Insight into interface behaviors to building a phase-boundary-matched

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Insight into interface behaviors to building a phaseboundary-matched Na-ion direct liquid fuel cell Yinshi Li, Y. Feng, and X.D. Sun ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b02084 • Publication Date (Web): 01 Sep 2018 Downloaded from http://pubs.acs.org on September 2, 2018

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ACS Sustainable Chemistry & Engineering

Insight into interface behaviors to building phase-boundary-matched Na-ion direct liquid fuel cells

Yinshi Li* †, Ying Feng†, Xianda Sun† ,

†

Key Laboratory of Thermo-Fluid Science and Engineering of MOE, School of Energy and

Power Engineering, Xi’an Jiaotong University, No. 28 Xianning West Road, Xi’an, Shaanxi 710049, China * Corresponding Author. E-mail: [email protected] Tel.: (+86) 29 8266 5582; fax: (+86) 29 8266 5445.

Keywords: Fuel cell; Alkaline fuel cell; Triple-phase boundary; Double-phase boundary; Cation-exchange membrane; Anion-exchange membrane; Anion ionomer

ACS Paragon Plus Environment

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ABSTRACT: One of the targets associated with developing high-performance alkaline direct liquid fuel cells is to maximize the utilization of catalysts and minimize the limitation of anion materials that suffer from low ion conductivity and thermal and chemical instability. Herein, an ionomer-free and phase-boundary-matched Na-ion direct formate fuel cell (Na-DFFC) was reported to address this issue from the viewpoint of meeting interface transport behaviors for electrochemical reactions. A proof-of-concept phase-boundary-matched Na-DFFC including neither ionomers nor additional liquid electrolyte yields a peak power density as high as 45 mW cm , primarily because the dissociation and hydrolysis of formate on anode and the generation of -2

NaOH on cathode enable electrodes to possess sufficient Na and OH , thereby increasing the +

-

double-phase boundary density, leading to a high catalyst utilization. Additionally, a stable 60min constant current discharge at 90 C proves the conceptual feasibility of the high-temperature o

Na-DFFC. These results present a new scheme towards high-performance alkaline direct liquid fuel cells in terms of meeting all requirements of electrochemical kinetics and mass and charge transport characteristics.


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Introduction The intermittent nature of wind and solar power output calls for a robust mean for large-scale energy storage. Among the thermal, chemical and mechanical energy storages, chemicals as 1–5

energy carriers have been considered as one of the most prospective candidates, primarily owing to their high energy density, facile storage and transportation. When releasing energy, fuel cell 6–8

that directly converts chemical energy stored in fuel (e.g., alcohol and hydrogen) into electricity holds promise in mobile, stationary, and portable power applications considering its striking merits of high energy-conversion efficiency, fast refueling, and environmental friendliness, especially the alkaline direct liquid fuel cells (DLFCs).

13–16

9–12

This is because the alkaline

environment accelerates electrochemical kinetics of redox reactions even using non-noble metals and nonmetal catalysts, let alone suppressing corrosion of electrode materials.

17–21

The anion-exchange membrane (AEM) is usually used in alkaline DLFCs to conduct OH from -

cathode to anode,

22–25

the state-of-the-art AEM as compared with its counterpart, cation-exchange

membrane (CEM), however, is still limited by: i) the low ion conductivity which is much lower than the commercialized proton-exchange membrane (PEM); ii) the poor thermal stability, typically, less than 60 C; and iii) the weak chemical stability due to the direct nucleophilic o

displacement and Hoffmann elimination reaction.

26–28

One effective solution for this issue is to

replace AEM with CEM, such as cation-exchange membrane direct methanol fuel cell (DMFC),

29

cation-exchange membrane direct ethanol fuel cell (DEFC), and cation-exchange membrane 30

direct borohydride fuel cell (DBFC).

31

Considering the electrode reactions, in acid PEM DLFCs, creating triple-phase boundary (TPB, see Figure 1a) that consists of H -conducting phase (typically, Nafion ionomer ), liquid phase +

32,33


3


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Figure 1. Schematic illustration of a) the conventional triple-phase boundary, b) interface without ion-conducting pathway, c) interface without reactant-delivering pathway, d) interface without electron-conducting pathway, e) quasi-quadruple-phase boundary, f) double-phase boundary, g) quadruple-phase boundary and h) triple-phase boundary. (e.g., methanol

34,35

carbon nanotube,

and ethanol ), and electron-conducting phase (typically, carbon black,

38,39

15,19

36,37

and graphene ) is of vital importance. The availability of electrode TPBs 40,41

directly determines the electrochemical reactions. However, there exist regions in which the interaction of catalyst/ionomer/reactant is not enough as a result of lacking: i) ion-conducting pathway as demonstrated in Figure 1b; ii) reactant-delivering pathway as illustrated in Figure 1c; and iii) the electron-conducting pathway as shown in Figure 1d, leading to the deactivated triplephase boundary. In this context, the catalyst utilization of the conventional electrodes falls into a significantly low level of 20-35%. Let us look back to the alkaline CEM DLFCs. To ensure that 42


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the CEM DLFCs possess the advantages of alkaline media for electrode reactions, conventionally, the additional liquid electrolyte (typically NaOH) needs to be added to the fuel solution. Therefore, Na but not H as a charge carrier penetrates through membrane from anode +

+

to cathode. However, the demand of a high system energy density requires the concentration of 37

the added liquid electrolyte to be very low, even nought, resulting in a slow ionic mobility. In this regard, theoretically, the high-conductivity cation and anion ionomers need to be involved in the TPB to facilitate the conduction of Na and OH , respectively. As a result, the triple-phase +

-

boundary goes up to a quasi-quadruple-phase boundary (QPB), which consists of Na -conducting +

phase, OH -conducting phase, liquid phase, and electron-conducting phase as demonstrated in -

Figure 1e, thus leading to a much lower catalyst utilization. Moreover, similar to the AEM, the state-of-the-art anion ionomer also suffers from the low ion conductivity, as well as the thermal and chemical instability, further reducing the cell performance. Therefore, one of the targets in 27

developing high-performance alkaline CEM DLFC is how to maximize QPB density and minimize the limitation of anion ionomer. Apparently, along with striving for synthesizing high-performance ionomer materials, it is essential to develop new strategy to extend the so-called QPB density for maximizing the catalyst utilization. Herein, a phase-boundary-matched Na-ion direct formate fuel cell (Na-DFFC) was reported to address this issue from the viewpoint of meeting mass and charge transports for redox reactions. It has been demonstrated that this system yields a peak power density as high as 45 mW cm without anion/cation ionomer and additional liquid electrolyte, proving the -2

conceptual feasibility. Figure 2 illustrates the schematic of a phase-boundary-matched Na-ion direct formate fuel cell that is composed of a cation-exchange membrane sandwiched between anode and cathode


5


electrodes. On the anode, the sodium formate,

21,43,44

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HCOONa, as the fuel, was fed to anode

electrode. When HCOONa dissolves in liquid water, Na can be dissociated from sodium formate +

molecule, i.e.:

HCOONa ® HCOO- + Na +

(1)

Meanwhile, OH can be derived from the hydrolysis reaction of formate according to: -

37

HCOO- + H 2O « HCOOH + OH-

(2)

Equations (1) and (2) reveal that in sodium formate aqueous solution, there exist Na , OH , and +

-

HCOONa. The presence of the Na , OH , and HCOONa in fuel solution allows the mass-transport +

-

pathway (HCOONa) also to act as the charge-conducting pathway (Na and OH ) as shown in the +

-

bottom-left enlarger of Figure 2. Accordingly, the deactivated regions that are short of ionconducting pathway (see Figure 1b) or reactant-delivering pathway (see Figure 1c) can be markedly activated. This means that the complicated quasi-quadruple-phase boundary is simplified as double-phase boundary (DPB) as illustrated in Figure 1f. Namely, the formate oxidation reaction (FOR) can readily takes place at the catalyst/liquid interface. As such, theoretically, both anion ionomer and cation ionomer can be eliminated from the anode electrode. Therefore, a conceptual ionomer-free and phase-boundary-matched anode comes true. In the present work, considering the tradeoff between electron conduction and ion transports, the neutral polytetrafluoroethylene (PTFE) as the binder was incorporated into anode catalyst layer as demonstrated in Figure 2, wherein the FOR takes place:

HCOO- + 3OH - ® CO32- + 2H 2O + 2e -

(3)

Na , generated from anode fuel solution, as a charge carrier, migrates through cation-exchange +

membrane from anode to cathode. Meanwhile, on the cathode, the oxygen is reduced to produce


6

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Figure 2. Schematic illustration of the phase-boundary-matched Na-ion direct formate fuel cells. OH , the oxygen reduction reaction (ORR) can be expressed as: -

(1 2) O2 + H2O + 2e- ® 2OH-

(4)

Subsequently, the Na interacts with the OH to form NaOH. Accordingly, instead of both anion +

-

ionomer and cation ionomer, the existence of the alkaline solution supplies a fast chargeconducting pathway for Na and OH as illustrated in the bottom-right enlarger of Figure 2. +

-

Although appealing, noted that it cannot deliver oxygen quickly. As a result, unlike the anodic DPB, the cathodic QPB that comprises Na -conducting phase, OH -conducting phase, gas phase, +

-

and electron-conducting phase (see Figure 1g) can be just reduced to TPB as demonstrated in Figure 1h. That is, the ORR occurs at the catalyst/liquid/gas interface, thereby making a conceptual ionomer-free and phase-boundary-matched cathode become reality. When preparing cathode electrode in this work, the PTFE as a hydrophobic binder was impregnated into cathode catalyst layer to benefit the gas-liquid two-phase fluid flow as shown in Figure 2.


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To sum up, an ionomer-free and phase-boundary-matched Na-ion direct formate fuel cell is successfully established taking a view of meeting mass and charge transports for electrochemical reactions. Additionally, the use of the hydrophobic PTFE binder not only allows the Na-DFFC operating in a high temperature (˃60 C) but also increasing catalyst utilization, thereby o

promising a better cell performance. Experimental Section Nafion 211 membrane and Nafion solution were purchased from DuPont. Palladium chloride (PdCl ) and Vulcan XC-72 carbon black were from Sigma-Aldrich and Cabot, respectively. 2

Carbon paper was received from Toray. Polytetrafluoroethylene emulsion and Quaternary ammonia polysulfone (QAPS) ionomer solution were separately purchased from Sigma-Aldrich and Hephasenergy. NaOH, HCOONa and ethanol were from Aladdin. The cation-exchange membrane, Nafion 211, was treated to conduct Na . The membrane was +

first soaked in 2.5 M NaOH solution for 10 h, and then heated at 80 C for 1 h, subsequently o

cooled down to room temperature and rinsed in deionized (DI) water to remove the residual chemicals. The membrane electrode assembly comprises a Na -conducting cation-exchange membrane +

sandwiched between the home-made anode and cathode electrodes. The carbon-supported Pd nanoparticles (Pd/C) were synthesized by a simultaneous reduction method. Catalyst inks were 21

prepared by mixing the Pd/C with the polymer, such as PTFE, Nafion, and QAPS, using ethanol as the solvent, then the catalyst ink was stirred continuously 20 min in an ultrasonic bath to ensure that they were well dispersed. To fabricate anode and cathode electrodes, the as-prepared catalyst ink with the required loading was brushed onto a carbon paper-supported micro porous layer that consists of 4.0 mg cm carbon black and 40 wt.% PTFE. -2


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Cyclic voltammetry (CV) experiments were carried out by an electrochemical workstation (AUTOLAB PGSTAT302N, Eco Chemie B.V.) in a three-electrode electrochemical cell, in which the as-prepared electrode with and without ionomer, the Pt foil, and the Hg-HgO (MMO, 1.0 M KOH, 0.098V vs. SHE) electrode were used as the working electrode, the counter electrode, and the reference electrode, respectively. When conducting the electrochemical characterization, all the working electrodes have the same catalyst loading of 1.0 mg cm-2 and polymer content of 15 wt.%. The potential window for the CV tests was from -0.926 to 0.274 V, and CV tests were performed in 1.0 M NaOH solution at a scan rate of 50 mV s . -1

The as-prepared MEA was sandwiched between a pair of current collectors which contains a single serpentine flow field with 1.0 mm channel width, 0.5 mm channel depth, and 1.0 mm rib width. The assembled fuel cell is connected to the electrochemical workstation (Arbin BT-G, Arbin Instrument Inc.), and then the voltage-current curves are measured and recorded. Aqueous solutions of HCOONa were directly fed to anode by a peristaltic pump at 1.0 mL min . 99.5% -1

oxygen was supplied to cathode by a mass flow controller (Omega FMA series, USA). The electric heating rod is inserted in the current collectors to control operating temperature. Results and discussion To demonstrate the feasibility of the ionomer-free and phase-boundary-matched Na-ion direct formate fuel cell, the cell performances of Na-DFFCs were investigated as shown in Figure 3 at 90 C by testing different MEAs, which consisted of the same Na -conducting cation-exchange o

+

membrane, the same electrode catalysts, but different electrode polymers: the neutral polymer


9


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Figure 3. Polarization and power density curves of the Na-DFFC with and without ionomer. Anode: HCOONa, 1.0 M, 1.0 mL min ; Cathode: oxygen, 5 sccm, ambient pressure; Operating -1

temperature: 90 C; Ionomer-free Na-DFFC: 1.0-mg cm Pd/C and 15wt.%-PTFE anode +Nafion o

Pd

-2

211+1.0-mg cm Pd/C and 15wt.%-PTFE cathode; Bi-ionomer Na-DFFC: 1.0-mg cm Pd/C and Pd

-2

Pd

-2

15wt.%-bi-ionomer anode+Nafion 211+1.0-mg cm Pd/C and 15wt.%-bi-ionomer cathode. Pd

-2

(ionomer-free, represented by the square symbols) and the conductive ionomer mixed between cation ionomer (Nafion) and anion ionomer (QAPS) with an ion-exchange-capacity ratio of 1:1 (represented by the circle symbols), hereafter referenced as bi-ionomer. It can be seen from the figure that when feeding 1.0 M-HCOONa aqueous solution without additional liquid electrolyte to anode, the ionomer-free Na-DFFC that incorporates the neutral polymer as a binder into both anode and cathode electrodes yields a peak power density of 20 mW cm and a limiting current -2

density of 75 mA cm , indicating the conceptual rightness. While the peak power density and the -2


10

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limiting current density decreases respectively to 13 mW cm and 55 mA cm when replacing the -2

-2

neutral binder with the conductive bi-ionomer, suggesting the superiority of the ionomer-free Na-DFFC. This phenomenon can be mainly explained as follows. i) the enhanced charge transport: either the dissociation and hydrolysis of formate on anode or the generation of NaOH on cathode enables the ionomer-free electrodes to possess sufficient Na and OH , resulting in +

-

activating the disconnected ion-conducting pathway (Figures. 1b and 2) and promising a higher ionic mobility, thereby reducing the ohmic loss as compared with the bi-ionomer electrodes, as evidenced from the slopes of the polarization curves shown in Figure 3; and ii) the enhanced mass transfer and the increased electron conduction: when incorporating the conductive biionomer into catalyst layers, the catalyst nanoparticles were covered by both anion-ionomer film and cation-ionomer film to form the large agglomerates, lowering active sites. In contrast, the 19

neutral PTFE inclines to adhere but not cover the catalyst particles together to form the smaller and porous agglomerates,

19

leading to raising catalyst utilization and facilitating electron

conduction as a result of an increase in active sites, as confirmed by the electrochemical active surface area (ECSA) of the reduction peak area of the palladium oxide shown in Figure 4. The ECSA of the ionomer-free electrode is 28.2 m g , larger than that of the bi-ionomer electrode 2

-1

with 20.4 m g . On the other hand, it should be mentioned that due to the cathode two-phase 2

-1

fluid flow, including liquid water and gas oxygen, the hydrophobic PTFE is beneficial to reduce the oxygen transport resistance. More importantly, the stability of the neutral PTFE is capable of making the ionomer-free Na-DFFC operate at an elevated temperature, thus further improving cell performance. Additionally, even if a single ionomer was employed in electrode catalyst layers, owing to the above-mentioned reasons, the anion-ionomer/cation-ionomer Na-DFFC still


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yields a lower cell performance than the ionomer-free Na-DFFC does (see Figure S1, Supporting Information).

Figure 4. Cyclic voltammograms of electrodes with and without ionomer in 1.0 M NaOH solution, scan rate: 50 mV s . -1

Both anode DPB density and cathode TPB density are significantly affected by the electrode composition, including catalyst loading and polymer content. Figure 5 shows the effect of anode catalyst loading on cell performance. It can be found that when increasing Pd loading from 1.0 mg cm to 4.0 mg cm , the peak power density raises from 21 mW cm to 36 mW cm , and the -2

-2

-2

-2

limiting current density increases from 80 mA cm to 145 mA cm . This is because the increase -2

-2

in Pd loading leads to extending the catalyst/liquid interface, thereby increasing DPB density. However, when further increasing Pd loading to 5.0 mg cm , the peak power density and the -2


12

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limiting current density reduce to 31 mW cm and 130 mA cm , respectively. The decrease in -2

-2

cell performance is mainly attributed to the increased mass and charge transport resistances, caused by the increase in thickness of the catalyst layer with Pd loading. Figure 6 shows the

Figure 5. Polarization and power density curves of the ionomer-free Na-DFFC with different anode catalyst loadings. Anode fuel: HCOONa, 1.0 M, 1.0 mL min ; Cathode oxidant: oxygen, 5 -1

sccm, ambient pressure; Anode electrode: 1.0-5.0 mg cm Pd/C, 15 wt.% PTFE; Cathode -2

Pd

electrode: 2.0 mg cm Pd/C, 15 wt.% PTFE; CEM: Nafion 211; Operating temperature: 90 C. -2

o

Pd


13


Page 14 of 27

Figure 6. Polarization and power density curves of the ionomer-free Na-DFFC with different anode polymer contents. Anode fuel: HCOONa, 1.0 M, 1.0 mL min ; Cathode oxidant: oxygen, 5 -1

sccm, ambient pressure; Anode electrode: 4.0 mg cm Pd/C, 5-50 wt.% PTFE; Cathode Pd

-2

electrode: 2.0 mg cm Pd/C, 15 wt.% PTFE; CEM: Nafion 211; Operating temperature: 90 C. -2

o

Pd

effect of PTFE content in anode catalyst layer on cell performance. As observed, the peak power density reduces from 37 mW cm to 27 mW cm as the PTFE content varies from 5 wt.% to 50 -2

-2

wt.%. This monotonic decrease in cell performance can be ascribed to the fact that a higher PTFE content not only lowers the porosity of catalyst layer, thus increasing the mass and charge transport resistances in liquid solution, it also hinders the electron conduction by reducing the connectivity of nanoparticles, leading to a large cell resistance. Similarly, on the cathode, the cell performance first increases and then decreases with cathode catalyst loading as presented in


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Figure S2 (Supporting Information). However, unlike the anode, there exists an optimal cathode PTFE content to yield a best cell performance as exhibited in Figure S3 (Supporting Information), because the hydrophobic characteristics of the PTFE facilitates the oxygen transport on two-phase fluid-flow cathode. Optimizing operating conditions is an effective way to improve cell performance, including fuel concentration, oxygen flow rate, and operating temperature. The effect of sodium formate concentration ranging from 0.5 M to 3.0 M on cell performance was presented in Figure 7. As

Figure 7. Polarization and power density curves of the ionomer-free Na-DFFC with different fuel concentrations. Anode fuel: HCOONa, 0.5-3.0 M, 1.0 mL min ; Cathode oxidant: oxygen, 5 -1

sccm, ambient pressure; Anode electrode: 4.0 mg cm Pd/C, 5 wt.% PTFE; Cathode electrode: -2

Pd

4.0 mg cm Pd/C, 15 wt.% PTFE; CEM: Nafion 211; Operating temperature: 90 C. Pd

-2

o


15


Page 16 of 27

seen, a peak power density of 45 mW cm was achieved when fuel concentration increases to 2.0 -2

M. This is mainly attributed to the balance of the competitive adsorption among mass and charge existed in liquid solution.

43,45,46

To explore the effect of oxygen flow rate on cell performance, the

current-voltage curves were measured and demonstrated in Figure 8. It can be seen when rising oxygen flow rate from 5 sccm (standard cubic centimeter per minute) to 100 sccm, the peak power density decreases from 45 mW cm to 27 mW cm . Obviously, the oxygen fed to cathode -2

-2

can purge liquid water to enhance oxygen transport, but noticed that a too

Figure 8. Polarization and power density curves of the ionomer-free Na-DFFC with different oxygen flow rates. Anode fuel: HCOONa, 2.0 M, 1.0 mL min ; Cathode oxidant: oxygen, 5-100 -1

sccm, ambient pressure; Anode electrode: 4.0 mg cm Pd/C, 5 wt.% PTFE; Cathode electrode: Pd

-2

4.0 mg cm Pd/C, 15 wt.% PTFE; CEM: Nafion 211; Operating temperature: 90 C. Pd

-2

o


16

Page 17 of 27

high flow rate will decrease water content in cathode catalyst layer and water uptake in membrane, lowering the electrochemical kinetics of the ORR and the membrane conductivity, respectively. The starvation of liquid water in a certain cathode flow rate can be confirmed by humidifying oxygen as displayed in Figure S4 (Supporting Information), which suggests 5-sccm oxygen flow rate be sufficient in the present system. In addition, elevating cell operating temperature can i) lower activation loss of both the FOR and ORR, ii) enhance mass and charge transports, and hence significantly improving cell performance, as proved in Figure 9: the ionomer-free and phase-boundary-matched Na-DFFC yields a peak power density as high as 45 mW cm at 90 C, 105% higher than that at 30 C. o

o

0.9

30 oC 60 oC 90 oC

0.8

45 40

0.7

35

0.6

30

0.5

25

0.4

20

0.3

15

0.2

10

0.1

5

0.0

0

20

40

60

80

100

120

140

160

180

Power density (mW cm-2)

-2

Cell voltage (V)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0 200

Current density (mA cm-2)

Figure 9. Polarization and power density curves of the ionomer-free Na-DFFC with different operating temperatures. Anode fuel: HCOONa, 2.0 M, 1.0 mL min ; Cathode oxidant: oxygen, 5 -1


17


Page 18 of 27

sccm, ambient pressure; Anode electrode: 4.0 mg cm Pd/C, 5 wt.% PTFE; Cathode electrode: -2

Pd

4.0 mg cm Pd/C, 15 wt.% PTFE; CEM: Nafion 211; Operating temperature: 30-90 C. -2

o

Pd

Figure 10. Constant-current discharging behavior of the ionomer-free Na-DFFC. Anode fuel: HCOONa, 2.0 M, 1.0 mL min ; Cathode oxidant: oxygen, 5 sccm, ambient pressure; Anode -1

electrode: 4.0 mg cm Pd/C, 5 wt.% PTFE; Cathode electrode: 4.0 mg cm Pd/C, 15 wt.% Pd

-2

Pd

-2

PTFE; CEM: Nafion 211; Operating temperature: 90 C. o

To gain a better understanding of the high-temperature ionomer-free and phase-boundarymatched Na-ion direct formate fuel cell, the transient cell-voltage behavior of the fuel cell at a constant current density of 10 mA cm was tested and presented in Figure 10. As seen, when -2

feeding 2.0 M HCOONa to anode and dry pure oxygen to cathode at an operating temperature as high as 90 C, a stable 60-min constant current discharge is achieved even eliminating both o


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anion/cation ionomer and additional liquid electrolyte. Also, after discharging, the open-circuit voltage (OCV) recovers to its original state. All of them illustrate the conceptual feasibility of the high-temperature phase-boundary-matched Na-DFFC. Conclusion In summary, an issue associated with the development of alkaline direct liquid fuel cells is the problem of the low catalyst utilization in conventional electrodes, not to mention the state-of-theart anion materials still suffer from low ion conductivity and the thermal and chemical instability. To address these issues, rather than focusing on synthesizing high-performance catalyst/membrane/ionomer materials, we reported a phase-boundary-matched Na-ion direct formate fuel cell from the viewpoint of meeting mass and charge transports for redox reactions. A peak power density of 45 mW cm and a limiting current density of 190 mA cm are achieved -2

-2

at 90 C, suggesting the conceptual feasibility of the ionomer-free and phase-boundary-matched o

Na-DFFC. It was found that the ionomer-free Na-DFFC yields a better cell performance than the bi-ionomer Na-DFFC does. The superiority of the ionomer-free Na-DFFC can be attributed to i) the enhanced charge transport, ii) the enhanced mass transfer, and iii) the increased electron conduction. Moreover, the ionomer-free Na-DFFC demonstrates a stable 60-min constantcurrent discharge at 90 C, implying the potential of the high-temperature ionomer-free Nao

DFFC. All these results present a promising design route towards high-performance alkaline direct liquid fuel cells in view of meeting mass and charge transports for electrochemical reactions.


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ASSOCIATED CONTENT Supporting Information. Supporting Information including experimental procedures and polarization and power density curves (PDF) AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Author Contributions Y.S. conceived the idea. Y. and X.D. did the experiments. Y.S. wrote the paper. All the authors discussed the results. Notes The authors declare no competing financial interest. Acknowledgements This work was fully supported by the National Natural Science Foundation of China (51776156).


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Table of Content Graphic

Synopsis A phase-boundary-matched Na-ion direct formate fuel cell containing neither ionomers nor additional liquid electrolyte promises a high catalyst utilization


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Insight into interface behaviors to building a phase-boundary-matched

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