s2fieldsofstudy: Chemistry Engineering

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Electrochemical energy conversion: methanol fuel cell as example

Thermodynamic and kinetic limitations of the electrochemical energy conversion are presented for the case of a methanol/oxygen fuel cell. The detection of intermediates and products is demonstrated using insitu FTIR spectroscopy and online mass spectrometry. The bifunctional catalysis of methanol oxydation by PtRu model surfaces is explained. The formation of HCOOH and HCHO via parallel reaction pathways is discussed. An example of DMFC system technology is presented.

Introduction
One of the most interesting developments in Electrochemistry of the last decade is the conversion of chemical energy into electricity via fuel cells. The surprising progress in the field was only possible on the basis of intensive studies on the processes at the fuel anode and the oxygen cathode. This will be shown for a characteristic example, the direct oxidation of methanol. But a review on the development of methanol electrocatalysts is not at all the aim of this article.
The high energy content of the chemical reaction of methanol and oxygen can deliver for the electrochemical pathway under ideal conditions as much as 6 kWh per kg of methanol. In opposition to the electrochemical reaction of hydrogen, the oxidation of methanol requires special attention due to the difficulty in splitting the molecule, adsorbed at the catalyst surface, and the complication of several parallel reaction pathways.
The detection of intermediates and products as well as the reaction model in the form of parallel pathways is demonstrated. The simultaneous application of experimental methods such as Cyclic Voltammetry, Infrared Spectroscopy, on-line Mass Spectrometry and Scanning Tunneling Spectroscopy was determining for the progress made. In addition to the outline of Electrocatalysis, the kind of problems which have to be solved are discussed, in order to build not only a single cell but a complete fuel cell system. The development in the technology of one example of a Direct Methanol Fuel Cell (DMFC) will be shown. 1,2 The conversion of chemical energy into electric energy proceeds in our days in most cases via the formation of heat. Power stations consume fuels like carbon, oil and natural gas, producing electric energy and CO 2 as final chemical product. The efficiency of this conversion process is limited by the Carnot factor. Normally, only 30 to 40% of the maximum heat value, ∆H, of the chemical reaction can be transferred in the form of useful energy. In mobile applications, our car engines offer only 15 to 25% of the supplied energy at the wheels. The "cold combustion" in a fuel cell, using an electrochemical conversion, offers 40 to 70% of the free energy of reaction, ∆G = ∆H-T∆S. In Figure 1 it is shown that losses during the electrochemical reaction path are due to the potential drop at the electrolyte resistance R E and at the charge transfer resistances at anode and cathode. At low current densities relatively high cell voltages, i.e. high energy efficiencies can be obtained. In all cases of ∆S<0, the amount T∆S is equal to a loss in the form of heat. In the case of hydrogen as fuel, the loss of energy due to the change in entropy is 16.9% at 25 °C.

Electrochemical Reactions at Anode and Cathode of a Fuel Cell 1,3
Let's come back to the reactions in a fuel cell. As an example, we apply a methanol/water mixture as fuel and air as oxidant ( Figure 2). The total chemical reaction follows and is split into two steps at two different places. At the anode we have For the formation of the final product CO 2 , water has to be added. The 6 electrons are passing the load resistance of the outer circuit and are consumed at the cathode together with 6 protons. Via a diffusion of protons from the anode to the cathode, the electrolyte concentration stays unchanged.
For the reaction of one molecule of methanol, 3/2 O 2 molecules will be reduced to 3 H 2 O, and at a stationary mode, part of this water has to be introduced at the fuel compartment again. In addition, after the operating temperature is obtained, the heat formed (compare Figure 1) has to leave the cell via a cooling device. From Figure 2, it is clear that a closed methanol/air fuel cell system has to have an automatic control device, for circulating methanol, water, air, CO 2 and heat. The standard free energy of reaction (1) is ∆G = -692.2 kJmol -1 . From this follows for a 6 electron charge transfer a standard cell voltage of 1.21V and an energy density of 6 kWh/kg methanol.
A first view about the reaction kinetics can be obtained via cyclic voltammograms (CV), using elected electrode materials. Figure 3 presents the well known CV's' for methanol and its soluble intermediate products, formaldehyde and formic acid, for the case of a platinum electrode in sulfuric acid. The CV for hydrogen is given for comparison. 3 Three important informations are: (i) the oxidation does not start at the thermodynamic data near the hydrogen potential in the same electrolyte, (ii) the observed maxima have nothing to do with the Matsuda curves, determined by charge transfer and diffusion, 1 (iii) on the cathodic scan the maxima occur at more negative potentials. For more information, we now apply FTIR and MS analysis, and we study the influence of electrode material and surface structure.

Intermediates via FTIR and MS Analysis
It is more than a decade that Kunimatsu did find by analysis of insitu IR spectra, in CH 3 OH/H 2 SO 4 solution on platinum surfaces at 400 mV RHE, bands for linearly bonded CO. 4 In both cases in Figure 4, saturated solution of CO and methanol solution, one has a total coverage of 0.86. But the IR signal is smaller for methanol, suggesting that other particles than CO from the splitting of the methanol molecule must be adsorbed also. Improving the IR technique, Iwasita et al. 5 obtained spectra with bands of bridge bonded and multi bonded CO, with bands of COH and HCOOCH 3 . An example shows Figure 5.   HCOOCH 3 is a product of the solution reaction of HCOOH with methanol. The formation as function of potential is presented in Figure 6, a combination of a current/potential scan and an online MS analysis (DEMS). 6 Using a porous catalyst layer, supported by a porous membrane, as a window to the MS, formation and consumption of volatile species can be followed. The current/potential scan in the upper part of Figure 6 is taken at a porous PtRu surface in contact with methanol solution.  Here, the current peak is due to the oxidation of split H atoms. Below 300 mV, no CO 2 is formed as stated by the mass signal m/e = 44 (lower part of Figure 6). Above 300 mV

Bifunctional Catalysis by PtRu Model Electrode Surfaces
Already in 1964 Bockris and Wroblowa 8 and in 1970 Sandstede et al. 9 did demonstrate that at a given oxidation potential, one observes an increase in the rate of methanol oxidation if platinum is replaced by PtRu alloys. Due to the above observation that CO is an adsorbed intermediate, and due to the fact that the oxidation of CO requires a second oxygen atom, and Ru is splitting water at low potentials easier than Pt, Watanabe 10 assumed the following bifunctional mechanism: Ru-H 2 O → Ru-OH + H + + e - Pt-CO + Ru-OH → Pt + Ru + CO 2 + H + + e - From this model for the mechanism it is expected that the distribution of Ru and Pt sites in the surface of the catalyst must be a sensible factor for catalyst activity. One example would be the formation of small Ru islands on a Pt substrate as shown for the case of a Pt(111) platinum surface in Figure 7. The Ru clusters of this model electrode had been produced by evaporation of Ru under UHV conditions. 11 An other way of preparation is the spontaneous deposition of platinum and ruthenium, out of solutions with different ratios of Pt-and Ru-ion concentrations. Last, but not least, PtRu alloys should offer ideal combinations of bifunctional catalysts. In Figure 8, we have plotted results of different authors for smooth surfacec catalysts in the form of the activity as a function of the Ru-atom surface percentage. As data for activity, the current density after 20 min of a potential step to 500 mV RHE, is arbitrarily chosen.
With the alloys, we obviously have the optimum geometric positions, and under the present experimental conditions of methanol concentration and operating temperature, the highest, almost constant activity is obtained between 15 and 40% ruthenium.
Interestingly, for all smooth samples (smooth alloys and Pt(111)), one observes a current decrease with time. Figure 9 shows that this is not due to a change in surface structure. 12 The topography of the Pt(111)/Ru surface remains unchanged after 50 min of current load at 500 mV. The origin of the decay in current has still to be discovered. More important for the application in fuel cells is the respective behavior of porous catalyst surfaces. The electrode, used in the experiment of Figure 10, was prepared on a gold substrate with electrodeposition out of a 20 mmol L -1 H 2 PtCl 6 + 2 mmol L -1 RuCl 3 solution at 100 mV RHE for 15 min. 11 According to EDXS measurements the composition was Pt:Ru = 81:19. The constant current response is comparable with results of other authors. 13,14 The two little steps in current near 4 and 14 min of time are caused by the formation of CO 2 bubbles.

Parallel Pathways of Methanol Oxidation
Studying the different pathways of methanol oxidation on platinum in acid media via CO, COH and HCOOH, one important fact is certainly the formation of formaldehyde.
Here, the determination of small amounts is difficult. Using IR reflection spectra one obtains only very weak signals. Characteristic signals in mass spectrometry are also absent. An interesting contribution is presented by Ota, 15 using PtPt electrodes and standard analytical methods ( Figure 11). Following the rate of CO 2 formation and the concentrations of HCHO and HCOOH in solution with time, these data of at least three different parallel pathways become constant only after about 20 h. After this time, obviously the rates of formation for the two soluble intermediates become equal to the rates of consumption at the electrode.
It is to be expected that the contribution of different pathways not only change with time but that it is also sensible to electrode material and structure. Indeed, this has been proved recently by comparing faradaic currents and the rate of CO 2 formation for two different PtRu alloys. 16 For a simplified description of methanol pathways, we can use the diagram of Figure 12. For more detailed informations see Iwasita. 17

A 40 watt Methanol/Air Fuel Cell System as Example of Present Technology
The oxidation of methanol in acid electrolyte, as in the DMFC of Figure 2, has the advantage that according to equation (1) two molecules of water are formed per molecule of methanol. This fact supports the closed system operation where sufficient water has to be introduced in the fuel compartment, together with methanol. The 40 watt unit of Figure 13 is using its electronic device for     controlling the flow of methanol and air, for the circulating of water and for the adjustment of the operation temperature at e.g. 60 °C, including respective heat balance regulations. 18 In addition, the control device takes care for the exhaust of the final product CO 2 . An example of the remarkable long time performance is given in Figure 14.

Closing Remarks
Catalysts with increasing reactivity for methanol oxidation could be the basis of DMFC system applications not only for low power stand by uses like in the example of Figure 13, but for power devices in mobile units also. At present, the study of new type catalysts like PtRuMo or PtRuSn is been done in many laboratories. If new catalysts are able to break C-C bonds in a sufficient manner, even the use of ethanol as fuel would be possible.


extfieldsofstudy: Physics Medicine

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Rules for Minimal Atomic Multipole Expansion of Molecular Fields

A non-empirical minimal atomic multipole expansion (MAME) defines atomic charges or higher multipoles that reproduce electrostatic potential outside molecules. MAME eliminates problems associated with redundancy and with statistical sampling, and produces atomic multipoles in line with chemical intuition.

The problem of representing the electrostatic potential outside a molecule using atomic charges or higher atomic multipoles is very important for understanding intermolecular forces. Atomic partial charges, an important part of chemical intuition, are defined in many different ways for different purposes. Chemically-derived (CD) charges, such as Mulliken 1 or Löwdin 2 , often describe molecular fields poorly. 3 More recent schemes partition molecular density into atomic regions, which may or may not overlap. 4 Similar approaches have been developed for solids. 5 Most attractive for our purposes are potentialderived (PD) charges, which avoid representation of the density by producing the 'best' fit to the molecular potential directly. 6,7 Atomic dipoles and quadrupoles 7 are often used to increase accuracy in solvation problems 8 and force field calculations. 9 Induced atomic dipoles appear naturally in electronic polarization of molecular solids 10 to account for the small part of molecular polarization that is due to the deformation of atomic orbitals and is not captured by redistribution of charges.
Computational schemes for PD multipoles such as Merz-Kollman (MK), 11 CHelp 12 , or CHelpG 13 differ mainly in the sampling domain and the resulting atomic charges are strongly method-dependent. 14 Worse still, PD methods often yield atomic charges that are counterintuitive, such as negative charges on hydrogens in alkanes. 15 Higher multipoles only increase the redundancies inherent in distributed multipole analysis, improving on the accuracy of the field at the expense of instability in the multipole values. The severity of the problem can be somewhat reduced with SVD techniques, 16,14 or by introducing restraints. 17 Our approach does not use sampling and eliminates redundancies before they appear.
We approximate the true molecular potential, φ(r), as a sum of multipoles of strength q k centered at nuclear positions r i , where φ k (r) is the potential due to the kth multipole of unit strength: φ k (r) = 1/r for charge, (n · r)/r 3 for a dipole in the direction n, and so on. Since ∇ 2 φ approx = 0 everywhere except at r i , but ∇ 2 φ = 4πρ(r), the atomic multipole expansion can only be accurate in regions where ρ(r) ≈ 0. Furthermore, φ on any closed surface S on which ρ = 0, determines φ(r) everywhere outside S. We therefore choose S to be an isodensity surface, ρ(r) = f , where f is sufficiently small to ensure negligible charge beyond S, but with sufficient potential on S for a determining fit (Fig. 1). We chose φ approx to minimize over S which leads to a system of linear equations Atomic multipoles defined in this way are fully rotationally invariant, which is an issue with some PD schemes. 13 The error σ can be compared to φ, The crucial issue remaining is the choice of a set of multipoles. We choose a minimal set, usually one scalar value per atom, and add additional multipoles to describe lone pairs when necessary, based on the Lewis structure. This carefully chosen minimal atomic multipole expansion (MAME) set avoids redundancies but is within ∼ 1 mH everywhere beyond S.
We illustrate MAME with three molecules: n-pentane, which is a classic example of difficulties encountered in PD schemes; glycine (standard and zwitterion), as a typical application in biochemistry; and water, to see how general MAME rules apply to a small polar molecule. All densities and potentials are produced on a cubic mesh by the Gaussian 98 program, 18 at the B3LYP/aug-cc-pVTZ level (6-311++G** for pentane). Surface integrals (3) are computed by triangulation of S. The program runs within a few seconds, and is available on request. Figure 1 shows φ(r) on S for n-pentane. Red spots (φ > 0) show an excess of positive charge near each hydrogen, but all PD schemes tested yield some or all hydrogens negative. Closer inspection of Fig. 1 reveals that the positive regions occupy less solid angle around hydrogens than would be produced by a positive charge. Such a potential is consistent with a dipole with a negative charge pointing inwards.
Our first rule is therefore to assign a charge to all nuclei but protons, to which we assign a dipole moment instead. The hydrogen atom is special as its sole electron participates in the bond, leaving no electron density centered on the proton. This unique property of hydrogens is wellknown in X-ray structure analysis, which systematically underestimates the C-H bond lengths for this reason. Table 1 Partial atomic charges in n-pentane. f = 5×10 −4 au., error σ as in (2), (%)= (σ/φ), eφ = 3.6 mHartree (1mH = 27meV ∼ kT at 300K). "µ" indicates atomic dipoles, "µr" -dipoles restricted along H-C bonds.
Mulliken charges are intuitively meaningful but produce large errors in the potential (Table 1). PD charges are negative on some hydrogens and still give significant errors. Adding dipoles reduces the potential error, but at the cost of producing meaningless multipoles. 15 Our scheme with charges on all atoms produces similar (though better) results, but we do far better (line 2) when the charges on hydrogens are replaced with dipoles. All dipoles come out similar in magnitude (numbers in brackets, in au.) and point toward C within 20 • of the H-C bond. The hydrogen dipoles can be safely restricted to lie along the H-C bonds (last line) with the accuracy still better than that of charges. All multipoles have reasonable values, including small charges on carbons. Note that we have now described the field outside the molecule more accurately than any existing scheme, with only one parameter per nuclues (a charge on each carbon and a bond-directed dipole on each hydrogen).
The same choice of multipoles yields a 1.05 mH error (=2%) in the glycine zwitterion, (NH 3 ) + -CH 2 -COO − , down from 4% with charges alone and 4%-6% with standard PD schemes. The glycine zwitterion is highly polar with dipole µ = 10.3 D, which MAME recovers within 0.1% accuracy.   Table 2 lists MAME results for glycine in its standard form, NH 2 -CH 2 -COOH, and illustrates the need for special treatment of lone pairs. In the zwitterion, the NH 3 group is well-described by a charge on N and three dipoles on hydrogens, similar to methyls in pentane. The NH 2 group in glycine lacks one site, but has extra electron density associated with the lone pair. We thus assign a dipole moment to N, in addition to its charge, restricted along the sp 3 direction of the lone pair.
Similarly, each oxygen has two lone pairs. Two dipoles for the two lone pairs sum to just one dipole along the symmetry axis, leading to only one variational parameter. The potential of this single dipole, however, is axially symmetric, whereas the potential around the oxygen deviates from axial symmetry due to the particular orientation of the lone pairs. Such a deviation can be accounted for with a quadrupole moment on the oxygen. The finite system of charges sketched in the inset in Fig. 2 shows what is needed. Computing a multipole expansion of three charges we describe two lone pairs with two scalar parameters, a dipole µ r restricted along the symmetry axis, and a restricted quadrupole θ r which has angle β as a fixed parameter (β = 120 • for O= and 109.7 • for O-, due to sp 2 and sp 3 hybridization respectively). θ r and µ r are chosen negative with "−" pointing outside. Table  2 shows a clear advantage of such a multipole set.
To make the definition more transparent, θ r can be expressed in the conventional form as a carefully crafted combination of θ zz = θ r (3 cos 2 β/2 − 1), θ yy = θ r (3 sin 2 β/2 − 1), θ xx = −θ r , and θ xy = θ xz = θ yz = 0, which depend on a single scalar parameter θ r . Here z is along the symmetry axis, and the lone pairs are in the yz-plane. A single restricted quadrupole of strength θ r creates the potential φ quad (r) = θ r 3(r · n 1 ) 2 + 3(r · n 2 ) 2 − 2r 2 2r 5 , where n 1 and n 2 are the directions of the lone pairs. For the zwitterion, MAME does not require θ r on the oxygens, because of the resonance. Lone pairs in the sp 2 and sp 3 configurations lie in perpendicular planes, virtually destroying any asymmetry. MAME accuracy improves away from the molecule (Fig. 2). If f is too large (10 −3 ), there is a net charge inside S which strongly affects the asymptotic behavior. This can be repaired by fixing the total charge using a Lagrange multiplier. 16 Figure 2 demonstrates MAME's insensitivity to choice of f , provided the total charge is correct. Ratio of (φapprox − φ) to φ, both square-averaged over points with a given density ρ, which is an inverse measure of distance. The inset illustrates two lone pairs on oxygen represented with a combination of restricted dipole µr and quadrupole θr.
Last, we analyze the water molecule. The first rule leads to two dipoles on hydrogens pointing along the bonds, sketched in Fig. 3. The charge on oxygen is zero because the molecule is neutral. The dipoles are equal due to symmetry and require no calculation since their vector sum must yield the dipole moment of water, 1.847D (B3LYP/aug-cc-pVTZ value). This already reduces the error to 21%, from 45% with a single dipole on oxygen. The two distributed dipoles yield Θ xx − Θ yy = 4.06 DÅ for the quadrupole moment of water, whereas experiment 19 gives 5.12 DÅ. We note that Θ xx − Θ yy is the only invariant combination of quadrupole components, since the finite dipole makes them dependent on the center of coordinates. We next add µ r and θ r multipoles on the oxygen to describe the correction due to lone pairs, and we find excellent accuracy eσ = 0.59 mH (< 3%) on and beyond S (1.56Å from H and 2.11Å from O, f = 10 −4 ). The sp 3 choice of β = 109.5 • in θ r is crucial: accuracy deteriorates dramatically (to 12%) when β is changed to e.g. 180 • (θ r replaced with θ yy ). The sp 3 description of the oxygen lone pairs is appropriate due to invariance under unitary rotations of occupied orbitals. In conclusion, molecular fields are represented to chemical accuracy with a minimal set of atomic multipoles carefully chosen based on the Lewis structure of the molecule. All H atoms are represented as dipoles. Lone pairs are treated with extra multipoles, avoiding additional off-nuclear expansion sites. 20 The scheme yields multipole values that conform to chemical intuition, are unique, fully rotationally-invariant and free of sampling errors.
We thank K. Krogh-Jespersen, Z.G. Soos, M. Lobanov, and R. Gaudoin for enlightening discussions. This work was funded by AFOSR and the New Jersey Commision on Science and Technology.