## 4 Conclusions  
In the present study, CaMn\({}_{2}\)O\({}_{4}\) ultrafine powders with different morphologies were synthesised and their catalytic performance in photo-electrochemical water splitting was investigated. CaMn\({}_{2}\)O\({}_{4}\) powders with irregular-shapes and nanowire shapes were synthesised using a dry mechanochemical method and a hydrothermal method, respectively.  
To study their photo-electrocatalytic properties, CaMn\({}_{2}\)O\({}_{4}\) powders were embedded within polypyrrole to form an anode in a photo-electrochemical cell. It was found that irrespective of the morphology of CaMn\({}_{2}\)O\({}_{4}\), the presence of CaMn\({}_{2}\)O\({}_{4}\) in the electrode resulted in a higher dark and light current, in comparison to the control without CaMn\({}_{2}\)O\({}_{4}\). The evolution of O\({}_{2}\) gas from the electrode surface was also observed. Thus, CaMn\({}_{2}\)O\({}_{4}\) showed a catalytic property in the water oxidation reaction.  
Figure 7: Tafel plots of (i) CaMn\({}_{2}\)O\({}_{4}\) nanowires/PPy (with light illumination), (ii) CaMn\({}_{2}\)O\({}_{4}\) nanowires/PPy (without light illumination, (iii) irregular-shaped CaMn\({}_{2}\)O\({}_{4}\) particles/PPy (with light illumination), (iv) irregular-shaped CaMn\({}_{2}\)O\({}_{4}\) particles/PPy (without light illumination) (dark), (v) PPy only (with light illumination) and (vi) PPy only (without light illumination).  
Upon light illumination, the current density was increased and the evolution of O\({}_{2}\) gas also increased, demonstrating the photocatalytic nature of CaMn\({}_{2}\)O\({}_{4}\).  
The electrode with CaMn\({}_{2}\)O\({}_{4}\) nanowires exhibited higher dark- and light- current in comparison to the electrode with irregular-shaped CaMn\({}_{2}\)O\({}_{4}\) particles. Electrochemical impedance spectroscopy studies revealed that the electrochemically active area increased in the order: PPy alone < irregular-shaped CaMn\({}_{2}\)O\({}_{4}\) particles/PPy < CaMn\({}_{2}\)O\({}_{4}\) nanowires/PPy, which was identified as a reason for the higher dark and photocurrent of CaMn\({}_{2}\)O\({}_{4}\) nanowires/PPy electrode. However, surface-area-normalised data suggest that the influence of particle shapes on the light-driven catalysis of CaMn\({}_{2}\)O\({}_{4}\) is relatively weak. The result suggests that the difference in catalytic activities among the crystal facets of CaMn\({}_{2}\)O\({}_{4}\) is small.  
The following are available online at [[http://www.mdpi.com/2076-3417/9/11/2196/s1](http://www.mdpi.com/2076-3417/9/11/2196/s1)]([http://www.mdpi.com/2076-3417/9/11/2196/s1](http://www.mdpi.com/2076-3417/9/11/2196/s1)), Figures S1 and S2.  
Conceptualization, A.G., T.T. and G.F.S.; methodology, A.G., M.A.; writing--original draft preparation, A.G.; writing--review and editing, G.F.S., T.T., A.G.  
This research received no external funding.  
[PERSON] would like to recognise the PhD research fellowship from the Australian National University. Access to the facilities of the Centre for Advanced Microscopy (CAM) with funding through the Australian Microscopy and Microanalysis Research Facility (AMMRF) is gratefully acknowledged. MA thanks the Government of Iraq for a PhD scholarship. The authors thank A/Prof [PERSON] of the University of Wollongong for use of a GC gas analyser. The authors acknowledge the Australian National Fabrication Facility (ANFF) Materials Node for equipment use and [PERSON] for the design and printing of the custom-built apparatus. Support from the Australian Research Council Centre of Excellence Scheme (Project Number CE140100012) is gratefully acknowledged.  
The authors declare no conflict of interest.