / lactate, acetate, carbonate

Power consumption (kWh) for 1 Kg H2 65.4 22.8 23.3 23.3 18.0

MEA for the PME electrolyzer consists of Pd-(NiZn)/C on Ni foam anode (Pd loading 1 mg cm−2 ), (E-TEK) Pt/C cathode on carbon paper (Pt loading 2 mg cm−2 ) and Tokuyama A201 membrane (5 cm2 active area). b Product analysis by 13 C NMR spectroscopy and HPLC; all compounds are isolated as potassium salts. a The


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Scheme 6. Energy or hydrogen and chemicals from renewable resources by DAFCs and electrolyzers. Figure 13. Simplified working scheme of hydrogen and potassium acetate production from ethanol through the electrolysis of an alkaline ethanol solution on Pd-(Ni-Zn)/C anode and Pt/C cathode with an anion-exchange membrane.

reaction is the same as in Eq. 10):

Conclusions

C2 H5 OH+5OH− → CH3 COO− +4H2 O+4 e− anode E◦ = −0.72 V [12] C2 H5 OH + OH− → CH3 COO− + 2H2

E◦ = 0.10 V [13] The theoretical consumption of energy in an electrolytic ethanol cell (under reversible thermodynamic conditions), calculated at the standard potential of the cell is 2.58 Wh g−1 , much lower that the analogous consumption of a traditional water electrolyzer (33 Wh g−1 ). This means that an aqueous ethanol electrolyzer consumes 86% less energy than a water electrolyser to produce the same amount of hydrogen. The efficiency of an electrolytic cell can be defined by the following equation: ε=

or CO2 ) occurs; iv) the absence of oxygen evolution allows for the production of hydrogen under high pressures.

overall

E◦ E◦ = E E◦ + ηa + ηc + ηiR

Where E◦ represents the cell reversible voltage and E represents the actual measured cell voltage (sum of the reversible voltage and the anodic, cathodic and ohmic overvoltages). By convention, the efficiency of an electrolyzer capable of generating hydrogen can be expressed in kWh kg−1 of H2 . One kg of H2 has a higher heating value of 39.67 kWh, so a traditional water electrolyzer is considered to have 100% efficiency in conversion of electricity into hydrogen when the production of 1 Kg of hydrogen requires 39.67 kWh. According to our experiments, the electrolysis of ethanol (10 wt%) in 2 M KOH can proceed at effective potentials of 0.85 V at a current density of 1.6 kA m−2 with an energy consumption of 22.8 kWh required to produce 1 Kg of H2 . The process also yields 25 Kg of CH3 COOK. The production of 1 Kg of H2 gas would actually provide a profit of 9–10 € kg−1 H2 , based on the average costs of electricity (0.17 € kWh−1 from solar energy), bioethanol (400 € ton−1 ), KOH (600 € ton−1 ) and CH3 COOK (1000 € ton−1 ). Even after purification and separation processes, the overall process should be still highly profitable. Table I and scheme 5 reports relevant data obtained for the electrolysis of aqueous solutions of ethanol, ethylene glycol and glycerol and 1,2-propandiol at room temperature in a PME device containing a Pd-(Ni-Zn/C) anode electrocatalyst. Besides energy savings and potential economic profitability, the present technology offers further advantages: i) renewable alcohols, largely distributed and commercialized, can be used to produce hydrogen and be converted to the corresponding carboxylates; ii) no CO2 emission occurs at any stage of the process if ethanol is used; iii) ultra pure hydrogen is produced as no contamination by other gases (O2

In the current paper we report the development of new systems for the conversion of biomasses into either energy or hydrogen and chemicals. In particular, we have proved that direct alcohol fuel cells, capable of supplying elevated power densities, can function also as chemical reactors where alcohols are selectively converted into alkali metal carboxylates. Crucial factors to achieve this goal are the anode electrocatalysts, based on nanostructured Pd, and a strongly alkaline environment. Excellent selectivity’s have been achieved with ethanol and 1,2-propanediol converted to acetate and lactate, respectively. Glycerol is converted into a mixture of glycerate, tartronate, glycolate and oxalate (Scheme 6). All of these compounds are valuable fine chemicals. Moreover, we have discovered that nanostructured Pd-based anode electrocatalysts can be employed to produce both hydrogen and chemicals in an electrolyzer. In particular, this unique electrolysis process combines the electrooxidation of a renewable alcohol, such as ethanol, ethylene glycol, glycerol or 1,2-propanediol selectively into highly value-added carboxylic compounds with no need of chemical reagents, with the simultaneous evolution of ultrapure hydrogen at one third of the energy required by a traditional water electrolyzer (Scheme 6). While the technology described here has allowed the construction of small passive DEFC devices for portable electronics as well as micro-fuel cells, the development of devices capable of releasing power densities as high as 500 W for thousands of hours will require further material investigation. In particular the development of new anion-exchange membranes capable of working at temperatures >100◦ C. The temperature limit of the current membranes is below 80◦ C. Further research must also be directed to the design and development of electrocatalysts with improved chemoselectivity and activity for the oxidation of renewable diols such as 1,2-propanediol, 1,3-propanediol, to lactate and 3-hydroxy-propanate, which have the potential to serve as building blocks for the synthesis of bio-degradable polymers. With regards to OMFCs it is desirable to develop molecular electrocatalysts with less costly and earth-abundant metals such as Fe, Co, Ni and Ru. Such engineering of new organometallic complexes represents a promising route to the realization of fuel cells fed with more complex fuels such as sugars. As for application in the electrolysis of aqueous alcohol solutions, there is need to improve the catalyst stability over long periods at very high current densities. Amongst renewable resources, ethanol is certainly the most promising for hydrogen production, due to the possibility of production from the fermentation of biomasses or steam reforming of cellulose material with reasonable low energy costs. The use of such compounds allows electrolysis at potentials lower than 1 V, leading to electrical power savings as compared to conventional


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