Carbon supported nano-sized Pt–Pd and Pt–Co electrocatalysts for proton exchange membrane fuel cells

Nano-sized Pt–Pd/C and Pt–Co/C electrocatalysts have been synthesized and characterized by an alcohol-reduction process using ethylene glycol as the solvent and Vulcan XC-72R as the supporting material. While the Pt–Pd/C electrodes were compared with Pt/C (20 wt.% E-TEK) in terms of electrocatalytic activity towards oxidation of H₂, CO and H₂–CO mixtures, the Pt–Co/C electrodes were evaluated towards oxygen reduction reaction (ORR) and compared with Pt/C (20 wt.% E-TEK) and Pt–Co/C (20 wt.% E-TEK) and Pt/C (46 wt.% TKK) in a single cell. In addition, the Pt–Pd/C and Pt–Co/C electrocatalyst samples were characterized by XRD, XPS, TEM and electroanalytical methods. The TEM images of the carbon supported platinum alloy electrocatalysts show homogenous catalyst distribution with a particle size of about 3–4 nm. It was found that while the Pt–Pd/C electrocatalyst has superior CO tolerance compared to commercial catalyst, Pt–Co/C synthesized by polyol method has shown better activity and stability up to 60 °C compared to commercial catalysts. Single cell tests using the alloy catalysts coated on Nafion-212 membranes with H₂ and O₂ gases showed that the fuel cell performance in the activation and the ohmic regions are almost similar comparing conventional electrodes to Pt–Pd anode electrodes. However, conventional electrodes give a better performance in the ohmic region comparing to Pt–Co cathode. It is worth mentioning that these catalysts are less expensive compared to the commercial catalysts if only the platinum contents were considered.     

Development of non-noble Co–N–C electrocatalyst for high-temperature proton exchange membrane fuel cells

The development of a non-noble Co–N/MWCNT (MWCNT = multi-walled carbon nanotubes) electrocatalyst is achieved through the high-temperature pyrolysis method and successfully characterized by five-step physico-chemical analysis. By utilizing high-resolution analytical surface characterization methods, the chemical states of elements are determined, and the presence of Co-N_x sites is confirmed. ORR activity of a Co–N/MWCNT is found to be auspicious. The maximum number of transferred-electron (n) and the diffusion-limiting current density (j_d) are calculated as 3.95 and 4.53 mA· cm⁻², respectively. The catalyst is further evaluated under a single-cell test station. The test results show that the current and power density values of Co–N/MWCNT are found superior to those of the commercial Pt/C at the 150 °C and 160 °C (e.g., 57 vs. 69 mW· cm⁻² at 150 °C). Due to some stability issues, it is observed that the performance of the Co–N/MWCNT catalyst is slightly decreased while switching the temperature towards 180 °C.     

Evaluation of the stability and durability of Pt and Pt–Co/C catalysts for polymer electrolyte membrane fuel cells

Carbon supported Pt and Pt–Co nanoparticles were prepared by reduction of the metal precursors with NaBH₄. The activity for the oxygen reduction reaction (ORR) of the as-prepared Co-containing catalyst was higher than that of pure Pt. 30 h of constant potential operation at 0.8 V, repetitive potential cycling in the range 0.5–1.0 V and thermal treatments were carried out to evaluate their electrochemical stability. Loss of non-alloyed and, to a less extent, alloyed cobalt was observed after the durability tests with the Pt–Co/C catalyst. The loss in ORR activity following durability tests was higher in Pt–Co/C than in Pt/C, i.e. pure Pt showed higher electrochemical stability than the binary catalyst. The lower stability of the Pt–Co catalyst during repetitive potential cycling was not ascribed to Co loss, but to the dissolution–re-deposition of Pt, forming a surface layer of non-alloyed pure Pt. The lower activity of the Pt–Co catalyst than Pt following the thermal treatment, instead, was due to the presence of non-alloyed Co and its oxides on the catalyst surface, hindering the molecular oxygen to reach the Pt sites.     

Electrocatalytic activity of Pt–Pd electrocatalysts for the oxygen reduction reaction in proton exchange membrane fuel cells: Effect of supports

Pt–Pd electrocatalysts supported on different types of support including domestic Hicon Black (HB), multi-walled carbon nanotubes (MWCNT) and titania (TiO₂) were prepared by a combined approach of impregnation and seeding, and compared to that prepared using the commercial Vulcan XC-72 (C). Their oxygen reduction reaction (ORR) activities in an acid electrolyte (0.5 M H₂SO₄) and in a single proton exchange membrane (PEM) fuel cell were evaluated. The type of support was found to affect the Pt–Pd electrocatalyst morphology and ORR activity. The Pt–Pd/C electrocatalyst had the smallest Pt particle size, better catalyst dispersion and a higher Pt:Pd M ratio compared to that of other types of supported Pt–Pd electrocatalysts. However, both in the acid solution and in a single PEM fuel cell, the ORR activities of the Pt–Pd/HB and Pt–Pd/CNT electrocatalysts were comparable to that of the Pt–Pd/C one. The ORR pathway of all supported Pt–Pd electrocatalysts were close to the four-electron pathway.     

Performance and durability of Ni–Co alloy cermet anodes for solid oxide fuel cells

Ni alloys are examined as redox-resistant alternatives to pure Ni for solid oxide fuel cell (SOFC) anodes. Among the various candidate alloys, Ni–Co alloys are selected due to their thermochemical stability in the SOFC anode environment. Ni–Co alloy cermet anodes are prepared by ammonia co-precipitation, and their electrochemical performance and microstructure are evaluated. Ni–Co alloy anodes exhibit high durability against redox cycling, whilst the current-voltage characteristics are comparable to those of pure Ni cermet anodes. Microstructural observation reveals that cobalt-rich oxide layers on the outer surface of the Ni–Co alloy particles protect against further oxidation within the Ni alloy. In long-term durability tests using highly humidified hydrogen gas, the use of a Ni–Co cermet with Gd-doped CeO₂ suppresses degradation of the power generation performance. It is concluded that Ni–Co alloy cermet anodes are highly attractive for the development of robust SOFCs.     

Pt–Co nanoparticles anchored by ZrO2 for highly efficient and durable oxygen reduction reaction in H2-air fuel cells

Synthesis of Pt-based catalysts with high activity and durability for oxygen reduction reaction (ORR) remains a very challenging task in the field of fuel cells. Here, Co-doped Pt nanoparticles (NP) with surface-defect ZrO₂ are supported on the multi-walled carbon nanotubes (MWCNTs) (denoted as Pt–Co + ZrO₂/MWCNTs). The Pt–Co + ZrO₂/MWCNTs displays an ORR mass activity of 0.98 A mg_Pt^?1 at 0.9 V, which is 4.1-fold higher than that of the commercial Pt/C (0.238 A mg_Pt^?1). Further durability test shows that the Pt–Co + ZrO₂/MWCNTs remains nearly unchanged ORR mass activity after 50000 accelerated durability testings (ADTs). Based on the mass performance and surface performance, the fuel cell with Pt–Co + ZrO₂/MWCNTs cathode has far better power performance than that with commercial Pt/C. Moreover, the fuel cell with Pt–Co + ZrO₂/MWCNTs cathode undergo only a 6.1% maximum power loss after 50000 ADTs. However, that with commercial Pt/C cathode after 30000 ADTs has 39.6% maxinum power loss. More impressively, compared to the 220 mV loss of Pt/C after 30000 ADTs, the Pt–Co + ZrO₂/MWCNTs cathode also displays only 20 mV loss at 0.8 A/cm² after 50000 ADTs. The enhanced intrinsic activity of Pt–Co + ZrO₂/MWCNTs may be attributed to the Co-doped Pt NPs and interface effect of Co-doped Pt NPs and surface defect-rich ZrO₂.     

Highly stable Ti–Co–Phen/C catalyst as the cathode for proton exchange membrane fuel cells

A Ti–Co–Phen/C catalyst was prepared for polymer electrolyte membrane fuel cells (PEMFCs) without precious metals using a modified polymer complex (PC) method with 1,10-phenanthroline (Phen) as the nitrogen precursor. The oxygen reduction reaction (ORR) activity of the Ti–Co–Phen/C catalyst was significantly higher than the ORR activity of the Ti–Co/C catalyst prepared with the PC method because the former had a larger N surface content due to its highly dispersed Co species. The catalyst also exhibited excellent chemical stability in acidic media due to the probable strong interactions between the highly dispersed Ti and Co species. A H₂/O₂ PEMFC using the Ti–Co–Phen/C catalyst as the cathode demonstrated excellent cell performance. A 0.68 W cm⁻² maximum power density was obtained. The cell performance stability did not drop perceptibly during its 550-h lifetime at 0.5 V and its 300-h lifetime at 0.7 V. The prepared Ti–Co–Phen/C catalyst exhibited both high ORR activity and excellent performance stability, making it a promising alternative for the cathode catalysts in PEMFCs.     

Combined method to prepare core–shell structured catalyst for proton exchange membrane fuel cells

This paper reports a modified core–shell structured CuPd@Pt/C catalyst, which was synthesized by combining a two-step reduction method and chemical dealloying step using carbon black Vulcan XC-72R as the supporting material. The physical measurements confirmed that the final catalyst has a complete core–shell structure. The interaction between the core and the shell as well as the particle size, particle size distribution, and morphology of the catalyst particles were characterized by X-ray diffraction (XRD) and transmission electron microscopy (TEM). The results of inductively coupled plasma atomic emission spectroscopy (ICP-AES) and X-ray photoelectron spectroscopy (XPS) indicated that the Pt and Pd on the surface of the catalyst nanoparticles are mainly in the zero valence oxidation state. Cyclic voltammetry (CV) and rotating disk electrode (RDE) tests were used to measure the electrochemical performance, and the results showed that the modified CuPd@Pt/C catalyst has higher electrochemical catalytic activity in catalyzing the oxygen reduction reaction (ORR) than Pt/C, making it possible to reduce the usage of platinum and leading to a promising low-Pt catalyst for proton exchange membrane fuel cells (PEMFCs).     

Facile synthesis of supported Pt–Cu nanoparticles with surface enriched Pt as highly active cathode catalyst for proton exchange membrane fuel cells

Carbon supported Pt–Cu catalyst (PtCu/C) with surface enriched Pt was synthesized by annealing the Pt-deposited Cu particles. X-ray diffraction (XRD) results indicate the formation of disordered Pt–Cu alloy phase with a high level of Cu/Pt atomic ratio. X-ray photoelectron spectroscopy (XPS) and inductively coupled plasma (ICP) analysis confirm the surface enrichment of Pt. Electrochemical measurements show that PtCu/C has 3.7 times higher Pt mass activity toward the oxygen reduction reaction (ORR) than commercial Pt/C. The enhanced ORR activity of PtCu/C is attributed to the modified electronic properties of surface Pt atoms, which reduces the surface blocking of the ORR oxygenated species.     

Binary Pt–Pd and ternary Pt–Pd–Ru nanoelectrocatalysts for direct methanol fuel cells

Nano-sized binary and ternary alloys are synthesized by polyol process on Vulcan XC72-R support. Nanostructured binary Pt–Pd/C catalysts are prepared either by co-deposition or by depositing on each other. Ternary Pt–Pd–Ru/C catalysts are prepared by co-deposition. The structural characteristics of the nanocatalysts are examined by TEM and XRD. Their electrocatalytic activity toward methanol oxidation and CO stripping curves were measured by electrochemical measurements and compared with that of commercial Pt/C catalyst. The results show that the binary nanocatalyst prepared by depositing the Pt precursor colloids on Pd-Vulcan XC-72R are more active toward methanol oxidation than that of the co-deposited binary alloy nanocatalyst. The co-deposited ternary Pt–Pd–Ru/C nanocatalyst based membrane electrodes assembly shows higher power density compared to the binary nanocatalysts as well as commercial Pt/C catalyst in direct methanol fuel cell. Significantly higher catalytic activity of the nanocatalysts toward methanol oxidation compared to that of the commercial Pt/C is believed to be due to lower level of catalyst poisoning.     

Recent developments in bifunctional air electrodes for unitized regenerative proton exchange membrane fuel cells – A review

Unitized regenerative proton exchange membrane fuel cell (UR-PEMFC) technology has progressed in the recent past and has started appearing towards few applications. However, the UR-PEMFC viability is limited by its lower round-trip efficiency mainly due to several reasons such as sluggish air electrode reactions, lower performance/stability, higher materials cost etc. In this context, many approaches are being implemented for efficiency enhancement including design and development of effective bifunctional air electrodes (oxygen reduction and evolution reactions) materials both for fuel cell and electrolyzer modes as well as for optimization of operating condition for performance stability in real life applications. This review focusses on the recent developments of air electrode active materials design/development for performance improvement in UR-PEMFC. Among all developed electrode materials, the catalysts with Pt- and Ir-based metals still provided the maximum round-trip efficiency of about 50% at 500 mA cm^?2 in the unit cell.     

Electrophoretic deposition of carbon-supported octahedral Pt–Ni alloy nanoparticle catalysts for cathode in polymer electrolyte membrane fuel cells

The advanced electrochemical catalytic activity for oxygen reduction reaction (ORR) based on the octahedral Pt–Ni alloyed catalyst has been demonstrated. However, a means of fabricating catalyst electrodes for use in PEMFCs that is cost-effective, scalable, and maintains the high activity of Pt–Ni_alloy/C has remained out of reach. Electrophoretic deposition (EPD) is a colloidal production process that has a history of successful deployment at the industrial scale. Here, we report on the facile preparation of an effective and active cathode consisting of Pt–Ni alloy loaded on the carbon cloth substrate using the electrophoretic deposition (EPD) technique, in which the optimum applied voltages and suspension pH are systematically investigated to obtain the highly porous Pt–Ni_alloy/C catalyst electrode. In a half cell test, the EPD-made Pt–Ni_alloy/C catalyst electrodes fabricated at 45 V and in a solution with a pH of 9.0 yields the best performances. On the other, as an active cathode, the EPD-made Pt–Ni_alloy/C electrodes deliver a superior performance in single cell test, with the maximum power density reaches 7.16 W/mg_Pt, ～28.1% higher than that of the spray-made Pt/C conventional electrode. The outperformance is attributed to the significantly higher porosity and surface roughness of the EPD-made electrode.     

Comparative study of the break in process of post doped and sol–gel high temperature proton exchange membrane fuel cells

In this paper six High Temperature PEM (HTPEM) MEAs from two manufacturers have been tested. The MEAs are three Dapozol 77 from Danish Power Systems (DPS) with varying electrode composition and two Celtec P2100 and one Celtec P1000 from BASF. The break in process of the MEAs has been monitored using voltage measurements and impedance spectroscopy. The purpose of this study is twofold. One aim is to try and interpret the processes happening during break in. The other aim is to investigate whether the impedance spectra or the voltage profiles contain information that can be used to determine when an MEA has been broken in. To aid in the interpretation of the impedance spectra, equivalent circuit models are used. Three models are evaluated. The most detailed models produce the best fits but the most simple model is chosen, since it produces the most consistent results. The processes happening during break in cannot be determined with certainty but for the Celtec P MEAs the main changes seem to be related to improved electrode kinetics. Judging from the voltage and the fitted resistances, the Celtec P MEAs seem to have been broken in after 30 h. The Dapozol MEAs only undergo minor changes in impedance and voltage during the break in period. This may indicate that this MEA type can be used directly without the need for break in.     

Evaluation of Pt–Ru–Ni and Pt–Sn–Ni catalysts as anodes in direct ethanol fuel cells

In this study, the electrooxidation of ethanol on carbon supported Pt–Ru–Ni and Pt–Sn–Ni catalysts is electrochemically studied through cyclic voltammetry at 50 °C in direct ethanol fuel cells. All electrocatalysts are prepared using the ethylene glycol-reduction process and are chemically characterized by energy-dispersive X-ray analysis (EDX). For fuel cell evaluation, electrodes are prepared by the transfer-decal method. Nickel addition to the anode improves DEFC performance. When Pt₇₅Ru₁₅Ni₁₀/C is used as an anode catalyst, the current density obtained in the fuel cell is greater than that of all other investigated catalysts. Tri-metallic catalytic mixtures have a higher performance relative to bi-metallic catalysts. These results are in agreement with CV results that display greater activity for PtRuNi at higher potentials.     

Deposition of gold–titanium and gold–nickel coatings on electropolished 316L stainless steel bipolar plates for proton exchange membrane fuel cells

The effects of electropolishing and coating deposition on electrical resistance and chemical stability were studied for the stainless steel bipolar plates in proton exchange membrane fuel cell (PEMFC). A series of 316L stainless steel plates, selected as the substrate for a proton exchange membrane fuel cell (PEMFC) bipolar plate, were electropolished with a solution of H₂SO₄ and H₃PO₄ at temperatures ranging from 70 to 110 °C. The surface regions of the two electropolished stainless steel plates were coated with gold and either a titanium or nickel layer using electron beam evaporation. The electropolished stainless steel plates coated in 2-μm thick gold with a 0.1-μm titanium or nickel interlayer showed remarkably smooth and uniform surface morphologies in AFM and FE-SEM images compared to the surfaces of the plates that were coated after mechanical polishing only. The electrical resistance and water contact angle of the deposited stainless steel bipolar plates are strongly dependent on the surface modification treatments (i.e., mechanical polishing versus electropolishing). ICP-MS and XPS results indicate that after electropolishing, the coating layers show excellent chemical stability after exposure to an H₂SO₄ solution of pH 3. Finally, it was concluded that before coating deposition, the surface modification using electropolishing was very suitable for enhancing the electrical property and chemical stability of the stainless steel bipolar plate.     

Effects of Pt loading in the anode on the durability of a membrane–electrode assembly for polymer electrolyte membrane fuel cells during startup/shutdown cycling

A polymer electrolyte membrane fuel cell (PEMFC) stack of a fuel cell vehicle (FCV) is inevitably exposed to reverse current conditions, which are formed by the oxygen reduction reaction (ORR) induced at the anode with a hydrogen/air boundary during startup/shutdown processes. With an increase in the reverse current, the degradation rate of the cathode that experiences a highly corrosive condition (locally high potential) increases. In this work, the anode Pt loading is decreased from 0.4 to 0.1 mg cm⁻² to decrease the reverse current. The decrease in the anode Pt loading is found to decrease the hydrogen oxidation rates (HOR) during normal operation, but this loading decrease barely affected the cell performance. However, a decrease in the anode Pt loading can significantly decrease the reverse current, leading to a diminished cathode degradation rate during startup/shutdown cycling. It is revealed by slow decreases in the cell performance (i–V curves) and electrochemical active surface area (EAS), and a slow increase in the charge-transfer resistance (R_ct), which can be attributed to corrosion of the carbon support and dissolution/migration/agglomeration of the platinum catalyst.     

Investigation of corrosion properties with Ni–P/TiNO coating on aluminum alloy bipolar plates in proton exchange membrane fuel cell

Aluminum alloy bipolar plates have unique application potential in proton exchange membrane fuel cell (PEMFC) due to the characteristics of lightweight and low cost. However, extreme susceptibility to corrosion in PEMFC operation condition limits the application. To promote the corrosion resistance of aluminum alloy bipolar plates, a Ni–P/TiNO coating was prepared by electroless plating and closed field unbalanced magnetron sputter ion plating (CFUMSIP) technology on the 6061 Al substrate. The research results show that Ni–P interlayer improves the deposition effect of TiNO outer layer and increase the content of TiN and TiO_xN_y phases. Compared to Ni–P and TiNO single-layer coatings, the Ni–P/TiNO coating samples exhibited the lowest current density value of (1.10 ± 0.02) × 10^?6 A·cm^?2 in simulated PEMFC cathode environment. Additionally, potential cyclic polarization measurements were carried out aiming to evaluate the durability of the aluminum alloy bipolar plate during the PEMFC start-up/shut-up process. The results illustrate that the Ni–P/TiNO coating samples exhibit excellent stability and corrosion resistance.     

Corrosion resistant quaternary Al–Cr–Mo–N coating on type 316L stainless steel bipolar plates for proton exchange membrane fuel cells

Insufficient corrosion resistance, electrical conductivity and wettability of bipolar plates are some of the important issues affecting the performance of hydrogen fuel cells. To address these issues, an amorphous Al–Cr–Mo–N coating is deposited on type 316L stainless steel using direct current (DC) magnetron sputtering. The electrochemical corrosion behaviour is investigated under simulated fuel cell anode (H₂-purging) and cathode (air-purging) environment consisting of 0.5 M H₂SO₄ + 2 ppm NaF at 70 ± 2 °C. The corrosion current density is reduced to 0.02 μA cm⁻² comparable to the commercially used Ta/TaN coatings. The polarization resistance increases by two orders of magnitude and the interfacial contact resistance (ICR) reduces significantly due to the application of the coating. Further, the coating shows better water management due to high hydrophobicity than the bare stainless steel.