In order to guarantee the privacy and safety of data transactions in Wireless Sensor Networks (WSNs), secure key transportation and unique node identification have become major concerns. WSNs are deployed in a wide range of applications with a high demand for secure communications. When designing a secure key management protocol for WSNs, special attention should be given to the resource constraints of the devices and the scalability of the network. In this paper, we exploit public-key nature protocols to define a hybrid key establishment algorithm for symmetric key cryptography. We propose an Elliptic Curve Cryptography based implicit certificate scheme and show how to utilize the certificates for deriving pair-wise link keys in a WSN. By a performance and security analysis, we justify that the proposed scheme is well fitting with the functional and architectural features of WSNs. Both experimental results and theoretical analysis show that the proposed key establishment protocol is viable to deploy in a real-time WSN application.