# Presentation: 504254

## Slide 1

## Electrochemical Hydrogen Storage and CaptureStephen M. LipkaCenter for Applied Energy ResearchUniversity of KentuckyLexington, KY 40511DOE/NSF-EPSCoR Conference 2004Experimental Program to Stimulate Competitive ResearchArgonne National LaboratoryJune 14-16, 2004

## Electrochemical Hydrogen Storage Methodology

**Electrochemical Hydrogen Storage Involves Cathodic Charging of Carbonaceous Electrode (with Catalyst) in Suitable Aqueous Electrolyte**
**Electrode usually Mechanical Mixture of Carbon and a Metal Additive (micron-sized metal powders such as Cu, Ni and Pd or nano-sized particles decorating the carbon surface)**** **
**Charged Cathodically for Prescribed Period of Time to Load with Hydrogen**
**Adsorbed Hydrogen is Stored “on” or “in” Carbon Via Hydrogen Dissociation and  Spillover**
**Gravimetric Density of Hydrogen Stored Determined Directly by Coulombs of Charge Released During Discharge (Oxidation) to Specified Cut-off Potential**

## Hydrogen Storage Mechanism

**Metal Acts as a Catalyst, Promoting Molecular-Dissociation via Spillover from the Dissociation Site to the Carbon Surface:**
**Ni + H****2****O + e****-**** = NiH****ad**** + OH****-**** ****	****	****	****(1)**
**NiH****ad**** + SWNT = SWNTH****ad**** + Ni****	****	****(2)**
**SWNT = single-wall nanotube**

## Summary of Reported Electrochemical Hydrogen Storage in Carbon Materials

| Lit. Citation |  |
| --- | --- |
| 1 | C. Nutzenadel, et. al., Electrochemical and Solid-State Letters, 2 (1) 30 (1999) |
| 2 | K. Jurewicz, et. al., Electrochemical and Solid-State Letters, 4 (3) A27 (2001) |
| 3 | X. Qin, et. al., Electrochemical and Solid-State Letters, 3 (12) 532 (2000) |
| 4 | S. M. Lee, et. al., Synthetic Metals, 113, 209 (2000) |
| 5 | N. Rajalakshmi, et. al., Electrochimica Acta, 45, 4511 (2000) |
| 6 | H. S. Youn, et. al., International Journal of Hydrogen Energy, 27, 937 (2002) |
| 7 | G. P. Dai, et. al.,  Electrochemical and Solid-State Letters, 5 (4), E13 (2002) |
| 8 | X. P. Gao, et. al., Electrochemical and Solid-State Letters, 4 (10), A173 (2001) |
| 9 | X. Yan, et. al., Chemical Physics Letters, 372, 336 (2003) |
| 10 | J. M. Skowronski, et. al., Advanced Materials, 15 (1), 55 (2003) |
| 11 | X. Chen, et. al., International Journal of Hydrogen Energy, 29, 743 (2004) |
| 12 | J. Liu, et. al., Abs. 130, pres. at 204th ECS Meeting, Orlando, FL, Oct. 12-16, 2003 |
| 13 | G-P. Dai, et. al., Nano Letters, 2 (5) 503 (2002) |
| 14 | I. Lombardi, et. al., Electrochemical and Solid-State Letters, 7(5) A115 (2004) |

## Summary of Reported Electrochemical Hydrogen Storage in Carbon Materials

| Material | Wgt% H | Experimental Conditions | Lit. Cit. |
| --- | --- | --- | --- |
| SWNT/Pd powder (1:4) | 0.39 | 6 M KOH, room temp, 1 atm, Idisch = 2500-5000 mA/g | 1 |
| Activated Carbon 
(1200 -1500 m2/g) | 1.5 | 6 M KOH, room temp, 1 atm, Idisch = 100 mA/g | 2 |
| MWNT/Ni  (1:10) | 0.7 | 6 M KOH, room temp, 1 atm, Idisch = 200-1000 mA/g | 3 |
| CNT/Ni powder (4:5) | 0.34 | 6 M KOH, room temp, 1 atm, Idisch =  0.4 mA | 4 |
| SWNT/Cu (1:3) | 2.9 | 30% KOH, room temp, 1 atm, Idisch =  10-100 mA | 5 |
| HNO3 treated CNF/ Ni powder (1:9) | 0.27 | 6 M KOH, room temp, 1 atm, Idisch = 100 mA/g | 6 |
| SWNT/Ni powder (1:3) | 1.2 | 6 M KOH, room temp, 1 atm, Idisch = 400 mA/g | 7 |
| CNT/Ni-P particles + Ni powder (1:3) | 1.1 | 6 N KOH, room temp, 1 atm, Idisch = 1000 mA/g | 8* |
| CNF/Ni-P particles (< 76 wgt.%) | 0.30 | 6 M KOH, room temp, 1 atm, Idisch = 1000 mA/g | 9* |
| MWNT (electrochem. oxidized) | 1.61 | 18 M H2SO4, 20 oC, 1 atm, cyclic voltammetry | 10 |
| CNT/Ni powder (1:20)
CNF/Ni powder (1:20) | 1.0
0.25 | 6 N KOH, room temp, 1 atm, Idisch = 800 mA/g | 11* |
| MWNT/Cu particles (electroless) | 6.1 | 6 M KOH, room temp.,1 atm , Idisch  = 1500 mA/g | 12 |
| SWNT pressed into Ni foam | 1.84 | 6 M KOH, room temp., 1 atm , Idisch  = 25 mA/g | 13 |
| Electrochem. Ox.  MWNT in  HNO3  | 0.3 | 6 M KOH and 0.3 M H2SO4,CV and Idisch  = 20 mA/g | 14 |

*** same research group**

## Experimental Variables

**Type of Carbon (Activated Carbon, Nanotubes,.....)**
**Electrolyte (KOH, H****2****SO****4****......)**
**Catalyst Type (Cu, Ni, Pd), Amount and Size**
**Charging Conditions (Galvanostatic, Rate, Potentiostatic,....)**
**Ambient (Air) or Pressure (H****2****) **

## Where’s the Hydrogen?

**Believed that Storage of Hydrogen in Nanotubes Similar to Gas Phase Experiments where Tubes Adsorb (or Absorb) Hydrogen by Increasing Pressure**
**Not Clear from Literature Whether Hydrogen is Stored Molecularly or Atomically or if Hydrogen is In or On Carbon Nanotube (Positron Annihilation?)**
**If Hydrogen is “Intercalated” into Nanotube, Series of Reactions Must take place, which include Physisorption, Chemisorption, Dissociation, Absorption and Diffusion.  Any One of in Sequence may be Rate-limiting**

## Carbon Electrode Fabrication 

**Carbon Materials**
**Purified SWNTs obtained from tubes@Rice**
**MWNTs obtained from Applied Sciences, Inc. (OH)**
**MWNTs Washed repeatedly with HCl to Remove any Metal Residue**
**Nanotubes Prepared into Free-standing Binderless Paper using Dispersion/Filtering Technique**
**Metal Catalysts Adsorbed onto Nanotube Mats by Washing with Selected Metal Salt Solution (Cu, Ni and Pd)**
**Mat Rinsed with Water to Remove Excess Metal Salt**
**Reduction under Hydrogen at 200 and 1100 ****o****C**

## Electrochemical Test Conditions

**Metal-doped Nanotube Paper Held in High-purity Nickel Screen**
**Three- electrode Cell using Ni Screen Counter and Hg/HgO/KOH Reference Electrodes**
**31 wgt.% KOH (Flooded)**
**Charged Cathodically at 200 or 1000 mA/g**
**After Desired Coulombs of Charge Passed, Cell Open-circuited for 5 Minutes**
**Discharge Capacity Measured at 200 mA/g to – 0.2V vs Hg/HgO/KOH**
**Charge/Discharge Performed under Ambient Conditions (Air) or under Hydrogen Pressure**

## Schematic of Electrochemical Cell

**1 - Nickel mesh counter electrode**
**2 - Hg/HgO/KOH reference electrode**
**3 - Carbon nanotube working electrode in nickel screen**
**4 - Electrolyte (6 M KOH).**
**5 - Three-way valve**
**6 - Pressure gauge**

** ****Amount of Hydrogen Stored Calculated **
**  ****from Discharge Capacity (mAh/g)**
** ****F = ItA/Wz     where:**
**F = 9.64846 x 10****4**** C/equiv.**
**I = current (A)**
**t = time (s)**
**A = atomic weight (amu)**
**z = number of electrons**
**W = weight (g)**

**1 C****H**** = 1.04466 x 10****-5 ****g**

## Cyclic Voltammogram for SWNT Electrode8.5 wt.% Pd (200 oC) in 31% KOH, Air

## Charge/Discharge Regimen

**201**

**243**

**299 **

**300**

**5 min. Open Circuit**

**360**

**480**

**SWNT 8.5 wt.% Pd, 200**** o****C @ 5.4 atm.**

**I****Charge**** = 1000 mA/g **
**I****Discharge**** = 200 mA/g**

## Effect of Pd Content on Electrochemical Potential

## Effect of Hydrogen Pressure and Pd Content on Capacity

## Effect of Charge Rate on Capacity8.5 wt.% Pd, 5.4 atm. H2

## Effect of Charging Coulombs on Capacity (8.5 wt.%, 200 oC) 5.4 atm. H2

## Effect of Pd Content on CapacitySWNT, 5.4 atm. Hydrogen

## Effect of Hydrogen Pressure on Electrochemical Hydrogen Storage (SWNT/Ni)

| Material | *Test Conditions | Capacity
(mAh/g) | Hydrogen
(wt. %) |
| --- | --- | --- | --- |
| SWNT – deposited Ni | Ambient (air) | 30.4 | 0.11 |
| SWNT – deposited Ni | 1.4 atm H2 | 25.8 | 0.09 |
| SWNT – deposited Ni | 5.4 atm H2 | 44.0 | 0.16 |

***Charged for 50 min. @ 1000 mA/g, discharged @ 200 mA/g to – 0.2V vs. Hg/HgO.  **
** ****Charge/discharge protocol corresponds to all of the data.  Ambient = air at 1 atm.**

## Effect of Hydrogen Pressure on 

**Electrochemical Hydrogen Storage (200 ****o****C Pd)**

***Charged for 50 min. @ 1000 mA/g, 5 min. @ Open Circuit,  discharged @ 200 mA/g to – 0.2V**
** ****vs. Hg/HgO; Charge/discharge protocol corresponds to all data.  Ambient = air at 1 atm.**

## Effect of Nanotube Type on Capacity

| Material | *Test Conditions | Capacity (mAh/g)
 | Hydrogen 
(wt. %)
 |
| --- | --- | --- | --- |
| MWNT/SWNT (6:4) <2% Pd | Ambient (air) | 9.9 | 0.04 |
| MWNT/SWNT (6:4) <2% Pd | 1.4 atm H2 | 15.2 | 0.06 |
| MWNT/SWNT (6:4) <2% Pd | 5.4 atm H2 | 116.2 | 0.43 |

***Charged for 50 min. @ 1000 mA/g, discharged @ 200 mA/g to – 0.2V vs. Hg/HgO.  **
** ****Charge/discharge protocol corresponds to all of the data.  Ambient = air at 1 atm.**

## Summary

**Demonstrated electrochemical method to reversibly charge carbon nanotubes with hydrogen**
**Pd-doped Nanotube Shows Promise as a Rechargeable Battery Electrode (e.g. NiOOH/H****2****)**
**Amount of Metal Catalyst *****and***** its Distribution on the Nanotube Surface Affect Hydrogen Storage Capacity**
**Potential Hysteresis (Charge/Discharge) Increased with Decreased Pd Loading**
**Reversible Hydrogen Storage Demonstrated for 100+ Cycles**
**Coulombic Efficiencies ~ 0.60**

## Summary

**Electrochemical Hydrogen Storage Capacity is Affected by Nanotube Type, Catalyst Type and Content and Hydrogen Pressure**
**Pd decorated Nanotubes were Most Effective in Storing Hydrogen when Compared to Other Metals (w/ 8.5% Pd @ 5.4 atm. Displaying the Highest Storage Capacity)**
**Modest Hydrogen Pressure (1.4 atm) Generally Produced Little or no Increase in Hydrogen Storage**
_**Small**_** Amount of Pd (1.1%) Produced 1600% Increase in Hydrogen Storage Capacity at Hydrogen Pressure of 5.4 atm**
**In General, Slower Charging Rates (200 mA/g) Displayed Larger Discharge Capacity**

## Acknowledgements

**Dr. David Firsich, Inorganic Specialists, Inc. for Preparing Catalyst-doped Nanotube Paper**

## Future Strategies?

**Optimization of Metal Doping of Nanocarbons**
**Improve Distribution of Catalyst and Location of Catalyst **
**Chemical Precipitation using Surface-treated Tubes to Provide Specific Sites for Catalysts to Affix ) e.g. Sulfonic Acid Groups, CVD, Electrodeposition**
**Sulfonic Acid Groups First Substituted onto the Nanotubes; Attach at the Tube Ends, the Only Place Where any Bonded Group *****can***** Attach on a SWNT**
**Rinse Nanotubes with a Metal Catalyst Solution in which Attached Sulfonic Acid groups scavenge soluble metal ions (as they do in ion-exchange materials), affixing them to the tube ends.****   ****Nanotubes then rinsed with water to remove excess metal ions, metal-modified nanotubes are reduced in hydrogen to create active catalyst.**

- In another experiment, a nanotube sample was _not_ rinsed with water after treating it with palladium solution; rather, the palladium solution was allowed to dry on the sample.   This increased the final palladium content to 8.5%.   This 8-fold increase in palladium did not produce an 8-fold increase in hydrogen storage, presumably because the palladium was inefficiently utilized.   The result points out that the amount of metal catalyst *and* its distribution will affect hydrogen storage

- When we use this technique to apply palladium catalyst to single-wall nanotubes (Rice), the nanotube product has a 1.1% palladium content.   This _small_ amount of palladium produces a 1700% increase in hydrogen storage at a hydrogen pressure of 5.4 atm., as shown in the table below.    In fact, this 1.1% palladium sample stores as much hydrogen as that claimed in a report(4) where 400 times more palladium was used.

## Electrochemical Hydrogen Storage

**Electrochemical Hydrogen Storage Involves Cathodic Charging of Carbonaceous Electrode (with Catalyst) in Appropriate Aqueous Electrolyte**
**Adsorbed Hydrogen is Stored “on” or “in” Carbon Via Hydrogen Dissociation and  Spillover**
**Gravimetric Density of Hydrogen Stored is Measured Directly by Coulombs of Charge Passed During Discharge (Oxidation)**

## Effect of Reduction Temperature on Capacity for Pd Decorated SWNTs

## Charge/Discharge Efficiency (SWNT 8.5 wt.%, 200 oC) 80 psi

## Effect of Reduction Temperature on Charge Profile (Pd Decorated SWNTs) ?psi

**Measured Capacity is Sum of Capacities from All Reducible Species on the Electrode Surface During Discharge**

**An 8-fold increase in Pd did not produce 8-fold increase in hydrogen storage, presumably**
** ****because the Pd De-wetted from the Nanotube Surface or was Inefficiently Utilized**

## Summary

**Highest Gravimetric Hydrogen Density Stored Electrochemically Achieved for SWNT Containing 8.5 wgt.% Pd @ 5.4 atm Hydrogen **
**Coulombic Efficiencies ~ 0.60**
_**Small**_** Amount of Pd (1.1%) Produced 1600% Increase in Hydrogen Storage Capacity at Hydrogen Pressure of 5.4 atm**