- Email: [email protected]
In operando measurement of localised cathode potential to mitigate DMFC temporary degradation
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 3 ( 2 0 1 8 ) 9 7 9 7 e9 8 0 2
Available online at www.sciencedirect.com
ScienceDirect journal homepage: www.elsevier.com/locate/he
Short Communication
In operando measurement of localised cathode potential to mitigate DMFC temporary degradation C. Rabissi a,*, E. Brightman b, G. Hinds b, A. Casalegno a a b
Politecnico di Milano, Energy Department, Via Lambruschini 4, Milano, Italy National Physical Laboratory, Hampton Road, Teddington, Middlesex, TW11 0LW, United Kingdom
article info
abstract
Article history:
An innovative external reference electrode technique has been applied to the cathode of an
Received 11 October 2017
operating DMFC in order to identify variations in electrode potential across the active area
Received in revised form
of the cell. The evolution of cathode potential at two different locations in the cell was
28 March 2018
monitored during operation, with the primary focus on studying the potential dynamics
Accepted 8 April 2018
during the temporary degradation recovery procedure, the so-called refresh cycle. The re-
Available online 27 April 2018
sults highlight for the first time a non-uniform local recovery of temporary degradation at the cathode during refresh cycles, associated with varying rates of platinum oxide reduction
Keywords:
across the cell, which could lead to current density redistribution and contribute to an
Direct methanol fuel cell
uneven degradation of the components. The technique shows great promise for the
Cathode potential
improvement of long term DMFC performance via optimisation of refresh cycle protocols.
Temporary degradation
© 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Reference electrode Platinum oxides
Introduction Liquid-fed direct methanol fuel cell (DMFC) technology has several advantages over conventional hydrogen-fed polymer electrolyte membrane fuel cells (PEMFCs); methanol is a higher energy density liquid, stable under a wide range of conditions and easier to store and transport. However, the slower methanol oxidation reaction (MOR), together with more complex liquid transport and fuel crossover phenomena, results in markedly lower efficiency of DMFCs [1]. This limits them at present to portable and low power applications, although efforts are ongoing to improve the technology. The significant anode overpotential for the MOR [1,2] results in a relatively
low current density and a higher cathode potential during operation. It is therefore critical to distinguish the contribution of each electrode to the overall cell voltage, particularly during the study of temporary and permanent degradation of DMFC cathodes, where the former is mostly related to platinum oxide formation [3,4] and the latter to mechanisms such as Ostwald ripening and Pt dissolution [2,5e8]. Moreover, strongly heterogeneous degradation across the membrane electrode assembly (MEA) has been reported in [2], which was attributed by the authors to spatial variations in operating conditions at both anode and cathode, emphasising the need for further investigation of localised performance and degradation.
* Corresponding author. E-mail address: [email protected] (C. Rabissi). https://doi.org/10.1016/j.ijhydene.2018.04.043 0360-3199/© 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
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For these reasons, in previous work [9] the authors demonstrated the effectiveness of an innovative throughplane reference electrode technique in performing localised in operando measurement of anode overpotential during DMFC operation, beyond what had been conventionally possible using the cathode as an internal pseudo-hydrogen reference electrode. Moreover, as discussed in [9], the direct connection of such reference electrodes on the catalyst layers active surface permits to overcome the major limitations of traditional fuel cell reference electrodes: ohmic drop, potential distribution due to electrode misalignment and edge effects in the membrane are negated. The investigation focused particularly on the short operation breaks (known as refresh cycles) typically performed to recover temporary degradation during operation [2]. It was demonstrated that anode operation is responsible for more than half of long-term temporary degradation, highlighting the homogeneous ineffectiveness of refresh cycles in fully recovering anode performance. At the same time, analysis of the anode potential during refresh cycles revealed the presence of hydrogen evolution in half of the cell; it is likely that such a localised process could introduce unpredictable heterogeneities in cathode operation. This paper focuses on the investigation of cathode overpotential during DMFC operation, using the same locallyresolved reference electrode approach. Analysis of localised temporary degradation is carried out over timescales ranging from minutes to hundreds of hours, as well as during fast transient refresh cycles, in order to highlight critical local degradation mechanisms and potential mitigation strategies.
Experimental DMFC MEAs manufactured by EWII Fuel Cells A/S of 25 cm2 active area were used for this investigation. The membrane was Nafion® 115 and the anode and cathode catalyst loadings were respectively 1.8 mg cm 2 (PtRu alloy) and 1.2 mg cm 2 (Pt), both coated with Sigracet® SGL35DC diffusion layers (overall thickness 325 mm, 20% PTFE content) and provided with a microporous layer (MPL). In order to investigate phenomena associated to actual nominal operation, operating conditions and protocols have been fixed to those indicated by MEAs manufacturer. Thus, anode and cathode were respectively fed with 1.0 M methanol solution and air saturated with water vapour at ambient temperature (stoichiometries equal to 6 and 3 respectively at 0.25 A cm 2) in a counter-flow configuration through triple serpentine flow plates. Nominal current density and cell operating temperature were respectively 0.25 A cm 2 and 75 C. The overall experimental setup, thoroughly described in [2], is schematized in Fig. 1A, including reference electrodes measurement at both anode, discussed in [9], and cathode of the fuel cell. The local reference electrode measurement setup is thoroughly explained in [10] and carefully follows the procedures established in [9], this time also providing measurements on the cathode side of the cell. Briefly, the in situ reference electrode is based on a salt bridge made from fully watersaturated Nafion® tubing, which directly connects the external surface of the GDL with an electrolyte solution in
Fig. 1 e Schematic diagram of A) experimental setup and B) cathode DMFC flow-plate (showing RHE locations).
which a Gaskatel HydroFlex® reversible hydrogen electrode (RHE) is immersed. At the appropriate locations within the GDL, a small amount (about 2 mL) of Nafion® polymer solution is dispersed in order to realise the necessary proton conductive path to the catalyst layer. In order to achieve an insight into the local evolution of cathode potential, two measurement points were employed, close to the inlet and outlet of the cell as shown in Fig. 1B, analogous to the previous anode measurements [9]. The evolution of cathode potential at the two measurement locations was monitored during a 200-h degradation test, adopting periodic refresh cycles every 20 min and a full refresh procedure [2] after 100 h. The refresh cycles consist of 1 min interruptions to galvanic cell operation, using a combination of 30 s at open circuit voltage (OCV) and 30 s of air feeding interruption period at the cathode (named as air-break period in the following), while keeping anode flow uninterrupted. Analysis of cathode potential
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dynamics during a refresh cycle was performed in order to investigate the effects on the cathode potential of the strongly localised hydrogen evolution mechanism highlighted in [9]. The full refresh procedure consists instead in a prolonged operation interruption (~16 h) keeping an uninterrupted low mass flow of methanol solution (~0.3 g min 1) in the anode serpentine, while maintaining the cell temperature at 75 C and both cathode inlet and outlet manifolds plugged to avoid excessive oxygen infiltrations [2]. As reported in [9], no noticeable effects on cell operation of the presence of the RHEs were observed over the 0e0.4 A cm 2 current density range. The uncertainty in the potential measured by the reference electrodes is estimated to be ±5 mV. All potentials in this work are quoted relative to the RHE.
Results and discussion In order to analyse the transient decay and recovery of cell performance over relatively short timescales, the evolution of cell and cathode potential at both inlet and outlet over a period of 40 min of cell operation, including a refresh cycle, is shown in Fig. 2A. Just after the refresh cycle (20 min in Fig. 2A), a fast initial overshoot in cell voltage occurs, together with a noticeable recovery in performance. The anode potential measurements in [9] revealed that this was dominated by the anode response, possibly due to the depletion of the small quantity of hydrogen evolved during the air-break period of the refresh cycle itself [10e12]. The cathode potential measurements reported here are consistent with this hypothesis, with no increase in cathode potential observed during the fast transient immediately following the refresh cycle either at inlet or outlet of the cell (Fig. 2A). In fact, the cathode potential at each location show a steady or monotonically decreasing trend. As mentioned previously and already discussed in [13], the relatively high DMFC anode potential (close to 0.4 V [9]) necessitates operation at a lower current density (0.25 A cm 2) resulting in a higher cathode potential than for a PEMFC. Indeed, excluding the initial transient following the refresh cycle, these measurements confirm that the cathode operates at a potential slightly lower than 0.9 V at the outlet and about 50 mV higher than this value at the inlet. This difference results from the combination of the higher oxygen concentration [13] togheter with higher membrane loss due to dry air feeding, higher anode loss at anode outlet [9] and lower methanol crossover. During each single uninterrupted operation period (i.e. from 1 min to 20 min and from 21 min to 40 min) the cell voltage decreases by about 20 mV. The local cathode potential measurements reveal much more heterogeneous behaviour, with the cathode potential remaining almost constant at the inlet but decreasing by about 30 mV at the outlet (Fig. 2A). This is in contrast to the local anode measurements for the same cell in [9], which showed a much more homogeneous trend. The effect of the refresh cycle on the cathode potential also varies significantly with position; the interruption (at 20 min in Fig. 2A) results in a noticeable performance recovery close
Fig. 2 e Evolution of cell voltage (black curve) and local cathode potential at inlet (green curve) and outlet (yellow curve) during 40 min operation, including a refresh cycle after 20 min (A) and during 200 h operation with refresh cycles every 20 min, including a full refresh after 100 h (B). (For interpretation of the references to color/colour in this figure legend, the reader is referred to the Web version of this article).
to the outlet (yellow curve in Fig. 2A), but has nearly no effect at the inlet (green curve in Fig. 2A). In order to quantify the cathode temporary degradation not recovered by refresh cycles, the test was extended to 200 h, as shown in Fig. 2B. The recovery in cell voltage following the full refresh at 100 h was about 40 mV, consistent with previous work which also showed that more than 30 mV of this value could be attributed to relatively homogeneous recovery in anode potential across the area of the cell [9]. However, the local cathode potential measurements in Fig. 2B reveal that the effectiveness of the refresh cycles in recovering cathode performance varies strongly from inlet to outlet of the cell. The full refresh produces a 30 mV recovery in cathode potential at the inlet (green curve, Fig. 2B) but no change at the outlet (yellow curve, Fig. 2B), where the periodic refresh cycles appear to have been fully effective in recovering temporary degradation. In order to better explain such interesting behaviour, a deeper investigation of potential evolution during refresh cycles was performed. The main mechanism for cathode temporary
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degradation is known to be platinum surface oxidation, which is enhanced at the relatively high potential of DMFC operation [3,4] and tends to progressively decrease the active surface area for the oxygen reduction reaction (ORR). For this reason refresh cycles typically comprise short periods at low cell voltages (in this case corresponding to the air-break period), designed to force the cathode potential to values lower than 0.5 V where platinum oxide reduction is significantly enhanced [3,4]. In Fig. 3, the evolution of cell voltage (solid black curve) with time during the standard refresh cycle recommended by the manufacturer and adopted in this work is shown, together with the measurements of local cathode potential at the inlet and outlet of the cell (solid green and yellow curves respectively). The dashed red line indicates the target cathode potential during the air-break period of the refresh cycle, below which effective reduction of platinum oxide is known to occur [3]. At the beginning of the refresh cycle (at 0 s in Fig. 3) the electronic load is switched off while maintaining the flow of reactants, thus the OCV (typically about 0.85 V in DMFC with this operating conditions) is reached after several seconds. Interestingly, cathode mixed potential does not change significantly upon moving from cell operation at 0.25 A cm 2 to OCV, due to the large increase in the rate of methanol crossover when fuel consumption at the anode is interrupted [13]; moreover, during the OCV period the cathode potential is relatively homogenous moving from cathode inlet to outlet. However, when the air flow is interrupted (after 30 s in Fig. 3), triggering a drop in cell voltage to just below 0.1 V, the cathode potential local dynamics show a very non-uniform evolution, highlighting the different behaviour of the cell moving from inlet to outlet. The cathode potential at the outlet (solid yellow curve in Fig. 3) drops to values below 0.5 V in less than 10 s, reaching a steady value close to 0.2 V after about 20 s. This demonstrates the effectiveness of the refresh cycle in the outlet region in reducing surface platinum oxide coverage generated during operation [3,4] and is consistent with:
Fig. 3 e Comparison of evolution of cell voltage and local cathode potential at inlet and outlet during standard (solid lines) and improved (dashed lines) refresh cycles.
the decreasing trend of local cathode potential at the outlet during each 20 min period of uninterrupted operation following a refresh cycle (yellow curve in Fig. 2A), most likely due to the more effective reduction of platinum oxide in this region. This exposes the reduced platinum surface to repeated oxidation during the following operation period, resulting in the periodic decrease and recovery of local cathode potential during each cycle; the almost constant trend of local cathode potential at the outlet during the longer term test (yellow curve in Fig. 2B), where no further recovery seems possible with a full refresh. This highlights a low overall cathode temporary degradation in this region during cell operation, confirming the effectiveness of the refresh cycle in periodically reducing platinum oxides. In contrast, the rate of decrease of local cathode potential at the inlet during the air-break period (solid green curve in Fig. 3, after 30 s) is significantly slower than that at the outlet and reaches only around 0.6 V after 25 s. This indicates a much lower effectiveness of the refresh cycle at the inlet of the cell and again this interpretation is consistent with: the relatively constant cathode potential at the inlet during each uninterrupted 20 min period of operation following a refresh cycle (green curve in Fig. 2A). A high cathode surface oxide coverage would be maintained during such a locally ineffective refresh cycle leading to a steady trend during operation, in contrast to the decreasing trend in cathode potential at the outlet (yellow curve in Fig. 2A) which is related to formation of fresh oxides; the steady decrease in local cathode potential at the inlet due to progressive platinum oxidation during the longer term test (green curve in Fig. 2B). In this case a noticeable recovery is possible with the full refresh, where a lower potential at the cathode inlet can actually be reached, confirming the presence of unrecovered cathode temporary degradation during cell operation. Such heterogeneous evolution of cathode potential during the air-break phase of the refresh cycle is consistent with the analysis of anode potential dynamics performed in [9]. A comprehensive schematization is reported in Fig. 4. Availability of residual oxygen close to the air inlet region during the air-break period [11,12], most likely due to diffusion from the air inlet manifold and humidification system, could trigger heterogeneous operation of the fuel cell during the refresh cycle. The availability of oxygen for localised ORR at the cathode inlet could be the reason for the slower decrease rate of local potential compared to that at the outlet. This is also consistent with galvanic MOR taking place at the corresponding anode outlet side, identified in [9] in a sudden anode potential peak. Oppositely, shortage of oxygen promotes MOR of crossed-over methanol at cathode outlet, actually behaving as an electrolytic area, leading to the discussed fast decrease of cathode local potential. This can promote electrolytic hydrogen evolution reaction (HER) at the corresponding anode inlet region, whose local potential was consistently found in [9] to decrease close to 0 V vs RHE.
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Overall, the results presented here demonstrate the large scope for optimisation of DMFC performance that could be achieved with the proposed localised RHE technique. The approach is particularly effective at the cathode, revealing heterogeneous potential dynamics that would not be otherwise noticeable at all. Further insights could be obtained by coupling an array of external reference electrodes with current distribution measurement (i.e. a segmented cell setup), which would facilitate a deeper understanding of the local phenomena discussed above. Other than the heterogeneous reduction of platinum oxide reported here, other possible contributions to the nonuniform performance evolution at the cathode may arise from adsorption and desorption of contaminants and/or uneven membrane humidification cycles, which would require dedicated investigation.
Conclusions Fig. 4 e Schematic diagram showing localised cathode operation and in-plane current after air-break during OCV conditions, together with localised anode hydrogen evolution. Adapted with permission from [9]. Proton transport occurs both through-plane and in-plane across the ionomer membrane, leading to a drop in electrolyte potential as suggested in [11], while electron transport occurs in-plane, as schematized in Fig. 4. The key observation here is that the standard refresh cycle procedure results in a spatially varying evolution of cathode potential over long periods, which could lead to redistribution of current density over time and, possibly, non-homogeneous degradation of MEA components. In order to confirm this hypothesis, a minor modification to the refresh cycle was tested on the same experimental setup, in order to avoid any oxygen diffusion to the cathode during the air-break period. This was achieved by closing a valve between the air inlet manifold and the cell itself during the air-break period. The corresponding dynamics during this modified refresh cycle are reported in Fig. 3 (dashed lines), together with that of the standard refresh cycle (solid lines) previously discussed. The modified refresh cycle results in an increase in both the magnitude and the rate of decrease of cell voltage during the air-break period (to around 0.05 V, dashed black curve in Fig. 3), which appears to be mostly due to the change in potential dynamics at the cathode inlet (green curves, Fig. 3). This is ascribed to the more rapid consumption of oxygen by crossed-over MOR and ORR in the absence of additional oxygen diffusion from the inlet manifold. The cathode potential at the inlet reaches a potential of 0.3 V in the 30 s of air-break, albeit still at a slower rate than that at the outlet (dashed yellow curve, Fig. 4). The effects of this type of modified refresh cycle on the recovery of temporary degradation are still to be verified in a long-term test. It should also be borne in mind that potential cycling between high and low values is known to be detrimental for cathode durability [6], implying the need for careful evaluation of permanent degradation mechanisms associated with such recovery strategies.
We have applied an innovative external reference electrode to a DMFC that enables for the first time accurate localised measurement of cathode potential across the active area of the cell. The durability of the measurement setup has been demonstrated with a 200-h degradation test in the dry-air inlet/humid-air outlet environment of a DMFC cathode. This approach provides new insights into DMFC degradation phenomena, highlighting significant spatial variation in the effectiveness of platinum oxide reduction at the cathode during refresh cycles. The technique shows great promise for the improvement of long term DMFC performance via optimisation of refresh cycle protocols.
Acknowledgments This work has been performed in the frame of the FCHeJU FP7 projects Second Act (EC Grant Agreement n 621216) and the FCH-JU FP7 project H2FC European Research Infrastructure (EC Grant Agreement n 284522) under user access project 2071. The authors would like to thank Vania Cocca for the helpful support in the experimental measurements.
references
[1] Ong BC, Kamarudin SK, Basri S. Direct liquid fuel cells: a review. Int J Hydrogen Energy 2017;42:10142e57. https:// doi.org/10.1016/J.IJHYDENE.2017.01.117. [2] Bresciani F, Rabissi C, Zago M, Gazdzicki P, Schulze M, taz L, et al. A combined in-situ and post-mortem Gue investigation on local permanent degradation in a direct methanol fuel cell. J Power Sources 2016;306:49e61. https:// doi.org/10.1016/j.jpowsour.2015.11.105. [3] Eickes C, Piela P, Davey J, Zelenay P. Recoverable cathode performance loss in direct methanol fuel cells. J Electrochem Soc 2006;153:A171. https://doi.org/10.1149/1.2136073.
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[4] Park J-Y, Scibioh MA, Kim S-K, Kim H-J, Oh I-H, Lee TG, et al. Investigations of performance degradation and mitigation strategies in direct methanol fuel cells. Int J Hydrogen Energy 2009;34:2043e51. https://doi.org/10.1016/ j.ijhydene.2008.10.092. [5] Park J-Y, Lee J-H, Sauk J, Son I. The operating mode dependence on electrochemical performance degradation of direct methanol fuel cells. Int J Hydrogen Energy 2008;33:4833e43. https://doi.org/10.1016/ j.ijhydene.2008.04.060. [6] Zhao M, Shi W, Wu B, Liu W, Liu J, Xing D, et al. Analysis of carbon-supported platinum through potential cycling and potential-static holding. Int J Hydrogen Energy 2014;39:13725e37. https://doi.org/10.1016/ j.ijhydene.2014.03.121. [7] Santasalo-Aarnio A, Borghei M, Anoshkin IV, Nasibulin AG, Kauppinen EI, Ruiz V, et al. Durability of different carbon nanomaterial supports with PtRu catalyst in a direct methanol fuel cell. Int J Hydrogen Energy 2012;37:3415e24. https://doi.org/10.1016/j.ijhydene.2011.11.009. [8] Wongyao N, Therdthianwong A, Therdthianwong S, Senthil Kumar SM, Scott K. A comparison of direct methanol fuel cell
[9]
[10]
[11]
[12]
[13]
degradation under different modes of operation. Int J Hydrogen Energy 2013;38:9464e73. https://doi.org/10.1016/ j.ijhydene.2012.07.058. Rabissi C, Brightman E, Hinds G, Casalegno A. In operando investigation of anode overpotential dynamics in direct methanol fuel cells. Int J Hydrogen Energy 2016;41:18221e5. https://doi.org/10.1016/j.ijhydene.2016.08.140. Hinds G, Brightman E. In situ mapping of electrode potential in a PEM fuel cell. Electrochem Commun 2012;17:26e9. https://doi.org/10.1016/j.elecom.2012.01.007. Kulikovsky AA, Schmitz H, Wippermann K, Mergel J, Fricke B, Sanders T, et al. DMFC: galvanic or electrolytic cell? Electrochem Commun 2006;8:754e60. https://doi.org/ 10.1016/j.elecom.2006.03.011. Bresciani F, Rabissi C, Casalegno A, Zago M, Marchesi R. Experimental investigation on DMFC temporary degradation. Politecnico di Milano 2014. https://doi.org/10.1016/ j.ijhydene.2014.09.072. Zago M, Bisello A, Baricci A, Rabissi C, Brightman E, Hinds G, et al. On the actual cathode mixed potential in direct methanol fuel cells. J Power Sources 2016. https://doi.org/ 10.1016/j.jpowsour.2016.06.093.
Available online at www.sciencedirect.com
ScienceDirect journal homepage: www.elsevier.com/locate/he
Short Communication
In operando measurement of localised cathode potential to mitigate DMFC temporary degradation C. Rabissi a,*, E. Brightman b, G. Hinds b, A. Casalegno a a b
Politecnico di Milano, Energy Department, Via Lambruschini 4, Milano, Italy National Physical Laboratory, Hampton Road, Teddington, Middlesex, TW11 0LW, United Kingdom
article info
abstract
Article history:
An innovative external reference electrode technique has been applied to the cathode of an
Received 11 October 2017
operating DMFC in order to identify variations in electrode potential across the active area
Received in revised form
of the cell. The evolution of cathode potential at two different locations in the cell was
28 March 2018
monitored during operation, with the primary focus on studying the potential dynamics
Accepted 8 April 2018
during the temporary degradation recovery procedure, the so-called refresh cycle. The re-
Available online 27 April 2018
sults highlight for the first time a non-uniform local recovery of temporary degradation at the cathode during refresh cycles, associated with varying rates of platinum oxide reduction
Keywords:
across the cell, which could lead to current density redistribution and contribute to an
Direct methanol fuel cell
uneven degradation of the components. The technique shows great promise for the
Cathode potential
improvement of long term DMFC performance via optimisation of refresh cycle protocols.
Temporary degradation
© 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Reference electrode Platinum oxides
Introduction Liquid-fed direct methanol fuel cell (DMFC) technology has several advantages over conventional hydrogen-fed polymer electrolyte membrane fuel cells (PEMFCs); methanol is a higher energy density liquid, stable under a wide range of conditions and easier to store and transport. However, the slower methanol oxidation reaction (MOR), together with more complex liquid transport and fuel crossover phenomena, results in markedly lower efficiency of DMFCs [1]. This limits them at present to portable and low power applications, although efforts are ongoing to improve the technology. The significant anode overpotential for the MOR [1,2] results in a relatively
low current density and a higher cathode potential during operation. It is therefore critical to distinguish the contribution of each electrode to the overall cell voltage, particularly during the study of temporary and permanent degradation of DMFC cathodes, where the former is mostly related to platinum oxide formation [3,4] and the latter to mechanisms such as Ostwald ripening and Pt dissolution [2,5e8]. Moreover, strongly heterogeneous degradation across the membrane electrode assembly (MEA) has been reported in [2], which was attributed by the authors to spatial variations in operating conditions at both anode and cathode, emphasising the need for further investigation of localised performance and degradation.
* Corresponding author. E-mail address: [email protected] (C. Rabissi). https://doi.org/10.1016/j.ijhydene.2018.04.043 0360-3199/© 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
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For these reasons, in previous work [9] the authors demonstrated the effectiveness of an innovative throughplane reference electrode technique in performing localised in operando measurement of anode overpotential during DMFC operation, beyond what had been conventionally possible using the cathode as an internal pseudo-hydrogen reference electrode. Moreover, as discussed in [9], the direct connection of such reference electrodes on the catalyst layers active surface permits to overcome the major limitations of traditional fuel cell reference electrodes: ohmic drop, potential distribution due to electrode misalignment and edge effects in the membrane are negated. The investigation focused particularly on the short operation breaks (known as refresh cycles) typically performed to recover temporary degradation during operation [2]. It was demonstrated that anode operation is responsible for more than half of long-term temporary degradation, highlighting the homogeneous ineffectiveness of refresh cycles in fully recovering anode performance. At the same time, analysis of the anode potential during refresh cycles revealed the presence of hydrogen evolution in half of the cell; it is likely that such a localised process could introduce unpredictable heterogeneities in cathode operation. This paper focuses on the investigation of cathode overpotential during DMFC operation, using the same locallyresolved reference electrode approach. Analysis of localised temporary degradation is carried out over timescales ranging from minutes to hundreds of hours, as well as during fast transient refresh cycles, in order to highlight critical local degradation mechanisms and potential mitigation strategies.
Experimental DMFC MEAs manufactured by EWII Fuel Cells A/S of 25 cm2 active area were used for this investigation. The membrane was Nafion® 115 and the anode and cathode catalyst loadings were respectively 1.8 mg cm 2 (PtRu alloy) and 1.2 mg cm 2 (Pt), both coated with Sigracet® SGL35DC diffusion layers (overall thickness 325 mm, 20% PTFE content) and provided with a microporous layer (MPL). In order to investigate phenomena associated to actual nominal operation, operating conditions and protocols have been fixed to those indicated by MEAs manufacturer. Thus, anode and cathode were respectively fed with 1.0 M methanol solution and air saturated with water vapour at ambient temperature (stoichiometries equal to 6 and 3 respectively at 0.25 A cm 2) in a counter-flow configuration through triple serpentine flow plates. Nominal current density and cell operating temperature were respectively 0.25 A cm 2 and 75 C. The overall experimental setup, thoroughly described in [2], is schematized in Fig. 1A, including reference electrodes measurement at both anode, discussed in [9], and cathode of the fuel cell. The local reference electrode measurement setup is thoroughly explained in [10] and carefully follows the procedures established in [9], this time also providing measurements on the cathode side of the cell. Briefly, the in situ reference electrode is based on a salt bridge made from fully watersaturated Nafion® tubing, which directly connects the external surface of the GDL with an electrolyte solution in
Fig. 1 e Schematic diagram of A) experimental setup and B) cathode DMFC flow-plate (showing RHE locations).
which a Gaskatel HydroFlex® reversible hydrogen electrode (RHE) is immersed. At the appropriate locations within the GDL, a small amount (about 2 mL) of Nafion® polymer solution is dispersed in order to realise the necessary proton conductive path to the catalyst layer. In order to achieve an insight into the local evolution of cathode potential, two measurement points were employed, close to the inlet and outlet of the cell as shown in Fig. 1B, analogous to the previous anode measurements [9]. The evolution of cathode potential at the two measurement locations was monitored during a 200-h degradation test, adopting periodic refresh cycles every 20 min and a full refresh procedure [2] after 100 h. The refresh cycles consist of 1 min interruptions to galvanic cell operation, using a combination of 30 s at open circuit voltage (OCV) and 30 s of air feeding interruption period at the cathode (named as air-break period in the following), while keeping anode flow uninterrupted. Analysis of cathode potential
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dynamics during a refresh cycle was performed in order to investigate the effects on the cathode potential of the strongly localised hydrogen evolution mechanism highlighted in [9]. The full refresh procedure consists instead in a prolonged operation interruption (~16 h) keeping an uninterrupted low mass flow of methanol solution (~0.3 g min 1) in the anode serpentine, while maintaining the cell temperature at 75 C and both cathode inlet and outlet manifolds plugged to avoid excessive oxygen infiltrations [2]. As reported in [9], no noticeable effects on cell operation of the presence of the RHEs were observed over the 0e0.4 A cm 2 current density range. The uncertainty in the potential measured by the reference electrodes is estimated to be ±5 mV. All potentials in this work are quoted relative to the RHE.
Results and discussion In order to analyse the transient decay and recovery of cell performance over relatively short timescales, the evolution of cell and cathode potential at both inlet and outlet over a period of 40 min of cell operation, including a refresh cycle, is shown in Fig. 2A. Just after the refresh cycle (20 min in Fig. 2A), a fast initial overshoot in cell voltage occurs, together with a noticeable recovery in performance. The anode potential measurements in [9] revealed that this was dominated by the anode response, possibly due to the depletion of the small quantity of hydrogen evolved during the air-break period of the refresh cycle itself [10e12]. The cathode potential measurements reported here are consistent with this hypothesis, with no increase in cathode potential observed during the fast transient immediately following the refresh cycle either at inlet or outlet of the cell (Fig. 2A). In fact, the cathode potential at each location show a steady or monotonically decreasing trend. As mentioned previously and already discussed in [13], the relatively high DMFC anode potential (close to 0.4 V [9]) necessitates operation at a lower current density (0.25 A cm 2) resulting in a higher cathode potential than for a PEMFC. Indeed, excluding the initial transient following the refresh cycle, these measurements confirm that the cathode operates at a potential slightly lower than 0.9 V at the outlet and about 50 mV higher than this value at the inlet. This difference results from the combination of the higher oxygen concentration [13] togheter with higher membrane loss due to dry air feeding, higher anode loss at anode outlet [9] and lower methanol crossover. During each single uninterrupted operation period (i.e. from 1 min to 20 min and from 21 min to 40 min) the cell voltage decreases by about 20 mV. The local cathode potential measurements reveal much more heterogeneous behaviour, with the cathode potential remaining almost constant at the inlet but decreasing by about 30 mV at the outlet (Fig. 2A). This is in contrast to the local anode measurements for the same cell in [9], which showed a much more homogeneous trend. The effect of the refresh cycle on the cathode potential also varies significantly with position; the interruption (at 20 min in Fig. 2A) results in a noticeable performance recovery close
Fig. 2 e Evolution of cell voltage (black curve) and local cathode potential at inlet (green curve) and outlet (yellow curve) during 40 min operation, including a refresh cycle after 20 min (A) and during 200 h operation with refresh cycles every 20 min, including a full refresh after 100 h (B). (For interpretation of the references to color/colour in this figure legend, the reader is referred to the Web version of this article).
to the outlet (yellow curve in Fig. 2A), but has nearly no effect at the inlet (green curve in Fig. 2A). In order to quantify the cathode temporary degradation not recovered by refresh cycles, the test was extended to 200 h, as shown in Fig. 2B. The recovery in cell voltage following the full refresh at 100 h was about 40 mV, consistent with previous work which also showed that more than 30 mV of this value could be attributed to relatively homogeneous recovery in anode potential across the area of the cell [9]. However, the local cathode potential measurements in Fig. 2B reveal that the effectiveness of the refresh cycles in recovering cathode performance varies strongly from inlet to outlet of the cell. The full refresh produces a 30 mV recovery in cathode potential at the inlet (green curve, Fig. 2B) but no change at the outlet (yellow curve, Fig. 2B), where the periodic refresh cycles appear to have been fully effective in recovering temporary degradation. In order to better explain such interesting behaviour, a deeper investigation of potential evolution during refresh cycles was performed. The main mechanism for cathode temporary
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degradation is known to be platinum surface oxidation, which is enhanced at the relatively high potential of DMFC operation [3,4] and tends to progressively decrease the active surface area for the oxygen reduction reaction (ORR). For this reason refresh cycles typically comprise short periods at low cell voltages (in this case corresponding to the air-break period), designed to force the cathode potential to values lower than 0.5 V where platinum oxide reduction is significantly enhanced [3,4]. In Fig. 3, the evolution of cell voltage (solid black curve) with time during the standard refresh cycle recommended by the manufacturer and adopted in this work is shown, together with the measurements of local cathode potential at the inlet and outlet of the cell (solid green and yellow curves respectively). The dashed red line indicates the target cathode potential during the air-break period of the refresh cycle, below which effective reduction of platinum oxide is known to occur [3]. At the beginning of the refresh cycle (at 0 s in Fig. 3) the electronic load is switched off while maintaining the flow of reactants, thus the OCV (typically about 0.85 V in DMFC with this operating conditions) is reached after several seconds. Interestingly, cathode mixed potential does not change significantly upon moving from cell operation at 0.25 A cm 2 to OCV, due to the large increase in the rate of methanol crossover when fuel consumption at the anode is interrupted [13]; moreover, during the OCV period the cathode potential is relatively homogenous moving from cathode inlet to outlet. However, when the air flow is interrupted (after 30 s in Fig. 3), triggering a drop in cell voltage to just below 0.1 V, the cathode potential local dynamics show a very non-uniform evolution, highlighting the different behaviour of the cell moving from inlet to outlet. The cathode potential at the outlet (solid yellow curve in Fig. 3) drops to values below 0.5 V in less than 10 s, reaching a steady value close to 0.2 V after about 20 s. This demonstrates the effectiveness of the refresh cycle in the outlet region in reducing surface platinum oxide coverage generated during operation [3,4] and is consistent with:
Fig. 3 e Comparison of evolution of cell voltage and local cathode potential at inlet and outlet during standard (solid lines) and improved (dashed lines) refresh cycles.
the decreasing trend of local cathode potential at the outlet during each 20 min period of uninterrupted operation following a refresh cycle (yellow curve in Fig. 2A), most likely due to the more effective reduction of platinum oxide in this region. This exposes the reduced platinum surface to repeated oxidation during the following operation period, resulting in the periodic decrease and recovery of local cathode potential during each cycle; the almost constant trend of local cathode potential at the outlet during the longer term test (yellow curve in Fig. 2B), where no further recovery seems possible with a full refresh. This highlights a low overall cathode temporary degradation in this region during cell operation, confirming the effectiveness of the refresh cycle in periodically reducing platinum oxides. In contrast, the rate of decrease of local cathode potential at the inlet during the air-break period (solid green curve in Fig. 3, after 30 s) is significantly slower than that at the outlet and reaches only around 0.6 V after 25 s. This indicates a much lower effectiveness of the refresh cycle at the inlet of the cell and again this interpretation is consistent with: the relatively constant cathode potential at the inlet during each uninterrupted 20 min period of operation following a refresh cycle (green curve in Fig. 2A). A high cathode surface oxide coverage would be maintained during such a locally ineffective refresh cycle leading to a steady trend during operation, in contrast to the decreasing trend in cathode potential at the outlet (yellow curve in Fig. 2A) which is related to formation of fresh oxides; the steady decrease in local cathode potential at the inlet due to progressive platinum oxidation during the longer term test (green curve in Fig. 2B). In this case a noticeable recovery is possible with the full refresh, where a lower potential at the cathode inlet can actually be reached, confirming the presence of unrecovered cathode temporary degradation during cell operation. Such heterogeneous evolution of cathode potential during the air-break phase of the refresh cycle is consistent with the analysis of anode potential dynamics performed in [9]. A comprehensive schematization is reported in Fig. 4. Availability of residual oxygen close to the air inlet region during the air-break period [11,12], most likely due to diffusion from the air inlet manifold and humidification system, could trigger heterogeneous operation of the fuel cell during the refresh cycle. The availability of oxygen for localised ORR at the cathode inlet could be the reason for the slower decrease rate of local potential compared to that at the outlet. This is also consistent with galvanic MOR taking place at the corresponding anode outlet side, identified in [9] in a sudden anode potential peak. Oppositely, shortage of oxygen promotes MOR of crossed-over methanol at cathode outlet, actually behaving as an electrolytic area, leading to the discussed fast decrease of cathode local potential. This can promote electrolytic hydrogen evolution reaction (HER) at the corresponding anode inlet region, whose local potential was consistently found in [9] to decrease close to 0 V vs RHE.
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Overall, the results presented here demonstrate the large scope for optimisation of DMFC performance that could be achieved with the proposed localised RHE technique. The approach is particularly effective at the cathode, revealing heterogeneous potential dynamics that would not be otherwise noticeable at all. Further insights could be obtained by coupling an array of external reference electrodes with current distribution measurement (i.e. a segmented cell setup), which would facilitate a deeper understanding of the local phenomena discussed above. Other than the heterogeneous reduction of platinum oxide reported here, other possible contributions to the nonuniform performance evolution at the cathode may arise from adsorption and desorption of contaminants and/or uneven membrane humidification cycles, which would require dedicated investigation.
Conclusions Fig. 4 e Schematic diagram showing localised cathode operation and in-plane current after air-break during OCV conditions, together with localised anode hydrogen evolution. Adapted with permission from [9]. Proton transport occurs both through-plane and in-plane across the ionomer membrane, leading to a drop in electrolyte potential as suggested in [11], while electron transport occurs in-plane, as schematized in Fig. 4. The key observation here is that the standard refresh cycle procedure results in a spatially varying evolution of cathode potential over long periods, which could lead to redistribution of current density over time and, possibly, non-homogeneous degradation of MEA components. In order to confirm this hypothesis, a minor modification to the refresh cycle was tested on the same experimental setup, in order to avoid any oxygen diffusion to the cathode during the air-break period. This was achieved by closing a valve between the air inlet manifold and the cell itself during the air-break period. The corresponding dynamics during this modified refresh cycle are reported in Fig. 3 (dashed lines), together with that of the standard refresh cycle (solid lines) previously discussed. The modified refresh cycle results in an increase in both the magnitude and the rate of decrease of cell voltage during the air-break period (to around 0.05 V, dashed black curve in Fig. 3), which appears to be mostly due to the change in potential dynamics at the cathode inlet (green curves, Fig. 3). This is ascribed to the more rapid consumption of oxygen by crossed-over MOR and ORR in the absence of additional oxygen diffusion from the inlet manifold. The cathode potential at the inlet reaches a potential of 0.3 V in the 30 s of air-break, albeit still at a slower rate than that at the outlet (dashed yellow curve, Fig. 4). The effects of this type of modified refresh cycle on the recovery of temporary degradation are still to be verified in a long-term test. It should also be borne in mind that potential cycling between high and low values is known to be detrimental for cathode durability [6], implying the need for careful evaluation of permanent degradation mechanisms associated with such recovery strategies.
We have applied an innovative external reference electrode to a DMFC that enables for the first time accurate localised measurement of cathode potential across the active area of the cell. The durability of the measurement setup has been demonstrated with a 200-h degradation test in the dry-air inlet/humid-air outlet environment of a DMFC cathode. This approach provides new insights into DMFC degradation phenomena, highlighting significant spatial variation in the effectiveness of platinum oxide reduction at the cathode during refresh cycles. The technique shows great promise for the improvement of long term DMFC performance via optimisation of refresh cycle protocols.
Acknowledgments This work has been performed in the frame of the FCHeJU FP7 projects Second Act (EC Grant Agreement n 621216) and the FCH-JU FP7 project H2FC European Research Infrastructure (EC Grant Agreement n 284522) under user access project 2071. The authors would like to thank Vania Cocca for the helpful support in the experimental measurements.
references
[1] Ong BC, Kamarudin SK, Basri S. Direct liquid fuel cells: a review. Int J Hydrogen Energy 2017;42:10142e57. https:// doi.org/10.1016/J.IJHYDENE.2017.01.117. [2] Bresciani F, Rabissi C, Zago M, Gazdzicki P, Schulze M, taz L, et al. A combined in-situ and post-mortem Gue investigation on local permanent degradation in a direct methanol fuel cell. J Power Sources 2016;306:49e61. https:// doi.org/10.1016/j.jpowsour.2015.11.105. [3] Eickes C, Piela P, Davey J, Zelenay P. Recoverable cathode performance loss in direct methanol fuel cells. J Electrochem Soc 2006;153:A171. https://doi.org/10.1149/1.2136073.
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[4] Park J-Y, Scibioh MA, Kim S-K, Kim H-J, Oh I-H, Lee TG, et al. Investigations of performance degradation and mitigation strategies in direct methanol fuel cells. Int J Hydrogen Energy 2009;34:2043e51. https://doi.org/10.1016/ j.ijhydene.2008.10.092. [5] Park J-Y, Lee J-H, Sauk J, Son I. The operating mode dependence on electrochemical performance degradation of direct methanol fuel cells. Int J Hydrogen Energy 2008;33:4833e43. https://doi.org/10.1016/ j.ijhydene.2008.04.060. [6] Zhao M, Shi W, Wu B, Liu W, Liu J, Xing D, et al. Analysis of carbon-supported platinum through potential cycling and potential-static holding. Int J Hydrogen Energy 2014;39:13725e37. https://doi.org/10.1016/ j.ijhydene.2014.03.121. [7] Santasalo-Aarnio A, Borghei M, Anoshkin IV, Nasibulin AG, Kauppinen EI, Ruiz V, et al. Durability of different carbon nanomaterial supports with PtRu catalyst in a direct methanol fuel cell. Int J Hydrogen Energy 2012;37:3415e24. https://doi.org/10.1016/j.ijhydene.2011.11.009. [8] Wongyao N, Therdthianwong A, Therdthianwong S, Senthil Kumar SM, Scott K. A comparison of direct methanol fuel cell
[9]
[10]
[11]
[12]
[13]
degradation under different modes of operation. Int J Hydrogen Energy 2013;38:9464e73. https://doi.org/10.1016/ j.ijhydene.2012.07.058. Rabissi C, Brightman E, Hinds G, Casalegno A. In operando investigation of anode overpotential dynamics in direct methanol fuel cells. Int J Hydrogen Energy 2016;41:18221e5. https://doi.org/10.1016/j.ijhydene.2016.08.140. Hinds G, Brightman E. In situ mapping of electrode potential in a PEM fuel cell. Electrochem Commun 2012;17:26e9. https://doi.org/10.1016/j.elecom.2012.01.007. Kulikovsky AA, Schmitz H, Wippermann K, Mergel J, Fricke B, Sanders T, et al. DMFC: galvanic or electrolytic cell? Electrochem Commun 2006;8:754e60. https://doi.org/ 10.1016/j.elecom.2006.03.011. Bresciani F, Rabissi C, Casalegno A, Zago M, Marchesi R. Experimental investigation on DMFC temporary degradation. Politecnico di Milano 2014. https://doi.org/10.1016/ j.ijhydene.2014.09.072. Zago M, Bisello A, Baricci A, Rabissi C, Brightman E, Hinds G, et al. On the actual cathode mixed potential in direct methanol fuel cells. J Power Sources 2016. https://doi.org/ 10.1016/j.jpowsour.2016.06.093.