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Researchers demonstrate high performance and stability of non-precious metal ORR catalysts in acidic PEM fuel cells

Researchers at Case Western University led by Prof. Liming Dai have demonstrated that rationally designed, metal-free, nitrogen-doped carbon nanotubes and their graphene composites exhibit significantly better long-term operational stabilities and comparable gravimetric power densities with respect to the best non-precious metal catalyst (NPMC) in acidic polymer electrolyte membrane (PEM) fuel cells.

The researchers said that this work, which advances their earlier work on high- performance NPMCs for fuel cells (e.g., earlier post, earlier post), represents a major breakthrough in removing the bottlenecks to translate low-cost, metal-free, carbon-based ORR (oxygen reduction reaction) catalysts to commercial reality in affordable and durable fuel cells. An open-access paper on their work appears in the online journal Science Advances (an offspring of the journal Science).

The molecular oxygen reduction reaction (ORR) is important to many fields, such as energy conversion (for example, fuel cells, metal-air batteries, and solar cells), corrosion, and biology. For fuel cells to generate electricity by electrochemically reducing oxygen and oxidizing fuel into water, cathodic oxygen reduction plays an essential role in producing electricity and is a key limiting factor on the fuel cell performance. To construct fuel cells of practical significance, efficient catalysts are required to promote the ORR at cathode. Traditionally, platinum has been regarded as the best catalyst for fuel cells, although it still suffers from multiple drawbacks, including its susceptibility to time-dependent drift and MeOH crossover and CO poisoning effects. However, the large-scale practical application of fuel cells cannot be realized if the expensive platinum-based electrocatalysts for ORR cannot be replaced by other efficient, low-cost, and durable electrodes.

… Along with intensive research efforts of more than half a century in developing non-precious metal ORR catalysts, a new class of metal-free ORR catalysts based on carbon nanomaterials has been discovered and attracted worldwide attention, which, as alternative ORR catalysts, could markedly reduce the cost and increase the efficiency of fuel cells. In particular, it was found that vertically aligned nitrogen-doped carbon nanotube (VA-NCNT) arrays can act as a metal-free electrode to catalyze a 4e ORR process with a three times higher electrocatalytic activity and better long-term stability than commercially available platinum/C electrodes … in an alkaline electrochemical cell. These carbon-based metal-free ORR catalysts are also free from the CO poisoning and methanol crossover effects.

… High electrocatalytic activity comparable or even superior to commercial Pt/C electrodes and excellent tolerance to MeOH crossover and CO poisoning effects have been demonstrated for many of the carbon-based metal-free ORR catalysts in electrochemical half-cells with alkaline electrolytes. Nevertheless, the large-scale applications of the carbon-based metal-free ORR catalysts in practical fuel cells cannot be realized if they do not have an adequate long-term durability and high ORR performance in acidic polymer electrolyte membrane (PEM) fuel cells, which currently serve as the mainstream fuel cell technology of great potential for large-scale applications in both transport and stationary systems.

—Shui et al.

Although carbon-based metal-free ORR catalysts have previously been found to be less effective in acidic electrolytes, in their new study the team demonstrated that both the VA-NCNT array and a rationally designed nitrogen-doped graphene/CNT composite (N-G-CNT) as the cathode catalysts in acidic PEM fuel cells exhibited high gravimetric current density comparable to the most active NPMCs.

Because carbon is much more anti-corrosive to acids than most transition metals, the VA-NCNT array and N-G-CNT composite showed a “significantly durable performance,” even with pure H2/O2 gases, in acidic PEM fuel cells, outperforming their NPMC counterparts.

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Electrocatalytic activities of the VA-NCNT catalyst in acidic electrolyte (O2-saturated 0.1 M HClO4) by half-cell tests. (A) LSV curves, (B) Tafel plot and (C) electron-transfer number of the VA-NCNT compared with Fe/N/C and Pt/C(20%) electrocatalysts by RRDE at scan rate of 10 mV s-1 and a rotation speed of 1600 rpm. (D) Long time stability, and tolerance to (E) carbon monoxide and (F) methanol of metal-free catalyst VA-NCNT compared with Fe/N/C and Pt/C(20%) electrocatalysts at 0.5 V (vs. RHE). CO ( flow 100 mL s-1) was injected into the electrolytes (100 mL) at the time of 200 s and stopped at the 500 s. Methanol (10 mL) was injected into the electrolytes (100 mL) at the time of 200 s. Click to enlarge.   Electrocatalytic activities of the carbon-based metal-free N-G-CNT catalysts in acidic electrolyte (O2-saturated 0.1 M HClO4) by half-cell tests. (A) LSV curves. (B) Tafel plot and (C) electron-transfer number of the N-G-CNT compared with Fe/N/C and Pt/C(20%) electrocatalysts by RDE at scan rate of 10 mV-1 and a rotation speed of 1600 rpm. (D) Long time stability, and tolerance to (E) carbon monoxide and (F) methanol of metal-free catalyst N-G-CNT compared with Fe/N/C and Pt/C(20%) electrocatalysts at 0.5 V (vs. RHE). CO ( flow 100 mL s-1) was injected into the electrolytes (100 mL) at the time of 200 s and stopped at the 500 s. Methanol (10 mL) was injected into the electrolytes (100 mL) at the time of 200 s. Click to enlarge.

To carry out the performance evaluation of VA-NCNTs in PEM fuel cells, the team made the VA-NCNT arrays (80 μm in height, a surface packing density of 0.16 mg cm−2) into a membrane electrode assembly (MEA) at the highest allowable catalyst loading of 0.16 mg cm−2.

The resulting MEA containing the VA-NCNT metal-free ORR electrocatalysts was evaluated in an acidic PEM fuel cell operating with the Nafion electrolyte and pure H2/O2 gases.

For the VA-NCNT MEA, significantly high gravimetric current densities were observed: 35 A g−1 at 0.8 V, 145 A g−1 at 0.6 V, and 1550 A g−1 at 0.2 V (Fig. 1E). The peak power density was 320 W g−1 for the VA-NCNT MEA, outperforming or comparable to even the most active NPMC catalysts.

Gravimetric activities of NPMCs in PEM fuel cells
Materials Current at 0.8V
(A g-1)
Current at 0.2 V
(A g-1)
Peak power density
(W g-1)
Catalyst loading
(mg cm-2)
O2-H2
back pressure
(bar)
FeCo/N/C 15 700 200 2 1.0
Fe/N/C 8/100 800/2500 233/400 3.9/0.9 0.5
Fe/N/C 15 325 80 4 1.3
VA-NCNT 35 1550 320 0.16 1.5
N-G-CNT+KB 30 1500 300 0.5 1.5
The gravimetric activities of various transition metal–derived NPMCs compared with the metal-free VA-NCNT and N-G-CNT + KB in PEM fuel cells. All the data in the table have also been scaled by the electrode surface area.

For the nitrogen-doped graphene composite catalyst, the team first prepared metal-free graphene oxide (GO) suspension, then mixed it with oxidized CNT suspension to produce metal-free porous N-doped graphene and CNT composites (N-G-CNT) through freeze-drying, followed by annealing at 800 °C in NH3. The N-G-CNT–based catalyst ink for MEAs was then prepared by mixing 2.5 mg of N-G-CNT catalyst with 10 mg of carbon black particles and 375 mg of Nafion solution (5%) in 1.5 ml of deionized water and isopropanol mixture (volume ratio = 1:2). Thereafter, the ink was sonicated for 10 min and stirred overnight, then painted onto a 5-cm2 GDL as the cathode electrode, and assembled into a MEA with a Pt/C-coated GDL as the anode and an intermediate layer of proton-conductive membrane as the separator for subsequent testing.

They noted that several synergistic effects can arise from the fabrication process to maximize the utilization of catalyst sites in the N-G-CNT composite:

  • N-G can prevent N-CNTs from the formation of the bundle structure to facilitate the dispersion of N-CNTs by anchoring individual N-CNTs on the graphene sheets.

  • N-CNTs can effectively prevent the N-G sheets from restacking by dispersing CNTs on the graphene basal plane to make more rigid curved N-G-CNT sheets than the N-G sheets.

  • The addition of carbon black (Ketjenblack, KB) not only further separates N-G-CNT sheets in the catalyst layer but also induces continued porous multichannel pathways between the N-G-CNT sheets for efficient O2 diffusion.

The N-G-CNT showed excellent electrocatalytic performance in 0.1 M KOH, even better than the commercial Pt/C electrode, with a better stability as well as a higher tolerance to MeOH crossover and CO poisoning effects than the Pt catalyst.

87324_web
A: Carbon black agglomerates maintain a clear distance between graphene sheets imbedded with carbon nanotubes, allowing oxygen and electrolyte to flow through and speeding the oxygen-reduction reaction. B: Without the agglomerates, the sheets stack closely, stalling the reaction. Click to enlarge.

As far as we are aware, these results are the highest records for metal-free graphene and CNT ORR catalysts. As expected, the N-G-CNT composite also exhibited much better ORR performance than that of N-CNT and N-G catalysts in both the alkaline and acidic media because of its unique foam-like 3D architecture formed in the thin composite layer on the RDE electrode even without the addition of carbon black in the absence of mechanical compression because 3D carbon networks have been previously demonstrated to facilitate electrocatalytic activities.

—Shui et al.

Graphene provides enormous surface area to speed chemical reactions, nanotubes enhance conductivity, and carbon black separates the graphene sheets for free flow of the electrolyte and oxygen, which greatly increased performance and efficiency. The researchers found that those advantages were lost when they allowed composite sheets to arrange themselves in tight stacks with little room between layers.

Dai’s lab continues to fine-tune the materials and structure as well as investigate the use of non-metal catalysts in more areas of clean energy.

Resources

  • Jianglan Shui, Min Wang, Feng Du, Liming Dai (2015) “N-doped carbon nanomaterials are durable catalysts for oxygen reduction reaction in acidic fuel cells” Science Advances Vol. 1 no. 1 doi: 10.1126/sciadv.1400129

Comments

HarveyD

Will we soon have longer lasting lower cost FCs?

gorr

Now the thing is to produce a lot of hydrogen gas for cheap with a new breakthrough method so I will be able to buy a fuelcell car that cost less then an ice car.

CheeseEater88

I think the thing holding FCs back is the cost and the space of the tank. Stacks are pretty cheap if mass produced if the government's numbers are right, but it doesn't hurt to be cheaper.

The cost of the tanks will drop rapidly as industry tools up for carbon fiber.

The other thing holding FCs back the silly notion of driving a Hindenburg Blimp of a car. I've heard that reference by very educated individuals in the car world.(as they sit atop 20x the energy their liquid fueled vehicles)Same can be said for BEVs, people are just afraid of the unfamiliar.

I know car companies are excited for FCs. I've met with a few individuals that have worked on them. One was from Toyota, and he stressed how important the launch of their vehicle was, that is hand built on their supercar line. Another was with Daimler, talked about H2 storage in America VS Europe. How we are likely to tap wind and other off peak power.


What excites me most, is that as long as battery packs, and cell stacks are modular, there is no reason not to repair/replace them. Lending to a much longer vehicle life... 20 years perhaps? ICEs are more of a pay less now, more later over time. BEVs/FCs are pay a lot now, less over time.... So, the longer you can go with one, the better your wallet feels

Engineer-Poet

At current and near-term prices for H2, FCEVs are a "pay a lot now, keep paying a lot for quite some time" proposition.  Their fuel also has very spotty availability.  Compared to the Tesla which can go just about anywhere by hopping from campground to campground to recharge, FCEVs are toys and will remain toys for some time.

Worse, FCEVs will never be "green" until H2 from carbon-free sources is cheaper than H2 from SMR or coal gasification.  That seems likely to happen around the 43rd of never.

CheeseEater88

Yes, but can every one afford a 100K car? And "a lot" is still relatively less than ICE, or at worse on parity with.

Most automotive companies are very confident in FCs. I believe Japan and London decided its cheaper and more efficient to do lay infrastructure for FCs than EVs.

If batteries are at $100/kwH, why aren't we seeing decent market penetration? I guess once Tesla's Gigafactory comes on line, we will know the fate of the car market.

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