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Effect of P H 2 on PEMFC performance. T = 37 °C; Pt–Ru anode at low (a) and high (b) P H 2 values. (Conditions: f anode = 400 cc/min; f cathode = 400 cc/min). (c) Effect of cell potential, U, on the Nafion 117 conductivity, σ, at various P H 2 values. Conditions as in (a).
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The role of the non-linear conductivity of fully hydrated Nafion membranes is discussed on the performance of state-of-the-art PEM fuel cells. It is shown that the Nafion conductivity contains two components, one constant corresponding to proton transport in the aqueous phase of the membrane, the other exponentially dependent on potential, linearly...
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... shown below, both γ and U c are dependent on P H 2 , and thus the double inequality (14) also defines the P H 2 range where steady-state-multiplicity is obtained. Fig. 2 shows the effect of the feed H 2 partial pressure P H 2,in on the U-I behaviour. The exit P H 2 value, P H 2 , exit , is also given in the figure. Steady-state multiplicity appears up to P H 2,in =1.5 kPa (Fig. 2a) but the characteristic dU /dI slope decrease persists for all P H 2 ,in curves up to the highest P H 2 ,in value ...
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... c are dependent on P H 2 , and thus the double inequality (14) also defines the P H 2 range where steady-state-multiplicity is obtained. Fig. 2 shows the effect of the feed H 2 partial pressure P H 2,in on the U-I behaviour. The exit P H 2 value, P H 2 , exit , is also given in the figure. Steady-state multiplicity appears up to P H 2,in =1.5 kPa (Fig. 2a) but the characteristic dU /dI slope decrease persists for all P H 2 ,in curves up to the highest P H 2 ,in value investigated of 100 kPa (Fig. ...
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... the effect of the feed H 2 partial pressure P H 2,in on the U-I behaviour. The exit P H 2 value, P H 2 , exit , is also given in the figure. Steady-state multiplicity appears up to P H 2,in =1.5 kPa (Fig. 2a) but the characteristic dU /dI slope decrease persists for all P H 2 ,in curves up to the highest P H 2 ,in value investigated of 100 kPa (Fig. ...
Citations
... It should be noticed that, i-E curve of PtÀ Ru/Gr shows instabilities at potentials lower than 0.3 V. In the literature, this kind of instabilities and multiplicities have been associated with the humidification of the membrane, [39] the transport of protons through the polymer membrane, [40][41][42][43] the variation of oxygen content on the cathode side, [44] the CO poisoning on the anode side [45] or even with a possible reduced feed stream humidification. [46] In our case, we believe that the observed behavior of i-E curve (which was taken at a scan rate of 20 mV s À 1 ) is mainly due to an oscillatory behavior associated with a continuous CO adsorption/desorption process. ...
In this study, we explore the effect of carbon support on the performance of anodic electrodes for polymer electrolyte membrane fuel cells (PEMFCs). Pt−Ru catalysts supported on different carbon materials were synthesized by wet impregnation method and characterized by means of Raman, XRD, TEM, and N2 adsorption analysis. The electrocatalytic activity of the prepared catalysts was studied during hydrogen oxidation reaction experiments (HOR) as well as during the operation of a fuel cell using pure H2, and H2‐carbon monoxide anode feeding. The results show that the crystalline phase and the point of zero charge (pzc) of the carbon supports play an important role in the metal particle size and consequently in the catalyst electrocatalytic activity. Carbon supports with amorphous phase and high values of pzc, result in catalysts with large metal particle size leading to low electrocatalytic activity. On the other hand, using graphene derivative or carbon nanotubes (CNT) in different catalysts generates smaller metal particle size which results in higher electrocatalytic activity. Choose your carbon wisely: The effect of carbon support on the performance of anodic electrodes for polymer electrolyte membrane fuel cells (PEMFCs) is studied. The results show that the crystalline phase and the point of zero charge (pzc) of the carbon supports play an important role in the metal particle size and consequently in the catalyst electrocatalytic activity.
... In the PEMFC literature there are a number of reports of SSM with various mechanisms proposed (see for example [19][20][21][22][23][24]). The noteworthy paper by Kulikovsky et al. [19] shows local NDR at the cathode side using a segmented fuel cell setup. ...
... Here U 0 is the open circuit voltage, and r e is the ohmic resistance from both ionic migration across the electrolyte and electrical resistance of the anode. In [3] the electrolyte resistance was allowed to be a function of potential in order to include the hypothesis of a potential dependent electrolyte resistance in PEMFCs given in [23,24]. However since no such behaviour has been observed in SOFCs this potential dependence has not been included in the present model. ...
... (23)), E f and E r are the activation energies for the forward and backward reactions, R is the universal gas constant, λ is a symmetry factor, F is Faraday's constant, and η is the overpotential. From Eqs. (22) and (24) we have that both ∂h/∂Z > 0 and ∂h/∂η > 0 for an exothermic reactant. ...
Nonlinear dynamics play a vital role in electrochemical systems and there are many examples in the literature of systems which exhibit interesting behaviour such as multiple steady states and autonomous oscillations. Within the fuel cell community nonlinear dynamics of proton exchange membrane fuel cells have been studied in depth, however other types of fuel cell have not received nearly as much attention. Since fuel cell systems are characterised by a wide variety of physicochemical processes operating on multiple time scales, it is crucial that the nonlinear effects are understood in order to better design and operate them. This work presents the first experimental results on global negative differential resistance and steady state multiplicity in solid oxide fuel cells operating on hydrogen under high fuel utilisation, alongside a basic prototype model that has been adapted from the literature in order to explore the origins of this behaviour. Results are discussed within the context of theoretical predictions from the literature and the prototype model presented within.
... A remarkable phenomenon of PEMFCs using Nafion as polymer electrolyte is occurring under specific conditions. This phenomenon has been already reported in the past [3][4][5][6] using low temperature fuel cells operating with H 2 as fuel, Pt-Ru/C as anode and Pt/C as cathode. During this phenomenon PEMFCs often exhibit steady state multiplicity i.e. the I-U curves have such a shape that, over a certain current range cell potential, two U values correspond to the same fixed current [3,5]. ...
... During this phenomenon PEMFCs often exhibit steady state multiplicity i.e. the I-U curves have such a shape that, over a certain current range cell potential, two U values correspond to the same fixed current [3,5]. This phenomenon cannot be described by any of the known classical mathematical expressions for overpotential [3][4][5]. ...
... Until now, steady state multiplicities have been studied as a function of the fuel concentration, flow rate and partial pressure of hydrogen [3][4][5][6]. It has been proposed that the phenomenon results from the significant dependence of the Nafion membrane conductivity on cell potential. ...
... A deeper knowledge of the interplay between mass, charge and heat transport coupled to electrochemical kinetics in unusual scenarios will contribute to a better understanding of PEFC technology. According to Hanke-Rauschenbach et al. [63], three main phenomena leading to nonlinear PEFC dynamics can be distinguished: i) coupled water and proton transport in the membrane, ii) electrochemical surface kinetics, and iii) interaction between reactant mass transport and water two-phase flow [18,19,[64][65][66][67][68][69][70][71][72][73][74][75]. Below we review some relevant contributions that have addressed PEFC dynamics induced by changes in membrane hydration under low feed humidification, i.e., belonging to point i) above. ...
Proper water management plays an essential role in the performance and durability of Polymer Electrolyte Fuel Cells (PEFCs), but it is challenged by the variety of water transport phenomena that take place in them. Previous experimental work has shown the existence of fluctuations between low and high current density levels in PEFCs operated with wet hydrogen and dry air feed. The alternation between both performance states is accompanied by strong changes in the high frequency resistance due to the cyclic hydration and dehydration of the membrane. This peculiar scenario is examined here considering liquid water distributions from neutron imaging and predictions from a 3D two-phase non-isothermal model. The results show that the hydration-dehydration cycles are triggered by the periodic condensation and shedding of liquid water at the anode inlet. The liquid water input humidifies the anode channel and offsets the membrane dry-out induced by the cathode stream, thus leading to the high-performance state. When liquid water is flushed out of the anode channel, the dehydration process takes over, and the cell eventually returns to the low-performance state. The predicted amplitude of the current oscillations grows with decreasing hydrogen and increasing air flow rates, in agreement with previous experimental data.
... Increasing the humidification beyond 1 16D 2 b , causes an additional saddlenode bifurcation to take place, resulting in hysteresis. Hysteresis has been observed experimentally in PEMFCs, along with both current and voltage oscillations (10,11). However, to the authors' knowledge, hysteretic behaviour has only been theoretically investigated for SOFCs (12,13). ...
... This observation is remarkably similar to the two stable steady states predicted by the present model, where the equilibrium point e − = (α − , β − ) represents a high power output, while the equilibrium point e 0 = (α 0 , β 0 ) represents a low power output state. The experiments in (10,11) show hysteresis between two steady states, namely, the high and low power output states, as the bifurcation parameter is varied. These results parallel the full e-e hysteresis loops described in Section 5, which jump between the equilibrium point e − = (α − , β − ), representing the high power output state, and the equilibrium point e 0 = (α 0 , β 0 ), representing the low power output state. ...
In this article, we address the phenomenon of temporal, self-sustained oscillations for a solid oxide fuel cell which utilises
a humidified methane fuel stream. Our objective is to uncover the fundamental mechanisms giving rise to current oscillations
that have been observed experimentally. To this end, we develop a model based on the fundamental chemical kinetics and transfer
processes which take place within the fuel cell. This leads to a three-dimensional dynamical system, which, under typical
operating conditions, is rationally reducible to a planar dynamical system. This planar dynamical system was studied in (Sands
et al., Proc. R. Soc. Lond. A 470 (2014)) for the case where the parameter c ¯ 0 ≪ 1 c¯0≪1, corresponding with a weakly humidified fuel stream. In the present article, the structural dynamics of the planar dynamical
system for the case where the parameter c ¯ 0 = O ( 1 ) c¯0=O(1), which corresponds with a humidified fuel stream, are studied in detail. Self-sustained oscillations are shown to arise through
Hopf bifurcations in this planar dynamical system, and the key parameter ranges for the occurrence of such oscillations are
identified. Fuel stream humidification is also shown to significantly alter the fuel cell dynamics, leading to hysteresis.
... Obwohl ein solches Verhalten für eine elektrochemische Zelle sehr ungewöhnlich ist, wurde es bereits bei den Gleichgewichts-Polarisationskurven einer PEFC unter bestimmten Bedingungen beobachtet. Untersuchungen ermöglichten es, diesen Effekt der Potenzialabhängigkeit der Leitfähigkeit der Nafion -Membran zuzuordnen [81,82]. Das Auftreten dieses Verhaltens bei den hier durchgeführten Untersuchungen an einer Wasserstoffpumpe schließt die ORR als alleinigen Ursprung dieses Effekts aus. ...
In der vorliegenden Arbeit wird die Methode der EC-AFM erweitert, um aus
zeit- und ortsaufgelösten Strom-Transienten tiefere Einblicke vor allem in die Dynamik des Systems im Nanometerbereich zu erhalten. Hierzu wird die AFM-Spitze am Ausgang eines protonenleitfähigen Kanals einer Nafion®-Membran platziert und ein Spannungssprung angelegt. Die daraus resultierende Strom-Antwort spiegelt die Eigenschaften des Systems wider und wird durch elektrochemische Reaktionen, kapazitive Effekte und den Einfluss der Membran bestimmt. Die Interpretation der Strom-Zeit-Kurven erfolgt zunächst in der Zeit-Domäne. Mithilfe der Fourier-Transformation können die Strom-Zeit-Kurven und die Spannungs-Zeit-Kurven in die Frequenz-Domäne überführt werden. Hieraus wird anschließend die Möglichkeit untersucht das elektrochemische Impedanzspektrum des Systems zu berechnen, da dies die Differenzierung der verschiedenen Effekte im Frequenzraum ermöglichen kann.
... Recent work has shown that the normal monotonically decreasing I-V curves obtained for P H 2 values in the range 5-100 kPa change very significantly for P H 2 values below 3 kPa [32] and new interesting features appear which include the development of bistability with one branch corresponding to positive dV/dI slopes at low cell potentials [32][33][34][35][36]. The latter implies a negative real part of the impedance which cannot be described, at least directly, by any of the classical types of overpotential [37]. ...
... The latter implies a negative real part of the impedance which cannot be described, at least directly, by any of the classical types of overpotential [37]. It has been shown that this peculiar I-V behavior is not due to activation or diffusion overpotential but reflects the overpotential developed in the fully hydrated Nafion membrane itself operating in the low P H 2 range [32][33][34][35][36]. A somehow similar behavior has been observed in local impedance spectra under low air stoichiometry in H 2 /air PEMFCs [38,39] and has been modeled in terms of oxygen depletion and oscillations in the cathode flow channels [39,40]. ...
... All these features are consistent with the two proton transport mechanism [1,7,8,[27][28][29], with the Grotthus bulk mechanism dominating at high P H 2 values and the transport along sulfonate groups dominating at low P H 2 values. This latter transport mechanism appears to be due to proton tunneling between adjacent sulfonate groups [32][33][34][35], an assignment supported both by DFT calculations [27] which have confirmed that proton transfer between SO 3 sites is an important proton transfer mechanism, and by the observed strong isotope affect [35]. This proton tunneling mechanism has been also shown to be consistent with the observed conductivity dependence on thickness and with the non-linear and in fact positive slope parts of the V-I plots [32][33][34][35][36]. ...
The proton transport mechanism in fully hydrated Nafion 117 membranes was examined via electrochemical impedance spectroscopy (EIS) and steady-state current–potential measurements both in a symmetric H2, Pt|Nafion|Pt, H2 cell and in a H2, Pt|Nafion|Pt, air PEM fuel cell with hydrogen partial pressure values, PH2, varied between 0.5kPa and 100kPa. In agreement with recent studies it is found that for low PH2 values the steady-state current–potential curves exhibit bistability and regions of positive slope. In these regions the Nyquist plots are found to exhibit negative real part impedance with a large imaginary component, while the Bode plots show a pronounced negative phase shift. These observations are consistent with the mechanism involving two parallel routes of proton conduction in fully hydrated Nafion membranes, one due to proton migration in the aqueous phase, the other due to proton transfer, probably involving tunneling, between adjacent sulfonate groups in narrow pores. The former mechanism dominates at high PH2 values and the latter dominates in the low PH2 region where the real part of the impedance is negative.
Over the last decade, nonlinear phenomena in low-temperature fuel cells as well as high-temperature fuel cells have been reported in the open literature. Experimental and theoretical studies found multiple steady states as well as periodic oscillations. The present article gives an overview of publications on this subject. Instead of sorting the analyses according to the types of fuel cells, this work used the source of the nonlinearity for classification. In the first part of the contribution, a very simple prototype fuel cell model is introduced. The model helps to give a qualitative explanation of the majority of nonlinear effects reported in literature. It is further used to identify potential sources of nonlinear behavior in reaction kinetics, membrane properties, and mass transport mechanisms. A classification scheme that is based on types of negative differential resistance (NDR) and was originally introduced by K. Krischer in Modern Aspects of Electrochemistry (Vol. 32, p. 1, Plenum Press, 1999) for electrochemical systems is applied to fuel cells. The second part of the work classifies the findings from literature according to their NDR type. Instabilities resulting not from electrochemistry but from other mechanisms such as water formation and reactant starvation are also discussed.
The repulsive Coulombic forces exerted between proton wave particles in electrochemical and physical systems are examined, together with the attractive ion-induced dipole forces between protons and neighboring neutral particles, e.g. neutrons in the case of physical systems (nuclei). It is shown that when protons and neutrons are treated as harmonic oscillators with the same kinetic and potential energy, then two roots exist for their vibrational velocity. One root corresponds to negligible relativistic corrections (v/c≪1) and unstable nuclei, the other to significant relativistic corrections (v/c≈1) and to formation of stable nuclei. It is shown that the first root corresponds to protons in chemical–electrochemical systems and the second (relativistic) root corresponds to protons in nuclei. In the latter case the formation of stable nuclei is due to the attractive ion-induced dipole forces and to the pronounced increase in mass and gravitational forces. The latter, together with the ion-induced dipole forces, counterbalance the strong repulsive Coulombic forces. This leads to the analytical computation of the energy of formation of the 4He and 2H nuclei and of the gravitational constant. All three computed values are in quantitative agreement with experiment.
With the help of a simple, spatially lumped, and isothermal model, we qualitatively predict and discuss bistable current-voltage
characteristics of proton exchange membrane (PEM) fuel cells operated with low humidified feed gases. The cell is found to
exhibit current-voltage curves with pronounced local extrema in a parameter range that is of practical interest when operated
at constant feed gas flow rates. This causes steady-state multiplicities in the potentiostatic and rheostatic operation mode.
The reason for the predicted behavior is an autocatalytical water production mechanism. For small inlet relative humidities
or even no humidification, the current-voltage curve possesses an isolated high-current branch, which was recently observed
experimentally. A parameter range is predicted, wherein such current-voltage characteristics can be observed. Furthermore,
we review selected experimental studies on this topic and discuss their results against our findings.
