novel cata lyst structures employing pt at ultra … · catapult deliverable report d.4.6 –...

Grant agreement no.: 325268

CATAPULT Deliverable Report D.4.6 – Demonstration in an MEA of a hybrid catalyst layer achieving >0.25 A/mg Pt 1

CATAPULT

NOVEL CATALYST STRUCTURES EMPLOYING PT AT ULTRA LOW AND ZERO LOADINGS

FOR AUTOMOTIVE MEAS

Grant agreement no.: 325268 Start date: 01.06.2013 – Duration: 36 months

Project Coordinator: Deborah Jones – CNRS

DELIVERABLE REPORT

D4.6 – DEMONSTRATION IN AN MEA OF A HYBRID CATALYST LAYER ACHIEVING >0.25 A/mg Pt

Due Date 31st May 2016

Author (s)

Ian Harkness, Jonathan Sharman, Dharshini Fongalland (JMFC)

Frédéric Jaouen, Anna Schuppert, Nastaran Ranjbar, Deborah Jones

(UM2, CNRS)

Workpackage 4

Workpackage leader Johnson Matthey Fuel Cells

Lead Beneficiary Johnson Matthey Fuel Cells

Date released by WP leader 22nd July 2016

Date released by Coordinator 22nd July 2016

DISSEMINATION LEVEL

PU Public X

PP Restricted to other programme participants (including the Commission Services)

RE Restricted to a group specified by the consortium (including the Commission Services)

CO Confidential, only for members of the consortium (including the Commission Services)

NATURE OF THE DELIVERABLE

R Report X

P Prototype

D Demonstrator

O Other



REVISIONS

Version Date Changed by Comments

SUMMARY

Keywords non-PGM catalyst, MOF, hybrid catalyst layer, oxygen reduction reaction

Abstract Non-PGM S-MOF derived Fe-NC catalysts were combined with conventional Pt on carbon

black catalysts into hybrid cathode catalyst layers and incorporated into MEAs that were

tested under automotive-relevant conditions. The performance of the best MEA was

slightly less than that which would be expected from the Pt on carbon catalyst in the

absence of the non-PGM catalyst. It is shown that this is due to a reduction in the

accessibility of the Pt on carbon catalyst, which reduces its effectiveness in the mixed

layer at the composition used in this work. The results obtained show that addition of

high ionomer levels is beneficial and it is possible that even more would increase

performance further. The hybrid layer performance also improves strongly with time on

test, so a different break-in procedure may also give a much higher performance. In

general, however, it is thought that the lack of structure in the non-PGM catalyst will

limit the performance of hybrid layers and this should be addressed in future work.

Work on hybrid catalysts formed from non-PGM Fe-N-C catalysts with small (<2%)

additions of Pt from WP5 is also reported here and shows that although the added Pt is

not active for the ORR, the stability of the hybrid catalyst is significantly improved. It is

hypothesised that this is due to benign decomposition of hydrogen peroxide by the

added Pt. The peroxide is formed at the Fe catalytic sites and causes site degradation in

the absence of the Pt. The performance of the layers formed with the hybrid catalysts

should also be improved by re-structuring of the layers. Further reductions in Pt content

or increases in non-PGM catalyst activity would be needed however to hit the target of

0.25 A/mgPt.



D4.6 – DEMONSTRATION IN AN MEA OF A HYBRID CATALYST

LAYER ACHIEVING >0.25 A/mg Pt

1. Introduction

This deliverable reports work in Task 4.4 ‘creation and testing of hybrid catalyst layers’ on cathode layers that

were formed by the combination of non-PGM containing catalysts, generated in WP5, with more conventional

platinum-containing catalysts (Part I). For completeness, work from Task 5.4 in WP5 is also reported here on the

complementary approach pursued in WP5 where a hybrid catalyst was made by adding a very small amount of Pt

to a non-PGM containing catalyst and then a layer made from the hybrid catalyst (Part II).

At the outset of the programme it was envisaged that the hybrid layers in this task would be formed using non-

PGM catalysts in combination with novel nanofibrous catalysts from Task 4.1. The limited availability and

reproducibility of the nanofibrous materials together with the difficulty in making high performance catalyst

layers from the nanofibres meant that it was decided to proceed with an investigation of the value of the hybrid

approach using more freely available, and better understood, conventional carbon black supported platinum

catalysts.

2. Part 1: Hybrid Catalyst Layers

a. MEA fabrication

The non-PGM catalyst used in this work was based on [Zn(eIm)2] – rho where eIm is 2-ethylimidazole. This was

one of the materials down selected in WP5 as being of particular merit. The catalyst was generated by pyrolysis of

this MOF under ammonia, after milling with iron acetate and 1,10 phenathroline. The pyrolysis was carried out

using a “flash” pyrolysis technique in which the MOF sits under a flow of argon in a cold portion of the furnace

until the furnace has reached the target temperature, the gas is then switched to ammonia and the MOF is

pushed into the hot zone for 15 minutes, the product is then pulled out of the hot zone and rapidly cooled under

argon. It has been found that this approach, which more closely mimics the technique used at CNRS, University

Montpellier, leads to better performing non-PGM catalysts.

Figure 2.1 shows the mercury intrusion porosimetry of a non-PGM catalyst compared to that of a conventional

platinum on carbon black. It is clear that the non-PGM catalyst is about half as porous as the conventional catalyst

and contains almost no porosity at pore sizes less than 10 µm. This shows that the non-PGM catalyst packs into a

relatively dense structure and the absence of smaller porosity means that it has relatively large non-porous

domains. This is not an ideal structure for a catalyst layer because the mass transport of oxygen to the active sites

within the layer will be poor. This strongly suggests that it would be better in a hybrid layer to mix the non-PGM

and conventional catalysts together so that the more open structure of the conventional carbon black can help

mitigate the expected poor mass transport properties of the non-PGM layer. In the proposal for this programme

it was envisaged that the hybrid layer would be a bilayer structure with a non-PGM layer on the GDL side of a

conventional layer. The porosity data in Figure 2.1 show that this would be a poor design, as the low porosity non-

PGM layer would prevent oxygen access to the conventional layer. As a result of this observation a mixed layer

was prepared.



Figure 2.1: Cumulative intrusion volume as a function of pore diameter, as determined by mercury porosimetry for an S-MOF

catalyst and a comparative, conventional Pt/C

A 1:1 mixture of the pyrolysed MOF and a 30% Pt/C catalyst was blended in a dual asymmetric centrifugal

laboratory mixer. After wetting the mixture with a minimum amount of water, to eliminate any combustion risk,

an alcoholic solution of 1000EW perfluorosulphonic acid ionomer was then added (40% propano-1-ol in water).

This mixture was then further processed in the mixer for 30 minutes to produce an ink. The processing time was

chosen by monitoring the particle size distribution of the ink using light scattering and stopping when maximum

particle break-up had been achieved. Two ionomer levels were used: 80% and 136% with respect to total carbon,

as these are ionomer levels that were used in the work on non-PGM layers in WP5. The inks were then bar-coated

onto SGL 35BC (a carbon fibre based gas diffusion substrate) to form a catalysed electrode with a target platinum

loading of 0.1 mg Pt/cm². This electrode was then hot-pressed onto a 17 µm automotive membrane which had

been pre-prepared with a 0.1 mg Pt/cm² anode catalyst layer.

b. MEA testing

The MEAs were tested in a cell with a 49 cm² active area. Air polarisation curves were collected at 80°C and 170

kPag at both 100% and 30% RH. These are the extremes of the conditions that were defined for this programme

in report D2.2 on test protocols. In order to aid understanding of the losses, polarisation curves were also

collected using heliox (21% oxygen in helium) and using pure oxygen as the gas at the cathode inlet. Air and

oxygen polarisation curves are shown in Figure 2.2. These show two clear points. Firstly, that an ionomer loading

of 136% is far superior to 80% and, secondly, that both samples show a very large improvement (show by the

arrows) when the tests are repeated. The improvement is apparent in both the air and the oxygen performance

curves. The improvement in the oxygen performance must be due to an increase in the number of active sites

that results from the higher ionomer loading. Since ionomer cannot create active sites, only utilise ones that

already exist, it appears that 80% ionomer is insufficient to coat all the available area in the catalyst layer. In other

words, at 80% ionomer, some of the area is inaccessible to protons and therefore electrochemically inactive.



Figure 2.2: Air and oxygen polarisation curves for the hybrid layers with differing levels of ionomer. Each sample was tested

twice with each oxidant. 0.1 mg Pt/cm² on the cathode.

Figure 2.3: Heliox and oxygen gains for the hybrid layers with differing levels of ionomer. Each sample was tested twice with

each oxidant. 0.1 mg Pt/cm² on the cathode.

The air and heliox gains for the same tests are shown in Figure 2.3. The heliox gain is the increase in performance

when the oxidant is changed from air to heliox. The oxygen gain is increase in performance when the oxidant is

changed from heliox to oxygen. The heliox gain is a measure of the gas phase mass transport loss and the oxygen

gain a measure of the dissolved phase mass transport loss.

Figure 2.3 shows that in addition to the increase in catalyst utilisation, the higher ionomer loading causes a

beneficial reduction in the oxygen gain, which means a decrease in the dissolved phase mass transport losses.

This is a result of the increased catalyst utilisation, because if there are more active sites then each one has to



provide less current and therefore the oxygen flux required for each site is lower. As long as the ionomer loading

increases the number of active sites more than it increases the ionomer film thickness on top of each site then

the dissolved phase mass transport will improve. The heliox gains do not give any clear difference between the

ionomer loadings, suggesting that the additional ionomer is neither reducing the porosity of the layer nor causing

water to build up within the porosity.

The effect of repeating the polarisation tests is very similar to having more ionomer in the catalyst layer. More

active sites appear accessible and dissolved phase mass transport is improved as a result. This could be caused by

the catalyst layer progressively wetting up and water reaching more and more active sites, allowing them to

become active by supplying protons.

For the 80% ionomer loading, this effect was studied in more detail. Cyclic voltammetry, after poisoning the Pt

surface with CO, was carried out for two samples, one of which had been run for a short time, the other of which

had been run for a longer time. The apparent surface area of the catalyst increased from 54 m²/g Pt for the short

time MEA to 73 m²/g Pt for the long-running MEA. Since the CO test will only measure the electrochemically

active Pt area (and not the non-PGM active sites), this shows that the increase in activity can indeed be assigned

to an increase in availability of platinum sites. The value for the same catalyst in a conventional, non-hybrid Pt/C

catalyst layer is 84 m²/g Pt. This shows that even after the longer testing not all of the platinum sites are

accessible in the hybrid layer. This kind of very slow wetting-up does not happen with conventional Pt/C layers in

the absence of the non-PGM catalyst.

The current interrupt resistances taken during the polarisation testing are shown in Figure 2.4. The current

interrupt resistances are in good agreement with results from conventional catalyst layers, but there is some

evidence that the membrane is slightly under-hydrated with the lower ionomer loading in the hybrid layer,

resulting in a slightly elevated resistance relative to the other MEA. Repeat testing causes a small reduction in

resistance for both MEAs, which supports the “wetting-up” improvement mechanism proposed above.

Figure 2.4: Current-interrupt resistances measured during the polarisation experiments

The polarisation curves collected at 30% RH are shown in Figure 2.5. The beneficial effect of higher ionomer is

even more marked under these conditions. For the higher ionomer loading, as for the wetter conditions, there is

a benefit in repeating the test, however for the lower ionomer loading there is no benefit in repetition. It is

possible that this ionomer loading is insufficient to promote wetting up of the catalyst layer at this humidity.



Figure 2.5: Air and oxygen polarisation curves for the hybrid layers with differing levels of ionomer measured at reduced

humidity. Each sample was tested twice with each oxidant. 0.1 mg Pt/cm² on the cathode.

Figure 2.6: Heliox and oxygen gains for the hybrid layers with differing levels of ionomer at reduced humidity. Each sample

was tested twice with each oxidant.

The gains shown in Figure 2.6 show that for the higher ionomer loading a higher number of active sites results,

shown by the higher oxygen performance in Figure 2.5, and that this, as expected, leads to better mass transport

and lower oxygen gains.



Figure 2.7: Current-interrupt resistances measured during the polarisation experiments at reduced humidity

The MEA wetting up that occurs with repeated testing increases the difference between the two ionomer

loadings, by improvements in both the number and accessibility of active sites. For the lower ionomer loading

MEA, the number of sites appears to remain constant in the repeat test but their accessibility is decreased. A

possible explanation of this would be that the oxygen permeability of ionomer decreases when it is dehydrated.

The current interrupt results shown in Figure 2.7 support this, in that they show that not only are both MEAs less

hydrated than at 100%, but that the lower ionomer sample appears to get even drier with repeated testing. This

dehydration is shown by the increase in the current interrupt resistance.

The figures above show that a functional MEA can be made using the hybrid approach, but a key aim of this work

was to see if there is any beneficial synergy between the non-PGM catalyst and a platinum-containing catalyst.

The CATAPULT reference MEA contains a very much higher loading of platinum than is present in this MEA so

comparison with that would not be appropriate, therefore the best hybrid MEA is compared to reference data on

a conventional Pt/C catalyst layer at 0.1 mg/cm² made in the laboratory at JMFC. As the CATAPULT conditions are

not standard at JMFC this data set, which was generated prior to the programme, is not at exactly the same

conditions (the test was carried out at 100 kPag rather than 170 kPag); nevertheless, the comparison is very

useful.

Figure 2.8: Polarisation curves on air and oxygen at 100% RH for the hybrid layer at 136% ionomer and a JMFC reference.

Both MEAs have 0.1 mg Pt/cm² on the cathode.



Comparison of the oxygen performance shows that the electro-catalytic performance of both layers is very similar.

The slightly higher pressure of the CATAPULT testing should be expected to give an increase in current of 35%

(=270/200), but this is not apparent. This suggests that the addition of the non-PGM catalyst is causing a

reduction in the Pt/C activity that outweighs any activity that the non-PGM catalyst is providing. The oxygen and

heliox performance of the hybrid is clearly inferior to the conventional layer suggesting that the mass transport in

both the dissolved and gas phases is inferior. The target for this deliverable was that the mass activity of platinum

in the hybrid layer should be boosted by the presence of the non-PGM catalyst, to a value of 0.25 A/mg @ 0.9V. It

is clear from the fact that the activity of the hybrid is slightly lower in the low current region, when run on pure

oxygen (Figure 2.8), that this has not been achieved. The reasons for this are believed to be the lack of structure

in the non-PGM catalyst inhibiting the mass transport of oxygen into the layer. The lack of porosity of the non-

PGM material and the thickness of the hybrid layer, as a result of the addition of the non-PGM material, make

oxygen transport into the hybrid much more difficult, thus reducing the effectiveness of the conventional Pt/C

catalyst, by restricting access to the Pt sites. The high degree of hydrophilicity of the non-PGM material also

appears to be upsetting the water balance within the layer and leading to the very slow conditioning behaviour of

the conventional catalyst. It is possible that this is due to the highly hydrophilic non-PGM catalyst preferentially

absorbing water in the layer and thus dehydrating the conventional catalyst and rendering it inactive. This effect

is at least partially reversed by running the cell for longer, but the apparent metal area of the hybrid layer implies

that this is still occurring to some extent, even after repeated testing. These results should not be taken as

meaning that it is impossible to successfully combine the Pt and non-Pt catalysts, but it means that a re-design of

the non-PGM material using the learning from this project is necessary.

It is clear from the results above that there is a very strong effect of ionomer loading and this is worthy of further

investigation.

3. Part II: Hybrid Catalysts

The work in Section 2 above describes the combination of different catalyst types in a single layer; the approach

in this section however, is to combine two catalyst sites on the same support material. While Fe-N-C catalysts are

stable during potential cycling in acidic media and under PEMFC environment,[1] explained by the strong co-

ordination of Fe ions by nitrogen ligands, Fe-N-C catalysts have hitherto been unstable in PEMFC operation. The

release of minute amounts of H2O2 during the ORR and subsequent Fenton reactions between Fe active sites and

H2O2 to produce radical oxygen species is one possible degradation path;[2] the radical oxygen species attacking

the ionomer in the cathode, the active sites and/or the hosting N-doped carbon.

To overcome this issue, we investigated layers made from a hybrid catalyst of Fe-N-C and a small amount of Pt. Pt

is an excellent catalyst for the electro-reduction of peroxide to water and could act as radical scavenger or

secondary site with a protective function for the primary Fe sites. The small addition of Pt to Fe-N-C catalysts is

economically interesting if the resulting hybrid catalyst results in figures of gPt/kW higher than otherwise reached

with conventional Pt/C catalysts, or improved stability. In addition, the areal power density (Wcm-2) should be

sufficiently high, otherwise the lower cost offered by the catalyst is offset by the increased cost in bipolar plates

and membranes to reach a given stack power.[3]

Using 1-2 % Pt on the Fe-N-C catalyst obtained by pyrolyzing the precursor 1%Fe/20 phen/80 ZIF-8 in Ar at 1050°C

did not increase the catalyst activity (this Pt is inactive due to incomplete reduction or encapsulation by a carbon

layer) but stabilized the Fe-N-C catalyst (Figures 3.1 and 3.2). 1% Pt on this Fe-N-C catalyst is at the threshold for

stabilization, some batches showing full stability in operation while others were only partially stable. The increase

in current density at 0.5 V during the first 10-20 hours of test is assigned to improved mass-transport, while the

activity at 0.8 V did not increase. The effect is also initially observed on Fe-N-C (no Pt), but the current density

starts to decrease after ca 5 h due to the combined effect of improved mass-transport but decreased ORR

catalytic activity.



Figure 3.1: Stability of the hybrid Pt/Fe-N-C catalysts for 50 h at 0.5 V (without correction for Ohmic drop in the membrane).

A: FeNC, B: H2-FeNC (0 wt% Pt but FeNC was subjected to the same heat-treatment in H2/N2 as used to reduce the Pt salt); C:

0.5%Pt-FeNC; D: 1.0% Pt-FeNC; E: 2.0% Pt-FeNC.

0 400 800 1200 1600 20000.2

0.3

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0.5

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1.0

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a initial after 50 hr at 0.5 V

Cel

l pot

entia

l / V

i (mA/cm²)

1%Pt - 1%FeNC (Ar - H2/N

2)

FeNC initial

i (m

A/c

m²)

Time / hour

b

Figure 3.2: Polarisation curves before/after 50 h at 0.5 V (Chronoamperometry test shown in b) for an MEA with a cathode

comprising 4 mg cm-2

of 1%Pt-1%Fe-N-C pyrolyzed in Ar, then in H2/N2 after impregnation with the Pt salt. The initial

polarisation curve for the reference Fe-N-C catalyst is also shown (green curve, Fe/phen/ZIF-8 pyrolyzed in Ar at 1050°C, then

in H2/N2 in the same conditions as used to reduce the Pt salt).

For the 1%Pt/1%Fe-N-C hybrid catalyst described above, the current density observed at 0.9 V in a PEMFC is

typically 2 mAcm-2 for a 4 mgcm-2 cathode loading, see Figure 3.3 (i.e. 40 µg Pt cm-2), resulting in an overall

apparent mass activity of 50 A per gram Pt (0.05 A/mg Pt). This number has a very different meaning than that

reported for classical Pt/C catalysts, as the ORR activity entirely originates from Fe-N-C while the Pt only stabilizes

the Fe-N-C catalyst, because it is inactive in this particular hybrid Pt/Fe-N-C catalyst. The lack of activity of Pt here

is assigned to incomplete reduction of the Pt salt during treatment in diluted H2/N2. The apparent mass activity



figure of 50 A per gram of Pt for such hybrid catalysts could thus be increased if either i) the Pt mass is decreased

while maintaining the stability during operation of the hybrid Pt/Fe-N-C, or ii) the Fe-N-C activity is significantly

increased and it can be stabilized by the same amount of Pt. The second option was further investigated during

CATAPULT.

0.4

0.6

0.8

1.0

1 10 100 1000i (mA/cm²)

iR-f

ree

cell

pote

ntia

l / V

Fe-N-C 1wt Pt / Fe-N-C

Figure 3.3: Polarisation curves for an MEA with a cathode comprising 4 mg cm

-2 of 1%Pt-1%Fe-N-C pyrolyzed in Ar, then in

H2/N2 after impregnation with the Pt salt. The initial polarisation curve for the reference Fe-N-C catalyst is also shown (dotted

curve, Fe/phen/ZIF-8 pyrolyzed in Ar at 1050°C, then in H2/N2 in the same conditions as used to reduce the Pt salt).

After demonstrating the stabilization effect of the Argon-pyrolyzed Fe-N-C catalyst by a small Pt addition, we

investigated whether the effect could also be observed on the more active, but intrinsically less stable, NH3-

pyrolyzed Fe-N-C catalysts. Figure 3.4 shows that this could not be achieved, with the initial part of the durability

test showing a decrease in performance (in contrast to Pt-hybridisation of Ar-pyrolyzed FeNC catalysts), until the

NH3-pyrolyzed Fe-N-C catalyst reached the same performance as an Ar-pyrolyzed Fe-N-C catalyst, at which point

the current-vs-time curve became stable (Figure 3.4b). The twofold higher area in micropores for the NH3-

pyrolyzed Fe-N-C catalysts versus the Ar-pyrolyzed Fe-N-C catalysts may account for this different behaviour. It is

hypothesised that the Fe based active sites are hosted in deeper micropores and are therefore less-efficiently

stabilized by Pt particles situated outside the micropores, or that the deeper micropores lead to a higher, more

damaging local concentration of H2O2 in the micropores.

A longer durability test of 200 h was also performed on a 2%Pt-1%FeNC Ar-pyrolyzed hybrid catalyst, with 1%Fe-

N-C prepared from 1%Fe/20% phen/ 80% ZIF-8, pyrolyzed in Ar at 1050°C, then in H2/N2. Figure 3.5 shows the

current vs. time response at 0.5 V, showing increased performance over the first 75 h of operation and a slight

performance increase after 100-200 h operation. It was verified with initial and final polarization curves that the

final activity at 0.8 V had decreased only slightly, and that Pt in the hybrid catalyst was still inactive (CO stripping

measurement, no signal). The increased current density at 0.5 V over time is thus not due to improved electro-

catalytic activity but improved charge or mass transport (O2, protons or electrons) across the thick cathode. This

kind of improvement is also seen during first 5-7 h with bare Fe-N-C but is thereafter offset by strongly decreased

electrode kinetics with time.

When comparing initial and final polarisation curves related to Figure 3.5, an improvement is observed at any

potential < 0.75 V, while the rate of potential decrease in the electro-catalytic activity domain at high potential

was 18 mV/200 hr = 90 µV/hr (averaged over 200 h, by reading the initial and final potential at current density of

10 mA cm-2). It is not known however, if this slight decay occurred continuously over 200 hr, or if it is assigned to a

sudden loss during the first hours of operation. This might be the case if some defective Fe-based catalytic sites

are not stable in the acidic medium of PEMFC (no acid leaching of the catalysts was performed before MEA

preparation). Consecutive polarisation curves recorded every 50 h will provide this information in the near future.

• In practice, the durability performance targeted for MS 3 (< 10 µV/hr over 500 h) is reached with this

hybrid catalyst in the region of interest for PEMFC operation (< 0.75 V).



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100

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400500

600700

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900

10001100

b

Cel

l vol

tage

/ V

Current density / mA cm-2

initialafter 50 hr

2%Pt - 1%FeNC (Ar-NH3-H

2/N

2)a

Cur

rent

den

sity

/ m

A c

m-2

Time / hour Figure 3.4: Polarisation curves before/after 50 h at 0.5 V for an MEA with a cathode comprising 4 mg cm

-2 of 2%Pt-1%Fe-N-C

pyrolyzed in Ar, then NH3, then H2/N2. The 3rd

pyrolysis was performed after the Fe-N-C catalyst had been impregnated with

the Pt salt.

0 20 40 60 80 100 120 140 160 180 2000

100

200

300

400

500

600

700

800

900

1000

1100

FeNC 2Pt-FeNC

I /m

A c

m-2

time /hour (h) Figure 3.5: Current density as a function of time for an MEA with a cathode comprising 4 mg cm

-2 of 2%Pt-1%Fe-N-C

pyrolyzed in Ar, impregnated with the Pt salt, then pyrolyzed in H2/N2 (orange curve).

To further investigate the effect of the reducing conditions during the second firing treatment, we compared the

activity of the hybrid catalyst 1%Pt/Fe-N-C, obtained after the 2nd pyrolysis in H2 / N2 (discussed previously) to

that obtained after a second pyrolysis in more reducing conditions. Figure 3.6 shows that the initial activity and

performance was greatly improved after the second pyrolysis in the more reducing condition. This novel hybrid

catalyst was prepared just before the end of CATAPULT and has not been investigated in more detail. The origin

of enhanced activity after highly reductive conditions must therefore be further investigated. It may be due to

increased ORR activity of the Fe-N-C catalyst, or due to complete reduction of the Pt salt to metallic Pt (in contrast



to the case with H2 in N2, where Pt is inactive, see black dotted curve in Figure 3.6), or due to a combination of

both effects.

With a current density of ca 5.4 mA cm-2 at 0.9 V for a 4 mg cm-2 cathode loading (i.e. 40 µg Pt cm-2), the apparent

Pt mass activity of this novel hybrid catalyst is 135 A per gram Pt (0.135 A/mg Pt). While this is close to the mass

activity of 2-3nm Pt nanoparticles (fully reduced Pt) on a regular carbon support, we do not know yet if the ORR

activity of this novel hybrid catalyst can be interpreted in this way or not.

We observed previously that the ORR activity of Fe-N-C was greatly improved after a second pyrolysis in H2/N2,

thus it is possible that the ORR activity of Fe-N-C after a second pyrolysis in more reducing conditions accounts for

the major part of the ORR activity observed for the blue curve in Figure 3.6. Investigating a second key reference

sample, namely 1%Pt/N-C obtained after a 2nd pyrolysis in the highly reducing condition, will indicate the role of

Fe and its contribution to the overall ORR activity in this advanced hybrid Pt/FeNC catalyst. Evolution of activity

and power performance with time will also need to be determined.

1 10 100 1000

0.7

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1Pt/Fe-N-C 2nd pyrolysis in high reducing conditions 1Pt/Fe-N-C 2nd pyrolysis in H

2/N

2

1Pt/N-C 2nd pyrolysis in H2/N

2

iR-c

orre

cted

pot

entia

l / V


a

iR-c

orre

cted

pot

entia

l / V


Figure 3.6: Initial polarisation curves for MEAs with a cathode comprising 4 mg cm

-2 of 1%Pt-1%Fe-N-C pyrolyzed in Ar, then

in a highly reducing condition (blue curve) and, for comparison, from the previous hybrid catalyst 1%Pt-1%Fe-N-C pyrolyzed

in Ar, then H2 / N2 (grey curve). For reference, the polarization curve for 1%Pt-N-C (no iron) pyrolyzed in Ar, then H2 / N2 is

also shown (black dots). PEMFC test conditions: H2, O2, 100% RH, 1 bar gauge pressure, 80°C cell temperature.

4. Summary and Conclusions

a. Hybrid Catalyst Layers

Hybrid layers with a non-PGM catalyst and a conventional Pt/C catalyst have been successfully tested in 50 cm2

active area MEAs.

A higher ionomer loading increases the hybrid layer performance both by increasing the number of active sites

and by improving the accessibility of these sites.



The performance of the hybrid layers in their current un-optimised state is not higher than can be achieved with a

fully optimised 0.1 mg/cm² Pt on carbon black layer. This is because not all of the platinum sites are being used in

the hybrid layers and this outweighs any benefit from the active sites present in the non-PGM catalyst component.

The lack of structure in the non-PGM material is believed to be the cause of the reduction in effectiveness of the

conventional Pt/C material, but the very high hydrophilicity may also be playing a role. Thus, to meet the project

target of 0.25 A/mg Pt @ 0.9V, a re-design of the structure of non-PGM catalyst would be required. The active

sites present in the non-PGM material need to be presented in a more accessible manner by forming them on a

more structured carbon, or by other means.

The performance of the hybrid layers improves with time on test, because the dehydrating effect of the non-PGM

material on the conventional catalyst is partially mitigated by the build-up of product water.

b. Hybrid Catalysts

Fe-N-C catalysts prepared via pyrolysis in Ar were stabilized during PEMFC operation for at least 200 h by

deposition of 1-2% Pt (Pt II salt) and firing in H2/N2. The stabilisation by this electrochemically-inactive Pt (as

demonstrated by CO stripping) of Fe-N-C catalysts brings hope for the future of such systems. The stabilization

mechanism is unclear however.

To exceed the mass activity target of 0.25 A/mgPt for this deliverable, the hybrid Pt/Fe-N-C catalysts must target

further reductions in Pt content, higher ORR activity of stabilizable Fe-N-C (Co-N-C) catalysts, and/or improved

transport properties of the Fe-N-C cathode layers.

Higher ORR activity was reached for another hybrid Pt/Fe-N-C catalyst by switching to more reducing conditions

than H2/N2. The origin of enhanced ORR activity of this second type of hybrid catalyst is as yet unknown, since the

reductive treatment also modifies the Fe-N-C catalyst, which could explain the improved ORR activity. Overall, the

apparent mass activity of Pt (apparent overall activity for Pt and Fe-N-C divided by Pt loading) is ca 0.135 A/mgPt.

Thus, further improvements can be expected by combining the benefits of a more structured hybrid layer from

Part I, as indicated above, with the stabilised hybrid catalysts documented in Part II of this report.

5. Recommendations

a. Hybrid Catalyst Layers 1. Create the same non-PGM active sites on a more porous, more highly-structured support.

2. In the short term, increase the ionomer loading in the layer and precondition the MEA under wetter

conditions.

b. Hybrid Catalysts 1. Ultimately, replace Pt with another cheaper element (e.g. Pd) in hybrid metal/Fe(Co)-N-C catalysts.

2. Decrease the Pt content needed for stabilization of Fe-N-C by better Pt dispersion and by using ORR-

active fully reduced Pt created via chemical reduction of a Pt salt.

3. Increase the Fe-N-C ORR activity using Fe-N-C catalysts that can be stabilized by Pt.



6. References 1. Choi, C.H., et al., Minimizing Operando Demetallation of Fe-N-C Electrocatalysts in Acidic Medium. ACS

Catal., 2015, 6: p. 3136-3146.

2. Goellner, V., et al., Degradation by Hydrogen Peroxide of Metal-Nitrogen-Carbon Catalysts for Oxygen

Reduction. Journal of the electrochemical society, 2015, 162: p. H403-H414.

3. Kongkanand, A. and M.F. Mathias, The priority and challenge of high-power performance of low-platinum

proton-exchange membrane fuel cells. J. Phys. Chem. Lett., 2016, 7: p. 1127-1137.

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