The shape of the CV curve varies with the electrolyte and the property of the material used. As you are using a neutral aqueous electrolyte and an EDLC material, it should give a square shape which you are able to produce until 0.3V. That means your material is reversible for supercapacitor application up to 0.3 V in 6M NaOH. Try other alkaline and acid electrolytes where you may be able to increase the potential window. The CV curves should be free from water redox peaks and improper cleaning method of Nickel foam can produce redox peaks. Need to take a CV of a cleaned nickel foam before coating and determine whether it consist of any redox peak. The redox peak shouldn't be an issue as you are getting a square peak up to 0.3 V; so try to change the molarity of electrolyte or use different electrolytes.

This is nothing more than a brainstorm but have you tried different scan rates? 

in 6M KOH using Ni Foam as a current collector, oxygen evolution starts after ~0.4 to 0.45V. Then I think you only can go to 0.4V. 

Kenichi, yes I have tried.

Saad, Could you elaborate your answer, is there literature that can prove this 0.45 V limit?

Ni foam can work also as an electrode. Did you perform cycling of empty electrode? Also you should perform it in different scan rates.

Dear Yasun, Can you show the CV that you got...?

What was your scan rate?  As Dr Valerii mentioned you can conduct CV test for Ni foam itself (With different scan rates, with slow scan rate, it is more probable that Ni could be undergoing redox reaction)...

With increased scan rate (Passivation layer formation concept), coated materials thickness,  you can avoid the reaction of Ni...  

http://www.ingentaconnect.com/content/asp/jnan/2014/00000004/00000001/art00010

https://www.researchgate.net/post/How_can_we_reduce_oxidation_reduction_current_of_Ni_foam_during_CV_scans_within_0_to_05_V_in_6_M_KOH_Is_pretreatment_with_HCl_enough

As far as I know, square shape CV means high capacitance and it depends on your electrode performance( pay attention that Graphene oxide is not conduvtive). however with suitable scan rate and electrolyte you can obtain square shape CV diagram. best regards

The shape of the CV curve varies with the electrolyte and the property of the material used. As you are using a neutral aqueous electrolyte and an EDLC material, it should give a square shape which you are able to produce until 0.3V. That means your material is reversible for supercapacitor application up to 0.3 V in 6M NaOH. Try other alkaline and acid electrolytes where you may be able to increase the potential window. The CV curves should be free from water redox peaks and improper cleaning method of Nickel foam can produce redox peaks. Need to take a CV of a cleaned nickel foam before coating and determine whether it consist of any redox peak. The redox peak shouldn't be an issue as you are getting a square peak up to 0.3 V; so try to change the molarity of electrolyte or use different electrolytes.

Thank you Dr Valerii, Dr Saad, Syam, Hossein and Sridar....I have studied valuable facts that you point out here. I hope Syam is totally correct in this case due to it might impure Ni foam and alkaline electrolyte makes oxidation. 

Sridar I have attached different graphs in different trials but its shows square shape in low voltage (from 0.1v to -0.1v) with some noise. In 0 V to 0.7 V the shape is changed as well as its shown oxidation. I think if I changed my electrolyte (6M NaOH) into neutral electrolyte like NaCl, I could remove the oxidation current. I am expecting your perfect suggestions in this time as well

and one more thing to know.....do you guys normalising the CV curves from the mass of the capacitive materials?

After looking into your CVs, one thing is for sure that your electrode is prone to side reactions. To investigate whether it is due to electrode material or electrolyte, you can dilute your electrolyte. And noise in the CVs is suggestive to poor material attachment or poor electrical connections or your material coating is much thicker. Also, I am just wandering, how come graphene oxide is giving that much high current, it must be reduced some how. As far as you are  playing with GO, which is known to have abundant functional groups (-COOH, -OH, =O, etc, depends on route of preparation), it is a bit difficult to see perfect rectangular shape. By the way why do you need perfect rectangular shape?

If you will use chloride solution, it is possible to obtain another one process Ni0 Ni2++2e. I suggest to use inert materials as an substrate (Pt, Au) in order to exclude other processes. As for normalization, it is common rule to estimate specific values for active materials for power sources.

Thank you, Dr Valerii and Vipin,

And there is a little thing to clarify.  If I going for publication do we need to show our CV graphs within high potential gap like 1 V ? Most of the papers have shown their CV and GCD both in 1 V potential difference (1st link), But I have found there are some with 0.4 V gap (2nd link).

http://pubs.acs.org/doi/pdf/10.1021/am507656q

http://pubs.rsc.org/en/content/articlepdf/2015/ra/c5ra11711a

if we can insert CV and GCD graphs within 0.4 V potential window based on Supercapacitor I can easily go for publication.

Yasun, in 3 electrode system measurements (as 2nd paper), the potential window is from 0 to 0.4 V, whereas if you need to assemble the symmetric or asymmetric supercapacitor device (2 electrodes system, as 1st paper) you can go from 0 to 1 V or more (depends on electrodes and electrolyte).

You can find both in the attached paper

The research shouldn't be paper oriented. If we intent to research for publication, it will loose its purpose. Please check the positive experimental points of your material. Try to extend the same in the next set of experiments, there it is, results are ready for publication. Voltage window and capacitance are not the sole criteria for publication. Check the capacitance retention, peak currents, mode of diffusion and so on. In one of the field, if your results have an upper edge, it is eligible for publication. Please, focus the manuscript according to it.