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.
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
|Current at 0.2 V
|Peak power density
|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.
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.
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