We report the successful development of Ti_(29)V_(62−x)Ni_9Cr_x (x = 0, 6, 12) body centered cubic metal hydride (MH) electrodes by addressing vanadium corrosion and dissolution in KOH solutions. By identifying oxygen as the primary source of corrosion and eliminating oxygen with an Ar-purged cell, the Cr-free Ti_(29)V_(62)Ni_9 alloy electrode achieved a maximum capacity of 594 mAh g^(-1), double the capacity of commercial AB_5 MH electrodes. With coin cells designed to minimize oxygen evolution, the cycle stability of a Ti_(29)V_(62)Ni_9 alloy electrode was greatly improved with either vanadate ion additions to the electrolyte or Cr-substitution in the alloy. Together, both approaches resulted in a reversible capacity of around 500 mAh g^(−1) for at least 200 cycles. We performed energy density calculations for a 100 W h MH–air cell utilizing the high capacity Ti_(29)V_(62−x)Ni_9Cr_x electrodes and found that these cells are comparable in energy density to state-of-the-art Li-ion batteries

Fultz, Brent

Tan, Hongjin

Weadock, Nicholas J.

Yang, Heng

Caltech Authors - Main

1Supplementary Information
High capacity V-based metal hydride electrodes for rechargeable batteries 
Heng Yang, Nicholas J. Weadock, Hongjin Tan, and Brent Fultz
California Institute of Technology, Pasadena, California 91125, USA
The microstructures of the Ti29V62-xNi9Crx (x = 0, 6, and 12) alloys were characterized by x-
ray diffraction (XRD), scanning electron microscopy (SEM) with backscattered electron imaging 
(BES), and energy dispersive x-ray spectroscopy (EDS). Alloy samples characterized by XRD 
were pulverized to a fine powder via the procedures outlined in the main text. For SEM and EDS, 
the samples were mounted in graphite and polished to a mirror finish.
Fig. S1 Lattice parameter, a, of Ti29V62-xNi9Crx (x = 0, 6, and 12) alloys
Measuring the lattice parameter is challenging because of the broad, overlapping peaks from 
regions of the BCC phase with different chemical compositions. The lattice parameter, a, of the 
majority V-rich region clearly decreases with higher Cr content, however. From our analysis of 
the backscattered electron images, we estimate the fraction of the Ni-rich region to be about 9 
vol%. The small amount of the Ni-rich region, the small number of reflections in the measured 2θ 
range, and the broad, overlapping peaks makes analysis of the lattice parameter of this region 
Electronic Supplementary Material (ESI) for Journal of Materials Chemistry A.
This journal is © The Royal Society of Chemistry 2017
2difficult. Given that the second peak has nearly disappeared in the x=12 sample, we estimate that 
the lattice parameter of the Ni-rich region is slightly smaller, on the order of 0.3065 – 0.307 nm.
Fig. S2 SEM images and EDS results of (a) Ti29V62Ni9 and (b) Ti29V56Ni9Cr6 alloy ingots. The 
compositions reported are the average compositions of the V-rich (dark) and Ni-rich (light) 
regions as determined by EDS.
Electrochemical cells were assembled using electrodes of the Ti29V62-xNi9Crx (x = 0, 6, and 
12) alloys. Three types of cells were used: (1) 3-electrode beaker cells assembled in air, (2) a 3-
electrode Ar purged cell, and (3) coin cells assembled in air. In addition to electrochemical 
cycling, elemental analysis of the electrolyte was performed with inductively coupled plasma 
mass spectrometry (ICP-MS), and the surface oxidation states of the cycled electrodes were 
characterized by x-ray photoelectron spectroscopy (XPS).  
3Fig. S3 Discharge curve of Ti29V62Ni9 electrode in KOH solutions.
Figure S3 shows the discharge curves of Ti29V62Ni9 electrodes in pristine 1 M KOH, 6.9 M 
KOH, and 6.9 M KOH with 500 mM KVO3. All three electrodes were assembled in air in three-
electrode beaker cells. The Hg/HgO reference electrodes were prepared with the corresponding 
KOH solutions. The experiments were performed by discharging at a small current of 10 mA/g 
based on the loading of the alloy powder. These cells were discharged without prior charging, so 
the capacity is a result of metal oxidation rather than oxidation of absorbed hydrogen. According 
to the Pourbaix diagram, vanadium oxidation to vanadate ions is expected at around -0.9 V vs. 
Hg/HgO (pH =15) for a pure vanadium electrode. The corresponding dissolution potential for the 
alloy is approximately -0.7 V in 6.9 M KOH solution, consistent with that reported in the main 
text (Fig. 3a). Increasing the vanadate ion concentration in the electrolyte, or reducing the pH, 
both shift the dissolution potential to a more positive value, as predicted by the Pourbaix diagram. 
Slower kinetics may also contribute to the shift in dissolution potential, however. 
4Table S1 ICP-MS results of the KOH solutions in which Ti29V62-xNi9Crx (x = 0 and 12) alloy 
electrodes were stored for 2 days.
% Loss from electrode
Alloy composition Ti V Ni Cr
Ti29V62Ni9 0.04 6.77 0.11  -
Ti29V50Ni9Cr12 0.09 2.47 0.07 0.15
5
Fig. S4 XPS spectra of V, Cr, Ni, and Ti prior to charge and after full discharge of Ti29V62-xNi9Crx 
(x = 0, 6, and 12) alloys. Boxed numbers correspond to the state of charge indicated in the main 
text Fig. 3a. 
6The XPS spectra for the Ti29V62-xNi9Crx (x = 0, 6, 12) electrodes before cycling (spectra [1]) and 
after charging and discharging to -0.50 V (spectra [4]) are presented in Fig. S4. The V 2p3/2 
metallic peaks disappear for the Ti29V62Ni9 and Ti29V56Ni9Cr6 electrodes, and the oxide 2p3/2 
peaks shift to a higher binding energy. This shift corresponds to an increase in the oxidation state 
of the V. The surface oxide is likely a mixed oxide and manifests as a broad peak. There are no 
shifts in the Ti 2p3/2 or 2p1/2 peaks; the native oxide layer appears to be stable in the electrolyte 
environment. The XPS spectra for Ni show a disappearance of the metallic 2p3/2 and 2p1/2 peaks 
and emergence of the corresponding Ni(OH)2 peaks. The NiO 2p3/2 and 2p1/2 peaks persist during 
cycling. The broad Cr 2p3/2 and 2p1/2 peaks have a small shift to higher energy and an increase in 
amplitude, indicating that a CrOx layer has formed on the surface. Similar to V, the CrOx layer is 
likely a mixed oxide.  
Fig. S5 Cycle performance of Ti29V62Ni9Crx (x = 0, 6, and 12) alloy electrodes in various 
electrolytes. All tests were performed in air-saturated beaker cells. Cells were charged to 800 
mAh/g, then discharged following the three-step procedure to -0.75 V.
7Fig. S6 Operation potential for the Ni(OH)2/NiO(OH) electrode used for coin cell assembly. 
Current density is based on the mass loading of the Ni(OH)2 electrode powder mixture. Ea and Ec 
correspond to the plateau potentials on charge and discharge, respectively. The shaded area 
corresponds to the applied current density range for the paired MH electrode (10 to 100 mA/g 
based on 3 mg/cm2 loading).
The design of the coin cells considered the following factors: (1) with a small internal 
volume, the cells can be sealed in air with negligible amount of oxygen trapped inside the cells; 
(2) the MH electrode is paired with a much larger Ni(OH)2 positive electrode, so that the positive 
electrode does not evolve oxygen during cell overcharge and the potential remain relatively stable 
as a reference electrode. This configuration is also more sensitive to capacity degradation of the 
MH electrode. Figure S6 shows the operation potential of the Ni(OH)2/NiOOH electrode used in 
this study. These electrodes were obtained from BASF (BASF-Ovonic, Rochester Hills, MI, USA) 
and punched into disks with a 1.27 cm diameter. The capacity of each electrode is about 24 mAh. 
The results shown in Fig. S6 were obtained by first charging a Ni(OH)2 electrode to 12 mAh, 
converting part of the Ni(OH)2 to NiOOH. The Ni(OH)2/NiOOH electrode was then charged at 
the specific current density shown in Fig. S6 for 7200 s, rested for 1200 s, and discharged at the 
same current density for 7200 s. The charge (Ea) and discharge (Ec) plateau potentials were 
recorded and plotted in Fig. S6. The smallest current for the discharge of the coin cells was 20 
8mA/g, and the vanadium dissolution potential for Cr-free alloy is at about -0.7 V versus Hg/HgO 
(Fig. S1 and Fig. 3a). For these reasons, and to allow a voltage window as large as possible, the 
cut-off voltage for all coin cells was set at 1.10 V. All coin cells were assembled with the half-
charged cathodes.
The Ti29V50Ni9Cr12 electrodes in Fig. 5a, b were charged for 550 mAh/g, which is higher 
than its maximum discharge capacity as measured in Fig. 4. The Coulombic efficiency of these 
coin cells are also less than 100%. As a result, a small amount of hydrogen is evolved during 
charging, which may be consumed on the positive electrode by reducing NiO(OH) to Ni(OH)2.1 
Fig. S7 Charge/discharge curves of Ti29V62Ni9 and Ti29V50Ni9Cr12 electrodes in the coin cell 
configuration. The cells were charged for 550 mAh/g and three-step discharged at 100, 40 and 20 
mA/g to 1.10 V. The decrease of discharge capacity at the high current step shows that the rate 
capability of the MH electrode decreases with cycling, despite excellent capacity retention.
Section 3: Energy density calculations of a MH-Air battery cell
Energy density calculations of the MH-air batteries are based on a prismatic cell design.  This 
MH-air cell has three electrodes: oxygen-reduction-reaction (ORR) electrode, oxygen-evolution-
reaction (OER) electrode, and a MH anode. The ORR electrode is a commercial alkaline fuel cell 
air electrode from Electric Fuel (Electric Fuel Limited, Bet Shemesh, Israel) with the catalytic 
9MnO2 on a substrate film of PTFE.2 The OER electrode is made from Monel mesh. The MH 
anode is made by pressing MH alloy powder onto a nickel mesh substrate. Figure S8 shows the 
layout of the two-sided prismatic cell. The cell consists of two ORR-MH-OER stacks with the 
OER electrodes facing the interior of the cell. The OER electrodes are separated by a channel 
through which electrolyte flows. Flowing electrolyte is utilized to compensate for local changes 
in electrolyte concentration and pH due to water loss or generation as the cell charges or 
discharges. 
The cell was designed to achieve 100 Wh with an overall dimension of 20.2 cm x 10.2 
cm x 1.13 cm. The cell discharge potential is determined from the ORR overpotential data as a 
function of cell current density, provided by the vendor of ORR electrode, and our cycling data 
for the MH anode. The air electrode potential is presented in Fig. S9. The current density needed 
from the ORR electrode is chosen to match the specifications of the MH anode for a given 
discharge rate. The total weight of the cell also includes those of passive components of electrode 
substrates, separators, frame, and seal materials. Key input parameters of this model are the MH 
anode thickness and MH anode specific discharge capacity. Other input parameters including 
dimension of the prismatic cell, dimensions of the two air electrodes, and the cell discharge C-
rate, are preset based on project targets and some of the preliminary lab testing results.
Based on the preliminary experimental results of the cell similar to this design, the MH 
anode specific discharge capacity is in a range of 150 – 550 mAh/g, and the practical thickness of 
the anode is above 300 microns. The model varies the MH anode capacity from 0 – 800 mAh/g, 
and thickness from 0.3 – 5 mm. Varying these two parameters leads to a 2D contour plot of cell 
level gravimetric and volumetric energy density as shown in the main text Fig. 7. The colored 
lines are regions of constant energy density in Wh/kg or Wh/L. MH anodes containing Ti29V62-
xNi9Crx (x = 0, 6, 12) were demonstrated in the manuscript to have a specific capacity of more 
than 400 mAh/g, making it possible to achieve 200 Wh/kg. From the sensitivity analysis of Fig. 7, 
there is also an optimized range of the MH anode thickness in this prismatic model, based on the 
targeted energy density. For example, with a target of 200 Wh/kg, the MH anode thickness is 
optimally 2 – 2.5 mm. The inputs and results from an individual calculation for the contour plots 
of Fig. 7 are provided in Table S2 below. The input values in the EXAMPLE CASE column are 
10
based on the specifications determined by Fig. S9 and our MH cycling data. For this example, an 
anode capacity of 400 mAh/g and thickness of 2.2 mm are chosen. The nominal cathode current 
is set to match the specifications of the MH anode with a discharge rate of C/3. The model then 
returns a specific energy density of 207 Wh/kg and a volumetric energy density of 427 Wh/L.
 
Figure S8. Top view (left) and 3D side view (right) drawings of the 100-Wh MH-air cell 
(dimensions not proportional).
Figure S9: Plot of electrode potential versus current density for the air electrode manufactured by 
Electric Fuel.2
11
Table S2: Input values and energy density results for an example case of a 100 Wh MH-Air cell.
Cathode density 4.00E+03 kg/m^3 Cell width 2.02E-01 m Cathode nominal current 1.30E+03 A/m^2
Anode density 4.00E+03 kg/m^3 Cell length 1.02E-01 m Cathode nominal potential -1.44E-01 V
KOH Density 1.25E+03 kg/m^3 Cell thickness w/o frame 9.30E-03 m  vs. Hg/HgO
Plastic density 9.00E+02 kg/m^3 Cell thickness w/ frame 1.13E-02 m Anode capacity 4.00E+02 Ah/kg
Stainless steel density 8.00E+03 kg/m^3 Frame area on air electrode 2.90E-03 m^2 Anode nominal potential -8.50E-01 V
Aluminum density 2.70E+03 kg/m^3 Active area, 2 sides 3.42E-02 m^2 vs. Hg/HgO
Nickel mesh density 2.50E+02 kg/m^3 Frame volume 1.15E-05 m^3 Anode thickness 2.20E-03 m
Cell Width 2.00E-01 m Whole cell volume 2.33E-04 m^3 Cell nominal potential 7.06E-01 V
Cell Length 1.00E-01 m Frame weight 1.03E-02 kg Corresponding anode C-rate 3.16E-01 1/h
Frame width 5.00E-03 m Cathode weight 8.00E-02 kg Corresponding cell current 4.45E+01 A
Cathode thickness 5.00E-04 m Anode weight 3.52E-01 kg Cell Power 3.14E+01 W
Anode thickness 2.20E-03 m Aux electrode weight 2.50E-03 kg Cell theoretical capacity 1.41E+02 Ah
Aux electrode thickness 5.00E-04 m Cell total weight w/o KOH 4.45E-01 kg Cell nominal energy 9.94E+01 Wh
Separator thickness 1.20E-04 m Cell total weight w/ KOH 4.81E-01 kg Specific energy 2.07E+02 Wh/kg
Frame thickness 1.00E-03 m Volumetric energy 4.27E+02 Wh/L
Flow chn thickness 2.92E-03 m
Cooling thickness 5.00E-04 m
KOH to MH mass ratio 2.05E-01 g/g
Table of Cell Parameters
INPUTs OUTPUTs EXAMPLE CASE
1 Purushothaman, B. K. & Wainright, J. S. Analysis of Pressure Variations in a Low-
Pressure Nickel-Hydrogen Battery– Part 2: Cells with Metal Hydride Storage. J. Power 
Sources 206, 421-428, doi:10.1016/j.jpowsour.2012.01.149 (2012).
2 "Electric Fuel Air Electrodes" http://electric-fuel.com/rd/zinc-air/air-electrode/ (2015). 


High capacity V-based metal hydride electrodes for rechargeable batteries

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High capacity V-based metal hydride electrodes for rechargeable batteries

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